A quantitative Visualization tool for large wind tunnel experiments

Abstract

The development of a measurement system to visualize complex flows in large scale wind tunnel tests is described. The method is based on the three-dimensional reconstruction of tracer path lines. This approach allows to visualize and quantify complex three-dimensional and time dependent flows in a full three-dimensional measurement volume. The objective is to develop a system practicable for wind tunnel test. The set-up complexity and the measurement effort must therefore be kept as simple and low as possible. Ideally the method allows for an on-line quantitative visualization of the flow. This requires simple and effective algorithms to keep the computational time low. Two cameras and a continuous illumination of the entire measurement volume are used to record the flow tracers. The camera shutter is therefore set to the maximum exposure time (∼1/frame rate) to produce consecutive images, in which a moving particle leaves a continuous string of connected path segments, forming a complete path line. The corresponding path lines from both camera views are used to reconstruct the path line in three dimensions. The reconstruction consist of the following steps. A reference image is subtracted from the actual image to enhance the contrast, the image is filtered to reduce the residual noise and a threshold operation is applied for the image segmentation. The path segments are identified based on the necessary connectivity condition inbetween frames; the follow-on segment in the current frame must be connected with the segment in the preceding frame. The two endpoints of the identified path segment are extracted and the boundary pixels are corrected for the distortion caused by the camera. After no follow-on segment for a path line can be found, the path centerline is approximated with a cubic spline. Corresponding path lines from both camera views are found based on the epipolar condition applied to the endpoints. Thereafter the path line is reconstructed pointwise in three dimensions and a three-dimensional cubic spline is calculated as path line representation. Thereafter the information inherent in the reconstructed path lines is extracted. The path segment length in conjunction with the exposure time gives an estimate on the flow velocity and the shape of the path line gives information on