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1
Andersson, Robin. "Flow Over Large-Scale Naturally Rough Surfaces." Licentiate thesis, Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-136.
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The fluid mechanical field of rough surface flows has been developed ever since the first experiments by Haagen (1854) and Darcy (1857). Although old, the area still holds merit and a surprising amount of information have to this day yet to be fully understood, which surely is a proof of its complexity. Many equations and CFD tools still rely on old, albeit reliable, concepts for simplifying the flow to be able to handle the effects of surface roughness. This notion is, however, likely to change within a not so unforeseeable future. The advancement of computer power has opened the door for more advanced CFD tools such as Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES). It can be argued that once a given flow situation has been fully accessible by numerical simulations, it is likely to be fully understood within a few years 1 . However, DNS is still limited to small scales of roughness and relatively low Reynolds number which is in contrast with given hydropower conditions today. The hydropower industry annually supplies Sweden with about 45% of its electricity production, and tunnels of various types are regularly used for conveying water to or from turbines within hydropower stations. The tunnels are a vital part of the system and their survival is of the essence. Depending on the manner of excavation, the walls of the tunnels regularly exhibit a roughness, this roughness may range from a few mm to m, which is true especially if the tunnel have been subjected to damage. For natural roughness e.g. hydropower tunnels, there is no clear way to distinguish between rough surface flows and flow past obstacles. Yet, to be able to distinguish between the two cases has proven to be important. This work is aimed to increase the understanding of how the wall roughness affects the flow, and how to treat it numerically. Paper A employs the use of pressure sensors to evaluate local deviations in pressure as well as head loss due to the surface roughness. Paper B is aimed at using PIV to evaluate the flow using averaging techniques and characteristic length scales. Paper C Further investigates the data from the PIV and pressure measurements and Evaluates the possibility to use basic but versatile turbulence models to evaluate the flow in such tunnels.
2
Watt, Robert McFarlane. "Effects of surface roughness on the boundary-layer characteristics of turbine aerofoils." Thesis, University of Oxford, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.330065.
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McClain, Stephen Taylor. "A discrete-element model for turbulent flow over randomly-rough surfaces." Diss., Mississippi State : Mississippi State University, 2002. http://library.msstate.edu/etd/show.asp?etd=etd-04032002-140007.
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Thakkar, Manan. "Investigation of turbulent flow over irregular rough surfaces using direct numerical simulations." Thesis, University of Southampton, 2017. https://eprints.soton.ac.uk/415836/.
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Incompressible turbulent flow in irregular rough channels is investigated using a finite-difference direct numerical simulation code which includes an iterative embedded boundary treatment to resolve the roughness. Seventeen industrially relevant rough surfaces with a wide variation in surface topography are considered. Various studies are conducted to understand the flow physics and the relationship between key flow parameters and surface topography. Studies at low values of friction Reynolds number, Reτ, for a single surface, show that the flow is laminar up to Reτ = 89 and begins to develop quasi-periodic fluctuations at Reτ = 89.5. Fluctuations in the three velocity components continue to grow until Reτ = 91, and the flow is turbulent for Reτ ≥ 92. Transition depends on the surface topography as some roughness peaks trigger fluctuations before others. For all the surfaces, mean and turbulent flow statistics are computed at Reτ = 180, for which the flow is fully turbulent but transitionally rough. All surfaces are scaled to the same physical roughness height. Nevertheless, a wide range of roughness function, ∆U+, values is obtained, indicating that it depends not only on the roughness height but also on the detailed roughness topography. Other mean and turbulence flow statistics also vary considerably depending on the surface topography. Next, based on the simulation results database at Reτ = 180, a newly formulated method, that determines which surface topographical properties are important and how new properties can be added to an empirical model, is tested. Optimised models with several roughness parameters are systematically developed for ∆U+ and profile peak turbulent kinetic energy. In determining ∆U+, besides the known parameters of solidity and skewness, it is shown that the streamwise correlation length and rms roughness height are also significant. The peak turbulent kinetic energy is determined by the skewness and rms roughness height, along with the mean forward-facing surface angle and spanwise effective slope. A Reynolds number dependence study is conducted for a single surface, wherein the roughness height in viscous units, k+, is varied from the transitionally rough to the fully-rough regime in the range 3.75 ≤ k+ ≤ 120. Excellent agreement with the experimental data of Nikuradse (Laws of flow in rough pipes, NACA Technical Memorandum 1292, 1933) is observed. The value of equivalent sand-grain roughness height, k+s,eq, thus obtained is close to the mean peak-to-valley height.
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LIU, WEN. "TRANSPORT PHENOMENA ASSOCIATED WITH LIQUID METAL FLOW OVER TOPOGRAPHICALLY MODIFIED SURFACES." UKnowledge, 2012. http://uknowledge.uky.edu/me_etds/16.
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Brazing and soldering, as advanced manufacturing processes, are of significant importance to industrial applications. It is widely accepted that joining by brazing or soldering is possible if a liquid metal wets the solids to be joined. Wetting, hence spreading and capillary action of liquid metal (often called filler) is of significant importance. Good wetting is required to distribute liquid metal over/between the substrate materials for a successful bonding.
Topographically altered surfaces have been used to exploit novel wetting phenomena and associated capillary actions, such as imbibitions (a penetration of a liquid front over/through a rough, patterned surface). Modification of surface roughness may be considered as a venue to tune and control the spreading behavior of the liquids. Modeling of spreading of liquids on rough surface, in particular liquid metals is to a large extent unexplored and constitutes a cutting edge research topic.
In this dissertation the imbibitions of liquid metal has been considered as pertained to the metal bonding processes involving brazing and soldering fillers. First, a detailed review of fundamentals and the recent progress in studies of non-reactive and reactive wetting/capillary phenomena has been provided. An imbibition phenomenon has been experimentally achieved for organic liquids and molten metals during spreading over topographically modified intermetallic surfaces. It is demonstrated that the kinetics of such an imbibition over rough surfaces follows the Washburn-type law during the main spreading stage. The Washburn-type theoretical modeling framework has been established for both isotropic and anisotropic non-reactive imbibition of liquid systems over rough surfaces. The rough surface domain is considered as a porous-like medium and the associated surface topographical features have been characterized either theoretically or experimentally through corresponding permeability, porosity and tortuosity. Phenomenological records and empirical data have been utilized to verify the constructed model. The agreement between predictions and empirical evidence appears to be good. Moreover, a reactive wetting in a high temperature brazing process has been studied for both polished and rough surfaces. A linear relation between the propagating triple line and the time has been established, with spreading dominated by a strong chemical reaction.
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Grissom, Dustin Leonard. "A Study of Sound Generated by a Turbulent Wall Jet Flow Over Rough Surfaces." Diss., Virginia Tech, 2007. http://hdl.handle.net/10919/28336.
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The far field acoustics generated by turbulent flow over rough surfaces has been experimentally investigated in an acoustically treated wall jet facility. The facility allows direct measurement of the far field sound from small patches of surface roughness, without contamination from edge or other aerodynamic noise sources. The facility is capable of generating turbulent boundary layer flows with momentum thickness Reynolds numbers between 450 and 1160. The variation of surface conditions tested cover the range from hydrodynamically smooth surfaces through most of the transitional range, with h+ variations from 3 to 85. Single microphone narrow band acoustic spectra, measured in the far field, show sound levels as much as 15 dB above the background from 0.186 m2 roughness patches. The measurements revealed the spectral shape and level variations with flow velocity, boundary layer thickness, and roughness size; providing the first data set large enough to assess the affects of many aerodynamic properties on the acoustic spectra. Increases in the size of grit type roughness produced significant increases in acoustic levels. Patches of hydrodynamically smooth roughness generated measurable acoustic levels, confirming that acoustic scattering is at least one of the physical mechanisms responsible for roughness noise. The shapes of the measured spectra show a strong dependence on the form of the surface roughness. The acoustic spectra generated by periodic two-dimensional surfaces have a much narrower louder peak than that generated by three-dimensional grit type roughness. Measurements also show the orientation of the two-dimensional surface significantly affects the acoustic levels and directivity.
