Characteristics of tip vortex of a low Reynolds number rotor using large eddy simulation*

We perform large eddy simulation to investigate the characteristics of tip vortices trailing from a low Reynolds number rotor at different pitch angles. The rotor consists of two blades featuring a circular-arc cambered profile with no twist along the span. The Reynolds number based on the tip speed and chord length is 34,200. The characteristics of the tip vortex is analyzed by aligning instantaneous flow fields with respect to the identified vortex center to account for the wandering motion of the vortex. During the formation of the tip vortex, separation of the crossing flow from the pressure side to the suction side at the blade tip creates complex flow containing multiple eddies. At a low angle of attack, the interaction of these small-scale vortices is observed inside the tip vortex even farther downstream, resulting in a wide high turbulence area inside the tip vortex. However, when the pitch angle is increased, small-scale vortices in the blade shear layer and at the tip merge into the primary vortex. In this case, the region with high turbulence intensity is confined only inside the core. The result from the snapshot POD method indicates that the dominant mode for the velocity fluctuations has two pairs of counter-rotating vortices. This mode rotates in the same direction as that of the vortex and modifies the core structure in a way that induces asymmetry inside the core.