LAGOON simplified (2-wheel) nose landing gear Benchmark for ...

LAnding Gear nOise database and CAA validatiON

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LAGOON simplified (2-wheel) nose landing gear Benchmark for Airframe Noise Computations Problem Statement

Contributor

ONERA

This document contains 9 pages

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Revision Table

Issue V1

Issue Date

Modifications

18.04.2011

 Airbus France. This document is the property of Airbus France and shall not be distributed or reproduced without the formal approval of Airbus France.

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Table of contents 1

CONTACTS

4

2

INTRODUCTION

4

3

EXPERIMENTAL CONFIGURATION

4

4

SUMMARY OF AVAILABLE EXPERIMENTAL DATA

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5

NUMERICAL DOMAIN

5

6

5.1

INTRODUCTION

5

5.2

CLOSED-SECTION AERODYNAMIC CONFIGURATION (F2)

5

5.3

OPEN-JET AERODYNAMIC CONFIGURATION (CEPRA19)

6

5.4

HYBRID CONFIGURATION

6

5.5

CONCLUSION

7

COMPUTATIONAL PROCESS

7

6.1

TRANSITION TRIPPING

7

6.2

TRANSIENT DATA

7

6.3

CONVERGENCE

7

6.4

OTHER METRICS

7

7

EXPECTED RESULTS FOR COMPARISON WITH MEASUREMENTS

8

8

DATABASE DIFFUSION

8

9

REFERENCES

9


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1

CONTACTS Eric MANOHA Onera, France +33 1 46 73 48 09 Email: [email protected]

2

Bastien CARUELLE Airbus-SA, France + 33 5 61 18 51 82 Email: [email protected]

INTRODUCTION

The objectives of the LAGOON project is to assess and validate advanced CFD/CAA methods against an experimental aerodynamic and acoustic database on a simplified (2-wheel) nose landing gear geometry [1-2]. It is funded and coordinated by Airbus, and also involves Onera, DLR and Southampton University. This document is a proposal for a CFD/CAA airframe noise benchmark problem using this simplified landing gear. The objective of this benchmark problem is to rely on an an extensive aerodynamic/acoustic experimental database to compare and validate advanced combined CFD / CAA methods for the prediction of the unsteady flow field (including noise sources, noise generation mechanisms, etc.) and the resulting acoustic field, for a simplified (but representative) 2-wheel landing gear. The experimental database has been gathered by Onera in its own facilities in the framework of the LAGOON project. It is described in details in the document [3] which, with the present “Problem Statement”, is available for downloading on the BANC web site. The database (numerical files) will be provided on demand to any contributor. The property of the database will protected by a Non Disclosue Agreement” signed by the contributor, Onera and Airbus. The present document describes the the problem statement and criteria for meeting benchmark goals.

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EXPERIMENTAL CONFIGURATION

The tested model is a simplified 2-wheel landing gear, icluding a main leg, axle and two wheels. The scale approximately corresponds to 1:2.5 of a nose LG for an Airbus A320 aircraft (wheel diameter equal to 300 mm, main leg heigth about 690 mm). The configuration does not include any part of the lower fuselage section. Figure 1 shows the shape of the model and the test setups in Onera’s windtunnel F2 (closed section, aerodynamics) and CEPRA19 (open-jet, acoustics). The reference axis of the F2 windtunnel is used for all experimental results and numerical activities in the benchmark. The configuration chosen for the benchmark problem corresponds to M = 0.23.

Figure 1 : Left : CAD drawings of the model. Test setups in F2 (centre) and in CEPRA19 (right)  Airbus France. This document is the property of Airbus France and shall not be distributed or reproduced without the formal approval of Airbus France.

