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EXTENDING STEREO P.I.V. TO MEASURE VELOCITY GRADIENTS AT A WALL AND A FREE SURFACE

Chuong Vinh Nguyen
Department of Civil & Environmental Engineering, Ritsumeikan University Noji Higashi 1-1-1, Kusatsu, Shiga 525-8577, Japan; Department of Mechanical Engineering Monash University VIC 3800, Australia

Tuy N.M. Phan
Department of Civil & Environmental Engineering, Ritsumeikan University Noji Higashi 1-1-1, Kusatsu, Shiga 525-8577, Japan

John Craig Wells
Department of Civil Engineering Ritsumeikan University 1-1-lNojihigashi, Kusatsu, Shiga, 525-8577 Japan

Ryuichi Nagaosa
Institute of Environmental Management Technology AIST Tsukuba West 16-1 Onogawa, Tsukuba 305-8569, Japan

Abstract

Velocity gradient is typically estimated in Particle Image Velocimetry (PIV) by differentiating a measured velocity field, which amplifies noise in the measured velocities. If gradients near a boundary are sought, such noise is usually greater than in bulk fluid, because of small tracer displacement, intense deformation of tracer patterns, and laser reflection. We consider here the application of Particle Image Distortion (PID) to directly calculate velocity gradients at a fixed wall and at a non-fluctuating free surface. Results from synthetic 2D PIV images suggest this "PIV/IG" technique ("Interface Gradiometry") achieves higher SNR and accuracy than velocity differentiation. Also, we have developed a procedure to reconstruct three-dimensional velocity gradients from PIV/IG data obtained in stereo views. At a fixed wall, these equations simplify considerably thanks to the no-slip condition. Experimental data from the bottom wall of turbulent open channel flow appear to suffer from a form of pixel locking. As with standard PIV, this shows the importance of adequate tracer diameter in the images, and of sufficient seeding density.