The variation of sound levels with flow velocity and roughness size suggests the acoustic field is significantly affected by changes in the near wall flow due to the presence of the roughness. Current models of noise generated by rough surfaces predict the general trends seen in measurements for flows over grit and two-dimensional roughness in the range of 20<h+<50. However, in cases with a low Reynolds number or large grit size, where the roughness is likely to significantly affect the hydrodynamic pressure field, the scattering models did not perform as well.<br>Ph. D.
7
Letherman, Sophie Bella. "Turbulence modelling of oscillatory flows over smooth and rough surfaces." Thesis, University of Manchester, 2000. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488128.
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This study investigates turbulence models for application to boundary layer flows. Firstly, steady channel flow and transient pipe flows are considered. Calculations of a low-Reynolds-number k-epsilon model, a k-epsilon-S model (a strain parameter model which has not been applied to unsteady flows previously) and a Reynolds Stress Transport model are compared with experimental and DNS data. The eddy viscosity turbulence models (k-epsilon, k-epsilon-S) satisfactorily predict the mean flow parameters of steady channel flow. However the k-epsilon-S model proves superior in comparison with turbulence quantities. Near to the pipe wall, the k-epsilon-S model best captures the details of periodic pipe flow detail, whereas in the outer flow region the RSTM gives closest agreement with the experimental data. The high-Reynolds-number k-epsilon and k-l eddy viscosity turbulence models are examined in a separate study of oscillatory flows over smooth and rough beds. The computations are considered over a wider range of experimental parameters than previously investigated. The turbulence models are assessed by comparison with field measurements and laboratory data sets including a new set of experimental measurements. Both models predict the bed shear stress and velocity adequately, but the k-epsilon model emerges as the superior scheme when considering turbulence quantities. An attempt is made to quantify the uncertainty in the Reynolds shear stress and eddy viscosity experimental data. The k-epsilon model calculations more frequently lie within the experimental uncertainty bands. However this uncertainty range is wide; any improvement would require a corresponding improvement in the experimental resolution of rough bed flows.
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Smith, Benjamin Scott. "Wall Jet Boundary Layer Flows Over Smooth and Rough Surfaces." Diss., Virginia Tech, 2008. http://hdl.handle.net/10919/27597.
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The aerodynamic flow and fluctuating surface pressure of a plane, turbulent, two-dimensional wall jet flow into still air over smooth and rough surfaces has been investigated in a recently constructed wall jet wind tunnel testing facility. The facility has been shown to produce a wall jet flow with Reynolds numbers based on the momentum thickness, Re<SUB>&delta</SUB> = &deltaU<SUB>m</SUB>/&nu, of between 395 and 1100 and nozzle exit Reynolds numbers, Re<SUB>j</SUB> = U<SUB>m</SUB>b/&nu, of between 16000 and 45000. The wall jet flow properties (&delta, &delta<SUP>*</SUP>, &theta, y<SUB>1</SUB><SUB>/</SUB><SUB>2</SUB>, U<SUB>m</SUB>, u<SUP>*</SUP>, etc.) were measured and characterized over a wide range of initial flow conditions and measurement locations relative to the wall jet source. These flow properties were measured for flow over a smooth flow surface and for flow over roughness patches of finite extent. The patches used in the current study varied in length from 305 mm to 914 mm (between 24 and 72 times the nozzle height, b) and were placed so that the leading edge of the patch was fixed at 1257 mm (x/b = 99) downstream of the wall jet source. These roughness patches were of a random sand grain roughness type and the roughness grain size was varied throughout this experiment. The tests covered roughness Reynolds numbers (k<SUP>+</SUP>) ranging from less than 2 to over 158 (covering the entire range of rough wall flow regimes from hydrodynamically smooth to fully rough). For the wall jet flows over 305 mm long patches of roughness, the displacement and momentum thicknesses were found to vary noticeably with the roughness grain size, but the maximum velocity, mixing layer length scale, y<SUB>1</SUB><SUB>/</SUB><SUB>2</SUB>, and the boundary layer thickness were not seen to vary in a consistent, determinable way. Velocity spectra taken at a range of initial flow conditions and at several distinct heights above the flow surface showed a limited scaling dependency on the skin friction velocity near the flow surface.
The spectral density of the surface pressure of the wall jet flow, which is not believed to have been previously investigated for smooth or rough surfaces, showed distinct differences with that seen in a conventional boundary layer flow, especially at low frequencies. This difference is believed to be due to the presence of a mixing layer in the wall jet flow. Both the spectral shape and level were heavily affected by the variation in roughness grain size. This effect was most notable in overlap region of the spectrum. Attempts to scale the wall jet surface pressure spectra using outer and inner variables were successful for the smooth wall flows. The scaling of the rough wall jet flow surface pressure proved to be much more difficult, and conventional scaling techniques used for ordinary turbulent boundary layer surface pressure spectra were not able to account for the changes in roughness present during the current study. An empirical scaling scheme was proposed, but was only marginally effective at scaling the rough wall surface pressure.<br>Ph. D.
9
Alhinai, Almajd. "An investigation of classifying the flow over rough surfaces into k- and d- type in turbulent channel flow." Thesis, University of Sheffield, 2015. http://etheses.whiterose.ac.uk/11255/.
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This thesis is concerned with the classification of roughness into k- and d- type in turbulent channel flow. Despite the practical importance of this type of flow, the literature review suggest that advancements in the field have been slow due to the difficulty of making accurate measurements close to the wall when using experimental methods. In recent years, numerical modelling has provided a good alternative to studying this type of flow. In this work, an Implicit Large Eddy Simulation (ILES) approach was developed to carry out numerical simulations for turbulent channel flow over rough surfaces. The application was developed based on the Finite Element Method and implemented using the Multi-Physics platform COMSOL. Verification and validation of the numerical model was carried out to asses the predictive capabilities of the model, including sensitivity analysis to quantify the uncertainty and comparison with results from literature to validate the model. In our analysis, we considered rough surfaces with square and triangular roughness elements with a constant roughness height and varying distributions of the roughness elements. The results demonstrated that the model is capable of resolving the coherent large eddy structures associated with the k- and d- type behaviours. The classification reported here is based on the coherent structures associated with the k- and d- type behaviours. Furthermore, we investigated the effects of roughness geometry on the k- and d- type behaviours. To this end, flow visualizations were used to study the interaction between the inner and outer layer of the flow. The results demonstrated that the geometry of the roughness elements has little effect on the coherent structures associated with the k- and d-type behaviours, these effects of the roughness geometry are confined to the inner region. However, the results show that the roughness geometry has a strong influence on the interaction between the inner and outer flow regions.
10
Seddighi-Moormani, Mehdi. "Study of turbulence and wall shear stress in unsteady flow over smooth and rough wall surfaces." Thesis, University of Aberdeen, 2011. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=166096.
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Flows over hydraulically smooth walls are predominant in turbulence studies whereas real surfaces in engineering applications are often rough. This is important because turbulent flows close to the two types of surface can exhibit large differences. Unfortunately, neither experimental studies nor theoretical studies based on conventional computational fluid dynamics (CFD) can give sufficiently accurate, detailed information about unsteady turbulent flow behaviour close to solid surfaces, even for smooth wall cases. In this thesis, therefore, use is made of a state of the art computational method “Direct Numerical Simulation (DNS)” to investigate the unsteady flows. An “in-house” DNS computer code is developed for the study reported in this thesis. Spatial discretization in the code is achieved using a second order, finite difference method. The semi-implicit (Runge-Kutta & Crank-Nicholson) time advancement is incorporated into the fractional-step method. A Fast Fourier Transform solver is used for solving the Poisson equation. An efficient immersed Boundary Method (IBM) is used for treating the roughness. The code is parallelized using a Message Passing Interface (MPI) and it is adopted for use on a distributed-memory computer cluster at University of Aberdeen as well as for use at the UK’s national high-performance computing service, HECToR. As one of the first DNS of accelerating/decelerating flows over smooth and rough walls, the study has produced detailed new information on turbulence behaviours which can be used for turbulence model development and validations. The detailed data have enabled better understanding of the flow physics to be developed. The results revealed strong non-equilibrium and anisotropic behaviours of turbulence dynamics in such flows. The preliminary results on the rough wall flow show the response of turbulence in the core and wall regions, and the relationship between the axial and the other components are significantly different from those in smooth wall flows.