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SUMMARY OF AVAILABLE EXPERIMENTAL DATA

The aerodynamic experimental data comprise: • steady and unsteady wall pressures provided by 64 static pressure taps, 27 Kulite pressure sensors and 2 microphones onboard the model, • highly resolved planar steady flow fields through particle image velocimetry (2D PIV), • steady flow fields (average and RMS) through Laser Doppler velocimetry (2D and 3D LDV) • unsteady velocity at isolated points through X-hot wire and 2D LDV, • combined 2-point (X-hot wire and 2D LDV) unsteady velocity measurements (space-time correlations) The acoustic experimental data comprise farfield noise measurements provided by two circular arrays of 12 microphones (flyover and sideline), each centered on the model origin at a distance of about 6 m. All experimental data are described in details in the document [3]. Moreover, the provided package contains the model CAD, several documents and a selection of photographs of the model in both windtunnels.

5 5.1

NUMERICAL DOMAIN INTRODUCTION

Computational aeroacoustic simulations of the LAGOON LG model are solicited. Although prediction of the acoustic field is the ultimate goal, submissions targeting only the near field steady and unsteady flow are also welcomed. An experimental database collected in two different facilities obviously raises the critical question of what configuration and flow conditions are to be considered for the simulations. In the following, we compare 3 different options 5.2

CLOSED-SECTION AERODYNAMIC CONFIGURATION (F2)

If the main objective is to facilitate the comparison of the steady/unsteady CFD results to the aerodynamic database for code validation, then the computations conditions should be as close as possible to the F2 conditions, including the wind tunnel geometry (floor, ceiling, lateral walls) and the corresponding infinite thermodynamic conditions. However, the use of such CFD computation for further farfield acoustic prediction could be jeopardized by the presence of spurious acoustic reflections on the solid walls. In the F2 test set-up, the model is mounted in an inverted position (main strut parallel to the Z axis) on the floor of the tunnel (Figure 1, centre). The F2 test section is 1.8 m high, 1.4 m wide, and the section is 5 m long. The lateral walls are parallel, whereas the floor and ceiling have a constant divergent slope of 0.29° to compensate from the WT boundary layers. The model centre is centered on the test section axis at X = 0 (2.5 m downstream of the section entry). The tunnel-floor wall should be treated as a viscous boundary. It is left to the contributors decision whereas the three other solid walls in the computation are considered as inviscid or viscous surfaces.

The flow conditions at the entrance of the test section (X = + 2.5 m) should be taken as : Static pressure P0 = 99447.7 Pa Static temperature: T0 = 293.56 °K Density: ρ0 = 1.18 kg/m3 Mach: M = 0.23  Airbus France. This document is the property of Airbus France and shall not be distributed or reproduced without the formal approval of Airbus France.

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It should be noted that the geometry of the convergent section of the WT, located upstream the test section, is not part of the standard database, but it can be provided to the contributors on demand. 5.3

OPEN-JET AERODYNAMIC CONFIGURATION (CEPRA19)

On the other hand, if the objective is to directly validate the whole CFD/CAA process against the farfield noise levels measured in the anechoic facility, then the computation should consider the LG in the CEPRA19 conditions, say without solid walls. But in this case, the comparison of aerodynamics with the F2 measurements could be biased. In this configuration, the model is immersed in a circular open jet with a diametre of 2 m.(Figure 2, right). The model centre is centered on the test section axis at 1.932 m of the WT nozzle plane. The flow conditions at the entrance of the test section (X = + 1.932 m) should be taken as : Static pressure P0 = 96772.3 Pa Static temperature: T0 = 288.39 °K Density: ρ0 = 1.18 kg/m3 Mach: M = 0.23 It should be underlined that the simulation of the full experimental environment, including the convergent, nozzle and open free jet (with associated shear layers), the collector and the surrounding anechoic room, is out of the scope of the benchmark. However, some additional geometrical data of this environment can be provided to the contributors on demand. 5.4

HYBRID CONFIGURATION

Obviously the most comfortable situation would be to achieve two separate conmputations corresponding to both configurations with the same tools. However, this will be out-of-reach for most contributors, and any other hybrid configuration between both approaches will be welcome. A possible compromise is given hereafter. It relies on two assumptions : •

•

despite obvious differences in the model setup and tunnel configuration between the closed-wall tunnel (F2) and the open-jet facility (CEPRA19), only tiny differences were predicted and observed between the mean flows in both WTs, and it is assumed that these differences have little impact on the farfield sound, almost all aerodynamic data, which are required to validate the CFD activity, were obtained in the F2 closed section WT.

