Fluid Dynamics of Interacting Blade TipVortices With a Ground Plane

Timothy E. Lee∗ J. Gordon Leishman† Manikandan Ramasamy‡

Department of Aerospace EngineeringGlenn L. Martin Institute of Technology

University of Maryland, College Park, MD 20742

Abstract

Experiments were conducted using flow visualization andphase-resolved digital particle image velocimetry (DPIV)to examine the fluid dynamic behavior of a rotor wake asit interacted with a horizontal ground plane. The experi-ments were conducted using a small rotor with a groundplane that could be moved to as close as 25% of the ro-tor radius to the rotor plane. Visualization of the rotorwake and the wake/ground plane interactions was per-formed by illuminating a seeded flow using a Nd:YAGlaser, phase-locked to the rotational frequency of the ro-tor. Two-component DPIV measurements were obtainedat four rotor heights off the ground. Measurements at sev-eral wake ages were obtained to examine the dynamic fea-tures of the wake/surface interaction process. The resultsshowed that the overall downwash induced by the rotorbecomes an unsteady radial wall jet defined between theground plane and the rotor wake boundary. The experi-ments have shown that the individual tip vortices createsubstantial unsteady shear stresses on the ground plane,in some cases exceeding twice the time-averaged values.Viscous processes such as vortex straining and turbulenceappear to be significant factors in understanding fluid dy-namics of the resulting flow near the ground. At the higherrotor heights the vortex filaments diffuse under the actionof viscosity and turbulence before reaching the ground. Atthe lower rotor heights, turbulence in the developing walljet quickly shears the coherent vortices, accelerating theirdiffusion. Corresponding measurements of rotor perfor-mance showed the expected reduction in rotor power re-quired and an increase in rotor thrust for in-ground-effectoperations.

∗Graduate Research Assistant. telee@umd.edu†Minta Martin Professor. leishman@umd.edu‡Assistant Research Scientist. mani@umd.edu

Presented at the 64th Annual Forum of the American HelicopterSociety, Montreal, Canada, April 29–May 1, 2008. c©2008 bythe authors except where noted. Published by the AHS Interna-tional Inc. with permission.

NomenclatureA Rotor disk areac Blade chordCT Rotor thrust coefficient, = T/ρAΩ2R2

CP Rotor power coefficient, = P/ρAΩ3R3

Nb Number of bladesP Rotor powerr Radial distanceR Radius of bladeT Rotor thrustvh Hover induced velocity, =

√T/2ρA

ρ Flow densityσ Rotor solidity, = Nbc/πRψ Azimuth angleΩ Rotational speed of the rotor

IntroductionThe performance of all types of rotorcraft will be affectedby the interference of the ground or any other boundarythat may constrain the flow into the rotor or otherwise al-ter the natural development of the rotor wake. The issue of“ground effect” is of concern in actual flight operations aswell as in the ground testing of rotors. This rotor/surfaceinterference effect has long been recognized, but the de-tailed aerodynamics at the rotor or with the developingflow near the ground are still not fully understood. Thereare many dependencies and interdependencies affectingrotor flows in-ground-effect operation, least of all beingthe effects of rotor thrust and proximity of the rotor tippath plane to the ground. Interference effects can be ob-tained both in hover and forward flight, but the effects aregenerally strongest in the hovering state.

From a performance perspective, it is well known thatwhen a rotor operates in-ground-effect (IGE) its thrustis increased for a given power supplied—see Kussner(Ref. 1), Zbrozek (Ref. 2), Betz (Ref. 3), Knight andHefner (Ref. 4), Cheeseman and Bennett (Ref. 5),Fradenburgh (Refs. 6, 7), Stepniewski and Keys (Ref. 8)

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Figure 1: Schematics of the general behavior of therotor wake when operating in: (a) Hover/very-low for-ward speed, (b) Low-speed forward flight.

and Prouty (Ref. 9). Hayden (Ref. 10) summarizesflight test measurements using the standardized techniqueof Lewis (Ref. 11). The results suggest significant effectsfor rotor heights of one diameter off the ground, wherepower required can be reduced by up to 30% relative toout-of-ground-effect operation at the same thrust (i.e., atthe same aircraft weight). Most of this power reduction isinduced in nature, but there is also some small reduction inprofile power because the blades operate at slightly lowerangles of attack to produce the same thrust. Measurementshave also shown secondary dependencies on blade load-ing, blade aspect ratio, and blade twist.

Flow visualization of rotors operating IGE have beenperformed by Fradenburgh (Ref. 7), Taylor (Ref. 12),Curtiss et al. (Refs. 14, 13), and Light and Norman(Ref. 15). Consider the schematics shown in Fig. 1—because the ground must always be a streamline to theflow, the rotor wake slipstream tends to rapidly expand ra-dially away from the rotor as it approaches the ground.This interference effect on the rotor wake development al-ters the induced velocity in the plane of the rotor, and soaffects the rotor thrust and power. At low forward speeds,a region of flow recirculation can be formed upstream ofthe rotor, which can increase induced power requirementsrelative to hover or free-air conditions. Ground effect isusually negligible for advance ratios greater than about0.10 at any rotor height off the ground.

