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AEROACOUSTICS OF FLUID FLOW MACHINERY: EXPERIMENTAL TECHNIQUES FOR NOISE

CHARACTERIZATION AND SOURCE DETECTION

T. H. Carolus

University of Siegen Department of Fluid- and Thermodynamics,

57068 Siegen Germany

Email: thomas.carolus@uni-siegen.de

ABSTRACT The working principle of any type of fluid flow machinery is based on flows with consider-able speed and complex structures. Hence the major noise source of any fluid flow machinery is flow induced. This contribution mainly deals with experimental methods characterizing and detecting aeroacoustic sound sources in air handling machinery. In a first section various pro-cedures for sound power measurement of fans are discussed. They are the basis for acoustic machine characterization. The main part of the presentation, however, deals with experimen-tal methods for acoustic source detection. Simple flow visualisation techniques and high reso-lution hot wire anemometry are employed to provide information on temporally and spatially varying inflow quantities. Inflow parameters, such as turbulent intensity and correlation length, are essential inputs in broad band noise prediction models. Spatial flow distortions give rise to tones. Foot prints of noise related flow phenomena are unsteady surface pressures. The instrumentation of wetted surfaces, even in the rotating frame of reference, with micro-pressure sensors is used to derive spectra and spatial coherence of flow induced unsteady pressures. They facilitate the detection of mechanisms in the machine by which broad band and tonal noise is most likely generated.

1 INTRODUCTION

The working principle of any type of fluid flow machinery is based on flows with consider-able high speed that need to be deflected and retarded or accelerated in the machine. Unfortu-nately, even if the shape of a machine like e.g. a radial fan impeller is geometrically very sim-ple, the flow is spatially inhomogeneous and/or temporal unsteady. That is the main reason for flow induced noise, most often the dominant noise source in these machines. This contribution deals with the acoustics from “low pressure” air handling turbo-machines. Figs. 1 and 2 show a selection of typical machines. “High pressure” machines like

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turbo-compressors, gas turbines or aircraft engines are not considered in this paper although similar principles may apply.

Figure 1: Left: Radial automotive heating, ventilating, air conditioning (HVAC) fan (Bosch); middle: Industrial radial fan (after ebm Papst); right: Axial locomotive cooling fan (Voith).

Figure 2: Left: Small wind turbine (Skystream); right: Wells turbine for ocean wave energy conversion (Voith/Wavegen, [1]). Lowson’s classification for wind turbine noise prediction methods [2] can be generalized for fans and other turbomachinery, Figure 3. Each class corresponds to typical experimental tech-niques, either for pure validation of the prediction or for creating a data base necessary in semi-empirical sub-models. For instance, • Validation of Class I prediction methods require accurate measurement methods of the

machine’s overall sound power. Sound power determination is often a challenge, particu-larly when the turbomachine is not placed in an acoustically well defined environment.

• Class II-methods are based on semi-empirical sub-models derived from generic experi-ments; energy spectra of turbulence induced fluctuating forces on wetted flat surfaces, the wake shape downstream of a generic blade cascade, etc. are of interest; prior to modelling it is often essential to properly identify the dominant noise mechanism(s) in the machine. This requires experimental source identification.

• Class III-prediction methods utilise, by definition, full information about the unsteady flow field around the detailed geometry and yield the acoustic field. This is the realm of computational aeroacoustics (CAA). Ideally no experimental input is necessary but, of

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course, validation of intermediate source- and resulting sound field quantities may require very detailed measurements.

CLASS IBasic maschine parameters• type• diameter• speed• flow rate• pressure rise

CLASS II•Separate consideration of various noise generationmechanisms

•Simplified fan geometry, flowfield (e.g. blade ⇒ flat plate)

•Simplified flow analysis

CLASS III•Common considerationof various noisegeneration mechanisms

•Detailed fan geometry•High fidelity flow fielddata (e.g. from unsteadyCFD)

Acoustic models

(SPECTRAL) SOUND POWER

Simple algebraic function(correlation)

Figure 3: Classification of noise prediction methods for fans (following Lowson [2]).

2 METHODS OF SOUND POWER MEASUREMENT Turbomachinery may radiate sound from their fluid ports into free field or attached duct work but also via their housing. It is essential to decide which sound power level is of interest. Ta-ble 1 summarizes some definitions as used for fans.

Table 1: Definitions of sound power levels for fans (DIN 45635 T38 [3]) Characterizes sound power radiated from ....

