A particle tracking system for wake survey

Harris, Carl John Joseph (1993) A particle tracking system for wake survey. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis presents the results of a study undertaken to evaluate the feasibility of a low-cost Particle Tracking Velocimetry system to measure two dimensional steady flow vectors. Emphasis was placed on the suitability of the technique for ship propulsion research. -- Experiments were conducted in a cavitation tunnel. Flow illumination was provided by an air-cooled Argon-ion laser with a measured output of 30 mW. An oscillating mirror was used to redirect and spread the laser light into a sheet. The water was seeded with 40 μm Aluminium-Silicate micro-spheres and particle images recorded using a standard Charge Coupled Device (CCD) video camera. High frequency sweeping of the laser beam produced multiple exposures within a single video field. This allowed the system to be used with flow speeds beyond the limit normally imposed by the standard 30 Hz. video framing rate. A desktop computer with a frame-grabber board was used to digitize selected video fields. Particle tracks were analyzed individually using off-the-shelf image analysis software. Velocity was computed by dividing the measured length of the track by the shutter speed of the camera. -- Experiments were carried out in four phases. Measurements were made in uniform flow and found to be accurate and repeatable to within a single pixel. This gave a resolution of 0.072 m/s for a flow speed of 1.656 m/s and a 140 mm x 110 mm field of view. Experiments were conducted to determine the flow field around a smooth cylinder at a Reynolds number of 72,700. The results showed good agreement with potential theory in the region upstream of the separation point. -- To demonstrate applicability of the work to ship propulsion research a set of experiments were done to measure the simulated nominal wake in the region behind a model ship skeg. The static thrust and torque of a four bladed B-screw propeller, both with and without the skeg, was measured in the cavitation tunnel. The effect of the skeg on propeller performance has been evaluated and discussed. -- For the case of no skeg, a comparison is made between the experimental results and predictions of performance as given by lifting surface theory. The experimental values of thrust and torque were 22% and 12% higher respectively than what was predicted by lifting surface theory assuming a uniform wake. -- For the case with the skeg the lifting surface theory, using as input the wake determined by the particle tracking experiments, showed qualitative agreement with experimental measurements in predicting increases in blade thrust and torque in the stalled flow behind the skeg.

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