Measurement of velocity and velocity derivatives based on pattern trackingin 3D LIF images

Pattern tracking in consecutive 3D LIF images based on least squares matchi ng (LSM) of grey levels has been developed recently for velocity and veloci ty gradient measurements. The shortcomings of this method are dearly shown. The present article presents an improvement:on:this method by introducing a local multi-patch (LMP) technique through the LSM approach. The method is validated using the flow field of a turbulent channel flow obtained by dir ect numerical simulation (DNS) and a synthetic image with grey-level patter ns. The results show that LMP matching allows the determination of the velo city and the velocity gradient fields with high accuracy including the seco nd derivatives. Measurements of a round non-buoyant jet are presented which demonstrate the-good performance of the method when applied under laborato ry conditions. This method can also be applied on two-dimensional images pr ovided that the flow is strictly two-dimensional.