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1
Farrell, Brian F., e Petros J. Ioannou. "Emergence of Jets from Turbulence in the Shallow-Water Equations on an Equatorial Beta Plane". Journal of the Atmospheric Sciences 66, n. 10 (1 ottobre 2009): 3197–207. http://dx.doi.org/10.1175/2009jas2941.1.
Testo completoAbstract (sommario):
Abstract Coherent jets, such as the Jovian banded winds, are a prominent feature of rotating turbulence. Shallow-water turbulence models capture the essential mechanism of jet formation, which is systematic eddy momentum flux directed up the mean velocity gradient. Understanding how this systematic eddy flux convergence is maintained and how the mean zonal flow and the eddy field mutually adjust to produce the observed jet structure constitutes a fundamental theoretical problem. In this work a shallow-water equatorial beta-plane model implementation of stochastic structural stability theory (SSST) is used to study the mechanism of zonal jet formation. In SSST a stochastic model for the ensemble-mean turbulent eddy fluxes is coupled with an equation for the mean jet dynamics to produce a nonlinear model of the mutual adjustment between the field of turbulent eddies and the zonal jets. In weak turbulence, and for parameters appropriate to Jupiter, both prograde and retrograde equatorial jets are found to be stable solutions of the SSST system, but only the prograde equatorial jet remains stable in strong turbulence. In addition to the equatorial jet, multiple midlatitude zonal jets are also maintained in these stable SSST equilibria. These midlatitude jets have structure and spacing in agreement with observed zonal jets and exhibit the observed robust reversals in sign of both absolute and potential vorticity gradient.
2
Farrell, Brian F., e Petros J. Ioannou. "Formation of Jets by Baroclinic Turbulence". Journal of the Atmospheric Sciences 65, n. 11 (1 novembre 2008): 3353–75. http://dx.doi.org/10.1175/2008jas2611.1.
Testo completoAbstract (sommario):
Abstract Turbulent fluids are frequently observed to spontaneously self-organize into large spatial-scale jets; geophysical examples of this phenomenon include the Jovian banded winds and the earth’s polar-front jet. These relatively steady large-scale jets arise from and are maintained by the smaller spatial- and temporal-scale turbulence with which they coexist. Frequently these jets are found to be adjusted into marginally stable states that support large transient growth. In this work, a comprehensive theory for the interaction of jets with turbulence, stochastic structural stability theory (SSST), is applied to the two-layer baroclinic model with the object of elucidating the physical mechanism producing and maintaining baroclinic jets, understanding how jet amplitude, structure, and spacing is controlled, understanding the role of parameters such as the temperature gradient and static stability in determining jet structure, understanding the phenomenon of abrupt reorganization of jet structure as a function of parameter change, and understanding the general mechanism by which turbulent jets adjust to marginally stable states supporting large transient growth. When the mean thermal forcing is weak so that the mean jet is stable in the absence of turbulence, jets emerge as an instability of the coupled system consisting of the mean jet dynamics and the ensemble mean eddy dynamics. Destabilization of this SSST coupled system occurs as a critical turbulence level is exceeded. At supercritical turbulence levels the unstable jet grows, at first exponentially, but eventually equilibrates nonlinearly into stable states of mutual adjustment between the mean flow and turbulence. The jet structure, amplitude, and spacing can be inferred from these equilibria. With weak mean thermal forcing and weak but supercritical turbulence levels, the equilibrium jet structure is nearly barotropic. Under strong mean thermal forcing, so that the mean jet is unstable in the absence of turbulence, marginally stable highly nonnormal equilibria emerge that support high transient growth and produce power-law relations between, for example, heat flux and temperature gradient. The origin of this power-law behavior can be traced to the nonnormality of the adjusted states. As the stochastic excitation, mean baroclinic forcing, or the static stability are changed, meridionally confined jets that are in equilibrium at a given meridional wavenumber abruptly reorganize to another meridional wavenumber at critical values of these parameters. The equilibrium jets obtained with this theory are in remarkable agreement with equilibrium jets obtained in simulations of baroclinic turbulence, and the phenomenon of discontinuous reorganization of confined jets has important implications for storm-track reorganization and abrupt climate change.
3
Schneider, Wilhelm. "Decay of momentum flux in submerged jets". Journal of Fluid Mechanics 154 (maggio 1985): 91–110. http://dx.doi.org/10.1017/s0022112085001434.
Testo completoAbstract (sommario):
Slender laminar and turbulent, plane and axisymmetric jets emerging from orifices in plane or conical walls are studied at large distances from the orifices. The entrainment of momentum coupled with the entrainment of volume into a jet is determined, and its effect on the flow field is analysed by combining inner and outer expansions with a multiple scaling approach.In turbulent (plane or axisymmetric) jets, the axial velocity decreases more rapidly than predicted by classical boundary-layer solutions, and the momentum flux vanishes as the distance from the orifice tends to infinity. The analysis unveils a source of discrepancies in previous experimental data on turbulent jets.In a laminar plane jet, the momentum flux changes but little. In a laminar axisymmetric jet, the momentum flux changes slowly, yet considerably. When a critical distance from the orifice is approached, the momentum flux in the jet becomes very small, the jet diameter very large, and a toroidal viscous eddy is predicted. The structure of the flow field is briefly discussed.
4
Kobayashi, Hiromichi, Hiroki Shionoya e Yoshihiro Okuno. "Turbulent duct flows in a liquid metal magnetohydrodynamic power generator". Journal of Fluid Mechanics 713 (17 ottobre 2012): 243–70. http://dx.doi.org/10.1017/jfm.2012.456.
Testo completoAbstract (sommario):
AbstractWe numerically assess the influence of non-uniform magnetic flux density and connected load resistance on turbulent duct flows in a liquid metal magnetohydrodynamic (MHD) electrical power generator. When increasing the magnetic flux density (or Hartmann number), an M-shaped velocity profile develops in the plane perpendicular to the magnetic field; the maximum velocity in the sidewall layer of the M-shaped profile increases to maintain the flow rate. Under the conditions of a relaminarized flow, the turbulence structures align along the magnetic field and flow repeatedly like a von Kármán vortex sheet. At higher Hartmann numbers, the wall-shear stress in the sidewall layer increases and the sidewall jets transit to turbulence. The sidewall jets in the MHD turbulent duct flows have profiles similar to the non-MHD wall jets, i.e. a mean velocity profile with outer scaling, Reynolds shear stress with the opposite sign in a sidewall jet, and two maxima for the turbulent intensities in a sidewall jet. The Lorentz force suppresses the vortices of the secondary mean flow near the Hartmann layer for low Hartmann numbers, whereas the secondary vortices remain near the Hartmann layer for high Hartmann numbers. An optimal load resistance (or load factor) to obtain a maximum electrical efficiency exists, because the strong Lorentz force for a low load factor and unextracted eddy currents for a high load factor reduce efficiency. When the value of the load factor is changed, the profiles of mean velocity and r.m.s. for the optimal load factor produce almost the same profiles as the high load factor near the open-circuit condition.
5
Kamenkovich, Igor, Pavel Berloff e Joseph Pedlosky. "Role of Eddy Forcing in the Dynamics of Multiple Zonal Jets in a Model of the North Atlantic". Journal of Physical Oceanography 39, n. 6 (1 giugno 2009): 1361–79. http://dx.doi.org/10.1175/2008jpo4096.1.
Testo completoAbstract (sommario):
Abstract Multiple zonal jets are observed in satellite data–based estimates of oceanic velocities, float measurements, and high-resolution numerical simulations of the ocean circulation. This study makes a step toward understanding the dynamics of these jets in the real ocean by analyzing the vertical structure and dynamical balances within multiple zonal jets simulated in an eddy-resolving primitive equation model of the North Atlantic. In particular, the authors focus on the role of eddy flux convergences (“eddy forcing”) in supporting the buoyancy and relative/potential vorticity (PV) anomalies associated with the jets. The results suggest a central role of baroclinic eddies in the barotropic and baroclinic dynamics of the jets, and significant differences in the effects of eddy forcing between the subtropical and subpolar gyres. Additionally, diabatic potential vorticity sources and sinks, associated with vertical diffusion, are shown to play an important role in supporting the potential vorticity anomalies. The resulting potential vorticity profile does not resemble a “PV staircase”—a distinct meridional structure observed in some idealized studies of geostrophic turbulence.
6
Bakas, Nikolaos A., e Petros J. Ioannou. "On the Mechanism Underlying the Spontaneous Emergence of Barotropic Zonal Jets". Journal of the Atmospheric Sciences 70, n. 7 (1 luglio 2013): 2251–71. http://dx.doi.org/10.1175/jas-d-12-0102.1.
Testo completoAbstract (sommario):
Abstract Zonal jets are commonly observed to spontaneously emerge in a β-plane channel from a background of turbulence that is sustained in a statistical steady state by homogeneous stochastic excitation and dissipation of vorticity. The mechanism for jet formation is examined in this work within the statistical wave–mean flow interaction framework of stochastic structural stability theory (SSST) that makes predictions for the emergence of zonal jets in β-plane turbulence. Using the coupled dynamical SSST system that governs the joint evolution of the second-order statistics and the mean flow, the structural stability of the spatially homogeneous statistical equilibrium with no mean zonal jets is studied. It is shown that close to the structural stability boundary, the eddy–mean flow dynamics can be split into two competing processes. The first, which is shearing of the eddies by the local shear described by Orr dynamics in a β plane, is shown in the limit of infinitesimal shear to lead to the formation of jets. The second, which is momentum flux divergence resulting from lateral wave propagation on the nonuniform local mean vorticity gradient, is shown to oppose jet formation. The upgradient momentum fluxes due to shearing of the eddies are shown to act exactly as negative viscosity for an anisotropic forcing and as negative hyperviscosity for isotropic forcing. The downgradient fluxes due to wave flux divergence are shown to act hyperdiffusively.
7
Farrell, Brian F., e Petros J. Ioannou. "Sensitivity of Perturbation Variance and Fluxes in Turbulent Jets to Changes in the Mean Jet". Journal of the Atmospheric Sciences 61, n. 21 (1 novembre 2004): 2644–52. http://dx.doi.org/10.1175/jas3256.1.
Testo completoAbstract (sommario):
Abstract Synoptic-scale eddy variance and fluxes of heat and momentum in midlatitude jets are sensitive to small changes in mean jet velocity, dissipation, and static stability. In this work the change in the jet producing the greatest increase in variance or flux is determined. Remarkably, a single jet structure change completely characterizes the sensitivity of a chosen quadratic statistical quantity to modification of the mean jet in the sense that an arbitrary change in the jet influences a chosen statistical quantity in proportion to the projection of the change on this single optimal structure. The method used extends previous work in which storm track statistics were obtained using a stochastic model of jet turbulence.
