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TASKQUARTERLYvol.19,No2,2015,pp NUMERICAL ANALYSIS OF HIGH-SPEED IMPULSIVE(HSI) NOISE OF PZL W3-A SOKÓŁ (FALCON) HELICOPTER MAIN ROTOR IN FORWARD FLIGHT OSKARSZULC 1,PIOTRDOERFFER 1,FERNANDO TEJERO 1,JERZYŻÓŁTAK 2 ANDJACEKMAŁECKI 3 1 InstituteofFluid-FlowMachinery,PolishAcademyofSciences Fiszera 14, Gdansk, Poland 2 InstituteofAviation Al. Krakowska 110/114, Warszawa, Poland 3 PZL-ŚwidnikS.A.,AugustaWestland Al. Lotników Polskich 1, Świdnik, Poland (received: 20 January; revised: 23 February 2015; accepted: 25 February 2015; published online: 3 April 2015) Abstract: The paper presents the results of a numerical simulation of the flow and acoustic field generated by the PZL W3-A Sokół (Falcon) helicopter main rotor in high-speed forward flight conditions based on the URANS approach and the chimera overlapping grids technique. A refined CFD model(40+ million of control volumes, 600+ blocks chimera mesh) was designed to resolve the flow-field together with the low-frequency content of the acoustic pressure spectrum in the near-field of the rotor blades to allow high-speed impulsive(hsi) noise prediction. Detailed 3D data was recorded for one rotor revolution(approx. 3 TB) allowing exceptional insight into the physical mechanisms initiating the occurrence and development of the HSI noise phenomenon. Keywords: aerodynamics, CFD, forward flight, helicopter rotor, HSI noise, shock wave 1. Introduction PZL W3 Sokół (Falcon) is a Polish, medium-size, twin-engine, multipurpose helicopter constructed by PZL-Świdnik, now a member of the British-Italian AugustaWestland company(figure 1). This first helicopter fully designed and serially produced in Poland has been continuously in service since The originalmainrotordesignhasservedformorethan25yearsandisstilloperating in hundreds of machines sold all over the world. The increasing significance of 182 O.Szulc,P.Doerffer,F.Tejero,J.ŻółtakandJ.Małecki the fuel consumption and noise emission restrictions forces the design of an improved version of the helicopter with enhanced performance and reduced fly-over noise. A completely new, 4-bladed main rotor(based on the ILH family of profiles recently developed at the Instytut Lotnictwa in Warsaw[1]) for the modernized W3-A Sokół (Falcon) helicopter is designed and manufactured by the Instytut Lotnictwa and PZL-Świdnik, verified experimentally through scale model wind tunnel tests by the Instytut Lotnictwa and tested numerically by the Institute of Fluid-Flow Machinery in Gdansk. Figure 1. PZL W3-A Sokół (Falcon) of the Polish Tatra Volunteer Search and Rescue The unrestricted part of the work (described in the article) contains aerodynamic and aero-acoustic results of a numerical simulation of the original 4- bladed NACA rotor in high-speed forward flight. Over the duration of the project detailed simulations of both NACA and ILH rotors were performed and compared for different low- and high-speed forward flight conditions. The numerical results suggestthatthenew,ilhrotorismoreefficientandemitslessnoiseinflight comparedtotheoldernacadesign,butsofarthedatacouldnotbepublished due to the confidentiality issues. Hence, the aerodynamic results of the high-speed case will be compared with very limited flight-test data, while for the acoustic part itisonlythecfdresultsthatareavailable,bothfortheoriginalnacarotoronly. 2. Physical and numerical modeling 2.1. FLOWer code from DLR The present investigation was carried out with the FLOWer solver from DLR[2]. It is a modern, parallel, block-structured, cell-centered code solving Favre-averaged Navier-Stokes equations with various turbulence models. The ROT/CHIMERA version of the code allows using the chimera overlapping grids technique and moving meshes. From various turbulence closures implemented in FLOWer, a two-equation, low-reynolds k-ω turbulence model of LEA(Linear Explicit Algebraic Stress Model) was chosen[3]. The numerical algorithm uses