Published online by Cambridge University Press: 14 December 2007
Direct numerical simulation data of a Mach 2.9, 24○ compression ramp configuration are used to analyse the shock motion. The motion can be observed from the animated DNS data available with the online version of the paper and from wall-pressure and mass-flux signals measured in the free stream. The characteristic low frequency is in the range of (0.007–0.013) U∞/δ, as found previously. The shock motion also exhibits high-frequency, of O(U∞/δ), small-amplitude spanwise wrinkling, which is mainly caused by the spanwise non-uniformity of turbulent structures in the incoming boundary layer. In studying the low-frequency streamwise oscillation, conditional statistics show that there is no significant difference in the properties of the incoming boundary layer when the shock location is upstream or downstream. The spanwise-mean separation point also undergoes a low-frequency motion and is found to be highly correlated with the shock motion. A small correlation is found between the low-momentum structures in the incoming boundary layer and the separation point. Correlations among the spanwise-mean separation point, reattachment point and the shock location indicate that the low-frequency shock unsteadiness is influenced by the downstream flow. Movies are available with the online version of the paper.
Movie 1. This movie shows a three-dimensional view of the 24-degree compression ramp interaction with Mach number 3 and Reyonlds number based on the momentum thickness of the incoming boundary layer 2300. Flow is from lower-left to upper-right. Iso-surface of the magnitude of the pressure gradient is shown to visualize the shock. Data rate is 100 kHz (or 1 Uinf/delta, where delta is the incoming boundary layer thickness). The black triangle in the movie is a reference point to see the streamwise shock motion. The relatively high-frequency spanwise wrinkling motion of the shock near the shock foot region and the low-frequency streamwise shock oscillation is seen.
Movie 2. This movie shows a plan-view (top-view) of the 24-degree compression ramp interaction with Mach number 3 and Reyonlds number based on the momentum thickness of the incoming boundary layer 2300. The plan is at a wall-normal location of 0.9 delta away from the wall, where delta is the incoming boundary layer thickness. Flow is from left to right. Contours of the magnitude of the pressure gradient is shown to visualize the spanwise wrinkling shock motion. Data rate is 100 kHz (or 1 Uinf/delta). The frequency of the spanwise wrinkling shock motion is seen to be of order 1 Uinf/delta with a magnitude of about 0.5 delta.