Laminar boundary layer separation: Instability and associated phenomena

Dovgal, A. V.; Козлов, В. В.; Michalke, A.

doi:10.1016/0376-0421(94)90003-5

Cited by 286 publications

(208 citation statements)

References 61 publications

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“…When merging occurs frequently enough, this doubling of the streamwise wavelength of the vortices leads to the subharmonic frequency peak noted in experiments [17,20,55]. Merging is associated with subharmonic instabilities in the separated shear layer [50] and also occurs in free-shear layers [57][58][59][60][61]. By acoustically forcing a separation bubble at the subharmonic frequency of its natural shedding frequency, Kurelek [20] was able to promote the merging process and observe a significant increase in the number of merging events in the separated shear layer as compared to when the bubble was left unforced.…”

Section: Figure 21mentioning

confidence: 86%

“…While both geometries yield similarly behaved LSBs, by studying separation bubbles over a flat plate an experimentalist benefits from being able to make measurements over a much simpler geometry. Separation bubbles also form in other geometries, such a backward facing steps and humps [50], but this chapter will focus on the airfoil and flat plate geometries. Over an airfoil, separation bubbles may be either short or long depending on their effect on the mean surface pressure distribution [5,7,11].…”

Section: Figure 21mentioning

confidence: 99%

“…Spectra of velocity fluctuations in the separated shear layer exhibit strong peaks at the frequency of vortex shedding [10,17,18,26], but can have additional peaks at low frequencies which are associated with bubble 'flapping' [32,48,50,53,54], and/or peaks at the subharmonic of the shedding frequency which are associated with vortex merging [17,20,55]. Bubble flapping is associated with fluctuations within the separation bubble that are not related to the shedding of shear layer vortices [17], and result in an unsteady up and down motion of the separated shear layer [31,48].…”

Section: Figure 21mentioning

confidence: 99%

See 2 more Smart Citations

Effects of free-stream turbulence intensity on laminar separation bubbles

Istvan

Kurelek

Yarusevych

2016

46th AIAA Fluid Dynamics Conference

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Section: Figure 21mentioning

confidence: 86%

Section: Figure 21mentioning

confidence: 99%

Section: Figure 21mentioning

confidence: 99%

See 1 more Smart Citation

Effects of free-stream turbulence intensity on laminar separation bubbles

Istvan

Kurelek

Yarusevych

2016

46th AIAA Fluid Dynamics Conference

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“…In the separation region, the eigenfunctions developed a third peak at the inflection point of the base flow profile. Dovgal, Kozlov & Michalke (1994), inducing a separation bubble by a rectangular hump in a 2-D boundary layer, observed TS modes turned into eigen-oscillations of the separation bubble and featured three maxima. 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.…”

Section: Modification Of the Linear Instability Modesmentioning

confidence: 98%

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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“…1,2) The instability causes the shear layer to roll up into a train of vortices rapidly, so it can therefore govern the downstream turbulent reattachment. In addition, the flow around the separation location is highly sensitive to external disturbances.…”

Section: Introductionmentioning

confidence: 99%

Experimental Study on the Separation Control of Turbulent Boundary Layers by Vortex Excitation

Asai¹,

Shimada²,

Imai³

2005

Trans. Japan Soc. Aero. S Sci.

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Artificial disturbances are introduced to a turbulent boundary layer separating at a convex corner to see how excited vortices control the turbulent separation. The results clearly show that excitation of vortices with appropriate scale can effectively suppress the separation of turbulent boundary layer, not unlike the case of laminar separation. However, strong disturbance growth due to Kelvin-Helmholtz instability, which decisively dominates development of separation bubble for laminar separation, does not occur for turbulent separation. Therefore direct excitation of energetic vortices around the separation point is required for suppression of turbulent separation.

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Laminar boundary layer separation: Instability and associated phenomena

Cited by 286 publications

References 61 publications

Effects of free-stream turbulence intensity on laminar separation bubbles

Effects of free-stream turbulence intensity on laminar separation bubbles

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

Experimental Study on the Separation Control of Turbulent Boundary Layers by Vortex Excitation

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