asemi-discreteapproach,utilizinga2 nd orderfinite-volumeformulationforthe spatialdiscretizationanda2 nd orderimplicitdual-time-stepping(withexplicit5- stage Runge-Kutta) method for integration in time. A scalar artificial dissipation model of Jameson is implemented to damp numerical oscillations. A time step equaltothetimeneededforarotationby0.25 ofazimuth(i.e.1440timesteps Numerical Analysis of High-Speed Impulsive(HSI) Noise of PZL W3-A Sokół perperiodofrotation)andacflnumber 10.0fortheinternalR-Kstages was set for the dual time-stepping scheme. At each physical time step the density residual gained a drop of 3.0 orders of magnitude within several iterations, which proved to be sufficient to obtain an accurate, unsteady flow-field around the rotor Rotor geometry The first approximation is to abandon the influence of the fuselage and the tailrotorandtoisolatethemainrotorblades.theelasticdeformationsduetothe air-loadsareneglectedintheoverallpictureaswell.therotorofthepzlw3- A Sokół (Falcon) helicopter consists of 4 blades(based on the NACA 23012M airfoil)havingaradiusofr=7.85mandlinearlytwistedfrom0 attherootup to 12 atthetiplocation(figure2).apartfromtherootarea,thetrimming tabsandthetipregion,thechordisequaltoc=0.44m.therotorrotatesinthe clockwise direction(as seen from above). In forward flight the rotor blades not onlyrotatearoundtheazimuth,butalsopitchandflap(thelead-lagmotionisnot consideredhere).theazimuthangleisassumedtobe0 forthefirstbladewhen it is pointing in the direction opposite to the flight direction. In forward flight the shaft normal plane is additionally inclined to the flight direction(inflow) at a constant angle. Figure2.NumericalmodelofthemainrotorofPZLW3-A Sokół (Falcon) 2.3. Chimera component grids The main idea of the chimera technique implemented in the FLOWer code is to easily generate grids for complex configurations by decomposing them into simple, independent parts[4]. The only limitation is that all component meshes 184 O.Szulc,P.Doerffer,F.Tejero,J.ŻółtakandJ.Małecki should overlap each other to allow inter-grid communication. In case of rotors in forward flight the chimera overlapping grids technique allows easy control of the rigid motion of the blades(translation, rotation, pitch and flap) preventing any grid deformation. The component overlapped grids for the PZL W3-A Sokół (Falcon) helicopter rotor were generated using a script based approach and the Interactive Grid Generator(IGG) from Numeca International allowing semiautomatic meshing. Three component grids created for a single rotor blade(root, center and tip areas) are placed within the background Cartesian mesh. The remaining three blades are generated automatically by the FLOWer solver. The Cartesian background grid is designed as a cuboid with the dimensions of 16.4R 18.2R 18.2R. Consequently, the far-field surface is located at least 8.0R away from the rotor in every direction(figure 3). 32 computational blocks contain volumes(25%ofthetotalnumberofcells).thecoreofthe background grid surrounding the rotor blades is divided into 4 computational blocks( , volumes)extendingbeyondthebladetipto1.1r and reaching ±0.22R above and below the rotor plane(figure 4). A uniform Cartesian grid(dimensions of volumes: 0.16c 0.16c 0.16c) located in this refined area constitutes an acoustic box designed to support propagation of the acoustic pressure waves resolved in space up to 950 Hz(5 points per wavelength) anddetectedupto2380hz(2pointsperwavelength).awayfromtherotor the grid spacing is more relaxed. The background grid component undergoes translation with forward flight velocity and a constant tilt by a shaft angle. Figure 3. Background component grid Numerical Analysis of High-Speed Impulsive(HSI) Noise of PZL W3-A Sokół Figure 4. Acoustic box Theregionofthebladeroot(Figure5)ismeshedusinganO-typegrid in stream-wise and H-type grid in crosswise directions. It spans from the surface for2chordlengths(0.88m)inthenormaldirectionand1.5chords(0.66m)in theradialdirection.itconsistsof13blocksand volumesperblade.the majority of the blade is covered by the center(figure 6) chimera component grid of a C-type in streamwise and H-type in crosswise directions. It spans from the surfacefor2chordlengths(0.88m)inthenormaldirectionandconsistsof118 blocksand volumesperblade.theclose-upatthebottomleftcorner Figure 5. Blade root component grid 186 O.Szulc,P.Doerffer,F.Tejero,J.