046

Dynamic Stall on a Fully Equipped Helicopter Model

K. Kindler1,†, K. Mulleners1, and M. Raffel1

1Institute of Aerodynamic and Flow Technology, DLR Gottingen, Germany†Current address: Max Planck Institute for Marine Microbiology, Bremen, Germany

Abstract

Three-dimensional dynamic stall was observed onthe rotor of a fully equipped sub-scale helicoptermodel in a wind tunnel. Results of stereoscopicparticle image velocimetry measurements on the re-treating blade clearly revealed the large-scale dy-namic stall vortex at 50% and 60% blade radius.The location of the axes of small-scale shear layervortices that constitute the dynamic stall vortexwere extracted. Based on the spatial distribution ofthese small-scale vortices, the characteristic large-scale structure was found to be unexpectedly com-pact. Along with an apparent spanwise flexion ofthe dynamic stall vortex, these results suggested therotational motion to have a stabilising effect on theformation and convection of this predominant fea-ture. The velocity field information which is nowavailable is also believed to be valuable for valida-tion of recent computational fluid dynamics inves-tigations of three-dimensional dynamic stall.

1 Introduction

Dynamic stall (DS) on an airfoil comprises a seriesof complex aerodynamic phenomena in response toan unsteady change of the angle of attack [2, 8, 9].It is accompanied by a lift overshoot and delayedmassive flow separation with respect to static stall.The salient feature of the unsteadily separating flowis the formation and convection of a large-scale co-herent structure referred to as the dynamic stall vor-tex. The most prominent example can be observedon the retreating blades of an helicopter rotor inforward flight.

Hitherto, experimental investigations have beenlimited to two-dimensional DS on oscillating air-foils [10, 8, 1, 14] and DS on finite wings of smallaspect ratio [20, 3]. Computational fluid dynam-ics (CFD) investigations on the other hand, havebeen constrained by challenges in turbulence mod-

elling in unsteady flows and limited computationalperformance. Only recently, CFD studies of three-dimensional DS on large aspect-ratio planforms,i.e. generic helicopter blades have become avail-able (cf. [19]). Previous two-dimensional DS studieshave lead to a deeper understanding of the compli-cated flow phenomena involved, but they have alsoclearly demonstrated the need to include and exam-ine the inherent three-dimensionality of the rotoroperating environment. Finite wing and rotationaleffects as well as the influence of blade lag motionhave to be considered for a comprehensive under-standing of DS on helicopter rotors.

Three-dimensional DS on finite wings is essen-tially characterised by the interaction of the tip vor-tices and the DS vortex which results in the emer-gence of a localised arched separation region in themid-span section of the wing [6, 17, 11]. Due tothe downwash induced by the wing tip vortices theeffective angle of attack is increased in the mid-span region leading to a faster growth of the DSvortex here. The latter consists of a combinationof the rolled-up shear layer and the remnants ofseveral vortices generated by a shear layer instabil-ity [12, 18]. The spanwise variation of the growthrate causes the DS vortex to arch away from thewings surface, yielding a bucking shape resemblinga capital omega (cf. [6]). The outboard segmentsare pinned to the surface and are weaker than thedominant central part of the vortex, which can bepresumed quasi-two-dimensional. In the outboardregion the three-dimensionality – and thus the com-plexity – of the flow field increases due to the influ-ence of the wing tip vortex and the accompaniedtip arching of the predominantly tranverse dynamicstall vortex.

When passing over to the rotor environment, theconcurrence of yaw angles and rotation leads to avariation in the effective angles of incidence alongthe span. The influence of the yaw angle and therotation of a large aspect ratio blade on DS were

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investigated numerically by Spentzos [19]. To theauthors’ knowledge, this is the only available (nu-merical) study of the subject and it reveals a asym-metric arched DS vortex which is shifted towardsthe outboard side of the blade.

Further advancement of DS prediction and mod-elling with regard to helicopter rotors requires fur-ther numerical studies on three-dimensional DS ina rotating frame of reference as well as additionalexperimental investigations to validate and comple-ment the numerical results. In the framework of theEU-project GoAHEAD (Generation of AdvancedHelicopter Experimental Aerodynamic Databasefor CFD code validation) a fully equipped model ofa generic medium size transport helicopter was in-vestigated in the Large Low-speed Facility (LLF) ofthe German-Dutch Wind Tunnels (DNW) [13, 16].In particular, phase-locked, high-resolution, three-component velocity field data of the dynamic stallvortex were obtained directly within the rotatingsystem. Thereby, a first quantitative insight intoDS on helicopter rotors has become available andwill be presented and discussed here. Furthermore,based on the gained insights and the experimental

difficulties encountered, further avenues of inquirywill be suggested.

