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James T. HeineckExperimental Physics BranchNASA Ames Research Center

Moffett Field, CA

Gloria K. YamauchiArmy / NASA Rotorcraft Division

NASA Ames Research CenterMoffett Field, CA

Alan J. WadcockAerospace Computing, Inc.

Los Altos, CA

Luiz LourencoFlorida State/Florida A&M University

Tallahassee, FL

Anita I. AbregoArmy / NASA Rotorcraft Division

NASA Ames Research CenterMoffett Field, CA


Three-component velocity measurements in thewake of a two-bladed rotor in hover were made usingthe Particle Image Velocimetry (PIV) technique. Thevelocity fields were oriented in a plane normal to therotor disk and were acquired for wake ages rangingfrom 0 to 270 deg. for one rotor thrust condition. Theexperimental setup and processing of theinstantaneous images are discussed. Data averagingschemes are demonstrated to mitigate the smearingeffects of vortex wander. The effect of vortex wanderon the measured vortex core size is presented.


The key to accurate predictions of rotorcraftaerodynamics, acoustics, and dynamics lies in theaccurate representation of the rotor wake. The vorticalwakes computed by rotorcraft CFD analyses typicallysuffer from numerical dissipation before the first bladepassage. With some a priori knowledge of the waketrajectory, grid points can be concentrated along thetrajectory to minimize this dissipation.Comprehensive rotorcraft analyses based on lifting-line theory rely on classical vortex models and/orsemi-empirical information about the tip vortexstructure. Until the location, size, and strength of thetrailed tip vortex can be measured over a range of

Presented at the American Helicopter Society 56thAnnual Forum, Virginia Beach, Virginia, May 2-4,2000. Copyright 2000 by the American HelicopterSociety, Inc. All rights reserved.

wake ages, the analyses will continue to be adjustedon a trial and error basis to correctly predict bladeairloads, acoustics, dynamics, and performance. Usingthe laser light sheet technique, tip vortex location canbe acquired in a straightforward manner. Measuringwake velocities and vortex core size, however, hasbeen difficult and tedious using point-measurementtechniques such as laser velocimetry. The ParticleImage Velocimetry (PIV) technique has proven to bean efficient method for acquiring velocitymeasurements over relatively large areas and volumesfor rotating flow-fields. The majority of PIV workreported to date has been restricted to 2-componentvelocity measurements acquired from the wake of arotor in forward flight (Refs. 1-12). Ref. 10 describesan experiment where three-component velocitymeasurements were made on a rotor in forward flight,although only preliminary results are shown.Although PIV has become an increasingly favoredtechnique for acquiring flow field information on rotorwakes, there has been minimal information in theliterature regarding methods of extracting criticalparameters such as vortex core size and strength fromPIV data.

The present work had two objectives. The firstobjective was to gain experience in applying thethree-component (3D) PIV technique to a simple rotorconfiguration. A two-bladed, untwisted rotoroperating at moderate thrust in hover was selected forthis investigation. Restricting the test conditions tohover allowed greater freedom, compared to forwardflight testing, in locating the cameras and laser sheet.Three-component PIV permits both cameras to bepositioned in the forward scatter position relative tothe laser light sheet thereby maximizing the qualityof the particle images. The second objective was to

obtain vortex core size and document the effects ofvortex wander.

This paper provides a description of the testfacility and the model used for the experiment, adiscussion of the test conditions, a detailed descriptionof the PIV measurement set-up, and the dataprocessing technique and procedures. Specific findingsfrom this experiment are discussed.

Facility and Model Description

The experiment was performed at NASA AmesResearch Center in the U.S. ArmyAeroflightdynamics Directorate Hover Test Chamber.The following sections describe the test chamber androtor model.

Hover Test Chamber: The chamber floor is 26 ftx 32 ft. The chamber ceiling height is adjustable andwas positioned approximately 6 ft above the rotorhub for this experiment (Fig. 1(a)). Two large roll-updoors on opposite sides of the chamber were eachraised 32 inches to allow the rotor to draw in air fromthe outside. Air filters were used to minimize debrisentering the chamber from beneath the roll-up doors.The rotor wake was exhausted upward through anaperture in the chamber ceiling and then to theexterior through openings near the top of thechamber.

