Free-surface flow structures of space-time correlation of coherent vortices generated behind dune bed

3-D space-time structures of the separated and kolk-boil vortices behind dune were discussed by evaluating conventional correlation coefficient and also by using conditional sampling techniques. Instantaneous velocity-velocity combinations at different points were simultaneously measured by means of 2 sets of fiber-optic laser Doppler anemometers and an automatic traversing system. The refined and physical models of coherent vortices were proposed in the present study.