Main rotor blade tip vortex characterization

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Coletta, M.
De Gregorio, F.
Visingardi, A.
Iluso, G.
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The paper illustrates a research activity carried out at CIRA with the aim to characterize experimentally and numerically the blade tip vortices of a small scale four-bladed isolated rotor in hover flight and to evaluate their decay process during the convection of the wake downstream. 2C-2D PIV measurements were carried out below the rotor disk down to a distance of one radius. The numerical simulations were aimed at assessing the modelling capabilities and the accuracy of a free-wake Boundary Element Methodology. Several detection criteria were investigated in order to identify a suitable one for the analysis of PIV data. The ?2 vortex was selected as the most robust and reliable criterion and was applied to both experimental and numerical results. The tip vortices were characterised in terms of vorticity, circulation, swirl velocity, core radius and trajectory. The rotor wake mean velocity field and the instantaneous vortex characteristics were investigated. The experimental/numerical comparisons showed a reasonable agreement in the estimation of the mean velocity inside the rotor wake, whereas the BEM simulations predicted and under-estimated effect of the diffusion thus generating a smaller shear layer region with respect to the experiment. The numerical simulations provided a clear picture of the filament vortex trajectory interested by complex interaction starting at about a distance of z/R=-0.5. The time evolution of the tip vortices was investigated in terms of net circulation and swirl velocity. The PIV tip vortex characteristics showed a linear mild decay up to the region interested by vortex pairing and coalescence, where a sudden decrease, characterised by a large data scattering, occurred. The numerical modelling predicted a hyperbolic decay of the swirl velocity down to z/R=-0.4 followed by an almost constant decay. Instead, the calculated net circulation showed a gradual decrease throughout the whole wake development. The comparisons showed discrepancies in the region immediately downstream the rotor disk but significant similarities beyond z/R=-0.5.