Experimental Study of Oscillating Sd8020 Foil for Propulsion

Abstract The thrust producing performance and efficiency of an SD8020 oscillating foil with a symmetrical saw-tooth angle of attack pitching profile was studied through force and torque measurements, as well as dye flow visualization, in the water tunnel at low Reynolds number of 13,000-16,000. The propulsive efficiency and thrust coefficient of the pitching foil were determined as a function of the Strouhal number, pitch amplitude and angular frequency. A propulsive efficiency of 30% was obtained experimentally at low Strouhal numbers. The flow visualization has revealed different wake patterns at various Strouhal numbers and can be classified into three regimes – a drag wake, a transition wake and a thrust wake. The drag wake consists of a combination of a regular Kármán street and an array of ‘primary’ stop-start vortices, whereas the thrust wake consists of a reverse Kármán vortex street, commonly observed in swimming fish. The transition wake regime, which occurs at approximately 0.2 < St < 0.5, is interpreted as a momentum balanced wake, where the thrust developed by the foil approximately balances its produced drag. This wake was observed to either consist of an inclined vortex street, or a paired vortex pattern. Based on the force and efficiency data collected, increasing pitch amplitude and angular frequency was associated with a decrease in propulsive efficiency and an increase in thrust forces produced. A high efficiency value of 0.3, accompanied by a thrust coefficient of order one is found at a low pitch amplitude of 10°, angular frequency of 0.79 rad/s and Strouhal number of 0.05. This presented the best conditions for thrust production observed at low Strouhal and Reynolds numbers.