8
Thompson, Andrew F., e William R. Young. "Two-Layer Baroclinic Eddy Heat Fluxes: Zonal Flows and Energy Balance". Journal of the Atmospheric Sciences 64, n. 9 (1 settembre 2007): 3214–31. http://dx.doi.org/10.1175/jas4000.1.
Testo completoAbstract (sommario):
Abstract The eddy heat flux generated by statistically equilibrated baroclinic turbulence supported on a uniform, horizontal temperature gradient is examined using a two-layer β-plane quasigeostrophic model. The dependence of the eddy diffusivity of temperature, Dτ, on external parameters such as β, bottom friction κ, the deformation radius λ, and the velocity jump 2U, is provided by numerical simulations at 110 different points in the parameter space β* = βλ2/U and κ* = κλ/U. There is a special “pivot” value of β*, βpiv* ≈ 11/16, at which Dτ depends weakly on κ*. But otherwise Dτ has a complicated dependence on both β* and κ*, highlighted by the fact that reducing κ* leads to increases (decreases) in Dτ if β is less than (greater than) βpiv*. Existing heat-flux parameterizations, based on Kolmogorov cascade theories, predict that Dτ is nonzero and independent of κ* in the limit κ* → 0. Simulations show indications of this regime provided that κ* ≤ 0.04 and 0.25 ≤ β* ≤ 0.5. All important length scales in this problem, namely the mixing length, the scale of the energy containing eddies, the Rhines scale, and the spacing of the zonal jets, converge to a common value as bottom friction is reduced. The mixing length and jet spacing do not decouple in the parameter regime considered here, as predicted by cascade theories. The convergence of these length scales is due to the formation of jet-scale eddies that align along the eastward jets. The baroclinic component of these eddies helps force the zonal mean flow, which occurs through nonzero Reynolds stress correlations in the upper layer, as opposed to the barotropic mode. This behavior suggests that the dynamics of the inverse barotropic cascade are insufficient to fully describe baroclinic turbulence.
9
Marke, Tobias, Susanne Crewell, Vera Schemann, Jan H. Schween e Minttu Tuononen. "Long-Term Observations and High-Resolution Modeling of Midlatitude Nocturnal Boundary Layer Processes Connected to Low-Level Jets". Journal of Applied Meteorology and Climatology 57, n. 5 (maggio 2018): 1155–70. http://dx.doi.org/10.1175/jamc-d-17-0341.1.
Testo completoAbstract (sommario):
AbstractLow-level-jet (LLJ) periods are investigated by exploiting a long-term record of ground-based remote sensing Doppler wind lidar measurements supported by tower observations and surface flux measurements at the Jülich Observatory for Cloud Evolution (JOYCE), a midlatitude site in western Germany. LLJs were found 13% of the time during continuous observations over more than 4 yr. The climatological behavior of the LLJs shows a prevailing nighttime appearance of the jets, with a median height of 375 m and a median wind speed of 8.8 m s−1 at the jet nose. Significant turbulence below the jet nose only occurs for high bulk wind shear, which is an important parameter for describing the turbulent characteristics of the jets. The numerous LLJs (16% of all jets) in the range of wind-turbine rotor heights below 200 m demonstrate the importance of LLJs and the associated intermittent turbulence for wind-energy applications. Also, a decrease in surface fluxes and an accumulation of carbon dioxide are observed if LLJs are present. A comprehensive analysis of an LLJ case shows the influence of the surrounding topography, dominated by an open pit mine and a 200-m-high hill, on the wind observed at JOYCE. High-resolution large-eddy simulations that complement the observations show that the spatial distribution of the wind field exhibits variations connected with the orographic flow depending on the wind direction, causing high variability in the long-term measurements of the vertical velocity.
10
Bachman, Scott D. "A Geometric Perspective on the Modulation of Potential Energy Release by a Lateral Potential Vorticity Gradient". Fluids 5, n. 3 (28 agosto 2020): 142. http://dx.doi.org/10.3390/fluids5030142.
Testo completoAbstract (sommario):
The release of available potential energy by growing baroclinic instability requires the slope of the eddy fluxes to be shallower than that of mean density surfaces, where the amount of energy released depends on both the flux angle and the distance of fluid parcel excursions against the background density gradient. The presence of a lateral potential vorticity (PV) gradient is known to affect the growth rate and energy release by baroclinic instability, but often makes the mathematics of formal linear stability analysis intractable. Here the effects of a lateral PV gradient on baroclinic growth are examined by considering its effects on the slope of the eddy fluxes. It is shown that the PV gradient systematically shifts the unstable modes toward higher wavenumbers and creates a cutoff to the instability at large scales, both of which steepen the eddy flux angle and limit the amount of energy released. This effect may contribute to the severe inhibition of baroclinic turbulence in systems dominated by barotropic jets, making them less likely to transition to turbulence-dominated flow regimes.
11
Smith, K. Shafer. "Eddy Amplitudes in Baroclinic Turbulence Driven by Nonzonal Mean Flow: Shear Dispersion of Potential Vorticity". Journal of Physical Oceanography 37, n. 4 (1 aprile 2007): 1037–50. http://dx.doi.org/10.1175/jpo3030.1.
Testo completoAbstract (sommario):
Abstract As in the midlatitude atmosphere, midocean eddies are primarily generated by baroclinically unstable mean currents. In contrast to the atmosphere, however, oceanic currents are significantly nonzonal. Even weak nonzonal currents are linearly unstable since β does not suppress growing meridional waves. Theories for the nonlinear equilibration of baroclinic instability, and hence theories for the amplitudes of midocean eddies, must therefore take into account the different dynamics of nonzonal flow. It is shown here that the amplitude of fully developed baroclinic turbulence due to nonzonal shears differs from that due to zonal shears primarily in the nature of the eddy generation. Since β will act to create large-scale zonal jet structures regardless of the generation source, the nature of eddy fluxes of potential vorticity (the source of eddy energy) in the zonal and meridional directions are fundamentally different. The cross-jet mixing has been shown previously to obey a mixing-length scaling, and this corresponds to the generation due to unstable zonal flow. The along-jet mixing, which corresponds to the generation due to the meridional shear, is shown here to be best described by a shear dispersion model. The resulting flux is orders of magnitude higher than in the cross-jet direction, and thus eddy energies driven by baroclinically unstable mean flows with a nonzero meridional component are much larger. This provides an explanation for recently reported results. Moreover, given recent observational and modeling studies showing the ubiquitous presence of zonal jets in the oceans, the results presented here indicate a powerful source of eddy energy.
12
Akselvoll, Knut, e Parviz Moin. "Large-eddy simulation of turbulent confined coannular jets". Journal of Fluid Mechanics 315 (25 maggio 1996): 387–411. http://dx.doi.org/10.1017/s0022112096002479.
Testo completoAbstract (sommario):
Large-eddy simulation (LES) was used to study mixing of turbulent, coannular jets discharging into a sudden expansion. This geometry resembles that of a coaxial jet-combustor, and the goal of the calculation was to gain some insight into the phenomena leading to lean blow-out (LBO) in such combustion devices. This is a first step in a series of calculations, where the focus is on the fluid dynamical aspects of the mixing process in the combustion chamber. The effects of swirl, chemical reactions and heat release were not taken into account. Mixing of fuel and oxidizer was studied by tracking a passive scalar introduced in the central jet. The dynamic subgrid-scale (DM) model was used to model both the subgrid-scale stresses and the subgrid-scale scalar flux. The Reynolds number was 38000, based on the bulk velocity and diameter of the combustion chamber. Mean velocities and Reynolds stresses are in good agreement with experimental data. Animated results clearly show that intermittent pockets of fuel-rich fluid (from the central jet) are able to cross the annular jet, virtually undiluted, into the recirculation zone. Most of the fuel-rich fluid is, however, entrained into the recirculation zone near the instantaneous reattachment point. Fuel trapped in the recirculation zone is, for the most part, entrained back into the step shear layer close to the base of the burner.
13
WESTERWEEL, J., C. FUKUSHIMA, J. M. PEDERSEN e J. C. R. HUNT. "Momentum and scalar transport at the turbulent/non-turbulent interface of a jet". Journal of Fluid Mechanics 631 (17 luglio 2009): 199–230. http://dx.doi.org/10.1017/s0022112009006600.
Testo completoAbstract (sommario):
Conditionally sampled measurements with particle image velocimetry (PIV) of a turbulent round submerged liquid jet in a laboratory have been taken at Re = 2 × 103 between 60 and 100 nozzle diameters from the nozzle in order to investigate the dynamics and transport processes at the continuous and well-defined bounding interface between the turbulent and non-turbulent regions of flow. The jet carries a fluorescent dye measured with planar laser-induced fluorescence (LIF), and the surface discontinuity in the scalar concentration is identified as the fluctuating turbulent jet interface. Thence the mean outward ‘boundary entrainment’ velocity is derived and shown to be a constant fraction (about 0.07) of the the mean jet velocity on the centreline. Profiles of the conditional mean velocity, mean scalar and momentum flux show that at the interface there are clear discontinuities in the mean axial velocity and mean scalar and a tendency towards a singularity in mean vorticity. These actual or asymptotic discontinuities are consistent with the conditional mean momentum and scalar transport equations integrated across the interface. Measurements of the fluxes of turbulent kinetic energy and enstrophy are consistent with computations by Mathew & Basu (Phys. Fluids, vol. 14, 2002, pp. 2065–2072) in showing that for a jet flow (without forcing) the entrainment process is dominated by small-scale eddying at the highly sheared interface (‘nibbling’), with large-scale engulfing making a small (less than 10%) contribution consistent with concentration measurements showing that the interior of the jet is well mixed. (Turbulent jets differ greatly from the free shear layer in this respect.) To explain the difference between velocity and scalar profiles, their conditional mean gradients are defined in terms of a local eddy viscosity and eddy diffusivity and the momentum and scalar fluxes inside the interface. Since the eddy diffusivity is larger than the eddy viscosity, the scalar profile is flatter inside the interface so that the scalar discontinuity is relatively greater than the mean velocity discontinuity. Theoretical arguments, following Hunt, Eames & Westerweel (in Proc. of the IUTAM Symp. on Computational Physics and New Perspectives in Turbulence, ed. Y. Kaneda, vol. 4, 2008, pp. 331–338, Springer), are proposed for how the vortex sheet develops, how the internal structure of the interface layer relates to the inhomogeneous rotational and irrotational motions on each side and why the dominant entrainment process of jets and wakes differs from that of free shear layers.
14
Li, Q., G. J. Page e J. J. McGuirk. "Large-eddy simulation of twin impinging jets in cross-flow". Aeronautical Journal 111, n. 1117 (marzo 2007): 195–206. http://dx.doi.org/10.1017/s0001924000004450.
