Comparative numerical and experimental investigations of the vortex ring phenomenon in rotorcraft

Newman, Simon, Brown, Richard, Perry, John, Lewis, Steven, Orchard, Matthew and Modha, Ajay (2001) Comparative numerical and experimental investigations of the vortex ring phenomenon in rotorcraft. American Helicopter Society, 57TH Annual Forum, Washington DC, USA. 08 - 10 May 2001. 21 pp .

Abstract

The boundary for the onset of vortex ring flow breakdown in low speed flight is derived from a physical hypothesis expressed as an extension of simple momentum theory. Predictions are adjusted to accord with measurements found in the literature and with some further deductions made regarding the nature of the flow in the vortex ring state. The opportunity is taken to investigate the details of the flow using a new numerical analysis of the unsteady rotor wake. By maintaining diffusion of vorticity at very low levels, the numerical approach fulfils an essential requirement for modelling the vortex ring condition, since vortices can persist for a long time in the vicinity of the rotor disk. The numerical analysis is compared with test data in the literature as well as with recently generated model rotor data. The numerical analysis supports the assumptions made in deducing the boundary for the onset of vortex ring. The complexity of the flow within the vortex ring/turbulent wake flow states is also revealed. The work has shown that that it is possible to calculate and understand the details of the flow in the vortex ring and turbulent wake regimes. The numerical analysis suggests that apparently minor variables can affect the flow, and that variations in these variables may account for some of the contradictions in past results that have made these regimes somewhat mysterious.

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