Response of an Axisynmetric Separation Bubble to Sinusoidal Forcing. Effects of Forcing Frequency, Forcing Level and Reynolds Number.

昌幸, 清水; 勝, 木谷; 修, 望月; 康司, 井門

doi:10.1299/kikaib.59.721

Cited by 3 publications

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“…Figure 5 presents the evolution of L * R with respect to St A . Results are in good agreement with previous observations [17,19,22]. Indeed, L * R decreases strongly at low actuation frequencies, reaching a minimum L * R,min ≈ 1.5 (i.e.…”

Section: Flow Forcing and Its Effects On Scalingsupporting

confidence: 92%

“…Among these methods, those based on a periodic forcing have received particular attention, because of their ability to interact with the instabilities of the separated shear layer. A large corpus of experimental works [17][18][19][20][21] as well as numerical studies [4] has shown that periodic actuators can be more or less effective at modifying the shape of the separation, depending on how the frequency of the forcing compares to the characteristic frequency of the natural convective instability. With respect to this natural threshold, low actuation frequencies generally create a train of coherent counterotating vortices [22], which enhances the growth of the separated shear layer and reduces L R [23].…”

Section: Introductionmentioning

confidence: 99%

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Mass entrainment-based model for separating flows

et al. 2018

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Recent studies have shown that entrainment effectively describes the behaviour of natural and forced separating/reattaching flows developing behind bluff bodies, potentially paving the way to new, scalable separation control strategies. In this perspective, we propose a new interpretative framework for separated flows, based on mass entrainment. The cornerstone of the approach is an original model of the mean flow, representing it as a stationary vortex scaling with the mean recirculation length. We test our model on a set of mean separated topologies, obtained by forcing the flow over a descending ramp with a rack of synthetic jets. Our results show that both the circulation of the vortex and its characteristic size scale simply with the intensity of the backflow (i.e. the amount of mass going through the recirculation region). This suggests that the vortex model captures the essential functioning of mean mass entrainment, and that it could be used to model and/or predict the mean properties of separated flows. In addition, we use the vortex model to show that the backflow (i.e. an integral quantity) can be estimated from a single wall-pressure measurement (i.e. a pointwise quantity). Since the backflow also appears to be anticorrelated to the characteristic velocity of the synthetic jets, this finding suggests that industrially deployable, closed-loop control systems based on mass entrainment might be within reach.

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Section: Flow Forcing and Its Effects On Scalingsupporting

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Mass entrainment-based model for separating flows

et al. 2018

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The control of flow separation by periodic excitation

Greenblatt

Wygnanski

2000

Progress in Aerospace Sciences

897

429

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The structure and control of a turbulent reattaching flow

Sigurdson

1995

J. Fluid Mech.

144

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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 using 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. The former consists of an interaction of the shear layer vorticity with itself, the latter with its images that result because of the presence of a wall. In order to determine the optimum forcing frequency, a method of frequency scaling is proposed which correlates data for a variety of bubbles and supports an analogy with Kármán vortex shedding.

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Response of an Axisynmetric Separation Bubble to Sinusoidal Forcing. Effects of Forcing Frequency, Forcing Level and Reynolds Number.

Cited by 3 publications

References 0 publications

Mass entrainment-based model for separating flows

Mass entrainment-based model for separating flows

The control of flow separation by periodic excitation

The structure and control of a turbulent reattaching flow

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