Mach number and wall thermal boundary condition effects on near-wall compressible turbulence

We investigate the effects of thermal boundary conditions and Mach number on turbulence close to walls. In particular, we study the near-wall asymptotic behavior for adiabatic and pseudo-adiabatic walls, and compare to the asymptotic behavior recently found near isothermal cold walls (Baranwal et al. (2022)). This is done by analyzing a new large database of highly-resolved direct numerical simulations of turbulent channels with different wall thermal conditions and centerline Mach numbers. We observe that the asymptotic power-law behavior of Reynolds stresses as well as heat fluxes does change with both centerline Mach number and thermal-condition at the wall. Power-law exponents transition from their analytical expansion for solenoidal fields to those for non-solenoidal field as the Mach number is increased, though this transition is found to be dependent on the thermal boundary conditions. The correlation coefficients between velocity and temperature are also found to be affected by these factors. Consistent with recent proposals on universal behavior of compressible turbulence, we find that dilatation at the wall is the key scaling parameter for this power-law exponents providing a universal functional law which can provide a basis for general models of near-wall behavior.Comment: 24 pages, 15 figures, Under consideration for publication in Journal of Fluid Mechanic

On the Modelling of High Speed Turbulent Flows with Applications towards Reentry Ablation

The fluid dynamics of an ablating hypersonic turbulent boundary layer is a complex process with strong coupling to the surface response. The overarching objective of our research is to improve the accuracy of turbulent heat flux and shear stress modeling for this class of flow. Our approach is to first establish physics based transport equation frameworks for model development, and then perform model driven experiments to isolate underlying phenomena. Among the complications is roughness and streamline curvature induced mechanical non- equilibrium. In this presentation, a description of a recent Mach 5 experimental campaign focused on characterizing the role of these complications on the Reynolds stresses is given first. This is followed by a discussion of the impact on modeling and control. The results from the study are encouraging from the perspectives that (1) existing Reynolds stress transport and large-eddy methods show promise in capturing the observed processes provided suitable understanding of the turbulence structure is achieved and (2) the mechanisms appear controllable