The wandering motion of tip vortices trailed from a hovering helicopter rotor is described. This aperiodicity is known to cause errors in the determination of vortex properties that are crucial inputs for refined aerodynamic analyses of helicopter rotors. Measurements of blade tip vortices up to 260 deg vortex age using stereo particle-image velocimetry (PIV) indicate that this aperiodicity is anisotropic. We describe an analytical model that captures this anisotropic behavior. The analysis approximates the helical wake as a series of vortex rings that are allowed to interact with each other. The vorticity in the rings is a function of the blade loading. Vortex core growth is modeled by accounting for vortex filament strain and by using an empirical model for viscous diffusion. The sensitivity of the analysis to the choice of initial vortex core radius, viscosity parameter, time step, and number of rings shed is explored. Analytical predictions of the orientation of anisotropy correlated with experimental measurements within 10%. The analysis can be used as a computationally inexpensive method to generate probability distribution functions for vortex core positions that can then be used to correct for aperiodicity in measurements.

Non-equilibrium plasma generated from positive-pulsed nanosecond electrical discharges into desiccated air is simulated in this paper using a multi-dimensional, multi-physics plasma solver. A pin-to-pin electrode configuration is used with a fixed 5.2 mm gap spacing. Peak pulse voltages range between 10.2 and 22.5 kV. Care is taken to match the exact electrode profile from the experiments, and adjust the electron collision frequency so that breakdown limits closely match those from corresponding experimental results. The optimized numerical simulations predict qualitative streamer structure that is in close agreement with experimental observations. Quantitative measurements of atomic oxygen at the anode tip and qualitative estimates of streamer gas heating are closely matched by simulations. The model results are used to provide insight into the spatial and temporal development of the transient plasma. The work performed in this paper delivers a numerical tool that can be extremely useful to link the post-discharge plasma properties to low-temperature plasma ignition mechanisms that are of great interest for the automotive industry.

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This paper focuses on the multi-dimensional simulation of non-equilibrium plasma generated by nanosecond pulsed discharge in air, at pressure values higher than atmospheric. Voltage profiles and electrode geometry closely match those from a complementary experimental study. Simulations highlight the transition between different post-discharge plasma regimes at increasing pressure and tie the characteristics of the streamers to the electric field distribution in the gap between the electrodes. Results from simulations match experimental observations and qualitatively capture the experimental trend in terms of regime transition pressure and structure of the streamers. As a result, this paper validates a numerical tool that captures the physical and chemical properties of the low-temperature plasma and contributes to expand the understanding of low-temperature plasma ignition processes.

A Mach-scale rotor system, 2.03 m in diameter, was built and hover tested in three configurations: two-bladed single rotor, four-bladed single rotor, and two-bladed coaxial counter-rotating rotor. The blades were untwisted with a VR-12 airfoil profile and a constant chord of 76.2 mm with a 3.8 mm trailing-edge tab. The hubs were rigid and had a vertical spacing of 13.8% rotor radius. Individual rotor steady and vibratory hub loads as well as lower-rotor pushrod loads were measured for several blade loadings up to 0.095. Mean loads were used to analyze rotor performance with an analytical momentum theory model as well as to validate an in-house, free-vortex wake model. Statistical analysis of the measured data revealed clear trends with a known confidence level. Because of mutual interference, the upper and lower rotors of the coaxial configuration consumed 18 and 49% more induced power than that of an isolated two-bladed rotor. The coaxial counter-rotating configuration was found to consume 6% less induced power than an isolated four-bladed single rotor of equal solidity. While torque-balanced, the upper rotor was found to produce 54% of the total system thrust regardless of blade loading. Rotor performance was not affected with an unbalanced torque of up to 5%. The free-vortex wake model was used to gain insight into the flow physics responsible for the interference effects by exploring the radial inflow and thrust distributions.