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Conrad, Jeffrey G. "Propagation of vertically polarized waves over rough ocean surfaces." Thesis, Monterey, California. Naval Postgraduate School, 1997. http://hdl.handle.net/10945/8867.
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Approved for public release; distribution is unlimited<br>The problem of propagation of vertically polarized radiowaves in an inhomogeneous atmosphere and over rough ocean surfaces is solved using the parabolic equation method. The solution of the parabolic equation is accomplished through the use of the Fourier split-step algorithm. Formulation of the equations is based upon (1) recognizing that the Fourier kernels of the transform equations in the split step algorithm represent planes waves and (2) compensating for the effects of rough ocean surfaces by using a rough surface reduction factor directly in the spectral domain. To accomplish this a redefinition of the Fourier transform pair is done to ensure mathematical consistency. The formulation also incorporates the first and second derivatives of the refractivity index to accommodate steep gradients in the refractivity profile. Hanning windows are used in both the spatial and wavenumber domains to contain computational requirements. The effects on propagation by varying parameters such as wave heights, computational domain ceilings, frequency and step size are investigated
12
Pruin, Bernard. "Wave propagation over one- and two-dimensional rough surfaces." Thesis, University of Cambridge, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.624138.
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Awadallah, Ra'id S. M. S. "Rough Surface Scattering and Propagation over Rough Terrain in Ducting Environments." Diss., Virginia Tech, 1998. http://hdl.handle.net/10919/30549.
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The problem of rough surface scattering and propagation over rough terrain in
ducting environments has been receiving considerable attention in the literature. One
popular method of modeling this problem is the parabolic wave equation (PWE) method.
In this method, the Helmholtz wave equation is replaced by a PWE under the assumption
of predominant forward propagation and scattering. The resulting PWE subjected to the
appropriate boundary condition(s) is then solved, given an initial field distribution, using
marching techniques such as the split-step Fourier algorithm. As is obvious from the
assumption on which it is based, the accuracy of the PWE approximation deteriorates in
situations involving appreciable scattering away from the near-forward direction, i.e.
when the terrain under consideration is considerably rough. The backscattered field is
neglected in all PWE-based models.
An alternative and more rigorous method for modeling the problem under
consideration is the boundary integral equation (BIE) method, which is formulated in two
steps. The first step involves setting up an integral equation (the magnetic field integral
equation, MFIE, or the electric field integral equation EFIE) governing currents induced
on the rough surface by the incident field and solving for these currents numerically. The
resulting currents are then used in the appropriate radiation integrals to calculate the field
scattered by the surface everywhere in space. The BIE method accounts for all orders of
multiple scattering on the rough surface and predicts the scattered field in all directions in
space (including the backscattering direction) in an exact manner.
In homogeneous media, the implementation of the BIE approach is
straightforward since the kernel (Green's function or its normal derivative) which appears
in the integral equation and the radiation integrals is well known. This is not the case,
however, in inhomogeneous media (ducting environments) where the Green's function is
not readily known. Due to this fact, there has been no attempt, up to our knowledge, at
using the BIE (except under the parabolic approximation) to model the problem under
consideration prior to the work presented in this thesis.
In this thesis, a closed-form approximation of the Green's function for a two-
dimensional ducting environment formed by the presence of a linear-square refractivity
profile is derived
using the asymptotic methods of stationary phase and steepest descents. This Green's
function is then modified to more closely model the one associated with a physical
ducting medium, in which the refractivity profile decreases up to a certain height,
beyond which it becomes constant. This modified Green's function is then used in the
BIE approach to study low grazing angle (LGA) propagation over rough surfaces in the
aforementioned ducting environment. The numerical method used to solve the MFIE
governing the surface currents is MOMI, which is a very robust and efficient method that
does not require matrix storage or inversion.
The proposed method is meant as a benchmark for people studying forward
propagation over rough surfaces using the parabolic wave equation (PWE). Rough
surface scattering results obtained via the PWE/split-step approach are compared to those
obtained via the BIE/MOMI approach in ducting environments. These comparisons
clearly show the shortcomings of the PWE/split-step approach.<br>Ph. D.
14
Harris, Joseph H. "Stability of the flow over a rough, rotating disk." Thesis, University of Warwick, 2013. http://wrap.warwick.ac.uk/60307/.
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This thesis is concerned with discovering the effect of a distributed roughness on the boundary-layer stability of a rotating disk. The investigation uses both a local, linear stability analysis and machined aluminium disks rotating in water in conjunction with a hot-film anemometer system. The stability analysis applies a sinusoidal function to the surface of the disk which mimics anisotropic roughness similar to a grooved record. The new surface is used with the governing equations in order to calculate the new mean flow profiles for the now grooved surface at a variety of roughnesses. These new flow profiles are then used in the stability analysis. The results show that the roughness has the effect of increasing the stability of the cross-flow instability mechanism by decreasing the velocity of the radial wall jet. Conversely, increasing roughness levels cause the growth of the streamlinecurvature instability mechanism, something which is probably caused by a thickening of the boundary-layer seen in the velocity profiles. These two outcomes result in a predicted switch of the dominant instability mechanism on the disk. The experimental arrangement confirms the results of the mean velocity profiles, and appears to show the appearance of the enlarged streamline-curvature instability at higher roughness levels. This instability appears as a small burst of frequencies at low Reynolds numbers centred on the numerically predicted neutral curve lobe. This burst dies down as it moves downstream, but appears to increase the amount of energy in the flow which hastens the onset of the cross-flow instability earlier than predicted. Before the emergence of this other mode at lower roughness levels, the roughness appears to delay the onset of the spiral vortices by pushing back the location of the initial cross-flow instability. The experimental results also see a decrease in the number of spiral vortices seen around the circumference of the disk as roughness is increased. This result is thought to be due to the decrease in the growth rate of the cross-flow instability rather than any switch in the neutral curve positions.
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Campbell, Lorna J. "Double-averaged open-channel flow over regular rough beds." Thesis, University of Aberdeen, 2005. http://digitool.abdn.ac.uk/R?func=search-advanced-go&find_code1=WSN&request1=AAIU214372.
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The majority of analysis of hydraulically rough open-channel flows has been founded on the temporally-averaged momentum equations (RANS). However, the structure of shallow open-channel flows over hydraulically rough beds, together with the near-bed region of deeper open-channel flows with rough beds awaits clarification. The purpose of this project was to conduct an extensive programme of laboratory-based experiments to gauge the applicability of the double-averaged momentum equations for shallow open-channel flows over simple rough beds (transverse square bars with varying centre-to-centre pitch). Proper assessment of the double-averaged equations requires comprehensive measurement of fluid velocities. Therefore, detailed Particle Image Velocimetry (PIV) measurements have been obtained of the streamwise and bed-normal velocity components, including their variation in both temporal and spatial domains. The results show that double-averaging is a powerful tool for the analysis of hydraulically rough flows. For a range of isolated flow types, the vertical distribution of the double-averaged streamwise velocity follows a linear trend between bars. Quadrant analysis has been applied to the spatial, rather than temporal, fluctuations of velocity components for the first time, and double-averaging analysis has revealed areas of intense local momentum transfer despite negligible global momentum exchange over the averaging window. This thesis also reports the discovery of an instability at the transition between wake interference and isolated roughness flows at which the overall properties of the flow are dramatically altered.
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Showkat, Ali Syamir Alihan. "Flow over and past porous surfaces." Thesis, University of Bristol, 2018. http://hdl.handle.net/1983/86527ebe-9574-425b-a1c4-6152f07cf4b4.
