On the Interpretation of Space-Time Correlation Measurements in Shear Flow

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The problem of deducing the convective velocities for the turbulent fluctuations from cross correlation measurements using two hot wires is re-examined. It is shown that for large scale motions the path of the maximum space-time correlation will, in general, deviate from the local mean streamline because of the distorting effect of the mean shear on the turbulent vorticity field. Calculations are presented to show that this effect can account for the shapes of the curves of maximum correlation in a boundary layer as reported by Favre some years ago. These same considerations would indicate that the convective velocity of the large scale disturbances should be measured along the lines of maximum correlation rather than along the streamlines. The deduction of the general form of the eddies from the space-time correlation measurements is also examined. In the case of two hot wires set on a line orthogonal to the surface, the negative optimum delay for maximum correlation is shown to be consistent with the presence of eddies aligned in a downstream direction at a relatively small angle to the surface.