Role of large coherent structures in turbulent compressible mixing

Abstract

Turbulent compressible shear layers of round supersonic jets were excited by placing an open cavity near the nozzle exit, which produced pressure oscillations due to flow-induced cavity resonance. As a result of the excitation, large coherent structures were created in the highly compressible shear layers, making it possible to study the effect of such structures on turbulent compressible mixing. For this study, pressure-matched Mach 2 jets were used under both nonreacting and afterburning conditions. By varying the cavity dimensions, we could apply excitation over a wide range of frequencies and manipulate the coherent structure dynamics in the near field of the jets. Mie-scattering flow visualization images revealed that large coherent structures were generated when high-amplitude excitation occurred at frequencies close to the jet preferred mode, which was near the Strouhal number of about one-half. Growth rate of nonreacting shear layers was quantified as a function of excitation frequency; and, for afterburning jets, the change in global flame luminosity due to excitation was measured. It was observed that the growth rate was drastically increased in the near field when the coherent structures were organized. The afterburning characteristics were also modified by coherent structures and their dynamics. Under these conditions, afterburning intensity increased when the excitation frequency was higher than the preferred mode frequency and decreased at lower frequencies. The results suggest that the initial size of the coherent structures not only determined the rate of large-scale entrainment, but also modified molecular-level mixing by affecting the timing of large-structure breakdown.