Compressibility effects in a turbulent annular mixing layer. Part 2. Mixing of a passive scalar

The mixing of fuel and oxidizer in a mixing layer between high-speed stream s is important in many applications, especially air-breathing propulsion sy stems. The details of this process in a turbulent annular mixing layer are studied with direct numerical simulation. Convective Mach numbers of the si mulations range from M-c = 0.1 to M-c = 1.8. Visualizations of the scalar h eld show that at low Mach numbers large intrusions of nearly pure ambient o r core fluid span the mixing region, whereas at higher Mach numbers these i ntrusions are suppressed. Increasing the Mach number is found to change the mixture fraction probability density function from non-marching to marchin g and the mixing efficiency from 0.5 at M-c = 0.1 to 0.67 at M-c = 1.5. Sca lar concentration fluctuations and the axial velocity fluctuations become h ighly correlated as the Mach number increases and a suppressed role of pres sure in the axial momentum equation is found to be responsible for this. An isotropy of scalar flux increases with M-c, and is explained via the suppre ssion of transverse turbulence lengthscale.