Application of PIV in a Hypersonic Double-Ramp Flow

Abstract

†† † . The flow over a double compression is investigated by means of schlieren visualization, quantitative infrared thermography and particle image velocimetry. While schlieren and infrared thermography are well-established measurement techniques for high-speed flows, PIV is far from common practice in the hypersonic flow regime. The implementation of PIV with nanometric particles and microsecond pulse separation allows the measurement of the instantaneous planar velocity field over a two-dimensional double compression ramp in a Mach 7 free stream produced with a Ludwieg tube facility. The wind tunnel free stream conditions are investigated as well as the tracer particle response to a shock wave. The PIV technique proves to be a valuable diagnostic tool returning quantitative information with relatively high spatial resolution. The investigated flow configuration is a 2D double compression ramp with a variable second ramp angle. Shock-shock interactions are found to be of Edney type VI or V for the lowest or highest ramp angle respectively. A single heat transfer peak is detected on the second ramp, associated to the type VI interaction, while the type V interaction shows a double heat transfer peak. Shock wave angles measured with PIV are in good agreement with theory and are consistent with schlieren visualization. Under adiabatic flow hypothesis the Mach number planar distribution can be inferred from the PIV data. Also in this case the results are in agreement with compressible flow theory. I. Introduction DVANCES in diagnostic techniques for hypersonic flows are driven by the need to extend the capabilities of ground-based flow investigations of re-entry vehicle aerodynamics. However, the objective of simulating reentry flow conditions with wind-tunnels can be only partly achieved. Even more critical is the issue of obtaining satisfactory and reliable measurements in the hypersonic regime. Among the limiting factors are the fact that the flow can be simulated over a very short time (typically milliseconds) requiring fast response measurement techniques. A relatively high data yield is desirable, given the cost of operation of hypersonic facilities. Moreover, the flow properties variation is extremely large as well as the range of flow scales due to the occurrence of shock waves and thin viscous layers associated to the high Reynolds number. In order to understand the origin of the aerodynamic heat loads on a re-entry vehicle it is necessary not only to diagnose the surface temperature of a windtunnel model but also to describe the flow around the model at the origin of the loads. Schlieren and shadowgraph techniques, together with interferometry are currently used as standard techniques for the visualization of high-speed flows 1 . InfraRed Thermography 2 showed to be a suitable technique for the measurement of transient heat loads. Particle Image Velocimetry offers the possibility to quantify the flow velocity field inside a thin plane during a relatively short time interval 3 and is therefore suited to describe not only the flow streamlines, but also compressible features such as shock waves and shear layers including flow unsteady effects and turbulence. The application of PIV in high-speed flows has been pioneered by Moraitis and Riethmuller 4 and Kompenhans and Hocker 5 . However the application of the digital version of the technique has been applied with success in supersonic flows only in the recent years 6,7,8 .The extension of PIV in the hypersonic regime involves that a number of critical issues are addressed, namely the selection of appropriate seeding particles, their dispersion in the flow and the measurement of particle image recordings with large spatial variation in particle image density. The present study proposes a challenging application of PIV to investigate the flow over a double compression ramp configuration in a Mach 7 flow produced with a Ludwieg tube facility 9 . The investigation aims first at the assessment of the PIV technique in the hypersonic limit where extreme compressibility effects are encountered in combination with important technical limitations (hypervelocity, short run time, hot-pressurized storage gas conditions). Technical issues such as the