Optimum suppression of fluid forces by controlling the separated shear layer. Control of shear layers on two sides of a square prism.

This paper deals with the suppression of the fluid forces by controlling a shear layer on one side separated from a square prism. The control of the separated shear layer was established by setting up a small circular cylinder (the control cylinder) in it on one side. Experimental data were collected to examine the effects on the fluid forces and vortex shedding frequency due to variation of the position and diameter of the control cylinder. The results show that (i) the maximum reduction of the time-mean drag and fluctuating lift and drag occurred when the control cylinder was located near what would ordinarily be considered the outer boundary of the shear layer; (ii) the control of the separated shear layer by means of a small cylinder appeared to be effective in suppressing the fluctuating lift and drag rather than the time-mean drag; (iii) in the case of the control cylinder of 6 mm in diameter, the time-mean drag was reduced to about 30 percent, and the fluctuating lift and drag were reduced to approximately 95 and 75 percent, respectively; (iv) the fluid forces and the frequency of vortex shedding of the square prism were mainly dependent on the characteristics of a very thin region near the outer boundary of the shear layer.

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Scaling of square-prism shear layers

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.443 ◽

2018 ◽

Vol 849 ◽

pp. 1096-1119 ◽

Cited By ~ 8

Author(s):

D. C. Lander ◽

D. M. Moore ◽

C. W. Letchford ◽

M. Amitay

Keyword(s):

Boundary Layer ◽

Shear Layer ◽

Power Law ◽

Frequency Ratio ◽

Shear Layers ◽

Nonlinear Dependence ◽

Front Face ◽

Length Scale ◽

Square Prism ◽

Ratio Scaling

Scaling characteristics, essential to the mechanisms of transition in square-prism shear layers, were explored experimentally. In particular, the evolution of the dominant instability modes as a function of Reynolds number were reported in the range $1.5\times 10^{4}\lesssim Re_{D}\lesssim 7.5\times 10^{4}$. It was found that the ratio between the shear layer frequency and the shedding frequency obeys a power-law scaling relation. Adherence to the power-law relationship, which was derived from hot-wire measurements, has been supported by two additional and independent scaling considerations, namely, by particle image velocimetry measurements to observe the evolution of length and velocity scales in the shear layer during transition, and by comparison to direct numerical simulations to illuminate the properties of the front-face boundary layer. The nonlinear dependence of the shear layer instability frequency is sustained by the influence of $Re_{D}$ on the thickness of the laminar front-face boundary layer. In corroboration with the original scaling argument for the circular cylinder, the length scale of the shear layer was the only source of nonlinearity in the frequency ratio scaling, within the range of Reynolds numbers reported. The frequency ratio scaling may therefore be understood by the influence of $Re_{D}$ on the appropriate length scale of the shear layer. This length scale was observed to be the momentum thickness evaluated at a transition point, defined where the Kelvin–Helmholtz instability saturates.

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Coherent structures and their frequency signature in the separated shear layer on the top of a square cylinder

Proceedings of the International Symposium on Turbulence, Heat and Mass Transfer ◽

10.1615/ichmt.2006.turbulheatmasstransf.920 ◽

2006 ◽

Author(s):

C. Brun ◽

S. Aubrun

Keyword(s):

Shear Layer ◽

Coherent Structures ◽

Square Cylinder ◽

Separated Shear Layer

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Countercurrent shear layer control of flame stabilization

10.2514/6.1997-1808 ◽

1997 ◽

Author(s):

E. Koc-Alkislar ◽

L. Lourenco ◽

A. Krothapalli ◽

P. Strykowski ◽

E. Koc-Alkislar ◽

...

Keyword(s):

Shear Layer ◽

Flame Stabilization ◽

Countercurrent Shear ◽

Layer Control

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Vibration reduction of a sphere through shear-layer control

Journal of Fluids and Structures ◽

10.1016/j.jfluidstructs.2021.103325 ◽

2021 ◽

Vol 105 ◽

pp. 103325

Author(s):

Thomas McQueen ◽

Jisheng Zhao ◽

John Sheridan ◽

Mark C. Thompson

Keyword(s):

Shear Layer ◽

Vibration Reduction ◽

Layer Control

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Exploitation of Subharmonics for Separated Shear Layer Control on a High-Lift Low-Pressure Turbine Using Acoustic Forcing

Journal of Turbomachinery ◽

10.1115/1.4025586 ◽

2013 ◽

Vol 136 (5) ◽

Cited By ~ 6

Author(s):

Chiara Bernardini ◽

Stuart I. Benton ◽

Jen-Ping Chen ◽

Jeffrey P. Bons

Keyword(s):

