DNS of compressible turbulent flow in convergent-divergent nozzles with isothermal wall

Compressible turbulent flow through canonical convergent-divergent nozzles with circular cross-section, isothermal walls, with and without bell-shaped divergent portion is investigated by means of Direct numerical simulations. The goal of the present study is to demonstrate the effects of flow expansion (and local flow compression) in these nozzles, which have substantial transonic regions, on the mean flow and the turbulence structure. The efficacy of the kinematic or incompressible displacement thickness as a possible lengthscale for computing nondimensional pressure gradient parameters which are indicators of self-similar regions in these flows is shown. Elongation of near-wall flow structures and the dominance of vortex stretching as local flow topology are found in regions of expansion. This is accompanied by a decrease in Reynolds stresses. Regions of local flow compression are found in the divergent portion of the bell-shaped nozzle where vortex compression is the dominant flow topology. A local increase in the streamwise Reynolds stress is found in this region.