AIAA 2014-3190 AIAA Aviation 16-20 June 2014, Atlanta, GA 20th AIAA/CEAS Aeroacoustics Conference
A coupling of computational methods for CROR installation effects L. Sanders*, D.-C. Mincu* and W. Denis* ONERA – The French Aerospace Lab, F-92322 Châtillon, France P. L. Vitagliano† and M. Minervino† CIRA – the Italian Aerospace Research Centre, Capua CE, Italy
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J. Kennedy‡, P. Eret‡ and G. Bennett§ Trinity College Dublin – Ireland
This paper deals with the computation of CROR installation effects in terms of tone noise within three separated steps. The first step aims at computing the aeroacoustic sources of a CROR by a CFD approach, namely a URANS method applied to counter-rotative propellers. The second step computes the acoustic field radiated by the isolated CROR. It is based on the Ffowcs-Williams and Hawkings (FW-H) surface integral method and uses the blade pressure fluctuations computed by CFD as input data. The last step computes the acoustic field scattered by the aircraft geometry within the Boundary Element Method using the incident acoustic field previously computed. This approach in three steps is applied on a 1/7th scale model of a regional aircraft design equipped with two CRORs and tested in windtunnel. The paper first focuses on the aeroacoustic study of the CROR including the effects of pylon and incidence. The numerical prediction of the CROR noise in take-off conditions is then compared to the experimental measurements of the isolated engine. The isolated CROR noise computation is globally underestimated compared to the measurements because it doesn’t take into account the acoustic reflections occuring in the windtunnel but a qualitative agreement is found between the computation and the measurements. Finally, the computation of CROR installation effects is presented in one of the aircraft configuration tested in windtunnel, showing a global increase of the tones when the CROR is installed in comparison with the isolated configuration.
Nomenclature AoA B c c0 BPF CROR Jn k m MT Mx r R1, R2 SPL
Angle of Attack blades number blade chord length at the root ambient speed of sound Blade Passing Frequency Contra-Rotating Open Rotor Bessel function of order n order of load harmonic sound harmonic blade tip rotational Mach number axial Mach number distance from center of CROR to observer Front, Rear blade radius Sound Pressure Level
*
Research Engineer, CFD and Aeroacoustic Department. Corresponding author:
[email protected] Aerodynamics Engineer, Fluid Mechanics Department. ‡ Research Fellow, Department of Mechanical and Manufacturing Engineering. § Assistant Professor, Department of Mechanical and Manufacturing Engineering, Senior AIAA Member. 1 American Institute of Aeronautics and Astronautics †
Copyright © 2014 by __________________ (author or designee). Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
z0
, θ (2) Ω
source radius/tip radius observer angles blade location at t = 0 rotor angular speed
I. Introduction and outline
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T
he Contra-Rotating Open Rotor (CROR) technology offers fuel burn reduction and is seen as a good alternative to turbofan engines for the propulsion of future low to medium range aircrafts. Nonetheless, this technology generates important noise which constitutes a major obstacle to its development. Reducing noise emission is required and several approaches are possible. On one hand, theoretical, experimental and numerical works on isolated CROR aim at a comprehensive knowledge of the noise generation and high accuracy of the noise prediction. On the other hand, the fuselage of the aircraft, in the vicinity of the CROR, has a significant impact on the radiated noise. It can induce acoustic shielding or amplification. Therefore new aircraft configurations can be designed from the optimisation of the engine integration aiming at reducing the external noise. This topics have been addressed by the WENEMOR project1. Many configurations of CROR installation on a scale model aircraft have been tested in Pininfarina wind tunnel (Turin, ITALY): pusher and tractor modes, empennage geometries, pylon length and azimutal orientation have been widely evaluated1, 2. In addition to the parametric study, this campaign affords an experimental database for the assessment of the computation of CROR installation effects. Given the lower cost of the numerical approach, it is adapted to optimisation problems but requires validation by experiments. The stakes of the present work – still in process and restricted to tonal noise – are the computation of the CROR installation effects and more specifically its validation by experimental measurements from WENEMOR. At the moment, the validation is focused on the noise of the engine only. It affords to assess the computation of the aeroacoustic sources of one CROR, basing the whole process of computing the CROR installation effects on the aircraft. This process relies on three steps which are detailed in the following paragraph. The third paragraph deals with the WENEMOR tests and the experimental database used for validation. In paragraph IV, an extended aeroacoustic analysis of the CROR model is achieved including the effects of the pylon and incidence then a comparison between the noise computation and the acoustic measurements of the CROR engine is achieved in paragraph V. Finally, paragraph VI presents the results of the CROR installation effects computed for one of the aircraft geometry addressed by WENEMOR, and thus assessing its capacities in acoustic shielding.
