Experiments on the Evolution of Large-Scale Structures in Compressible Shear Layers

The evolution of the large-scale turbulent structure in compressible shear layers has been studied experimentally using a double-pulsed planar laser-induced fluorescence imaging technique. The technique provided cross-sectional digital images of the large-scale structure and of its evolution a short time later. Two-dimensional cross-correlation 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 large-scale 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 large-scale 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 high-speed stream (fast mode). In shear layers with both freestreams supersonic, the convective velocity tends towards the velocity of the low-speed 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. Roller-type 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 three-dimensional and distorted very slowly as they propagated downstream.