Turbulent/non-turbulent interfaces in temporally evolving compressible planar jets

Turbulent/non-turbulent interfaces (TNTIs) in compressible jets are studied with direct numerical simulations of temporally evolving compressible planar jets with jet Mach numbers MJ of 0.6, 1.6, and 2.6 ejected with a jet initial pressure equal to the ambient pressure. The flow properties near the TNTI are investigated with statistics computed on the local interfacial coordinate. The layer thicknesses are about 10-13η for the TNTI layer, 3η for the viscous superlayer, and 7-10η for the turbulent sublayer (TSL), where η is the Kolmogorov scale on the jet centerline. The TSL thickness divided by η decreases from 10 to 7 as MJ increases. The turbulent fluid is characterized with lower density, higher temperature, and lower pressure than the non-turbulent fluid, where these properties sharply change within the TNTI layer. The rate of change in internal energy near the TNTI is proportional to the initial kinetic energy of the jet, where the internal energy at the outer edge of the TNTI layer changes because of the diffusive/dilatational effects. The movement of entrained fluid is similar in compressible and incompressible jets. Compressibility affects the total entrainment rate via the total surface area of the TNTI, where the surface area of the TNTI per unit area of the plane perpendicular to the cross-streamwise direction decreases from 9.5 to 7.0 as MJ increases. Strongly compressive waves appear in the non-turbulent region at a high Mach number, where the imprints of these waves are found within the TNTI layer as strong pressure/temperature correlation and large values of pressure skewness.