Testo completoAbstract (sommario):
The flow-field beneath a jet-borne vertical landing aircraft is highly complex and unsteady. large-eddy simulation is a suitable tool to predict both the mean flow and unsteady fluctuations. This work aims to evaluate the suitability of LES by applying it to two multiple jet impingement problems: the first is a simple twin impinging jet in cross-flow, while the second includes a circular intake. The numerical method uses a compressible solver on a mixed element unstructured mesh. The smoothing terms in the spatial flux are kept small by the use of a monitor function sensitive to vorticity and divergence. The WALE subgrid scale model is utilised. The simpler jet impingement case shows good agreement with experiment for mean velocity and normal stresses. Analysis of time histories in the jet shear layer and near impingement gives a dominant frequency at a Strouhal number of 0·1, somewhat lower than normally observed in free jets. The jet impingement case with an intake also gives good agreement with experimental velocity measurements, although the expansion of the grid ahead of the jets does reduce the accuracy in this region. Turbulent eddies are observed entering the intake with significant swirl. This is in qualitative agreement with experimental visualisation. The results show that LES could be a suitable tool when applied to multiple jet impingement with realistic aircraft geometry.
15
Prabha, Thara V., Monique Y. Leclerc, Anandakumar Karipot e David Y. Hollinger. "Low-Frequency Effects on Eddy Covariance Fluxes under the Influence of a Low-Level Jet". Journal of Applied Meteorology and Climatology 46, n. 3 (1 marzo 2007): 338–52. http://dx.doi.org/10.1175/jam2461.1.
Testo completoAbstract (sommario):
Abstract Turbulent bursts observed over a tall forest canopy during the initiation of a nocturnal low-level jet (LLJ) are studied with the help of wavelet analysis. The burst of turbulence is observed in response to a shear instability associated with the initiation of LLJ. Turbulent kinetic energy, momentum, and CO2-rich cold air are transferred downward by large eddies with length scales that are higher than the LLJ height. Microfronts are observed over the canopy as a secondary instability that enhances the mixing processes within and above the canopy. The scale-dependent wavelet correlation analysis reveals that countergradient fluxes result from low frequencies, whereas cogradient flux is associated with high-frequency turbulent motions. The countergradient flux is initially noted at low frequencies, and, through coherent motions, it is transferred to smaller scales with a nearly 20-min delay. The countergradient flux dominates at the initiation of the event and reduces net flux, whereas enhanced cogradient flux at the decay of the event increases the net flux. The wavelet correlation coefficient corresponding to cogradient and countergradient fluxes is applied to segregate three regions of the spectra corresponding to “turbulent,” “coherent,” and “noncoherent” large scales. These findings are used to examine the implications on eddy covariance flux measurements.
16
Landel, Julien R., C. P. Caulfield e Andrew W. Woods. "Meandering due to large eddies and the statistically self-similar dynamics of quasi-two-dimensional jets". Journal of Fluid Mechanics 692 (6 gennaio 2012): 347–68. http://dx.doi.org/10.1017/jfm.2011.518.
Testo completoAbstract (sommario):
AbstractWe investigate experimentally the structure of quasi-two-dimensional plane turbulent jets discharged vertically from a slot of width $d$ into a fluid confined between two relatively close rigid boundaries with gap $W\ensuremath{\sim} O(d)$. At large vertical distances $z\gg W$ the jet structure consists of a meandering core with large counter-rotating eddies, which develop on alternate sides of the core. Using particle image velocimetry, we observe an inverse cascade typical of quasi-two-dimensional turbulence where both the core and the eddies grow linearly with $z$ and travel at an average speed proportional to ${z}^{\ensuremath{-} 1/ 2} $. However, although the present study concerns quasi-two-dimensional confined jets, the jets are self-similar and the mean properties are consistent with both experimental results and theoretical models of the time-averaged properties of fully unconfined planar two-dimensional jets. We believe that the dynamics of the interacting core and large eddies accounts for the Gaussian profile of the mean vertical velocity as shown by the spatial statistical distribution of the core and eddy structure. The lateral excursions (caused by the propagating eddies) of this high-speed central core produce a Gaussian distribution for the time-averaged vertical velocity. In addition, we find that approximately 75 % of the total momentum flux of the jet is contained within the core. The eddies travel substantially slower (at approximately 25 % of the maximum speed of the core) at each height and their growth is primarily attributed to entrainment of ambient fluid. The frequency of occurrence of the eddies decreases in a stepwise manner due to merging, with a well-defined minimum value of the corresponding Strouhal number $\mathit{St}\geq 0. 07$.
17
HADŽIABDIĆ, M., e K. HANJALIĆ. "Vortical structures and heat transfer in a round impinging jet". Journal of Fluid Mechanics 596 (17 gennaio 2008): 221–60. http://dx.doi.org/10.1017/s002211200700955x.
Testo completoAbstract (sommario):
In order to gain a better insight into flow, vortical and turbulence structure and their correlation with the local heat transfer in impinging flows, we performed large-eddy simulations (LES) of a round normally impinging jet issuing from a long pipe at Reynolds number Re = 20000 at the orifice-to-plate distance H = 2D, where D is the jet-nozzle diameter. This configuration was chosen to match previous experiments in which several phenomena have been detected, but the underlying physics remained obscure because of limitations in the measuring techniques applied. The instantaneous velocity and temperature fields, generated by the LES, revealed interesting time and spatial dynamics of the vorticity and eddy structures and their imprints on the target wall, characterized by tilting and breaking of the edge ring vortices before impingement, flapping, precessing, splitting and pairing of the stagnation point/line, local unsteady separation and flow reversal at the onset of radial jet spreading, streaks pairing and branching in the near-wall region of the radial jets, and others. The LES data provided also a basis for plausible explanations of some of the experimentally detected statistically-averaged flow features such as double peaks in the Nusselt number and the negative production of turbulence energy in the stagnation region. The simulations, performed with an in-house unstructured finite-volume code T-FlowS, using second-order-accuracy discretization schemes for space and time and the dynamic subgrid-scale stress/flux model for unresolved motion, showed large sensitivity of the results to the grid resolution especially in the wall vicinity, suggesting care must be taken in interpreting LES results in impinging flows.
18
Huang, Xuan, Ling-ling Wang e Jin Xu. "Numerical Study on Dynamical Structures and the Destratification of Vertical Turbulent Jets in Stratified Environment". Water 12, n. 8 (23 luglio 2020): 2085. http://dx.doi.org/10.3390/w12082085.
Testo completoAbstract (sommario):
The law of pollutant emission and diffusion in stratified waters is a common issue. In this paper, numerical study on the interaction between vertical turbulent jets and the pycnocline is carried out to study the problems of jet’s emission through the large eddy simulation (LES). A trigonometric function disturbance (TFD) method is developed to ensure the velocity distribution of the jet in the horizontal plane yield to Gaussian profile. Numerical simulations are carried out in the range of 1.11 < Frp < 4.77, corresponding to 1393 < Rep < 5979, where the Froude number Frp and the Reynolds number Rep are defined at the entrance of pycnocline. The coherent structure and internal waves are observed at the pycnocline during the process of jets impinging. After the impingement, the destratification effects can be found. It can be found that Frp = 3 is a threshold value for the interaction between jets and the pycnocline. When Frp > 3, the interaction becomes intensely. Furthermore, the fitting formula of the radial momentum flux dissipation rate that is used to describe the decay of energy contained by the jets during the impinging process, is established through the dimensionless analysis. As a result, the influence range of the jet on the horizontal plane can be evaluated by Rep. It is also found that the destratification of jets is mainly affected by the velocity of the internal wave induced by jets. In addition, by employing the dimensionless time T related to that velocity, the law of destratification varies with dimensionless time is obtained, which can be summarized as follows: Due to the influence of the first internal wave, the thickness of the pycnocline increases rapidly and reaches a critical value at T = 1.4, after that, the increase of the thickness of the pycnocline becomes linear.
19
Sullivan, Peter P., James B. Edson, Tihomir Hristov e James C. McWilliams. "Large-Eddy Simulations and Observations of Atmospheric Marine Boundary Layers above Nonequilibrium Surface Waves". Journal of the Atmospheric Sciences 65, n. 4 (1 aprile 2008): 1225–45. http://dx.doi.org/10.1175/2007jas2427.1.
Testo completoAbstract (sommario):
Abstract Winds and waves in marine boundary layers are often in an unsettled state when fast-running swell generated by distant storms propagates into local regions and modifies the overlying turbulent fields. A large-eddy simulation (LES) model with the capability to resolve a moving sinusoidal wave at its lower boundary is developed to investigate this low-wind/fast-wave regime. It is used to simulate idealized situations with wind following and opposing fast-propagating waves (swell), and stationary bumps. LES predicts momentum transfer from the ocean to the atmosphere for wind following swell, and this can greatly modify the turbulence production mechanism in the marine surface layer. In certain circumstances the generation of a low-level jet reduces the mean shear between the surface layer and the PBL top, resulting in a near collapse of turbulence in the PBL. When light winds oppose the propagating swell, turbulence levels increase over the depth of the boundary layer and the surface drag increases by a factor of 4 compared to a flat surface. The mean wind profile, turbulence variances, and vertical momentum flux are then dependent on the state of the wave field. The LES results are compared with measurements from the Coupled Boundary Layers Air–Sea Transfer (CBLAST) field campaign. A quadrant analysis of the momentum flux from CBLAST verifies a wave age dependence predicted by the LES solutions. The measured bulk drag coefficient CD then depends on wind speed and wave state. In situations with light wind following swell, CD is approximately 50% lower than values obtained from standard bulk parameterizations that have no sea state dependence. In extreme cases with light wind and persistent swell, CD < 0.
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Bernstein, Joseph, e Brian Farrell. "Low-Frequency Variability in a Turbulent Baroclinic Jet: Eddy–Mean Flow Interactions in a Two-Level Model". Journal of the Atmospheric Sciences 67, n. 2 (1 febbraio 2010): 452–67. http://dx.doi.org/10.1175/2009jas3170.1.
Testo completoAbstract (sommario):
Abstract The origin of low-frequency variability in the midlatitude jet is investigated using a two-level baroclinic channel model. The model state fields are separated into slow and fast components using intermediate time- scale averaging. In the equation for the fast variables the nonlinear wave–wave interactions are parameterized as a stochastic excitation. The slowly varying ensemble mean eddy fluxes obtained from the resulting stochastic turbulence model are coupled with the slowly varying mean flow dynamics. This forms a coupled set of deterministic equations on the slow time scale that governs the dynamics of the eddy–mean flow interaction. The equilibria of this coupled system are found as a function of the excitation strength, which controls the level of turbulence. At low levels of turbulence the equilibrated flow with zonally symmetric mean forcing remains zonally symmetric, but as excitation increases it undergoes zonal symmetry-breaking bifurcations. Time-dependent flows arising from these bifurcations take the form of westward-propagating wavelike structures resembling blocking patterns. Features of these waves are characteristic of blocking in both observations and atmospheric general circulation model simulations including retrogression, eddy variance concentrated upstream of the waves, and eddy momentum flux forcing the waves.
