Subcritical Flow Mechanism And Buzz Suppression Method Of An External-Compression Supersonic Inlet

Subcritical operation is common to an external-compression supersonic inlet and the buzz,which is easily triggered in that situation,is a kind of catastrophic flow unsteadiness.Using theory,three-dimensional simulation,and wind-tunnel experiment,this project studies systematically the features and mechanisms regarding subcritical inlet flow and further develops a novel strategy for suppressing supersonic inlet buzz.First of all,a rectangular external-compression supersonic inlet with partially isentropic compression is carefully studied at its design Mach number.It is indicated that the inlet operates at the little buzz state after going through the supercritical state and the steady subcritical state.Later on,the big buzz,which is similar to the little buzz in frequency,appears along with the little buzz.But as the occurrence of little buzz reduces,the continuous big buzz takes place eventually.Unlike the prevailing viewpoint,the present little buzz proves to belong to the Dailey instability.Also,it is found that a single shear layer from a ?-shaped terminal shock with an upstream Mach number below 1.60 is not qualified to trigger the Ferri instability.The evolution of inlet flow under an overspeed condition is then investigated.Results show that the interaction between the ramp shock and the cowl-induced bow shock appears as regular reflection,Mach reflection,and ?-shaped pattern successively in the early throttling process.In contrast to the previous observations,the present Mach reflection is fairly stable.As the exit blockage goes beyond the limit,flow instability occurs and three types of buzz of similar frequencies but of different oscillating behaviors and amplitudes are observed afterwards.It is shown that both the little buzz and big buzz are characterized by the separation oscillation,whereas the newfound medium buzz features the bow-shock oscillation.Further analysis reveals that the buzz diversity is closely related to the internal flow division caused by the ingestion of a strong shear layer.Based on the above-mentioned supersonic inlet,a new model with a sharp-tipped cowl plate and one with two compression ramps are further designed to learn the effects of compression shock pattern.Tests show that it doesn't change the basic subcritical flow pattern whether the cowl lip is blunt or not.However,both the onset of subcritical operation and that of buzz shift to a lower throttle threshold when the cowl becomes sharp.By contrast,the results from the latter model indicate that its buzz flow at the design mode is mostly similar to the original overspeed case due to the ingestion of strong shear layers.But it is because of the Mach number discontinuity across the second ramp shock that the medium buzz disappears at high throttling ratios.Finally,in order to overcome the disadvantages of traditional buzz control methods,a suppression technique based on distributed ramp bleeding is developed and tested.It is proved that the new method is capable of remarkably improving the subcritical stability at both the design mode and the overspeed mode through bypass flow removal naturally varying with the pressure gradient and the bleeding range.Simultaneously,violent buzz is found completely eliminated over the entire throttling process in this way.Further simulations reveal that the undesired air leakage at the near-critical state is not beyond 1%of the inlet flow.Actually,it causes no obvious decrease in the inlet flow rate.Moreover,the accompanying total pressure drop and drag increase are both within 0.4%.Additionally,the modified subcritical flow suggests that the buzz origins are not necessarily limited to the two known sources and the cowlside backflow is also an alternative.