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1
Johari, H. "Cavitation on hydrofoils with sinusoidal leading edge." Journal of Physics: Conference Series 656 (December 3, 2015): 012155. http://dx.doi.org/10.1088/1742-6596/656/1/012155.
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Mehraban, A. A., and Mohammad Hassan Djavareshkian. "Experimental study of low Reynolds number effects on aerodynamics of smooth and sinusoidal leading-edge wings in the vicinity of the ground." Journal of Mechanical Engineering and Sciences 15, no. 2 (June 18, 2021): 8205–18. http://dx.doi.org/10.15282/jmes.15.2.2021.19.0644.
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Present study experimentally investigates the effects of ground clearance and Reynolds number on aerodynamic coefficients of smooth and sinusoidal leading-edge wings. Wind tunnel tests are conducted over a wide range of angles of attack from zero to 36 degrees, low Reynolds numbers of 30,000, 45,000 and 60,000, and also ground clearances of 0.5, 1 and ∞. Results showed that reduction of ground clearance and increment of Reynolds number cause the lift coefficient and the lift to drag ratio of both wings to be enhanced. Furthermore, the effects of Reynolds number and ground clearance on the smooth leading-edge wing are more than the sinusoidal leading-edge one. In addition, the sinusoidal leading-edge wing shows an excellent performance in the poststall region due to producing a higher lift and also by delaying the stall angle compared to the smooth leading-edge wing.
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Zhang, Ri-Kui, and Van Dam Jie-Zhi Wu. "Aerodynamic characteristics of wind turbine blades with a sinusoidal leading edge." Wind Energy 15, no. 3 (July 31, 2011): 407–24. http://dx.doi.org/10.1002/we.479.
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Chen, Huang, Chong Pan, and JinJun Wang. "Effects of sinusoidal leading edge on delta wing performance and mechanism." Science China Technological Sciences 56, no. 3 (January 16, 2013): 772–79. http://dx.doi.org/10.1007/s11431-013-5143-3.
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UEKI, Ayami, Takahiro YASUDA, and Hisato MINAGAWA. "The effect of wake from forward wing on performance of wing with liner leading edge and one with sinusoidal leading edge." Proceedings of the Fluids engineering conference 2020 (2020): OS03–01. http://dx.doi.org/10.1299/jsmefed.2020.os03-01.
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Feng, Feng, Xiang Ru Cheng, Xiang Yang Qi, and Xin Chang. "Hydrodynamic Performance of Leading-Edge Tubercle Three-Dimensional Airfoil." Applied Mechanics and Materials 152-154 (January 2012): 1509–15. http://dx.doi.org/10.4028/www.scientific.net/amm.152-154.1509.
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Based on RANS method, this paper studied leading-edge tubercle three-dimensional airfoil, which had effect on hydrodynamic performance of three-dimensional airfoil. Both section configurations of the two three-dimensional airfoil models were NACA0020 airfoil. The research method was numerical simulation. First, the leading-edge profile of the first airfoil model was normal. To get stalling angle of the first model, it analyzed hydrodynamic performance of the first model under different angle of attacks at Re=1.35*105. Then, the second model had a sinusoidal leading-edge profile. The second model chose the same Reynolds number. By comparison the numerical calculation results between the first and the second model, the stalling angle of second model delays 3°than the normal airfoil, and the lift coefficient of the second model increases 11.92% than the normal model. The results have laid the foundation for optimization design of leading-edge tubercle three-dimensional airfoil.
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Ozen, C. A., and D. Rockwell. "Control of vortical structures on a flapping wing via a sinusoidal leading-edge." Physics of Fluids 22, no. 2 (February 2010): 021701. http://dx.doi.org/10.1063/1.3304539.
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FUKUI, Keita, Takahiro YASUDA, Hisato NINAGAWA, and Ryo KURIMOTO. "The improvement of the wing performance using the wing with sinusoidal leading edge." Proceedings of Conference of Kansai Branch 2018.93 (2018): P027. http://dx.doi.org/10.1299/jsmekansai.2018.93.p027.
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Aftab, S. M. A., and K. A. Ahmad. "CFD study on NACA 4415 airfoil implementing spherical and sinusoidal Tubercle Leading Edge." PLOS ONE 12, no. 8 (August 29, 2017): e0183456. http://dx.doi.org/10.1371/journal.pone.0183456.
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Aftab, S. M. A., and K. A. Ahmad. "Correction: CFD study on NACA 4415 airfoil implementing spherical and sinusoidal Tubercle Leading Edge." PLOS ONE 12, no. 11 (November 21, 2017): e0188792. http://dx.doi.org/10.1371/journal.pone.0188792.
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FUKUI, Keita, Takahiro YASUDA, Hisato MINAGAWA, and Ryo KURIMOTO. "A study on the improvement of the wing performance by using sinusoidal leading edge." Proceedings of the Fluids engineering conference 2018 (2018): OS1–12. http://dx.doi.org/10.1299/jsmefed.2018.os1-12.
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Chaitanya, P., P. Joseph, S. Narayanan, C. Vanderwel, J. Turner, J. W. Kim, and B. Ganapathisubramani. "Performance and mechanism of sinusoidal leading edge serrations for the reduction of turbulence–aerofoil interaction noise." Journal of Fluid Mechanics 818 (April 4, 2017): 435–64. http://dx.doi.org/10.1017/jfm.2017.141.
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This paper presents the results of a detailed experimental investigation into the effectiveness of sinusoidal leading edge serrations on aerofoils for the reduction of the noise generated by the interaction with turbulent flow. A detailed parametric study is performed to investigate the sensitivity of the noise reductions to the serration amplitude and wavelength. The study is primarily performed on flat plates in an idealized turbulent flow, which we demonstrate captures the same behaviour as when identical serrations are introduced onto three-dimensional aerofoils. The influence on the noise reduction of the turbulence integral length scale is also studied. An optimum serration wavelength is identified whereby maximum noise reductions are obtained, corresponding to when the transverse integral length scale is approximately one-fourth the serration wavelength. This paper proves that, at the optimum serration wavelength, adjacent valley sources are excited incoherently. One of the most important findings of this paper is that, at the optimum serration wavelength, the sound power radiation from the serrated aerofoil varies inversely proportional to the Strouhal number $St_{h}=fh/U$, where $f$, $h$ and $U$ are frequency, serration amplitude and flow speed, respectively. A simple model is proposed to explain this behaviour. Noise reductions are observed to generally increase with increasing frequency until the frequency at which aerofoil self-noise dominates the interaction noise. Leading edge serrations are also shown to reduce aerofoil self-noise. The mechanism for this phenomenon is explored through particle image velocimetry measurements. Finally, the lift and drag of the serrated aerofoil are obtained through direct measurement and compared against the straight edge baseline aerofoil. It is shown that aerodynamic performance is not substantially degraded by the introduction of the leading edge serrations on the aerofoil.
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Kim, Jae Wook, Sina Haeri, and Phillip F. Joseph. "On the reduction of aerofoil–turbulence interaction noise associated with wavy leading edges." Journal of Fluid Mechanics 792 (March 3, 2016): 526–52. http://dx.doi.org/10.1017/jfm.2016.95.
