Material Derivative Measurements in High-Speed Flows by Four-Pulse Tomographic PIV

Material Derivative Measurements in High-Speed Flows by Four-Pulse Tomographic PIV

Lynch, K.
Scarano, F.

2013-07-03

A tomographic PIV system is introduced for the instantaneous measurement of the material derivative of velocity (VMD). The system is able to operate with very short temporal separation and is therefore suitable for applications in high-speed flows. The method of operation consists of the imaging of a measurement volume using an array of 12 CCD cameras and two double-cavity laser systems. Four independent recordings of particle images are captured by decomposing the system into three separate tomographic PIV systems comprised of four cameras. A discussion is made that compares the present working principle with other methods used to separate the light scattered from multiple pulses, namely by polarization. Various approaches are compared to determine the optimal utilization of four-pulse data to measure the VMD: the Eulerian and Lagrangian schemes are compared with the recently introduced fluid trajectory correlation (FTC) technique from the authors (Lynch and Scarano, 2013). The comparison focuses on the behavior of the schemes with respect to truncation errors and how the error estimates for four-pulse data are modified from those typically applied to image sequence data from a time-resolved PIV experiment. The analysis of synthetic images of a translating vortex clearly shows the envelope of applicability of the different schemes and the structure of the measurement errors introduced by truncation. The 12-camera tomographic system in four-pulse configuration is employed to measure the wake of an axisymmetric truncated base with an afterbody at a Reynolds number of 68,000. The system calibration accuracy and the baseline measurement uncertainty of the velocity are evaluated by performing a test with a negligible time delay between the independent tomographic PIV systems. The comparative performance of the material derivative schemes is estimated by appealing to a physical property of the material derivative field. The results indicate that a 12-camera system can be employed for material derivative evaluation using a variety of estimation schemes. Among these schemes, the FTC technique is found to be the least susceptible to the growth of truncation errors and is thus suitable for measurement at large temporal intervals which are necessary to suppress random errors.

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Conference proceedings

conference paper

(c) 2013 Lynch, K.; Scarano, F.