Effect of chemical reactions on decaying isotropic turbulence

There have been many studies of turbulent combustion flows, however the interaction between turbulent motion and the chemical reactions that occur in hypersonic flows has not been studied. In these flows, the rate of product formation depends almost exclusively on the temperature, and small temperature fluctuations may produce large changes in the rate of product formation. To study this process, we perform direct numerical simulations of reacting isotropic turbulence decay under conditions typical of a hypersonic turbulent boundary layer flow. We find that there is a positive feedback between the turbulence and exothermic reactions. That is, positive temperature fluctuations increase the reaction rate, thereby increasing the heat released by the reaction, which further increases the temperature. Simultaneously, the pressure increases causing localized expansions and compressions that feed the turbulent kinetic energy. The Reynolds stress budget shows that the feedback occurs through the pressure-strain term. We also find that the strength of the feedback depends on how much heat is released, the rate at which it is released, and the turbulent Mach number. The feedback process is negative for endothermic reactions, and temperature fluctuations are damped. © 1998 American Institute of Physics. @S1070-6631~98!00307-9#