Figure 1. Use of the Boundary Element Method for installed engine. Left : general case, right : present application to CROR.
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II. Method and description of the computational methods A. Method Engine integration on aircraft requires an evaluation of the acoustic installation effect whatever the engine type. The Boundary Element Method (BEM) can take into account complex geometries such as an aircraft empennage. Its cost is proportional to the third power of the frequency of the acoustic source. As long as the dominating frequencies of the engine noise are not too high, the BEM method has an affordable cost. In the case of the present CROR, the ratio between the typical acoustic wavelength and geometric length, about 0.02, is acceptable for computation cost. The left side of Figure 1 illustrates the general approach with BEM in the case of engine noise. The engine acoustic source is assumed to be isolated from the scattering aircraft surface and its acoustic emission is known. The BEM computes the pressure scattered by the aircraft surface from the incident field. This method is applied to CRORs (right side of Figure 1) by computing the incident acoustic field with a Ffowcs-Williams and Hawkings (FW-H) surface integral method applied to the blade pressure fluctuations computed by CFD. The delimitation between the acoustic source surface and the scattering surface is tricky in the case of an engine installed close to the fuselage with a short pylon. The blades mainly contribute to the noise emission but, to some extent, the hub also contributes to increase the global noise. Thus it can be included in the source surface along which the FW-H integration is achieved (see §IV.B.4). The pylon theoretically scatters the noise from the propellers as the fuselage, but it also induces a significant distorsion in the flow seen by the blades and mostly the front row, therefore it modifies the acoustic sources (see §IV.B.5). The present work singled out the study of this latter effect. B. Computational Fluid Dynamics (CFD) The objective of the CFD is to compute the aeroacoustic sources of the propellers. The Unsteady Reynolds Averaged Navier-Stokes simulation (URANS) is suitable for the calculation of the unsteady flow of propellers. In the present work, it is achieved with CIRA’s multi-block structured solver U-ZEN3,4 using the k-ω TNT turbulence model5. Front and rear propeller meshes rotate in opposite direction and the resulting non-conformal interface (sliding mesh) is updated at each time step. A snapshot of the simulation of the isolated CROR is shown by Figure 2 and Figure 3 exhibits the coarse mesh (about 900 000 cells in the computation domain) firstly used for the computation of the CROR with pylon. A finer mesh (about 7 millions of cells) have secondly been used for the CFD of the CROR whose outputs have been used as input of the acoustic computations described further.
