A compressible Reynolds-averaged Navier-Stokes solver is used to simulate a hovering miciorotor operating at a very low tip Reynolds number in ground effect, with the primary objective of demonstrating its capability to provide good flowfield predictions for a hovering rotor operating close to the ground. The simulations are done using a system of overset meshes that are strategically placed to accurately capture the formation and evolution of tip vortex to sufficiently long duration. The computations are validated with experimentally measured mean thrust and power This is then followed by a detailed validation of the tip-vortex flowfield with the experimental particle-image-velocimetry data. In addition, the computational-fluid-dynamics (CFD) data are used to explore the details of wake characteristics, wake trajectory, and groundwake interactions at various rotor heights. The following are specific observations/conclusions that can be drawn from the present work: (1) The computed rotor thrust and power coefficients show good comparison with the available experimental data. With decreasing rotor height above the ground, the thrust increases at a nominally constant power. (2) The phase-averaged azimuthal-vorticity contours for different wake ages show good comparison with the experimental results in terms of wake trajectory and vortex strength. An instantaneous snapshot of the azimuthal-vorticity contour shows that the vortices are strong even after six blade passages for h/R = 1.5. (3) Predicted time-averaged and phase-averaged radial-velocity profiles at several radial locations show reasonable correlation with the experimental data for all rotor heights. CFD predicts a marginally higher maximum radial velocity and at a slightly larger wall distance than that measured in the experiment. (4) Flow visualization with the aid of q-criterion and analysis of wake trajectory shows large amount of instabilities developing in the tip vortices at later wake ages. The wake shows increased scatter for larger rotor heights; however, the rms deviations of the trajectory from the phase-averaged radial and vertical positions are smaller at larger rotor heights. (5) The tip vortices decay rapidly in the early wake ages, but they decay at a much slower rate as the wake expands near the ground. The tip vortices for smaller rotor heights show faster decay compared to that for larger rotor heights. (6) The interaction of the tip vortex with the ground is observed for all rotor heights. The flow is found to be highly aperiodic. The increased induced velocity from the tip vortex causes the jet flow on the ground to separate and form a separation bubble. The separation bubble formed at one wake age is seen to peel off at a later wake age, forming an opposite-sign vortex. (7) Eddy-viscosity contours show that the turbulence levels near the ground increase with a decrease in rotor height. Ground-friction velocities are seen to be higher directly below the tip vortices and are larger for smaller rotor heights. The current work is an initial study to understand the flow physics of a rotor operating in ground effect. As a continuing work, the current authors are performing studies to simulate the full brownout problem by coupling the solution obtained from the present simulations to a sediment tracking code. These simulations are expected to provide further insight on how various flow physics discussed in this paper impact the brownout problem.
Detailed computational investigation of a hovering microscale rotor in ground effect
Detaillierte numerische Untersuchung eines schwebenden, mikroskaligen Rotors unter Bodenffekt
AIAA Journal ; 51 , 4 ; 893-909
2013
17 Seiten, 22 Bilder, 2 Tabellen, 31 Quellen
Aufsatz (Zeitschrift)
Englisch
Detailed Computational Investigation of a Hovering Microscale Rotor in Ground Effect
Online Contents | 2013
|Computational Investigation of Micro Hovering Rotor Aerodynamics
British Library Conference Proceedings | 2006
|Computational investigation of micro hovering rotor aerodynamics
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