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An aerofoil leading-edge profile based on wavy (sinusoidal) protuberances/tubercles is investigated to understand the mechanisms by which they are able to reduce the noise produced through the interaction with turbulent mean flow. Numerical simulations are performed for non-lifting flat-plate aerofoils with straight and wavy leading edges (denoted by SLE and WLE, respectively) subjected to impinging turbulence that is synthetically generated in the upstream zone (free-stream Mach number of 0.24). Full three-dimensional Euler (inviscid) solutions are computed for this study thereby eliminating self-noise components. A high-order accurate finite-difference method and artefact-free boundary conditions are used in the current simulations. Various statistical analysis methods, including frequency spectra, are implemented to aid the understanding of the noise-reduction mechanisms. It is found with WLEs, unlike the SLE, that the surface pressure fluctuations along the leading edge exhibit a significant source-cutoff effect due to geometric obliqueness which leads to reduced levels of radiated sound pressure. It is also found that there exists a phase interference effect particularly prevalent between the peak and the hill centre of the WLE geometry, which contributes to the noise reduction in the mid- to high-frequency range.
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Chaitanya, P., P. Joseph, S. Narayanan, and J. W. Kim. "Aerofoil broadband noise reductions through double-wavelength leading-edge serrations: a new control concept." Journal of Fluid Mechanics 855 (September 14, 2018): 131–51. http://dx.doi.org/10.1017/jfm.2018.620.
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Aerofoils operating in a turbulent flow generate broadband noise by scattering vorticity into sound at the leading edge. Previous work has demonstrated the effectiveness by which serrations, or undulations, introduced onto the leading edge, can substantially reduce broadband leading-edge noise. All of this work has focused on sinusoidal (single-wavelength) leading-edge serration profiles. In this paper, a new leading-edge serration geometry is proposed which provides significantly greater noise reductions compared to the maximum noise reductions achievable by single-wavelength serrations of the same amplitude. This is achieved through destructive interference between different parts of the aerofoil leading edge, and therefore involves a fundamentally different noise reduction mechanism from conventional single-wavelength serrations. The new leading-edge serration profiles simply comprise the superposition of two single-wavelength components of different wavelength, amplitude and phase with the objective of forming two roots that are sufficiently close together and separated in the streamwise direction. Compact sources located at these root locations then interfere, leading to less efficient radiation than single-wavelength geometries. A detailed parametric study is performed experimentally to investigate the sensitivity of the noise reductions to the profile geometry. A simple model is proposed to explain the noise reduction mechanism for these double-wavelength serration profiles and shown to be in close agreement with the measured noise reduction spectra. The study is primarily performed on flat plates in an idealized turbulent flow. The paper concludes by introducing the double-wavelength serration on a 10 % thick aerofoil, where near-identical noise reductions are obtained compared to the flat plate.
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ZHANG, RI-KUI, JIE-ZHI WU, and SHI-YI CHEN. "A NEW ACTIVE CONTROL STRATEGY FOR WIND-TURBINE BLADES UNDER OFF-DESIGN CONDITIONS." International Journal of Modern Physics: Conference Series 19 (January 2012): 283–92. http://dx.doi.org/10.1142/s2010194512008872.
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A new active control strategy for wind-turbine blades under off-design conditions has been investigated in this paper. According to our previous work, in comparison with the traditional straight leading-edge blade, a new kind of bionic blades with a sinusoidal leading edge can significantly enhance the turbine's power output at high speed inflows. However, the wavy leading-edge shape is unfavorable under the design operating conditions since an early boundary-layer separation is inevitable for a wind-turbine blade because of the geometric disturbances of the leading-edge tubercles. But for the present active control, the deflect in wavy leading-edge blades can be eliminated by introducing a series of small flat delta wings as the control units, since delta wings can also generate powerful leading-edge vortices. As a preliminary test, our numerical results show that, the shaft-torque fluctuation in the turbine's stall region can be improved from 27.8% for a straight leading-edge blade (no control) to 8.9% for the present active control; and by adjusting the control parameters, the control units nearly have not any negative effect on the blade's shaft torque under the design conditions. We believe that, as an auxiliary tool of the conventional control strategies, the present active control approach may be favorable to generate a more stable and more controllable power output for wind turbines under all operating conditions (even in the yawed inflows).
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Xing, Shi-Long, He-Yong Xu, Ming-Sheng Ma, and Zheng-Yin Ye. "Inflatable Leading Edge-Based Dynamic Stall Control considering Fluid-Structure Interaction." International Journal of Aerospace Engineering 2020 (February 20, 2020): 1–28. http://dx.doi.org/10.1155/2020/9046542.
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The inflatable leading edge (ILE) is explored as a dynamic stall control concept. A fluid-structure interaction (FSI) numerical method for the elastic membrane structure is constructed based on unsteady Reynolds-averaged Navier-Stokes (URANS) and a mass-spring-damper (MSD) structural dynamic model. Radial basis function- (RBF-) based mesh deformation algorithm and Laplacian and optimization-based mesh smoothing algorithm are adopted in flowfield simulations to achieve the pitching oscillation of the airfoil and to ensure the mesh quality. An airfoil is considered at a freestream Mach number of 0.3 and chord-based Reynolds number of 3.92×106. The airfoil is pitched about its quarter-chord axis at a sinusoidal motion. The numerical results indicate that the ILE can change the radius of curvature of the airfoil leading edge, which could reduce the streamwise adverse pressure gradient and suppress the formation of dynamic stall vortex (DSV). Although the maximum lift coefficient of the airfoil is slightly reduced during the control process, the maximum drag and pitching moment coefficients of the airfoil are greatly reduced by up to 66% and 75.2%, respectively. The relative position of the ILE has a significant influence on its control effect. The control laws of inflation and deflation also affect the control ability of the ILE.
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Chang, Xin, Xin Ning Wang, and Xiang Ru Cheng. "Research on Hydrodynamic Performance of Three-Dimensional Airfoil with Tubercles on Leading-Edge." Applied Mechanics and Materials 575 (June 2014): 405–13. http://dx.doi.org/10.4028/www.scientific.net/amm.575.405.
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This paper aims to improve and control hydrodynamic performance of three-dimensional airfoils and investigate hydrodynamic performance of three-dimensional airfoil with tubercles on leading-edge by imitating the sinusoidal leading-edge systematically. Based on the DES method, a series of parameters, such as amplitudes and numbers of tubercles, had been studied via the FLUENT software with model constructed by ICEM software and divided by structural grid. According to the results, the amplitudes significantly affect the hydrodynamic performance of three-dimensional airfoil. With maintaining other conditions,tubercle airfoils can make stall angle delay, raise the lift and the drag ratio coefficient. Especially, if there is a bigger attack angle, it is better to reduce resistance and save energy, which will be a cornerstone for further study. It is of vital importance to find out appropriate amplitudes and numbers of tubercles to achieve further progress in hydrodynamic performance of three-dimensional airfoil.
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Goruney, T., and D. Rockwell. "Flow past a delta wing with a sinusoidal leading edge: near-surface topology and flow structure." Experiments in Fluids 47, no. 2 (May 10, 2009): 321–31. http://dx.doi.org/10.1007/s00348-009-0666-x.
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Asli, Majid, Behnam Mashhadi Gholamali, and Abolghasem Mesgarpour Tousi. "Numerical Analysis of Wind Turbine Airfoil Aerodynamic Performance with Leading Edge Bump." Mathematical Problems in Engineering 2015 (2015): 1–8. http://dx.doi.org/10.1155/2015/493253.
