Research On Flow Characteristics And Control Rules Of Rectangular Isolators

This paper studied flow characteristics and control rules of rectangular scramjet isolators under asymmetric supersonic flow with a combination of experiments and numerical simulations.The flow characteristic of the high aspect ratio rectangular cross-section isolator was investigated with direct connect wind tunnel experiment and 3D numerical simulation. The results showed that the flow characteristic of the high aspect ratio isolator under asymmetric supersonic flow, such as the shape of shock trains, the shock-holding capability and the rule of wall pressure, have significant difference from the low aspect ratio isolator. When AR≥3.5, a modified Waltrup correlation is obtained by replacing the isolator's height with the equivalent diameter in the classic Waltrup correlation. The precision of the modified Waltrup correlation to calculate the length of the shock train in the high aspect ratio isolator is much better. The research results of the low aspect ratio isolator could not be simply applied to the design of the high aspect ratio isolator.Several flow control devices, such as the ramps (the full ramp, the swept ramp and the sweep forward ramp), the bleeding slot and the passive bleeding bypass, which could effectively improve the shock-holding capability and shorten the isolator, were developed with the experiment and numerical simulation. In particular, experimental results indicated that the maximum back-pressure is improved by 9.89 percent for the isolator with a swept ramp. The length of the isolator with a swept ramp is reduced by 15 percent for the same back-pressure. The total pressure recovery ratio of the isolator with a swept ramp is improved by 2.3 percent. The key point about the difference between the swept ramp and the other ramps is that the swept ramp generates a cascade of streamwise vortices in the near wall region of the supersonic boundary layer. The downwash region of the stream vortices alters the properties of a supersonic boundary layer by injecting high momentum flow into the low momentum portion of the boundary layer. This"energizes"the turbulent boundary layer rendering it less apt to succumb to the negative effects of developing in an adverse pressure gradient. The bleeding slot bleeds out the low-momentum flows near the slot to reduce shock-wave/turbulent-boundary-layer interactions. So the compression of shock trains becomes stronger and shock trains are shortened. The passive bleeding bypass, which could make use of the tremendous pressure difference between the bypass and the outlet of the high aspect ratio isolator, bleeds out the separated flow in the rear part of the isolator to shorten the shock train.Compared with the bleeding slot, it doesn't cost external energy and mass flow. Locations and parameters of the ramps, the bleeding slot and the passive bleeding bypass have great influence on the length of shock trains. They should be considered reasonablely in order to obtain the the high performance isolator.In order to analyze the response of the wall pressure in an isolator when the shock train is subjected to a periodic motion at a low frequency, the isolator experiment using wall pressure fluctuation measurements and statistically analysis was conducted in a blow-down supersonic wind tunnel under asymmetric incoming flow with the periodic back pressure generated by a shuttled tail cone. It was found that the isolator effectively isolates the periodic back pressure fluctuation from affecting upstream undisturbed flow. Wall pressure fluctuations are due to the propagation of wave fronts of the second acoustic mode, but they are subjected to an oscillating shock train in the most part of the shock oscillation region. Attenuation of wall pressure fluctuations on the lower wall with thick boundary layer accords with the exponential law, but it fluctuates on the upper wall with thin boundary layer in the shock oscillation region.