Extension of Applicable Flow Velocity Range for High-Speed Impinging Jet by Optical Flow Analysis

Abstract

Laser velocimetry techniques such as Particle Image Velocimetry (PIV) and Laser Doppler Velocimeter (LDV) capture the scattered light from the laser beam on tracer particles added to the fluid. However, these laser velocimetry techniques can be difficult to apply to high-speed fluids, where adding tracer particles and tracking particles are difficult. This study attempts to analyze images of high-speed impinging jet flows by applying the optical-flow image analysis to typical shadowgraph time-series images to calculate the velocity field without tracer particles. In the image analysis of such high-speed fluid phenomena, the spatiotemporal resolution of a high-speed video camera is a major issue. This study investigates the range of flow velocities for which the optical flow image analysis can be applied to high-speed impinging jets in the range of NPR = 1.5 to 2.7, using the latest Phantom high-speed video camera with high spatiotemporal resolution. The optical flow image analysis allows all NPRs to clearly capture the shear layer oscillations of the jet and the advection of vortices caused by the shear layer instability and the secondary vortices near the wall, respectively. Furthermore, the peak frequency of the optical flow associated with the vortex shedding coincides well with that of the wall pressure. These results show the possibility of extracting velocity fields in high-speed flows by the optical flow image analysis without tracer particles based on shadowgraph images.