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Aerodynamic performance improvement of wind turbine blade is the key process to improve wind turbine performance in electricity generated and energy conversion in renewable energy sources concept. The flow behavior on wind turbine blades profile and the relevant phenomena like stall can be improved by some modifications. In the present paper, Humpback Whales flippers leading edge protuberances model as a novel passive stall control method was investigated on S809 as a thick airfoil. The airfoil was numerically analyzed by CFD method in Reynolds number of 106and aerodynamic coefficients in static angle of attacks were validated with the experimental data reported by Somers in NREL. Therefore, computational results for modified airfoil with sinusoidal wavy leading edge were presented. The results revealed that, at low angles of attacks before the stall region, lift coefficient decreases slightly rather than baseline model. However, the modified airfoil has a smooth stall trend while baseline airfoil lift coefficient decreases sharply due to the separation which occurred on suction side. According to the flow physics over the airfoils, leading edge bumps act as vortex generator so vortices containing high level of momentum make the flow remain attached to the surface of the airfoil at high angle of attack and prevent it from having a deep stall.
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Amiet, R. K. "Airfoil leading-edge suction and energy conservation for compressible flow." Journal of Fluid Mechanics 289 (April 25, 1995): 227–42. http://dx.doi.org/10.1017/s0022112095001315.
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When a flat-plate airfoil at zero angle of attack encounters a vertical gust in an otherwise uniform flow, it experiences a force along the chord. This leading-edge suction force is examined for compressible flow with a time-dependent gust. A simple derivation of the thrust force is based on the fact that the leading edge is a singular point so that the flow here is dominated by the leading-edge dipole strength. From the viewpoint of a fluid-fixed observer the fluid does work on the airfoil, and this energy must come from the incident gust. Demonstrating energy conservation is not surprising, but it gives a better understanding of the relationship between the individual energy terms. The derivation shows that the acoustic energy can be calculated using compact assumptions at low frequency, but that it must be calculated non-compactly at high frequency. For a general gust the work done on the airfoil is shown to equal the energy taken from the fluid, the energy transfer occurring at the leading edge. For a sinusoidal gust the energy contained in the incident gust is shown to equal the sum of the energy remaining in the wake, the work done on the airfoil and the acoustic energy radiated away. The relative proportions of the incident energy going to these three energy types depends on the gust frequency, the acoustic radiation becoming more efficient as the frequency increases. For a fixed gust frequency, the thrust force goes to zero at a Mach number of one, and for an incident gust consisting of vorticity on the airfoil axis, the entire energy of the gust is radiated as acoustic energy at this Mach number.
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Hansen, Kristy L., Nikan Rostamzadeh, Richard M. Kelso, and Bassam B. Dally. "Evolution of the streamwise vortices generated between leading edge tubercles." Journal of Fluid Mechanics 788 (January 12, 2016): 730–66. http://dx.doi.org/10.1017/jfm.2015.611.
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Sinusoidal modifications to the leading edge of a foil, or tubercles, have been shown to improve aerodynamic performance under certain flow conditions. One of the mechanisms of performance enhancement is believed to be the generation of streamwise vortices, which improve the momentum exchange in the boundary layer. This experimental and numerical study investigates the formation and evolution of these streamwise vortices at a low Reynolds number of $Re=2230$, providing insight into both the averaged and time-dependent flow patterns. Furthermore, the strength of the vortices is quantified through calculation of the vorticity and circulation, and it is found that the circulation increases in the downstream direction. There is strong agreement between the experimental and numerical observations, and this allows close examination of the flow structure. The results demonstrate that the presence of strong pressure gradients near the leading edge gives rise to a significant surface flux of vorticity in this region. As soon as this vorticity is created, it is stretched, tilted and diffused in a highly three-dimensional manner. These processes lead to the generation of a pair of streamwise vortices between the tubercle peaks. A horseshoe-shaped separation zone is shown to initiate behind a tubercle trough, and this region of separation is bounded by a canopy of boundary-layer vorticity. Along the sides of this shear layer canopy, a continued influx of boundary-layer vorticity occurs, resulting in an increase in circulation of the primary streamwise vortices in the downstream direction. Flow visualisation and particle image velocimetry studies support these observations and demonstrate that the flow characteristics vary with time, particularly near the trailing edge and at a higher angle of attack. Numerical evaluation of the lift and drag coefficients reveals that, for this particular flow regime, the performance of a foil with tubercles is slightly better than that of an unmodified foil.
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Chen, Shuling, Yan Liu, Changzhi Han, Shiqiang Yan, and Zhichao Hong. "Numerical Investigation of Turbine Blades with Leading-Edge Tubercles in Uniform Current." Water 13, no. 16 (August 13, 2021): 2205. http://dx.doi.org/10.3390/w13162205.
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Inspired by the tubercles on humpback whale flippers, leading-edge tubercles have been incorporated into the design of wings and turbine blades in an attempt to improve their hydrodynamic performance. Although promising improvements, especially in terms of the stall performance, have been demonstrated in the limited research that exists to date, the effectiveness of the leading-edge tubercles seems to be influenced by the base blade. This paper focuses on the introduction of sinusoidal leading-edge tubercles to a base blade developed from the classic NACA0018 airfoil, and numerically investigates the effectiveness of leading-edge tubercles on the hydrodynamics associated with the blade in uniform current with different attack angles. Both the macroscopic parameters, such as the lift and drag forces, and the micro-scale flow characteristics, including the vortex and flow separation, are analyzed. The results indicate that the leading-edge tubercles brings a significant influence on the hydrodynamic forces acting on the blade when subjected to an attack angle greater than 15°. This study also reveals the important role of the turbulence and flow separation on hydrodynamic loading on the blade and the considerable influence of the tubercles on such micro-scale flow characteristics. Although the conditions applied in this work are relatively ideal (e.g., the blade is fixed in a uniform flow and the end effect is ignored), the satisfactory agreement between the numerical and corresponding experimental data implies that the results are acceptable. This work builds a good reference for our future work on the hydrodynamic performance of tidal turbines which adopt this kind of blade for operating in both uniform and shearing currents.
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KIYA, M., M. SHIMIZU, and O. MOCHIZUKI. "Sinusoidal forcing of a turbulent separation bubble." Journal of Fluid Mechanics 342 (July 10, 1997): 119–39. http://dx.doi.org/10.1017/s0022112097005521.
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A turbulent separation bubble is forced by single- and double-frequency sinusoidal disturbances, with the emphasis placed on the reattachment length as a function of the forcing amplitude and frequency. The separation bubble is that formed along the side of a blunt circular cylinder with a square leading edge. In single-frequency forcing, the reattachment length attains a minimum at a particular forcing frequency, F, which scales with the frequency of shedding of vortices from the reattachment region of the separated shear layer. A flow model is presented to interpret the frequency F. Forcing of sufficiently high amplitude eliminates the recirculating region in a range of the forcing frequency. Flow visualization and a survey of the mean flow and turbulence properties demonstrate how the flow in the separated shear layer is modified by the forcing. In double-frequency forcing, the superposition of the F-component on its higher or subharmonic components is considered. A non-resonant combination of the two frequencies is also considered.
