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Freestream

About: Freestream is a research topic. Over the lifetime, 3428 publications have been published within this topic receiving 56147 citations.


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TL;DR: In this article, the effects of Reynolds number (Re), velocity ratio (V R ) and Strouhal number (St) on the behavior of synthetic jets were studied. But the results were limited to a laminar boundary layer with a range of actuator operating conditions and freestream velocities.
Abstract: Dye flow visualisation of circular synthetic jets was carried out in laminar boundary layers developing over a flat plate at a range of actuator operating conditions and freestream velocities of 0.05 and 0.1ms -1 . The purpose of this work was to study the interaction of synthetic jets with the boundary layer and the nature of vortical structures produced as a result of this interaction. The effects of Reynolds number (Re), velocity ratio (V R ) and Strouhal number (St) on the behaviour of synthetic jets were studied. At low Re and V R , the vortical structures produced by synthetic jets appear as highly stretched hairpin vortices attached to the wall. At intermediate Re and V R , these structures roll up into vortex rings which experience a considerable amount of tilting and stretching as they enter the boundary layer. These vortex rings will eventually propagate outside the boundary layer hence the influence of the synthetic jets on the near wall flow will be confined in the near field of the jet exit. At high Re and V R , the vortex rings appear to experience a certain amount of tilting but no obvious stretching. They penetrate the edge of the boundary layer quickly, producing very limited impact on the near wall flow. Hence it is believed that the hairpin vortices produced at low Re and V R are likely to be the desirable structures for effective flow separation control. In this paper, a vortex model was also described to explain the mechanism of vortex tilting.

46 citations

Journal ArticleDOI
TL;DR: In this article, the mass velocity of the freestream molecules with respect to the local coordinates is measured by unit vectors in the x, y, and z directions, respectively.
Abstract: 6= freestream temperature =|5| = the x, y, and z components of the mass velocity of the freestream molecules with respect to the local coordinates, respectively = Cartesian stationary frame of reference = local Cartesian coordinates, fixed on the moving differential area = unit vectors in the x, y, and z directions, respectively = azimuth, measured from negative X axis = mn^, freestream density = momentum accommodation coefficients, Eq. (12) and Eq. (13) — tangential component of momentum flux, Eqs. (6b) and (6c), or shearing stress acting on the differential area, Eqs. (14b) and (14c) = latitude, measured from positive Y axis SubscriptsK = components in the X, Y, and Z directions, respectively i = quantity impinging to the sphere r = quantity reflected from the sphere w = value at sphere surface x, y, z = components in the x, y, and z directions, respectively.

46 citations

Journal ArticleDOI
TL;DR: In this article, the authors compared the Reynolds-averaged Navier-Stokes and detached-eddy simulation models to obtain a detailed comparison of the secondary structures of the diamond-injector flowfield.
Abstract: Sonic transverse gaseous injection into a Mach 5.0 freestream flow was numerically simulated using two-equation and detached-eddy turbulence models. Circular- and diamond-shaped injectors were investigated in this study. The numerical simulations were compared with available experimental results and it was determined that both the Reynolds-averaged Navier-Stokes and detached-eddy simulation models captured the secondary flow structure. A detailed comparison of the secondary structures was performed for both injectors. Two new vortex structures of practical importance were observed in the diamond-injector flowfield. First, a leading-edge mixing mechanism was identified. Second, a trapped lateral counter-rotating vortex pair was produced. These new structures were observed in both Reynolds-averaged Navier-Stokes and detached-eddy simulation simulations. The detached-eddy simulations indicated that the large-scale structures observed in the plume/wake region of the flowfield were more organized in the diamond-injector test case. To better understand the secondary flow advection mechanism, the magnitudes of the terms in the compressible vorticity transport equation were compared. The diamond-injector structured-grid Reynolds-averaged Navier-Stokes solution was used as a baseline for this study. The inviscid compressibility, vortex-stretching, and baroclinic-torque terms were dominant. Downstream of the barrel-shock region, the baroclinic term was found to diminish when compared with the other inviscid terms. Planar-averaged results for the transport quantities confirmed this behavior. Vortex stretching was found to persist the longest.

46 citations

Journal ArticleDOI
TL;DR: In this article, the effects of large scale high freestream turbulence intensity and exit Reynolds number on the surface heat transfer distribution of a turbine vane in a 2D linear cascade at realistic engine Mach numbers were investigated.
Abstract: This paper experimentally and numerically investigates the effects of large scale high freestream turbulence intensity and exit Reynolds number on the surface heat transfer distribution of a turbine vane in a 2D linear cascade at realistic engine Mach numbers. A passive turbulence grid was used to generate a freestream turbulence level of 16% and integral length scale normalized by the vane pitch of 0.23 at the cascade inlet. The base line turbulence level and integral length scale normalized by the vane pitch at the cascade inlet were measured to be 2% and 0.05, respectively. Surface heat transfer measurements were made at the midspan of the vane using thin film gauges. Experiments were performed at exit Mach numbers of 0.55, 0.75, and 1.01, which represent flow conditions below, near, and above nominal conditions. The exit Mach numbers tested correspond to exit Reynolds numbers of 910 5 , 1.0510 6 , and 1.510 6 based on a vane chord. The experimental results showed that the large scale high freestream turbulence augmented the heat transfer on both the pressure and suction sides of the vane as compared to the low freestream turbulence case and promoted a slightly earlier boundary layer transition on the suction surface for exit Mach 0.55 and 0.75. At nominal conditions, exit Mach 0.75, average heat transfer augmentations of 52% and 25% were observed on the pressure and suction sides of the vane, respectively. An increased Reynolds number was found to induce an earlier boundary layer transition on the vane suction surface and to increase heat transfer levels on the suction and pressure surfaces. On the suction side, the boundary layer transition length was also found to be affected by increase changes in Reynolds number. The experimental results also compared well with analytical correlations and computational fluid dynamics predictions. DOI: 10.1115/1.2952381

46 citations

Journal ArticleDOI
TL;DR: In this article, the effects of freestream turbulence intensity on the mean topology and transition characteristics of laminar separation bubbles forming over the suction side of a NACA 0018 airfoil are investigated.
Abstract: The effects of freestream turbulence intensity on the mean topology and transition characteristics of laminar separation bubbles forming over the suction side of a NACA 0018 airfoil are investigate...

46 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023195
2022350
2021108
2020113
201986
2018118