Measurement and Prediction of the Separated Flow on a Rotor at High Advance Ratio
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Flow separation is prevalent in a number of aerospace applications, but because of complex non-linearities in the governing equations, the resulting aerodynamic forces are challenging to model. This mathematical limitation is particularly impactful in the field of high speed rotorcraft. When a rotor operates at high advance ratio, a regime typical of high speed (and low power) flight, the blades of the rotor are subject to several unsteady motions that incur flow separation, including high pitch inputs and a region of reverse flow that occupies much of the rotor's retreating side. The aerodynamic forces in these regions are dominated by large-scale, coherent vortex structures that are poorly captured by conventional aerodynamics theories. The purpose of this work is to understand the physics of flow separation on high advance ratio rotors, and to leverage this understanding into a low-order, physics-based model for use in rotorcraft design applications.
The current work approaches this goal by identifying, understanding, and ultimately modeling the coherent flow structures present on a representative, sub-scale rotor system operating at high advance ratio. Flowfield measurements on this rotor revealed the presence of two distinct vortex structures, a
sharp-edge'' vortex and a blunt-edge'' vortex, believed to dominate unsteady loading in the reverse flow region. The sharp-edge vortex was studied via a high-fidelity numerical simulation, and its growth was found to be dominated by 2-D mechanisms of vorticity transport. The insignificance of 3-D effects was attributed to a mutual cancellation of Coriolis forces and spanwise convection/tilting, a feature unique to reverse flow. Likewise, the blunt-edge vortex was studied in a series of 2-D surging and pitching wing experiments; its formation was found to largely depend on the unsteady, "external" features of the flow, most notably the trailing wake. Together, these observations led to the development of a 2-D discrete vortex model capable of predicting the strength of the sharp-edge vortex and the timing of the blunt-edge vortex. The model has a computation time on the order of seconds, features only a single empirical parameter, and captures the fundamental physical mechanisms at play on a rotor at high advance ratio.