Direct numerical simulation of hypersonic boundary layer transition over a blunt cone with a small angle of attack

摘要

The direct numerical simulation of boundary layer transition over a 5 degrees half-cone-angle blunt cone is performed. The free-stream Mach number is 6 and the angle of attack is 1 degrees. Random wall blow-and-suction perturbations are used to trigger the transition. Different from the authors' previous work [Li et al., AIAA J, 46, 2899 (2008)], the whole boundary layer flow over the cone is simulated (while in the author's previous work, only two 45 degrees regions around the leeward and the windward sections are simulated,). The transition location on the cone surface is determined through the rapid increase in skin fraction coefficient (C-f). The transition line on the cone surface shows a nonmonotonic curve and the transition is delayed in the range of 20 degrees <= theta <= 30 degrees (theta = 0 degrees is the leeward section), The mechanism of the delayed transition is studied by using joint frequency spectrum analysis and linear stability theory (LST). It is shown that the growth rates of unstable waves of the second mode are suppressed in the range of 20 degrees <= theta <= 30 degrees, which leads to the delayed transition location, Very low frequency waves (VLFWs) are found in the time series recorded just before the transition location, and the periodic times of VLFWs are about one order larger than those of ordinary Mack second mode waves. Band-pass filter is used to analyze the low frequency waves, and they are deemed as the effect of large scale nonlinear perturbations triggered by LST waves when they are strong enough.