Affiliations: Department of Civil & Environmental Engineering,
Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan. E-mail:
jwells@se.ritsumei.ac.jp | Currently Department of Mechanical Engineering, Monash
University, Clayton, Victoria 3800, Australia
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 to directly
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.
