Isotropic turbulence of variable-density incompressible flows

In the present study, the effects of density variations on structures developing in an isotropic incompressible turbulent flow are investigated. Statistical analyses are carried out on data sets obtained from direct numerical simulations of forced turbulence. The discretized variable -density incompressible Navier-Stokes equations are time advanced with a Fourier -Fourier spectral solver coupled with a semi -implicit second order in time Runge-Kutta scheme. Turbulence is forced using an extension of the Lundgren method to the variable -density equations including mass diffusion effects. Numerical evidence shows that the introduction of a variable -density field into a turbulent field modifies the coherent structures and the energy spectrum in the inertial range. The analysis of probability density functions of velocity gradients and Lagrangian acceleration suggests an increase in time and space intermittency beyond a threshold density ratio associated with the two -fluid mixture. These modifications are not captured by the classical scaling laws of skewness and flatness factors given in the literature for the constant -density flow case. The energy spectra preserve the Kolmogorov slope while exhibiting an energy level alteration within the smallest scales of the inertial range. This region corresponds to the range of modes where the energy levels of the Rayleigh -Taylor instability criterion are the highest, giving some physical arguments that the aforementioned structural modifications may be attributed to Rayleigh -Taylor -like instabilities.