TAYLOR HYPOTHESIS AND 2-POINT COHERENCE MEASUREMENTS

Experimentally obtained time coherence has traditionally been interpre ted as streamwise one-dimensional spatial coherence through Taylor's h ypothesis. We calculate corrections to the high-wavenumber part of the coherence to account for the errors caused by the deviation from Tayl or's hypothesis in high-intensity turbulent flows. The small-scale tur bulence is assumed to be frozen and convected by a fluctuating convect ion velocity. Both Lumley's two-term approximation and the Gaussian ap proximation are used in the calculations. In general, we find that the coherence for cross-stream separations is significantly overestimated by the direct use of Taylor's hypothesis, the error increasing with w avenumber; that for streamwise separations is underestimated. The anal yses are compared with cross-stream coherence measurements in the atmo spheric surface layer. Our results indicate that predictions from Luml ey's approximation yield better agreement with experimental data for c ross-stream separations than those from the Gaussian model. Our study suggests that reliable measurement of two-point spatial coherence can be achieved only for scales not too small compared to the sensor separ ation.