Rotating three-dimensional velocimetry

Flow evolution over helicopter rotors, wind turbine blades, and insect wings are unsteady, three-dimensional (3D), and influenced by phenomena unique to the rotating frame of reference (FoR), e.g., Coriolis and centrifugal forces. Conventional 3D-PIV techniques are unable to fully characterize these rotating FoR physics, since the measurements are limited to a fixed FoR of a relatively small volume through which the rotor blade or wing traverses intermittently. In this paper, a new “Rotating Three-Dimensional Velocimetry (R3DV)” technique is proposed to address these gaps. R3DV consists of 3D measurements made with a single stationary plenoptic camera in combination with a hub-mounted mirror that aligns the camera’s field of view with a rotating wing. In post-processing R3DV data, a rotational volumetric calibration method is developed to account for image acquisition through a rotating mirror. Rotating FoR volumes are then reconstructed using the Multiplicative Algebraic Reconstruction Technique (MART) algorithm with the adapted calibration scheme and subsequently cross-correlated to derive a 3D velocity field. R3DV was experimentally demonstrated in a study of 3D unsteady flow over an impulsively rotated flat-plate wing. Prominent flow features like the formation and shedding of the primary and secondary leading-edge vortices (LEVs) were observed, which corroborate well with the existing literature on rotating wings. The time-resolved variation of LEV velocity profiles and circulation with azimuthal angle exhibited expected trends. The ability to quantify 3D and time-resolved velocity fields in the rotating FoR demonstrates the feasibility of adopting R3DV as a technique to investigate rotating flows. Graphic abstract