Tip Noise Passive Control in Low Mach Number Axial Fan

Tip Noise Passive Control in Low MachNumber Axial FanStefano BIANCHIXI Israeli Symposium of Jet Engines and Gas TurbinesHaifa, Israel, October 25 2012

Outlines of civil aircraft issues

Introduction- Typically launch/recovery support personnel can beexposed to brutal acoustic loads of up to 150 dBA. Eachlaunch typically involves a 30 sec average mil powerexposure (occasionally full AB).- The carrier deck personnel may experience up to 200launches/recoveries per 12 hr duty shift.Picture US NavyThe goal of engines noise reduction is considered of high importance.

- For high by pass ratio engines turbine noise accountonly during the Landing operation, as the TO thrust isexerted mainly by the fan.- Low bpr engines are more senstive to turbine noise,even for TO operation.NASA report: FS-1999-07-003-GRC- In low bpr engines turbine noise is comparablewith the rear component of compressor noise:directed roughly to the engine side.Rolls-Royce, 2006

Techniques and concepts for tip treatment, a first review (ii)• During the last decade have appeared several passive noise control concepts basedon the blade tip modifications by means of anti-vortex appendagesanti-vortex devices as end-plates at the tiporiginally inspired by the technique developed for the control of tip vortex and induced dragreduction in aircraft wings, also used as anti-vortex devices for catamaran hullsKarman end-plateBoeing, blended wingletsF2005, rear winglets

Conventional end-plate design• Base-line solution for the tip treatments, Corsini et al. (2006, 2007), defined a bladeconfiguration with use of tip appendages to implement a control on leakage flowrotor AC90/6/TF tip blade section modified byan end-plate on pressure surface with squaretailtrailing edgeTF end-platetip blade section is locally thickened of a factor3:1 wrt maximum thickness at the tip of datumbladeAccording to the theory behind the end-plate design, thisdimension was chosen as the reference radial dimension ofleakage vortex to be controlled approx. 0.2 0.1 blade span,(Inoue et al., 1986) (Corsini et al., 2004).FMGroup @ DMA-Sapienza

Analysis on inner workings of TF end-plates, tip leakage vortex bursting• This finding was in accordance with the existence of a vortex breakdownCriterion for VB detection based on vortex swirl analysisan interpretive criterion to detect the appearance of vortex breakdown makes use of the Rossbynumber (or inverse swirl parameter) (Ito et al., 1985)Ro = V /(r W)Ro1.61.4design pointfor a confined vortex the scales are:r , radial distance from the vortex axis at whichthe swirl velocity was at its maximumin accord with characteristic viscouslength scale Leibovich (1984)V = waTLV, was taken to be the axial velocity1.210.8W, be the rotation rate in the wing-tipvortices due to the solid body rotation-likestructure (estimated near the vortex centre).0.60.40.4 0.5 0.6 0.7 0.8fraction of chordbreakdown regionFMGroup @ DMA-Sapienza

End-plates for tip leakage vortex swirl control, design goals• The work is dedicated to sketch new concepts for the shaping (design)of the anti-vortex tip end-plate• The rationale is to designate a variation of end-plate thickness, in order toimplement a manipulation of the chord-wise evolution of the leakage vortex swirlVisualization and Analytics Center for Enabling Technologies(VACET) DOE, USAAircraft Vortex Manipulation,Manipulation of the blade vortex interactionof helicopters with nonplanar blade tips on the main rotorInstitute of Aeronautics and Astronautics, Aachen, Germany• The aim is to promote an enhancement or a detriment of near-axis swirl by influencingthe momentum transfer from the leakage flow and by inducing some wavinessinto the leakage vortex trajectory, as attempted in delta-wing platform designSrigrarom and Kurosaka (2000)FMGroup @ DMA-Sapienza

End-plate for Multiple Vortex Breakdown MVBconcept rationaleCorsini A, Rispoli F & Sheard AG, A MeridionalFan, Patent granted No. GB 2452104, July(2009)Rossby number definition for a confined vortexTLV rotational frequency and length measuresgiven by Rain (1954) and Lakshminarayana (1970)Leakage flow modelled as a two-dimensional floworthogonal to the chord lineAC90/6 fansdatumTF end-plateThe TLV Rossby number could beexpressed along the blade chord as afunction of:- gap geometry, (height & width)TFvte end-plate- kinetic energy of the leakage flowMVB end-plate- friction factor which is itself a function ofleakage flow Reynolds number, Regap- local blade load conditionsRo(xc)=f [tip gap, blade load, leakage flow, end-plate thickness]FMGroup @ DMA-Sapienza