Ground interference effects are also apparent in windtunnel tests on rotors, where the effects of the tunnel wallscan alter the flow at the rotor. Ganzer and Rae (Ref. 16)and Lehman and Besold (Ref. 17) have studied the ef-fects experimentally. For tunnel dimensions that are atleast twice the diameter of the rotor, the effects of the windtunnel walls are small for operating advance ratios greaterthan 0.1. Generally, it must always be assumed that theeffects of the tunnel walls will lead to some flow recir-

Figure 2: The fluid dynamics of the rotor wakein-ground-effect is important for understanding theproblem of brownout. (Image courtesy of Optical AirData Systems.)

culation at lower advance ratios, making reliable free-airmeasurements of rotor performance difficult or impossibleeven if suitable corrections could be derived.

Besides the interference effects of the ground on ro-tor performance, the velocity of the induced flow nearthe ground may reach high enough thresholds to lift uploose surface materials such as dirt and sand, resulting ina problem that is known as “brownout.” Brownout canquickly obscure the pilots field of view, and has resultedin several helicopter accidents. Figure 2 shows how a ra-dially and vertically expanding dust cloud is produced bya helicopter landing over a sandy surface. The details ofthe development of the dust cloud, however, is known todiffer substantially from rotorcraft to rotorcraft and withsurface type and condition. Brownout is fundamentally acomplex physical phenomenon because it can potentiallybe affected by several interdependent parameters, includ-ing but not limited to: rotor disk loading, blade loading,number and placement of rotors, number of blades, bladetwist, blade tip shape, and fuselage shape.

Ground effect problems have received considerable at-tention from a modeling perspective. Cheeseman andBennett (Ref. 5) replaced the rotor by a simple sourcewith a corresponding image source to simulate ground ef-fect. Knight and Hefner (Ref. 4) and Rossow (Ref. 18)used a similar approach but with a vortex cylinder model.More recent studies of the flows associated with rotorsoperating IGE have used both Lagrangian vortex track-ing methods and Eularian grid-based computational fluiddynamic (CFD) methods. Vortex wake models havebeen based on potential flow theory, assuming vorticityconservation even near the ground plane. Such invis-cid models are described by DuWaldt (Ref. 19), Saberiand Maisel (Ref. 20), Quackenbush and Wachspress(Ref. 21), Graber et al. (Ref. 22), Itoga et al. (Ref. 23),

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and Griffiths and Leishman (Ref. 24). While reason-able predictions of wake behavior in hover and in forwardflight has been demonstrated, more limited success hasbeen achieved in correctly predicting rotor performanceIGE when compared to experimental measurements. Thisreflects the inherent complexity and strongly viscous na-ture of the flow field for IGE operations. Reynolds Av-eraged Navier–Stokes (RANS) approaches such as thatdiscussed by Moulton et al. (Ref. 25) seem promisingfor predicting such problems. Vorticity transport modelsseem to have met with reasonable success in predictingboth ground effect aerodynamics (Ref. 26) and brownout(Ref. 27).

The present paper describes results from an ongoing ex-perimental study using phase-resolved digital particle im-age velocimetry (DPIV) to examine in detail the fluid dy-namic behavior of the rotor wake with its embedded bladetip vortices as it interacts with a ground plane. The workwas a precursor to three-component, dual-plane, two-phase flow measurements of rotors operating over sur-faces covered with loose sediment. Such an understandingis a prerequisite to better predicting the induced veloc-ity field and performance of rotors operating in-ground-effect, and eventually to further the understanding theproblem of “brownout.” Predictive models for brownoutare still being developed, and they require better valida-tion of rotor aerodynamics in-ground-effect and of thetwo-phase flow interactions of vortices with the sedimentbed. Ultimately, the real mechanisms responsible for par-ticulate uplift in a non-steady turbulent flow environmentmust be understood. The present work is focused on un-derstanding using DPIV the near-wall fluid behavior as thetip vortices interact with the radially expanding turbulentjet flow on the ground plane.

Description of the Experiment

Rotor Setup

The experiment was conducted with a small, two-bladedrotor in which the rotor tip-path-plane plane was mountedparallel to a ground plane. The rotor blades were madeof composite carbon fiber, and had circular arc, camberedairfoil sections. The radius of the blade was 86 mm, witha uniform chord of 19 mm with no taper. No blade twistwas used. The rotor had a solidity σ of 0.14. A drawingthe rotor is shown in Fig. 3, and photograph of the rotorand ground plane setup is shown in Fig. 4. The rotationalspeed of the rotor was measured with a Hall-effect sensormounted on the rotor shaft, which provided a once-per-revolution trigger signal to the data acquisition system.The data acquisition system ensured precise phase lock-ing to less than ±0.1 degrees.

Figure 3: Drawing of the rotor and test stand.

Figure 4: Experimental setup of the rotor and theground plane.

The ground plane was a circular disk that was threetimes the diameter of the rotor, and was painted flat blackto minimize reflections from the laser sheet. The groundplane was mounted on a mechanical traverse so that itcould be moved to any distance from the rotor plane, andalso set to any inclination. In all of the present experi-ments, the rotor tip-path-plane was adjusted so that it wasperfectly parallel to the ground plane.

The rotor was operated at a rotational frequency of be-tween 35 Hz and 60 Hz, giving a maximum tip speed of32.42 ms−1. The nominal operating tip Mach number andReynolds number based on chord were up to 0.095 and42,170, respectively.

