Large eddy simulation of vortical flow in a half-ducted propeller fan

Structure and unsteady behavior of the vortical flow in a half-ducted propeller fan, in which only a rear part of a rotor is covered by a shroud, have been investigated by a large eddy simulation (LES) with a subgrid-scale model of Smagorinsky. The vortical flow structure is visualized by a semi-analytic method for identifying vortex cores based on the critical-point theory. The simulation shows that the tip vortex plays a major role in the structure and unsteady behavior of the vortical flow in the half-ducted propeller fan. The tip vortex starts to be formed on the blade tip suction side near the midchord and grows nearly in the tangential direction, thus impinging on the pressure surface near the tip of the adjacent blade. The spiral-type breakdown of the tip vortex occurs near the midpitch, so that the tip vortex twists and turns violently with time. The very large movement of the tip vortex causes the high pressure fluctuation in the interference region between the tip vortex and the main stream.