Abstract
An investigation into 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 allows 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 varPsi _{{text {v}}}= {70}^circ in case of PIV and CFD, and up to varPsi _{{text {v}}}= {200}^circ in case of BOS are considered. The vortex locations are obtained through all three techniques. The unsteadiness of the vortices was obtained by the experimental results, whereas CFD provides an average solution. An increased position scatter was observed during the downstroke of the pitch cycle with both experimental methods and was found to be in good agreement. In the second part, the PIV velocity data are compared to common vortex models. An approach to link 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.Graphic abstract
Highlights
Blade tip vortices are a dominant feature of rotor wakes
Even with a much larger field of view (FOV), it is not possible to derive a reliable value for Γ∞ e.g., due to the influence of the connected shear layer from the rotor blades (Goerttler et al 2020)
Vortices from a sub-scale rotor were investigated by means of Background oriented schlieren (BOS) and particle image velocimetry (PIV) measurements
Summary
Blade tip vortices are a dominant feature of rotor wakes. They form a complex, helical structure and are the cause for many undesired effects: Their interaction with blades or other vortices can lead to increased acoustic emissions, trigger stall on the blades, or may give rise to vibrations in the rotorcraft.Older measurements used visualization techniques (Gray 1956) or point wise methods as, for example, laser doppler velocimetry measurements (Scully and Sullivan 1972). Blade tip vortices are a dominant feature of rotor wakes. A major advancement was achieved by the use of particle image velocimetry (PIV), which is a technique to measure velocities in a plane or even in a volume. Light ray through vortex center Shift on zD zA x sensor zB z. The deflection, in combination with the geometry of the BOS setup, leads to an apparent shift sx of the background pattern on the image sensor: sx = f zD zD + zA − f x = S x. A shift toward the vortex center is present on the image sensor. The light ray passing through the center of the vortex is not deflected since no gradients perpendicular to its path occur. The apparent shift in the measurement images can be computed by a correlation of each image with an undisturbed reference image of the background
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