Preliminary Study of Shock Train in a Curved Variable-Section Diffuser

Wind-tunnel tests are conducted to study the characteristics of the shock train in the curved variable-section diffuser for hypersonic inlets. The test model is equipped with a forebody, a contracting entrance, a dump mixing duct, and an aft plug. Tests are performed at nominal freestream Mach numbers of 4.0, 5.0, and 6.0, and the corresponding inlet Mach numbers of the diffuser are 2.05, 2.59, and 3.06, respectively. Results indicate that at an inlet Mach number of 2.05, the surface pressure distributions in the shock train are similar at different backpressure ratios and can be well predicted by the modified Waltrup formula, but the length of the shock train is increased by 32 % due to the curved duct, the incident shock waves, and the incident expansion waves at the inlet plane. At higher inlet Mach numbers, the similarity disappears and the distributions of the surface pressure are not easy to predict. At different measurement points in the shock train, the instantaneous surface pressures vary obviously and almost synchronously, suggesting the oscillatory motions of the shock train. At an inlet Mach number of 2.59, the base frequency of the oscillations is about 19 Hz and the maximum fluctuating range of the instantaneous surface pressure is as high as 21.6% of the ideal pressure rise of the shock train. The static pressure fluctuations at different points in the shock train correlate strongly but are almost unrelated with those of the exit survey point, which indicates that the oscillation of the shock train is not likely induced by the pressure fluctuations far downstream of the shock train.