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Gehrke, Alexander, Guillaume Guyon-Crozier, and Karen Mulleners. "Genetic Algorithm Based Optimization of Wing Rotation in Hover." Fluids 3, no. 3 (August 15, 2018): 59. http://dx.doi.org/10.3390/fluids3030059.
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The pitching kinematics of an experimental hovering flapping wing setup are optimized by means of a genetic algorithm. The pitching kinematics of the setup are parameterized with seven degrees of freedom to allow for complex non-linear and non-harmonic pitching motions. Two optimization objectives are considered. The first objective is maximum stroke average efficiency, and the second objective is maximum stroke average lift. The solutions for both optimization scenarios converge within less than 30 generations based on the evaluation of their fitness. The pitching kinematics of the best individual of the initial and final population closely resemble each other for both optimization scenarios, but the optimal kinematics differ substantially between the two scenarios. The most efficient pitching motion is smoother and closer to a sinusoidal pitching motion, whereas the highest lift-generating pitching motion has sharper edges and is closer to a trapezoidal motion. In both solutions, the rotation or pitching motion is advanced with respect to the sinusoidal stroke motion. Velocity field measurements at selected phases during the flapping motions highlight why the obtained solutions are optimal for the two different optimization objectives. The most efficient pitching motion is characterized by a nearly constant and relatively low effective angle of attack at the start of the half stroke, which supports the formation of a leading edge vortex close to the airfoil surface, which remains bound for most of the half stroke. The highest lift-generating pitching motion has a larger effective angle of attack, which leads to the generation of a stronger leading edge vortex and higher lift coefficient than in the efficiency optimized scenario.
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Kiya, M., O. Mochizuki, Y. Ido, and H. Kosaku. "Structure of turbulent leading-edge separation bubble of a blunt circular cylinder ans its response to sinusoidal disturbances." Journal of Wind Engineering and Industrial Aerodynamics 49, no. 1-3 (December 1993): 227–36. http://dx.doi.org/10.1016/0167-6105(93)90018-j.
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Utama, I. Ketut Aria Pria, Dendy Satrio, Mukhtasor Mukhtasor, Mehmet Atlar, Weichao Shi, Ridho Hantoro, and Giles Thomas. "Numerical simulation of foil with leading-edge tubercle for vertical-axis tidal-current turbine." Journal of Mechanical Engineering and Sciences 14, no. 3 (September 30, 2020): 6982–92. http://dx.doi.org/10.15282/jmes.14.3.2020.02.0547.
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The main disadvantage of the vertical-axis turbine is its low coefficient of performance. The purpose of this work was to propose a method to improve this performance by investigating the hydrodynamic forces and the flow-field of a foil that was modified with a sinusoidal leading-edge tubercle. NACA 63(4)021 was chosen as the original foil since it has a symmetrical profile that is suitable for use on a vertical-axis tidal-current turbine. The study was conducted using a numerical simulation method with ANSYS-CFX Computational Fluid Dynamics (CFD) code to solve the incompressible Reynolds-Averaged Navier-Stokes (RANS) equations. Firstly, the simulation results of the original foil were validated with available experimental data. Secondly, the modified foils, with three configurations of tubercles, were modelled. From the simulation results, the tubercle foils, when compared with the original foil, had similar lift performances at low Angles of Attack (0-8 degrees of AoA), lower lift performances at medium AoA (8-19 degrees) and higher lift performances at high AoA (19-32 degrees). A tubercle foil with Height/Chord (H/C) of 0.05 can maintain the static stall condition until 32 degrees. Therefore, a vertical-axis turbine with tubercle-blades provides an opportunity to increase its performance by extending the operational range for extracting energy in the dynamic stall condition.
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Baghdad, Mohammed, Abdelkader Nehmar, and Ahmed Ouadha. "Numerical simulation of the flow over a tubercled wing." MATEC Web of Conferences 307 (2020): 01036. http://dx.doi.org/10.1051/matecconf/202030701036.
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The objective of the present study is to carry out a numerical study of the flow around a NACA0021 modified wing by the incorporation of sinusoidal tubercles on its leading edge at a Reynolds number equal to 225,000. The SST k-ω turbulence model is used as closure to the incompressible governing equations. Runs have been performed for several attack angles. Results show that for lower angles of attack, tubercles reduce the drag coefficient with a slight increase in lift.
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Bai, Chi-Jeng, Wei-Cheng Wang, and Po-Wei Chen. "The effects of sinusoidal leading edge of turbine blades on the power coefficient of horizontal-axis wind turbine (HAWT)." International Journal of Green Energy 13, no. 12 (May 4, 2016): 1193–200. http://dx.doi.org/10.1080/15435075.2016.1180624.
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Pérez-Torró, Rafael, and Jae Wook Kim. "A large-eddy simulation on a deep-stalled aerofoil with a wavy leading edge." Journal of Fluid Mechanics 813 (January 17, 2017): 23–52. http://dx.doi.org/10.1017/jfm.2016.841.
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A numerical investigation on the stalled flow characteristics of a NACA0021 aerofoil with a sinusoidal wavy leading edge (WLE) at chord-based Reynolds number $Re_{\infty }=1.2\times 10^{5}$ and angle of attack $\unicode[STIX]{x1D6FC}=20^{\circ }$ is presented in this paper. It is observed that laminar separation bubbles (LSBs) form at the trough areas of the WLE in a collocated fashion rather than uniformly/periodically distributed over the span. It is found that the distribution of LSBs and their influence on the aerodynamic forces is strongly dependent on the spanwise domain size of the simulation, i.e. the wavenumber of the WLE used. The creation of a pair of counter-rotating streamwise vortices from the WLE and their evolution as an interface/buffer between the LSBs and the adjacent fully separated shear layers are discussed in detail. The current simulation results confirm that an increased lift and a decreased drag are achieved by using the WLEs compared to the straight leading edge (SLE) case, as observed in previous experiments. Additionally, the WLE cases exhibit a significantly reduced level of unsteady fluctuations in aerodynamic forces at the frequency of periodic vortex shedding. The beneficial aerodynamic characteristics of the WLE cases are attributed to the following three major events observed in the current simulations: (i) the appearance of a large low-pressure zone near the leading edge created by the LSBs; (ii) the reattachment of flow behind the LSBs resulting in a decreased volume of the rear wake; and, (iii) the deterioration of von-Kármán (periodic) vortex shedding due to the breakdown of spanwise coherent structures.
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Gopinathan, Veerapathiran Thangaraj, John Bruce Ralphin Rose, and Mohanram Surya. "Investigation on the effect of leading edge tubercles of sweptback wing at low reynolds number." Mechanics & Industry 21, no. 6 (2020): 621. http://dx.doi.org/10.1051/meca/2020095.