Figure 2. Instantaneous pressure field on CROR from URANS
Figure 3. Surface mesh for the URANS computation of the CROR with pylon 3 American Institute of Aeronautics and Astronautics
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C. Ffowcs-Williams and Hawkings surface integral method (FW-H method) In the present case, the CRORs work in take-off conditions, e.g. with a low axial Mach number, and is mostly characterized by dipole sources on their blades. The acoustic sources in the flow (quadrupoles) are thus neglected and the FW-H integral method using the blade pressure fluctuations is quite accurate to compute the CROR noise emission. The noise computations are performed by solving the Ffowcs-Williams and Hawkings6 equation in the time domain with ONERA’s code KIM7 and a Fast Fourrier Transform (FFT) is applied to the acoustic signatures to provide the incident pressure in frequency domain. D. Boundary Element Method (BEM) The last computational step is to solve a scattering problem where the aircraft is the obstacle and the CROR is the source. In the present case, the low flow speed (28 m/s) is neglected and the scattering problem assumes an infinite space E of an inviscous fluid at rest in which Y is a source and Σ a solid surface (Figure 4). Ωe is the problem domain and Ωi the interior region. The exterior problem is governed by the Helmholtz equation : (1) ∆p ( x ) + k 2 p ( x ) = 0 where k is the wave number of the acoustic source Y. The Sommerfeld radiation condition applies at infinity and the rigid boundary condition on Σ is :
∂p = 0, x ∈ Σ ∂n
(2)
The integral solution of this problem relating functions defined on the boundary domain Σ is then :
x ∈ Ωe p( x ) G ( x , y ) p 1 ∂ ∂ ∫Σ p( y ) ∂n − ∂n ( y )G ( x, y )dy = 2 p( x ) x ∈ Σ x ∈ Ωi 0
(3)
with the free-space Green function G defined as:
G ( x, y ) =
e
− ik x − y
4π x − y
Figure 4. The scattering problem 8, 9
Onera’s solver BEMUSE solves the boundary integral problem with the Brakhage-Werner approach10 and the aircraft skin is discretized by a mesh composed of about 72 000 triangles.
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III. Experimental database for validation The WENEMOR project has been developed in response to the requirements of the European Clean Sky Joint Technology Initiative to assess the aero-acoustic noise emissions for an advanced regional open rotor aircraft configuration. A detailed presentation of the WENEMOR project is given in Ref.1. The consortium consists of 7 partners including two universities (Trinity College Dublin and Università Politecnica delle Marche), a large European wind tunnel facility (Pininfarina) and several SMEs (Eurotech, Teknosud, MicrodB and Paragon S.A.) with specific competences in design, manufacture, noise measurement and data analysis. The project utilises a design for an advanced regional aircraft configuration, developed within the Clean Sky Green Regional Aircraft project, with realistic modern blade profiles for the propulsion system (Figure 5 and Figure 6). The project designed, manufactured and tested a 1/7th scale model of the open rotor aircraft design. The model consists of a modular design featuring interchangeable tail pieces, variable fuselage length, engine pylon rotation and elongation and is controllable for angle of attack. A parametric study of the CROR noise emission is reported in Ref.2. In the scope of our validation objective, the configurations of interest are restricted to CROR in pusher mode and take-off conditions. Table 1 gives the characteristics of the two configurations this paper focuses on. The one named “Isolated CROR” corresponds to one CROR engine with pylon embedded into the floor without aircraft in the section. The other named “PS-A” is the reference aircraft geometry of the WENEMOR tests. The aircraft and CROR geometries used hereafter for the computations corresponds to those of the tests and have been provided by ALENIA and SNECMA. Case 1 2
Configuration ID Isolated CROR PS-A
Pylon Angle 15°
Pylon Length 480 mm
Tail T
Angle of Attack 0° 6°
Flow Speeds 28 m/s 28 m/s
Table 1. Two configurations tested in the Pininfarina windtunnel.
Figure 5. Model installed in the Pininfarina wind tunnel facility The Pininfarina Aerodynamic and Aeroacoustic Reasearch Center in Turin, Italy, contains a test section of 8m x 9.6m x 4.2m. Acoustic treatment of the wind tunnel has reduced background noise to 68.5 dBA at a flow velocity of 100 km/h (28 m/s). A considerable number of sensors was deployed for each test set-ups. The kinds of sensors or arrays is provided below : • Blade Kulites (on one engine, 12 sensors on each front and rear blade row) • Fuselage Microphones (20 flush mounted pressure sensors at the axial plane around the front blade row) • Near Field Traversing Array (5 microphones on a linear array moving to 7 planes centred about the front blade plane) • Far Field Linear Array (13 microphones covering angles from 30° to 150°) • Top array (78 microphones) • Lateral array (66 microphones) • Front array (30 microphones)
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In the scope of our acoustic validation purpose, the comparison between numerics and measurements focuses on the near field and far field arrays which offer a large range of data in the 3D space. Auto-spectra of the microphone pressures are achieved for direct comparisons in the frequency domain.
