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Flow separation

About: Flow separation is a research topic. Over the lifetime, 16708 publications have been published within this topic receiving 386926 citations.


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Proceedings ArticleDOI
06 Jan 2003
TL;DR: In this paper, a spanwise-oriented phased-plasma-array actuator is used for boundary-layer separation in a closed-circuit wind tunnel with a shaped insert on the opposite wall.
Abstract: Active flow control of boundary-layer separation using glow-discharge plasma actuators is studied experimentally. Separation is induced on a flat plate installed in a closed-circuit wind tunnel by a shaped insert on the opposite wall. The flow conditions represent flow over the suction surface of a modern low-pressure-turbine airfoil. The Reynolds number, based on wetted plate length and nominal exit velocity, is varied from 50,000 to 300,000, covering cruise to takeoff conditions. Low (0.2 percent) and high (2.5 percent) free-stream turbulence intensities are set using passive grids. A spanwise-oriented phased-plasma-array actuator, fabricated on a printed circuit board, is surface-flush-mounted upstream of the separation point and can provide forcing in a wide frequency range. Static surface pressure measurements and hot-wire anemometry of the base and controlled flows are performed and indicate that the glow-discharge plasma actuator is an effective device for separation control.

145 citations

Journal ArticleDOI
TL;DR: In this paper, a 3D numerical model for predicting steady, in the mean, turbulent flows through lateral intakes with rough walls is developed, validated, and employed in a parametric study.
Abstract: A three-dimensional (3D) numerical model for predicting steady, in the mean, turbulent flows through lateral intakes with rough walls is developed, validated, and employed in a parametric study. The method solves the Reynolds-averaged Navier-Stokes equations closed with the isotropic k-ω turbulence model of Wilcox, which resolves the near-wall flow and accounts for roughness effects in a straightforward manner. Calculations are carried out for flows through rectangular closed-duct and open-channel T-junctions. Comparisons of the predicted mean velocity field with laboratory measurements indicate that the model captures most experimental trends with reasonable accuracy. For the parametric study, flows are predicted for a range of discharge ratios, aspect ratios, and main channel-bed-roughness distributions. The numerical solutions are examined to elucidate the complex 3D flow patterns of lateral-intake flows, including zones of flow division, separation and reversal, vortices, and singular points within th...

145 citations

Journal ArticleDOI
TL;DR: In this paper, a 3D Lagrangian Coherent Structures (LCS) was used to analyze the dynamics of cavitation-vortex interactions in the Delft twisted hydrofoil.

145 citations

Journal ArticleDOI
TL;DR: In this paper, it is shown that the skin-friction drag in a fully developed channel can be sustained below that corresponding to the laminar profile when the flow is subjected to surface blowing and suction in the form of an upstream travelling wave.
Abstract: It is shown, by direct numerical simulations, that the skin-friction drag in a fully developed channel can be sustained below that corresponding to the laminar profile when the flow is subjected to surface blowing and suction in the form of an upstream travelling wave. A key mechanism that induces the sub-laminar drag is the creation of positive (negative) Reynolds shear stress in the wall region, where normally negative (positive) Reynolds shear stress is expected given the mean shear. This mechanism is contained in the linearized Navier–Stokes equations, thus allowing linear analysis of the observed phenomena. When applied to a fully developed turbulent channel flow, skin-friction drag is also significantly reduced by an upstream travelling wave, demonstrating that the surface blowing and suction in the form of such a wave is also effective in fully developed turbulent flows. Consideration of the energy budget shows a possibility of net drag reduction in turbulent channel flows with the present open-loop control.

145 citations

Journal ArticleDOI
TL;DR: In this paper, an experimental and theoretical study of a laminar separation bubble and the associated linear stability mechanisms was performed over a flat plate kept in a wind tunnel, with an imposed pressure gradient typical of an aerofoil.
Abstract: This is an experimental and theoretical Study of a laminar separation bubble and the associated linear stability mechanisms. Experiments were performed over a flat plate kept in a wind tunnel, with an imposed pressure gradient typical of an aerofoil that would involve a laminar separation bubble. The separation bubble was characterized by measurement of surface-pressure distribution and streamwise velocity using hot-wire anemometry. Single component hot-wire anemometry was also used for a detailed study of the transition dynamics. It was foundthat the so-called dead-air region in the front portion of the bubble corresponded to a region of small disturbance amplitudes, with the amplitude reaching a maximum value close to the reattachment point. An exponential growth rate of the disturbance was seen in the region upstream of the mean maximum height of the bubble, and this was indicative of a linear instability mechanism at work. An infinitesimal disturbance was impulsively introduced into the boundary layer upstream of separation location, and the wave packet was tracked (in an ensemble-averaged sense) while it was getting advected downstream. The disturbance was found to be convective in nature. Linear stability analyses (both the Orr-Sommerfeld and Rayleigh calculations) were performed for mean velocity profiles, starting from an attached adverse-pressure-gradient boundary layer all the way up to the front portion of the separation-bubble region (i.e. up to the end of the dead-air region in which linear evolution of the disturbance could be expected). The conclusion from the present work is that the primary instability mechanism in a separation bubble is inflectional in nature, and its origin can be traced back to upstream of the separation location. In other words, the inviscid inflectional instability of the separated shear layer should be logically seen as an extension of the instability of the upstream attached adverse-pressure-gradient boundary layer. This modifies the traditional view that pegs the origin of the instability in a separation bubble to the detached shear layer Outside the bubble, with its associated Kelvin-Helmholtz mechanism. We contendthat only when the separated shear layer has moved considerably away from the wall (and this happens near the maximum-height location of the mean bubble), a description by the Kelvin-Helmholtz instability paradigm, with its associated scaling principles, Could become relevant. We also propose a new scaling for the most amplified frequency for a wall-bounded shear layer in terms of the inflection-point height and the vorticity thickness and show it to be universal.

145 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023177
2022333
2021361
2020394
2019403
2018371