Influence of rotational accelerations on the transient lift generation and leading-edge vortex dynamics

Abstract

This dissertation aims to experimentally investigate the influence of rotational accelerations on the transient lift generation and flow field dynamics over a rotating wing. The research focuses on three specific objectives: (i) Understanding the influence of rectilinear acceleration on lift generation, (ii) Examining the effects of Euler acceleration on the transient dynamics over a rotating wing, and (iii) Decoupling the effects of Euler and Coriolis acceleration on the transient dynamics. Results show that the maximum lift and flow field characteristics, such as leading-edge vortex~(LEV) growth rate and separation azimuth, depend on both wing acceleration and steady-state velocity. Consequently, a new scaling parameter is introduced, defined as the ratio of Euler to convective acceleration. Based on the variation of the maximum lift coefficient with the scaling parameter, two distinct regimes are identified: (i) "Quasi-steady regime," where the maximum lift coefficient shows minimal variation and the dynamics are driven by significant secondary vorticity generation; and (ii) "Acceleration-dominated regime," where maximum lift coefficient increases with an increase in the scaling parameter, and secondary vorticity does not influence the LEV dynamics. Similar to the increase in Euler acceleration, decreasing Coriolis acceleration also leads to an increase in transient lift, primarily due to the increase in both non-circulatory and circulatory lift. The reduction in spanwise vorticity flux, coupled with enhanced shear layer vorticity transport, results in a higher LEV growth rate and, consequently, higher circulatory lift as Coriolis acceleration decreases. These findings have provided novel insights into the role of rotational accelerations in dictating the dynamics over an accelerating and rotating wing, which will be useful for researchers designing bio-inspired micro air vehicles and developing unsteady aerodynamic models.