Figure 6. CROR engine installed in the wind tunnel in pusher configuration. (Low-Tail empennage tested)
IV. Aeroacoustic analysis of the CROR model A. Preliminary analysis Each propeller of the CROR has 12 blades and a rotational speed equal to 2359 rpm in the considered take-off conditions. The semi-analytical model of CROR noise proposed by Hanson11 offers an insight into the parameters controlling the efficiency of the noise emitted by a CROR. The harmonic noise from a propeller is theoretically composed of an infinity of sound harmonics m, to which an infinity of load harmonics k contributes. But only few pairs (m, k) are associated to an efficient contribution. From the noise formula proposed by Hanson11 for a CROR with equal blade number (B) and rotational speed (Ω) on front and rear rotors, one can relate the acoustic pressure p emitted by a rotor at distance r from the center of the CROR to : (4) where Ar takes into account the relative mach number, offset and sweep of the blade section Ak takes into account the kth drag and lift harmonic coefficients, e.g. the loading sources The Bessel function term significantly controls the efficiency of the contribution of the harmonic (m, k). Bessel functions Jn(x) decays rapidly for |x| < |n|. As suggested by Hanson11, we can define a low-efficiency criterion for the sound radiated at θ = 90° from the blade tip (z0 = 1), which writes : |mBMT| < |m-2k|B This low-efficiency criteria underlines that, if m = 2k, then the efficiency is high. This corresponds to the Bessel function of order 0. Generally, the zero-order of the Bessel functions dominates the other orders whatever the directivity θ and radius z0 are, therefore one can expect that even sound harmonics dominate the CROR noise emission. In addition, J0(x) is maximum for x = 0, e.g. θ = 0 or θ = π . In other words, the flow direction is privileged. B. Isolated CROR 1. Blade loading The two propellers of the CROR have the same number of blades B, therefore each propeller blade sees 2*B passing blades during a round and has an angular periodicity of 2π/(2*B). Figure 7 shows the steady loading and the three first harmonics. The blade loadings are normalised by the maximum front blade steady loading. Steady loading 6 American Institute of Aeronautics and Astronautics
is quite similar between the two propellers, mainly concentrated on the leading edge but higher on the rear propeller. On the opposite, the unsteady loading is much weaker than the steady one and much higher on the rear rotor than on the front rotor. In particular, the first loading harmonic maximum is about thirty times stronger on the aft rotor than on the front rotor. This is due to the strong interaction of the wake and vortex impingement from the front blades on the aft blades in comparison with the interaction of the potential effect of the aft propeller with the front propeller. It is consistent with the maxima on the rear blade concentrated on its leading edge. The second loading harmonic pattern is similar to the first one. One can notice that on the front rotor the maximum is still located in the root region. Concerning the third loading harmonic, the pressure fluctuations are still higher on the aft rotor and the pressure fluctuations distribution on front and rear blade are quite similar to the previous ones.
