Abstract
A freestream Mach 2.9 flat-plate supersonic turbulent boundary layer subject to a “pure” adverse pressure gradient (APG) without the impact of wall curvatures is studied by direct numerical simulation and compared with a benchmark flow with zero pressure gradient. Due to APG, the streamwise velocity shows an increase in the near-wall region and a reduction in the outer boundary layer. The principal strain rate shows a sandwich distribution along the wall-normal direction. The mismatch between the temperature and velocity fluctuations in both the inner and the outer layer is observed. Enhanced LSMs (large-scale motions) and large velocity patches are the typical flow structures in the outer and inner boundary layer subject to APG, respectively. From the analysis of quadrant decomposition, the sweep events dominate in the near-wall region while ejection events dominate the rest of the boundary layer. It is found that the baroclinicity plays a significant role in the formation of the enhanced LSMs in the outer boundary layer and the near-wall velocity patches. The resulting amplified vorticity further drives the interactive motions of the outer fluid and inner fluid. The turbulent kinetic energy and turbulent Mach number profiles are amplified by APG and a second peak is observed in both profiles. Turbulent energy budget analysis demonstrates that both the production and viscous effects are strengthened in the near-wall region while in the outer layer, the production is significantly amplified and balanced by the increased convection and turbulent transport.
Highlights
Among internal and external flows, adverse pressure gradient (APG) is ubiquitous when interference, such as compression/shock waves, geometric curvatures are confronted
Due to APG, the streamwise velocity shows an increase in the near-wall region and a reduction in the outer boundary layer
In order to avoid the impact of coupling the effect of wall curvatures which would further introduce strong centrifugal effect, a freestream Mach 2.9 flat-plate supersonic turbulent boundary layer subject to APG (β = 2.15) induced by an external compression wave generator is studied through Direct numerical simulation (DNS)
Summary
Among internal and external flows, adverse pressure gradient (APG) is ubiquitous when interference, such as compression/shock waves, geometric curvatures are confronted. External compression waves induced APG (βmax = 5.8) are exerted on a flat-plate supersonic turbulent boundary layer by Fernando and Smits using a two-dimensional wave generator, results are compared with the supersonic boundary layer where a similar pressure gradient is produced by a concave wall Both the mean flow and the turbulent intensity are shown to have been strongly affected by the pressure gradient. In order to avoid the impact of coupling the effect of wall curvatures which would further introduce strong centrifugal effect, a freestream Mach 2.9 flat-plate supersonic turbulent boundary layer subject to APG (β = 2.15) induced by an external compression wave generator is studied through DNS. The TKE budgets show that the turbulence transport is altered predominantly compared to the ZPG case
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