Experiments on a two–dimensional laminar separation bubble

Häggmark, C. P.; Bakchinov, Andrey; Alfredsson, P. Henrik

doi:10.1098/rsta.2000.0704

Cited by 70 publications

(63 citation statements)

References 7 publications

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“…These and other works on boundary layers with separation bubbles thus indicate, the dominant peaks of the disturbances are located at the critical layer, these peaks increase with distance from the separation point and achieve maximum values which can be attributed to the critical layer. Häggmark et al (2000) experimentally studied a 2-D separation bubble on a flat plate by means of hot-wire anemometry and found that the streamwise velocity disturbances for both natural and forced cases exhibited three maxima; Diwan & Ramesh (2009) experimental results confirmed this observation.…”

Section: Modification Of the Linear Instability Modessupporting

confidence: 66%

“…Rist, Maucher & Wagner (1996) and Rist & Maucher (2002) observed that the primary growth of 2-D disturbances via the TS instability mechanism in the boundary layer upstream of the separation location may undergo a gradual switchover into an inviscid amplification similar to the Kelvin-Helmholtz mechanism. Häggmark, Bakchinov & Alfredsson (2000) experimentally studied a 2-D separation bubble on a flat plate and found the bubble highly susceptible to high frequency 2-D instability waves for both natural and forced conditions; with exponential growth rate of wave disturbances in the bubble. Rist (1993) suggested a 3-D oblique mode breakdown rather than a secondary instability of finite-amplitude 2-D waves.…”

Section: Natural Transitionmentioning

confidence: 99%

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Destabilisation and modification of Tollmien–Schlichting disturbances by a three-dimensional surface indentation

Mughal

Gowree

et al. 2017

J. Fluid Mech.

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We consider the influence of a smooth three-dimensional (3-D) indentation on the instability of an incompressible boundary layer by linear and nonlinear analyses. The numerical work was complemented by an experimental study to investigate indentations of approximately 11δ 99 and 22δ 99 width at depths of 45 %, 52 % and 60 % of δ 99 , where δ 99 indicates 99% boundary layer thickness. For these indentations a separation bubble confined within the indentation arises. Upstream of the indentation, spanwise-uniform Tollmien-Schlichting (TS) waves are assumed to exist, with the objective to investigate how the 3-D surface indentation modifies the 2-D TS disturbance. Numerical corroboration against experimental data reveals good quantitative agreement. Comparing the structure of the 3-D separation bubble to that created by a purely 2-D indentation, there are a number of topological changes particularly in the case of the widest indentation; more rapid amplification and modification of the upstream TS waves along the symmetry plane of the indentation is observed. For the shortest indentations, beyond a certain depth there are then no distinct topological changes of the separation bubbles and hence on flow instability. The destabilising mechanism is found to be due to the confined separation bubble and is attributed to the inflectional instability of the separated shear layer. Finally for the widest width indentation investigated (22δ 99 ), results of the linear analysis are compared with direct numerical simulations. A comparison with the traditional criteria of using N-factors to assess instability of properly 3-D disturbances reveals that a general indication of flow destabilisation and development of strongly nonlinear behaviour is indicated as N = 6 values are attained. However N-factors, based on linear models, can only be used to provide indications and severity of the destabilisation, since the process of disturbance breakdown to turbulence is inherently nonlinear and dependent on the magnitude and scope of the initial forcing.

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Section: Modification Of the Linear Instability Modessupporting

confidence: 66%

Section: Natural Transitionmentioning

confidence: 99%

Destabilisation and modification of Tollmien–Schlichting disturbances by a three-dimensional surface indentation

Mughal

Gowree

et al. 2017

J. Fluid Mech.

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“…Figure 7 shows an approximatively linear increase in the G max with respect to all of the three parameters. It is noteworthy that the aspect ratio of the smallest bubble is close to the ones experimentally measured for laminar separation bubbles on the suction surface of aerofoils at a large angle of attack, 29,30 whereas the largest bubble has a shape factor which is comparable to the one analyzed in Refs. 3 and 31.…”

Section: -3supporting

confidence: 62%

The effects of non-normality and nonlinearity of the Navier–Stokes operator on the dynamics of a large laminar separation bubble

