Investigation of tip vortex aperiodicity in hover

Previous research has indicated aperiodicity in the positions of tip vortices emitted from a helicopter rotor blade in hover. The objective of the current study is to develop an analysis of the tip vortex aperiodicity in hover and to validate it with measurements on a reduced-scale, 1m diameter, four-bladed rotor. A “vortex ring emitter model” (VREM) was developed to study the statistics of the tip vortices emitted from a rotor blade during hover. In order to better model the rotor wake, a number of independent vortex blobs were used to describe a vortex ring. An empirical model for viscosity was also considered which helped model the core radius growth of the vortex ring with vortex age. A parametric analysis was then performed to obtain a comprehensive qualitative and quantitative convergence study of the time step, viscosity parameter, initial core size, number of rings shed, number of blobs and overlap factor. It was observed that the solution converged rapidly for all the parameters used. The locations of tip vortex cores for vortex ages ranging from 0◦ to 260◦ were measured on the reduced-scale rotor using a stereo PIV system. The blade loading for the reduced scaled rotor was Ct /σ = 0.044 and the blade rotational speed was 1520 RPM, which corresponds to a tip Reynolds number of 248,000. The 95 % confidence region for the position of tip vortex cores exhibited an anisotropic, aperiodic pattern, approximating an ellipse. It was seen that the principal axis of this ellipse appeared to be aligned perpendicular to the slipstream boundary. The analytical model showed good correlation with experimental data in terms of the orientation and extent of the anisotropy. Moreover, an estimate of the total thrust produced and spanwise loading along the rotor blade was also obtained and compared with Blade Element Momentum Theory (BEMT). It was seen that by using more blobs to represent a vortex ring, the solution converged to the BEMT estimate.