LW1 ... inlet, outlet and casing into free field LW2 ... casing into free field LW3 ... inlet into attached duct LW4 ... outlet into attached duct LW5 ... inlet into free field LW6 ... outlet into free field LW7 ... inlet and casing into free field LW8 ... outlet and casing into free field

Ideally, for sound power determination a turbomachine is placed in an acoustically well de-fined environment, e.g. in an anechoic or semi-anechoic room. Figure 4 shows a test rig which allows determining Lw5 of an axial fan assembly (assuming that the sound power radiated by the housing is negligible). A number of microphones are placed on the surface of a rectangu-lar parallelepiped with area S enclosing the equivalent sound source (grey box covering the fan inlet) at standardized distances. The sound power level is then the spatial average of the sound pressure levels as measured at each microphone and a surface area correction

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0

10 dB.W pSL L lgS

= +

(1)

This method is rather standard for all kind of sound power measurements of technical sound sources. More easily LW3 and LW4 of fans can be determined with the duct method. Figure 5 shows a test rig according to EN ISO 5136 [5]. The basic idea is that sound radiated into the duct is propagating without reflections at the end of the duct (therefore the anechoic termina-tions). Then, one microphone is sufficient to measure the relevant sound pressure, hence the sound power transmitted through the cross-sectional plane where the microphone is placed. The microphone may be moved in the plane during measurement to average out possible circumferential duct modes.

Figure 4: Determination of sound power level Lw5 of a fan - precision method for semi-anechoic rooms, requiring 16 microphone positions (DIN EN ISO 3745 [4]).

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Figure 5: Duct test rig for determination of LW3 and LW4; (1) undisturbed inflow domain, (2) volume flow rate meter, (3) anechoic termination, (4) microphone, (5) static pressure tap, (6) fan, (7) adjustable throttle, (8) honey comb, see also DIN EN ISO 5136 [5].

In another standard method the turbomachine is placed in a reverberant room (DIN EN ISO 3741 [6]). This standard also describes a method if the environment is neither completely an-echoic nor reverberant, being the case in most in situ measurements. A calibrated reference sound source (RSS) with a known sound power level Lw,RSS is positioned at the location of the machine. With the RSS radiating but the machine switched off one measures the sound pres-sure level of the RSS Lp,RSS at a given listener position away from the source. In a second step the RSS is replaced by the running machine which produces a sound pressure level Lp. The desired sound power level of the machine then becomes w w,RSS p p ,RSSL L ( L L )= + − dB. (2) An alternative is the direct measurement of sound intensity around the machine. The energy flux through a surface covering the machine (i.e. the sound intensity) is recorded by a sound intensity probe, Figure 6. By integration one obtains the overall sound power level [7, 8 and 9]. The application of this method has been reported for in situ sound power determina-tion even of large steam turbines. One has to keep in mind that the probe must not be placed in flows.

Figure 6: Two-microphone based sound intensity probe (Brüel&Kjær).

mic 1 separator mic 2

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3 MEASUREMENT OF SOUND RELEVANT FLOW QUANTITIES In some class II-prediction methods a flow immersed flat plate serves as a first approximation of a turbomachinery blade. As guidance for experimental techniques we start with a rather simple model originally proposed by Sharland [10] and based on even earlier aeroacousticans like Doak [11] and Curle [12]. A rigid flat plate radiates sound power into the free field due to pressure fluctuations imposed by a flow above both surfaces, Figure 7, as

( ) ( ) ( )2

21 2 1 2 1 23

0

3 C

A

fW f p , , f A , , f d dc

π ∆ ξ ξ ξ ξ ξ ξρ

′= ⋅ ⋅ ∫∫ . (3)

ρ is the fluid density, c0 the speed of sound, f the frequency. 1 2dA d dξ ξ= is the radiating sur-face. Essential terms in eq. 3 are • The mean square pressure fluctuations in terms of a difference between top and bottom of

the plate 2p∆ ′ • The correlation area Ac of the pressure fluctuations (by definition, within Ac the fluctuat-

ing pressures are completely correlated with each other). Note that, in essence, pressure fluctuation times an area forms a fluctuating force which is the source of the so-called dipole sound in aeroacoustics.

Figure 7: Sound radiated from a rigid flat plate due to flow induced pressure fluctuations (schematically). There are a number of mechanisms which cause stochastically unsteady pressure or forces on an ‘infinitely’ long wing section (Figure 8): • Incident turbulence • Blade surface turbulent boundary layer / blade surface interaction • Blade surface turbulent boundary layer / trailing edge interaction • Flow separation

Figure 8: Airfoil with flow phenomena relevant for flow induced sound; (1) incident turbulence, (2) blade surface turbulent boundary layer / blade surface interaction, (3) blade surface turbulent boundary layer / trailing edge interaction, (4) flow separation.