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Craske, John, e Maarten van Reeuwijk. "Energy dispersion in turbulent jets. Part 1. Direct simulation of steady and unsteady jets". Journal of Fluid Mechanics 763 (18 dicembre 2014): 500–537. http://dx.doi.org/10.1017/jfm.2014.640.
Testo completoAbstract (sommario):
AbstractWe study the physics of unsteady turbulent jets using direct numerical simulation (DNS) by introducing an instantaneous step change (both up and down) in the source momentum flux. Our focus is on the propagation speed and rate of spread of the resulting front. We show that accurate prediction of the propagation speed requires information about the energy flux in addition to the momentum flux in the jet. Our observations suggest that the evolution of a front in a jet is a self-similar process that accords with the classical dispersive scaling$z\sim \sqrt{t}$. In the analysis of the problem we demonstrate that the use of a momentum–energy framework of the kind used by Priestley & Ball (Q. J. R. Meteorol. Soc., vol. 81, 1955, pp. 144–157) has several advantages over the classical mass–momentum formulation. In this regard we generalise the approach of Kaminskiet al. (J. Fluid Mech., vol. 526, 2005, pp. 361–376) to unsteady problems, neglecting only viscous effects and relatively small boundary terms in the governing equations. Our results show that dispersion originating from the radial dependence of longitudinal velocity plays a fundamental role in longitudinal transport. Indeed, one is able to find dispersion in the steady state, although it has received little attention because its effects can then be absorbed into the entrainment coefficient. Specifically, we identify two types of dispersion. Type I dispersion exists in a steady state and determines the rate at which energy is transported relative to the rate at which momentum is transported. In unsteady jets type I dispersion is responsible for the separation of characteristic curves and thus the hyperbolic, rather than parabolic, nature of the governing equations, in the absence of longitudinal mixing. Type II dispersion is equivalent to Taylor dispersion and results in the longitudinal mixing of the front. This mixing is achieved by a deformation of the self-similar profiles that one finds in steady jets. Using a comparison with the local eddy viscosity, and by examining dimensionless fluxes in the vicinity of the front, we show that type II dispersion provides a dominant source of longitudinal mixing.
22
Mehta, R. C. "Numerical simulation of high-speed turbulent reacting coaxial jet flow using a structured grid". Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering 214, n. 6 (1 giugno 2000): 359–68. http://dx.doi.org/10.1243/0954410001531944.
Testo completoAbstract (sommario):
Reacting flowfields are described by compressible turbulent Reynolds-averaged Navier-Stokes equations augmented with appropriate species continuity equations that provide for the convection, diffusion and production of each chemical species. The closure of the system of equations is achieved using a two-equation turbulence model. A single-step overall fast chemical reaction combustion model based on the eddy break-up concept is employed for the turbulence-chemistry interaction. A finite volume discretization is carried out in spatial coordinates to compute inviscid and viscous flux vectors. A multistage Runge-Kutta time-stepping scheme is used to obtain a steady state solution. The numerical algorithm is developed by taking into consideration the structured grid arrangement for a turbulent chemically reacting coaxial jet. The numerical scheme is shown to be computationally fact, easy to program and efficient. A supersonic diffusion flame is analysed and the results are compared with the available experimental data.
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Zhou, Bowen, e Fotini Katopodes Chow. "Large-Eddy Simulation of the Stable Boundary Layer with Explicit Filtering and Reconstruction Turbulence Modeling". Journal of the Atmospheric Sciences 68, n. 9 (1 settembre 2011): 2142–55. http://dx.doi.org/10.1175/2011jas3693.1.
Testo completoAbstract (sommario):
Abstract Large-eddy simulation (LES) of the stably stratified atmospheric boundary layer is performed using an explicit filtering and reconstruction approach with a finite difference method. Turbulent stresses are split into the resolvable subfilter-scale and subgrid-scale stresses. The former are recovered from a reconstruction approach, and the latter are represented by a dynamic eddy-viscosity model. The resulting dynamic reconstruction model (DRM) can sustain resolved turbulence with less stringent resolution requirements than conventional closure models, even under strong atmospheric stability. This is achieved by proper representation of subfilter-scale (SFS) backscatter of turbulent kinetic energy (TKE). The flow structure and turbulence statistics for the moderately stable boundary layer (SBL) are analyzed with high-resolution simulations. The DRM simulations show good agreement with established empirical formulations such as flux and gradient-based surface similarity, even at relatively coarse resolution. Similar results can be obtained with traditional closure models at the cost of higher resolution. SBL turbulence under strong stability is also explored. Simulations show an intermittent presence of elevated TKE below the low-level jet. Overall, the explicit filtering and reconstruction approach is advantageous for simulations of the SBL. At coarse resolution, it can extend the working range of LES to stronger stability, while maintaining agreement to similarity theory; at fine resolution, good agreement with theoretical formulations provides confidence in the results and allows for detailed investigation of the flow structure under moderate to strong stability conditions.
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Sullivan, Peter P., e James C. McWilliams. "Langmuir turbulence and filament frontogenesis in the oceanic surface boundary layer". Journal of Fluid Mechanics 879 (1 ottobre 2019): 512–53. http://dx.doi.org/10.1017/jfm.2019.655.
Testo completoAbstract (sommario):
Submesoscale currents, small-scale turbulence and surface gravity waves co-exist in the upper ocean and interact in complex ways. To expose the couplings, the frontogenetic life cycle of an idealized cold dense submesoscale filament interacting with upper ocean Langmuir turbulence is investigated in large-eddy simulations (LESs) based on the incompressible wave-averaged equations. The simulations utilize large domains and fine meshes with $6.4\times 10^{9}$ grid points. Case studies are made with surface winds or surface cooling with waves oriented in across-filament (perpendicular) or down-filament (parallel) directions relative to the two-dimensional filament axis. The currents $u$, $v$ and $w$ are aligned with the across-filament, down-filament and vertical directions, respectively. Frontogenesis is induced by across-filament Lagrangian secondary circulations in the boundary layer, and it is shown to be strongly impacted by surface waves, in particular the propagation direction relative to the filament axis. In a horizontally heterogeneous boundary layer, surface waves induce both mean and fluctuating Stokes-drift vortex forces that modify a linear, hydrostatic turbulent thermal wind (TTW) approximation for momentum. Down-filament winds and waves are found to be especially impactful, they significantly reduce the peak level of frontogenesis by fragmenting the filament into primary and secondary down-welling sites in a broad frontal zone over a width ${\sim}500~\text{m}$. At peak frontogenesis, opposing down-filament jets $\langle v\rangle$ overlie each other resulting in a vigorous vertical shear layer $\unicode[STIX]{x2202}_{z}\langle v\rangle$ with large vertical momentum flux $\langle v^{\prime }w^{\prime }\rangle$. Filament arrest is induced by a lateral shear instability that generates horizontal momentum flux $\langle u^{\prime }v^{\prime }\rangle$ at low wavenumbers. The turbulent vertical velocity patterns, indicative of coherent Langmuir cells, change markedly across the horizontal domain with both across-filament and down-filament winds under the action of submesoscale currents.
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Shen, Weiqi, e Steven A. E. Miller. "Validation of a High-Order Large Eddy Simulation Solver for Acoustic Prediction of Supersonic Jet Flow". Journal of Theoretical and Computational Acoustics 28, n. 03 (settembre 2020): 1950023. http://dx.doi.org/10.1142/s2591728519500233.
Testo completoAbstract (sommario):
A high-order large eddy simulation (LES) code based on the flux reconstruction (FR) scheme is further developed for supersonic jet simulation. The FR scheme provides an efficient and easy-to-implement way to achieve high-order accuracy on an unstructured mesh. The order of accuracy and the shock capturing capability of the solver are validated with the isentropic Euler vortex and Sod’s shock tube problem. A heated under-expanded supersonic jet case from NASA’s Small Hot Jet Acoustic Rig (SHJAR) database is used for validation. The turbulence statistics along the nozzle centerline and lip-line are examined. We predict the acoustic radiation with the Ffowcs Williams and Hawkings method, which is integrated with our solver. The far-field acoustic predictions show reasonable agreement with the experimental measurement in the upstream and downstream directions, where the shock-associated noise and the large-scale turbulent mixing noise are dominant, respectively.
26
MENDEZ, S., e F. NICOUD. "Large-eddy simulation of a bi-periodic turbulent flow with effusion". Journal of Fluid Mechanics 598 (25 febbraio 2008): 27–65. http://dx.doi.org/10.1017/s0022112007009664.
Testo completoAbstract (sommario):
Large-eddy simulations of a generic turbulent flow with discrete effusion are reported. The computational domain is periodic in both streamwise and spanwise directions and contains both the injection and the suction sides. The blowing ratio is close to 1.2 while the Reynolds number in the aperture is of order 2600. The numerical results for this fully developed bi-periodic turbulent flow with effusion are compared to available experimental data from a large-scale spatially evolving isothermal configuration. It is shown that many features are shared by the two flow configurations. The main difference is related to the mean streamwise velocity profile, which is more flat for the bi-periodic situation where the cumulative effect of an infinite number of upstream jets is accounted for. The necessity of considering both sides of the plate is also established by analysing the vortical structure of the flow and some differences with the classical jet-in-crossflow case are highlighted. Finally, the numerical results are analysed in terms of wall modelling for full-coverage film cooling. For the operating point considered, it is demonstrated that the streamwise momentum flux is dominated by non-viscous effects, although the area where only the viscous shear stress contributes is very large given the small porosity value (4%).
27
Bourgault, Pascal, David Straub, Kevin Duquette, Louis-Philippe Nadeau e Bruno Tremblay. "Vertical Heat Fluxes beneath Idealized Sea Ice Leads in Large-Eddy Simulations: Comparison with Observations from the SHEBA Experiment". Journal of Physical Oceanography 50, n. 8 (1 agosto 2020): 2189–202. http://dx.doi.org/10.1175/jpo-d-19-0298.1.
