Combining Simultaneous Density and Velocity Measurements of Rotor Blade Tip Vortices under Cyclic Pitch Conditions

An investigation on blade tip vortices of a sub-scale rotor under cyclic pitch conditions is carried out. Background oriented schlieren (BOS), Particle image velocimetry (PIV), and computational fluid dynamics (CFD) are applied to the same test cases. This approach enables to combine the velocity data from PIV in a measurement plane, the density related data from BOS in a measurement volume and the comprehensive set of flow variables provided by unsteady detached eddy simulations. Vortices up to an age of Ψv = 70◦ in case of PIV and CFD, and up to Ψv = 200◦ in case of BOS are considered. The vortex locations are obtained from all three techniques. The unsteadiness of the vortices was obtained from the experimental results, whereas the CFD provides an average solution. An increased position scatter was observed during the downstroke of the pitch cycle by both experimental methods and in good agreement. In the second part, the PIV velocity data are compared to common vortex models. A link between the density distribution and the swirl velocity is applied to the measured data. Based on the CFD results it is shown that the assumption of isothermal flow yields better agreement between velocity and density than isentropic flow