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Steady loading
Front rotor
First harmonic
Aft rotor
Front rotor
Second harmonic
Front rotor
Aft rotor
Aft rotor Third harmonic
Front rotor
Aft rotor
Figure 7. Steady and harmonic blade loadings. (Normalized pressure harmonics ranked from 0 to 3) 2. Far-field noise The tones of this 12x12 CROR with equal rotation speed at front and rear rotors are theoritically made of several contributions at identical frequencies which are all multiples of the Blade Passing Frequency (BPF). For instance, the BPF noise is made of the steady loading noise of each propeller and the 2*BPF noise is made of (i) the first harmonic of the steady load noise and (ii) the first interaction tones of the propellers. The computed directivity of the CROR noise (left side of Figure 8) shows that the even sound harmonics are dominating with maxima towards the axis as expected from the preliminary analysis (§IV.A). This two tones correspond to the propellers interaction tones which dominate the global noise of the CROR. In the present case, the BPF noise levels are very low because of the weak steady load of the propellers. The aft rotor is much more noisy than the front one (right side of Figure 8) 7 American Institute of Aeronautics and Astronautics
and this is correlated with the blade loading analysis (previous section) showing higher unsteady loads on the aft blades due to the wake and vortex impingement from the front propeller.
BPF 2*BPF 3*BPF 4*BPF glob-dB
glob-dB FRONT glob-dB REAR
0
45
90
135
θ (°)
0
180
45
90
135
θ (°)
180
Figure 8. Far-field noise directivity. Left : contribution of the seven first harmonics, right : front and rear contribution to the global levels. 3. Near-field noise The near-field of a CROR is the region close to the propellers where the distance from the source to the observer is of the order of the size of the sources, e.g. chord and span of the blades. In that region, the acoustic field may be very different from the one observed in far field. Since the fuselage and pylon of the aircraft are quite close to the propellers, the acoustic near-field of the CROR deserves attention when one computes installation effects. Two lines of microphones survey the transition from acoustic near-field to acoustic far-field (Figure 9). On the left plot of Figure 9, near-field effects are clearly observed for the BPF and its third harmonic and the far-field is established beyond r = 3*R1, as r is the radial distance from the center of the two propellers. The near-field effects are negligible in this direction for the interaction tones at frequencies 2*BPF and 4*BPF. Generally in the area of the near-field, the steady load noise (BPF) is dominating. On the right plot of Figure 9, the latter is dominating along two chord lengths and far-field is established beyond five chord lengths. Since fuselage is about 1.3*R1 from the center of the CROR and the pylon is about 1.5*c from the front blades, attention should be paid to the scattering of the near-field which can induce higher levels of the steady load noise in far-field.
r
BPF 2*BPF 3*BPF 4*BPF 20*log(P/r)
BPF 2*BPF 3*BPF 4*BPF
R1
20 dB
d
0
SPL (dB)
20 dB SPL (dB)
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SPL (dB)
SPL (dB)
10 dB
1
3
5
7
9 11
0
5
r/R1
10
d/c
Figure 9. Acoustic near-field to acoustic far-field. Left : acoustic pressure along the radial direction in log scale from hub radius. Right : acoustic pressure in the upstream direction from the front leading edge in fonction of the chord length at the root 8 American Institute of Aeronautics and Astronautics
4. Effect of the hub The blades are embedded in the hub and both are rotating in the CFD computation and real engines. The whole hub is located in the vicinity of the blade acoustic sources and can be considered as part of the propeller aeroacoustic sources. Including the hub and the blades together in the FW-H solid surface integration shifts the computed farfield noise (Figure 10, left). The directivities of the global levels in far-field are globally increased from 25° to 130°. It results from the shift of the two first interaction tones (Figure 10, center and left). Indeed, the directivity of the first interaction tone is modified with increased levels from 80° to 130° and the levels of the second dominating interaction tone are increased. glob-dB - Propellers glob-dB - Propellers & Hub
2*BPF1 - Propellers 2*BPF1 - Propellers & Hub
4*BPF1 - Propellers 4*BPF1 - Propellers & Hub
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PdB
PdB
PdB
10 dB
10 dB
10 dB 0
45
90
θ (°)
135
180
0
45
90
θ (°)
135
180
0
45
90
θ (°)
135
180
