Jet mixing enhancement by high-amplitude fluidic actuation

Recent experiments have shown that properly designed high-amplitude, low mass flux pulsed slot jets blowing normal to a jet's shear layer near the nozzle can significantly alter the jet's development. In contrast to commonly used low-amplitude forcing, this strong excitation appears to overwhelm the turbulence, having nearly the same effect at high and low Reynolds numbers. It can, therefore, be studied in detail by direct numerical simulation. Direct numerical simulations of Mach 0.9, Reynolds number 3.6×10³ jets exhausting into quiescent fluid are conducted. Physically realistic slot jet actuators are included in the simulation by adding localized body-force terms to the governing equations. Three cases are considered in detail: a baseline unforced case and two cases that are forced with flapping modes at Strouhal numbers 0.2 and 0.4. (Sr = 0.4 was found to be the most amplified frequency in the unforced case.) Forcing at either frequency causes the jet to expand rapidly in the plane parallel with the actuators and to contract in the plane perpendicular to the actuators, as observed experimentally. It is found that the jet responds closer to the nozzle when forced at Sr = 0.4, but forcing at Sr = 0.2 is more effective at spreading the jet farther downstream. Several different measures of mixing (scalar dissipation, volume integrals of jet fluid mixture fraction, and point measurements of mixture fraction) are considered, and it is shown that by most, but not all, measures forcing at Sr = 0.2 is the more effective of the two at mixing.