Turbulent Burst around Experimental Spur Dike

Three-dimensional turbulent flow field was measured around an experimental spur dike by using a micro-acoustic Doppler velocimeter. Time fractions of turbulent burst events including outward interaction, ejection, inward interaction, and sweep were analyzed in each quadrant at the neighbor of the dike before and after the formation of scour hole. Over 80% of burst events near the bed have lower-order of magnitudes for both flat and scoured bed surfaces. Ejections and sweeps are prevalent before the local scour was initiated, and then outward interactions and sweeps are dominant after the equilibrium scour hole was formed. In the scour zone turbulence intensity in the vertical direction has reduced significantly, while it increased in the horizontal direction and facilitated bed load sediment transport. Conditional Reynolds stresses and high-order moments of turbulent velocities were analyzed along the thalweg. The magnitudes of u'-w' and u'-v' pairs were much greater than those of v'-w' pair in the scour zone. Ejections and sweeps were the higher order events contributing to the Reynolds stresses. Since sediment entrainment and transport are closely associated with turbulent bursts near bed, the contributions of high-order events to sediment motion greatly depend on localized turbulence intensity.