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This thesis is concerned with the application of porous treatments as a means of flow and aerodynamic noise reduction. An extensive experimental investigation is undertaken to study the effects of flow interaction with porous media, in particular in the context of the manipulation of flow over a flat plate and past the blunt trailing edges. Comprehensive boundary layer and wake measurements have been carried out for a long flat plate with solid and porous blunt trailing edges. Unsteady velocity and surface pressure measurements have also been performed to gain an in-depth understanding of the changes to the energy–frequency content and coherence of the boundary layer and wake structures as a result of the flow interaction with a porous treatment. The interaction of the flow with the porous substrate was found to significantly alter the energy cascade within the boundary layer. Results have shown that permeable treatments can effectively delay the vortex shedding and stabilize the flow over the blunt edge via mechanisms involving flow penetration into the porous medium and discharge into the near-wake region. It has also been shown that the porous treatment can effectively destroy the spanwise coherence of the boundary layer structures and suppress the velocity and pressure coherence, particularly at the vortex shedding frequency. The flow–porous scrubbing and its effects on the near-wall and large coherent structures have also been studied. The emergence of a quasi-periodic recirculating flow field inside highly permeable surface treatments has also been investigated. This study has identified several important mechanisms concerning the application of porous treatments and paves the way for further investigation into the interaction of the porous media with different flow fields and development of tailored porous treatments for improving the aerodynamic and aeroacoustic performance of different aero- and hydro-components.
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Harp, Susan R. "A computational method for evaluating cavitating flow between rough surfaces." Diss., Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/16838.
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Merrill, Craig F. "Spray generation for liquid wall jets over smooth and rough surfaces." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1998. http://handle.dtic.mil/100.2/ADA354473.
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Dissertation (Ph.D. in Mechanical Engineering) Naval Postgraduate School, September 1998.<br>"September 1998." Dissertation supervisor(s): T. Sarpkaya. Includes bibliographical references (p. 171-176). Also available online.
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Nilsson, Månz. "Radio-wave propagation modelling over rough sea surfaces and inhomogeneous atmosphere." Thesis, Karlstads universitet, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-84595.
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Walshe, John D. "CFD modelling of wind flow over complex and rough terrain." Thesis, Loughborough University, 2003. https://dspace.lboro.ac.uk/2134/7827.
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A model has been developed using the general-purpose Navier-Stokes solver CFX4 to simulate Atmospheric Boundary Layer flow over complex terrain. This model has been validated against the measured data from the Askervein Hill experiment, and has been shown to perform well. The CFD model is also compared to the WAsP linear model of wind flow over topography, and a significant improvement is noted for flow over complex topography. Boundary conditions, gridding issues and sensitivity to other solver parameters have all been investigated. An advanced roughness model has been developed to simulate flow over forest canopies, using a resistive body force within the canopy volume. The model is validated against measured data for simple 2D cases, and for a complex 3D case over real topography. The model is shown to give a more physically realistic profile for the wind speed in and just above forest canopies than the standard roughness length model used in most CFD simulations. An automated methodology for setting up CFD simulations using the models described has been developed. A custom pre-processing package to implement this has been written, to enable the use of the CFD methodology in a commercial environment.
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Yeo, K. S. "The stability of flow over flexible surfaces." Thesis, University of Cambridge, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.384471.
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Belcher, Stephen E. "Turbulent boundary layer flow over undulating surfaces." Thesis, University of Cambridge, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279606.
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Huang, Jui-Che. "Boundary layer receptivity of flow over compliant surfaces." Thesis, University of Liverpool, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.428289.
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Husain, C. N. "The effect of ridged roughness on momentum transfer." Thesis, University of Newcastle Upon Tyne, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380768.
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梁耀彰 and Yiu-cheong Leung. "Investigation of flows over grooved surfaces." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1987. http://hub.hku.hk/bib/B30425323.
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Leung, Yiu-cheong. "Investigation of flows over grooved surfaces /." [Hong Kong : University of Hong Kong], 1987. http://sunzi.lib.hku.hk/hkuto/record.jsp?B12361422.
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Andersson, Anders G. "Modelling flow with free and rough surfaces in the vicinity of hydropower plants." Doctoral thesis, Luleå tekniska universitet, Strömningslära och experimentell mekanik, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-25687.
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Flow with free and rough surfaces near hydropower stations is of interest for both engineering and environmental applications. Here, Computational Fluid Dynamics simulations of free surface flow and flow over rough surfaces in regulated rivers were performed in applications such fish migration, spillway design and flow over rough surfaces as in hydropower tunnels or natural channels. For all the investigated applications it is typical with very large geometrical scales, high flow rates and highly turbulent flow. Modelling boundaries such as free water surfaces and rough walls presents a challenge and was given special attention as well as the treatment of turbulence. Validation of the numerical simulations was performed in all cases with methods such as acoustic measurements with an Acoustic Doppler Current Profiler (ADCP), Acoustic Doppler Velocimeter (ADV) and optical measurements with Particle Image Velocimetry (PIV).Numerical simulations have been used to evaluate the flow downstream the Stornorrfors hydropower plant in Umeälven with regards to upstream migrating fish. Field measurements with an ADCP were performed and the measurements were used to validate the simulations. By adding a fish ladder in the simulations it was possible to investigate the attraction water created from the fishway at different positions and angles. An additional possibility to create better attraction water and improve the conditions for upstream migrating fish was simulated by guiding the spill water from the hydropower dam through a smaller passage from the old river bed.Fish population data from the same location was compared with flow fields from numerical simulations. The population data was compared with variables such as velocity, vorticity and turbulence intensity. A correlation between fish detections and turbulence intensity was shown.Simulations on the spilling from a dam were performed and compared to experimental results from a physical scale model. ADV was used to measure the velocity and validate the simulations. Two different spillway configurations were considered and simulations with both the Rigid Lid model and the Volume of Fluids method were carried out. Water levels, velocities and the shape of the water surface were compared between simulations and experiments. The simulations capture both qualitative features such as a vortex near the outlet and show good quantitative agreement with the experiments.A wall with large surface roughness was created by laser scanning a tunnel. One of the side walls was down-scaled and used to create a rough wall in a channel with rectangular cross-section for both a numerical model and an experimental model. Numerical simulations were performed and validated by PIV-measurements in the experimental model.The resolution of the geometry for the rough surface was lowered in two steps and numerical simulations were performed for flow over all three surfaces. The difference in flow fields in the bulk and near wall region was investigated as well as the difference in turbulent quantities which can provide good input for a new model for surface roughness in applications with very large surface roughness and high velocities such as flow in hydropower tunnels or natural channels and rivers.<br>Godkänd; 2013; 20130425 (aneane); Tillkännagivande disputation 2013-05-29 Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Anders G. Andersson Ämne: Strömningslära/Fluid Mechanics Avhandling: Modelling Flow with Free and Rough Surfaces in the Vicinity of Hydropower Plants Opponent: Associate Professor Nils Rüther, Dept of Hydraulic and Environmental Engineering, Norwegian University of Science and Technology, Trondheim, Norway Ordförande: Professor Staffan Lundström, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Torsdag den 20 juni 2013, kl 13.00 Plats: E231, Luleå tekniska universitet
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Kuhn, Simon. "Transport mechanisms in mixed convective flow over complex surfaces." Zürich : ETH, 2008. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=17627.
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Eberly, Lauren Elizabeth. "Internal Wave Generation Over Rough, Sloped Topography: An Experimental Study." BYU ScholarsArchive, 2012. https://scholarsarchive.byu.edu/etd/3437.
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Internal waves exist everywhere in stratified fluids - fluids whose density changes with depth. The two largest bodies of stratified fluid are the atmosphere and ocean. Internal waves are generated from a variety of mechanisms. One common mechanism is wind forcing over repeated sinusoidal topography, like a series of hills. When modeling these waves, linear theory has been employed due to its ease and low computational cost. However, recent research has shown that non-linear effects, such as boundary layer separation, may have a dramatic impact on wave generation. This research has consisted of experimentation on sloped, sinusoidal hills. As of yet, no experimental research has been done to characterize internal wave generation when repeated sinusoidal hills lie on a sloped surface such as a continental slope or a foothill. In order to perform this experiment, a laboratory was built which employed the synthetic schlieren method of wave visualization. Measurements were taken to find wind speed, boundary layer thickness, and density perturbation. From these data, an analysis was performed on wave propagation angle, wave amplitude, and pressure drag. The result of the analysis shows that when wind blows across a series of sloped sinusoidal hills, fluid becomes trapped in the troughs of the hills resulting in a lower apparent forcing amplitude. The generated waves contain less energy than linear predictions. Additionally, the sloped hills produce waves which propagate at an angle away from the viewer. A necessary correction, which shifts from the reference frame of the observer to the reference plane of the waves is described. When this correction is applied, it is shown that linear theory may only be applied for low Froude numbers. At high Froude numbers, the effect of the boundary layer is great enough that the wave characteristics deviate significantly from linear theory predictions. The analyzed data agrees well with previous studies which show a similar deviation from linear theory.