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Loads Balance

The rotor stand (Fig. 3) was mounted on a micro massbalance capable of measuring the thrust with a precisionof ±0.1 grams. A linear reaction sensor was used to mea-sure torque. The torque cell was separately calibrated byusing precision weights applied though a known arm. Adigital inclinometer was used to set the pitch angle of theblades at the 75% span location.

At each rotational speed, torque and thrust data werecollected over a 5 second period. The rotor was also testedat all conditions without the blades attached to the hubto determine tares. These tares were subtracted from thethrust and torque measurements taken with the blades at-tached.

To guarantee measurement precision and repeatabilityof the experiment, multiple tests were made at the samepitch angles and rotational speeds. The measured thrustwas found to be within the instrument precision of themass balance, while the measured torque had an uncer-tainty of ±3% of the mean value. The final balance datawere ensemble averaged, and used to find the thrust andpower coefficients.

Flow Visualization

For the flow visualization studies, a thermally producedmineral oil fog was used as a flow tracer material. The av-erage size of the tracer particles was about 0.20 microns indiameter, and were determined to be small enough not tohave any significant tracking errors in the flow (Ref. 28).Judicious adjustment of the flow rate from seeder and thelocation of the outlet nozzle was required to introduceconcentrations of tracer particles at the locations needed toclearly identify the key flow structures such as the tip vor-tices and vortex sheets, as well as the developing boundarylayer flow on the ground.

The illumination of the flow was performed using aNd:YAG laser, the pulse frequency being phased synchro-nized with the rotational frequency of the rotor. An artic-ulated optical arm transmitted the laser light to the regionof interest. A 500 mm cylindrical lens at the end of thearm converted the beam to a planar laser light sheet thatwas approximately 1 mm thick at its waist. Flow visu-alization images were taken with a Nikon 6.1 mega-pixelcamera with 60 mm focal length lens. The camera waspositioned so that it was exactly perpendicular to the laserlight sheet—see Fig. 5.

Many high-resolution images were taken at each rotoroperating state; only images showing all of the key fea-tures of the wake and the interactions with the groundplane were retained for further analysis. The phase ofthe trigger signal from the rotor could be altered to cap-ture images of the wake at different ages. i.e., at different

Figure 5: Schematic explaining the laser sheet andcamera setup for flow visualization and DPIV.

blade azimuth angles. The ground plane was positioned atseveral ground plane distances (z/R), typically z/R = 4.0,3.0, 2.5, 2.0, 1.75, 1.5, 1.25, 1.0, 0.9, 0.8, 0.75, 0.7, 0.6,0.5, 0.4, and 0.25. These conditions are referred to in thispaper as the in-ground-effect (IGE) results. Baseline testswere conducted without the ground plane present, whichare referred to as the out-of-ground effect (OGE) results.Not all types of measurements were made at all groundplane heights.

Digital Particle Image Velocimetry

Two-component flow measurements were made using a 2mega-pixel CCD camera for the DPIV imaging. A 532 nmband pass filter prevented the camera from receiving anyreflections other than the reflections from the seed par-ticles. A relatively small region of focus (100 mm-by-100 mm) was chosen, which required a 60 mm lens withan f-stop of 2.8. The interrogation window size was 16-by-16 pixels with a 50% overlap. Measurements could bemade to within a few millimeters of the ground surface, atwhich point laser reflections made it difficult to performsuccessful cross-correlations with the DPIV images.

DPIV imaging requires a calibration process to incor-porate the registration of the camera and its mapping fromthe object plane onto the image plane to correct for distor-tions from variable magnification across the image. Forthe present system, the single camera was mapped in theusual way for any PIV system. A nonlinear mapping func-tion was created from images of a calibration target. Thisprecision calibration target was made from regular grid ofwhite dots on a black anodized background. A fiducialreference point on the target defined the origin for all thecalibration images.

For the digital particle image velocimetry (DPIV) ex-periments, the entire test area was uniformly seeded with

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the tracer particles before each sequence of measure-ments. This is in contrast to the flow visualization ex-periments, which required concentrating tracer particlesin regions of the flow needed to clearly identify the tipvortices, vortex sheets, and shear layers.

The correlation algorithm used for the DPIV was basedon a deformation grid correlation method (Refs. 33, 34).This procedure is similar to the more common recursivetechnique, however, in this method the second window isdeformed in that it is both sheared and translated insteadof just undergoing a simple translation—see Fig. 6. Theprocedure starts with the correlation of an interrogationwindow of a defined size (say, 64-by-64), which is thefirst iteration. After the mean displacement of that regionis estimated, the interrogation window of the displacedimage is moved by integer pixel values for better corre-lation in the second iteration. This third iteration starts bymoving the interrogation window of the displaced imageby sub-pixel values based on the displacement estimatedfrom second iteration. Following this, the interrogationwindow is sheared twice (for integer and sub-pixel val-ues) based on the velocity magnitudes from the neighbor-ing nodes before performing a fourth and fifth iteration,respectively.

