The Experts below are selected from a list of 273 Experts worldwide ranked by ideXlab platform
Alexander Smits - One of the best experts on this subject based on the ideXlab platform.
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a supersonic turbulent boundary layer in an adverse Pressure Gradient
Journal of Fluid Mechanics, 1990Co-Authors: Emerick M Fernando, Alexander SmitsAbstract:This investigation describes the effects of an adverse Pressure Gradient on a flat plate supersonic turbulent boundary layer ( M f ≈ 2.9, β x ≈ 5.8, Re θ, ref ≈ 75600). Single normal hot wires and crossed wires were used to study the Reynolds stress behaviour, and the features of the large-scale structures in the boundary layer were investigated by measuring space–time correlations in the normal and spanwise directions. Both the mean flow and the turbulence were strongly affected by the Pressure Gradient. However, the turbulent stress ratios showed much less variation than the stresses, and the essential nature of the large-scale structures was unaffected by the Pressure Gradient. The wall Pressure distribution in the current experiment was designed to match the Pressure distribution on a previously studied curved-wall model where streamline curvature acted in combination with bulk compression. The addition of streamline curvature affects the turbulence strongly, although its influence on the mean velocity field is less pronounced and the modifications to the skin-friction distribution seem to follow the empirical correlations developed by Bradshaw (1974) reasonably well.
Ivan Marusic - One of the best experts on this subject based on the ideXlab platform.
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les of the adverse Pressure Gradient turbulent boundary layer
International Journal of Heat and Fluid Flow, 2013Co-Authors: M Inoue, Dale Pullin, Zambri Harun, Ivan MarusicAbstract:We describe large-eddy simulations (LES) of the flat-plate turbulent boundary layer in the presence of an adverse Pressure Gradient. The stretched-vortex subgrid-scale model is used in the domain of the flow coupled to a wall model that explicitly accounts for the presence of a finite Pressure Gradient. The LES are designed to match recent experiments conducted at the University of Melbourne wind tunnel where a plate section with zero Pressure Gradient is followed by section with constant adverse Pressure Gradient. First, LES are described at Reynolds numbers based on the local free-stream velocity and the local momentum thickness in the range 6560–13,900 chosen to match the experimental conditions. This is followed by a discussion of further LES at Reynolds numbers at approximately 10 times and 100 times these values, which are well out of range of present day direct numerical simulation and wall-resolved LES. For the lower Reynolds number runs, mean velocity profiles, one-point turbulent statistics of the velocity fluctuations, skin friction and the Clauser and acceleration parameters along the streamwise, adverse Pressure-Gradient domain are compared to the experimental measurements. For the full range of LES, the relationship of the skin-friction coefficient, in the form of the ratio of the local free-stream velocity to the local friction velocity, to both Reynolds number and the Clauser parameter is explored. At large Reynolds numbers, a region of collapse is found that is well described by a simple log-like empirical relationship over two orders of magnitude. This is expected to be useful for constant adverse-Pressure Gradient flows. It is concluded that the present adverse Pressure Gradient boundary layers are far from an equilibrium state.
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Pressure Gradient effects on the large scale structure of turbulent boundary layers
Journal of Fluid Mechanics, 2013Co-Authors: Zambri Harun, Jason P. Monty, Romain Mathis, Ivan MarusicAbstract:Research into high-Reynolds-number turbulent boundary layers in recent years has brought about a renewed interest in the larger-scale structures. It is now known that these structures emerge more prominently in the outer region not only due to increased Reynolds number (Metzger & Klewicki, Phys. Fluids , vol. 13(3), 2001, pp. 692–701; Hutchins & Marusic, J. Fluid Mech. , vol. 579, 2007, pp. 1–28), but also when a boundary layer is exposed to an adverse Pressure Gradient (Bradshaw, J. Fluid Mech. , vol. 29, 1967, pp. 625–645; Lee & Sung, J. Fluid Mech. , vol. 639, 2009, pp. 101–131). The latter case has not received as much attention in the literature. As such, this work investigates the modification of the large-scale features of boundary layers subjected to zero, adverse and favourable Pressure Gradients. It is first shown that the mean velocities, turbulence intensities and turbulence production are significantly different in the outer region across the three cases. Spectral and scale decomposition analyses confirm that the large scales are more energized throughout the entire adverse Pressure Gradient boundary layer, especially in the outer region. Although more energetic, there is a similar spectral distribution of energy in the wake region, implying the geometrical structure of the outer layer remains universal in all cases. Comparisons are also made of the amplitude modulation of small scales by the large-scale motions for the three Pressure Gradient cases. The wall-normal location of the zero-crossing of small-scale amplitude modulation is found to increase with increasing Pressure Gradient, yet this location continues to coincide with the large-scale energetic peak wall-normal location (as has been observed in zero Pressure Gradient boundary layers). The amplitude modulation effect is found to increase as Pressure Gradient is increased from favourable to adverse.
