Experiments on the Evolution of LargeScale Structures in Compressible Shear Layers
Abstract
The evolution of the largescale turbulent structure in compressible shear layers has been studied experimentally using a doublepulsed planar laserinduced fluorescence imaging technique. The technique provided crosssectional digital images of the largescale structure and of its evolution a short time later. Twodimensional crosscorrelation techniques were applied to measure the displacement of the structure, from which the convective velocity was computed. Eight flow cases with the 'symmetric' convective Mach number =M_{c} ranging from 0.24 to 0.86 were studied. The experimental results were compared against the 'symmetric' model of the largescale structure, which predicts that the two convective Mach numbers experienced by the structure, M_{c1} and M_{c2}, are approximately equal. The experiments show that at low =M _{c}, the largescale structure travels with a convective velocity consistent with that predicted by the symmetric model. For =M _{c}>0.3, however, a dramatic departure from the symmetric model occurs. For shear layers with one freestream supersonic and the other subsonic, the convective velocity tends towards the velocity of the highspeed stream (fast mode). In shear layers with both freestreams supersonic, the convective velocity tends towards the velocity of the lowspeed stream (slow mode). In either case, M _{c1} and M_{c2 } differ greatly from each other, in contrast to the symmetric model. These asymmetric modes may have a profound effect on compressible mixing and on combustion. In addition, the fast mode has a direct impact on noise generation in supersonic jets. When the dependence of the M _{c1}M_{c2 } relation on speed of sound ratio is taken into account, the deviation of the experimental results from the symmetric model is shown to be a monotonically increasing function of =M_{c }. This leads to an approximate relation for predicting M_{c1} and M_{c2} given the freestream velocities and speeds of sound. Rollertype structures were observed up to =M_{c}=0.54. At higher =M_{c}, the structures became less organized. At =M_{c}=0.48, the structures in the spanwise plane were observed to be of comparable size to the structures in the transverse plane. In addition, they appeared to be highly threedimensional and distorted very slowly as they propagated downstream.
 Publication:

Ph.D. Thesis
 Pub Date:
 November 1995
 Bibcode:
 1995PhDT.......140B
 Keywords:

 MIXING LAYERS;
 Engineering: Mechanical; Engineering: Aerospace; Physics: Fluid and Plasma