2 Experimental Methods

The helicopter model consisted of a scaled NH90fuselage including all control surfaces, main, andtail rotor. A 40% Mach-scaled ONERA 7AD mainrotor was used with parabolic tips and included arotor hub. The blade radius was R = 2.1m withc = 0.14m reference chord length, yielding an as-pect ratio Æ = 15. The main rotor had a build-ininclination angle of −5 with respect to the fuse-lage and the blades featured τ = −8.3/R lineartwist. The model was positioned in the centre ofthe 8× 6m2 closed test section of the wind tunnel.

Since the test conditions for DS were consid-ered high-risk for the integrity of the model, stallonset was approached by varying the trim parame-ters at a free-stream and rotor tip Mach number ofMa∞ = 0.259 and MaMR = 0.617 respectively, andα = −3.95 angle of attack. The angular velocityof the main rotor was Ω = 101.1 rad/s correspond-ing to an advance ratio µ = U∞/(ΩR) = 0.42 with

r/R = 0.5

r/R = 0.6

r/R = 0.9

x

y

z

U∞

-−→Ω

Figure 1: Schematic representation of the experimental configuration and the positions of the measurementplanes on the retreating blade at ψv = 272.3.

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Figure 2: The footprint of dynamic stall in the three measurement planes at r/R =0.5, 0.6, and 0.9, atψ = 272.3 in azimuth; the scaled, phase averaged out-of-plane velocity component v is colour-coded whilethe in-plane components u, w are depicted as stream lines.

U∞ = 88.3m/s. When the power, vibration, con-trol, and actuator force limits were reached, a pitchlink force spike in the range of ψ = 180 to 270 wasencountered indicating stall inception1.

Stereoscopic PIV data was acquired at three ra-dial positions, r/R =0.5, 0.6, and 0.9 (figure 1) onthe retreating blade at ψ = 270 to 276 using stan-dard procedures [15]. Illumination was provided bya double-cavity Nd:YAG laser with a pulse energy of2×280mJ fanned out into a light sheet and directedinto the test section from the ceiling of the wind tun-nel. In order to reduce light reflections on the uppersurface of the blade, the light sheet access was lo-cated far downstream from the model in order toachieve an approximately tangential impingementon the surface of the blade. Due to the limited opti-cal access, the observation regions on the blade wereinclined by 22, 21, and 3 at r/R =0.5, 0.6, and0.9 with respect to chord. Series of N = 100 imageswere acquired through lateral windows in the wind

tunnel wall using a pair of high-resolution pco.2000cameras with 200mm lenses. Image acquisition waspulse-triggered by the main rotor with the camerasoperating at a multiple of the rotor frequency. Thesize of the fields of view were 0.23m× 0.22m to0.164m× 0.164m. The corresponding spatial reso-lutions varied between 1.4mm and 1mm.

3 Results and Discussion

3.1 Phase averaged flow fields

At a free-stream and rotor Mach number ofMa∞ =0.259 and MaMR = 0.617 respectively, a large-scale coherent DS vortex can be clearly identi-fied at r/R = 0.5 and 0.6 whereas at the mostoutboard position r/R = 0.9 a fully separatedflow is observed, which is moreover strongly three-dimensional (cf. figure 2). In this region the bladetip vortex interacts with the outboard segment of

1Conventionally ψ = 0 denotes the rear, i.e. downstream, position of the blade.

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a

b

c

Figure 3: The scaled, phase averaged velocity (left) and the in-plane vorticity component (right) atψ = 272.3 and r/R = 0.5 (a), 0.6 (b), and 0.9 (c).

the DS vortex.

More detailed views of the phase averaged ve-locity fields2 along with the in-plane vorticity fieldsare depicted in figure 3. It can be readily observedthat the chord-wise position of the DS vortex is vir-tually unchanged between r/R = 0.5 and 0.6. Inthis spatial region the DS vortex thus appears to

be stretched virtually parallel to the leading edge,which is in good agreement with recent CFD resultsby Spentzos [19]. However, this large-scale struc-ture is noticeably flatter at r/R = 0.6 than at mid-span while the associated region of concentrated in-plane vorticity ωy is distinctly enlarged. Further-more, the radial or out-of-plane velocity component

2The free stream velocity U∞ has been subtracted in all vector fields. The out-of-plane component is normalised as(v − v)/U∞ with v taken far from the blade surface.

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at r/R = 0.5 is strongly negative, i.e. directed out-board, in a concentrated region covered by the DSvortex (see figure 3a). The area of strong outboardswirling flow is flanked at both sides by patches ofinboard directed flow. The latter is still valid atr/R = 0.6 (figure 3b). However, the strength of theoutboard directed velocity component is attenuatedand the extent of the region covered by it is reduced.