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Kazemi, Ehsan. "Numerical modelling of turbulent free surface flows over rough and porous beds using the smoothed particle hydrodynamics method." Thesis, University of Sheffield, 2017. http://etheses.whiterose.ac.uk/19579/.
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Understanding turbulent flow structure in open channel flows is an important issue for Civil Engineers who study the transport of water, sediments and contaminants in rivers. In the present study, turbulent flows over rough impermeable and porous beds are studied numerically using the Smoothed Particle Hydrodynamics (SPH) method. A comprehensive review is carried out on the methods of turbulence modelling and treatment of bed boundary in open channel flows in order to identify the limitations of the existing particle models developed in this area. 2D macroscopic SPH models are developed for simulating turbulent free surface flows over rough impermeable and porous beds under various flow conditions. For the case of impermeable beds, a drag force model is proposed to take the effect of bed roughness into account, while for the case of porous beds, macroscopic governing equations are developed based on the SPH formulation, incorporating the effects of drag and porosity. To simulate the effect of turbulence on the average flow field, a Macroscopic SPH-mixing-length (MSPH-ML) model is proposed based on the Large Eddy Simulation (LES) concept where the mixing-length approach is applied to estimate the eddy-viscosity rather than employing the standard Smagorinsky model. The difficulty in reproducing steady uniform free surface flow is tackled by introducing novel inflow/outflow techniques for the situations in which the flow quantities are unknown at the inflow and outflow boundaries. The performance of these models is tested by simulating different engineering problems with an insight developed into turbulence modelling and bed/interface boundary treatment. The accuracy of the models is tested by comparing the predicted quantities such as flow velocity, water surface elevation, and turbulent shear stress with existing experimental data. The limitations of the models are mainly attributed to the macroscopic representation of the roughness layer and porous bed, difficulty in the determination of the values of the empirical coefficients in the closure terms, and limitations with the use of fine computational resolution. On the other hand, the main strength of the model is describing the complicated processes occuring at the bed using simple and practical computational treatments so that the momentum transfer is estimated accurately. It is shown that if the closure terms in the momentum equation which represent the effect of bed drag and flow turbulence are determined carefully based on the physical conditions of bed and flow, the model is capable of being employed for different civil engineering applications.
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Sapkota, Deependra. "TRIPPING OF THE BOUNDARY LAYER DEVELOPMENT LENGTH OVER ROUGH AND FULLY TURBULENT SUBCRITICAL FLUME." OpenSIUC, 2015. https://opensiuc.lib.siu.edu/theses/1801.
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The distance required for flow entering a laboratory channel to become fully-developed and uniform can be substantial. Given the need to establish fully-developed uniform flow, if the length of a laboratory channel is not substantial then it likely that the flume cannot be used to conduct open-channel flow research. In laboratory studies where the channel bed is hydraulically rough, the noted problem can be lessened by minimizing the length over which the flow becomes fully-developed and uniform (Lunif). For this study it is hypothesized that if bed material with a roughness height (ks, ∆) is placed at the channel entrance and ks, ∆ is greater has the roughness height of bed material placed throughout the channel (ks, bed) then Lunif can be reduced. The length over which the larger bed material is referred to as the tripping zone length (∆). A second hypothesis for this study is that if ∆ is longer, then Lunif will be shorter. The primary objective of this study is to test the above mentioned hypothesis and to develop a relationship for predicting Lunif as a function of Δ. For this study, physical tests were performed in a rectangular Plexiglas flume with a variable slope. The flume was 6.1 m long, 45.7 cm wide, and 45.7 cm deep. The channel has smooth walls and the bed was lined with gravel (median particle size, d50 = 8.5 mm or 22 mm). Similarly tripping zone was lined with gravel of larger size (median particle size, d50 = 13 mm or 58 mm).Twelve tests were conducted for the study. For each test, longitudinal point velocity measurements (u) were made along the channel center, at five elevations (z), and at twelve longitudinal stations (x). An Acoustic Doppler Velocimeter was used to measure u. Lunif was determined by considering four indications of flow uniformity. Results indicate that having a tripping zone decreases Lunif and the magnitude of the decrease in Lunif was dependent on ∆. A function is presented for predicting Lunif /H = f (Rep, Fr, and Δ/H) where Rep is the Reynold's particle number, Fr is the Froude number and H is the flow depth.
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Lucey, Anthony Denis. "Hydroelastic instability of flexible surfaces." Thesis, University of Exeter, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235966.
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Langley, A. J. "Vortical flows on wavy surfaces." Thesis, University of Cambridge, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.377190.
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Jelly, Thomas. "A critical assessment of turbulent flow over textured superhydrophobic surfaces." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/24113.
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Over the past century, a sustained effort has been expended on the research and development of surfaces that reduce the amount of drag experienced by a fluid as it passes by, motivated by both environmental and economic savings. Superhydrophobic surfaces have recently emerged as an attractive means to reduce the levels of skin-friction drag under both laminar and turbulent flow conditions. A superhydrophobic state is attained naturally or synthetically through a combination of surface topology and surface chemistry and can, in some cases, support a free-stress gas-liquid interface. In the presence of bulk fluid motion, the interfaces permit a finite slip velocity which has been credited to the reduction of the average wall shear stress. The fundamental drag reduction mechanism, however, remains unclear. In order to accurately resolve the full spectrum of turbulent scales, direct numerical simulations of fully turbulent channel flow over superhydrophobic textures at a friction Reynolds number of Reτ ≈ 180 were conducted. The instantaneous flow fields were subject to triple decomposition which permits statistical quantities to be accumulated in a phase-averaged form. From these phase-averaged statistics the mean, periodic and stochastic fluid motions can be considered independently. Following a detailed statistical analysis, the contributions of the mean, periodic and stochastic fluid motions towards the local levels of wall shear stress were determined by the derivation and evaluation of an appropriate skin-friction identity. In addition, a new modification to superhydrophobic surfaces is investigated by means of meandering the surface topology in the streamwise direction. Relative to a streamwise-aligned topology, it was anticipated that superior drag reduction would be achieved due to the addition of an oscillatory spanwise motion to the mean flow.
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Chen, Zhuo. "Scalar dispersion in turbulent open channel flows over smooth and rough beds." Thesis, Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/44896.
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Study of passive dispersion of a neutral scalar in turbulentflows is highly important due
to its numerous applications in the areas of turbulent flow visualization, turbulent heat
transfer and transport of pollutants and other substances in the environment. Over the past
few decades, many analytical, numerical, and experimental studies have been conducted
on this topic to obtain a better understanding of the physical process. In the present work,
Large Eddy Simulations (LES) of scalar dispersion in turbulent flow over smooth and rough
channels is conducted to contribute to the further understanding of the relation between the
turbulent velocity field and the concentration field.
The LES results from the present work showed good agreement with a recently com-pleted experimental study(Rahman and Webster [2005]). An in-depth comparison of in-stantaneous concentration and velocity fields revealed thecorrelation between scalar dis-persion and coherent structures of the turbulent flow. Also,a three dimensional visual-ization of concentration iso-surfaces at different instants provided a good picture of the
concentration structures transported as a result of hairpin vortices of turbulent flow, which
is quite difficult to accomplish using experimental studies.
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Mansour-Tehrani, Mehrdad. "Spacial distribution and scaling of bursting events in boundary layer turbulence over smooth and rough surfaces." Thesis, University College London (University of London), 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.261297.
