Axisymmetric flare-induced separation of high-speed transitional boundary layers

Abstract

This paper aims to develop the understanding of laminar-turbulent transition in separated compressible flow. To this end, a fundamental experimental study of flare-induced shock/boundary-layer interactions (SBLIs) is conducted in a gun tunnel at a freestream Mach number of M∞=8.9 and a Reynolds number of Re∞/m=47.4x10. A blunted cylinder with a nose radius of 25mm is used to establish an axisymmetric reference laminar flow which then permits a controlled environment for assessing three-dimensionalities inherent to the transitional cases. The boundary layer development, which is dominated by the entropy layer effect on the blunt nose, results in edge conditions at the flare location of ME≈3.4, ReE/m≈3.3x10 and δ=2mm. Data presented are based on time-resolved surface heat transfer measured in the transverse direction and at an axial location of x=213mm from the nose leading edge. An 8o flare is positioned 7mm downstream of the gauges allowing to measure the spanwise characteristics of a well-established separated laminar boundary layer – with the separation located 15mm upstream of the flare. This is then followed by experiments using a discrete roughness element on the nose as a boundary layer trip, which reproduces a turbulent wedge as a result of the amalgamation of turbulent spots. In this case, the interaction downstream of the turbulent flow remains attached. Eventually, the most challenging case consists in using a boundary layer trip which permits to produce a train of isolated turbulent spots. High-speed schlieren videos obtained simultaneously with the heat transfer measurements show the intermittent attached/separated estate of the interaction as the incoming spots go through it.