On the perseverance of a quasi-two-dimensional eddy-structure in a turbulent mixing layer

Abstract

Strong external disturbances were introduced into a mixing layer in order to test the formation of the quasi two-dimensional coherent eddies and their survival under less than ideal conditions. Velocity and temperature correlation measurements, flow visualization, and the simultaneous use of a large number of sensors suggest that these eddies are very stable in the range of Reynolds numbers considered and they persevere in spite of the external buffeting imposed. Some measurements were carried out in a mixing layer between two parallel streams and some in a mixing layer entraining quiescent surrounding fluid. In both cases the large eddies could be described. as vortex rolls spanning the test section; these rolls may be contorted and sometimes skewed, but they are basically two-dimensional.

Streamwise vortex structure in plane mixing layers

“…It is interesting to compare our results for the spacing of the streamwise vortices with velocity correlation measurements by Wygnanski et al (1979) and Browand & Troutt (1980). From the measurements of Wygnanski et al a t the centre of the layer, we estimate a value of Az/S, x 0.5 for the location of the minimum cross-correlation coefficient.…”

Effects of free-stream turbulence intensity on laminar separation bubbles

“…The effects of turbulence intensity on free-shear flows were investigated by Wygnanski et al [129], who inserted a turbulence generating grid in one of the streams upstream of the trailing edge of a splitter plate. The authors found that roll-up of the shear layer persisted with the increase in FSTI, despite previous hypotheses and results suggesting that free-stream turbulence intensity inhibits two-dimensional roll-up, and rather leads directly to a three-dimensional flow [118].…”

Numerical simulation of sound generated by vortex pairing in a mixing layer

“…Three-dimensional effects appear downstream of the rst few pairings. Experimentally, to isolate these two-dimensional structuresand to study their role, 16,17 the mixing layers can be forced at the frequency corresponding to their natural rollup and its subharmonics.…”