Test Stand and Rotor: Fig. 1(b) shows the two-bladed rotor and the Rotary Wing Test Stand (RWTS)installed in the hover chamber. The RWTS has two90-HP electric motors and a 2.5:1 transmission. Aflexible coupling between the input shaft and dummybalance was instrumented to measure torque. A rotaryshaft encoder served to provide the synchronizationtrigger for the PIV cameras and laser.

A two-bladed rotor was installed on the RWTS.The blades have a rectangular planform, zero twist,and a chord of 7.5 inches. The rotor diameter is 7.5 ft.The outer 50 percent of each aluminum blade is aNACA 0012 profile; the profile linearly tapers to aNACA 0020 from 0.5R to 0.2R (Ref. 13). The rotorhub consists of 2 steel halves which are clampedtogether to hold the blades at the desired pitch setting.The hub-and-blade assembly is designed for a zero-degree coning angle. To minimize recirculation in thechamber, blade pitch was manually adjusted to anegative angle resulting in a downward thrust andupwardly convected wake. The rotor hub wasapproximately 7.5 ft above the chamber floor. Asimilar installation was used in Ref. 13, whichprovides additional details about the rotor and teststand.

Fig. 1(a). Illustration of test stand and chamber.

Fig. 1(b). Photograph of test stand installed in hover chamber.

Fig. 1. Hover chamber test installation.

Test Conditions: Rotor rpm was chosen to be870 rpm (14.5Hz) to maximize the thrust while notexceeding the maximum frame rate of the PIVcameras. Since a rotor balance was not installed forthis test, a thrust condition matching the conditiontested in Ref. 13 was selected. All data were thereforeacquired for 8 deg. collective pitch, corresponding toa rotor thrust coefficient of 0.005. PIV data wereacquired from 0 deg. to 270 deg. wake age inincrements of 30 degrees. Five hundred image pairs,or realizations, were recorded in groups of 50 at eachwake age.

Three-Component PIV Method Description

PIV measures fluid velocities by time-sequentialimaging of individual tracer particles in the flow. Alaser light sheet illuminates the particles. The cross-

correlation of two successive images yields themagnitude and direction of the particle displacement.

Originally, PIV was developed using a singlecamera that viewed the laser light sheet normal to theplane. The resulting data was an array of two-component velocity vectors. One error source in 2DPIV is caused by out-of-plane motion of the particles.Near the center of the image area, where the viewangle is nearly 90 deg. to the plane, the error isnegligible. Toward the edges of the image the viewangle decreases from the ideal 90-deg. angle such thatthe motion perpendicular to the sheet becomesdetectable. This perspective error led the PIVcommunity to believe that the method was sensitiveenough to exploit the so-called outof-plane error toderive the third component of velocity (Ref. 14). Byusing two cameras to view the laser sheet obliquely,the movement through the laser sheet can be recorded.Each camera has a different perspective of the particlemotion from Ref. exposure (T0) to Delayed exposure(T1) in the region of interest (Fig. 2). The thirdcomponent of velocity can be calculated usingphotogrammetric equations. The method becameknown as Stereoscopic PIV or 3D PIV.

Fig. 2. Diagram showing the pe rspec t i veeffect of particles in 3D motion f o rStereoscopic PIV.

The technique demands both critical focus andmaximum brightness of the particle image.Conventional lens mounts create a plane of focus inobject space that is parallel with both the image plane

and the lens plane. An optical challenge arises whenthe camera views the plane of laser light obliquely.Using conventional focusing, a small lens aperture(large f value) is required to produce the large depth offield necessary to maintain discrete particle images.Unfortunately, a small lens aperture drasticallyreduces both the particle image intensity and imageresolution. For this reason a new approach wasnecessary.

Scheimpflug focusing solved this conundrum.(Ref 15). Fig. 3 illustrates the basic principle, whichstates that when the object plane, the lens plane andthe image plane meet on a common line in space,focus is achieved. Photogrammetic equations werederived specifically for 3D PIV using theScheimpflug technique (Ref. 16).

Fig. 3. The Scheimpflug condition.

Experimental Set-up

The cameras were mounted horizontally on aplane 3 inches above the rotor disc. Fig. 4 shows anoverview of the system. The angle of view for eachcamera relative to the laser light sheet was 36 deg.This angle maximized the brightness of the particleimage by capitalizing on the efficiency of laser lightin forward-scatter without sacrificing accuracy. Fig. 5illustrates the position of the image area relative tothe blade tip. The image height is twelve inchesoutboard and approximately 16 inches inboard due toperspect