The coherent structure of the flow in the wall region was investigated by means of a comparative flow visualization method in a turbulent open channel flow.
The experimental results suggest that (1) scale-shaped patterns of three dimensional structure are formed near the upper edge of the linear sublayer by the effect of coherent motions near the wall, (2) low speed streaks lifted from the linear aublayer make a pair in most cases, and the spanwise dimension of the pair is the order of 10-30ν/Uτ.

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