Structures of Density Fluctuations at a Low-Mach-Number Turbulent Boundary Layer by DNS

We performed a direct numerical simulation of a low-Mach-number turbulent boundary layer using fundamental equations of compressible flow to investigate the relation between vortex structures and the density distribution. A fully developed turbulent boundary layer is reproduced in the simulation. From the turbulence statistics and instantaneous structures, we identified different features in the three regions of a near-wall field, far field and a flow field outside the turbulent boundary layer. We then observed fine-scale structures of the density fluctuation that are strongly related to turbulent vortices in the vicinity of the wall. In addition, there are large-scale density structures in the upper boundary layer. The large-scale structures seem to correlate with the fine-scale structures close to the wall, with there being a non-steady larger-scale density fluctuation profile in the outer region of the boundary layer.