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Aerodynamic efficiency of an airplane wing can be improved either by increasing its lift generation tendency or by reducing the drag. Recently, Bio-inspired designs have been received greater attention for the geometric modifications of airplane wings. One of the bio-inspired designs contains sinusoidal Humpback Whale (HW) tubercles, i.e., protuberances exist at the wing leading edge (LE). The tubercles have excellent flow control characteristics at low Reynolds numbers. The present work describes about the effect of tubercles on swept back wing performance at various Angle of Attack (AoA). NACA 0015 and NACA 4415 airfoils are used for swept back wing design with sweep angle about 30°. The modified wings (HUMP 0015 A, HUMP 0015 B, HUMP 4415 A, HUMP 4415 B) are designed with two amplitude to wavelength ratios (η) of 0.1 & 0.24 for the performance analysis. It is a novel effort to analyze the tubercle vortices along the span that induce additional flow energy especially, behind the tubercles peak and trough region. Subsequently, Co-efficient of Lift (CL), Co-efficient of Drag (CD) and boundary layer pressure gradients also predicted for modified and baseline (smooth LE) models in the pre & post-stall regimes. It was observed that the tubercles increase the performance of swept back wings by the enhanced CL/CD ratio in the pre-stall AoA region. Interestingly, the flow separation region behind the centerline of tubercles and formation of Laminar Separation Bubbles (LSB) were asymmetric because of the sweep.
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Brown, Edward, Leo M. Carlin, Claus Nerlov, Cristina Lo Celso, and Alastair W. Poole. "Multiple membrane extrusion sites drive megakaryocyte migration into bone marrow blood vessels." Life Science Alliance 1, no. 2 (May 2018): e201800061. http://dx.doi.org/10.26508/lsa.201800061.
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Platelets, cells central to hemostasis and thrombosis, are formed from parent cell megakaryocytes. Although the process is highly efficient in vivo, our ability to generate them in vitro is still remarkably inefficient. We proposed that greater understanding of the process in vivo is needed and used an imaging approach, intravital correlative light electron microscopy, to visualize platelet generation in bone marrow in the living mouse. In contrast to current understanding, we found that most megakaryocytes enter the sinusoidal space as large protrusions rather than extruding fine proplatelet extensions. The mechanism for large protrusion migration also differed from that of proplatelet extension. In vitro, proplatelets extend by sliding of dense bundles of microtubules, whereas in vivo our data showed the absence of microtubule bundles in the large protrusion, but the presence of multiple fusion points between the internal membrane and the plasma membrane, at the leading edge of the protruding cell. Mass membrane fusion, therefore, drives megakaryocyte large protrusions into the sinusoid, significantly revising our understanding of the fundamental biology of platelet formation in vivo.
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Fukui, Keita, Takahiro Yasuda, Hisato Minagawa, and Ryo Kurimoto. "Improvement in wing performance using a wing with sinusoidal leading edge (The effect of the wing camber and the wing camber position)." Journal of Aero Aqua Bio-mechanisms 8, no. 1 (2019): 13–19. http://dx.doi.org/10.5226/jabmech.8.13.
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Uddin, Md Kutub, and Rabindra Nath Mondal. "Radiation-Conduction Interaction of Steady Streamwise Surface Temperature Variations on Vertical Free Convection." ISRN Applied Mathematics 2011 (December 14, 2011): 1–11. http://dx.doi.org/10.5402/2011/927416.
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The combined effects of the steady free convective boundary-layer flow induced by a vertical heated surface in the presence of sinusoidal surface temperature variations about a constant mean value with the effect of radiation are examined. The problem is studied using fully numerical techniques. The surface rate of heat transfer eventually alternates in sign with distance from the leading edge, but no separation occurs unless the amplitude of the thermal modulation is sufficiently high. Numerical results are obtained for different values of the physical parameters, the radiation parameter Rd, the Prandtl number Pr, and the surface temperature wave amplitude a. It is found that both the local shear stress and the rate of heat transfer decrease when values of Rd increase.
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Al-Bakri, Basim, and Radwan Aljuhashy. "The aerodynamics of the wavy blade under the effect of fluctuated wind flow." FME Transactions 49, no. 3 (2021): 704–10. http://dx.doi.org/10.5937/fme2103704a.
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In the present study, the influence of the wavy edge blade on aerodynamic characteristics for the flow of blades at Reynolds number (Re) of 8×105 is numerically investigated based on the unsteady wind flow. Aerodynamic characteristics of a (sinusoidal leading edge) wavy NACA0015 aerofoil blade are carried out using ICEM 19.1 and ANSYS fluent. The numerical simulation is conducted then validated by experimental data with steady wind flow. This is conducted by employing the same Reynold's number in the experimental work. While, the unsteady flow was numerically performed at 1 Hz frequency of wind flow conditions. The main findings from this work show that the wavy blade can behave better in turbulent wind conditions with the maximum lift coefficient of 0.73 compared to 0.621 for the normal blade. However, the findings declare that the wavy blade stalled earlier than the normal one in the unsteady flow case. Similarly, it stalled at 12° angle of attack earlier than the normal one which was stalled at 14° in the steady flow case.
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Stango, Robert J., Lienjing Chen, and Vikram Cariapa. "Automated Deburring with a Filamentary Brush: Prescribed Burr Geometry." Journal of Manufacturing Science and Engineering 121, no. 3 (August 1, 1999): 385–92. http://dx.doi.org/10.1115/1.2832693.
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In this paper, a dynamic model for removal of edge burrs with a compliant brushing tool is reported. Description of the burr geometry is assumed to be known through on-line measurement methods such as a computer vision system in the flexible manufacturing cell. Dynamic response of the brush/workpiece system is evaluated on the basis of experimentally obtained data. Master Curves are introduced as machining descriptors which characterize the incremental burr removal performance of the brush/workpiece system, leading to the development of an analytical dynamic model for orthogonal burr removal using a finite-width brushing tool. Based upon the dynamic model for material removal, a control strategy for automatic deburring is presented for burr configurations having constant height as well as variable height. A closed-form solution for transverse brush feed rate is obtained which is applicable for removal of burrs having variable height, as described by suitable geometry functions. For illustrative purposes, simulations are carried out for a straight-edge burr profile and sinusoidal burr geometry. Results are reported which identify important relationships among brush feed rate, brush penetration depth, and brush rotational speed. In order to help assess the validity of the proposed analytical model and control strategy, experimental results are reported for a combination ramp/straight-edge burr configuration. The results demonstrate generally good correlation between the predicted and actual profile for the edge burr that has been machined. In addition, some important observations include; (1) burr removal is most rapidly carried out by using the highest brush speed and deepest brush/workpiece penetration depth, subject to the condition that the brush fiber is not damaged, (2) Currently available polymer abrasive brushing tools exhibit very slow machining characteristics and must be improved in order to be used in a production environment where burr size is appreciable, (3) Material removal characteristics of the leading and trailing edge of brushes may be a source of error which merits further investigation.
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Sato, Saya, Hiroshi Yokoyama, and Akiyoshi Iida. "Control of Flow around an Oscillating Plate for Lift Enhancement by Plasma Actuators." Applied Sciences 9, no. 4 (February 22, 2019): 776. http://dx.doi.org/10.3390/app9040776.
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During insect flight, a feathering motion of the wing’s controls vortex shedding for lift enhancement. In this study, in order to control the flow around a wing flapping with simplified sinusoidal motion, plasma actuators were introduced to simplify the complex feathering motion. In a wind tunnel, a smoke-wire method was enacted to visualize the flow fields around an oscillating plate with an attack angle of 4° in a uniform flow for the baseline and controlled cases. The actuator placed around the leading edge was found to suppress the flow separation on the top surface. Numerical simulations were performed to investigate the control effects on the fluctuating lift, where the control effects by the intermittently driven actuator were also predicted. The actuator installed on the top surface throughout the up-stroke motion was found to suppress vortex shedding from the trailing edge, which resulted in an 11% lift enhancement compared to the baseline case. In regard to the effects of the installation position, it was found that the actuator placed on the top surface was effective, compared to the cases for installation on the bottom surface or both surfaces.
