Behavior of turbulent spots when they are placed in a turbulent environment is investigated through wind-tunnel experiments. The turbulent spot, introduced by a short-duration emission of a jet from a small hole in the flat plate, grows downstream and moves into a turbulent environment. The turbulent environment is created by either a trip wire or an area of rough surface on the plate surface. Experimental results obtained by applying conditional sampling technique show the spreading of a low velocity region in the spanwise direction at locations away from the wall. The amount of spreading is affected. by the method used for tripping the boundary layer. It is also shown that there is a low velocity region near the center of a turbulent spot, which grows in the vertical direction as it is convected downstream, but does not grow in the spanwise direction. The results indicate the existence of an inclined vortex-pair structure near the center of a turbulent spot.

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Turbulence measurements of open-channel flows with vegetated zone at a half channel width were conducted by making use of a two-component laser Doppler anemometer (LDA). It was found that the intensity of secondary currents and the turbulence energy increase with increase of the Froude numbers. The turbulence is advected spanwisely near the free surface by the secondary currents, which are generated by surface vortices. The horizontal vortices near the free surface are generated by shear instability which increases with an increase of the. Froude numbers. The wall shear stress is calculated by the equation of motion with the aid of an empirical drag coefficient in vegetated zone. The calculated wall shear stress agrees well with the experimental values.

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