Linear stability of real-fluid mixing layers at supercritical pressures

Abstract

Linear stability analysis is a useful tool for the exploration of the initial evolution of flow motions in mixing layers. A real fluid mixing layer exhibits strong property variations and, thus, may present stability behaviors distinct from its ideal gas counterpart. The present study carries out spatial and temporal stability analyses of nitrogen mixing layers at supercritical conditions, with special attention to the density stratification induced by the temperature and velocity gradients across the mixing layer. The differences between the ideal gas and real fluid approaches are discussed. The maximum spatial growth rate and the most unstable frequency evaluated based on the real fluid density profile are found to be substantially lower than their ideal gas counterparts near the critical point, where an inflection of the density distribution occurs in the mixing layer. Across the inflection point, the strong density stratification arising from the real fluid effect tends to stabilize the mixing layer. The maximum growth rate and the most unstable frequency do not show a monotonic trend with the ratios of temperature and density. In the absence of the inflection point, however, the mixing layer is destabilized and features a substantially higher maximum spatial growth rate at lower ratios of density and temperature. The most unstable frequency and the maximum spatial growth rate increase with increasing pressure. The real fluid effect diminishes when the pressure is away from the critical value or when there is no inflection point in the density profile. The temporal stability analysis also indicates that a detailed density distribution plays a key role in dictating the stability characteristics of mixing layers at supercritical pressures.