Boundary layer forcing on a rotating wing at low Reynolds numbers

Two separate experiments were conducted on a three-bladed NACA0012 rotor operating at a blade tip Reynolds number ranging from 44–110 \(\times 10^{3}\) using phase-locked infrared thermography (IRT) coupled with force and torque measurements. The first experiment consisted of a parametric study on the impact of forcing boundary layer transition using roughness placed on the suction side of the aerofoil in a hover configuration. The roughness height varied from 52 to 220 \(\upmu\)m and was placed all at 10% chord over the entire span of the blade. Force and torque measurements confirmed a roughness height that could lead to a performance increase due to the suppression or reduction of a laminar separation bubble. Moreover, IRT measurements showed the formation of turbulent wedges behind the roughness elements at critical roughness Reynolds numbers based on empirical correlations from the literature. The second set of experiments investigated the effects of freestream turbulence (FST) on the performance and flow development of the same rotary wing in an advancing configuration. FST was generated in an open section wind tunnel using grids and was characterised using hot-wire anemometry. When the rotary wing was subjected to FST, an increase in thrust and efficiency was observed, which could be due to the FST suppressing laminar flow separation by inducing early transition since IRT measurements indicated an advancement of the transition region, confirming performance improvement with earlier transition, where the excrescence drag due to the roughness elements would not be present in the freestream turbulence forcing case.