Visualization and measurement of the tip vortex core of a rotor blade in hover

Thompson, T.L.; Komerath, Narayanan; Gray, Robin B.

doi:10.2514/3.45710

Cited by 33 publications

(7 citation statements)

References 6 publications

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“…These images are of theatrical fog illuminated by sheets of white light from lamps operating essentially in 16-microsecond pulses. These observations correspond very well with measurements made using laser velocimetry and with light sheet visualization in the wakes of a small single-bladed rotor in hover 6 and a two-bladed rotor in forward flight [8][9][10] . In these facilities, the periodicity of the vortex system was clearly demonstrated by long-exposure still photographs of a strobed laser sheet synchronized with the rotor 6 ; comparisons of single-shot vs. ensemble-averaged laser sheet images of the core 21 particle arrival deficit intervals 21 and particle arrival statistics 6 .…”

Section: Observations On Tip Vortex Persistencesupporting

confidence: 73%

“…Thompson et al 6 used Laser Velocimetry to examine the vortex formation process and vortex properties in the near wake of a single bladed rotor. They showed that as the tip vortex formed, discrete secondary features corresponding to vortex sheets were visible within the core of the vortex.…”

Section: Secondary Structure In the Tip Vortex Corementioning

confidence: 99%

“…Applications include the prediction of blade-vortex interaction noise 1,2 , improvement of rotor performance 3,4,5,6,7 , prediction of mutual interaction between wakes, and wake /rotor /airframe interactions involving the main rotor(s), fuselage and tail rotor 8,9 and predicting wake-ground interactions which are important to improve handling qualities and reduce debris hazards in near-ground operation 10 . This paper examines issues related to the formation and the decay of vortices from rotor blades in the light of recent measurements and analyses.…”

Section: Introductionmentioning

confidence: 99%

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On the Formation and Decay of Rotor Blade Tip Vortices

Komerath

Wong

Ganesh

2004

34th AIAA Fluid Dynamics Conference and Exhibit

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Models for the rate of decay of rotor tip vortices, based on near-wake data and computations, are in conflict with the observed persistence of rotor tip vortices for many revolutions. A correlation of data from the literature shows that the measured tip vortex strength is only 40% of the strength expected from the peak of the blade bound circulation, for blades with sharply-cut straight-edged tips, over a wide range of tip Mach and Reynolds numbers. Explanations of this drop in the near wake through turbulence models, are seen to be both counter-intuitive and in conflict with measurements of turbulent fluctuations. Using flow visualization and velocimetry in the wake of a 2-bladed rotor in low-speed forward flight, explanations are shown for these mysteries. Shear layer development at the blade tip, and the twist distribution of the blade have a strong influence on the fraction of the blade tip vorticity lost into the inboard counter-rotating wake. The observations of elliptic core shape and of multiple local extrema of axial velocity deficit, are shown to be features of the shear-layer separation and rollup processes. Blade-to-blade differences in vortex evolution from the same rotor are traced to surface texture differences at the blade tip. Results in the literature reporting 90 t0 100% tip circulation recovery for such blades, are seen to be in cases where the induced velocity due to the tip vortex could not be distinguished from that due to the inboard sheet.

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Section: Observations On Tip Vortex Persistencesupporting

confidence: 73%

Section: Secondary Structure In the Tip Vortex Corementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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On the Formation and Decay of Rotor Blade Tip Vortices

Komerath

Wong

Ganesh

2004

34th AIAA Fluid Dynamics Conference and Exhibit

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“…That is, up until the first blade passage (360 • ), diffusion causes the core radius to increase with increasing vortex Data source age. Aside from the studies of Thompson, Komerath & Gray (1988) and Richard et al (2008), the core radius is shown to decrease after the first blade passage due to vortex stretching induced by the oncoming blade. The corresponding vortex Reynolds number of our study was found to range between 8.3 × 10 4 and 8.7 × 10 4 over the vortex ages studied.…”

Section: Vortex Characteristicsmentioning

confidence: 93%

A study of the turbulence within a spiralling vortex filament using proper orthogonal decomposition

