The Near Wake Of A 2-Bladed Rotor In Forward Flight

The velocity fields in the near wake and the blade tip flowfield of a square-tipped, rectangular planform, 2-bladed teetering rotor have been measured, in low-speed forward flight. A technique for establishing periodicity of the flowfield from laser velocimeter data is shown. The vortex trajectories and wake structure in low-speed forward flight are shown to a vortex age of 488 degrees. Harmonic variations of the axial velocity distributions in the tip vortices are observed. The issue of wake rollup and transition from near to far wake is addressed. It is seen that with periodicity ensured, phase-resolved measurements prove the persistence of the vortex to large ages, as well as the absence of turbulence inside the core of the rotor tip vortex. Introduction One of the most daunting challenges for helicopter aerodynamicists is accurately predicting rotor loads, noise and vibration. Due to problems with numerical dissipation, firstprinciples-based full-physics computations of rotor aerodynamics are not able to capture the structure and evolution of rotor tip vortices beyond about 90 degrees of vortex age. Consequently, computational models of rotor wakes rely on empirical parameters derived from experimental data. There is limited data on the tip vortices of rotary wings, and the usage of vortex models from fixed-wing wake measurements increases the level of uncertainty. There is also considerable confusion in the literature about the rate of decay of vortices, especially in the rotary wing case. Fixed-wing vortices are known to persist for durations on the order of 1 minute or more, so that following aircraft have been known to encounter strong vortices as far as 15 kilometers behind large preceding aircraft. However, several attempts to measure rotary wing vortices, reported in the literature, have been unable to track the vortices beyond roughly 180 degrees of age, with severe “diffusion” or dissipation of the core reported by 90 degrees. Since this does not correspond to predictions using expected values of fluid mechanics variables, turbulence inside the core, and turbulent diffusion, have been postulated as the causes for measured decay rates. This in turn implies the need for extensive development of computational fluid mechanics methods to accurately capture turbulent transport in vortexdominated flows. Other papers have reported “meandering” and “jitter” of vortices, and again the question is whether these are fundamental phenomena or results of the particular features of the experiments in question. In this paper, we continue to show measurements where periodicity is ensured by careful experiment design, and the periodicity of the measurements are proven. The results of such measurements continue to show that the rotor tip vortex does indeed persist as should be expected. Current vortex models used in routine aerodynamic prediction are two-dimensional and ignore the presence of a strong velocity directed along the axis of the vortex. Continuing measurements reported by Thompson et al [1] and Liou et al [2], Mahalingam et al [3] showed that the presence of axial velocity is of first-order importance in the prediction of vortex airframe interactions. There is, however, disagreement on the direction and magnitude of the velocity. Batchelor [4] examined the axial velocity in the core of trailing vortices from a theoretical perspective. He has shown that lack of viscosity results in a jet-like profile, while a wake-like profile is induced by the presence of viscosity. Experiments on fixed and rotary wings have shown jet-like and wake-like axial velocity profiles [5 – 10]. Since all vortex models require empirical data, accurate measurements of the vortex properties are vital. Unfortunately, measurements of vortex properties may not always be reliable. Perhaps the largest source of error comes from aperiodicity. In many rotor facilities, the flow field does not repeat with the expected n/rev variations. The meander is often due to recirculation or strong vortex interactions with the facility’s walls. This results in long-timescale fluctuations which appear chaotic. The effect of this on single point measurements is to smooth the vortex core properties when the data Presented at the AHS Aeromechanics Specialists Meeting, Atlanta, Georgia, November 13 14, 2000. Copyright © 2000 by the American Helicopter Society, Inc. All rights reserved. are ensemble averaged. Core properties, therefore, appear to diffuse or dissipate rapidly since the high gradients are smeared over many rotor cycles. Several experimentalists have attempted to compensate for the observed meander in both fixed and rotary wing experiments [5, 12 13]. These methods used either a gating criteria based on vortex location during the analysis phase, or a mathematical or statistical correction based on the observed levels of vortex meander. However, experiments have shown that if facility recirculation is removed, the rotor wake can be extremely periodic [14], even at realistic full-scale blade Reynolds numbers and tip Mach numbers. Therefore, it is very important to quantify the levels of vortex meander to ensure that “clean” measurements of the vortex core properties are made. The near wake of a two-bladed isolated rotor was studied using Laser Doppler Velocimetry (LDV). Periodicity in the setup is excellent, thus it provides an environment for clean measurements. The vortex core properties as well as the vortex trajectories were examined. Vortex core properties for wake ages up 110° on the front, Retreating Blade Side (RBS) and the rear of the rotor were measured. On the Advancing Blade Side (ABS), vortex core properties were measured up to 488° of wake age. Experimental Setup Tests were conducted in the John Harper 7’x9’ tunnel at Georgia Tech. It is a closed circuit tunnel powered by a 600hp electric motor. The turbulence level in the tunnel is 0.5%. The rotor is an untwisted, constant chord NACA 0015 twobladed teetering rotor with a 45.72 cm radius and a chord of 8.57 cm. Rotor solidity is 0.12 and collective is fixed at 10°. The rotor runs at 1050 RPM and is spun by a 2hp electric motor mounted to a frame above the test section. The tip speed is 50 m/s, and the advance ratio is 0.10. The corresponding thrust for this condition is 0.0089. An optical trigger produces a 1/revolution pulse. This pulse is used for phase averaging the data. The green line (514nm) of a 5W argon-ion laser was used for single component LDV. Fiber optics transmit the beams to a 83mm diameter probe in the test section. 350mm and 700mm lenses were used to focus the beams and also collect the scattered light. The probe is mounted to a three axis linear traverse. The traverse and probe are outside of the rotor wake to avoid adversely influencing the flow. Atomized mineral oil was used for seeding. Velocity measurements are sorted into bins by rotor azimuth at the time of arrival. The characteristics of the vortex in forward flight depend on the where in the rotor cycle they were shed. Therefore, measurements were made at four azimuthal locations to capture all of the possible shedding patterns. Figure 1 shows the measurement planes located on the ABS (ψ = 90), the front (ψ = 180), the RBS (ψ = 270) and the rear of the rotor (ψ = 0). Each plane was oriented so that the axial and circumferential velocities were perpendicular and parallel respectively to the plane. Figure 1 Harper Tunnel test section. Also shown are the measurement grids. Flow Quality within the Harper Tunnel As previously mentioned, quantifying the level of aperiodicity is of extreme importance on measurements of the vortex core properties. Flow visualization and LDV measurements were used to examine the periodicity of the flow in the Harper Tunnel [10]. A snapshot of the flow at the front of the rotor is shown in Fig. 2. Visible in the image is the blade and rotor hub as well as three tip-vortices. The blade is coming out of the page and rotor azimuth is 210°. The flow was frozen using a 20ns light pulse from a copper vapor laser. Four images of the same rotor azimuth and from several different runs were superimposed (Fig. 3). There is little to no change in the locations of the vortices, demonstrating that the flow is not only periodic, but also highly repeatable.