Once the flow velocities were estimated after these fiveiterations, the window was split into four equal windows(of size 32-by-32). These windows were moved by the av-erage displacement estimated from the final iteration (us-ing a window size of 64-by-64) before starting the firstiteration at this resolution. This procedure was continueduntil the resolution required to properly resolve the flowfield was reached. The second interrogation window wasdeformed until the particles remain at the same locationafter the correlation. This technique, especially the in-troduction of shear, has been shown (Ref. 34) to be themost appropriate for measuring the high velocity gradi-ents found inside rotor wake flows, and especially at thenear-wall flow region for a rotor operating IGE.

Figure 6: DPIV image processing technique.

Spurious flow velocity vectors in the results were re-moved using standard deviation filter, followed by medianfilter, and finally by mean filter based on 5-by-5 neighbor-ing nodes. The spurious vectors were then replaced withinterpolated vectors. Care was taken to minimize the num-ber of interpolated vectors. 100 DPIV image pairs wereacquired at each ground plane distance and for each wakeage. Any image that had spurious vectors of more than1% of the total was eliminated. Typically, this reduced thetotal number of instantaneous velocity maps to between93 and 95. The results were then simple phase-averagedbefore further processing.

Results and Discussion

Rotor Performance Measurements

Measurements of rotor performance (thrust and power)were made for different rotor heights off the ground. Boththe thrust and power changed during these tests becausethey were made at a fixed blade pitch. From the powerpolars of CT versus CP, the results were interpolated tofind the power required at a constant thrust, as well as thethrust produced at a constant power.

Figure 7 shows the relationship between the thrust pro-duced in-ground-effect, TIGE, to that for out-of-ground-effect operation, TOGE, for a constant power. There waslittle effect on thrust for heights of more than three ro-tor radii, which is an expected result for any rotor basedon all prior measurements of rotor performance IGE. Forheights below one diameter, the rotor thrust increasedmore rapidly, rising to as high as 1.4TOGE at z/R =0.25R. There was some small dependency on power set-ting within the range that could be tested, but the resultswere practically the same.

Figure 7: Measurements of rotor thrust at a constantpower show a substantial increase in thrust for opera-tion near to the ground plane.

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Figure 8: Measurements of rotor power at a constantthrust show a rapid decrease in power for operationnear to the ground plane.

Figure 8 shows the relationship between PIGE and POGEfor constant rotor thrust. Nearly a 40% reduction in powerwas achieved at z/R = 0.25R. There was some evidencethat the results were dependent on the operating state ofthe rotor, with the higher thrusts (and hence higher bladeloadings) producing a slightly more significant effect onrotor IGE performance.

Flow Visualization

Flow visualization was conducted for both rotor out-of-ground-effect (OGE) and rotor in-ground-effect (IGE) op-eration. Digital images of the rotor wake and surface flowwere taken for different rotor heights and at different bladephase angles. Judicious seeding of the flow was requiredto produce the best images that most clearly identified thekey flow features, including the blade tip vortices and tur-bulent surface wall jet. The operating conditions for therotor at the different heights off the ground are given inTable 1.

z/R CT /σ CP/σ Test conditions∞ (OGE) 0.0917 0.01569 Flow viz.

2.5 0.0916 0.01582 Flow viz.2 0.0946 0.01607 Flow viz.

1.5 0.0997 0.01629 Flow viz., DPIV1.25 0.1030 0.01636 Flow viz.

1 0.1067 0.01646 Flow viz., DPIV0.75 0.1099 0.01637 Flow viz.0.5 0.1153 0.01636 Flow viz., DPIV0.25 0.1329 0.01679 Flow viz., DPIV

Table 1: The operating conditions for the rotor at thedifferent heights off the ground.

OGE Operation

Figure 9 shows a representative flow visualization imageof the rotor wake for OGE operation. All of the basic fea-tures of a hovering rotor wake are present here, includingthe blade tip vortices and the turbulent vortex sheets. Thesignature of the tip vortices appear as spiral bands of tracerparticles with clear voids at the vortex cores resulting fromthe action of centrifugal and Coriolis accelerations on theparticles.

These vortices then convect along the slipstream bound-ary that separate the higher velocities inside the rotorwake from the external quiescent flow. Because a twobladed rotor was used, successive tip vortices below therotor are 180 degrees of age apart. For OGE operation,the tip vortices remained distinct (with clear tracer par-ticle voids) until they were about three rotor revolutionsold. As expected, the was some evidence of aperiodicity(i.e., deviations from the identical flow state from rotorrevolution-to-revolution) in the rotor wake, this being aninherent characteristic of all types of helicoidal vorticalwakes (Ref. 29).

The presence of the turbulent sheets trailing from the

Figure 9: Representative flow visualization of the rotorwake for hover out-of-ground-effect.

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inboard parts of the blade can also be seen in Fig. 9, thesebeing relatively thick because of the lower chord Reynoldsnumber at the blades. These vortex sheets convect axiallymore quickly than the blade tip vortices, and so interactwith the older tip vortices as they age in the flow. Theinner parts of the vortex sheets remain relatively distinctto two or three rotor revolutions. The wake further down-stream becomes filled with the folded and merged vortexsheets, and the tip vortices have spun down under the ac-tion of viscosity and turbulence. As a result, the far wakeand becomes relatively turbulent overall and begins to ex-pand slowly, eventually becoming a turbulent jet-like flowcontaining fairly diffused zones of residual vorticity.