Figure 10. Influence of including the hub in the computation of the far-field noise: global levels (left), 2*BPF levels (center), 4*BPF levels (right) 5. Effect of the pylon and incidence Nacelles of CRORs in pusher mode are mounted on the aircraft fuselage by a pylon located upstream the front blades. The pylon induces a distortion in the upstream flow of the propeller and a spatial modification of the steady load of the blades (mainly the front blades). The noise emission of the steady load is thus modified. Another cause of modification of the upstream flow is the incidence of the aircraft during take-off for instance. Figure 11 compares the effect of the pylon and/or incidence on the noise emission of the steady load of the propellers at the BPF. All results of this section are presented on a half-sphere oriented toward the direction of the ground with respect to the incidence, and X being the flow direction. It appears that the pylon has a much stronger influence than the incidence (figures b. and c. compared to a.). Whereas steady load noise of the isolated CROR is very low because of a weak load of the propellers, the pylon induces a significant contribution of the BPF to the global noise. Figure 12 separates the respective noise contribution of the front (left) and rear (right) rotors and confirms that the main increase of the noise comes from the front propeller. In addition, despite the lower noise effect of the incidence at BPF, the incidence combined to the pylon significantly increases the BPF noise levels (Figure 11d). As for the 2*BPF, the effects of pylon and incidence are slightly different. The first harmonic of the steady load noise (2*BPF) is logically increased by the pylon effect, as observed previously with the steady load noise and the first interaction tone is also weakly increased by the pylon, leading to a global increase of the SPLs (Figure 13a-b). But the incidence induces the highest increase of the 2*BPF levels (Figure 13b-c). Looking at the front and rear rotor contribution to the noise at 2*BPF (Figure 14), it appears that the rear rotor has the highest noise emission. During a 360° rotation of propeller, since the steady load of the front propeller is spatially modified by the incidence, the intensity of the tip vortex from a front blade varies, and so do the load fluctuations of the front propeller, which are seen by the rear blades. This can explain the increase of the interaction tone at 2*BPF from the rear propeller. Finally, combining the effects of the pylon and the incidence significantly increase the 2*BPF levels (Figure 13d). This configuration is expected to be the noisest.
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X
a. isolated
X
b. with pylon
X
c. with incidence (AoA = 6°)
X
d. with pylon and incidence (AoA = 6°)
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Figure 11. Effect of the pylon and incidence on the BPF frequency in far-field noise
a. Front rotor noise ar BPF
b. Rear rotor noise at BPF
Figure 12. Far-field noise contribution of the rotors at the BPF frequency in the case of the CROR with pylon
a. isolated
b. with pylon
c. with incidence (AoA = 6°)
d. with pylon and incidence (AoA = 6°)
Figure 13. Effect of the pylon and incidence on the 2*BPF frequency in far-field noise
a. Front rotor noise ar 2*BPF
b. Rear rotor noise at 2*BPF
Figure 14. Far-field noise contribution of the rotors at the 2*BPF frequency in the case of the CROR with incidence (AoA = 6°)
V. Comparison of the CROR noise prediction with the measurements During the WENEMOR windtunnel test, noise measurements of the CROR model without any fuselage have been achieved. In this configuration, the model engine is attached to the floor by its pylon. Figure 15 (left) gives an insight of this configuration. Several arrays have been used during the tests (see §III) and three of them have been 10 American Institute of Aeronautics and Astronautics
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selected for the comparison with the computations. The first one is the moveable near-field array located at 1.5D of the model center, D being the front propeller diameter. The central microphone of the array have been selected at the various positions of the array so that the near-field comparison is achieved along a linear side line (Figure 15, right). Figure 16 focuses on the Linear array (left) equipped with 13 microphones and the Top array (right) equipped with 78 microphones in which 6 microphones (blue labels) were selected for the comparison with the acoustic computation.
Figure 15. Location of the arrays. Left : Linear and Top arrays in the wind tunnel (only the CROR and floor are visualized with the arrays). Right : selected microphones of the Near-field array.
