Compressibility effects in turbulent nonpremixed reacting shear flows.

摘要

Direct simulations of the turbulent shear layer are performed for subsonic-to-supersonic Mach numbers both in the non-reacting and reacting cases. Large computational grids and high-order accuracy enable fully-developed turbulence in a self-similar state to be achieved with profiles of mean velocity and turbulence intensities that agree well with laboratory experiments. The DNS results are used to discriminate between compressibility effects due to high-speed, unequal free-stream density and heat release.; The thickness growth rate of the shear layer exhibits a large reduction with increasing values of the convective Mach number, M_c. DNS and analysis are used to explain this stabilizing effect of compressibility. The following physical mechanism for the compressibility effect is offered: the finite speed of sound in compressible flow introduces a finite time delay in the transmission of pressure signals from one point to an adjacent point and, the consequent decorrelation results in inhibited inter-component energy transfer.; DNS is also used to study the effect of different free stream densities parametrized by the density ratio, $s=r2/r1.$ It is found that the temporal growth rate of the vorticity thickness exhibits a small reduction. However, the momentum thickness growth rate decreases strongly for $s\ne 1.$ The peak value of the shear stress, $uu,$ is observed to be insensitive to changes in s. The dividing streamline of the shear layer is observed to move into the low-density stream. An analysis is performed to explain this shift and the consequent reduction in momentum thickness growth rate.; An infinitely fast, irreversible reaction between methane and air is assumed for the reacting simulations. In the reacting cases a reduction of the growth rate of the mixing layer is observed with increasing heat release. An analysis is performed to relate the mean density variation to the reduced growth rate. Scalar and scalar dissipation statistics exhibit significant effects of heat release. For example, unlike the non-reacting case, averages of scalar dissipation conditioned on scalar depend on the value of the scalar in reacting cases. Scalings for the averaged turbulent and scalar dissipation based on the vorticity thickness as length scale are proposed.