Active forcing of an axisymmetric leading-edge turbulent separation bubble

Kiya, Masaru; Shimizu, Masaki; Mochizuki, Osamu; Ido, Yasushi; Tezuka, Hideaki

doi:10.2514/6.1993-3245

Cited by 11 publications

(13 citation statements)

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“…This implies that there is a lower limit on the Strouhal number, below which vortices will be formed at the end of (or beyond) the recirculation region and entrainment into the recirculation region will be minimal. This leads to the effect of forcing on the reattachment length diminishing for low Strouhal numbers as is observed in the literature [8,17].…”

Section: F Scaling Parameterssupporting

confidence: 56%

“…The decrease in strength leads to a decrease in the effect on the reattachment length as is visible for increasing Strouhal numbers in regime C in Figure 14 (in the direction of arrow c). Following the literature, it is assumed that the reattachment length stabilises at the unforced length (x R = x R,0 ) for high Strouhal numbers [17].…”

Section: G Reconsideration Of Forcing Regimesmentioning

confidence: 99%

“…where f is the forcing frequency, H is the step height and U ∞ is the free-stream velocity. The same scaling is used by Kiya et al [17], who investigated forcing of the leading-edge separation of a blunt axisymmetric body using synthetic jets. They found that (for a constant Strouhal number) an increase in free-stream Reynolds number or in forcing amplitude increases the effect of forcing.…”

Section: Introductionmentioning

confidence: 99%

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Entrainment effects in periodic forcing of the flow over a backward-facing step

2017

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The effect of the Strouhal number on periodic forcing of the flow over a backward-facing step (height, H) is investigated experimentally. Forcing is applied by a synthetic jet at the edge of the step at Strouhal numbers ranging from 0.21 < StH < 1.98 (StH = f H/U∞) at a Reynolds number of ReH = HU∞/ν = 41000. In the literature, the effect of Strouhal number on the reattachment length is often divided into low-and high frequency actuation, referring to different frequency modes in the unforced flow. In the present paper, variations with Strouhal number are explained based on entrainment rather than frequency modes. The reattachment length is shown to decrease linearly with entrainment. Entrainment is driven by vortices generated by the forcing and locally entrainment is shown to be qualitatively similar to circulation for all cases considered. Total circulation (and therewith entrainment and the effect on the reattachment length) is shown to decrease with Strouhal number whereas this is not predicted by models based on frequency modes. An empirical model for the (decay of) circulation is derived by tracking vortices in phase-locked data. This model is used to decipher relevant scaling parameters that explain the variations in circulation, entrainment and reattachment length. Three regimes of Strouhal number are identified. A low-Strouhal-number regime is observed for which vortices are formed at a late stage relative to the recirculation region, causing a decrease in effectiveness. For high Strouhal numbers vortices are being re-ingested into the actuator or are packed so close together that they cancel each other, both decreasing the effectiveness of forcing. In the intermediate regime a vortex train is formed of which the decay of circulation increases for increasing Strouhal number. The scaling of this decay fully explains the observed variation in reattachment length. The observations on entrainment made in this study are expected to also hold for periodic forcing of other bluff-body flows. * G.Bharath@southampton.ac.uk arXiv:1702.06895v1 [physics.flu-dyn]

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Section: F Scaling Parameterssupporting

confidence: 56%

Section: G Reconsideration Of Forcing Regimesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Entrainment effects in periodic forcing of the flow over a backward-facing step

2017

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“…Use of sound waves to influence the reattachment process was examined by several researchers (Bhattacharjee et al 1986;Roos and Kegelman 1986;Cooper et al 1986;Zaman et al 1987;Nishioka et al 1990). In particular, the introduction of a local forcing in the vicinity of the separation edge has been contemplated (Sigurdson and Roshko 1988;Kiya et al 1993). These experimental efforts utilized a smallamplitude localized jet flow very close to the square-cut leading-edge of the blunt body.…”

