Flow Bistability Downstream of Three-Dimensional Double Backward Facing Steps at Zero-Degree Sideslip

“…They have been observed in the separated flows over the rear window of a notch-back vehicle (Lawson, Garry & Faucompret 2007) or past a three-dimensional double backward facing step (Herry, Keirsbulck & Paquet 2011). High degrees of asymmetry have also been obtained in experiments and numerical simulations of the flow past three-dimensional geometries despite the symmetry of the set-up (Lee, Paik & Lee 2009;Wassen, Eichinger & Thiele 2010); they might be the signature of a comparable instability.…”

mentioning

confidence: 92%

Turbulent wake past a three-dimensional blunt body. Part 2. Experimental sensitivity analysis

2014

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The sensitivity of the flow around three-dimensional blunt geometry is investigated experimentally at Reynolds number $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}9.2\times 10^4$. Vertical and horizontal control cylinders are used to disturb the natural flow which is the superposition of two reflectional symmetry breaking states (see Part 1 of this study, Grandemange, Gohlke & Cadot, J. Fluid Mech., vol. 722, 2013b, pp. 51–84). When the perturbation breaks the symmetry of the set-up, it can select one of the two asymmetric topologies so that a mean side force is found. When the reflectional symmetry is preserved, some positions of horizontal and vertical control cylinders alter the natural bi-stability of the flow which may result in drag reduction. In addition, it is found that the horizontal perturbation affects the lift force especially when the top and bottom mixing layers are disturbed. The ability of the disturbances to suppress the bi-stable behaviour is discussed and, introducing a formalism of induced drag, a quantification of the impact on the drag of the cross-flow forces observed for the natural bi-stable wake is suggested. Finally, a general concept for a control strategy of separated flows past three-dimensional bluff bodies can be drawn up from these analyses.

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“…Therefore, lots of studies have been dedicated to characterize the large-and small-scale fluctuations of the turbulent wakes. However recent experimental studies ( [1], [2], [3]) have shown that three-dimensional turbulent wake flows additionaly exhibit bistability properties associated to the breaking of reflectional symmetries. Such bistability property is more commonly observed in laminar flows developing in closed geometry ( [4], [5]).…”

Section: Introductionmentioning

confidence: 99%

Global wake instabilities of low aspect-ratio flat-plates

Marquet¹,

Larsson²

2015

European Journal of Mechanics - B/Fluids

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This paper investigates the linear destabilization of three-dimensional steady wakes developing behind flate plates placed normal to the incoming flow. Plates characterized by low length-towidth ratio L are considered here. By varying this aspect ratio in the range 1 ≤ L ≤ 6 three destabilization scenarios are identified. For very low aspect ratio 1 ≤ L ≤ 2, the flow is first destabilized, when increasing the Reynolds number, by a steady global mode that breaks the top/bottom planar reflectional symmetry. The symmetric steady flow bifurcates, via a pitchfork bifurcation, towards an asymmetric steady wakes, similarly to the case of axisymmetric wakes behind sphere and disks. For long aspect ratio, 2.5 ≤ L ≤ 6, the first unstable mode also breaks the top/bottom symmetry but is unsteady. A Hopf bifurcation occurs, as for the wake developing behind a two-dimensional circular cylinder. Finally an intermediate regime 2 ≤ L ≤ 2.5 is found for which the flow gets first unstable to an unsteady mode that breaks the left/right planar reflectional symmetry.

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Flow Bistability Downstream of Three-Dimensional Double Backward Facing Steps at Zero-Degree Sideslip

Cited by 55 publications

References 6 publications

Influence of Short Rear End Tapers on the Unsteady Base Pressure of a Simplified Ground Vehicle

Influence of Short Rear End Tapers on the Unsteady Base Pressure of a Simplified Ground Vehicle

Turbulent wake past a three-dimensional blunt body. Part 2. Experimental sensitivity analysis

Global wake instabilities of low aspect-ratio flat-plates

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