Testo completoAbstract (sommario):
AbstractLarge-eddy simulations (Δx = Δz = 1 m) are used to examine vertical ocean heat fluxes driven by mechanical and buoyancy forcing across idealized sea ice leads. Forcing parameters approximate conditions from a shear event during the Surface Heat Budget of the Arctic (SHEBA) experiment in March 1998. In situ measurements near the lead showed isopycnal displacements of 14 m and turbulent vertical heat fluxes up to 400 W m−2, both of which were attributed to a strong cyclonic stress curl localized along the lead axis. By contrast, the large-eddy simulations show cyclonic shear across the lead to produce no turbulence, with vertical heat transport instead related to an overturning cell that connects a broad upwelling near the lead to downwelling farther away. Anticyclonic forcing produces an opposite-signed overturning cell, but with an intense, narrow downwelling jet and strong turbulent heat fluxes (~100 W m−2) near the lead. For both signs of curl, domain-integrated heat transport due to the overturning cells is somewhat larger than the turbulent heat flux, the latter being confined to the vicinity of the lead. Buoyancy forcing related to sea ice formation in the lead was found to increase both the turbulent and the cell-related heat fluxes (by up to 50% and 10%, respectively). Vertical isopycnal displacements for the upwelling case were found to increase linearly with the strength of the forcing. Possible reasons for the discrepancies with the observations include finer scale variation in the surface ocean stress and turbulence associated with the formation of a ridge during the shear event.
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Landel, Julien R., C. P. Caulfield e Andrew W. Woods. "Streamwise dispersion and mixing in quasi-two-dimensional steady turbulent jets". Journal of Fluid Mechanics 711 (12 settembre 2012): 212–58. http://dx.doi.org/10.1017/jfm.2012.388.
Testo completoAbstract (sommario):
AbstractWe investigate experimentally and theoretically the streamwise transport and dispersion properties of steady quasi-two-dimensional plane turbulent jets discharged vertically from a slot of width $d$ into a fluid confined between two relatively close rigid boundaries with gap $W\ensuremath{\sim} O(d)$. We model the evolution in time and space of the concentration of passive tracers released in these jets using a one-dimensional time-dependent effective advection–diffusion equation. We make a mixing length hypothesis to model the streamwise turbulent eddy diffusivity such that it scales like $b(z){ \overline{w} }_{m} (z)$, where $z$ is the streamwise coordinate, $b$ is the jet width, ${ \overline{w} }_{m} $ is the maximum time-averaged vertical velocity. Under these assumptions, the effective advection–diffusion equation for $\phi (z, t)$, the horizontal integral of the ensemble-averaged concentration, is of the form ${\partial }_{t} \phi + {K}_{a} {\text{} {M}_{0} \text{} }^{1/ 2} {\partial }_{z} \left(\phi / {z}^{1/ 2} \right)= {K}_{d} {\text{} {M}_{0} \text{} }^{1/ 2} {\partial }_{z} \left({z}^{1/ 2} {\partial }_{z} \phi \right)$, where $t$ is time, ${K}_{a} $ (the advection parameter) and ${K}_{d} $ (the dispersion parameter) are empirical dimensionless parameters which quantify the importance of advection and dispersion, respectively, and ${M}_{0} $ is the source momentum flux. We find analytical solutions to this equation for $\phi $ in the cases of a constant-flux release and an instantaneous finite-volume release. We also give an integral formulation for the more general case of a time-dependent release, which we solve analytically when tracers are released at a constant flux over a finite period of time. From our experimental results, whose concentration distributions agree with the model, we find that ${K}_{a} = 1. 65\pm 0. 10$ and ${K}_{d} = 0. 09\pm 0. 02$, for both finite-volume releases and constant-flux releases using either dye or virtual passive tracers. The experiments also show that streamwise dispersion increases in time as ${t}^{2/ 3} $. As a result, in the case of finite-volume releases more than 50 % of the total volume of tracers is transported ahead of the purely advective front (i.e. the front location of the tracer distribution if all dispersion mechanisms are ignored and considering a ‘top-hat’ mean velocity profile in the jet); and in the case of constant-flux releases, at each instant in time, approximately 10 % of the total volume of tracers is transported ahead of the advective front.
29
Wu, Lichuan, Anna Rutgersson e Erik Nilsson. "Atmospheric Boundary Layer Turbulence Closure Scheme for Wind-Following Swell Conditions". Journal of the Atmospheric Sciences 74, n. 7 (1 luglio 2017): 2363–82. http://dx.doi.org/10.1175/jas-d-16-0308.1.
Testo completoAbstract (sommario):
Abstract Over the ocean, atmospheric boundary layer turbulence can be altered by underlying waves. Under swell conditions, the impact of waves on the atmosphere is more complicated compared to that under wind-wave conditions. Based on large-eddy simulation (LES), the wind-following swell impact on the atmospheric boundary layer is investigated through three terms: swell-induced surface momentum flux, the vertical profile of swell-induced momentum flux, and the swell impact on atmospheric mixing. The swell-induced surface momentum flux displays a decreasing trend with increasing atmospheric convection. The swell-induced momentum flux decays approximately exponentially with height. Compared with atmospheric convection, the decay coefficient is more sensitive to wave age. Atmospheric mixing is enhanced under swell conditions relative to a flat stationary surface. The swell impact on the atmospheric boundary layer is incorporated into a turbulence closure parameterization through the three terms. The modified turbulence closure parameterization is introduced into a single-column atmospheric model to simulate LES cases. Adding only the swell impact on the atmospheric mixing has a limited influence on wind profiles. Adding both the impact of swell on the atmospheric mixing and the profile of swell-induced momentum flux significantly improves the agreement between the 1D atmospheric simulation results and the LES results, to some extent simulating the wave-induced low-level wind jet. It is concluded that the swell impact should be included in atmospheric numerical models.
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Roullet, G., J. C. McWilliams, X. Capet e M. J. Molemaker. "Properties of Steady Geostrophic Turbulence with Isopycnal Outcropping". Journal of Physical Oceanography 42, n. 1 (1 gennaio 2012): 18–38. http://dx.doi.org/10.1175/jpo-d-11-09.1.
Testo completoAbstract (sommario):
Abstract High-resolution simulations of β-channel, zonal-jet, baroclinic turbulence with a three-dimensional quasigeostrophic (QG) model including surface potential vorticity (PV) are analyzed with emphasis on the competing role of interior and surface PV (associated with isopycnal outcropping). Two distinct regimes are considered: a Phillips case, where the PV gradient changes sign twice in the interior, and a Charney case, where the PV gradient changes sign in the interior and at the surface. The Phillips case is typical of the simplified turbulence test beds that have been widely used to investigate the effect of ocean eddies on ocean tracer distribution and fluxes. The Charney case shares many similarities with recent high-resolution primitive equation simulations. The main difference between the two regimes is indeed an energization of submesoscale turbulence near the surface. The energy cycle is analyzed in the (k, z) plane, where k is the horizontal wavenumber. In the two regimes, the large-scale buoyancy forcing is the primary source of mechanical energy. It sustains an energy cycle in which baroclinic instability converts more available potential energy (APE) to kinetic energy (KE) than the APE directly injected by the forcing. This is due to a conversion of KE to APE at the scale of arrest. All the KE is dissipated at the bottom at large scales, in the limit of infinite resolution and despite the submesoscales energizing in the Charney case. The eddy PV flux is largest at the scale of arrest in both cases. The eddy diffusivity is very smooth but highly nonuniform. The eddy-induced circulation acts to flatten the mean isopycnals in both cases.
31
Momen, Mostafa, Elie Bou-Zeid, Marc B. Parlange e Marco Giometto. "Modulation of Mean Wind and Turbulence in the Atmospheric Boundary Layer by Baroclinicity". Journal of the Atmospheric Sciences 75, n. 11 (28 settembre 2018): 3797–821. http://dx.doi.org/10.1175/jas-d-18-0159.1.
Testo completoAbstract (sommario):
Abstract This paper investigates the effects of baroclinic pressure gradients on mean flow and turbulence in the diabatic atmospheric boundary layer (ABL). Large-eddy simulations are conducted where the direction of the baroclinicity, its strength, and the surface buoyancy flux are systematically varied to examine their interacting effects. The thermal wind vector, which represents the vertical change in the geostrophic wind vector resulting from horizontal temperature gradients, significantly influences the velocity profiles, the Ekman turning, and the strength and location of the low-level jet (LLJ). For instance, cold advection and positive (negative) geostrophic shear increased (decreased) friction velocity and changed the LLJ elevation. Given the baroclinicity strength and direction under neutral conditions, a simple reduced model is proposed and validated here to predict the general trends of baroclinic mean winds. The baroclinic effects on turbulence intensity and structure are even more intricate, with turbulent kinetic energy (TKE) profiles displaying an increase of TKE magnitude with height for some cases. When a fixed destabilizing surface heat flux is added, a positive geostrophic shear favors streamwise aligned roll-type structures, which are typical of neutral ABLs. Conversely, a negative geostrophic shear promotes cell-type structures, which are typical of strongly unstable ABLs. Furthermore, baroclinicity increases shear in the outer ABL and tends to make the outer flow more neutral by decreasing the Richardson flux number. These findings are consequential for meteorological measurements and the wind-energy industry, among others: baroclinicity alters the mean wind profiles, the TKE, coherent structures, and the stability of the ABL, and its effects need to be considered.
32
Xu, Liang, Xu Zhao, Lei Xi, Yonghao Ma, Jianmin Gao e Yunlong Li. "Large-Eddy Simulation Study of Flow and Heat Transfer in Swirling and Non-Swirling Impinging Jets on a Semi-Cylinder Concave Target". Applied Sciences 11, n. 15 (3 agosto 2021): 7167. http://dx.doi.org/10.3390/app11157167.
Testo completoAbstract (sommario):
Swirling impinging jet (SIJ) is considered as an effective means to achieve uniform cooling at high heat transfer rates, and the complex flow structure and its mechanism of enhancing heat transfer have attracted much attention in recent years. The large eddy simulation (LES) technique is employed to analyze the flow fields of swirling and non-swirling impinging jet emanating from a hole with four spiral and straight grooves, respectively, at a relatively high Reynolds number (Re) of 16,000 and a small jet spacing of H/D = 2 on a concave surface with uniform heat flux. Firstly, this work analyzes two different sub-grid stress models, and LES with the wall-adapting local eddy-viscosity model (WALEM) is established for accurately predicting flow and heat transfer performance of SIJ on a flat surface. The complex flow field structures, spectral characteristics, time-averaged flow characteristics and heat transfer on the target surface for the swirling and non-swirling impinging jets are compared in detail using the established method. The results show that small-scale recirculation vortices near the wall change the nearby flow into an unstable microwave state, resulting in small-scale fluctuation of the local Nusselt number (Nu) of the wall. There is a stable recirculation vortex at the stagnation point of the target surface, and the axial and radial fluctuating speeds are consistent with the fluctuating wall temperature. With the increase in the radial radius away from the stagnation point, the main frequency of the fluctuation of wall temperature coincides with the main frequency of the fluctuation of radial fluctuating velocity at x/D = 0.5. Compared with 0° straight hole, 45° spiral hole has a larger fluctuating speed because of speed deflection, resulting in a larger turbulence intensity and a stronger air transport capacity. The heat transfer intensity of the 45° spiral hole on the target surface is slightly improved within 5–10%.