IGE Operation

In all cases, it was found that the presence of the groundplane significantly altered the geometry of the developingrotor wake compared to the OGE case (Fig. 9), the wakeexpanding (stretching) significantly radially outward as itapproaches the ground surface. A simple schematic of thegeneral flow developments is shown in Fig. 10. On theground plane, the flow becomes a wall jet with a bound-ary layer close to the surface and a developing shear flowregion above with significant turbulent mixing.

Figure 11 shows flow visualization results for eight ro-tor heights above the ground plane. The images weretaken for the same blade phase and are scaled in both thevertical and horizontal dimensions so that all the images

Figure 10: General development of the flow below therotor in ground effect operation.

can be directly compared with each other. Notice that thewake distortion becomes particularly pronounced at thelower rotor heights, where the wake quickly expands radi-ally outward within the first rotor revolution after its for-mation.

For the highest height of z/R = 2.5R, the initial devel-opment of the wake was found to be similar to the OGEcase—compare Fig. 11(a) with Fig. 9. However, the tipvortices were noted to be axially little closer together thanfor the OGE case, and exhibited some evidence of pairingor bundling, which is a common characteristic of wakeswith reduced helicoidal pitch and more closely spacedvortices.

By the time that the far wake approaches the groundplane it had already become relatively turbulent and thetip vortices have diffused under the action of viscosity andturbulence. The flow near the ground, therefore, becomesa developing turbulent wall jet that is relatively free of co-herent vortical flow structures but was notably unsteady(aperiodic). This jet expands radially away from the rotor,and exhibits the features of both a boundary layer and afree shear layer. Quantification of the resulting flow veloc-ities and the boundary layer profiles was measured usingusing DPIV, which is discussed later.

Figure 11(b) shows flow visualization results for z/R =2.0R. In this case, the older blade tip vortices (i.e., 3 rotorrevolutions old) approach the ground plane before show-ing any evidence of spinning down and diffusing. As theyinteract with the ground, the vortices undergo a stretchingprocess (Refs. 30, 31), which results in vorticity intensifi-cation that counters normal diffusive behavior, at least fora while. As a result, in this case the tracer particle voidsignatures of the tip vortices were apparent in the flow vi-sualization to about five rotor revolutions. The initial de-velopment of the surface wall jet, therefore, is different inthat it now contains significant embedded vorticity fromthe tip vortices and, to a lesser extent, additional vorticitycontributions from the vortex sheets.

Figure 11(c) shows results for z/R = 1.5R. These re-sults are similar to the z/R = 2.0R case, but the tip vor-tices were only visible up to about four rotor revolutionsold. There were now significantly large eddies present inthe wall surface flow. Lowering the height to z/R = 1.25Rproduced significant pairing of the older vortices as theyapproached the wall—see Fig. 11(d). This pairing wasperiodic, the vortices spinning around each other and un-dergoing some evidence of viscous merging as they weresheared in the developing wall flow.

The vortex sheets trailed from the inner parts of theblades were also more evident near the wall in this case.The vortex sheets were seen to remain embedded insidethe developing boundary layer, at least initially. The re-sulting wall jet for this condition seemed to have signifi-cantly higher levels of vorticity with a variety of eddies at

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(a) Rotor height z/R = 2.5 (b) Rotor height z/R = 2.0

(c) Rotor height z/R = 1.5 (d) Rotor height z/R = 1.25

(e) Rotor height z/R = 1.0 (f) Rotor height z/R = 0.75

(g) Rotor height z/R = 0.5 (h) Rotor height z/R = 0.25

Figure 11: Flow visualization of the rotor wake for several rotor heights off the ground plane: (a) z/R = 2.5, (b) z/R= 2.0, (c) z/R = 1.5, (d) z/R = 1.25, (e) z/R = 1.0, (f) z/R = 0.75, (g) z/R = 0.5, and (h) z/R = 0.25.

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various scales. The resulting flow suggested the develop-ment of a marginally thicker turbulent wall jet and shearflow as it expanded away from the rotor.

Lowering the rotor to z/R = 1.0R off the ground showedthat the older tip vortices were now being significantlystretched as they came into proximity with the ground—see Fig. 11(e). In this case, the older tip vortices were ac-tually stretched to about twice their initial length, whichobviously results in considerable vorticity intensification.This stretching process decreases the viscous core size ofthe vortices and increases their peak swirl velocities rela-tive to those at the previous rotor revolution.

The reduction of the size of the seed void to values closeto those seen after the first rotor revolution suggests thatthe stretching effect (which conserves circulation) is in-deed a powerful mechanism in prolonging the life of thetip vortices during their entrainment into the surface walljet. However, it should be noted that the seed void doesnot represent the dimensions of the viscous core but sim-ply an equilibrium position for the rotating seed particles.Nevertheless, the presence of a seed void suggests highvorticity and is evidence that the tip vortex is still presentin the flow. The signatures of the vortex sheets can alsobe seen in the resulting sheared flow zone at the groundsurface flow, perhaps as far away as two rotor radii.