Shear Layer ◽

Separation Bubble ◽

Boundary Layer Separation ◽

Low Pressure ◽

Linear Instability ◽

Laminar Separation ◽

Significant Control ◽

Low Pressure Turbine ◽

Separated Shear Layer ◽

Sound Control

The mechanism of separation control by sound excitation is investigated on the aft-loaded low-pressure turbine (LPT) blade profile, the L1A, which experiences a large boundary layer separation at low Reynolds numbers. Previous work by the authors has shown that on a laminar separation bubble such as that experienced by the front-loaded L2F profile, sound excitation control has its best performance at the most unstable frequency of the shear layer due to the exploitation of the linear instability mechanism. The different loading distribution on the L1A increases the distance of the separated shear layer from the wall and the exploitation of the same linear mechanism is no longer effective in these conditions. However, significant control authority is found in the range of the first subharmonic of the natural unstable frequency. The amplitude of forced excitation required for significant wake loss reduction is higher than that needed when exploiting linear instability, but unlike the latter case, no threshold amplitude is found. The fluid-dynamics mechanisms under these conditions are investigated by particle image velocimetry (PIV) measurements. Phase-locked PIV data gives insight into the growth and development of structures as they are shed from the shear layer and merge to lock into the excited frequency. Unlike near-wall laminar separation sound control, it is found that when such large separated shear layers occur, sound excitation at subharmonics of the fundamental frequency is still effective with high-Tu levels.

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Dual-position excitation technique in flow control over an airfoil at low speeds

International Journal of Numerical Methods for Heat &amp Fluid Flow ◽

10.1108/hff-05-2018-0195 ◽

2018 ◽

Vol 30 (9) ◽

pp. 4141-4154

Author(s):

Abbas Ebrahimi ◽

Majid Hajipour ◽

Kamran Ghamkhar

Keyword(s):

Flow Control ◽

Shear Layer ◽

Control Method ◽

Lift Coefficient ◽

Shear Layers ◽

Control Flow ◽

Plasma Actuators ◽

Dbd Plasma ◽

Surface Shear ◽

Content Type

PurposeThe purpose of this paper is to control flow separation over a NACA 4415 airfoil by applying unsteady forces to the separated shear layers using dielectric barrier discharge (DBD) plasma actuators. This novel flow control method is studied under conditions which the airfoil angle of attack is 18°, and Reynolds number based on chord length is 5.5 × 105.Design/methodology/approachLarge eddy simulation of the turbulent flow is used to capture vortical structures through the airfoil wake. Power spectral density analysis of the baseline flow indicates dominant natural frequencies associated with “shear layer mode” and “wake mode.” The wake mode frequency is used simultaneously to excite separated shear layers at both the upper surface and the trailing edge of the airfoil (dual-position excitation), and it is also used singly to excite the upper surface shear layer (single-position excitation).FindingsBased on the results, actuations manipulate the shear layers instabilities and change the wake patterns considerably. It is revealed that in the single-position excitation case, the vortices shed from the upper surface shear layer are more coherent than the dual-position excitation case. The maximum value of lift coefficient and lift-to-drag ratio is achieved, respectively, by single-position excitation as well as dual-position excitation.Originality/valueThe paper contributes to the understanding and progress of DBD plasma actuators for flow control applications. Further, this research could be a beneficial solution for the promising design of advanced low speed flying vehicles.

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Scaling of separated shear layers: an investigation of mass entrainment

Journal of Fluid Mechanics ◽

10.1017/jfm.2017.455 ◽

2017 ◽

Vol 826 ◽

pp. 851-887 ◽

Cited By ~ 6

Author(s):

Francesco Stella ◽

Nicolas Mazellier ◽

Azeddine Kourta

Keyword(s):

Shear Layer ◽

Large Scale ◽

Separated Flow ◽

Layer Growth ◽

Upper And Lower Bounds ◽

Stream Velocity ◽

Physical Parameters ◽

Separated Shear Layer ◽

Mass Entrainment ◽

Recirculation Length

We report an experimental investigation of the separating/reattaching flow over a descending ramp with a $25^{\circ }$ expansion angle. Emphasis is given to mass entrainment through the boundaries of the separated shear layer emanating from the upper edge of the ramp. For this purpose, the turbulent/non-turbulent interface and the separation line inferred from image-based analysis are used respectively to mark the upper and lower bounds of the separated shear layer. The main objective of this study is to identify the physical parameters that scale the development of the separated shear layer, by giving a specific emphasis to the investigation of mass entrainment. Our results emphasise the multiscale nature of mass entrainment through the separated shear layer. The recirculation length $L_{R}$, step height $h$ and free-stream velocity $U_{\infty }$ are the dominant scales that organise the separated flow (and related large-scale quantities as pressure distribution or shear layer growth rate) and set mean mass fluxes. However, local viscous mechanisms seem to be responsible for most of local mass entrainment. Furthermore, it is shown that large-scale mass entrainment is driven by incoming boundary layer properties, since $L_{R}$ scales with $Re_{\unicode[STIX]{x1D703}}$, and in particular by its turbulent state. Surprisingly, the relationships evidenced in this study suggest that these dependencies are established over a large distance upstream of separation and that they might also extend to small scales, at which viscous entrainment is dominant. If confirmed by additional studies, our findings would open new perspectives for designing effective separation control systems.

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