(a) Microphones 1 to 13 of the Linear array from upstream to downstream (top view)
(b) Selected microphones of the Top array for comparison with CROR noise prediction (top view)
Figure 16. Selected far-field microphones of the windtunnel tests for comparison with CROR noise prediction Figure 17 to Figure 19 compare the noise measurements (dotted lines) of the first four tones (BPF, 2*BPF, 3*BPF, 4*BPF) with those computed (solid lines) for each selected array. In near-field (Figure 17), there is a good agreement of BPF and 3*BPF whereas the 2*BPF patterns are different. The trend is similar for the 4*BPF, but the computations globally underestimate the levels by about 10 dB. The linear array is parrallel to the previous microphone line but is longer and located in the acoustic far-field. Computations and measurements show a same trend for the 4 tones but, globally, the computations unevenly underestimate the noise levels by about 1 to 10 dB (Figure 18). This underestimation is attributed to the free-field conditions of the computations compared to the closed section of the windtunnel, whose walls have a low acoustic absorption. We assume that multiple reflections occur during the measurements and increase the measured SPL in an unknown extend. The uneven development of the experimental SPLs along the linear array confirms this assumption. Looking at Top array (Figure 19), the computations still underestimate the measurements. The assumption of multiple reflections seems confirmed. The better agreement of the SPLs in near-field can also be related to this assumption, nonetheless the discrepancies of the 2*BPF patterns is unexplained and the lower SPLs of the computed 4*BPF suggests a possible limit of CFD to compute with accuracy the higest harmonic loads participating to the noise intensity since the sample frequency for the 4*BPF is 16 time steps by period. As a matter of fact, the global qualitative agreement between computed tones 11 American Institute of Aeronautics and Astronautics
and measurement are satisfying for the computation of the CROR installation effect. However, an evaluation of the acoustic reflections in terms of SPL, in function of the location of the microphones in the wind tunnel with respect to the location of the CROR model, is necessary and will be achieved in a future work.
10
15
20
25
30
Num. 4*BPF Exp. 4*BPF
35
5
Microphone Number
10
15
20
25
30
SPL (dB)
Num. 3*BPF Exp. 3*BPF
SPL (dB)
10 dB
5
35
5
Microphone Number
10
15
20
25
30
35
5
Microphone Number
10
15
20
25
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Microphone Number
Figure 17. CROR noise prediction compared to measurements of the near-field array
SPL (dB)
Num. BPF Exp. BPF
Num. 3*BPF Exp. 3*BPF
Num. 2*BPF Exp. 2*BPF
Num. 4*BPF Exp. 4*BPF
20 dB
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Microphone Number
Microphone Number
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Figure 18. CROR noise prediction compared to measurements of the linear array Num. 2*BPF Exp. 2*BPF
Num. BPF Exp. BPF
1
18
SPL (dB)
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Num. 2*BPF Exp. 2*BPF
SPL (dB)
SPL (dB)
Num. BPF Exp. BPF
36
Num. 4*BPF Exp. 4*BPF
Num. 3*BPF Exp. 3*BPF
55 66 48
20 dB
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0 80
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80 0
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0 80
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Figure 19. CROR noise prediction compared to measurements of the top array (selected microphones)
VI. Acoustic shielding by the aircraft model The aircraft model tested in the WENEMOR project is equipped with two CRORs. In paragraph §IV.B.5, it has been shown that the pylon induces a significant increase of the SPLs and a noise pattern which is not axisymmetric anymore. Left and right CRORs of the aircraft model have thus a different contribution to the global noise. In a first step, only the contribution of the left CROR is computed and no effect of incidence is taken into account. So the computation of the CROR installation effect have been achieved using (i) the aircraft geometry named PS-A (see Table 1), and (ii) the URANS simulation of the CROR with pylon without incidence. Figure 20 shows a part of the aircraft mesh and the blades and hubs surfaces used for the FW-H integration in order to compute the incident field from the left CROR. In this application, the true azimutal pylon angle is 15° but it has been neglected in the acoustic 12 American Institute of Aeronautics and Astronautics
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computations when positionning the CROR and the aircraft together so that the effect of the pylon in terms of acoustic emission is slightly shifted in azimuth. In addition, the pylons and nacelles of the CRORs are excluded from the scattering surface. Indeed, the nacelle is considered as a part of the CROR acoustic sources whose contribution to global noise is not significant. As for the pylon, its influence on the modification of the acoustic sources have been chosen as a priority. Finally, given that the flow has a low speed (28 m/s), its convection effect on acoustic waves is neglected.
