Adaptive-Time-Step High-Frame-Rate Particle Image Velocimetry

Abstract

With the availability of high-frame-rate (HFR) PIV systems, it is possible to capture time series of particle images at rates which can exceed the necessary frequency to temporally resolve essential flow. In this case, some of the recorded frames can be skipped, in order to obtain an adequate time step between two images, leading to favorable values for pixel displacement. In this paper a technique for decreasing pixel locking in complex flows based on an adaptive time step processing procedure is proposed. If PIV data are taken at much higher frequency than needed, the processing can be repeated for multiple time steps, ensuring the optimal pixel displacement for every interrogation spot. A smaller time step is used for large velocities and larger time steps are used in the areas of low velocity. To illustrate this technique, a sample data set was acquired of the flow on the suction side and the region just downstream of a NACA 0015 hydrofoil at a high angle of attack (16 degrees), ensuring a wide range of velocities within the PIV field of view. The experiments were performed in the University of New Hampshire High-Speed Cavitation Tunnel – HiCaT, at a comparatively low speed (2 m/s) in order to increase the range of possible time steps for post processing. HFR PIV data were obtained at 1800 frames per second for a field of view of approximately 100 mm × 100 mm. The data were processed multiple times, each time skipping between 0 and 19 frames, resulting in time steps between 1 to 20 times of the original time step between two images. As expected, there is a significant difference between the results for mean velocities with different time step processing, most notably in the region of the suction side of the foil. The RMS velocity fields also show a similar trend. In order to obtain a more accurate representation of the mean flow, the data for each interrogation spot were combined based on the pixel displacement values. The resulting velocity field was compared to that obtained with single-time-step PIV. A “map” was created showing what time step was selected for each interrogation spot. Suggestions were made on what time steps should be selected for the post processing in order to decrease the post processing computational time. A discussion was provided on the feasibility of using the technique for instantaneous velocity fields. Using ATS PIV may be beneficial in flows where both large and small velocities are of importance, e.g., flows with separation.