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Peluso, Emmanuele, Riccardo Rossi, Andrea Murari, Pasqualino Gaudio, and Michela Gelfusa. "Alternative Detection of n = 1 Modes Slowing Down on ASDEX Upgrade." Applied Sciences 10, no. 21 (November 6, 2020): 7891. http://dx.doi.org/10.3390/app10217891.
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Disruptions in tokamaks are very often associated with the slowing down of magneto-hydrodynamic (MHD) instabilities and their subsequent locking to the wall. To improve the understanding of the chain of events ending with a disruption, a statistically robust and physically based criterion has been devised to track the slowing down of modes with toroidal mode numbers n = 1 and mostly poloidal mode number m = 2, providing an alternative and earlier detection tool compared to simple threshold based indicators. A database of 370 discharges of axially symmetric divertor experiment—upgrade (AUG) has been studied and results compared with other indicators used in real time. The estimator is based on a weighted average value of the fast Fourier transform of the perturbed radial n = 1 magnetic field, caused by the rotation of the modes. The use of a carrier sinusoidal wave helps alleviating the spurious influence of non-sinusoidal magnetic perturbations induced by other instabilities like Edge localized modes (ELMs). The indicator constitutes a good candidate for further studies including machine learning approaches for mitigation and avoidance since, by deploying it systematically to evaluate the time instance for the expected locking, multi-machine databases can be populated. Furthermore, it can be thought as a contribution to a wider approach to dynamically tracking the chain of events leading to disruptions.
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Miyara, Akio. "Flow Dynamics and Heat Transfer of Wavy Condensate Film." Journal of Heat Transfer 123, no. 3 (August 14, 2000): 492–500. http://dx.doi.org/10.1115/1.1370522.
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Wave evolution and heat transfer behavior of a wavy condensate film down a vertical wall have been investigated by a finite difference method, in which the algorithm is based on the HSMAC method, and a staggered grid fixed on a physical space is employed. For the moving interface, newly proposed methods are used. A random perturbation of the film thickness is generated near the leading edge. The perturbation quickly diminishes once and small-amplitude long waves are propagated downstream. Then the amplitude of the wave increases rapidly at a certain position, and the wave shape changes from a sinusoidal wave to a pulse-like solitary wave which is composed of a large-amplitude wave and capillary waves. A circulation flow occurs in the large wave and it affects the temperature field. The heat transfer is enhanced by space-time film thickness variation and convection effects.
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Brooks, Seth A., and Melissa A. Green. "Experimental Study of Body-Fin Interaction and Vortex Dynamics Generated by a Two Degree-Of-Freedom Fish Model." Biomimetics 4, no. 4 (October 8, 2019): 67. http://dx.doi.org/10.3390/biomimetics4040067.
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Oscillatory modes of swimming are used by a majority of aquatic swimmers to generate thrust. This work seeks to understand the phenomenological relationship between the body and caudal fin for fast and efficient thunniform swimming. Phase-averaged velocity data was collected and analyzed in order to understand the effects of body-fin kinematics on the wake behind a two degree-of-freedom fish model. The model is based on the yellowfin tuna (Thunnus albacares) which is known to be both fast and efficient. Velocity data was obtained along the side of the tail and caudal fin region as well as in the wake downstream of the caudal fin. Body-generated vortices were found to be small and have an insignificant effect on the caudal fin wake. The evolution of leading edge vortices formed on the caudal fin varied depending on the body-fin kinematics. The circulation produced at the trailing edge during each half-cycle was found to be relatively insensitive to the freestream velocity, but also varied with body-fin kinematics. Overall, the generation of vorticity in the wake was found to dependent on the trailing edge motion profile and velocity. Even relatively minor deviations from the commonly used model of sinusoidal motion is shown to change the strength and organization of coherent structures in the wake, which have been shown in the literature to be related to performance metrics such as thrust and efficiency.
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Baik, Yeon Sik, Luis P. Bernal, Kenneth Granlund, and Michael V. Ol. "Unsteady force generation and vortex dynamics of pitching and plunging aerofoils." Journal of Fluid Mechanics 709 (August 6, 2012): 37–68. http://dx.doi.org/10.1017/jfm.2012.318.
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AbstractExperimental studies of the flow topology, leading-edge vortex dynamics and unsteady force produced by pitching and plunging flat-plate aerofoils in forward flight at Reynolds numbers in the range 5000–20 000 are described. We consider the effects of varying frequency and plunge amplitude for the same effective angle-of-attack time history. The effective angle-of-attack history is a sinusoidal oscillation in the range $\ensuremath{-} 6$ to $2{2}^{\ensuremath{\circ} } $ with mean of ${8}^{\ensuremath{\circ} } $ and amplitude of $1{4}^{\ensuremath{\circ} } $. The reduced frequency is varied in the range 0.314–1.0 and the Strouhal number range is 0.10–0.48. Results show that for constant effective angle of attack, the flow evolution is independent of Strouhal number, and as the reduced frequency is increased the leading-edge vortex (LEV) separates later in phase during the downstroke. The LEV trajectory, circulation and area are reported. It is shown that the effective angle of attack and reduced frequency determine the flow evolution, and the Strouhal number is the main parameter determining the aerodynamic force acting on the aerofoil. At low Strouhal numbers, the lift coefficient is proportional to the effective angle of attack, indicating the validity of the quasi-steady approximation. Large values of force coefficients (${\ensuremath{\sim} }6$) are measured at high Strouhal number. The measurement results are compared with linear potential flow theory and found to be in reasonable agreement. During the downstroke, when the LEV is present, better agreement is found when the wake effect is ignored for both the lift and drag coefficients.
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Abderrahmane, Belkallouche, Tahar Rezoug, and Laurent Dala. "Passive control of cavity acoustics via the use of surface waviness at subsonic flow." Aircraft Engineering and Aerospace Technology 91, no. 2 (February 4, 2019): 296–308. http://dx.doi.org/10.1108/aeat-01-2018-0061.
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PurposeAircraft noise is dominant for residents near airports when planes fly at low altitudes such as during departure and landing. Flaps, wings, landing gear contribute significantly to the total sound emission. This paper aims to present a passive flow control (in the sense that there is no power input) to reduce the noise radiation induced by the flow over the cavity of the landing gear during take-off and landing.Design/methodology/approachThe understanding of the noise source mechanism is normally caused by the unsteady interactions between the cavity surface and the turbulent flows as well as some studies that have shown tonal noise because of cavity resonances; this tonal noise is dependent on cavity geometry and incoming flow that lead us to use of a sinusoidal surface modification application upstream of a cavity as a passive acoustics control device in approach conditions.FindingsIt is demonstrated that the proposed surface waviness showed a potential reduction in cavity resonance and in the overall sound pressure level at the majority of the points investigated in the low Mach number. Furthermore, optimum sinusoidal amplitude and frequency were determined by the means of a two-dimensional computational fluid dynamics analysis for a cavity with a length to depth ratio of four.Research limitations/implicationsThe noise control by surface waviness has not implemented in real flight test yet, as all the tests are conducted in the credible numerical simulation.Practical implicationsThe application of passive control method on the cavity requires a global aerodynamic study of the air frame is a matter of ongoing debate between aerodynamicists and acousticians. The latter is aimed at the reduction of the noise, whereas the former fears a corruption of flow conditions. To balance aerodynamic performance and acoustics, the use of the surface waviness in cavity leading edge is the most optimal solution.Social implicationsThe proposed leading-edge modification it has important theoretical basis and reference value for engineering application it can meet the demands of engineering practice. Particularly, to contribute to the reduce the aircraft noise adopted by the “European Visions 2020”.Originality/valueThe investigate cavity noise with and without surface waviness generation and propagation by using a hybrid approach, the computation of flow based on the large-eddy simulation method, is decoupled from the computation of sound, which can be performed during a post-processing based on Curle’s acoustic analogy as implemented in OpenFOAM.