Figure 20. Noise Sources surfaces of the CROR colored by steady pressure and part of the BEM mesh, namely the skin of the aircraft (only empennage is illustrated) A square matrix of virtual microphones has been located in a horizontal plane under the aircraft model at about one length of the aircraft and covering at least a conical solid angle with an apex angle of 2*56° from the CROR model center and oriented towards the ground (see the location of the aircraft in the left corner at the bottom of Figure 21). In Figure 21, the noise from the isolated CROR with the pylon (top of the figure) is compared on this map with the noise from the CROR installed on the aircraft model (bottom of the figure). The installation of the CROR globally increases the levels taken as a whole and does not modify the global noise pattern. In particular, no shielding effect is observed but a significant increase of the levels at 4*BPF is localised in the area where these levels are maxima (Figure 21d).
a. BPF
b. 2*BPF
c. 3*BPF
d. 4*BPF
Figure 21. Tone noise map under the aircraft model for the isolated CROR with pylon (top) and the installed CROR with pylon (bottom)
VII. Conclusion This paper has presented the computational methods for the prediction of CROR installation effects in terms of tone noise in the scope of the WENEMOR project which aims at predicting shielding effect of various aircraft geometries on CROR noise. Specifically, an aeroacoustic numerical study of the CROR model used during the WENEMOR windtunnel test has been achieved to define its noise emission. In addition, the impacts of (i) an upstream pylon corresponding to the pusher mode and (ii) the incidence of the mean flow, have been both assessed. The steady load noise is mostly modified by the pylon, whereas the first interaction tones are mostly influenced by 13 American Institute of Aeronautics and Astronautics
Downloaded by Peter Jordan on August 15, 2014 | http://arc.aiaa.org | DOI: 10.2514/6.2014-3190
the incidence. Combined effects – pylon and incidence – lead to a maximum of noise. The computed CROR noise emission has been compared to the measurements of the engine noise in the Pininfarina windtunnel during the WENEMOR experimental campaign. A global large underestimation of the computed SPLs, by about 5 to 10 dB, has been found in far field. At the moment, these discrepancies observed for the four first tones are attributed to the free-field conditions of the computation, with respect to the closed section of the windtunnel where multiple acoustics reflections are assumed to occur. This assumption is still to be confirmed in a future work by an evaluation of these multiple reflections. In addition, the highest discrepancy occurs at the highest computed frequency (4*BPF) and the time discretisation of the URANS simulation may be too long to accurately compute unsteady blade loadings at this frequency. Nonetheless, a qualitative agreement between computations and measurements has been found looking at the trend along a linear array for instance and validate the use of the CROR noise output for the computation of the installation effects with the Boundary Element Method (BEM). In conclusion, an example of such an application has been shown, where the tone noise levels taken as a whole are globally increased by the installation. The next steps consist in extending the computation of the CROR installation effects to several empennage configurations tested during the WENEMOR experimental campaign to assess their benefit in terms of acoustic shielding.
Acknowlegments This research has received funding from the European Community's Seventh Framework Programme (FP7/20072013) for the Clean Sky Joint Technology Initiative under grant agreement n°CSJU-GAM-GRA-2008-001. The WENEMOR experimental project’s database and the aircraft and CROR geometries have been used with permission of SNECMA and ALENIA. In particular, the authors would like to thank Nicolas Tantot from SNECMA and Massimiliano Di Giulio from ALENIA for their involvement in providing all the data necessary to this work.
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