Characteristics Of Forced Shock Train Oscillation In A Rectangular Isolator

The forced shock train oscillation has been investigated experimentally and computationally in a rectangular isolator.In the experiments,the fluctuating back-pressure was imposed in the downstream of the isolator using the fluctuated throttling system,which was self-designed.In the computational cases,the periodical pressure outlet condition was set in the outlet of the isolator by programming UDF.The research was conducted firstly under the steady back-pressure condition.Under the incoming Mach 2 condition,the shock train is symmetrical and the first bifurcated shock shows in ―λ‖ structure.Under the incoming Mach 3 condition,the shock train is asymmetrical and the first bifurcated shock shows in ―X‖ structure.The shock train has two different kinds of asymmetrical structure.The shock train self-excited oscillation was experimentally studied.It is found that the shock train oscillated at relative low frequency,which is less than 200 Hz.For the asymmetrical shock train,the frequency of the separated shock oscillation is independent of the magnitude of separation region.The response of the shock train to the downstream fluctuating back-pressure was studied.The results show that the fundamental frequency of the shock train oscillation is very close to the downstream excitation frequency,and the shock train motion is a forced oscillation.However,some other oscillation frequencies may occur when the self-excited oscillation is violently.The shock train structure keeps transforming during an oscillation period,including the strength of the separated shock,the height of the separated bubble and the position of the bifurcated point.The structure transformation is essentially induced by the varying pressure ratio of the separated shock.The pressure standard deviation distribution,the pressure main frequency distribution and the correlation to the back-pressure signals were analyzed.Based on the experimental phenomena,three possible methods for the shock train leading detection have been proposed under the fluctuating back-pressure condition.The most upstream position of the shock train leading edge is located where the excitation frequency content disappears or where the pressure standard deviation decreases to near-zero or the coherence estimate to back-pressure signals decrease sharply.The effect of the back-pressure frequency and amplitude on the shock train oscillation was explored.It can be summarized that the oscillation amplitude of the shock train decreases with the increasing excitation frequency,and increases with the increasing excitation amplitude.Consequently,the low frequency and high amplitude prevent the inlet unstart.Based on the simplified flow fields and some assumptions,a simple model for the shock train dynamics was developed.The amplitude of the forced shock train oscillation can be predicted by this theoretical model.Totally,the theoretical correlation can well predict the trend of the experimental and computational results.Considering the practical working process of the scramjet,the experiments were designed to compare several shock train leading edge detection methods.In addition,a new detection method based on differential pressure was proposed and tested.The current detection methods are not applicable for practical using due to some restrictions,and these restrictions are summarized in this paper.