Direct numerical simulations of transitional supersonic base flows

Transitional supersonic base flows at M = 2.46 are investigated using direct numerical simulations. Results are presented for Reynolds numbers based on the cylinder diameter Re_D = 3 × 10⁴-1 × 10⁵. As a consequence of flow instabilities, coherent structures develop that have a profound impact on the global flow behavior. Simulations with various circumferential domain sizes are conducted to investigate the effect of coherent structures associated with different azimuthal modes on the mean flow, in particular on the base pressure, which determines the base drag. Temporal spectra reveal that frequencies found in the axisymmetric mode can be related to dominant higher modes present in the flow. It is shown that azimuthal modes with low wave numbers cause a flat base pressure distribution and that the mean base pressure value increases when the most dominant modes are deliberately eliminated. Visualizations of instantaneous flow quantities and turbulence statistics at Re_D = 1 × 10⁵ show good agreement with experiments at a significantly higher Reynolds number. For these investigations, a high-order-accurate compressible Navier-Stokes solver in cylindrical coordinates developed specifically for this research was used.