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Ganti, Himakar, Manu Kamin e Prashant Khare. "Design Space Exploration of Turbulent Multiphase Flows Using Machine Learning-Based Surrogate Model". Energies 13, n. 17 (3 settembre 2020): 4565. http://dx.doi.org/10.3390/en13174565.
Testo completoAbstract (sommario):
This study focuses on establishing a surrogate model based on machine learning techniques to predict the time-averaged spatially distributed behaviors of vaporizing liquid jets in turbulent air crossflow for momentum flux ratios between 5 and 120. This surrogate model extends a previously developed Gaussian-process-based framework applicable to laminar flows to accommodate turbulent flows and demonstrates that in addition to detailed fields of primitive variables, second-order turbulence statistics can also be predicted using machine learning techniques. The framework proceeds in 3 steps—(1) design of experiment studies to identify training points and conducting high-fidelity calculations to build the training dataset; (2) Gaussian process regression (supervised training) for the range of operating conditions under consideration for gaseous and dispersed phase quantities; and (3) error quantification of the surrogate model by comparing the machine learning predictions with the truth model for test conditions (i.e., conditions not used for training). The framework was trained using data generated by high-fidelity large eddy simulation (LES)-based calculations (also referred to as the truth model), which solves the complete set of conservation equations for mass, momentum, energy, and species in an Eulerian reference frame, coupled with a Lagrangian solver that tracks the dispersed phase. Simulations were conducted for the range of momentum flux ratios between 5 and 120 for liquid water injected into crossflowing air at a pressure of 1 atm and temperature of 600 K. Results from the machine-learned surrogate model, also called emulations, were compared with the truth model under testing conditions identified by momentum flux ratios of 7 and 40. L1 errors for time-averaged field quantities, including velocity magnitudes, pressure, temperature, vapor fraction of the evaporated liquid, and turbulent kinetic energy in the gas phase, and spray penetration and Sauter mean diameters in the dispersed phase are reported. Speedup of 65 was achieved with this emulator when compared against LES simulation of the same test conditions with errors for all quantities below 14%, thus demonstrating the potential benefits of using machine learning techniques for design space exploration of devices that are based on turbulent multiphase fluid flows. This is the first effort of its kind in the literature that demonstrates the application of machine learning techniques on turbulent multiphase flows.
34
Hoecker-Martínez, Martín S., William D. Smyth e Eric D. Skyllingstad. "Oceanic Turbulent Energy Budget using Large-Eddy Simulation of a Wind Event during DYNAMO". Journal of Physical Oceanography 46, n. 3 (marzo 2016): 827–40. http://dx.doi.org/10.1175/jpo-d-15-0057.1.
Testo completoAbstract (sommario):
AbstractThe dominant processes governing ocean mixing during an active phase of the Madden–Julian oscillation are identified. Air–sea fluxes and upper-ocean currents and hydrography, measured aboard the R/V Revelle during boreal fall 2011 in the Indian Ocean at 0°, 80.5°E, are integrated by means of a large-eddy simulation (LES) to infer mixing mechanisms and quantify the resulting vertical property fluxes. In the simulation, wind accelerates the mixed layer, and shear mixes the momentum downward, causing the mixed layer base to descend. Turbulent kinetic energy gains due to shear production and Langmuir circulations are opposed by stirring gravity and frictional losses. The strongest stirring of buoyancy follows precipitation events and penetrates to the base of the mixed layer. The focus here is on the initial 24 h of an unusually strong wind burst that began on 24 November 2011. The model shows that Langmuir turbulence influences only the uppermost few meters of the ocean. Below the wave-energized region, shear instability responds to the integrated momentum flux into the mixed layer, lagging the initial onset of the storm. Shear below the mixed layer persists after the storm has weakened and decelerates the surface jet slowly (compared with the acceleration at the peak of the storm). Slow loss of momentum from the mixed layer extends the effect of the surface wind burst by energizing the fluid at the base of the mixed layer, thereby prolonging heat uptake due to the storm. Ocean turbulence and air–sea fluxes contribute to the cooling of the mixed layer approximately in the ratio 1:3, consistent with observations.
35
Bertin, F., P. Van Velthoven, A. Cremieu, R. Ney e R. Beugin. "UHF radar observation of strato-tropospheric transfers on the anticyclonic side of a jet streak". Annales Geophysicae 13, n. 11 (30 novembre 1995): 1229–36. http://dx.doi.org/10.1007/s00585-995-1229-y.
Testo completoAbstract (sommario):
Abstract. An observation by UHF ST radar of a subsidence pattern on the right side of the exit region of a jet streak is reported. The onset of the subsidence pattern occurred at 23:30 UTC on the 29 November 1991, when a downward motion was initiated above 14 km. The injections of stratospheric air in this region seem to have an intermittent nature; they occur during at least three intervals during the lifetime of the subsidence pattern. Comparison of these results with an ECMWF analysis suggests that it is an unfolding case. However, observation of turbulent intensities w' greater than 60 cm s–1 at the tropopause level also suggests the existence of a turbulent flux between the stratosphere and the troposphere. From the turbulence characteristics measured by the radar and the potential temperature profile obtained by radiosonde data, the eddy diffusivity at the tropopause level has been calculated. An eddy diffusion coefficient ranging between 5 and 7 m2 s–1 is found. From these values, and with the assumption of a climatological gradient of the volume mixing ratio of ozone in the lower stratosphere, it is possible to deduce a rough estimate of the amount of ozone injected from the stratosphere into the troposphere during this event. A rate of transfer of 1.5×1020 molecules of ozone per day and per square meter is found.
36
Nakamura, Noboru, e Da Zhu. "Finite-Amplitude Wave Activity and Diffusive Flux of Potential Vorticity in Eddy–Mean Flow Interaction". Journal of the Atmospheric Sciences 67, n. 9 (1 settembre 2010): 2701–16. http://dx.doi.org/10.1175/2010jas3432.1.
Testo completoAbstract (sommario):
Abstract An exact diagnostic formalism for finite-amplitude eddy–mean flow interaction is developed for barotropic and quasigeostrophic baroclinic flows on the beta plane. Based on the advection–diffusion–reaction equation for potential vorticity (PV), the formalism quantifies both advective and diffusive contributions to the mean flow modification by eddies, of which the latter is the focus of the present article. The present theory adopts a hybrid Eulerian–Lagrangian-mean description of the flow and defines finite-amplitude wave activity in terms of the areal displacement of PV contours from zonal symmetry. Unlike previous formalisms, wave activity is readily calculable from data and the local Eliassen–Palm relation does not involve cubic or higher-order terms in eddy amplitude. This leads to a natural finite-amplitude extension to the local nonacceleration theorem, as well as the global stability theorems, in the inviscid and unforced limit. The formalism incorporates mixing with effective diffusivity of PV, and the diffusive flux of PV is shown to be a sink of wave activity. The relationship between the advective and diffusive fluxes of PV and its implications for parameterization are discussed in the context of wave activity budget. If all momentum associated with wave activity were returned to the zonal-mean flow, a balanced eddy-free flow would ensue. It is shown that this hypothetical flow uREF is unaffected by the advective PV flux and is driven solely by the diffusive PV flux and forcing. For this reason, uREF, rather than the zonal-mean flow, is proposed as a diagnostic for the diffusive mean-flow modification. The formalism is applied to a freely decaying beta-plane turbulence to evaluate the contribution of the diffusive PV flux to the jet formation.
37
Schaller, Carsten, Fanny Kittler, Thomas Foken e Mathias Göckede. "Characterisation of short-term extreme methane fluxes related to non-turbulent mixing above an Arctic permafrost ecosystem". Atmospheric Chemistry and Physics 19, n. 6 (1 aprile 2019): 4041–59. http://dx.doi.org/10.5194/acp-19-4041-2019.
Testo completoAbstract (sommario):
Abstract. Methane (CH4) emissions from biogenic sources, such as Arctic permafrost wetlands, are associated with large uncertainties because of the high variability of fluxes in both space and time. This variability poses a challenge to monitoring CH4 fluxes with the eddy covariance (EC) technique, because this approach requires stationary signals from spatially homogeneous sources. Episodic outbursts of CH4 emissions, i.e. triggered by spontaneous outgassing of bubbles or venting of methane-rich air from lower levels due to shifts in atmospheric conditions, are particularly challenging to quantify. Such events typically last for only a few minutes, which is much shorter than the common averaging interval for EC (30 min). The steady-state assumption is jeopardised, which potentially leads to a non-negligible bias in the CH4 flux. Based on data from Chersky, NE Siberia, we tested and evaluated a flux calculation method based on wavelet analysis, which, in contrast to regular EC data processing, does not require steady-state conditions and is allowed to obtain fluxes over averaging periods as short as 1 min. Statistics on meteorological conditions before, during, and after the detected events revealed that it is atmospheric mixing that triggered such events rather than CH4 emission from the soil. By investigating individual events in more detail, we identified a potential influence of various mesoscale processes like gravity waves, low-level jets, weather fronts passing the site, and cold-air advection from a nearby mountain ridge as the dominating processes. The occurrence of extreme CH4 flux events over the summer season followed a seasonal course with a maximum in early August, which is strongly correlated with the maximum soil temperature. Overall, our findings demonstrate that wavelet analysis is a powerful method for resolving highly variable flux events on the order of minutes, and can therefore support the evaluation of EC flux data quality under non-steady-state conditions.
38
Huang, Jing, e Elie Bou-Zeid. "Turbulence and Vertical Fluxes in the Stable Atmospheric Boundary Layer. Part I: A Large-Eddy Simulation Study". Journal of the Atmospheric Sciences 70, n. 6 (29 maggio 2013): 1513–27. http://dx.doi.org/10.1175/jas-d-12-0167.1.
