A unified description of oblique waves in ideal and non-ideal steady supersonic flows around compressive and rarefactive corners

Abstract

According to classical gasdynamic theory, if a steady supersonic parallel flow encounters a sudden change in the wall slope, two very different phenomena may possibly occur. If the flow expands around a sharp corner, the well known isentropic Prandtl-Meyer fan is observed. Conversely, a shock wave occurs if the flow is compressed: for wedge angles smaller than the detachment value, which depends on the uniform upstream state, an oblique shock is originated at the corner; at larger deviation angles, a detached shock is formed. A unified description of these flows is presented here to extend the validity of the common β-ϑ (shock angle-deflection angle) diagram for shocked non-isentropic flows into the realm of isentropic expansions. The new graph allows for a straightforward identification of the wave angles for self-similar flow fields around compressive and rarefactive corners. Besides, it clarifies the relation between shock waves and rarefaction fans in the neighbourhood of the ϑ = 0 axis, where shock waves are weak enough to be fairly well approximated by isentropic compressions. At ϑ = 0, indeed, shock and rarefaction curves are demonstrated to be first order continuous. This result is interpreted in view of the bisector rule for oblique shock waves. Exemplary diagrams are reported for both ideal-gas flows, dilute-gas flows and non-ideal flows of dense vapours in the close proximity of the liquid-vapour saturation curve and critical point. The application of the new diagram is illustrated for the textbook case of the supersonic flow past a diamond-shaped airfoil.