The measurement of velocity gradients in fluid flow by laser light scattering

The motion of the speckle pattern in the far field limit is uniquely determined by the velocity gradient of the light scattering medium. This is the basic physical effect of a non-invasive method that allows the measurement of velocity gradients with high spatial and temporal resolution down to 75 m and 1 ms, respectively. We present three different measurement schemes for the analysis of the scattered light. The experimental results refer to mean gradients obtained from measurements on turbulent channel flows.