ŻółtakandJ.Małecki Figure 6. Blade center component grid Figure 7. Blade tip component grid of Figure 6 reveals the geometrical complexity of the surface mesh and block topologynearthetrimmingtablocatedatthetrailingedgeoftheblade(area depicted in Figure 2 as trimming tab ). The last component grid covers the area ofthebladetip(figure7).duetoaroundedtipshapeano-typegridisappliedin streamwise and crosswise directions. It spans from the surface for 2 chord lengths (0.88m)inthenormaldirectionandconsistof19blocksand volumesper blade.theclose-upatthebottomleftcorneroffigure7presentstherounded tipsurfacemeshinmoredetails(areadepictedinfigure2as tip ).Theroot, Numerical Analysis of High-Speed Impulsive(HSI) Noise of PZL W3-A Sokół center and tip components undergo all the prescribed motions: the translation with forward flight velocity, the rotation around the azimuth, a constant tilt by ashaftangle,theunsteadypitchandflap.acompletesetofmeshesconsists of632blocksand controlvolumes.thebladecomponentgrids(root green,center redandtip bluecolor)ofthefirstbladeareplacedinthe background grid(figure 8). The remaining three blades are set-up and managed by the FLOWer solver. Figure 8. Rotor chimera grid topology 2.4. Flight test conditions The high-speed flight test(266 km/h) of the PZL W3-A Sokół (Falcon) helicopterwasperformedat931mabovethesealevelinthetemperatureof5.2 C. Therotoroperatedat28rad/swiththetipMachnumberof0.66,thetipReynolds numberof ,theforwardflightMachnumberof0.22andtheforwardflight Reynoldsnumberof (theadvanceratioof0.34).duringtheflighttest the instantaneous values of rotor control angles were recorded(pitching, flapping and shaft tilt according to the flight direction) and applied in the simulation(see detailsin[5]). 3. Validation of the method 3.1. Caradonna-Tung model helicopter rotor in high-speed hover The aerodynamic validation of the FLOWer solver applied to rotorcraft flowsagainsttheexperimentaldatawasbasedonafamousnasatestcasefrom 1981 of the Caradonna-Tung(C-T), two-bladed model helicopter rotor in highspeed, transonic hover conditions, operating with a tip Mach number of and collectiveof8 [6].AQ-criterionvisualization(coloredbythevorticitymagnitude) 188 O.Szulc,P.Doerffer,F.Tejero,J.ŻółtakandJ.Małecki of the rotor flow-field and wake was extracted from the FLOWer numerical solution based on a structured grid and Spalart-Allmaras(SA) turbulence model (Figure 9). On the other hand, an exemplary pressure coefficient distribution c p obtainedusingagaintheflowercodeandthechimeraoverlappinggrids technique(sa turbulence closure) is compared in Figure 10 with the experimental data measured at the chordwise cross-section of the blade r/r = 0.89(more information in[7 9]). The solutions presented in Figures 9 and 10, based on the block-structured and chimera grids are of equal quality(compared with the experimental data) as the previous numerical results obtained using two other CFD codes: SPARC from the University of Karlsruhe(Germany)[7, 10 12] and Fine/Turbo from Numeca Int.