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In principle a turbomachinery blade can be considered as a finite wing section - with addi-tional 3D-boundary layer effects where it is attached to other structures such as a hub or front- and backplate or at the blade tip. Moreover, the blade may encounter radially or circumferen-tially non-uniform inflow distortions which produce periodically unsteady blade forces. This gives rise to experiments that aim at the • Unsteady velocity field as ingested by the turbomachine • Unsteady pressure distribution on wetted surfaces as e.g. the blades including their space-

time correlation.

3.1 Inflow velocity field - inflow distortions Naively one expects the flow sucked in by a turbomachine from free field to be undisturbed. Simple smoke visualization as shown in Figure 9 serves to unveil unexpected structures origi-nating either in the free field far upstream of the machine or by self induction due to the spin-ning impeller. The smoke is either released from a smoke generator via a nozzle (Figure 9 left) or generated by a grid of heated wires coated with special oil (Figure 9 right). For a pre-liminary quantification a 3D-hot film probe is employed to find the distribution of the time-averaged velocity in the inlet cross-sectional plane, Figure 10 (left). Apparently in this case there is a vertical flow velocity component associated with the vortex type structure. Those vortices may be cut by the downstream rotating blades, resulting in more or less periodic blade forces and eventually tonal sound. Figure 11 depicts a fan assembly with grid upstream of the rotor. This grid generates a turbulent inflow to the rotor that can be described by statistical quantities. Figure 12 (left) shows time-averaged through flow velocities xc downstream of the grid as measured by one 1D hot wire probe traversed along the domain. Cleary visible is the wake behind the struts of the grid and the jet in the gap. That means the turbulence is not homogenous in the vicinity of the grid as confirmed by the turbulence intensity Tu along a radius from hub to tip, Figure 12 (right). Correlation of two time-synchronous signals from a pair of hot wire probes allows determining the axial and circumferential length scales Λx and Λu of the grid generated turbu-lence.

3.2 Surface pressure measurements Here we exclusively discuss surface pressure measurements by an array of pressure sensors. Figure 13 shows an isolated airfoil section which is tested stationary in a wind-tunnel for blade development. Because of space constraints the miniature microphones are not flush mounted but placed outside of the airfoil in side branches of internal channels. To avoid re-flections the channels are connected to very long elastic tubes with open terminations. The price to pay is that the transfer function of the complete system has to be determined by cali-bration. The pin holes on the airfoil in Figure 13 are spaced for high chordwise and lateral resolution in the trailing edge region of the airfoil, aiming at the sound generating mechanism “turbulent boundary layer / trailing edge interaction” and eventually trailing edge noise.

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Figure 9: Flow ingested into by an axial fan: Flow visualization with smoke filaments [13].

Figure 10: Left: 3D hot film probe in inlet cross-sectional plane; right: Measured distribution of local time-averaged vertical velocity [14].

Figure 11: Axial fan assembly with turbulence generating upstream grid.

z

x ,m

cc

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0.0 0.1 0.2 0.3 0.4 0.5 0.60.2

0.3

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]

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c x [m

/s],

Tu [%

]

0

8

16

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32

40

48

Λx

[mm

], Λ

u [m

m]

Figure 12: Measured turbulent inflow parameters with one and a pair of 1D hot-wire probes. Left: Local time-averaged axial velocity distribution downstream of the grid. Right: turbulent inflow statistics: : xc , : Tu , ∆: xΛ ,∇: uΛ ; the bars indicate a spatial averaging of the quantity in circumferential direction; after [15].

1 2 3

Figure 13: Isolated airfoil for wind-tunnel testing; unsteady surface pressure measurements; (1) pin holes linked by internal channels to (2) remote miniature microphones (Knowles Acoustics Type FG-23329-P07), (3) long elastic tubes as anechoic determination; [16]. Even more challenging is the measurement of fluctuating pressures in the rotating frame of reference. Figure 14 shows the manufacture and instrumentation of an axial fan rotor. Here a slip ring assembly with silver coated contact surfaces and silver brushes was used to transmit the signals to the fixed laboratory frame of reference. The electric noise due to the signal transmission from the rotating into the stationary systems needs to be checked. Prior to meas-uring, each sensor is calibrated using a press-on adapter with a miniature acoustic driver cali-brated by a reference microphone before. Depending on the size of the sensitive area of the sensor the surface pressure signals need to be corrected for the finite sensor see e.g. Smol’yakov [17].

x

x ,m

cc

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Figure 14: Upper: Grooved blade surface for flush mounted miniature microphones (Knowles Acoustics Type FG-23329-P07) acting as surface pressure probes, and instru-mented blade; lower: Impeller and in-situ calibration of the probes; from [18]. Figure 15 shows typical results. In this experiment, the blade tip clearance was varied from 0.1% of the rotor diameter to 0.5%. Obviously the power spectral density levels of the surface pressure fluctuations depend very much on location of the sensor on the blade. It is evident that the pressure fluctuations close to the tip are more affected by the tip clearance than the mid span/mid chord blade region: Larger tip clearance yields higher broad band surface pres-sure fluctuations. This is reflected accordingly in the observed radiated sound pressure spectra (not shown here).