Consequently, the idea is to work with an hybrid geometry which (i) would be as close as possible to the closed-section WT conditions, but (ii) would allow further acoustic extrapolation and fair comparison to the open-jet WT measurements. For this purpose the CFD configuration should: • •

•

use the thermodynamic infinite upstream conditions of the closed-section WT, include the WT floor wall (from the entrance to the end of the test section) considered as a viscous boundary, because of the expected influence of the floor boundary layer with the LG main leg, exclude the WT lateral walls and ceiling, so that the influence of noise reflections on the noise sources is minimized.

With this compromise, it is expected that the farfield acoustic levels predicted from the flow computed in these “semi-closed” conditions (possibly rescaled from the infinite upstream thermodynamic conditions observed in CEPRA19) will be comparable to the noise levels measured in CEPRA19 (possibly corrected from the acoustic effect of the infinite rigid plane).


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5.5

CONCLUSION

Finally, it is left to the contributors decision to rely on their own priorities to choose a configuration for their computations. This is why both geometries and thermodynamic conditions are described above.

6

COMPUTATIONAL PROCESS

The objective of this part is to offer some guidance to the contributors in order to facilitate further comparisons of their computations with the experimental database and with other numerical contributions. 6.1

TRANSITION TRIPPING

Tripping devices were placed on all cylindrical elements (axle, strut) and wheels of the model, and their positions were adjusted after several acenaphten visualisations, to ensure that the transition of boundary layers from laminar flow to turbulent flow is effective. The nature and positions of these tripping devices are precisely described in [3], so that the contibutors are able to reproduce this transition in their computations, which is of primary importance regarding the simulation of the noise sources generation. 6.2

TRANSIENT DATA

For highly interactive and complex flows such as those encountered over landing gears, the transient flow is of significant duration and can easily be mistaken with the expected final wellestablished unsteady flow. Inclusion of this initial transient segment in both the sampled time records and the construction of the mean quantities irrevocably corrupt the computed results. Contributors must ensure that, upon the start of time-accurate simulations, a minimum duration of the computed data are discarded and not used for any post processing purpose. One reasonable estimation of this duration is about 25 transit time of the flow over one LG wheel diameter. Based on a free stream M = 0.23, this corresponds to a physical duration of about 100 ms. In any case, authors should ensure that sufficient time is allowed for their CAA tools to fully converge prior to acquiring data for post processing. 6.3

CONVERGENCE

The convergence behavior of the simulated flow field is of paramount importance and should be clearly demonstrated by every submitting author. As a global indicator, the lift and drag behavior on the complete LG (or on some gear subcomponents), should be tabulated and reported. To clearly demonstrate convergence, the authors should extend the time duration of the initial computational record by 40%, reprocess the data, and then show the percentage of change in the selected metrics. Demonstration of grid convergence is highly desirable but in most instances difficult to achieve. In addition to the finest resolution possible, all authors are encouraged to run their simulations at other (coarser) resolutions and report the incurred changes. 6.4

OTHER METRICS

All submissions should provide the total grid count (e.g. number of nodes, cells, etc.) as well as the cell size nearest to the gear surface at select locations on the main strut, axle and wheels. The computational domain should be described precisely, including the total dimensions, the stretching coefficients and the boundary conditions, especially the non-reflecting B.C. if any. Additional data regarding the computational resources that should be furnished by the authors are: a) the type and  Airbus France. This document is the property of Airbus France and shall not be distributed or reproduced without the formal approval of Airbus France.

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number of CPUs utilized, b) the type of data communications used, c) the clock time required to obtain 1000 time steps.

