Modulation of large-scale motions on turbulent/non-turbulent interface in spatially developing compressible mixing layer

Direct numerical simulations (DNSs) are conducted to investigate the modulations of large-scale motions (LSMs) on the turbulent/non-turbulent interfaces (TNTIs) in spatially developing compressible mixing layers with convective Mach numbers (Mc) of 0.4 and 0.8. Turbulent statistics, including velocity profiles, turbulent Mach number, normalized growth rate, Reynolds stress, and velocity spectrum, are analyzed to validate the DNS data. At the shear layer center, large-scale high- and low-speed structures are observed, with spanwise rollers being suppressed as the Mach number increases. At the upper layer, the TNTI elevates above the low-speed (negative fluctuating streamwise velocity) large-scale motions (nLSMs) and sinks above the high-speed (positive fluctuating streamwise velocity) large-scale motions (pLSMs). The conditional averages based on LSMs reveal the modulations of LSMs on TNTIs. Across the upper TNTI, nLSMs stimulate positive (upward) transverse velocity and pLSMs stimulate negative (downward) transverse velocity. Under the influence of nLSMs, the jumps in velocity, turbulent kinetic energy (TKE), and vorticity magnitude are larger, as compared to pLSMs. As the convective Mach number increases, small-scale variables are suppressed, while the modulations of LSMs on TNTIs become more pronounced. The lower TNTI exhibits opposite behaviors. It is less affected by LSMs, with less shear and less intense rotation. The jumps of temperature and density increase with increasing convective Mach number. The effect of LSMs on the temperature and density jumps is significant at Mc=0.8.