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Joseph, Liselle AnnMarie. "Pressure Fluctuations in a High-Reynolds-Number Turbulent Boundary Layer over Rough Surfaces of Different Configurations." Diss., Virginia Tech, 2017. http://hdl.handle.net/10919/79630.
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The pressure fluctuations under a high Reynolds Number, rough-wall, turbulent, boundary layer have been studied in the Virginia Tech Stability Wind Tunnel. Rough surfaces of varying element height (1-mm, 3-mm), shape (hemispheres, cylinders) and spacing (5.5-mm, 10.4-mm, 16.5-mm) were investigated in order to ascertain how the turbulent pressure fluctuations change with changes in roughness geometry. Rough surfaces which contain two types of elements are investigated and relationships between the combination surface and the individual surfaces have been uncovered. Measurements of the wall pressure fluctuations were made using pinhole microphones and hotwire measurements were made to obtain the velocity and turbulence field.
Among the principal findings is the development of two scaling laws for the low frequency pressure fluctuations. Both of these are based on the idea that the defect between the edge velocity and some local boundary layer velocity sustains the pressure fluctuations in the outer regions of the flow. The first scaling uses the broadband convection velocity as the local velocity of the large scale pressure fluctuations. The second scaling uses the mean boundary layer velocity. Both these scalings appear more robust than the previously proposed scalings for the low frequency region and are able to scale the pressure spectra of all the data to within 3.5-dB.
In addition, it was proven that the high frequency shear friction velocity scaling of Meyers et al. (2015) is universal to rough surfaces of different element shape and density. Physical insights into the shear friction velocity, on which this scaling is based, have been revealed. This includes an empirical formula which estimates the element pressure drag coefficient from the roughness density and the Reynolds number.
The slopes in the mid-frequency region were found to vary with element density and microphone location such that a useful scaling could not be determined for this region. The possibility of an overlap region is explored and the expectation of a -1 slope is disproved. It is hypothesised that an evanescent decay of the mid-frequency pressure fluctuations occurs between their actual location and the wall where they are measured. A method for accounting for this decay is presented in order to scale the pressure fluctuations in this region.
Lastly, a piecewise interpolation function for the pressure spectrum of rough wall turbulent boundary layers was proposed. This analytical function is based on the low frequency scaling on mean velocity and the high frequency scaling of Meyers et al. (2015) The mid-frequency is estimated by a spline interpolation between these two regions.<br>Ph. D.
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Repasky, Russell James. "Turbulent Boundary Layers over Rough Surfaces: Large Structure Velocity Scaling and Driver Implications for Acoustic Metamaterials." Thesis, Virginia Tech, 2019. http://hdl.handle.net/10919/90796.
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Turbulent boundary layer and metamaterial properties were explored to initiate the viability of controlling acoustic waves driven by pressure fluctuations from flow. A turbulent boundary layer scaling analysis was performed on zero-pressure-gradient turbulent boundary layers over rough surfaces, for 30,000≤〖Re〗_θ≤100,000. Relationships between fluctuating pressures and velocities were explored through the pressure Poisson equation. Certain scaling laws were implemented in attempts to collapse velocity spectra and turbulence profiles. Such analyses were performed to justify a proper scaling of the low-frequency region of the wall-pressure spectrum. Such frequencies are commonly associated with eddies containing the largest length scales. This study compared three scaling methods proposed in literature: The low-frequency classical scaling (velocity scale U_τ, length scale δ), the convection velocity scaling (U_e-U ̅_c, δ), and the Zagarola-Smits scaling (U_e-U ̅, δ). A default scaling (U_e, δ) was also selected as a baseline case for comparison. At some level, the classical scaling best collapsed rough and smooth wall Reynolds stress profiles. Low-pass filtering of the scaled turbulence profiles improved the rough-wall scaling of the Zagarola-Smits and convection velocity laws. However, inconsistent scaled results between the pressure and velocity requires a more rigorous pressure Poisson analysis. The selection of a proper scaling law gives insight into turbulent boundary layers as possible sources for acoustic metamaterials. A quiescent (no flow) experiment was conducted to measure the capabilities of a metamaterial in retaining acoustic surface waves. A point source speaker provided an acoustic input while the resulting sound waves were measured with a probe microphone. Acoustic surface waves were found via Fourier analysis in time and space. Standing acoustic surface waves were identified. Membrane response properties were measured to obtain source condition characteristics for turbulent boundary layers once the metamaterial is exposed to flow.<br>Master of Science<br>Aerodynamicists are often concerned with interactions between fluids and solids, such as an aircraft wing gliding through air. Due to frictional effects, the relative velocity of the air on the solid-surface is negligible. This results in a layer of slower moving fluid near the surface referred to as a boundary layer. Boundary layers regularly occur in the fluid-solid interface, and account for a sufficient amount of noise and drag on aircraft. To compensate for increases in drag, engines are required to produce increased amounts of power. This leads to higher fuel consumption and increased costs. Additionally, most boundary layers in nature are turbulent, or chaotic. Therefore, it is difficult to predict the exact paths of air molecules as they travel within a boundary layer. Because of its intriguing physics and impacts on economic costs, turbulent boundary layers have been a popular research topic. This study analyzed air pressure and velocity measurements of turbulent boundary layers. Relationships between the two were drawn, which fostered a discussion of future works in the field. Mainly, the simultaneous measurements of pressure on the surface and boundary layer velocity can be performed with understanding of the Pressure Poisson equation. This equation is a mathematical representation of the boundary layer pressure on the surface. This study also explored the possibility of turbulent-boundary-layer-driven-acoustic-metamaterials. Acoustic metamaterials contain hundreds of cavities which can collectively manipulate passing sound waves. A facility was developed at Virginia Tech to measure this effect, with aid from a similar laboratory at Exeter University. Microphone measurements showed the reduction of sound wave speed across the metamaterial, showing promise in acoustic manipulation. Applications in metamaterials in the altering of sound caused by turbulent boundary layers were also explored and discussed.
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Keays, Steven J. "Turbulent fluid flow and heat transfer in annular passages with rough surfaces and moving cores." Thesis, University of Ottawa (Canada), 1993. http://hdl.handle.net/10393/6827.
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This study investigates the behaviour of fully developed fluid flows in concentric annuli with rough surfaces and moving cores, by analytically predicting the effects of surface roughness, core velocity and turbulent motion on the momentum and heat transfer characteristics of the flow, such as friction forces, velocity profiles, temperature profiles and heat transfer. The analytical predictions are produced by a mathematical model based on an adaptation of Prandtl's mixing length theories and on the data of previous studies of turbulent flow in annuli and rectangular channels which were artificially roughened. The computer program developed for this study employs an iterative process to match velocity and temperature profiles with force and energy balances and calculates the desired momentum and thermal characteristics. The results indicate excellent correlation with previous experimental data on rough annuli with static cores and smooth annuli with moving cores. From these results, the combined effects of surface roughness and moving cores are inferred. New equations are proposed for predicting the radius of maximum fluid velocity and the effects of core velocity on roughness, in terms of annulus radius ratio, roughness geometry, flow's Reynolds number and core velocity. The study demonstrates the advantages, under certain conditions, of using roughness elements to increase heat transfer and enhance the overall efficiency in the energy transfer processes involved.
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Shetty, Sanat Achanna. "Liquid spreading and thin film flows over complex surfaces /." Access abstract and link to full text, 1995. http://0-wwwlib.umi.com.library.utulsa.edu/dissertations/fullcit/9610610.
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Bomminayuni, Sandeep Kumar. "Large eddy simulation of turbulent flow over a rough bed using the immersed boundary method." Thesis, Georgia Institute of Technology, 2010. http://hdl.handle.net/1853/34821.
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Study of turbulent flow over a rough bed is highly important due to its numerous applications in the areas of sediment transport and pollutant discharge in streams, rivers and channels. Over the past few decades, many experimental studies have been conducted in this respect to understand the underlying phenomenon. However, there is a scarcity in the number of computational studies conducted on this topic. Therefore, a Large Eddy Simulation (LES) of turbulent flow over a rough channel bed was conducted to contribute further understanding of the influence of bed roughness on turbulent flow properties. For this purpose, an efficient, second order accurate 'immersed boundary method' was implemented into the LES code Hydro3d-GT, and validated for flow past bluff bodies.
