Abstract: Mass exchange characteristics of cavities for scramjetapplications within a supersonic cold flow are investigated.Calculations are carried out using the hybrid RANS/LES method to predict unsteadystate flowfields. The key cavity parameters include cavityaft wall angle and cavity length to depth ratio (L/D). The hybrid model captures the large scale unsteadiness and reveals the evolvementof cavity free shear layer. For pressure fluctuation spectra, thetheoretical prediction agrees quite well with some calculated data.The hybridRANS/LES data suggest that both the cavity shape and the size have a significanteffect on the mass exchange processes. Cavities with larger L/D have a longerresidence time. For cavities with smaller L/D, the residence time reduces as theaft wall angle decreases. The opposite is true for cavities with largerL/D. These calculated results provide some new insight for cavityflamehoder design.