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.
Instantaneous laser Doppler velocimeter using a fast wavelength tracking Michelson interferometer.
A new species of laser Doppler velocimeter is presented based on wavelength detection with a two-beam interferometer. A finite optical path difference, Deltaphi, of the interferometer creates a sensitivity of the interference to wavelength changes. By means of a fast feedback system, the phase, Deltaphi/lambda is kept constant. Wavelength changes are immediately answered by changes of Deltaphi generated with a Pockels cell. The Pockels cell voltage represents a continuous signal proportional to the instantaneous velocity component of the scattering object. A time resolution of 1 micros or lower can be obtained. The field of applications includes velocity recordings in gas flows containing tracer particles as well as velocity recordings of moving surfaces. A number of experiments are described demonstrating the performance of the system.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.