Journal ArticleDOI
Control of flow separation by acoustic excitation
M. Nishioka,M. Asai,S. Yoshida +2 more
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In this article, the response to the incident sound and the resulting flow instability were studied experimentally and therically on the basis of the linear stability theory, for a flat-plate airfoil, at a chord Reynolds number R c =4 x 10 4.Abstract:
To control the leading-edge flow separation on an airfoil by means of acoustic excitation, the response to the incident sound and the resulting flow instability are studied experimentally and therically on the basis of the linear stability theory, for a flat-plate airfoil, at a chord Reynolds number R c =4 x 10 4 .read more
Citations
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Journal ArticleDOI
The control of flow separation by periodic excitation
TL;DR: In this article, a review of the control of flow separation from solid surfaces by periodic excitation is presented, with an emphasis on experimentation relating to hydrodynamic excitation, although acoustic methods as well as traditional boundary layer control, such as steady blowing and suction are discussed in order to provide an appropriate historical context for recent developments.
Journal ArticleDOI
Laminar boundary layer separation: Instability and associated phenomena
TL;DR: In this paper, a review of studies of laminar separated flows is presented, where the authors show that a mean flow pattern in a separated flow as well as its unsteady properties depend primarily on the instability and other "transitional" phenomena associated with a laminars separation.
Journal ArticleDOI
Control of turbulent separated flow over a backward-facing step by local forcing
K.B. Chun,Hyung Jin Sung +1 more
TL;DR: In this paper, the effect of local forcing on the flow structure was scrutinized by altering the forcing amplitude (0 ⩽ A� 0.07) and forcing frequency (0⩽ St====== Hαγγαγαβαγβαβγα βαγ βαββ ββββα ββααβ β ββ βγ ββγβ βα βγββγ βγγ β βγα αββδ ββΔ βγΔβα α
Journal ArticleDOI
Transverse combustion instabilities: Acoustic, fluid mechanic, and flame processes
TL;DR: In this paper, the authors present a review of transverse acoustic wave motions in air-breathing systems and discuss issues associated with simulating or scaling instabilities, either in subscale experimental geometries or by attempting to understand instability physics using identical axial oscillations of the same frequency as the transverse mode of interest.
Journal ArticleDOI
The Structure and Control of a Turbulent Reattaching Flow
Abstract: An experimental study was made of the effect of a periodic velocity perturbation on the separation bubble downstream of the sharp-edged blunt face of a circular cylinder aligned coaxially with the free stream. Velocity fluctuations were produced with an acoustic driver located within the cylinder and a small circumferential gap located immediately downstream of the fixed separation line to allow communication with the external flow. The flow could be considerably modified when forced at frequencies lower than the initial Kelvin-Helmholtz frequencies of the free shear layer, and with associated vortex wavelengths comparable to the bubble height. Reattachment length, bubble height, pressure at separation, and average pressure on the face were all reduced. The effects on the large-scale structures were studied on flow photographs obtained by the smoke-wire technique. The forcing increased the entrainment near the leading edge. It was concluded that the final vortex of the shear layer before reattachment is an important element of the flow structure. There are two different instabilities involved, the Kelvin-Helmholtz instability of the free shear layer and the “shedding” type instability of the entire bubble. A method of frequency scaling is proposed which correlates data for a variety of bubbles and supports an analogy with Karman vortex shedding.
References
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Journal ArticleDOI
Scattering of acoustic waves into Tollmien-Schlichting waves by small streamwise variations in surface geometry
TL;DR: By using the triple-deck scaling of Stewartson (1969) and Messiter (1970) it was shown that small but relatively sudden surface geometry variations that produce only very weak static pressure variations can nevertheless produce strong coupling between an externally imposed acoustic disturbance and a spatially growing Tollmien-Schlichting wave as discussed by the authors.
Journal ArticleDOI
The Structure and Control of a Turbulent Reattaching Flow
Abstract: An experimental study was made of the effect of a periodic velocity perturbation on the separation bubble downstream of the sharp-edged blunt face of a circular cylinder aligned coaxially with the free stream. Velocity fluctuations were produced with an acoustic driver located within the cylinder and a small circumferential gap located immediately downstream of the fixed separation line to allow communication with the external flow. The flow could be considerably modified when forced at frequencies lower than the initial Kelvin-Helmholtz frequencies of the free shear layer, and with associated vortex wavelengths comparable to the bubble height. Reattachment length, bubble height, pressure at separation, and average pressure on the face were all reduced. The effects on the large-scale structures were studied on flow photographs obtained by the smoke-wire technique. The forcing increased the entrainment near the leading edge. It was concluded that the final vortex of the shear layer before reattachment is an important element of the flow structure. There are two different instabilities involved, the Kelvin-Helmholtz instability of the free shear layer and the “shedding” type instability of the entire bubble. A method of frequency scaling is proposed which correlates data for a variety of bubbles and supports an analogy with Karman vortex shedding.
Journal ArticleDOI
Effect of acoustic excitation on the flow over a low-Re airfoil
TL;DR: In this paper, wind-tunnel measurements of lift, drag, and wake velocity spectra were carried out under (tonal) acoustic excitation for a smooth airfoil in the chord-Reynolds-number Re(c) range of 40,000-140,000.
Journal ArticleDOI
Boundary-layer receptivity to unsteady pressure gradients: experiments and overview
M. Nishioka,M. V. Morkovin +1 more
TL;DR: In this paper, it is proposed that a likely effective receptivity mechanism rests on the fact that under realistic conditions A varies with distance x along any body of finite thickness, A(x), and introduces thereby additional characteristic lengths which can match λTS.