LES results from the present study showed excellent agreement with previous experimental studies on flow over rough beds. An in-depth analysis of time varying turbulent quantities (like the velocity fluctuations) revealed the presence of coherent structures in the flow. Also, a three dimensional visualization of the turbulent structures provided a good picture of the flow, especially in the near bed region, which is quite difficult to accomplish using experimental studies.
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Mensire, Rémy. "Hydrodynamics of oil in contact with an aqueous foam : wetting, imbibition dynamics and flow in rough confined media." Thesis, Paris Est, 2016. http://www.theses.fr/2016PESC1137/document.
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L’extraction de matières premières du sol à des fins énergétiques (récupération assistée d’huile) et environnementales (dépollution des sols) fait l’objet de recherches intensives en lien avec des thématiques telles que la séquestration du carbone ou la fracturation hydraulique. L’objectif est de trouver des méthodes moins destructives, moins gourmandes en matériel et en énergie, mais aussi plus efficaces et moins coûteuses. Nous proposons d’étudier une méthode alternative aux moyens conventionnels avec l’utilisation de mousses aqueuses comme agent extracteur d’huile. Les mousses aqueuses sont souvent utilisées en présence d'huile : dans des applications quotidiennes comme la cosmétique et la détergence, mais aussi dans des domaines moins connus comme la décontamination des centrales nucléaires ou l’industrie pétrolière. Ainsi, des tensioactifs et du gaz sont couramment injectés dans le sol afin d'améliorer les procédés de récupération de pétrole. Nous explicitons deux mécanismes d'extraction que nous quantifions en termes d'efficacité et de stabilité. Tout d'abord, la mousse peut aspirer de l'huile en son sein, comme le ferait une éponge. Ensuite, lorsque celle-ci est mise en écoulement, elle peut entraîner de l'huile confinée dans la rugosité d'une surface par cisaillement. Notre étude s’appuie en particulier sur une analyse théorique et expérimentale, à la fois multi-échelle, statique et dynamique pour laquelle nous avons systématiquement fait varier les paramètres géométriques (configuration de l'huile, taille des bulles et fraction volumique de liquide dans la mousse) et physico-chimiques (tensions interfaciales, rigidité des interfaces entre bulles et viscosité)<br>The extraction of raw materials from the soil for energetical (enhanced oil recovery) and environmental purposes (soil remediation) is the subject of intense fundamental and applied research. This field is related to other important topics, such as carbon sequestration and hydraulic fracturing. The goal is to find fewer destructive, as well as energy and material-saving methods. These techniques should also be cost-effective and more efficient. To find a substitution to conventional means, we study an alternative method that puts aqueous foams on the map as the extraction material. Aqueous foams are often used in numerous daily applications, such as cosmetics and detergency, but also in less known fields, such as the decommissioning of nuclear power plants and the oil industry. Thus, surfactants and gas are commonly injected into the soil to improve the recovery processes of oil. We explain two extraction mechanisms that we quantify in terms of efficiency and stability. On one hand, the foam is able to absorb oil, similarly to a solid sponge. On the other hand, when a flow of foam is induced, the foam can entrain oil confined in the roughness of a surface by shearing the oil-water interface. Our work especially lies on a theoretical and experimental analysis, which is multiscale, static and dynamic. We systematically vary the geometrical parameters (oil configuration, bubble size and liquid fraction in the foam) and the physical and chemical parameters (interfacial tensions, interfacial rigidity and viscosity)
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Perkins, Richard Mark. "PIV Measurements of Turbulent Flow in a Rectangular Channel over Superhydrophobic Surfaces with Riblets." BYU ScholarsArchive, 2014. https://scholarsarchive.byu.edu/etd/5547.
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In this thesis I investigate characteristics of turbulent flow in a channel where one of the walls has riblets, superhydrophobic microribs, or a hybrid surface with traditional riblets built on a superhydrophobic microrib surface. PIV measurements are used to find the velocity profile, the turbulent statistics, and shear stress profile in the rectangular channel with one wall having a structured test surface. Both riblets and superhydrophobic surfaces can each provide a reduction in the wall shear stress in a turbulent channel flow. Characterizing the features of the flow using particle image velocimetry (PIV) is the focus of this research. Superhydrophobicity results from the combination of a hydrophobic coating applied to a surface with microrib structures, resulting in a very low surface energy, such that the fluid does not penetrate in between the structures. The micro-rib structures are aligned in the streamwise flow direction. The riblets are larger than the micro-rib structure by an order of magnitude and protrude into the flow. All the test surfaces were produced on silicon wafers using photolithographic techniques. Pressure in the channel is maintained below the Laplace pressure for all testing, creating sustainable air pockets between the microribs. Velocity profiles, turbulent statistics, shear stress profiles, and friction factors are presented. Measurements were acquired for Reynolds numbers ranging from 4.5x10^3 to 2.0x10^4. Modest drag reductions were observed for the riblet surfaces. Substantial drag increase occurred over the superhydrophobic surfaces. The hybrid surfaces showed the greatest drag reduction. Turbulence production was strongly reduced during riblet and hybrid tests.
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Amin, Abolfazl. "Three-Dimensional Numerical Simulations of Liquid Laminar Flow Over Superhydrophobic Surfaces with Post Geometries." BYU ScholarsArchive, 2011. https://scholarsarchive.byu.edu/etd/2634.
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Frictional resistance reduction of liquid flow over surfaces has recently become a more important topic of research in the field of fluid dynamics. Scientific and technological progress and continued interest in nano and micro-technology have required new developments and approaches related to reducing frictional resistance, especially in liquid flow through nano and micro-channels. The application of superhydrophobic surfaces could be very effective in achieving the desired flow through such small channels. Superhydrophobic surfaces are created by intentionally creating roughnesses on the surface and applying a uniform hydrophobic coating to the entire surface. Liquid droplet tests have revealed that because of the trapped air within the cavities such surfaces could have contact angles as high as 179°. Such a property gives superhydrophobic surfaces liquid repelling characteristics making them very suitable for frictional resistance reduction in liquid flow through nano or micro-channels, provided wetting of the cavities could be avoided. This study presents 3-D numerical simulation results of liquid laminar flow over post patterned superhydrophobic surfaces. The research was performed in three phases, 1) pressure-driven flow with square micro-posts, 2) Couette flow with square micro-posts, and 3) pressure-driven flow with rectangular micro-posts at various aspect ratios. In phases (1) and (2) the influences of important parameters such as the cavity fraction, in the range of 0.0-0.9998, and the relative module width, from 0.01 to 1.5, on frictional resistance reduction in the creeping flow regime were explored. Phase (1) also addressed the effect of varying Reynolds number from 1 to 2500 on frictional resistance. Phase (3) was conducted for aspect ratios of 1/8, 1/4, 1/2, 2, 4, and 8 also in the creeping flow regime. The obtained results suggest that important parameters such as cavity fraction (relative area of the cavities), relative module width (combined post and cavity width relative to the channel hydraulic diameter), and the Reynolds number have great influence on the frictional resistance reduction. For pressure-driven flow at cavity fraction 0.9998, reductions as high as 97% in the frictional resistance were predicted compared with the classical channel flow. This reduction is directly related to the significant reduction in liquid-solid contact area. With respect to the effect of relative module width on the overall frictional resistance, a reduction of 93% in the frictional resistance was observed as the relative module width was increased from 0.1 to 1.5. This is indicative of the importance of the relative spacing size of the posts/cavities compared to the channel size in micro-channel liquid flow. The overall frictional resistance for post-patterned superhydrophobic surfaces was found to be independent of the Reynolds number up to a value of nominally 40 after which the non-dimensional frictional resistance increased at high values of the Reynolds number. However, at very high cavity fractions the frictional resistance was independent of Reynolds number only up to about 4. When the driving mechanism was a Couette flow, similar to the pressure-driven flow, as the cavity fraction and the relative module width increased the frictional resistance on the superhydrophobic surface decreased. At a cavity fraction of 0.9998 the reduction in the non-dimensional frictional resistance was approximately 96%, which was only 1% different from the similar pressure-driven scenario. However, a more significant difference was observed between the slip velocities for the two flow types, and it was determined that the pressure-driven flow resulted in greater apparent slip velocities than Couette flow. A maximum difference in normalized slip between the two scenarios of approximately 20% was obtained at relative module width 0.1 and Reynolds number 1. Results for superhydrophobic surfaces with rectangular micro-posts approached those reported in the literature for micro-ribs as the aspect ratio of the posts increased. When the flow was perpendicular to the long side of the posts, and as the aspect ratio increased, the frictional resistance approached previously published transverse rib results. Similarly, when the liquid flow direction was parallel to the long side of the posts, the frictional resistance results also approached those of the previously published longitudinal ribs as the aspect ratio increased.