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Kumar, Vivek V., and Dilip A. Shah. "Application of Tubercles in Wind Turbine Blades: A Review." Applied Mechanics and Materials 867 (July 2017): 254–60. http://dx.doi.org/10.4028/www.scientific.net/amm.867.254.
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Due to the rapid depletion of conventional energy resources like fossil fuels and their harmful effects on the environment, there is an urgent need to seek alternative and sustainable energy sources. Wind energy is considered as one of the efficient source of energy which can be converted to useful form of energy like electrical energy. Though the field of wind engineering has developed in the recent era there is still scope for improvement in the effective utilization of energy. Energy efficiency in wind turbine is largely determined by the aerodynamics of the turbine blades and the characteristics of the turbulent fluid flow. The objective of this paper is to have a review on the improvement of Horizontal Axis Wind Turbine (HAWT) blade design by incorporating biomimetics into blades. Biomimetics is the field of science in which we adapt designs from nature to solve modern problems. The morphology of the wing-like flipper of the humpback whale (Megaptera novaeangliae) has potential for aerodynamic applications. Instead of straight leading edges like that of conventional hydrofoils, the humpback whale flipper has a number of sinusoidal rounded bumps, called tubercles arranged periodically along the leading edge. The presence of tubercles modifies the flow over the blade surface, creating vortices between the tubercles. These vortices interact with the flow over the tubercle and accelerate that flow, helping to maintain a partially attached boundary layer. This aerodynamic effect can delay stall to higher angles of attack, increase lift and reduce drag compared to the post-stall condition of conventional airfoils. The modified airfoil is characterized by a superior lift/drag ratio (L/D ratio) due to greater boundary layer attachment from vortices energizing the boundary layer.
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Hong, Liu, Huo Fupeng, and Chen Zuoyi. "Analysing and Optimizing the Aerodynamic Performance of Wind Turbine Blades Using Injected-Air Jets at Variable Frequency and Amplitude for Flow Control." Wind Engineering 29, no. 4 (June 2005): 331–39. http://dx.doi.org/10.1260/030952405774857860.
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Optimum aerodynamic performance of a wind turbine blade demands that the angle of attack of the relative wind on the blade remains at its optimum value. For turbines operating at constant speed, a change in wind speed causes the angle of attack to change immediately and the aerodynamic performance to decrease. Even with variable speed rotors, intrinsic time delays and inertia have similar effects. Improving the efficiency of wind turbines under variable operating conditions is one of the most important areas of research in wind power technology. This paper presents findings of an experimental study in which an oscillating air jet located at the leading edge of the suction surface of an aerofoil was used to improve the aerodynamic performance. The mean air-mass flowing through the jet during each sinusoidal period of oscillation equalled zero; i.e. the jet both blew and sucked. Experiments investigated the effects of the frequency, momentum and location of the jet stream, and the profile of the turbine blade. The study shows significant increase in the lift coefficient, especially in the stall region, under certain conditions. These findings may have important implications for wind turbine technology.
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Hardin, L. W., F. O. Carta, and J. M. Verdon. "Unsteady Aerodynamic Measurements on a Rotating Compressor Blade Row at Low Mach Number." Journal of Turbomachinery 109, no. 4 (October 1, 1987): 499–507. http://dx.doi.org/10.1115/1.3262139.
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An experiment was conducted on a heavily instrumented isolated model compressor rotor to study the unsteady aerodynamic response of the blade row to a controlled pitching oscillation of all blades in an undistorted flow, and to a circumferential inlet flow distortion with nonoscillating blades. To accomplish this, miniature pressure transducers were embedded in the blades and the unsteady pressure time histories were recorded. Both phases of the experiment were performed over a wide range of flow coefficient, from Cx/Um = 0.6 to 0.95 in 0.05 steps, and data were taken at each condition for sinusoidal disturbances characterized by one, two, and four per revolution waves. Steady-state data were acquired for flow coefficients from 0.55 to 0.99 in 0.05 steps. In this paper the steady and unsteady results of the portion of this experiment dealing with oscillating blades are compared with analytical predictions, and the steady results are compared with experimental data from previous work. Although the model blades were instrumented at five spanwise stations, only the midspan measurements will be presented herein. The measured pressures for nonoscillating blades were in good agreement with the steady potential flow predictions (and with previous steady experimental data) when the measured exit angle was imposed as the downstream boundary condition for the analysis. It was found that a quasi-steady approach yielded marginally acceptable agreement with the experimental results for the lowest frequency tested. For the higher reduced frequencies, the experimental data could not be modeled in this manner. In contrast, a comparison of the measurements with the Verdon–Caspar unsteady potential flow theory produced generally good agreement except near the leading edge at high mean incidence (i.e., at low flow coefficient). At high incidence the blades in this experiment had very high steady pressure gradients near the leading edge and it is suspected that this may be responsible for the lack of agreement. The agreement was somewhat better at the higher frequencies.
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Arefin, Md Arman, Avijit Mallik, and Md Asfaquzzaman. "Renewable energy–assisted hybrid three-wheeler: A numerical investigation." Advances in Mechanical Engineering 10, no. 12 (December 2018): 168781401881437. http://dx.doi.org/10.1177/1687814018814372.
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This research is to investigate the various parameters and conditions of implementing solar and wind energy in engine-driven three-wheelers. No reliable study of using both these energies in three-wheelers is found in the literature so far. In this article, a numerical study is conducted for using both these energies in a plugged-in hybrid three-wheeler. From the analysis, it is found that both the renewable energies will provide approximately 54% of total energy to run the vehicle altogether. A novel computational fluid dynamics simulation for a wind turbine is conducted to verify the theoretical results. Here the wind turbine blade has simple aerofoil look with sinusoidal leading edge and dimpled surface. The vehicle will not only reduce the pressure on fuel and national grid electricity; but, also will reduce the emission by a large amount. A custom drive cycle along with drive power demand is obtained using vehicle properties and city road conditions and comparing with Asian urban drive cycle. A detailed feasibility analysis of the vehicle is analyzed considering various costs, weight, and emissions, and a future researching infrastructure is also proposed. Finally, some recommendations are provided which can be considered for future research interests.
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Miklosovic, David S., Mark M. Murray, and Laurens E. Howle. "Experimental Evaluation of Sinusoidal Leading Edges." Journal of Aircraft 44, no. 4 (July 2007): 1404–8. http://dx.doi.org/10.2514/1.30303.
