An uncoupled method based on an implicit dual time-step approach for nonequilibrium flows is developed.Flow and reaction are separated by using operator-splitting method.Flow equations are solved by a dual time-step approach and source equations are solved by 2nd order trapezoidal formulas.A source-eliminating method is designed to remove error imposed to the solution by chemical source,which is very effective to assure time precision of flow-solving.Analysis and computations suggest that the method maintains efficiency of dual time-step approach as well as precision of unsteady computational results.

Self-sustained oscillation of supersonic cold flowover cavities of scramjets is investigated.Unsteady flowfields are calculated by hybrid RANS/LES method.The key cavity parameters,cavity aft wall angle andthe ratio of cavitylengthto depth,are considered.The hybrid model captures the large scale unsteady characteristics and reveals the evolution process of cavityfree shearlayer.Pressure fluctuation spectra agree well with theoretical prediction.The hybrid RANS/LES data indicate that both cavity shape and size have significant effects on oscillation.Asthe ratio of cavitylengthto depth decreases,the oscillations tend to concentrate on several modes,and the frequency of maximumoscillation amplitude shifts to higher values.Most oscillations are greatly attenuate as the cavity aft wall angle decreases.Compared with a steep aft wall cavity,only highfrequency mode remains inthe cavity with small aft wall angle.