An Investigation on Rotational Augmentation by means of CFD

Abstract

A total of eight 3D CFD simulations of a 10m-radius extruded blade (constant chord), using the S809 airfoil, in pure rotational motion (without vortex recirculation) have been performed at two angular velocities (Ωlow=12.1rpm≈1/3Ωhigh) and four blade pitch angles (from full attached flow, i.e. at AoA 0°, to high and full stall conditions at 15.24°). Two blade radial locations were chosen as r/R=0.3 and r/R=0.5 for the analysis of rotation effects. For this purpose, a set of 2D CFD simulations of both radial locations has been performed at consistent conditions for each blade test case. The flow solver used for the simulations was the DLR TAU code, prescribing the RANS Spalart-Allmaras turbulence model for a steady state case. The Analysis of the results shows a remarkable increase in lift coefficient from the 3D CFD blade simulations w.r.t. the corresponding 2D CFD airfoil simulations, otherwise known as rotational augmentation. These increments specially occur at the most inboard part of the blade and have been found strongly related to the inertial Coriolis force acting on the flow in chordwise direction. Inside the boundary layer at the thickest point of the upper blade airfoil, an overshooting of the chordwise velocities is to be seen in the presence of radial velocities towards the blade tip, both increasing in magnitude with the angle of attack. This increase is precisely proportional to the values of rotational augmentation. For every blade test case, rotational augmentation has also been found inversely proportional to the blade radius, which also agrees with the effects of Coriolis forces after the Coriolis coefficient definition inside the Lindenburg model. The most pronounced increase in lift coefficient at both radial locations is to be seen with the blade in high stall. In these cases, a remarkable increase in radial velocities occurs along with a delay in boundary layer separation as the Coriolis forces increase in magnitude. Important phenomena have also been found such as the presence of radial velocities towards the blade root in the limits of the boundary layer for the test cases below high stall, as well as a significant development of rotational augmentation for the special cases of AoA 0° and full stall.