Mula

Tinney

2015

J. Fluid Mech.

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The stability and turbulence characteristics of a vortex filament emanating from a single-bladed rotor in hover are investigated using proper orthogonal decomposition (POD). The rotor is operated at a tip chord Reynolds number and tip Mach number of 218 000 and 0.23, respectively, and with a blade loading of C T /σ = 0.066. Inplane components of the velocity field (normal to the axis of the vortex filament) are captured by way of two-dimensional particle image velocimetry with corrections for vortex wander being performed using the Γ 1 method. The first POD mode alone is found to encompass nearly 75 % of the energy for all vortex ages studied and is determined using a grid of sufficient resolution to avoid numerical integration errors in the decomposition. The findings reveal an equal balance between the axisymmetric and helical modes during vortex roll-up, which immediately transitions to helical mode dominance at all other vortex ages. This helical mode is one of the modes of the elliptic instability. The spatial eigenfunctions of the first few Fourier-azimuthal modes associated with the most energetic POD mode is shown to be sensitive to the choice of the wander correction technique used. Higher Fourier-azimuthal modes are observed in the outer portions of the vortex and appeared not to be affected by the choice of the wander correction technique used.

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“…Thompson et al [25] performed detailed measurements with a laser Doppler velocimeter in the tip region and in the tip vortex core of a single-bladed subscale rotor in hover. The data exhibited evidence of secondary structure inside the rotational core of the vortex.…”

Section: Rotary-wing Tip Vortex Studies Experimental Studiesmentioning

confidence: 99%

Computational Investigation of Microscale Coaxial-Rotor Aerodynamics in Hover

Lakshminarayan

Baeder

2010

Journal of Aircraft

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In this work, a compressible Reynolds-Averaged Navier Stokes (RANS) solver is extended to investigate the aerodynamics of a micro-scale coaxial rotor configuration in hover. This required the following modifications to the solver: implementation of a time-accurate low Mach preconditioner, implementation of a sliding mesh interface boundary condition, improvements in the grid connectivity and parallelization of the code.First, an extensive validation study on the prediction capability of the solver is performed on a hovering micro-scale single rotor, for which performance data and wake characteristics have been measured experimentally. The thrust and power are reasonably well predicted for different leading and trailing geometries.Blunt leading edge geometries show poorer performance compared to the sharp leading edge geometries; the simulations show that this is mainly because of the large pressure drag acting at the blunt front. The tip vortex trajectory and velocity profiles are also well captured. The predicted swirl velocities in the wake for the micro-rotor are found to be significantly larger as compared to those for a full-scale rotor, which could be one of the reasons for additional power loss in the smaller scale rotors. The use of twist and taper is studied computationally and is seen to improve the performance of micro-rotor blades.Next, the solver is applied to simulate the aerodynamics of a full-scale coaxial rotor configuration in hover, for which performance data is available from experiments. The global quantities such as thrust and power are predicted reasonably well. In the torque trimmed situation, the top rotor shares significant percentage of the total thrust at lower thrust levels, which decreases to about 55% of the total thrust at higher thrust values. The simulations reveal that the interaction between the rotor systems is seen to generate significant impulses in the instantaneous thrust and power. The characteristic signature of this impulse is explained in terms of the blade thickness effect and loading effect, as well as blade-vortex interactions for the bottom rotor (wake effect).Finally, the RANS solver is applied to investigate the aerodynamics of a micro-scale coaxial rotor configuration in hover. The overall performance is well predicted. The interaction between the rotor systems is again seen to generate 3-8% fluctuation in the instantaneous thrust and power. The wake effect in the simulation is seen to be very prominent and the phasing of the impingement of the tip vortex from the top rotor upon the bottom rotor plays a significant role in the amount of unsteadiness on the bottom rotor. Interaction of the top rotor vortex and inboard sheet with the bottom rotor results in significant shedding on the bottom rotor blade, and this is believed to be caused by the of sharp leading edge geometry. Significant blade-vortex and vortex-vortex interactions are observed for coaxial systems.

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Visualization and measurement of the tip vortex core of a rotor blade in hover

Cited by 33 publications

References 6 publications

On the Formation and Decay of Rotor Blade Tip Vortices

On the Formation and Decay of Rotor Blade Tip Vortices

A study of the turbulence within a spiralling vortex filament using proper orthogonal decomposition

Computational Investigation of Microscale Coaxial-Rotor Aerodynamics in Hover

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