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Automated two dimensional flow visualization and coherent structure recognition

University of British Columbia

This paper describes an efficient method for extracting quantitative data from time sequences of fluid flow images. It also introduces a computer automated method for the identification of coherent structures in turbulent flow fields. This method eliminates subjective manual judgement in the crucial stage of coherent structure recognition. The surface motion on turbulent grid flow (Reynolds number 10") was visualized by recording images of tracer particles on a video tape. Each video frame was then digitized as a binary image using a microcomputer. By tracking and connecting the tracer paths through successive frames using a mainframe computer the flow history was reconstructed. Streak trajectories were then fitted by polynomials to give various flow parameters of interest over desired flow times. In particular the linear and angular velocities were determined as scatter points from which mesh fields were interpolated. Coherent structures were identified by thresholding the field of angular velocity. Using the interpolated mesh fields of linear velocities, each coherent structure was parametrized with the properties of size, average linear and angular velocities, and energy content. The flow dynamics and interactions are then discussed using these structure properties. The system was developed primarily to enhance data recognition for a new model of turbulence based on the energetics of coherent structures. It is also intended as a general technique to be used for other flow visualization and coherent structure studies. Applying the system to the initial stage of grid turbulence, it successfully recognized over 80% of all the coherent structures manually identified. Parameter results using these automatically identified structures were compared with established results and model predictions. Limitation and possible improvement on the present two dimensional system is discussed. Various aspects in extending the system to a three dimensional environment are also presented.

Text

2010-06-20

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http://hdl.handle.net/2429/25883

Physics

University of British Columbia

Graduate