Interaction between a Tollmien-Schlichting wave and a laminar separation bubble

Gruber, Karl Heinz; Bestek, H.; Fasel, Hermann F.

doi:10.2514/6.1987-1256

Cited by 24 publications

(9 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Besides bursting, the prediction of other characteristics of separated flow such as the onset of unsteadiness or three-dimensionalization of nominally two-dimensional LSBs has been attempted following this vein. The presence and dominance of Kelvin-Helmholtz (K-H) instability acting on the shear layer has been confirmed in a multitude of experimental (Dovgal, Kozlov & Michalke 1994) and numerical (Gruber, Bestek & Fasel 1987;Rist & Maucher 1994;Marxen, Lang & Rist 2012) investigations. External instability waves reaching the separated region experience a growth of several orders of magnitude, eventually leading to nonlinear effects and vortex shedding unless the initial amplitudes are very small.…”

Section: Introductionmentioning

confidence: 65%

The two classes of primary modal instability in laminar separation bubbles

2013

View full text Add to dashboard Cite

The self-excited global instability mechanisms existing in flat-plate laminar separation bubbles are studied here, in order to shed light on the causes of unsteadiness and threedimensionality of unforced, nominally two-dimensional separated flows. The presence of two known linear global mechanisms, namely an oscillator behaviour driven by local regions of absolute inflectional instability and a centrifugal instability giving rise to a steady three-dimensionalization of the bubble, is studied in a series of model separation bubbles. These results indicate that absolute instability, and consequently a global oscillator behaviour, does not exist for two-dimensional bubbles with a peak reversed-flow velocity below 12 % of the free-stream velocity. However, the threedimensional instability becomes active for recirculation levels as low as u rev ≈ 7 %. These findings suggest a route to the three-dimensionality and unsteadiness observed in experiments and simulations substantially different from that usually found in the literature of laminar separation bubbles, in which two-dimensional vortex shedding is followed by three-dimensionalization.

show abstract

Section: Introductionmentioning

confidence: 65%

The two classes of primary modal instability in laminar separation bubbles

2013

View full text Add to dashboard Cite

show abstract

“…The presence and dominance of Kelvin-Helmholtz instability acting on the shear layer has been confirmed in a multitude of experimental 11,12 and numerical investigations. 6,8,9,13,14 However, Kelvin-Helmholtz (K-H) instability alone does not suffice to explain either the occurrence or breathing or flapping bubble behavior, i.e. oscillations of the entire separation bubble at frequencies substantially lower than those predicted by locally-parallel linear stability theory.…”

Section: Introductionmentioning

confidence: 96%

On the two classes of global primary modal instability in laminar separation bubbles

Rodríguez

Gennaro

Juniper

2013

43rd Fluid Dynamics Conference

View full text Add to dashboard Cite

The self-excited global instability mechanisms existing in flat-plate laminar separation bubbles are studied here, in order to shed light on the causes of unsteadiness and threedimensionality of unforced, nominally two-dimensional separated flows. The presence of two known linear global mechanisms, namely an oscillator behavior driven by local regions of absolute inflectional instability and a centrifugal instability giving rise to a steady threedimensionalization of the bubble, is studied in a series of model separation bubbles. Present results indicate that absolute instability, and consequently a global oscillator behavior, does not exist for two-dimensional bubbles with a peak reversed-flow velocity below 12% of the free-stream velocity. However, the three-dimensional instability becomes active for recirculation levels as low as urev ≈ 7%. These findings suggest a route to the three-dimensionality and unsteadiness observed in experiments and simulations substantially different from that usually found in the literature, in which two-dimensional vortex shedding is followed by three-dimensionalization.

show abstract

“…Gruber, Bestek & Fasel (1987) prescribed a locally decelerated external velocity field at the free-stream boundary to generate the bubble, and hence simulated the spatial development of the TS wave. The nearly explosive growth of disturbance amplitudes was observed.…”

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.

View full text Add to dashboard Cite

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.

show abstract

Interaction between a Tollmien-Schlichting wave and a laminar separation bubble

Cited by 24 publications

References 0 publications

The two classes of primary modal instability in laminar separation bubbles

The two classes of primary modal instability in laminar separation bubbles

On the two classes of global primary modal instability in laminar separation bubbles

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

Contact Info

Product

Resources

About