Tree-dimensional Measurement of Thrbulence Structure in Open Channel Flows by Flow Visualization and Picture Processing

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First, the large scale structure of turbulence in an open channel flow was examined by flow visualization and picture analysis. A horizontal cross-section of the flow was illuminated by a sheet of light and a pair of photographs ware taken in a short span of time using the 35 mm movie camera. Then the illuminated cross-section was quickly shifted upward and another pair of pictures ware taken. 15 pairs of photographs ware taken from the channel bed to the water surface during about 1 second. The photographs ware digitized by a CCD digital camera and velocity vectors ware calculated by multi-step correlation technique. Characteristic features of turbulence structures near the channel bed and in the upper layer ware elucidated based on the three-dimensional distribution of the velocity and the vorticity.Secondly, the turbulence structures of the unsteady smooth open channel flow were examined by flow visualization and image processing. Based on the data obtained by the image processing, the characteristic features of the horseshoe vortex and parallel spiral flows with longitudinal axes ware elucidated.Thirdly, the flow patterns in three types of meandering channels with compound cross-section were examined by flow visualization and image processing. Based on the data obtained by the image processing, the characteristic features of the large scale turbulence structures in the best type of meandering channels with compound cross-section ware elucidated.