The remaining three rotor heights of z/R = 0.75R, 0.5Rand 0.25R—see Figs. 11(f), (g) and (h), respectively—arerelatively low rotor heights that would encountered witha conventional helicopter, but nevertheless provide a use-ful set of results in helping to understand the fundamen-tal flow physics of rotor IGE operations. These cases allresulted in relatively low helical spacing between succes-sive turns of the rotor wake, and so significant tip vor-tex pairing was produced as the wake began to interactwith the ground plane. The tip vortices are significantlystretched soon after their formation, and appeared to dif-fuse their vorticity at relatively young wake ages as theyinteracted with the ground. The resulting flow near theground seemed relatively free of vorticity but was clearlyvery turbulent.

DPIV Flow Field Measurements

DPIV measurements were made for four rotor heights offthe ground, namely z/R = 1.5, 1.0, 0.5 and 0.25. TheDPIV results were post-processed to obtain both phase-averaged and time-averaged velocity vectors, and the datawere also processed in terms of velocity profiles measurednormal to the ground plane at different radial distancesfrom the rotational axis of rotor.

Time-Averaged Flow Field Results

Time-averaged velocity field results are shown in Fig. 12in the form of flow vectors on a color-contoured back-ground for the four rotor heights off the ground. Onlyevery other flow vector has been plotted in these casesto prevent congestion. Because the rotor thrust variesfor all four IGE cases, the results shown here have beennormalized by the momentum theory factor for the av-erage induced velocity, i.e., by dividing by the factorvh =

√T/2ρA. This approach allows the measurements

to be better compared on the basis of constant rotor thrust.In all four cases, it is obvious that the rotor induced flow

is forced to expand radially outward as a wall jet as pre-viously shown with the flow visualization. The quiescentflow (blue zones) outside of the rotor wake boundary isapparent. For z/R = 1.5, as shown in Fig. 12(a), it can beseen that the flow velocities within the wake boundary arerelatively high (about twice as large than the value of vh,which is expected based on momentum theory considera-tions). These high velocities reach out to about r/R=1.5 atthis rotor height, and thereafter drop quickly. The thick-ness of the wall jet (including the shear layer) is about0.2R, and the thickness slowly increases as the wake ex-pands radially to distances beyond r/R = 1.5.

For the z/R = 1.0 rotor height case, as shown inFig. 12(b), the overall flow was found to be similar tothe previous IGE case, but now a much higher local-ized flow velocity was produced at the wall for radialstations between r/R=1.2 to 1.8. The highest flow ve-locities exceeded 2vh and occurred at about r/R = 1.5.Again, the surface velocities dropped off quickly beyondr/R = 1.6 and the wall jet expanded. Continuity consider-ations would dictate that the centerline velocities shoulddecrease inversely with distance in the fully developedwall jet far away from the rotor.

For the two lowest rotor heights of z/R = 0.5 and 0.25,as shown in Fig. 12(c) and (d), respectively, the aver-age flow velocities might be expected to increase becauseof the closer proximity of the rotor to the ground plane.However, interestingly enough the results show that thevelocities remain relatively low (for a nominally constantrotor thrust), the local peak velocity values actually de-creasing from the results at the higher rotor height of z/R= 1.0. For z/R = 0.5 there are two zones where the localvelocities exceeded 1.5vh, one at about r/R = 1.25 and theother at about r/R = 1.85.

These results reflect the overall nonlinear effects of theground on the flow field, where the resulting outflow fromthe rotor at the ground plane (i.e., the groundwash) de-pends on the influence of the average rotor induced down-wash combined with the local effects of the blade tip vor-tices and the vortex sheets. Much of the local increasein flow velocity near the ground seen in Fig. 12(b) is be-

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(a) Rotor height z/R = 1.5 (b) Rotor height z/R = 1.0

(c) Rotor height z/R = 0.5 (d) Rotor height z/R = 0.25

Figure 12: DPIV results of the time-averaged velocity field through a radial plane for different rotor heights off theground: (a) Rotor height at z/R = 1.5, (b) Rotor height at z/R = 1.0, (c) Rotor height at z/R = 0.5, (d ) Rotor heightat z/R = 0.25.

cause of the persistence of the tip vortices to relatively oldwake ages as a direct result of vortex stretching effects—see Figs. 11d and (e).

At the higher rotor heights the vortex filaments spindown and diffuse under the action of viscosity and tur-bulence before they can ever reach the ground; at thelower rotor heights the turbulence in the developing walljet quickly shears the vortices, which causes them to losetheir coherent flow structures within only 1.5 rotor revolu-tions of wake age. It is these two competing mechanismsthat suggest that for any rotor there will be some interme-diate rotor height off the ground where the groundwashvelocities will reach a maximum. This height will dependon the average downwash (a thrust and/or disk loading ef-fect) produced by the rotor, the strength of the tip vortices(a blade loading effect for a constant tip speed), and the

number of blades (i.e., the number of vortices reachingthe ground per unit time).

Phase-Averaged Flow Field Results

Phase-averaged velocity field results are shown in Fig. 13for a rotor height of z/R = 1.0R. These images representa snapshot of the flow field for different blade positions.From the sequence of images, it can be seen how the tipvortices convect along the slipstream boundary and intothe developing boundary layer flow at the ground plane.

