On the detection of coherent structures in turbulent flows

Experimental techniques for the detection of coherent structures in turbulent water flows are evaluated comparatively. The structure of wall-bounded and free turbulence is characterized theoretically; several Eulerian detection techniques (including an autocorrelation method and a second-quadrant analysis method similar to that of Lu and Willmarth, (1973) and a combination of hydrogen-bubble visualization and laser Doppler anemometry (LDA) are applied to boundary-layer, pipe, channel, and axisymmetric jet flows; and the results are presented in graphs and photographs and discussed. In general, the Eulerian methods are found to lack objectivity and to be of very limited appropriateness for detecting coherent structures. The second-quadrant technique, for examples, gives some results which correlate well with visual/LDA findings, but only for ejections, some bursts, and structures in the mixing-layer and outer development regions of jet flows.