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, uncertainty in the effective positions of velocity vectors, intense deformation of tracer patterns, and laser reflection. We consider here a modified form of the Particle Image Distortion (PID) method todirectly calculate velocity gradients at a fixed wall, and refer it as “PIV/IG” (“Interface Gradiometry”). Results from synthetic 2D PIV images suggest our method achieves higher SNR and accuracy than velocity differentiation. Also, we have developed a procedure to reconstruct three-dimensional velocity gradient at a fixed wall the two non-zero components from PIV/IG data obtained in stereo views; 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 underlines the importance of adequate tracer diameter in the images, sufficient seeding density, and of dynamic range of the camera sensor.
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Chuong V. Nguyen: He received his B.Sc. in Aeronautical Engineering in 2001 from Ho Chi Minh City University of Technology, Vietnam. In 2003, he received his M.Sc. in Civil and Environmental Engineering from Ritsumeikan University, Japan. Currently he is a Ph.D candidate in the Department of Mechanical Engineering, Monash University, Australia. His research interests are PIV and Holographic PIV for wall-bounded flows.
John C. Wells: He obtained a Doctorate in Mechanics from the University of Grenoble I, France, and has worked at Ritsumeikan University as an Associate Professor in the Department of Civil and Environmental Engineering since 1999. His research interests are wall turbulence and sediment transport micromechanics.
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Nguyen, C.V., Wells, J.C. Direct measurement of fluid velocity gradients at a wall by PIV image processing with stereo reconstruction. J Vis 9, 199–208 (2006). https://doi.org/10.1007/BF03181763
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DOI: https://doi.org/10.1007/BF03181763