Notice that distinguishable vorticity exists even afterthe vortices become several rotor revolution old and havebegun to interact with the developing turbulent flow onthe ground. It is apparent that the action of the wall jetvelocity, as well as the turbulent wake sheets and other

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(a) Phase angle = 0 (b) Phase angle = 30

(c) Phase angle = 60 (d) Phase angle = 90

(e) Phase angle = 120 (f) Phase angle = 150

Figure 13: Phase-averaged velocity field for z/R = 1.0: (a) Phase angle = 0, (b) Phase angle = 30, (c) Phase angle =60, (d) Phase angle = 90, (e) Phase angle = 120, and (f) Phase angle = 150.

turbulence in the developing flow, begins to shear the vor-tices. The vorticity contained in the vortices, therefore,quickly becomes more diffused. Nevertheless, it is appar-ent that closer to the rotor the vortices induce high local

velocities near the ground with considerable fluctuationsas each vortex sweeps by.

As previously mentioned, at higher rotor heights thevortex filaments undergo diffusion before they reach the

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(a) Rotor height z/R = 1.5 (b) Rotor height z/R = 1.0

(c) Rotor height z/R = 0.5 (d) Rotor height z/R = 0.25

Figure 14: Phase-averaged velocity field for different rotor heights off the ground at a phase angle of 0: (a) Rotorheight at z/R = 1.5, (b) Rotor height at z/R = 1.0, (c) Rotor height at z/R = 0.5, (d ) Rotor height at z/R = 0.25.

(a) Phase angle = 0 (b) Phase angle = 30

(c) Phase angle = 60 (d) Phase angle = 90

(e) Phase angle = 120 (f) Phase angle = 150

Figure 15: Detail of the tip vortex pairing and shearing in the developing flow near the ground flow.

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ground, so their effect on the development of the wall jetis relatively small. As the rotor height from the groundplane is lowered however, the tip vortices begin to reachthe ground before they spin down and so begin to inducehigher local mean surface velocities as well as higher ve-locity fluctuations in the flow—see the sequence in Fig. 14and compare with the time-averaged results in Fig. 12.If the vortices reach the ground, then they are stretchedradially, which increases their core vorticity and inducedvelocities—see Figs. 14(a) and (b). The persistence of thevortices to four or five rotor revolutions has already beennoted in Fig. 11(b).

Figures 14(c) and (d) show that at the lower rotorheights off the ground, the tip vortices are sheared as theyenter the wall jet. This shearing action quickly diffusesvorticity, and the signatures of the vortices are only appar-ent for up to two rotor revolutions. Nevertheless, at thelowest rotor height the wall jet is seen to contain signifi-cant fluctuating velocities and turbulence.

The process is detailed in Fig. 15, which shows DPIVresults at the ground plane for a series of increasing times.Notice that the vortices begin to pair and merge, but arealso sheared in the developing flow. The merging resultsin some locally high induced velocities on the surface, butthese are also diminished by the shearing action. The neteffect is a zone of residual vorticity within the developingwall jet that produces significant velocities at the wall.

Time-Averaged Surface Velocity ProfilesThe DPIV results were further processed to extract time-averaged radial velocity profiles as a function of distancenormal to the ground surface for several rotor heights offthe ground. Measurements at six radial stations at r/R =0.8, 1.0, 1.25, 1.5, 1.75 and 2.0 were examined. It wasfound that developing jet-like flow near the ground takeson the features of both a turbulent boundary layer and ashear region, as shown in the schematic in Fig. 16.

Figure 16: The jet-like flow at the ground exibits thefeatures of a thin turbulent boundary layer at the sur-face and a shear region above.

The boundary layer region is relatively thin, typicallyonly a few millimeters or less than 5% of rotor radius. Thesmall thickness of the boundary layer poses some chal-lenges for the DPIV because of the need for a very finespatial resolution near the surface while also minimizinglaser reflections from the surface. The results show thatmeasurements are feasible well into the boundary layerand that its profile can be properly resolved using DPIV.

Directly below the rotor, as shown in Fig. 17(a), the ra-dial flow has already become established, and by r/R =1.0 (Fig. 17(b)) the wall jet has already become well de-veloped. Notice that the highest surface flow velocities areobtained for a rotor height of z/R = 1.0. It is these veloc-ities and velocity gradients that are responsible for shear-ing the flow in the tip vortices. Moving radially outwards(Fig. 17(c) through (f)), the wall jet becomes thinner andreaches its peak velocity at about r/R = 1.5. Thereafter,the peak velocity decreases and the boundary layer be-comes slightly thicker.

Phase-Averaged Surface Velocity Profiles

Phase-averaged velocity profiles for a rotor height of z/R= 1.0 are shown in Fig. 18. Notice in these cases, that thesignatures of the convecting tip vortices are apparent, atleast up to about r/R = 1.25. The largest unsteady fluctua-tions near the ground plane occur at r/R = 1.0, where thereare local excursions of about 50% from the time-averagedvalues. The swirl velocity induced by the tip vortex insidethe wall jet is in the same direction as the groundwash ve-locity and results in overall augmentation of the local flowvelocities near the surface.

This mechanism of how the wake interacts with theground is important because it will affect the wall shearstress and so, ultimately, the “brownout” problem alludedto in the introduction of this paper through the changes tothe local thresholds that will govern the uplift of particles.Underestimation of the overall decay rate of a blade tipvortex, for instance, will result in increased entrainmentof loose surface sediment, and more extended regions ofsediment lift-up. The mechanisms of shear and turbulentdiffusion, however, will likely increase the mixing of anyparticles near the ground, again affecting the number ofparticles that are lofted into the flow. The shear fluctu-ations on the ground clearly become much smaller withincreasing radial distance from the rotor.

