|M.Sc Student||Yam Maor|
|Subject||Wind-Tunnel Investigation of Two Dimensional Sails|
|Department||Department of Aerospace Engineering||Supervisors||Professor Rimon Arieli|
|Dr. Baruch Karlean|
|Full Thesis text - in Hebrew|
Sails are used to provide motive power to maritime vessels by creating lift and drag. Inertial forces acting on sail elements are not negligible compared to forces imposed by the fluid, and significant shape changes occur due to flow conditions. Two-dimensional flexible sail-wings were investigated in a wind-tunnel at 20 m/s. The purpose of the work was to evaluate the applied data acquisition and processing method, and to analyze the sail-profile shape for four cloth-slack and three cloth-permeability values at pitch-angles between 0° and 23°. All sail types were tested without changes in the setup. Integral lift, drag and pitch moment acting on a model were measured using a tiltable rigid frame on a tripod balance and then converted to aerodynamic coefficients. Measurements accuracy and repeatability were suitable for comparative investigation.
Sail-profile at mid-span was illuminated and acquired using a high-speed video camera. Calibrated projective transformation obviated the need for a geodetic survey and for exact camera parameters estimation. Profile image was extracted from each frame of each run and processed by in-house custom-made software. Estimated profile shape resolution was better than 0.05% of chord. Average profile for all frames in each run was calculated and subtracted from that set. Setup design and processing software proved to be accurate, fast, robust, and economical.
Three regions of sail behavior were identified: small pitch angles with loose sail, random shape changes, and high measurement noise; mid-range pitch angles with taut sail, small vibrations, and low measurement noise; high pitch angles with taut sail, a distinct standing wave around the average profile, and high measurement noise.
Vibration amplitude and frequency varied with flow velocity. Increased cloth-permeability decreases lift and pitching-moment, also increases drag. It also shifts the center of pressure and the maximum camber location backwards, and diminishes the ‘standing-wave’ phenomenon, probably as seepage flow disrupts the structure of vortex shedding. Increase in cloth-slack increases the value of all aerodynamic coefficients, shifts both the center of pressure and the location of maximum camber backwards, and enhances the dynamic phenomena.
A fixed ratio between the standing wave frequency and the flooding flow velocity, meaning a constant Strouhal number, implies vortex shedding over a static geometry, and sets the relation of cause-and-effect between vortex shedding and standing wave vibrations.
Future research should require better resolution of pitch angles, quantifying levels of cloth permeability, and applying methods of flow visualization.