The Interdependence of Straining and Viscous Diffusion Effects on Vorticity in Rotor Flow Fields

An experiment was performed in an attempt to quantify the interdependence of straining and viscous diffusion effects on the development of rotor tip vortices. The properties of the blade tip vortices were measured in the wake of a hovering rotor, and compared to the results for the case when the wake approached a solid boundary. The presence of the boundary forced the tip vortex filaments to strain or stretch, allowing the effects of this process on the tip vortex developments to be studied and measured relative to the baseline case. It is shown that vortex stretching begins to decrease the viscous core size and, when the strain rates become large, this can balance the normal growth in the vortex core resulting from viscous diffusion. A parsimonious model for the effects of vortex stretching based on the conservation laws was used to correct the measurements, which brought the results into agreement with what would be expected on the basis of viscous diffusion alone. A byproduct of the process of progressively stretching the vortex filaments allowed their properties to be measured to older wake ages than would otherwise have been possible. The measured results were used to compare with other vortex measurements, and to augment previous correlations in an attempt to develop a more general vortex model that would be suitable for use in a variety of aeroacoustic applications. A new model is suggested that combines the effects of diffusion and strain on the vortex core growth. The empirical coefficients of this model have been derived based on the best available results from many sources of both fixed-wing and rotating-wing tip vortex measurements.