3.2 Sub-structures of the dynamic stall vortex

Although the DS vortex appears as a single com-pact vortical structure in the image sequence of fig-ure 3, it is essentially formed by the roll up of theshear layer that emerged at the interface betweenthe separated flow near the airfoil’s surface and thefree stream flow. Hence, the DS vortex is composedof a multitude of small-scale vortices generated by aprimary instability of this shear layer. To study theconstitution and compactness, as well as the cycleto cycle stability of the large-scale DS vortex, theindividual small-scale vortical structures were iden-tified and localised by means of a Galilean invariant,Eulerian vortex axis identification procedure. Theposition of the centre of vortical structures in the en-semble of velocity fields was determined utilising thefollowing scalar function introduced by Graftieauxet al. [5]:

Γ2(P ) =1

N

∑

S

[PM × (UM − UP )] ey

‖ PM ‖ · ‖ UM − UP ‖, (1)

where N is the number of points in the two dimen-sional neighbourhood S of any given point P in thex, z plane, M lies in S, ey is the unit vector in ydirection, and UP is the local convection velocityaround P . The local extrema of Γ2 are associatedwith the location of vortex centres3.

Unlike alternative vortex identification criteria,such as vorticity concentration, λ2, etc. , the Γ2

function does not require the evaluation of velocityfield gradients and is therefore less susceptible toexperimental noise. For a comprehensive review ofthe diversity and robustness of gradient-based vor-tex detection methods it should be referred to [7, 4].

The spatial distribution of all individual small-scale shear layer vortices extracted from the ensem-ble of instantaneous velocity fields at r/R = 0.5and 0.6 is depicted in figure 4 and overlayed bythe phase average in-plane velocity field. Addition-

ally, histograms represent the vertical and stream-wise distributions separately. Noticeably, especiallyfor readers familiar with two-dimensional DS ex-periments, is the very localised distribution of thesmall-scale structures. The conclusions here aretwofold. First, the strong confinement refers to aspatially very compact DS vortex which remains inclose proximity to the airfoil’s surface. This is some-what different from what is usually observed in two-dimensional measurements (cf. [12, 18]). Hence, therotation of the blade and the associated span-wisevelocity gradient seem to have a stabilising effect onthe DS vortex, preventing it to fall apart. Second,the spreading of the vortex positions due to the cy-cle to cycle variations must be relatively small.

a

b

Figure 4: The in-plan velocity, the centre positionsof sub-structures composing the DS vortex and thedistribution of centre positions at ψ = 272.3 forr/R = 0.5 (a), 0.6 (b).

3Note that Γ2 is not to be confused with the circulation of a vortex.

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4 Conclusions and Directions forFuture Experiments

Dynamic stall was directly observed on the retreat-ing blade of the rotor of a fully equipped sub-scale helicopter model during wind tunnel tests.The characteristic DS vortex was identified and ap-peared to be stretched almost parallel to the lead-ing edge in the region covering the measurementplanes at 50% and 60% blade radius. At r/R = 0.9the blade tip vortex interacted with the outboardsegment of the DS vortex yielding increased three-dimensionality and complexity of the flow field.Based on the identification and analysis of the in-dividual small-scale shear layer vortices composingthe DS vortex, the latter is found to be spatiallyconcentrated. This is attributed to a combinationof rotational and finite wing effects.

These first quantitative insights into the DSphenomenon on helicopter rotors are believed tobe highly valuable for validation of computationalfluid dynamics investigations. Nevertheless, exten-sive experimental efforts including the rotor oper-ating environment are required in order to con-tribute to the fundamental understanding of three-dimensional DS on helicopters. In this context, thespanwise development and deformation of the DSvortex needs to be further analysed. In particular,information about the interaction of the end seg-ment of the DS vortex with the reversed flow regionand the blade tip vortex is of utmost interest. Ad-ditional investigations of the whole DS life cycle bymeans of high-fidelity PIV are highly desirable in or-der to model and predict the complex aerodynamicphenomena attributed to DS. For this purpose itis envisaged to build the image acquisition systeminto the rotor hub and to provide a rotating lasersheet from above the rotor axis. By doing so, thecontinuous observation of the velocity field and thedevelopment of the characteristic DS vortex as afunction of azimuthal position will be possible.

Acknowledgements This work has been part of the Euro-

pean Commission funded project “Generation of Advanced

Helicopter Experimental Aerodynamic Database for CFD

code validation” (GoAhead). Valuable discussion and sup-

port by the project partners is greatfully acknowledged.

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