Measurement of the Velocity Gradient Tensor in Turbulent Flows

This article reviews the principal experimental methods currently available to simultaneously measure all the terms of the velocity gradient tensor of turbulent flows. These methods have been available only for a little more than 20 years. They have provided access to the most fundamental and defining properties of turbulence. The methods include small, multisensor, hot-wire probes that provide single-location, time-resolved measurements of the tensor and various optical arrangements, most of which provide the tensor information over a larger spatial extent but, in most cases, without resolution in time. Data-reduction algorithms, spatial-resolution issues, and the use of Taylor's hypothesis are addressed in evaluating the accuracy of the various methods.