A Two-Spot Sensor for Measurement of Dissipation by Means of Laser-Doppler Velocimetry

Abstract

An estimate of the dissipation rate for mechanical energy in a turbulent flow can be obtained by computing the variance of the temporal derivative of the fluid velocity measured at a single point. This technique is not suited to conventional laser-Doppler velocimetry because of inherent lower bounds on noise in the velocity measurements, which biases the dissipation estimates. A two-spot technique, described by George and Lumley, overcomes this difficulty. In this technique, laser-Doppler measurements of velocity are obtained at two points, which are separated in the spanwise direction by a distance larger than the size of the optical scatterers but smaller than the Kolmogorov scale. The covariance of the temporal derivative of the velocities measured at the two points provides an unbiased estimate of the dissipation, because the velocities at the two points are essentially identical, while the two noise records are uncorrelated. An oceangoing sensor based on this technique has been developed, and its success is demonstrated by experiments in the near-boundary region of a laboratory channel flow, where measurements of shear production provide a standard for evaluation of the dissipation estimates.