Experiments on a two–dimensional laminar separation bubble

2000 ◽  
Vol 358 (1777) ◽  
pp. 3193-3205 ◽  
Author(s):  
C. P. Häggmark ◽  
A. A. Bakchinov ◽  
P. H. Alfredsson
Keyword(s):  
Separation Bubble ◽  
Laminar Separation Bubble ◽  
Two Dimensional ◽  
Laminar Separation

Structure and dynamics of a laminar separation bubble near a wingtip

2021 ◽  
Vol 929 ◽  
Author(s):  
Connor E. Toppings ◽  
Serhiy Yarusevych
Keyword(s):  
Vortex Shedding ◽  
Vortex Dynamics ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Surface Flow ◽  
Laminar Separation Bubble ◽  
Two Dimensional ◽  
Flow Topology ◽  
Laminar Separation ◽  
The Mean

The three-dimensional flow topology of a laminar separation bubble forming on the suction surface of a semispan wing with an aspect ratio of $2.5$ and NACA 0018 airfoil section is characterised experimentally using surface pressure measurements and particle image velocimetry at a chord Reynolds number of $125\ 000$ . In the inboard region of the wing, the separation bubble is essentially two-dimensional, and the transition process in the separated shear layer leads to periodic vortex shedding, which dominates the bubble dynamics, similar to two-dimensional separation bubbles. However, progressive spanwise changes in the mean structure and vortex dynamics occur near the wingtip, leading to an open separation and eventual suppression of the bubble. In the immediate proximity of the wingtip, the boundary layer remains attached, no vortex shedding occurs and the flow remains laminar, terminating separation bubble formation. Despite variations in the mean separation bubble topology and vortex dynamics along the span, the fundamental shedding characteristics remain nearly invariant across the portion of the wing where vortex shedding occurs, and the flow appears to lock onto a common instability mode across the span, leading to minimal changes in the mean bubble characteristics despite notable changes in the effective angle of attack along the span. A comparison with available surface flow visualisations from previous studies indicates that the observed changes to the mean bubble footprint along the span of the wing are similar across different geometries and flow characteristics, suggesting similarities in the three-dimensional bubble topology and dynamics on finite wings.

Vortex formation and vortex breakup in a laminar separation bubble

2013 ◽  
Vol 728 ◽  
pp. 58-90 ◽  
Author(s):  
Olaf Marxen ◽  
Matthias Lang ◽  
Ulrich Rist
Keyword(s):  
Large Scale ◽  
Separation Bubble ◽  
Vortex Formation ◽  
Secondary Instability ◽  
Small Scale ◽  
Spanwise Direction ◽  
Laminar Separation Bubble ◽  
Two Dimensional ◽  
Instability Mechanism ◽  
Laminar Separation

AbstractThe convective primary amplification of a forced two-dimensional perturbation initiates the formation of essentially two-dimensional large-scale vortices in a laminar separation bubble. These vortices are then shed from the bubble with the forcing frequency. Immediately downstream of their formation, the vortices get distorted in the spanwise direction and quickly disintegrate into small-scale turbulence. The laminar–turbulent transition in a forced laminar separation bubble is dominated by this vortex formation and breakup process. Using numerical and experimental data, we give an in-depth characterization of this process in physical space as well as in Fourier space, exploiting the largely periodic character of the flow in time as well as in the spanwise direction. We present evidence that a combination of more than one secondary instability mechanism is active during this process. The first instability mechanism is the elliptic instability of vortex cores, leading to a spanwise deformation of the cores with a spanwise wavelength of the order of the size of the vortex. Another mechanism, potentially an instability of flow in between two consecutive vortices, is responsible for three-dimensionality in the braid region. The corresponding disturbances possess a much smaller spanwise wavelength as compared to those amplified through elliptic instability. The secondary instability mechanisms occur for both fundamental and subharmonic frequency, respectively, even in the absence of continuous forcing, indicative of temporal amplification in the region of vortex formation.

Loss Production Mechanisms in a Laminar Separation Bubble

2012 ◽  
Vol 89 (4) ◽  
pp. 547-562 ◽  
Author(s):  
Daniele Simoni ◽  
Marina Ubaldi ◽  
Pietro Zunino
Keyword(s):  
Separation Bubble ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Production Mechanisms
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