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We investigate experimentally the causes of jet plume instability and enhanced mixing observed in the exhaust of shock-containing convergent-divergent nozzles. Key features of the internal flow are the separation shock, separation shear layers, and pattern of alternating expansion and compression waves downstream of the shock. We focus on two possible reasons for this instability—the motion of the separation shock and the wave pattern downstream of the shock. The nozzle flow was generated in a planar facility with variable area ratio and pressure ratio, and the motion of the shock was tracked using time-resolved wall pressure measurements. The isolated effect of the wave pattern was investigated in a separate facility wherein a sonic shear layer, simulating the nozzle separation shear layer, was disturbed with compression and expansion waves emanating from a wavy wall. In both instances, the instability of the shear layer was characterized by time-resolved measurements of the total pressure. In the nozzle flow, the amplitude of shock motion increases with shock strength. Correlation of shock motion with shear layer total pressure is virtually absent for weak shocks but becomes significant for strong shocks. However, impingement of stationary waves on the shear layer had no impact on its growth rate. We conclude that the enhanced shear layer instability is strongly coupled to shock motion, and that the wave pattern by itself is not a cause of enhanced mixing. The occurrence of asymmetric separation at large shock strengths is a further contributor to the enhancement of instability. [ABSTRACT FROM AUTHOR]

Investigates the evolution of large turbulent eddies in seven supersonic shear layers with average convective Mach numbers and with large variation in density and velocity ratios. Use of a two-laser, single-detector planar laser-induced fluorescence technique for flow visualization and its evolution.

Presents experimental evidence of shocklets in a counterflow Mach 2 shear layer. Shock waves emanating from the turbulent structure; Analysis of the slope of the Mach waves suggesting that the turbulent structures are nonstationary, even though the shear layer is symmetric.

The characteristics and bicharacteristics of the linearized equations of motion for compressible fluid are identical to the wave fronts and rays, respectively, calculated by Fermat's least-time principle of geometric acoustics. The bicharacteristics emanating from the middle of a compressible shear layer curve so as to hinder communication in the axial direction. [ABSTRACT FROM AUTHOR]