Effects of Wind-Tunnel Noise on Array Measurements in Closed Test Sections

Transcription

1 . Effects of Wind-Tunnel Noise on Array Measurements in Closed Test Sections K. Ehrenfried 1, L. Koop 1, A. Henning 2 and K. Kaepernick 2 1 Institute of Aerodynamics and Flow Technology German Aerospace Center (DLR), Göttingen 2 Institut für Luft und Raumfahrt Technical University of Berlin klaus.ehrenfried@dlr.de 1

2 Overview Motivation Experimental setup Source maps Effects of background noise Wavenumber spectrum Algorithm to remove noise effect in source maps Improved results Background noise 2

3 Motivation Economic efficiency in aerodynamic and aeroacoustic testing Aerodynamic and aeroacoustic tests in parallel Acoustically optimized wind tunnels Open jet configuration in anechoic chamber Surfaces line with absorbing material Little background noise Aerodynamic measurements Closed test sections Well defined boundary conditions Ideal for comparison with numerical results = Aeroacoustic experiments in closed test sections Background noise 3

4 Experimental setup Inflow array and SCCH model Background noise 4

5 Experimental parameters Wind tunnel 2.0 m 1.4 m test section, hard side walls 30 m/s flow velocity Array 144 microphones on 9 spiral arms, 1 m diameter of microphone field 25 mm thick array body Electret microphones (RTI) 120 khz sampling frequency, 30 sec measurement time Wing model Swept-wing constant-chord half-model (SCCH model) 1.2 m span, 0.4 m chord Slat deployed, flap retracted, 7 degree angle of attack Background noise 5

6 Point of view Inflow array and SCCH model (upside down) Background noise 6

7 Array processing Delay-and-sum beamforming in frequency domain 4096 samples window length (rectangle), f = 29.3 Hz Diagonal removal 10 mm source map resolution Observation plane turned with SCCH model Array maps using local coordinates Assumption of homogeneous flow (U = 30 m/s) Model-frequency scaling 1:6 Background noise 7

8 Beamforming results f = 7500 Hz (narrow band) Background noise 8

9 Beamforming results f = Hz (narrow band) Background noise 9

10 Beamforming results f = Hz (narrow band) Background noise 10

11 Beamforming results f = Hz (1/3 octave) Background noise 11

12 Beamforming results f = 3750 Hz (narrow band) Background noise 12

13 Beamforming results f = Hz (narrow band) Background noise 13

14 Opposite side wall lined with absorbing layer 30 mm thick foam layer Background noise 14

15 Effect of absorbing layer Untreated side wall With foam layer f = Hz (narrow band) Background noise 15

16 Wavenumber spectrum at side wall Wavenumber vector k = (k x, k y, k z ) with k = 2πf M /c f M : Frequency in moving reference frame Uniform flow 2πf = 2πf M + k x U and k = (2πf k x U)/c Acoustic domain limited by ellipse k x k 0 = cos θ 1 + M cos θ and k y k 0 = sin θ 1 + M cos θ with k 0 = 2πf/c and 0 θ 2π Duct modes k z = mπ and W W, H: Width and height (2 m, 1.4 m) k y = nπ H Background noise 16

17 Acoustic domain U = 30 m/s (M 0.088), 720 duct modes cut on (f = 2900 Hz) Background noise 17

18 Estimation of the wavenumber spectrum Beamforming with an infinite focus distance Steering vector s j = exp [ i(k x x j + k y y j )] (x j, y j ): Position of j-th microphone Map in wavenumber space Normalization with k 0 = 2πf/c Summation over 1/3-octave of values for fix (k x /k 0, k y /k 0 ) Background noise 18

19 Maps in wavenumber space Untreated side wall With foam layer f = 3000 Hz (1/3-octave) Background noise 19

20 Method to remove background noise Split cross-spectral matrix in two parts: R = R 1 + R 2 R 1 represents waves from the model R 2 represents background noise Iterative procedure: R (0) = R, R (0) 1 = 0 and R (0) 2 = 0 1. Calculate source map in object plane and map in wavenumber space using current R (j) 2. Search absolute maximum over both maps 3. Construct a synthetic cross-spectral matrix using steering vector s belonging to this maximum DR(): Diagonal removal R s = DR(s s H ) Background noise 20

21 Method to remove background noise 4. Normalize synthetic cross-spectral matrix ( ) s R H R (j) s s = R s s H R s s 5. Set R (j+1) = R (j) R s 6. If absolute maximum belongs to source map in object plane then { R (j+1) 1 = R (j) 1 + R s } else { R (j+1) 2 = R (j) 2 + R s } Iteration can be stopped when maximum is below predetermined threshold (here: 200 iterations) Remaining R (j) is added to R (j) 1 and then R (j) 1 is taken as new cross-spectral matrix to replace the initial R Background noise 21

22 Method to remove background noise Object plane grid Wavenumber space grid Grids used in iterative procedure Background noise 22

23 Result f = Hz (narrow band), reduced cross-spectral matrix Background noise 23

24 Comparison Raw cross-spectral matrix Reduced cross-spectral matrix f = Hz (narrow band) Background noise 24

25 Concluding remarks Strong background noise present in wind tunnels with closed test sections Upstream propagating waves Waves cause artifacts in source maps Iterative method to remove artifacts caused by plane waves Further testing in other wind tunnels required Background noise 25