Aerodynamics of the Near Wake of a Helicopter Rotor

Raghav Mahalingam and Narayanan Komerath (email)

 
 
 

Objectives
Motivation
Experimental Details
Papers and Presentations
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What are we trying to do ? Objectives

The objective of this experiment is to characterize the wake of a 2-bladed rotor in forward flight using Laser Doppler Velocimetry. We intend to measure the 3-components of the velocity, component by component, and point by point, and from that, obtain the 3-d velocity field in the near-wake ( i.e. , vortex ages < 180 degrees) of the rotor. Some measurements for vortex ages up to 540 degrees will also be made to obtain the variation in the tip-vortex characteristics with vortex age, primarily on the front of the rotor.

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Why do we need to do this ? Motivation

Rotorcraft aerodynamic design is very much dependent on the capability to accurately predict rotor loads, performance and noise. This in turn needs an accurate knowledge of the wake shed by the rotor and its interaction with the airframe components, in particular the tip-vortex characteristics and its interaction with various surfaces. Also since most interactions occur within the first 200 degrees or so of vortex age it is very important to know the characteristics of the wake soon after it has been shed by the blade. We have an extensive database of rotor/airframe and rotor/wing interactions. During previous measurements we found that the tip-vortex in our particular setup does not diffuse and is very strong for vortex ages beyond 360 degrees. This needs to be confirmed with isolated rotor measurements too. The following paragraphs is an introduction from a recent paper presented at the Reno Aerospace Sciences conference. For a more indepth introduction please read the paper, "Measurements of the Near Wake of a Rotor in Forward Flight", AIAA-980692, Mahalingam and Komerath, 36th Aerospace Sciences Meeting and Exhibit, Reno, NV.

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Experimental Details

The experiments are carried out in the J.J.Harper wind-tunnel at Georgia Tech. This a closed-circuit tunnel with freestream turbulence levels of less than 0.5%. The rotor is a 2-bladed teetering rotor of 0.9144 m dia and 10 degree collective and a blade chord of 8.75cm. The rotor is suspended from the ceiling of the tunnel and inclined at an angle of 6 degrees to simulate forward flight conditions. The slender rotor shaft and the minimal-size teetering hub serve to produce a rotor configuration very close to the "isolated rotor" ideal. The rotor blades are untapered and untwisted with a NACA0012 section. The rotor RPM was 1050 for all the measurements, and the tip speed was 50 m/s. The advance ratio for all the experiments was 0.1. The thrust coefficient at these conditions is 0.0089.

Single-component LDV measurements are performed using a 83mm diameter fiber optic probe for transmitting the dual incident beams and receiving the scattered light in back-scatter. The wavelength of the incident beams was green ( 514 nm ), obtained from a continuous wattage 5W Argon ion laser coupled with a Colorburst module. It is demonstrated from the results that the periodicity and repeatability of the flow-field obviated the need for simultaneous three-component velocity measurement. The entire set of measurements will be performed at four shedding locations on the rotor cycle. The parameters used for non-dimensionalizing velocity and circulation are the tip speed and blade chord.

The fiber optic probe being used is a 83mm dia probe with either a 350mm or a 750mm focal length lens depending on the region of measurement. In forward flight it can be expected that the characteristics of the tip-vortex are different depending on where it is shed. The minimum number of measurement locations that cover all possible shedding patterns is four, the Advancing Blade Side ( ABS ), the front , the Retreating Blade Side ( RBS ) and the rear of the rotor wake. See figures below.Measurement Schematic on ABS   Measurement plan for entire near wake   Development of the near wake on the ABS

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Papers and Presentations   Mahalingam, R., and Komerath, N. M., Characterization of the Near-Wake of a Helicopter Rotor, AIAA Paper 98-2909, 29th AIAA Fluid Dynamics Meeting , June 1998, Albuquerque, NM. ( Click here for presentation )

Jain, R., Conlisk, A. T., Mahalingam, R., and Komerath, N. M., Interaction of Tip-Vortices in the Wake of a Two-Bladed Rotor, Proceedings of the 54th Annual Forum of the American Helicopter Society,  May 1998, Washington D.C.

Mahalingam, R., and Komerath, N. M., Measurements of the Near-Wake of a Helicopter Rotor in Forward Flight, AIAA Paper 98-0692, 36th AIAA Aerospace Sciences Meeting, January 1998, Reno, NV.

Caradonna, F., Henley, E., Silva, M., Huang, S., Komerath, N.M., Reddy, U., Mahalingam, R., Funk, R., Wong, O., Ames, R., Darden, L., Villareal, L., Gregory, J., "An Experimental Study of a Rotor in Axial Flight". AHS Specialists' Meeting on Rotorcraft Aerodynamics and Aeroacoustics, Williamsburg, VA, October 1997. ( Click here for presentation ) , ( Click here to see a recontruction of the rotor-wake from flow images )

Mahalingam, R., " Structure of the Near Wake of a Rotor in Forward Flight and its Effect on Surface Interactions", Ph.D. thesis presentation, June 1999.

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