4 CASE STUDY: SOUND SOURCE DETECTION IN A RADIAL FAN

In spite of low circumferential Mach number the sound of isolated centrifugal fan impellers, Figure 16, is sometimes dominated by distinctive tones at blade passing frequency (BPF = number of blades times speed of impeller rotation) and its higher harmonics, Figure 17. In a recent project at the University of Siegen (Wolfram et al, [19]) we tried to throw some light on the mechanism for these unexpected tones. The sound spectra from three geometrical absolutely similar impellers operating at a large range of speed were measured and decomposed into Strouhal and Helmholtz number dependent functions. This led to the preliminary conclusion that the BPF related tones are exclusively flow-induced. Based on hot-wire and blade pressure fluctuation measurements, Figure 18, and a subsequent correlation analysis, coherent flow structures, different from the one associated with the principal azimuthal flow pattern due to the blade cascade, were de-tected. Eventually, numerical three-dimensional unsteady flow simulation and experimental flow visualization revealed an inlet vortex, Figure 19. It takes on a helical form, with the vor-tex core slowly varying its position with respect to the impeller centre. As the blades cut through that quasi-stationary helical vortex they encounter blade force fluctuations producing the BPF tones. The slow spin of the vortex core and the slow variation of vortex strength were identified as the reason for an amplitude modulation of the BPF tone.

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Figure 15: Power spectral density levels of surface pressures on a rotating fan blade; sensors on blade suction side; fan is operating at design point; red: small tip gap, green: large tip gap; from [18].

LM050 (50%) LM075 (75%) LM100 (100%)

Figure 16: Three geometrically similar radial impellers.

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0 500 1000 1500 2000 2500 3000

L p [dB

]

f [Hz]

1xBPF

2xBPFNozzleWhistling

L p[d

B]

f [Hz]

10 dB

Figure 17: Noise spectra of an isolated centrifugal impeller near design point; dominant tones are at multiples of BPF and at high frequency due to nozzle whistling; tones and the whis-tling are not related to each other, [19].

Figure 18: Left: Locations of the flush mounted miniature pressure transducers on the rotating blade surface; LE = leading, TE = trailing edge; right: Three stationary hot wire probes in the impeller inlet through hollow fan shaft and back plate.

5 SUMMARY AND CONCLUSIONS The first section of this contribution was dealing with various standard procedures for sound power measurement of fans and related turbomachinery. Prior to any measurement one care-fully has to decide which portion of the total sound power is of interest and can be determined with the method selected (e.g. sound power into free field from inlet, from outlet into an at-tached duct, etc.). Precision methods according to international standards yield are rated with an accuracy of 1 dB and require expensive test facilities with well defined acoustic properties. Hence, one has to be very cautious reporting and assessing absolute levels of in situ meas-urements.

LE

hubshroud

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Figure 19: Snapshots of a smoke filament at radial impeller intake and CFD-simulated iso-surfaces of vorticity at impeller intake (500 s-1, scaled with relative flow velocity); [19]. Measured overall sound power levels and even spectra of overall sound power do rarely allow localizing critical noise sources in the machine. For that we discussed some experimental methods such as simple flow visualisation, high resolution hot wire anemometry and unsteady surface pressure measurement techniques. They all aim at the quantification of flow phenom-ena that - as theories state - lead to flow induced sound. More advanced experimental methods have been published, not discussed here. E.g. unsteady particle image velocimetry (PIV) can capture unsteady flow velocities fields syn-chroneously in a complete plane. Disadvantages are the required seeding of the fluid with small particles and the still limited time resolution. Phased array techniques and acoustic holography allow the direct detection of sound sources. Inherent problems in application of these techniques for turbomachinery noise are the fact that the sound source may rotate with considerable speed, the existence of a flow field that scatters sound waves, difficulties in ac-cessing the sound radiating regions in the machine due to housing and attached duct work, and limitations in the spatial resolution. Nevertheless, a well known example of a successful application of the phased microphone array technique for source identification in wind tur-bines has been published in 2007 by Oerlemans et al [20], Fig. 20.

