Application of PIV in Acoustic and AeroacousticExperiments

Next to Flow-noise, buffeting by open cavities are the most dominant noise-sources in aviation and automotive aerodynamics. The buffeting phenomenon describes the self-sustained oscillations of a Helmholtz-Resonator evoked by shear-layer disturbances. The wide-banded disturbances, induced by the flow-separation at the leading-edge of the cavity opening, also contain the frequency-range which matches the resonance-frequency of the Helmholtz-Resonator. A detailed analysis of the phenomenon has been performed using High-Speed Stereo Particle Image Velocimetry measurements combined with coincident microphone measurements on a generic cavity model.

An arrangement of two consecutive orifices was investigated where the sound generation at the downstream orifice depends on the mean flow velocity and on the flow disturbance depending on the distance to the upstream orifice. Broadband noise simulations often utilized a hybrid CFD/CAA approach. In the CAA step the acousticfield is computed using acoustic perturbation equations. The turbulent fluctuations are induced via a broadband turbulence model. The objective of the current study is to validate the first step of such a hybrid approach, the steady flow simulation, with timeresolved particle image velocimetry measurements in the source region.

PIV versus CFD as Basis for Hybrid CAA

HS-PIV investigation of cavity resonance

Causality Correlation by means of simultaneous PIV and Microphone-Array measurements

Calibration of a p-u-probe using HS-PIVIn practical applications the velocity transducer is usually calibrated with respect to the pressure transducer of the p-u probe. However, the formulation of the acoustic impedance can be very complex if the plane wave propagation is not guaranteed. The idea of the present investigation is to obtain absolute levels of the particle velocity by means of particle image velocimetry measurements, sampled up to a frequency of 20 kHz.

Spectral representation of the measured particle velocity [m/s]. Depicted is SPL [dB] re: 50 nm/s measured with the high-speed PIV system in the near-field of the HF loudspeaker at 250 Hz.

Field of view

Detailed view of the laser light-sheet illuminating the near field region of the HF loudspeaker. In the background the long-distance microscope and the high-speed camera are visible.

Simultaneous multi plane PIV and microphone array measurements have been performed on a rod-airfoil configuration in an aeroacoustic wind tunnel with an open test section. The simultaneous measurement of near-field velocity fluctuations by PIV and far-field pressure fluctuations by microphone array measurements is successfully used to obtain the cross-correlation function between near-field and the acoustic far-field data.

Experimental setup of the Rod-Airfoil Experiment. The PIV system consists of 2 independent double pulse laser systems generating two coplanar light sheets, allowing the calculation of temporal velocity derivatives. The measurement setup consists of a symmetric NACA-0012 airfoil located one chord downstream of a rod with d/c = 0.1.

The temporal evolution of the cross correlation coefficients (Rv’;p’ ) with �. Top: [x/c; y/c] = [0:165 ; 0:05] near the cylinder wake. Bottom:[x/c ; y/c] = [0:95 ; 0] near the leading edge of the airfoil.

Spatial distribution of �*(x): |Rv’p’(�*)|(x) � Max |Rv’p’(�)| (x). Corresponds to group velocity of flow structures traveling c = 0.15 m.

Comparison between high-speed PIV and far-field measurement in a large anechoic room. Displayed is the sensitivity [V/ms-1] of the p-u probe measured with the HS-PIV system (+) and with the far-field procedure. The black line shows the calibration curve given by the manufacturer.

A. Henning1, K. Ehrenfried2, A. Heider2, L. Koop2 and C. Spehr2

1Technical University of Berlin. Current address:2. arne.henning@dlr.de2German Aerospace Center (DLR), Institute of Aerodynamics and Flow Technology (AS), Göttingen, Germany

Distribution of the fluctuating velocity for ReL= 3,5x105 standardized by opening length L. Vorticity colour coded.

The Reynolds-Number depen-dent frequency distribution calculated by microphone measurements. Additionally the vortex shedding frequencies calculated by the PIV Measurements are shown (black scatters).

Spatial distribution of distribution of the turbulent kinetic energy. Left: Results of the simulations, performed with an unstructured incompressible RANS code. The k-spot in the vicinity of the orifice may be attributed to the missing limitation of the production term of turbulent kinetic energy of the k-� model used. Right: HS- PIV results.

This work was supported by the LUFO IV – TEKOS research program

fm l/umean

L W[d

B]

100 101 102-20

0

20

40

60lA= 0.4 m 5 m/slA= 0.5 m 5 m/slA= 0.3 m 10 m/slA= 0.4 m 10 m/slA= 0.5 m 10 m/slA= 0.3 m 15 m/slA= 0.4 m 15 m/slA= 0.5 m 15 m/slA= 0.3 m 20 m/slA= 0.4 m 20 m/slA= 0.5 m 20 m/slA= 0.3 m 25 m/slA= 0.4 m 25 m/slA= 0.5 m 25 m/s

Results of the RPM Simulations.

p-u probe

Instantaneous distribution of the cross correlation coefficients (Ru’;p’ ,Rv’;p’ ) for � = 0 ms as a vector plot. Left: Cylinder wake. Right: Airfoil leading edge.

References:

Spehr, C., Henning, A., Siefert, M. and Kornow, O. (2009) “PIV versus CFD as Basis for Hybrid CAA”, 13th CEAS-ASC Workshop & 4th Scientific Workshop of X3-Noise, Bucharest, Romania.

Henning, A., Kaepernick, K., Ehrenfried, K., Koop, L., and Dillmann, A., “Investigation of Aeroacoustic Noise Generation by Simultaneous Particle Image Velocimetry and Microphone Measurement,” Experiments in Fluids, Vol. Vol. 348, 2008.

Henning, A., Koop, L., Ehrenfried, K., Lauterbach, A., and Kroeber, S., “Simultaneous Multiplane PIV and Microphone Array Measurements on a Rod-Airfoil Configuration,” 15th AIAA/CEAS Aeroacoustics Conference, Miami, USA, Vol. AIAA-2009-3184, 2009.

Henning, A., Koop, L., Ehrenfried, K., and Schröder, A., “Investigation of Aeroacoustic Noise Generation by Simultaneous Multiplane PIV and Microphone Array Measurements,” 8th International Symposium On Particle Image Velocimetry - PIV09, Vol. PIV09-0061, 2009.

Heider, A., Geisler, R., Henning, A., Kröber, S., Agocs, J. and Schröder, A. „Experimental investigations of flow-induced cavityresonances.“ In: Liepmann-Ludwieg-Seminar 2008, 2008-05-19, Santa Barbara, CA (USA).