Figure 19 shows the near-wall phase-averaged velocityprofiles for a rotor height of z/R = 1.0. In this case, re-sults are for the zone within 0.4 rotor radius (i.e., 34.4 mmor 1.35 inches) off the ground plane. The flow could bemeasured successfully down to about 0.015 radius (i.e.,1.29 mm or 0.051 inches) from the ground plane, at whichpoint surface reflections of the laser light (even from amatt-black surface) made it difficult to perform successful

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(a) r/R = 0.8 (b) r/R = 1.0

(c) r/R = 1.25 (d) r/R = 1.5

(e) r/R = 1.75 (f) r/R = 2.0

Figure 17: Time-averaged velocity profiles measured using DPIV at various radial distances from the rotor: (a) r/R= 0.8, (b) r/R = 1.0, (c) r/R = 1.25, (d) r/R = 1.5, (e) r/R = 1.75, (f) r/R = 2.0.

cross-correlations on the DPIV images.

Notice that the profiles near the ground show the fea-tures of a wall jet, with the boundary layer flow being rel-atively thin. The boundary layer thickness was estimatedin each case by using the location where the surface ve-locity reached its peak, the results of which are shown inFig. 20. Notice that the boundary layer becomes thinnermoving away from the rotor. While the initial thicknessdepends on the rotor height off the ground, the final thick-ness is the same in each case.

Conclusions

Experiments have been conducted using flow visualiza-tion and phase-resolved digital particle image velocime-try (DPIV) to examine the fluid dynamic behavior ofa rotor wake, with its embedded blade tip vortices andvortex sheets, as it interacted with a horizontal groundplane. Two-component DPIV measurements were ob-tained at four rotor heights off the ground, and allowedthe quantification of the flow within a few millimeters ofthe ground surface. Measurements at several wake ageswere also obtained to examine the dynamic features of the

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(a) r/R = 0.8 (b) r/R = 1.0

(c) r/R = 1.25 (d) r/R = 1.5

(e) r/R = 1.75 (f) r/R = 2.0

Figure 18: Phase-averaged velocity profiles measured using PIV at various radial distances for a rotor height of z/R= 1.0: (a) r/R = 0.8, (b) r/R = 1.0, (c) r/R = 1.25, (d) r/R = 1.5, (e) r/R = 1.75, (f) r/R = 2.0.

wake/surface interaction process.The following conclusions have been drawn from the

work:

1. Measurements of rotor performance showed the a re-duction in rotor power required for a constant thrustand an increase in the rotor thrust for a constantpower during in-ground-effect operations. These re-sults are typical for any hovering rotor operating inground effect, and do not appear to be affected sub-stantially by scale.

2. Flow visualization suggests that the overall down-

wash induced by the rotor is forced to escape as a jetbetween two boundaries defined by the ground planeand the wake boundary, the latter which at least ini-tially is defined by the path of the tip vortices. Asthe vortices age in the flow, the wake boundary be-comes much less distinct as it mixes with the devel-oping flow on the ground and produces a shear zone.

3. DPIV measurements of the flow were successfullymade near the ground plane, and down to about 1.5%of the rotor radius. The resulting flow velocity at theground plane closely resembled the form of a classic

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(a) r/R = 0.8 (b) r/R = 1.0

(c) r/R = 1.25 (d) r/R = 1.5

(e) r/R = 1.75 (f) r/R = 2.0

Figure 19: Near-wall phase-averaged velocity profiles at various radial distances for a rotor height of z/R = 1.0: (a)r/R = 0.8, (b) r/R = 1.0, (c) r/R = 1.25, (d) r/R = 1.5, (e) r/R = 1.75, (f) r/R = 2.0.

wall jet, which shows the characteristics of a turbu-lent boundary layer very near the wall and a shearflow away from the wall.

4. At the higher rotor heights the vortex filaments spindown (diffuse) under the action of viscosity and tur-bulence before they can reach the ground. At thelower rotor heights the turbulence in the develop-ing wall jet quickly shears the vortices, which causesthem to diffuse quickly.

5. In some cases, pairing and merging of the blade tipvortices occurred near the ground. However, the vor-

tices were also sheared in the developing wall jet.The residual velocity field was still sufficient to in-duce significant fluctuating shear on the wall.

6. The competing flow mechanisms (diffusion, stretch-ing and shearing) suggest that there will be some in-termediate rotor height off the ground where the sur-face flow (groundwash) velocities will reach a maxi-mum. This height will depend on the average down-wash (a disk loading effect) produced by the rotor,the strength of the tip vortices (a blade loading ef-fect), and the number of blades.

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Figure 20: Measurements of the radial developmentof the boundary layer thickness as a function of rotorheight off the ground.

Acknowledgments

This research was partly supported by the Army ResearchOffice (ARO) under grant W911NF0610394 and partlyby the Multi-University Research Initiative under GrantW911NF0410176. Additional support for the first authorwas provided by the Minta Martin Aeronautical Fund.

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