Publications Details
The influence of velocity gradients on PIV measurements of turbulence statistics: A preliminary study
Error in Particle Image Velocimetry (PIV) interrogation due to velocity gradients in turbulent flows was studied for both classical and advanced algorithms. Classical algorithms are considered to be digital cross-correlation analysis including discrete window offsets and, for the present work, advanced algorithms are those using image deformation to compensate for velocity gradients. Synthetic PIV simulations revealed substantial negative biases in the turbulent stress for classical algorithms even for velocity gradients within recommended PIV design limits. This bias worsens if the distribution of velocity gradients has a nonzero mean, and error in the mean velocity may be introduced as well. Conversely, advanced algorithms do not exhibit this bias error if the velocity gradients are linear. Nonlinear velocity gradients increase the error in classical algorithms and a significant negative bias in the turbulent stress arises for the advanced algorithm as well. Two experimental data sets showed substantially lower turbulent stresses for the classical algorithm compared with the advanced algorithm, as predicted. No new experimental design rules for advanced algorithms are yet proposed, but any such recommendation would concern second-order velocity derivatives rather than first order.