Hypersonic flow diagnostics
Filtered Rayleigh scattering (FRS) is a useful and proven tool for flow diagnostics, and a key aspect of this technique is that no seeding of the flow is needed. Laser Rayleigh scattering is obtained by elastic scattering of molecules and nanoscale particles. Measurements are made with a high power, short pulse laser that can freeze a flow field, in time, and generate quantitative images of density structures. In this technique, a molecular filter is placed in front of a detector, typically an intensified charge coupled device (CCD) camera or a photomultiplier tube, and the filter is used to modify the frequency spectrum of light scattered and is used to suppress unwanted background scattering, or to make quantitative measurements of flow properties and their spatial distributions. MetroLaser performed measurements in an oblique shock layer from a 2-D wedge at an approach flow Mach number of 4. The diagnostics, employing filtered Rayleigh scattering (FRS), is used obtain density and the measured rise in density across the oblique shock demonstrated that this technique is capable of obtaining quantitative measurements of density. This flow has several practical applications in high speed vehicle flight and propulsion systems and we use this flow to demonstrate the capability of the FRS technique to diagnose a complex flow field.