2010

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. The effects of non-normality and nonlinearity of the Navier-Stokes operator on the dynamics of a large laminar separation bubble. The effects of non-normality and nonlinearity of the Navier-Stokes operator on the dynamics of a large laminar separation bubble, 2010, 22 (014102), pp.15. <10.1063/1.3276903>. The effects of non-normality and nonlinearity of the Navier-Stokes operator on the dynamics of a large laminar separation bubble The effects of non-normality and nonlinearity of the two-dimensional Navier-Stokes differential operator on the dynamics of a large laminar separation bubble over a flat plate have been studied in both subcritical and slightly supercritical conditions. The global eigenvalue analysis and direct numerical simulations have been employed in order to investigate the linear and nonlinear stability of the flow. The steady-state solutions of the Navier-Stokes equations at supercritical and slightly subcritical Reynolds numbers have been computed by means of a continuation procedure. Topological flow changes on the base flow have been found to occur close to transition, supporting the hypothesis of some authors that unsteadiness of separated flows could be due to structural changes within the bubble. The global eigenvalue analysis and numerical simulations initialized with small amplitude perturbations have shown that the non-normality of convective modes allows the bubble to act as a strong amplifier of small disturbances. For subcritical conditions, nonlinear effects have been found to induce saturation of such an amplification, originating a wave-packet cycle similar to the one established in supercritical conditions, but which is eventually damped. A transient amplification of finite amplitude perturbations has been observed even in the attached region due to the high sensitivity of the flow to external forcing, as assessed by a linear sensitivity analysis. For supercritical conditions, the non-normality of the modes has been found to generate low-frequency oscillations ͑flapping͒ at large times. The dependence of such frequencies on the Reynolds number has been investigated and a scaling law based on a physical interpretation of the phenomenon has been provided, which is able to explain the onset of a secondary flapping frequency close to transition.

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“…8 represents the resulting contour map of the dimensionless velocity u/u ext (with u ext the external velocity to the boundary-layer). This shape of a Laminar Separation Bubble is well known, see for example Haggmark et al (2000). The presence of the LSB, characterized by negative to small positive values, can be easily spotted between x=c % 0:7 and x=c % 0:82.…”

Section: Velocity Measurementsmentioning

confidence: 95%

An experimental study of boundary-layer transition induced vibrations on a hydrofoil

Ducoin

Astolfi

Gobert

2012

Journal of Fluids and Structures

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International audienceThis paper aims at characterizing experimentally laminar to turbulent transition induced vibrations. Here, the transition is known to be triggered by a Laminar Separation Bubble that results from a laminar separation of the boundary-layer flow on a hydrofoil. In this study we consider two NACA66312 (Mod) laminar hydrofoils at low angles of incidence (mostly 2° and 4°) and Reynolds numbers ranging from Re=450 000 to 1 200 000, in order to get transitional regimes. The first hydrofoil, made of steel (E=2.1×1011 Pa), is referred to as the rigid hydrofoil, although it is seen to vibrate under the action of the LSB. To better understand the possible interaction between the flow and the foil vibrations, vibration measurements are repeated using a flexible hydrofoil (E=3×109 Pa) of same geometry (under zero loading) and in close configurations. The experiments are carried out at the French Naval Academy Research Institute (IRENav, France). Wall pressure and flow velocity measurements enable a characterization of the laminar separation bubble and the identification of a vortex shedding at a given frequency. It is hence shown that the boundary-layer transition induces important foil vibrations, whose characteristics in terms of frequency and amplitude depend on the vortex shedding frequency, and can be coupled with natural frequencies of the hydrofoils

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Experiments on a two–dimensional laminar separation bubble

Cited by 70 publications

References 7 publications

Destabilisation and modification of Tollmien–Schlichting disturbances by a three-dimensional surface indentation

Destabilisation and modification of Tollmien–Schlichting disturbances by a three-dimensional surface indentation

The effects of non-normality and nonlinearity of the Navier–Stokes operator on the dynamics of a large laminar separation bubble

An experimental study of boundary-layer transition induced vibrations on a hydrofoil

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