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a parametric study of adverse Pressure Gradient turbulent boundary layers
International Journal of Heat and Fluid Flow, 2011Co-Authors: Jason P. Monty, Zambri Harun, Ivan MarusicAbstract:There are many open questions regarding the behaviour of turbulent boundary layers subjected to Pressure Gradients and this is confounded by the large parameter space that may affect these flows. While there have been many valuable investigations conducted within this parameter space, there are still insufficient data to attempt to reduce this parameter space. Here, we consider a parametric study of adverse Pressure Gradient turbulent boundary layers where we restrict our attention to the Pressure Gradient parameter, β, the Reynolds number and the acceleration parameter, K. The statistics analyzed are limited to the streamwise fluctuating velocity. The data show that the mean velocity profile in strong Pressure Gradient boundary layers does not conform to the classical logarithmic law. Moreover, there appears to be no measurable logarithmic region in these cases. It is also found that the large-scale motions scaling with outer variables are energised by the Pressure Gradient. These increasingly strong large-scale motions are found to be the dominant contributor to the increase in turbulence intensity (scaled with friction velocity) with increasing Pressure Gradient across the boundary layer.
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constant adverse Pressure Gradient turbulent boundary layers
Proceedings of the 17th Australasian Fluid Mechanics Conference 2010, 2010Co-Authors: Zambri Harun, Jason P. Monty, Ivan MarusicAbstract:Significant progress has been made towards understanding the large scale features of wall-bounded shear flow in zero Pressure Gradient (ZPG) turbulent boundary layers (TBL). Here we consider their effects in adverse Pressure Gradient (APG) flows where the Pressure Gradient parameter is held constant and Reynolds number is varied. This is done by documenting the changes in the mean velocity, streamwise turbulence intensities and their associated spectral densities. Increased large-scale activity near the wall is seen with increasing Reynolds number and for this Pressure Gradient, the mean flow deviates from the classically regarded log-law. Introduction The case of the adverse Pressure Gradient boundary layer is of great importance since this must be the condition of a boundary layer prior to separation. As such, many boundary layer control strategies will be designed for implementation in APG conditions. While there are some features of APG boundary layers that are well-known, such as the stronger wake of the mean velocity profile and increased broadband turbulence intensity, u2/U2 τ in the logarithmic and wake region, there remain important features to be investigated. The large-scale structure of the flow is a case in point. Compared with the ZPG case, there is far less known about the large-scale features in APG boundary layers. This may be due, in part, to the greater number of variables pertinent to the APG case. In order to reduce the parameter space, the present investigation presents data with varying Reynolds numbers, Reτ = δUτ/ν (where δ is the boundary layer thickness, Uτ is the friction velocity and ν is the kinematic viscosity) and fixed Pressure Gradient parameter
Yuliang Liu - One of the best experts on this subject based on the ideXlab platform.
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Investigation on a Pressure-Gradient fiber laser hydrophone
Measurement Science and Technology, 2010Co-Authors: Wentao Zhang, Fang Li, Faxiang Zhang, Yuliang LiuAbstract:In this paper, a Pressure-Gradient fiber laser hydrophone is demonstrated. Two brass diaphragms are installed at the end of a metal cylinder as sensing elements. A distributed feedback fiber laser, fixed at the center of the two diaphragms, is elongated or shortened due to the acoustic wave. There are two orifices at the middle of the cylinder. So this structure can work as a Pressure-Gradient microphone in the acoustic field. Furthermore, the hydrostatic Pressure is self-compensated and an ultra-thin dimension is achieved. Theoretical analysis is given based on the electro-acoustic theory. Field trials are carried out to test the performance of the hydrophone. A sensitivity of 100 nm MPa −1 has been achieved. Due to the small dimensions, no directivity is found in the test.
Yuxin Zhao - One of the best experts on this subject based on the ideXlab platform.
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on the impact of adverse Pressure Gradient on the supersonic turbulent boundary layer
Physics of Fluids, 2016Co-Authors: Qiancheng Wang, Zhenguo Wang, Yuxin ZhaoAbstract:By employing the particle image velocimetry, the mean and turbulent characteristics of a Mach 2.95 turbulent boundary layer are experimentally investigated without the impact of curvature. The physical mechanism with which the streamwise adverse Pressure Gradient affects the supersonic boundary layer is revealed. The data are compared to that of the concave boundary layer with similar streamwise distributions of wall static Pressure to clarify the separate impacts of the adverse Pressure Gradient and the concave curvature. The logarithmic law is observed to be well preserved for both of the cases. The dip below the logarithmic law is not observed in present investigation. Theoretical analysis indicates that it could be the result of compromise between the opposite impacts of the compression wave and the increased turbulent intensity. Compared to the zero Pressure Gradient boundary layer, the principal strain rate and the turbulent intensities are increased by the adverse Pressure Gradient. The shear layer formed due the hairpin packets could be sharpened by the compression wave, which leads to higher principal strain rate and the associated turbulent level. Due to the additional impact of the centrifugal instability brought by the concave wall, even higher turbulent intensities than that of the adverse Pressure Gradient case are introduced. The existence of velocity modes within the zero Pressure Gradient boundary layer suggests that the large scale motions are statistically well organized. The generation of new velocity modes due to the adverse Pressure Gradient indicates that the turbulent structure is changed by the adverse Pressure Gradient, through which more turbulence production that cannot be effectively predicted by the Reynolds-stress transport equations could be brought.