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Raad, P. E., and J. W. White. "Entrance and Stationary Roughness Effects on the Load Carrying Capacity of a Wide Wedge Gas Bearing." Journal of Tribology 111, no. 1 (January 1, 1989): 41–48. http://dx.doi.org/10.1115/1.3261877.
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The objective of this work is the determination of the effects of surface roughness amplitude and inlet conditions on ultra-thin, compressible, isothermal, infinitely-wide gas bearings. The method of study is numerical in nature and consists of a second-order accurate finite difference solution of the Reynolds equation of lubrication without molecular slip for a range of bearing numbers spanning six orders of magnitude. The motivation for this work comes from the magnetic disk drive industry where ever decreasing head flying heights are being sought to increase the recording density. Past studies by these authors of the infinitely-wide air bearing with stationary roughness have shown that, unlike in the case of a smooth bearing, the load peaks at a finite bearing number comparable to that at which current rigid disk drives operate. This suggests that it may be possible to arrange the roughness pattern in such a way as to cause the slider to separate from the hard disk more rapidly, minimizing wear to both surfaces. A wedge bearing with stationary sinusoidal roughness is studied for different roughness amplitudes and two phases. It is shown that for all configurations considered, the load exhibits a peak unlike in the case of the smooth bearing where the load monotonically reaches a peak at an infinite gas bearing number. Two rough sliders with flat tapers smoothly attached to their leading edge are also studied to answer questions regarding the role that the inlet condition plays in the resulting magnitude and behavior of the generated load. The leading taper allows the bearing to dynamically determine the entrained flow rate and maximum pressure as well as to self prescribe the inlet condition at the leading edge of the first roughness wave. The inlet conditions prescribed by the developing flow in the flat taper region still give rise to a peak in the load. The addition of the smooth taper, however, causes an overall decrease (increase) in the load when the roughness waves are entirely above (below) the plane of the taper compared to the results of the rough bearing with no taper. It is demonstrated that all the considered roughness patterns result in a peak load at a finite bearing number. Of special interest are two bearing configurations: one composed of a smooth taper followed by a transversely roughened slider and the other is a rough slider with a transverse roughness pattern whose slope at the inlet of the bearing is negative. Both are shown to achieve a maximum lifting force at low bearing numbers, providing rapid separation while alleviating the narrow rail manufacturing problem.
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DAREKAR, RUPAD M., and SPENCER J. SHERWIN. "Flow past a square-section cylinder with a wavy stagnation face." Journal of Fluid Mechanics 426 (January 10, 2001): 263–95. http://dx.doi.org/10.1017/s0022112000002299.
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Numerical investigations have been performed for the flow past square-section cylinders with a spanwise geometric deformation leading to a stagnation face with a sinusoidal waviness. The computations were performed using a spectral/hp element solver over a range of Reynolds numbers from 10 to 150.Starting from fully developed shedding past a straight cylinder at a Reynolds number of 100, a sufficiently high waviness is impulsively introduced resulting in the stabilization of the near wake to a time-independent state. It is shown that the spanwise waviness sets up a cross-flow within the growing boundary layer on the leading-edge surface thereby generating streamwise and vertical components of vorticity. These additional components of vorticity appear in regions close to the inflection points of the wavy stagnation face where the spanwise vorticity is weakened. This redistribution of vorticity leads to the breakdown of the unsteady and staggered Kármán vortex wake into a steady and symmetric near-wake structure. The steady nature of the near wake is associated with a reduction in total drag of about 16% at a Reynolds number of 100 compared with the straight, non-wavy cylinder.Further increases in the amplitude of the waviness lead to the emergence of hairpin vortices from the near-wake region. This wake topology has similarities to the wake of a sphere at low Reynolds numbers. The physical structure of the wake due to the variation of the amplitude of the waviness is identified with five distinct regimes. Furthermore, the introduction of a waviness at a wavelength close to the mode A wavelength and the primary wavelength of the straight square-section cylinder leads to the suppression of the Kármán street at a minimal waviness amplitude.
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Vineeth, V. K., and Devendra Kumar Patel. "Propulsion performance and wake transitions of a customized heaving airfoil." International Journal of Modern Physics C 32, no. 09 (April 28, 2021): 2150117. http://dx.doi.org/10.1142/s0129183121501175.
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The application of a flapping wing mechanism offers a vast range of development possibilities for unmanned aerial vehicles (UAVs) and autonomous underwater vehicles (AUVs). The influence of wake transitions on flapping wing mechanism’s capabilities is not fully understood particularly at low Reynolds numbers. The numerical investigation of a symmetric airfoil performing sinusoidal heaving oscillations is performed to explore the wake transitions. The influence of heaving parameters on wake transitions when exposed to a constant velocity flow is investigated. The existence of reverse von Karman vortex street, deflected wake and chaotic wake is observed. The wake deflection is found to switch its direction before transforming into a chaotic wake. The coherent structures and its evolution with the flow are presented using proper orthogonal decomposition (POD). The underlying structures and their interactions for different wake situations are identified. Correlations for the nondimensional maximum velocity in the wake in terms of frequency and amplitude is proposed. The wake dynamics is found to depend significantly on the leading edge vortices. The time-varying velocity fluctuations in the flow field are presented and discussed in detail. The velocity fluctuation contours are used to identify the regions of momentum transfer. The transient nature of the flow field is studied using the phase plot. A transition route from the periodic to chaotic regime though a quasi-periodic regime is established using time series analysis. The wake transitions are observed to be more sensitive towards heaving frequency than the heaving amplitude.
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Ghoreyshi, M., A. D. H. Kim, A. Jirasek, A. J. Lofthouse, and R. M. Cummings. "Validation of CFD simulations for X-31 wind-tunnel models." Aeronautical Journal 119, no. 1214 (April 2015): 479–500. http://dx.doi.org/10.1017/s0001924000010575.
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AbstractComputational Fluid Dynamics (CFD) has become an attractive method of choice in the design of many aerospace vehicles because of advances in numerical algorithms and convergence acceleration methods. However, the flow around an advanced fighter aircraft is complicated and usually unsteady due to the presence of vortex-dominated flows. The accuracy and predictability of conventional turbulence models for these applications may be questionable and therefore results obtained from these models must be validated and evaluated on the basis of experimental data from wind tunnels and/or flight tests. This work aims to validate CFD simulations of X-31 wind-tunnel models with and without a belly-mounted sting. The sting setup facilitates forced sinusoidal oscillations in one of three modes of: pitch, yaw, and roll. However, the results show that measured aerodynamic data are altered by the turbulent wake behind the sting, even at small angles of attack. The high angle-of-attack flow around the X-31 is also very complicated and unsteady due to canard and wing vortices. Therefore, validation of CFD models for predicting these complex flows can be a very challenging task. The X-31 wind-tunnel experiments were carried out in the German Dutch low-speed wind tunnel at Braunschweig and include aerodynamic force and moment measurement as well as span-wise pressure distributions at locations of 60% and 70% chord length. This data set is used to validate the Cobalt and Kestrel flow solvers and the results are similar and match quiet well with experiments for small to moderate angles of attack. The main discrepancies between CFD and measurements occur close to the wing tip, where leading-edge flaps are located.