CFD survey detailsdatumThe authors solved the Reynolds-Averaged Navier-Stokesequations by an original parallel Multi-Grid Finite Elementflow solver, using C++ technology and libMesh librariesTFThe physics involved in the fluid dynamics ofincompressible 3D turbulent flows in a rotating frame ofreference is modelled with a non-linear k-e model, here inits topology-free low-Reynolds variant.The Finite Element flow solver uses second-order accurateapproximations in space for primary-turbulent variables.TFvteConcerning the solution strategy, the authors solved theNavier-Stokes and turbulence equations fully coupled. Thelinear solver uses a SOR preconditioned GMRES(5)technique.The mesh was built using a non-orthogonal body fittedcoordinate system with block-structured topology. Themesh consists of about 0.6 million linear hexahedralelements.Standard boundary condition settings were adopted, aspreviously used in studies on high-performance fansFMGroup @ DMA-Sapienza

datumMVB tip leakage flow surveyTLV detection helicity criterion (i)MVBTFTFvteIdentical TLV onset point wrt to datum and TFVortex core path deformation according to theenhancement/detriment of swirlFMGroup @ DMA-Sapienza

TLV helicity & 3D streamlinesMVB tip leakage flow surveyTLV streamlines (ii)datum TF TFvteswirl enhancementNo evidences of helicity inversionswirl detrimentNo evidences of separation cores, TLVgoes through the VB onset withoutactually burstingMVBFMGroup @ DMA-Sapienza

MVB tip leakage flow survey, TKE & n t (iii)Turbulent kinetic energy iso-surfacesMVB0.80.80.20.20.80.2datum rotor AC90/6/TF AC90/6/TFvteNormalized turbulent viscosity nt iso-surfacesnt_2: 10 2 nmol;nt_1: 5× 10 nmoln t _1n t _1n t _1MVBn t _2n t _2n t _1n t _2datum rotor AC90/6/TF AC90/6/TFvteFMGroup @ DMA-Sapienza

Turbulent kinetic energy iso-surfacesMVBLw dB(A)frequency (Hz)Normalized turbulent viscosity nt iso-surfacesThe noise has a broadband characteristicMVBTones are due to the tip leakage flow in thepressure sideBoundary layer separation appears in the suctionside at the trailing edgeFMGroup @ DMA-SapienzaFLAKT WOODS Ltd

operating point 8 m3/snarrow band (3 Hz) sound pressure level spectraMVB rotor performance assessmentaeroacoustics (ii)sound power spectra in one-third octave bandBPFmotor signatureFMGroup @ DMA-Sapienza

FMGroup @ DMA-SapienzaMVB rotor performance assessmentaeroacoustics (iii)

Experimental techniques, rig (i)Fan tested @ 8 – 6 m 3 /sAnechoic chamber BS848 p2Far-fieldmicrophoneNoise measurements @ farfield:Span traversed microphone(when operating in the rig)• distance = 2 rotor D from the fan section• six azimuthal positions = 0° = 90° = 75° = 60° = 45° = 30°Pressure measurements @ nearfield:• distance d=15 mm from the trailing edge2% of the chord• spanwise traversing microphone in 14positions (10% span each)FMGroup @ DMA-Sapienza

Experimental techniques, data post-processing (ii)Near-fieldFar-fieldHypotesis:- H(t,r) Linear throug the anechoic space- H(t,r) depends only from geometriestipS(t,1)H(t,1)Cross-correlation, between near- and far-fieldAuto&cross spectra G SS G NN G SNS(t,r)H(t,r)S(t,0)H(t,0)∑N(t)Coherence:Co2GGSS2SN GNNhubS(t,r)H sound (t,r)H psdSound (t,r)H(t,r)D(t,r)N(t,r)To far-fieldFMGroup @ DMA-Sapienza

datumNorm Freq (ʄ = f/BPF) ʄ = 1 - 7MVB rotor performance assessmentNear-field aeroacoustic sourcedissection (ii)TFMVBSource characterization – Spanwise Coherence map @ 30°Norm Freq (ʄ = f/BPF) ʄ = 1 - 7TFvteAC90/6/TFMVBFMGroup @ DMA-Sapienza

Conclusions• The passive control technique explored in the study was based on blade tips that hadbeen modified by the addition of anti-vortex appendages as end-plates.• The end-plate configuration aimed to control the chord-wise evolution of the leakagevortex swirl level by preventing the occurrence of tip-leakage vortex bursting by theenhancement of near-axis swirl.• The leakage-flow survey has shown that the variable-thickness configuration MVB wasable to exert control over near-axis leakage vortex swirl.As a result, although the critical Rossby number region is reached,the end-plate configuration provides effective control over leakage vortex bursting.• The comparative aerodynamic and aeroacoustic assessment, demonstratedthe overall gain for the investigated end-plates was found to be a consequenceof the implemented passive flow control concept acting on the tip leakage flow

Thank you!(Corsini A, Rispoli F & Sheard AG, A Meridional Fan, Patent granted No. GB 2452104, July2009)FMGroup @ DMA-Sapienza

ackup slideMVB rotor performance assessmentaerodynamics (i)FMGroup @ DMA-Sapienza

ackup slideMVB design concept, sensitivity analysessensitivity on tip gapsensitivity on lift @ blade tipFMGroup @ DMA-Sapienza