Testo completoAbstract (sommario):
Abstract This study seeks to quantitatively and qualitatively understand how stability affects transport in the continuously turbulent stably stratified atmospheric boundary layer, based on a suite of large-eddy simulations. The test cases are based on the one adopted by the Global Energy and Water Cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS) project, but with a largely expanded stability range where the gradient Richardson number (Rig) reaches up to around 1. The analysis is mainly focused on understanding the modification of turbulent structures and dynamics with increasing stability in order to improve the modeling of the stable atmospheric boundary layer in weather and climate models, a topic addressed in Part II of this work. It is found that at quasi equilibrium, an increase in stability results in stronger vertical gradients of the mean temperature, a lowered low-level jet, a decrease in vertical momentum transport, an increase in vertical buoyancy flux, and a shallower boundary layer. Analysis of coherent turbulent structures using two-point autocorrelation reveals that the autocorrelation of the streamwise velocity is horizontally anisotropic while the autocorrelation of the vertical velocity is relatively isotropic in the horizontal plane and its integral length scale decreases as stability increases. The effects of stability on the overall turbulent kinetic energy (TKE) and its budget terms are also investigated, and it is shown that the authors' large-eddy simulation results are in good agreement with previous experimental findings across varied stabilities. Finally, Nieuwstadt's local-scaling theory is reexamined and it is concluded that the height z is not a relevant scaling parameter and should be replaced by a constant length scale away from the surface, indicating that the z-less range starts lower than previously assumed.
39
Jiang, Qingfang, Peter Sullivan, Shouping Wang, James Doyle e Linwood Vincent. "Impact of Swell on Air–Sea Momentum Flux and Marine Boundary Layer under Low-Wind Conditions". Journal of the Atmospheric Sciences 73, n. 7 (24 giugno 2016): 2683–97. http://dx.doi.org/10.1175/jas-d-15-0200.1.
Testo completoAbstract (sommario):
Abstract The impact of fast-propagating swell on the air–sea momentum exchange and the marine boundary layer is examined based on multiple large-eddy simulations over a range of wind speed and swell parameters in the light-wind–fast-wave regime. A wave-driven supergeostrophic jet forms near the top of the wave boundary layer when the forwarding-pointing (i.e., negative) form drag associated with fast wind-following swell overpowers the positive surface shear stress. The magnitude of the form drag increases with the wavelength and slope and decreases with increasing wind speed, and the jet intensity in general increases with the magnitude of the surface form drag. The resulting negative vertical wind shear above the jet in turn enhances the turbulence aloft. The level of the wind maximum is found to be largely determined by the wavenumber and the ratio of the surface shear stress and form drag: the larger the magnitude of this ratio, the higher the altitude of the wind maximum. Although the simulated wind profile often closely follows the log law in the wave boundary layer, the surface stress derived from the logarithmic wind profile is significantly larger than the actual total surface stress in the presence of swell. Therefore, the Monin–Obukhov similarity theory is generally invalid over swell-dominated ocean. This is attributed to the wave-induced contribution to momentum flux, which decays roughly exponentially in the vertical and is largely independent of local wind shear.
40
Gastineau, Guillaume, Laurent Li e Hervé Le Treut. "Some Atmospheric Processes Governing the Large-Scale Tropical Circulation in Idealized Aquaplanet Simulations". Journal of the Atmospheric Sciences 68, n. 3 (1 marzo 2011): 553–75. http://dx.doi.org/10.1175/2010jas3439.1.
Testo completoAbstract (sommario):
Abstract The large-scale tropical atmospheric circulation is analyzed in idealized aquaplanet simulations using an atmospheric general circulation model. Idealized sea surface temperatures (SSTs) are used as lower-boundary conditions to provoke modifications of the atmospheric general circulation. Results show that 1) an increase in the meridional SST gradients of the tropical region drastically strengthens the Hadley circulation intensity, 2) the presence of equatorial zonal SST anomalies weakens the Hadley cells and reinforces the Walker circulation, and 3) a uniform SST warming causes small and nonsystematic changes of the Hadley and Walker circulations. In all simulations, the jet streams strengthen and move equatorward as the Hadley cells strengthen and become narrower. Some relevant mechanisms are then proposed to interpret the large range of behaviors obtained from the simulations. First, the zonal momentum transport by transient and stationary eddies is shown to modulate the eddy-driven jets, which causes the poleward displacements of the jet streams. Second, it is found that the Hadley circulation adjusts to the changes of the poleward moist static energy flux and gross moist static stability, associated with the geographical distribution of convection and midlatitude eddies. The Walker circulation intensity corresponds to the zonal moist static energy transport induced by the zonal anomalies of the turbulent fluxes and radiative cooling. These experiments provide some hints to understand a few robust changes of the atmospheric circulation simulated by ocean–atmosphere coupled models for future and past climates.
41
Pawar, Shashikant, e Devendra Kumar Patel. "Influence of moving plate velocity on conjugate heat transfer due to the impingement of an inclined slot jet". International Journal of Modern Physics C 30, n. 12 (dicembre 2019): 2050006. http://dx.doi.org/10.1142/s0129183120500060.
Testo completoAbstract (sommario):
In this paper, a dimensionless numerical study of the flow-field and heat transfer characteristics of an incompressible turbulent slot jet impinging obliquely over a moving surface of finite thickness is presented. Simulations were performed using [Formula: see text] eddy viscosity turbulence model. The temperature field was solved simultaneously in the solid and the fluid domain. For a fixed impingement distance and a fixed Reynolds number, the impingement angle ([Formula: see text]) and plate velocity ([Formula: see text]) were varied in the range of 30–75∘ and 0–0.3, respectively. In the results, the length of the potential core depends on the jet inclination, which increases with increase in jet angle. The jet angle and plate velocity have more influence on the uphill side compared to the downhill side. The location of stagnation displaces toward the uphill side as the inclination angle decreases, and the drifting of stagnation point is noted with the variation in plate velocity. The average skin-friction coefficient increases with increase in [Formula: see text] and [Formula: see text], and the influence of [Formula: see text] on the skin-friction coefficient is reduced as [Formula: see text] increases. The maximum Nusselt number ([Formula: see text]) increases with increase in [Formula: see text], and the drifting of [Formula: see text] is observed with increase in plate velocity. It is found that the average Nusselt number increases quickly with increase in plate velocity for lower angles of impingement. The distribution of local heat flux follows the same trend as the local Nusselt number.
42
Shen, Zhaoyi, e Yi Ming. "The Influence of Aerosol Absorption on the Extratropical Circulation". Journal of Climate 31, n. 15 (agosto 2018): 5961–75. http://dx.doi.org/10.1175/jcli-d-17-0839.1.
Testo completoAbstract (sommario):
This study examines how aerosol absorption affects the extratropical circulation by analyzing the response to a globally uniform increase in black carbon (BC) simulated with an atmospheric general circulation model forced by prescribed sea surface temperatures. The model includes aerosol direct and semidirect effects, but not indirect or cloud-absorption effects. BC-induced heating in the free troposphere stabilizes the midlatitude atmospheric column, which results in less energetic baroclinic eddies and thus reduced meridional energy transport at midlatitudes. Upper-tropospheric BC also decreases the meridional temperature gradient on the equatorward flank of the tropospheric jet and yields a weakening and poleward shift of the jet, while boundary layer BC has no significant influence on the large-scale circulation since most of the heating is diffused by turbulence in the boundary layer. The effectiveness of BC in altering circulation generally increases with height. Dry baroclinic eddy theories can explain most of the extratropical response to free-tropospheric BC. Specifically, the decrease in vertical eddy heat flux related to a more stable atmosphere is the main mechanism for reestablishing atmospheric energy balance in the presence of BC-induced heating. Similar temperature responses are found in a dry idealized model, which further confirms the dominant role of baroclinic eddies in driving the extratropical circulation changes. The strong atmospheric-only response to BC suggests that absorbing aerosols are capable of altering synoptic-scale weather patterns. Its height dependence highlights the importance of better constraining model-simulated aerosol vertical distributions with satellite and field measurements.
43
Semedo, Alvaro, Øyvind Saetra, Anna Rutgersson, Kimmo K. Kahma e Heidi Pettersson. "Wave-Induced Wind in the Marine Boundary Layer". Journal of the Atmospheric Sciences 66, n. 8 (1 agosto 2009): 2256–71. http://dx.doi.org/10.1175/2009jas3018.1.
Testo completoAbstract (sommario):
Abstract Recent field observations and large-eddy simulations have shown that the impact of fast swell on the marine atmospheric boundary layer (MABL) might be stronger than previously assumed. For low to moderate winds blowing in the same direction as the waves, swell propagates faster than the mean wind. The momentum flux above the sea surface will then have two major components: the turbulent shear stress, directed downward, and the swell-induced stress, directed upward. For sufficiently high wave age values, the wave-induced component becomes increasingly dominant, and the total momentum flux will be directed into the atmosphere. Recent field measurements have shown that this upward momentum transfer from the ocean into the atmosphere has a considerable impact on the surface layer flow dynamics and on the turbulence structure of the overall MABL. The vertical wind profile will no longer exhibit a logarithmic shape because an acceleration of the airflow near the surface will take place, generating a low-level wave-driven wind maximum (a wind jet). As waves propagate away from their generation area as swell, some of the wave momentum will be returned to the atmosphere in the form of wave-driven winds. A model that qualitatively reproduces the wave-following atmospheric flow and the wave-generated wind maximum, as seen from measurements, is proposed. The model assumes a stationary momentum and turbulent kinetic energy balance and uses the dampening of the waves at the surface to describe the momentum flux from the waves to the atmosphere. In this study, simultaneous observations of wind profiles, turbulent fluxes, and wave spectra during swell events are presented and compared with the model. In the absence of an established model for the linear damping ratio during swell conditions, the model is combined with observations to estimate the wave damping. For the cases in which the observations showed a pronounced swell signal and almost no wind waves, the agreement between observed and modeled wind profiles is remarkably good. The resulting attenuation length is found to be relatively short, which suggests that the estimated damping ratios are too large. The authors attribute this, at least partly, to processes not accounted for by the model, such as the existence of an atmospheric background wind. In the model, this extra momentum must be supplied by the waves in terms of a larger damping ratio.
44
Olivares, V., P. Salome, F. Combes, S. Hamer, P. Guillard, M. D. Lehnert, F. L. Polles et al. "Ubiquitous cold and massive filaments in cool core clusters". Astronomy & Astrophysics 631 (15 ottobre 2019): A22. http://dx.doi.org/10.1051/0004-6361/201935350.