(Belgium)[7]. The FLOWer results obtained for a hovering C-T rotor using the chimera set-up and LEA k-ω turbulence model Figure 9. Caradonna-Tung rotor wake Figure10.Pressurecoefficientc p Numerical Analysis of High-Speed Impulsive(HSI) Noise of PZL W3-A Sokół (a combination applied for the PZL W3-A Sokół (Falcon) and the AH-1G Cobra helicopter main rotor forward-flight simulations) are validated as well, but presentedwithmoredetailsin[8,9,13] AH-1G Cobra helicopter rotor in high-speed forward flight The aerodynamic validation of the forward flight capabilities of the FLOWer code and chimera overlapping grids technique using the LEA k-ω turbulence modelwasperformedagainsttheflighttestdatagatheredbycrossj.l.and WattsM.E.atNASAin1981fortheAH-1GCobrahelicopterequippedwith a 2-bladed, OLS main rotor in low-, medium- and high-speed flights[14]. In highspeedforwardflight(290km/h)therotoroperatedwiththetipmachnumberof 0.64andforwardflightMachnumberof0.24(advanceratioof0.38).Duringthe flighttesttheinstantaneousvaluesofthrustcoefficientc T andpressurecoefficient c p distributionsatseveralcross-sectionsofthebladeweremeasured.therecorded rotor control angles(pitching, flapping and shaft inclination regarding the flight direction) were applied in the simulation as well. The Q-criterion(colored by the vorticity magnitude) visualization of the flow-field of the rotor at the azimuthal positionof80 isextractedfromtheflowernumericalsolution(figure11). Servingasanexamplethenormalforcecoefficientc n vs.azimuthψandchordwise pressurecoefficientc p distributionarecomparedwiththeexperimentaldata measured at r/r = 0.86(more details in[8, 9]). The calculated averaged thrust coefficientc T isoverpredictedby20%.thisleadstotheconclusionofasignificant influence of the modeling simplifications related to the rigid blade assumption (noflexibility),lackofthefuselageandtailrotororabsenceofthemainrotor trimming.still,thedatamaybeusedforacomparativestudyofrotorsintermsof the aerodynamic and aero-acoustic performance. Contrary to the low-speed data, thepresentedcfdresultsforthehigh-speedflighttestareuniqueintermsofthe literature survey. 4. Numerical results 4.1. Flow-field of the PZL W3-A Sokół (Falcon) helicopter main rotor in high-speed forward flight The flow-field around the PZL W3-A Sokół (Falcon) helicopter main rotor in high-speed forward flight is visualized by an iso-surface of the Q-criterion (colored by the vorticity magnitude) in Figure 12. The blade tip and trimming tabs create strong vortices that interact with the blades leading to the perpendicular and parallel blade-vortex interactions. Due to high relative inflow velocity at the advancing side large areas of the supersonic flow emerge, terminated by shock waves, having a significant impact on the level of the generated high-speed impulsive(hsi)noise.therotorthrustc T andpowerc P coefficientsfluctuatein timewiththemeanvaluesequalto:c T = andC P = Themean componentoftheforceactingagainsttheweightofthehelicopterisequalto 7130kgandthemeanpowerisequalto2200HP.Itisworthtomentionthatthe 190 O.Szulc,P.Doerffer,F.Tejero,J.ŻółtakandJ.Małecki Figure11.AH-1GCobrarotorwake,pressurec p andnormalforcec n coefficients Figure 12. PZL W3-A Sokół (Falcon) helicopter rotor wake(as seen from below) Numerical Analysis of High-Speed Impulsive(HSI) Noise of PZL W3-A Sokół PZL W3-A Sokół (Falcon) helicopter had a take-off weight of 6100 kg(minus 150kgoftheconsumedfuel)andwasequippedwithenginesofthetotalpower of1800hp.thenumericalsimulationoftheflowpasttheisolatedrotorofthe PZL W3-A Sokół (Falcon) helicopter in high-speed forward flight leads to an overpredictionofthemeanthrustandpowerby 20%comparedtotheflighttest data of a complete helicopter(more details in[5]). It is worth mentioning that very similar deviations in the aerodynamic performance have been presented in subsection 3.2 devoted to the AH-1G Cobra helicopter main rotor in high-speed flight Acoustic field of the PZL W3-A Sokół (Falcon) helicopter main rotor in high-speed forward flight The acoustic post-processing of the flow-field of the PZL W3-A Sokół (Falcon) helicopter main rotor in high-speed forward flight is based on an analysis ofalargesetofoutput3ddatafiles(3tb)containingdensity,pressureand velocity components as well as turbulent quantities. The data is recorded for one rotationperiodoftherotorevery1 ofazimuth(thecfdsimulationisprogressed intimewith0.25 time-step)whilekeepingthespaceresolutionunmodified. It is worth mentioning that the acoustic box uniform grid refinement of the volume enclosing the rotor