Instantaneous planar measurements of velocity and large-scale vorticity and strain rate in an engine using particle-image velocimetry

Particle-image velocimetry (PIV) has been used in an engine to produce a virtually continuous two-dimensional velocity-vector map over a 12 × 32 mm area. The particle-seeded flow field in the clearance volume of a motored engine (600 r/min, 8:1 compression) was illuminated by a double-pulsed sheet of laser light (20-40μs pulse separation) oriented parallel to the piston. The illuminated particles (<1μm) were photographed at 78deg BTDC compression and 12deg ATDC with 1 × magnification, resulting in paired particle images separated by distances ∼200μm. The two-dimensional velocity distribution was determined by interrogating 0.9-mm square spots on a 0.5-mm grid spacing. The average particle image-pair displacement within each interrogation spot was determined by performing a spatial correlation, and thus the magnitude and direction of the average velocity within the interrogation spot was inferred from the light-pulse separation. Valid velocity data were obtained within 500μm of a surface. The PIV velocity fields were in general agreement with expectations based on ensemble-averaged laser-Doppler-velocimetry (LDV) data from the engine. The spatial resolution achieved was adequate to resolve the few-mm turbulence integral length scales that exist near TDC in engines. High-pass spatial filtering of the velocity-vector plots revealed eddy-like structures on the scale of 1-5 mm. Closely corresponding features were found in the large-scale vorticity fields evaluated from the PIV data. The large-scale strain rates in some instances appeared sufficient to affect the burning rate of lean flames.