7

EXPECTED RESULTS FOR COMPARISON WITH MEASUREMENTS

Due to the extensive amount of data provided in the experimental database, benchmarking and validation of the simulations will be limited to a selected subset of the aeroadynamic and acoustic database proposed hereafter in Table 1. Submitting authors should provide a full account of the comparison between their simulations and the highlighted data. All details concerning the location of the probes can be found in reference [3]. Moreover, any other experimental data provided in the database can be subject to comparison to CFD/CAA results by the contributor. Magnitude Static wall pressure

Unit

Domain

Pressure coefficients Positions of all static pressure taps, sorted (non dimensionnal) by groups on the model (Table 2 [3])

PSD of unsteady wall dB ref 4.10-10 Pa2/Hz pressure fluctuations Average/RMS (U, V, W)

velocity m/s

Average/RMS (U, W)

velocity m/s

PSD of velocity fluctua- (m/s)2/Hz tions (U, W)

Positions of unsteady wall pressure sensors (odd number only : #1, #3, ...,#27) 2D maps at Z = 0, Z = -104, Z = +104 U and V : domain of PIV2D (Fig. 13 [3]) W : domain of LDV3D (Fig. 15 [3]) 1D surveys corresponding to lines 2, 4, 7, 8, 9, 10, 12, 20 of Table 5 [3] (and corresponding zones in LDV2D-C#1.plt) LDV2D surveys described in Table 9 [3], 8035_1 (4 points), 8039_2 (3 points), 8039_9 (3 points)

Correlations of axial (m/s)2 (computation of Surveys in X, Y, Z directions described in component U (“Pseudo “Pseudo U” from CFD: Table 10-1&2 [3], 8060-1, 8060-2, 8060-3 U” for XHW) see § 11.4 [3]) Integral length scales

m

PSD of farfield noise

dB ref 4.10-10 Pa2/Hz

Farfield noise directivity dB ref 4.10-10 Pa2 (integration of PSD)

Positions of farfield microphones (odd number only #1, #3, ...,#11) in flyover and sideline Positions of all 24 farfield microphones in flyover and sideline

Table 1 : Proposition of selected subset of aeroadynamic and acoustic data for benchmarking and validation of the simulations

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DATABASE DIFFUSION

The LAGOON experimental database is the property of Airbus. The dissemination of the database is managed by Onera on behalf of Airbus. Applications for accessing the experimental database should be sent to Onera and Airbus (contacts in Paragraph 1) and then the data will be sent to the contributor by the most convenient way.  Airbus France. This document is the property of Airbus France and shall not be distributed or reproduced without the formal approval of Airbus France.

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Prior to the transfer, the contributor will sign a Non Disclosure Agreement (NDA) in which he will declare that:

9

•

the database will be only used for the purpose of validating the CFD/CAA techniques developed by the contributor,

•

the database will not be copied nor disseminated without the autorization of the database owner, even inside the contributor’s organization,

•

the database owner will be informed of any publication (with a copy sent by email) of activities achieved by the contributor involving comparisons of numerical predictions or experimental data with the LAGOON database (this is for information purpose only, not for authorization),

•

any publication involving the database will cite (i) the LAGOON project name in the abstract and (ii) the references [1] and [2] of the present document in the text.

REFERENCES

[1]

E. Manoha, J. Bulté, and B. Caruelle, “Lagoon : An Experimental Database for the Validation of CFD/CAA Methods for Landing Gear Noise Prediction”, AIAA-2008-2816, 14th AIAA/CEAS Aeroacoustics Conference, Vancouver, May 5-7, 2008

[2]

E. Manoha, J. Bulté, V. Ciobaca and B. Caruelle, “LAGOON: further analysis of aerodynamic experiments and early aeroacoustics results”, AIAA-2009-3277, 15th AIAA/CEAS Aeroacoustics Conference, Miami, May 11-13 2009

[3]

E. Manoha, “LAGOON simplified (2-wheel) nose landing gear - Configuration #1 - Experimental database”, LAGOON deliverable R12, 14.04.2011
