Numerical simulation of dense gas turbulent flows

Dense gas turbulent flows, of great interest for a wide range of engineering applications, exhibit physical phenomena that are still poorly understood and difficult to reproduce experimentally. In this work, we study for the first time the influence of dense gas effects on the structure of compressible turbulence by means of numerical simulations. The fluid considered is PP11, a heavy fluorocarbon, whose thermodynamic behavior is described by means of different equations of state to quantify the sensitivity of solutions to modelling choices. First, we considered the decay of compressible homogeneous isotropic turbulence. Temperature fluctuations are found to be negligible, whereas those of the speed of sound are large because of the strong dependence on density. The peculiar behavior of the speed of sound significantly modifies the structure of the turbulence, leading to the occurrence of expansion shocklets. The analysis of the contribution of the different structures to energy dissipation and enstrophy generation shows that, for a dense gas, high expansion regions play a role similar to high compression ones, unlike perfect gases, in which the observed behaviour is highly asymmetric. Then, we carried out numerical simulations of a supersonic turbulent channel flow for several values of Mach and Reynolds numbers. The results confirm the validity of the Morkovin' hypothesis. The introduction of a semi-local scaling, taking into account density and viscosity variations across the channel, allow to compare the wall-normal profiles of turbulent quantities (Reynolds stresses, anisotropy, energy budgets) with those observed in ideal gases. Nevertheless, the thermodynamic variables exhibit a different evolution between perfect and dense gases, since the high specific heats of the latter lead to a decoupling of dynamic and thermal effects, and to a behavior close to that of variable property incompressible fluids.