Study on the Performance of a Cycloidal Rotor in Hover at Different Reynolds Numbers

This paper studied the influence of the Reynolds number on lift, power, and efficiency of a three-bladed cycloidal rotor in hover. In a parameter study, Reynolds numbers from 10,000 to 600,000 were investigated. The fluid mechanics were solved using incompressible 2D unsteady Reynolds-averaged Navier–Stokes (URANS) computational fluid dynamics (CFD). The CFD model was first carefully validated using experimental data for a pitching airfoil undergoing dynamic stall. The model was then verified to reproduce the shapes of both the force and power curves and the wake flow of a landmark cyclorotor experimental study. Two different flow regimes were identified: for the first regime Re ranged from 10,000 to 100,000 and two dynamic flow separations occurred, the first at maximum pitch angle and bottommost position of the blade and the second shortly afterwards. The second flow regime had Re from 200,000 to 600,000 and avoided the first separation due to the increased Reynolds number. Both separations were visible in the flow as well as in the azimuthal lift distribution. Regardless of the flow regime, rotation averaged lift and power followed the predictions of momentum theory, except for Re of 500,000. At this Re, a significant drop in power was observed and corresponds to findings of a 20-year old experimental study which were at the time classified as possibly wrong. This particular behavior at Re of 500,000 is explained by a change in shape of the downwash, which avoids flow separation as the blade starts to travel upwards.