Testo completoAbstract (sommario):
Multi-phase filamentary structures around brightest cluster galaxies (BCG) are likely a key step of AGN-feedback. We observed molecular gas in three cool cluster cores, namely Centaurus, Abell S1101, and RXJ1539.5, and gathered ALMA (Atacama Large Millimeter/submillimeter Array) and MUSE (Multi Unit Spectroscopic Explorer) data for 12 other clusters. Those observations show clumpy, massive, and long (3−25 kpc) molecular filaments, preferentially located around the radio bubbles inflated by the AGN. Two objects show nuclear molecular disks. The optical nebula is certainly tracing the warm envelopes of cold molecular filaments. Surprisingly, the radial profile of the Hα/CO flux ratio is roughly constant for most of the objects, suggesting that (i) between 1.2 and 6 times more cold gas could be present and (ii) local processes must be responsible for the excitation. Projected velocities are between 100 and 400 km s−1, with disturbed kinematics and sometimes coherent gradients. This is likely due to the mixing in projection of several thin (and as yet) unresolved filaments. The velocity fields may be stirred by turbulence induced by bubbles, jets, or merger-induced sloshing. Velocity and dispersions are low, below the escape velocity. Cold clouds should eventually fall back and fuel the AGN. We compare the radial extent of the filaments, rfil, with the region where the X-ray gas can become thermally unstable. The filaments are always inside the low-entropy and short-cooling-time region, where tcool/tff < 20 (9 of 13 sources). The range of tcool/tff of 8−23 at rfil, is likely due to (i) a more complex gravitational potential affecting the free-fall time tff (sloshing, mergers, etc.) and (ii) the presence of inhomogeneities or uplifted gas in the ICM, affecting the cooling time tcool. For some of the sources, rfil lies where the ratio of the cooling time to the eddy-turnover time, tcool/teddy, is approximately unity.
45
Mott, Rebecca, Ivana Stiperski e Lindsey Nicholson. "Spatio-temporal flow variations driving heat exchange processes at a mountain glacier". Cryosphere 14, n. 12 (22 dicembre 2020): 4699–718. http://dx.doi.org/10.5194/tc-14-4699-2020.
Testo completoAbstract (sommario):
Abstract. Multi-scale interactions between the glacier surface, the overlying atmosphere, and the surrounding alpine terrain are highly complex and force temporally and spatially variable local glacier energy fluxes and melt rates. A comprehensive measurement campaign (Hintereisferner Experiment, HEFEX) was conducted during August 2018 with the aim to investigate spatial and temporal dynamics of the near-surface boundary layer and associated heat exchange processes close to the glacier surface during the melting season. The experimental set-up of five meteorological stations was designed to capture the spatial and temporal characteristics of the local wind system on the glacier and to quantify the contribution of horizontal heat advection from surrounding ice-free areas to the local energy flux variability at the glacier. Turbulence data suggest that temporal changes in the local wind system strongly affect the micrometeorology at the glacier surface. Persistent low-level katabatic flows during both night and daytime cause consistently low near-surface air temperatures with only small spatial variability. However, strong changes in the local thermodynamic characteristics occur when westerly flows disturbed this prevailing katabatic flow, forming across-glacier flows and facilitating warm-air advection from the surrounding ice-free areas. Such heat advection significantly increased near-surface air temperatures at the glacier, resulting in strong horizontal temperature gradients from the peripheral zones towards the centre line of the glacier. Despite generally lower near-surface wind speeds during across-glacier flow, peak horizontal heat advection from the peripheral zones towards the centre line and strong transport of turbulence from higher atmospheric layers downward resulted in enhanced turbulent heat exchange towards the glacier surface at the glacier centre line. Thus, at the centre line of the glacier, exposure to strong larger-scale westerly winds promoted heat exchange processes, potentially contributing to ice melt, while at the peripheral zones of the glacier, stronger sheltering from larger-scale flows allowed the preservation of a katabatic jet, which suppressed the efficiency of the across-glacier flow to drive heat exchange towards the glacier surface by decoupling low-level atmospheric layers from the flow aloft. A fuller explanation of the origin and structure of the across-glacier flow would require large-eddy simulations.
46
Spall, Robert E., Elgin A. Anderson e Jeffrey Allen. "Momentum Flux in Plane, Parallel Jets". Journal of Fluids Engineering 126, n. 4 (1 luglio 2004): 665–70. http://dx.doi.org/10.1115/1.1778717.
Testo completoAbstract (sommario):
The evolution of the streamwise momentum flux for two turbulent, plane, parallel jets discharging through slots in a direction normal to a wall was studied both numerically and experimentally. The numerical results, obtained by solving the Reynolds-averaged Navier-Stokes equations employing a standard k−ε turbulence model, predicted to within experimental error measured integrals of the momentum flux downstream of the merge point for jet spacing S/d=5. Integration of the streamwise component of the Reynolds-averaged Navier-Stokes equations over a control volume results in an integral constant that was evaluated numerically for jet spacings S/d=3, 5, 7, 9, and 11, and for different levels of turbulence kinetic energy and dissipation rate at the jet inlet boundaries. Results revealed that the integral constant is decreased as the jet spacing increases, and is also decreased as jet entrainment rates are increased due to higher levels of inlet turbulence kinetic energy, or alternatively, decreased levels of dissipation rate. Streamwise distance to the merge point was also found to decrease for increased levels of turbulence kinetic energy or decreased levels of dissipation rate at the jet inlet.
47
Li, Qian, Sukyoung Lee e Alexa Griesel. "Eddy Fluxes and Jet-Scale Overturning Circulations in the Indo–Western Pacific Southern Ocean". Journal of Physical Oceanography 46, n. 10 (ottobre 2016): 2943–59. http://dx.doi.org/10.1175/jpo-d-15-0241.1.
Testo completoAbstract (sommario):
AbstractThe relationship between Antarctic Circumpolar Current jets and eddy fluxes in the Indo–western Pacific Southern Ocean (90°–145°E) is investigated using an eddy-resolving model. In this region, transient eddy momentum flux convergence occurs at the latitude of the primary jet core, whereas eddy buoyancy flux is located over a broader region that encompasses the jet and the interjet minimum. In a small sector (120°–144°E) where jets are especially zonal, a spatial and temporal decomposition of the eddy fluxes further reveals that fast eddies act to accelerate the jet with the maximum eddy momentum flux convergence at the jet center, while slow eddies tend to decelerate the zonal current at the interjet minimum. Transformed Eulerian mean (TEM) diagnostics reveals that the eddy momentum contribution accelerates the jets at all model depths, whereas the buoyancy flux contribution decelerates the jets at depths below ~600 m. In ocean sectors where the jets are relatively well defined, there exist jet-scale overturning circulations with sinking motion on the equatorward flank and a rising motion on the poleward flank of the jets. These jet-scale TEM overturning circulations, which are also discernible in potential density coordinates, cannot be attributed to Ekman downwelling because the Ekman vertical velocities are much weaker and their meridional structure shares little resemblance to the rapidly varying jet-scale overturning pattern. Instead, the location and structure of these thermally indirect circulations suggest that they are driven by the eddy momentum flux convergence, much like the Ferrel cell in the atmosphere.
48
Farrell, Brian F., e Petros J. Ioannou. "A Theory of Baroclinic Turbulence". Journal of the Atmospheric Sciences 66, n. 8 (1 agosto 2009): 2444–54. http://dx.doi.org/10.1175/2009jas2989.1.
Testo completoAbstract (sommario):
Abstract Understanding the physical mechanism maintaining fluid turbulence remains a fundamental theoretical problem. The two-layer model is an analytically and computationally simple system in which the dynamics of turbulence can be conveniently studied; in this work, a maximally simplified model of the statistically steady turbulent state in this system is constructed to isolate and identify the essential mechanism of turbulence. In this minimally complex turbulence model the effects of nonlinearity are parameterized using an energetically consistent stochastic process that is white in both space and time, turbulent fluxes are obtained using a stochastic turbulence model (STM), and statistically steady turbulent states are identified using stochastic structural stability theory (SSST). These turbulent states are the fixed-point equilibria of the nonlinear SSST system. For parameter values typical of the midlatitude atmosphere, these equilibria predict the emergence of marginally stable eddy-driven baroclinic jets. The eddy variances and fluxes associated with these jets and the power-law scaling of eddy variances and fluxes are consistent with observations and simulations of baroclinic turbulence. This optimally simple model isolates the essential physics of baroclinic turbulence: maintenance of variance by transient perturbation growth, replenishment of the transiently growing subspace by nonlinear energetically conservative eddy–eddy scattering, and equilibration to a statistically steady state of marginal stability by a combination of nonlinear eddy-induced mean jet modification and eddy dissipation. These statistical equilibrium states provide a theory for the general circulation of baroclinically turbulent planetary atmospheres.
49
Farrell, Brian F., e Petros J. Ioannou. "Structure and Spacing of Jets in Barotropic Turbulence". Journal of the Atmospheric Sciences 64, n. 10 (1 ottobre 2007): 3652–65. http://dx.doi.org/10.1175/jas4016.1.
Testo completoAbstract (sommario):
Abstract Turbulent flows are often observed to be organized into large-spatial-scale jets such as the familiar zonal jets in the upper levels of the Jovian atmosphere. These relatively steady large-scale jets are not forced coherently but are maintained by the much smaller spatial- and temporal-scale turbulence with which they coexist. The turbulence maintaining the jets may arise from exogenous sources such as small-scale convection or from endogenous sources such as eddy generation associated with baroclinic development processes within the jet itself. Recently a comprehensive theory for the interaction of jets with turbulence has been developed called stochastic structural stability theory (SSST). In this work SSST is used to study the formation of multiple jets in barotropic turbulence in order to understand the physical mechanism producing and maintaining these jets and, specifically, to predict the jet amplitude, structure, and spacing. These jets are shown to be maintained by the continuous spectrum of shear waves and to be organized into stable attracting states in the mutually adjusted mean flow and turbulence fields. The jet structure, amplitude, and spacing and the turbulence level required for emergence of jets can be inferred from these equilibria. For weak but supercritical turbulence levels the jet scale is determined by the most unstable mode of the SSST system and the amplitude of the jets at equilibrium is determined by the balance between eddy forcing and mean flow dissipation. At stronger turbulence levels the jet amplitude saturates with jet spacing and amplitude satisfying the Rayleigh–Kuo stability condition that implies the Rhines scale. Equilibrium jets obtained with the SSST system are in remarkable agreement with equilibrium jets obtained in simulations of fully developed β-plane turbulence.
50
Nadiga, B. T. "Orientation of eddy fluxes in geostrophic turbulence". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 366, n. 1875 (30 aprile 2008): 2489–508. http://dx.doi.org/10.1098/rsta.2008.0058.
Testo completoAbstract (sommario):
Given its importance in parametrizing eddies, we consider the orientation of eddy flux of potential vorticity (PV) in geostrophic turbulence. We take two different points of view, a classical ensemble- or time-average point of view and a second scale decomposition point of view. A net alignment of the eddy flux of PV with the appropriate mean gradient or the large-scale gradient of PV is required. However, we find this alignment to be very weak. A key finding of our study is that in the scale decomposition approach, there is a strong correlation between the eddy flux and a nonlinear combination of resolved gradients. This strong correlation is absent in the classical decomposition. This finding points to a new model to parametrize the effects of eddies in global ocean circulation.