Luciano Castillo - One of the best experts on this subject based on the ideXlab platform.
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The rough favourable Pressure Gradient turbulent boundary layer
Journal of Fluid Mechanics, 2009Co-Authors: Brian Brzek, T. Gunnar Johansson, Luciano CastilloAbstract:Laser Doppler anemometry measurements of the mean velocity and Reynolds stresses are carried Out for a rough-surface favourable Pressure Gradient turbulent boundary layer. The experimental data is compared with smooth favourable Pressure Gradient and rough zero-Pressure Gradient data. The velocity and Reynolds stress profiles are normalized using various scalings Such as the friction velocity and free stream velocity. In the velocity profiles, the effects of roughness are removed when using the friction velocity. The effects of Pressure Gradient are not absorbed. When using the free stream velocity, the scaling is more effective absorbing the Pressure Gradient effects. However, the effects of roughness are almost removed, while the effects of Pressure Gradient are still observed on the outer flow, when the mean deficit velocity profiles are normalized by the U(infinity)delta(*)/delta scaling. Furthermore, when scaled with U(infinity)(2), the component of the Reynolds stress augments due to the rough Surface despite the imposed favourable Pressure Gradient; when using the friction velocity scaling it u(*)(2), it is dampened. It becomes 'flatter' in the inner region mainly due to the rough Surface, which destroys the coherent structures of the flow and promotes isotropy. Similarly, the Pressure Gradient imposed on the flow decreases the magnitude of the Reynolds stress profiles especially on the and - components for the u(*)(2) or U(infinity)(2) scaling. These effects are reflected in the boundary layer parameter delta(*)/delta, which increase due to roughness, but decrease due to the favourable Pressure Gradient. Additionally, the Pressure parameter Lambda found not to be in equilibrium, describes the development of the turbulent boundary layer, with no influence of the roughness linked to this parameter. These measurements are the first with an extensive number of downstream locations (11). This makes it possible to compute the required x-dependence for the production term and the wall shear stress from the full integrated boundary layer equation. The finding indicates that the skin friction coefficient depends oil the favourable Pressure Gradient condition and surface roughness.
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similarity analysis for turbulent boundary layer with Pressure Gradient outer flow
AIAA Journal, 2001Co-Authors: Luciano Castillo, William K GeorgeAbstract:The equilibrium-type similarity analysis of George and Castillo for the outer part of zero Pressure Gradient boundary layers has been extended to include boundary layers with Pressure Gradient. The constancy of a single new Pressure Gradient parameter is all that is necessary to characterize these new equilibrium turbulent boundary layers. Three major results are obtained: First, most Pressure Gradient boundary experiments appear to be equilibrium flows (by the new definition), and nonequilibrium flows appear to be the exception. Second, there appear to be only three values of the Pressure Gradient parameter: one for adverse Pressure Gradients, one for favorable Pressure Gradients, and one for zero Pressure Gradients. Third, correspondingly, there appear to be only three normalized velocity deficit profiles, exactly as suggested by the theory
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zero Pressure Gradient turbulent boundary layer
Applied Mechanics Reviews, 1997Co-Authors: William K George, Luciano CastilloAbstract:Of the many aspects of the long-studied field of turbulence, the zero-Pressure-Gradient boundary layer is probably the most investigated, and perhaps also the most reviewed. Turbulence is a fluid-dynamical phenomenon for which the dynamical equations are generally believed to be the Navier-Stokes equations, at least for a single-phase, Newtonian fluid. Despite this fact, these governing equations have been used in only the most cursory manner in the development of theories for the boundary layer, or in the validation of experimental data-bases. This article uses the Reynolds-averaged Navier-Stokes equations as the primary tool for evaluating theories and experiments for the zero-Pressure-Gradient turbulent boundary layer. Both classical and new theoretical ideas are reviewed, and most are found wanting. The experimental data as well is shown to have been contaminated by too much effort to confirm the classical theory and too little regard for the governing equations. Theoretical concepts and experiments are identified, however, which are consistent-both with each other and with the governing equations. This article has 77 references.
