Computational Predictions of Forces on Autorotors at High Reynolds Numbers

摘要

Autorotors are described as cylindrical bodies that fall through a fluid, due to a body force such as gravity. They are inclined to both rotate and generate a lifting force. Lift results from the vortex induced by the rotation, and is known as the Magnus force. Autorotors can be compared to gliding wings, but with a very different (and typically more inefficient) mechanism for creating the lifting vortex. Rotation can be produced through several forces including, drag, lift, and transient dynamics such as vortex shedding. The lifting force is known to be proportional to the rotation rate. Unfortunately, in most autorotors, the rotation is driven by drag-based forces yielding rather small lifting forces and dismal glide slopes. If lift-based forces were used to instead drive the rotation, there is evidence that the gliding performance can be vastly improved (although still not comparable to traditional wings). The motivation for this research is to use computational fluid dynamics to investigate the performance of autorotors. The objective is to validate the use of CFD models by comparing against existing flight-test experiments. In particular, URANS-based turbulence modeling is shown to provide reasonable accuracy. The reduced computational expense afforded by URANS greatly increases the feasibility of preliminary design studies, including allowing the potential for computational optimization. In the process of validating the CFD models, other questions are answered, such as the effect of turbulence on the Magnus force, a question for which there is little data in published literature.