Section: Fig I Flow Configuration and Relevant Coordinates I Hmentioning

confidence: 99%

Vortex simulation of leading-edge separation bubble with local forcing

1996

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A numerical study is made of the ftow control of leading-edge separation bubbles in a two-dimensional semi-infinite blunt plate, which is aligned parallel to the main stream. A version of the discrete-vortex method is utilized. A timedependent point source of small magnitude is introduced near the separation edge. The effect of local source forcing on the evolution of a leading-edge separation bubble is scrutinized by varying the strength and the forcing frequency of the source. It is found that the reattachment length attains a single minimum at lower forcing levels, while it shows double minima at moderately high forcing levels. Based on the numerical results, the mechanism of decreasing reattachment length is pursued. These findings are qualitatively consistent with the existing experimental results for a blunt circular cylinder.

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“…The initial spectrum of perturbations is modified by periodic excitation of the separation region by external acoustic forcing [1,2] or locally generated oscillations [3,4]. The stability of the separated flow to small-amplitude oscillations can be changed, e.g., by flow suction or by cooling of the body surface [5][6][7].…”

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confidence: 99%

Application of flow suction for controlling the shedding of large-scale vortices at boundary-layer separation

Dovgal

Sorokin

2006

J Appl Mech Tech Phys

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A wind-tunnel study of the influence of flow suction on laminar boundary-layer separation behind a two-dimensional step on the surface is performed. Hot-wire measurements are carried out at low subsonic flow velocities. It is demonstrated that this method of flow control allows suppressing the formation of large-scale vortices determined by global stability properties of the separation region.Emergence of local regions of laminar-flow separation (so-called separation bubbles) is often accompanied by generation of large-scale vortices, which are quasi-periodically shed from the separation zone in the external flow direction. Dominating in the unsteady flow, the vortices determine its fluctuating and, to a large extent, mean (time-averaged) parameters, which can be optimized by affecting the formation and dynamics of coherent vortex structures.The vortex motion emerging owing to evolution of disturbances of the separated shear layer can be controlled by changing their initial spectrum and local characteristics of flow stability. The initial spectrum of perturbations is modified by periodic excitation of the separation region by external acoustic forcing [1,2] or locally generated oscillations [3,4]. The stability of the separated flow to small-amplitude oscillations can be changed, e.g., by flow suction or by cooling of the body surface [5][6][7].A possibility of controlling quasi-periodic vortex formation is considered in the present paper, the vortices being induced by instability of the separation region to long-wave oscillations ("shedding"-type instability in terminology of [8]) rather than amplification of convective disturbances of the separated layer. The coherent vortex motion determined by the global flow properties at the scale of the entire separation region was examined experimentally in [9, 10]. One method of controlling flow separation in such a regime of instability was considered in [11], where vortex formation was found to depend on weak harmonic excitation of the flow in the frequency range of amplifying small perturbations of the separated boundary layer.A possible method of affecting the shedding of periodic vortices used in the present work is flow suction, which is a well-known flow-control technique, in particular, for modification of the transition to turbulence initiated by flow instability behind the boundary-layer separation point [5].Experimental Technique. The experiments were performed in a T-324 low-turbulent subsonic wind tunnel located at the Institute of Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Sciences. The wind tunnel has a closed test section 1 × 1 × 4 m, and the free-stream turbulence is lower than 0.04%. The experimental model schematically shown in Fig. 1 was previously used to study the formation of coherent vortices in the flow behind a rectangular step on a flat-plate surface [9][10][11]. The model consisting of two plates was mounted in the centerplane of the wind-tunnel test section at a zero angle of attack, and the level of ...

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Active forcing of an axisymmetric leading-edge turbulent separation bubble

Cited by 11 publications

References 0 publications

Entrainment effects in periodic forcing of the flow over a backward-facing step

Entrainment effects in periodic forcing of the flow over a backward-facing step

Vortex simulation of leading-edge separation bubble with local forcing

Application of flow suction for controlling the shedding of large-scale vortices at boundary-layer separation

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