Development of a quantitative flow visualization tool for applications in industrial wind tunnels

The development of a measurement system to visualize, classify (based on topological features) and quantify complex flows in large scale wind tunnel experiments is described. A new approach is sought where the topological features of the flow, e.g. stream lines, separation and reattachment regions, stagnation points and vortex lines are extracted directly and preferably visualized in realtime in a virtual wind tunnel environment. The system is based on a three dimensional particle tracking method (3D-PTV) using a stereo arrangement of 2 CCD cameras. A frame rate of 120 frames/s allows measurements at high flow velocities. For the 3D-PTV method an approach is taken where the tracer particles are recorded such that consecutive frames form continuous path lines. The 3-dimensional positions and shapes of the particle path lines are reconstructed by means of the epipolar constraint and the stereo camera model. The particle path segments which contain both velocity and topological information are then analysed to extract the relevant information. Neutrally buoyant helium bubbles are used as tracer particles. Matching the density of the ambient air, the bubbles are ideal flow tracers in this respect.