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.
Tomographic PIV investigation of roughness-induced transition in a hypersonic boundary layer
The disturbance generated by roughness elements in a hypersonic laminar boundary layer is investigated, with attention to its three-dimensional properties. The transition of the boundary layer is inspected with tomographic particle image velocimetry that is applied for the first time at Mach 7.5 inside a short duration hypersonic wind tunnel. A low aspect ratio cylindrical roughness element is installed on a flat plate, and experiments are conducted downstream of the element describing the mean velocity field and the turbulent fluctuations. Details of the experimental procedure needed to realize these measurements are discussed, along with the fluid dynamic behaviour of the perturbed hypersonic boundary layer.
PIV Investigation of Role of Boundary Layer Velocity Fluctuations in Unsteady Shock-Induced Separation
Particle image velocimetry measurements have been made in conjunction with fluctuating wall pressure measurements in a blunt fin-induced interaction in a Mach 5 flow. Results show that the separation shock foot motion correlates with the velocity fluctuations in the lower part of the boundary layer: positive fluctuations are associated with downstream motions of the shock foot, whereas negative fluctuations are associated with upstream motions of the shock foot. These results are consistent with an earlier study in the same facility with a compression ramp model.