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Gallagher, Michael William. "Coherent flow structures over mixed grain sized surfaces and their role in sediment transport." Thesis, University of Aberdeen, 1998. http://digitool.abdn.ac.uk/R?func=search-advanced-go&find_code1=WSN&request1=AAIU105984.
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The prediction of grain entrainment rates in river flows is of great importance to an engineer, as the movement of sediment can cause permanent changes to the form and character of a river. One of the key elements in achieving accurate prediction of grain entrainment rates is the understanding of the near bed flow regime, as research has revealed the existence of a number of organised flow structures which are thought to control the magnitude of shear stress applied to the bed surface. However, most of the knowledge gained on these turbulent flow structures has been from experiments conducted over smooth walls often at low Reynolds numbers. Many geophysical flows differ greatly from this scenario, having flows with high Reynolds numbers and boundaries which are rough and deformable. A series of flume experiments have therefore been conducted to investigate whether the flow structures identified over smooth walls are also present over mixed grain sized sediment beds. The first experiment used a new flow measurement technique known as particle image velocimetry to record flow field measurements over a fixed bed. Both horizontal and vertical planes of the flow were investigated (at different times) in order to obtain an understanding of the three dimensional nature of the flow. A second set of experiments was conducted over the same fixed bed, but also involved the introduction of sediment grains into the flow upstream of the measurement area. The same techniques were used to examine the flow in this case, thus enabling significant differences between the circumstances to be identified. A third set of experiments was conducted over a natural sediment and in this case flow field measurements were taken at periods of high and low transport rates, a difference in transport rate being associated with bed armouring. A short time series of near instantaneous velocity field measurements were recorded for each experimental condition to aid the identification of the flow structures. Visual inspection of the time series revealed that sweeps and ejections occur at different scales, the smallest scaling with grain size or bed features and the largest scaling with flow depth.
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Butt, Mohammad Usman [Verfasser], and Christoph [Gutachter] Egbers. "Experimental investigation of the flow over macroscopic hexagonal structured surfaces / Mohammad Usman Butt ; Gutachter: Christoph Egbers." Cottbus : BTU Cottbus - Senftenberg, 2014. http://d-nb.info/1114282901/34.
Full text
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Butt, Usman [Verfasser], and Christoph [Gutachter] Egbers. "Experimental investigation of the flow over macroscopic hexagonal structured surfaces / Mohammad Usman Butt ; Gutachter: Christoph Egbers." Cottbus : BTU Cottbus - Senftenberg, 2014. http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-30555.
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Senior, A. K. "A Numerical study of resistance in a rough walled channel flow where the ratio of roughness length scale to the depth of flow varies over a wide range." Thesis, School of Engineering and Applied Science, 2009. http://hdl.handle.net/1826/3892.
Full textAbstract:
Numerical calculations were performed over a variety of two-dimensional rib roughness
configurations in which the ratio of flow depth to roughness height was varied from 1.1
to 40. Periodically fully developed flow was achieved by employing periodic boundary
conditions and the effect of turbulence was accounted for by a two-layer model.
These calculations were used to test the hypothesis that any rough wall resistance may
be reduced to an equivalent wall shear stress located on a plane wall. The position of the
plane wall is determined by a novel method of prediction obtained by consideration of
strearnwise force moments. The resistance is then determined by three dynamically
significant length scales: the first (yo) specifies the position of the equivalent plane wall,
the second is the depth of flow h and the third is similar to Nikuradse's sand grain
roughness k,,. The latter length scale is however depth dependent and a universal
relationship is postulated:
ks
y,,
-,= F(Tkwhere
ksw is the asymptotic value of ks at very large flow depths. For the calculation of
friction factor, a resistance equation is proposed of the form typical of fully rough flows.
These postulates are supported by the numerical model results though further work
including physical experiments is required to confirm them.
Before applying the two-layer model to this problem it was tested on smooth rectangular
duct flows and Schlichting's (1936) long angle roughness experiments. The opportunity
was taken to further explore these flows, and in addition calculations were carried out
for Grass et al's ( 199 1) open channel rib roughness experiments.
The periodic boundary conditions were also applied to a larninar counter-flow plate-fin
heat exchanger.A novel source-sinka rrangemenfto r heat flux was developedi n order
to implement these boundary conditions.
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Krumbein, Benjamin [Verfasser], Cameron [Akademischer Betreuer] Tropea, Suad [Akademischer Betreuer] Jakirlić, and Amsini [Akademischer Betreuer] Sadiki. "A modeling framework for scale-resolving computations of turbulent flow over porous and rough walls / Benjamin Krumbein ; Cameron Tropea, Suad Jakirlić, Amsini Sadiki." Darmstadt : Universitäts- und Landesbibliothek Darmstadt, 2019. http://d-nb.info/1190818906/34.
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Klute, Sandra M. "The Development and Control of Axial Vortices over Swept Wings." Diss., Virginia Tech, 1999. http://hdl.handle.net/10919/29718.
Full textAbstract:
The natural unsteadiness in the post-breakdown flowfield of a 75° sweep delta wing at 40° angle of attack was studied with dual and single point hot-wire anemometry in the Engineering Science and Mechanics (ESM) Wind Tunnel at a Reynolds number Re = 210,000. Data were taken in five crossflow planes surrounding the wing's trailing edge. Results showed a dominant narrowband Strouhal frequency of St = 1.5 covering approximately 80% of the area with lower-intensity broadband secondary frequencies over 15% of that region. Cross-correlations between a fixed and traversing wire were calculated and phase and coherences mapped to determine the convection speed and trajectory of the helical mode instability. High-speed Particle Image Velocimetry (PIV) was conducted over a 75° sweep delta wing at 40° angle of attack in the ESM Water Tunnel II at Re = 45,000. Data were taken along the axis of the vortex in the breakdown flowfield at a speed of 0.1% of the convective time scale of the flow. Animations of instantaneous streamlines and velocity vectors revealed the impression of a helically spiralling vortex core on the measurement plane. Spectral analysis of the PIV data showed reduced frequencies which confirmed those found with the single-point measurements made in the ESM Wind Tunnel. The effect of four novel control surfaces on the breakdown flowfield of the delta wing was studied with surface pressure measurements along the axis of the vortex in the ESM Wind Tunnel. The apex flap was found effective and delayed vortex breakdown by 8° for a fixed wing. The flowfield was characterized over the delta wing executing a pitch-up maneuver at a reduced frequency of 0.06. Surface pressure measurements were taken in the ESM Wind Tunnel and Laser Doppler Velocimetry (LDV) was employed in the ESM Water Tunnel I as both the unmodified wing and then the wing with an apex flap deployed at an optimal angle <font face="Symbol">b</font> = 15° executed the pitch-up. Both sets of data confirmed the hysteresis of the flowfield. The LDV data, taken in two crossflow planes throughout the maneuver, showed an asymmetric breakdown development. As a practical extension of the study of the breakdown wake flowfield, hot-wire measurements were made over an F/A-18 model to determine the spectral characteristics of the flowfield. Three-dimensional vortex interactions were investigated with helium bubble flow visualization in the VPI Stability Tunnel.<br>Ph. D.