Application of Particle Image Velocimetry to High-Speed Supersonic Flows in a Shock Tunnel
Particle Image Velocimetry (PIV) has become an important tool in modern flowfield diagnostics, in particular for subsonic flows. PIV systems have also been used to measure supersonic flows in supersonic wind tunnels, but only recently it has been shown that PIV can also be a valuable tool for high-speed supersonic flows in a shock tunnel (Haertig et al., 2001). At the ISL shock tunnel a PIV system was installed and tested (Fig. 1). A shock tunnel is a transient wind-tunnel facility with short testing times of a few milliseconds and high supersonic flow velocities in the km/s range. The short flow duration time makes a high information density highly desirable. An application of PIV to such kind of flows is considerably more difficult than to a continuous flows, however, because no online adjustments of the particle seeding and the optics can be done. In this publication, PIV measurement results obtained for flows at Mach numbers of 4.5 and 6 with velocities of up to 1.8 km/s are presented. We measured the freestream flow of nozzles, supersonic wedge flows with an attached shock wave, and the supersonic flow around a sphere with a detached shock wave. To obtain a good spatial resolution a laser pulse separation of less than 0.5 μs is necessary. Solid particles (TiO2 and Al2O3) with a nominal diameter of about 0.3 μm were used for the seeding. The measured particle relaxation across shock waves was found to be consistent with theoretical estimations. Because of the strong particle density gradients in shock waves, the PIV correlation algorithm can contribute to considerable inaccuracies, however. Fig. 1. ISL shock-tunnel facility with PIV system.
Overview of PIV in supersonic Flows
The extension of particle image velocimetry to supersonic and hypersonic wind-tunnel flows has been achieved in the last decade. This was mainly possible with the advent of short interframing-time CCD cameras with temporal resolution allowing to obtain correlated particle images at flow velocities exceeding 1000m∕s. The most challenging aspects of PIV experiments in supersonic flows are still recognized as the seeding-particle-selection and seeding-distribution techniques. Also, the optical access for illumination and imaging require a specific attention since pressurized facilities offer limited optical access. The presence of shock waves in supersonic flows introduces regions where particle tracers slip with respect to the surrounding flow. Moreover, the particle seeding density becomes strongly nonuniform and particle-image blur can occur as a result of the strong refractive index variations. The present chapter reviews the physical and technical problems of PIV experiments and discusses the potential of such techniques on the basis of recent experiments performed in high-speed wind tunnels: double compression ramp at Mach 7 and shock-wave turbulent boundary interaction at Mach 2.
Millimeter-Wave Radar Clutter
Backscatter and attenuation theory clutter characteristics detection of targets immersed in clutter.