Figure 20: Wind turbine and microphone array platform. The noise sources in the rotor plane (averaged over several revolutions) are projected on the picture; from Oerlemans et al [20].

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6 REFERENCES [1] R. Starzmann, C. Moisel, T.H. Carolus, K. Tease, R. Arlitt. Assessment method for

sound radiated by cyclically operating Wells turbines. EWTEC 2011, 9th European Wave and Tidal Energy Conference Series, Southampton, 5-9 Sept. 2011

[2] M.V. Lowson. Assessment and prediction of wind turbine noise. Flow Solutions Report 92/19, ETSU W/13/00284/REP pp. 1-59, Dec. 1992

[3] DIN 45635 T38: Geräuschmessung an Maschinen; Luftschallemission; Hüllflächen-, Hallraum- und Kanal-Verfahren; Ventilatoren. April 1986

[4] DIN EN ISO 3745: Akustik - Bestimmung der Schallleistungspegel von Geräuschquel-len aus Schalldruckmessungen, Verfahren der Genauigkeitsklasse 1 für reflexionsarme Räume und Halbräume. January 2004

[5] DIN EN ISO 5136: Akustik - Bestimmung der von Ventilatoren in Kanäle abgestrahlten Schalleistung - Kanalverfahren. October 2003

[6] DIN EN ISO 3741: Akustik - Bestimmung der Schallleistungspegel von Geräuschquel-len aus Schalldruckmessungen, Hallraumverfahren der Genauigkeitsklasse 1. January 2001

[7] DIN EN ISO 9614-1: Akustik - Bestimmung des Schalleistungspegels von Geräusch-quellen aus Schallintensitätsmessungen - Teil 1: Messung an diskreten Punkten. June 1995

[8] DIN EN ISO 9614-2: Akustik - Bestimmung des Schalleistungspegels von Geräusch-quellen aus Schallintensitätsmessungen - Teil 2: Messung mit kontinuierlicher Abtas-tung. December 1996

[9] DIN EN ISO 9614-3: Akustik - Bestimmung des Schalleistungspegels von Geräusch-quellen aus Schallintensitätsmessungen - Teil 3: Scanning-Verfahren der Genauig-keitsklasse 1, April 2003

[10] I. J. Sharland. Sources of noise in axial flow fans. J. of Sound and Vibration, Vol. 1, No. 3 pp. 302-322, 1964

[11] P.E. Doak. Acoustic radiation from a turbulent fluid containing foreign bodies. Proc. Royal Society of London, Series A pp. 129-145, 1960

[12] S.N. Curle. The influence of solid boundaries upon aerodynamic sound. Proc. Roy. Soc. of London, Series A, Vol. 231 pp. 505-514, 1955

[13] M. Sturm, T. Carolus. Tonal Fan noise of an isolated axial fan rotor due to inhomoge-neous coherent structures at the intake. Fan2012, Senlis, France 2012

[14] M. Sturm, T. Carolus. Drehton bei freilaufenden Rotoren von Axialventilatoren: Ex-perimentelle und numerische Untersuchung der Entstehungsmechanismen. DFG Project CA170/9-1, Intermediate report No. F30 001 A of the Institute of Fluid- and Thermody-namics, University of Siegen, 2012

[15] T. Carolus, M. Schneider, H. Reese. Axial flow fan broad-band noise and prediction, J. of Sound and Vibration 300 (2007) pp. 50-70

[16] J. Winkler: Investigation of Trailing-Edge Blowing on Airfoils for Turbomachinery Broadband Noise Reduction, Shaker Verlag 2011 (Ph.D. Thesis University of Siegen)

[17] A.V. Smol’yakov, V.M. Tkachenko. The measurement of turbulent fluctuations. Springer-Verlag, Berlin, Heidelberg, New York, 1983

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[18] T. Zhu, T. Carolus: Akustische Nachberechnung von Ventilatoren; Arbeitspaket C -Experimente. FLT Nr. L 236, Final report No. F60 001 A of the Institute of Fluid- and Thermodynamics, University of Siegen, 2012

[19] D. Wolfram, T. Carolus: Experimental and numerical investigation of the unsteady flow field and tone generation in an isolated centrifugal fan impeller. J. of Sound and Vibra-tion 329 (2010) pp. 4380-4397

[20] S. Oerlemans, P. Sijtsma, B. Méndez López. Location and quantification of noise sources on a wind turbine. J. of Sound and Vibration 299 (2007) pp. 869 - 883