blades supports propagation of the acoustic pressure fluctuations up to a frequency of 950 Hz(5 points per wavelength) and detection uptoafrequencyof2380hz(2pointsperwavelength).outsideofthe acoustic box the resolution of the grid decreases, leading to a strong suppression of the acoustic waves. Since the rotorcraft high-speed impulsive noise phenomenon is low-frequency dominated and emitted mostly in the rotor plane the model constitutes a good approximation of the generation and near-field propagation of the acoustic signals[15]. An example snapshot of the instantaneous acoustic pressure field at aplaneperpendiculartotheshaftandlocated0.35mabovetherotorhubis presented in Figure 13. The pressure fluctuation scale is limited to a range from 100Pato100Painordertodistinctweakacousticwavesfromaveryintense background. Due to the coning and flapping motion the root and inner parts ofthebladesarelocatedbelow,whiletheouterpartsandtipsarepositioned above the chosen plane. A particularly strong HSI pressure impulse( 140 db) isgeneratedbytheadvancingbladeat90 ofazimuthandtransmittedinthe direction forward of the helicopter(marked as HSI noise in Figure 13). To quantify the aerodynamic noise of the rotor the overall sound pressure level OASPL [db]measureispresentedinfigure14.theaveragingprocessoftherootmean square(rms) of the acoustic pressure amplitude is based on the interpolation of thevalueontoauniform,2dplanargrid( , cells,resolution of0.1c)beingperpendiculartotheshaftandlocatedalso0.35mabovetherotor hub.duetohighadvanceratioof0.34theoasplmapishighlyasymmetrical with higher values shifted to the advancing side of the rotor. The maximum value oftheoasplrecordedintherotordiscatthisplaneisextremelyhighandequal 192 O.Szulc,P.Doerffer,F.Tejero,J.ŻółtakandJ.Małecki Figure 13. Instantaneous acoustic pressure Figure 14. Overall sound pressure level to158db,butstilllowerthanthemaximumvalueinthewholevolume(161db) locatedattheplanethatiscrossingtheadvancingbladetipat90 ofazimuth (0.58mabovetherotorhub).Themaximumnoiselevelisdecreasingto145dB whenprobingthepressurefartherawayfromtherotoratacirclewitharadiusof 8.635m(inscribedinthe acousticbox atthesameplanelocated0.58mabove therotorhub).alreadyat5mfromthetipoftheadvancingbladeat90 of azimuththenoiseleveldropsto 120dB. Numerical Analysis of High-Speed Impulsive(HSI) Noise of PZL W3-A Sokół The HSI noise phenomenon is strongly dependent on a maximum inflow Machnumberpresentattheadvancingbladeat90 ofazimuthcalledadvancing tipmachnumberma AT [16].Theshapeandamplitudeoftheacousticpulse arechangingnon-linearlywithincreasingma AT.Belowacriticalvalue(called the delocalization Mach number) the smooth acoustic waveform is symmetrical (elliptic). Above and at the delocalization Mach number the pulse changes dramatically into a shock waveform (hyperbolic). For the UH-1H helicopter rotor(2-bladed, rectangular and untwisted NACA 0012) operating in non-lifting, hoveringconditionsthedelocalizationisinitiatedbetweenma AT =0.88and0.90 [17]. For more advanced rotors(in terms of airfoil or blade tip planform, i.e. swept, taperedorthinned)thisborderisshiftedtoevenhighervaluesofma AT 0.92. In a rotating frame of reference the described transition occurs when a local supersonicpocket(relativemachnumberma r 1.0)locatedatthebladetip develops a connection with the non-linear sonic cylinder(surface of a relative MachnumberMa r =1.0).Forthisreason,thevisualizationoftheMa r isof greatimportanceinastudyofthehsinoiseandispresentedforthepzlw3- A Sokół (Falcon) helicopter rotor in Figure 15. In the analyzed high-speed flightwithma AT =0.88thesupersonicarea(Ma r 1.0)locatednearthetipof theadvancingbladeisnotconnectedwithanon-linersoniccylinder(ma r =1) leading to a HSI noise phenomenon far below delocalization. Figure15.RelativeMachnumberMa r 194 O.Szulc,P.Doerffer,F.Tejero,J.ŻółtakandJ.Małecki Although the shock wave is not delocalized, a very intense acoustic pressure pulse
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