FRAUNHOFER-INSTITUT FÜR Bauphysik IBP

IBP Report 506

37 (2010) New research results in brief

Seiji Adachi, Peter Brandstätt, Computational Dynamics John C. Simpson and Computational for Aircraft Noise Estimation

Introduction of small eddies that cannot be captured Fraunhofer Institute A new method for estimating aeroacous- with a simulation mesh. The wing used in of Building Physics IBP tic noise has been introduced to the this analysis has a slat deployed in front of Nobelstrasse 12, 70569 Stuttgart, Germany Fraunhofer Institute for Building Physics IBP. the main wing. The chord length, i. e., the Phone +49 711 970-00 It is a simulation study based on computa- length between the leading edge of the [email protected] tional (CFD) and computa- slat and the trailing edge of the main wing tional aeroacoustics (CAA). CFD is a tool to is 3.3 m. The flow was assumed to be two- Holzkirchen Branch simulate a flow numerically. Aeroacoustics dimensional. The velocity of the flow away Fraunhoferstr. 10, 83626 Valley, Germany is a sub-field of acoustics in which from the wing was set to 50 m/s. The wing Phone +49 8024 643-0 generated from a turbulent flow is studied. was tilted by ten degree to the direction of [email protected] In CAA, such noise is computed numerically. the flow. Fig. 1 (on next page) shows a sim- Kassel Branch ulation result, where a flow is represented Gottschalkstr. 28a, 34127 Kassel, Germany Due to recent development of low-noise by stream lines and is denoted in Phone +49 561 804-1870 aircraft engines, airframe noise has now color. Red and blue indicate high and low [email protected] become comparable to that from engines pressure, respectively. at landing phase. One major cause of air- www.ibp.fraunhofer.de frame noise is a wing and high lift devices Two main effects are observed at the wing attached to it. This short report summarizes due to the flow: drag and lift. High pres- This work is funded by the European Union within the a way to estimate aeroacoustic noise from sure contributing to drag occurs at the program “JTI Clean Sky Green regional aircraft”, grant agreement no. CSJU-GAM-GRA-2008-001. a wing. front of the slat, because the air is stagnant there. Lift is generated due to the fact that References Flow simulation pressure is low on the upper surface of the [1] Lesieur, M. and Metais, O.: New trends in large-eddy simulations of , Ann. Rev. Fluid Mech., The incompressible Navier-Stokes equations wing, while it is moderate and high on the 28 (1996) p. 45--82 were solved by assuming that the flow ve- lower surface. This is because flow velocity [2] Curle, N.: The influence of solid boundaries upon locity is sufficiently smaller than the sound is larger above the wing than below it. Af- aerodynamic sound. Proc. Roy. Soc., London, A231 (1955) p. 505–514 velocity. Large eddy simulation (LES) [1] ter the flow along the upper surface passes was adopted here as a turbulence model. the front part of the wing, it then leaves © Fraunhofer-Institut für Bauphysik IBP Nachdruck oder Verwendung von Textteilen oder Ab­ This is for correctly estimating the effect the surface (flow separation) and becomes bildungen nur mit unserer schriftlichen Genehmigung 1 2

turbulent. Eddies can be observed in the Acoustic simulation Sound pressure was monitored at a point wake. These result in unsteady pressure Acoustic radiation was calculated by solving 4.0 m downstream and 58 degree down- and give rise to a noise source. the wave equations with fluctuating pres- ward from the trailing edge of the main sure on the wing surface as a noise source wing. The waveform and spectrum are To investigate the characteristics of the [2], which was obtained in the flow simula- plotted in Fig. 4. We find that this estimated noise source more deeply, the pressure at tion. To realize a free field condition, a per- noise has large components in the low fre- two points, one on the upper and the other fectly matched layer (PML) was arranged quency region, which is about 20 to 40 Hz. on the lower surfaces, is analyzed. In Fig. 3, at the boundary of the calculation domain. It also has perceptible components in the the waveforms and spectra of the pressure This layer absorbs the wave radiating from middle frequency region that is about 150 on the upper and lower surfaces are plot- the wing and does not reflect it back to to 250 Hz.

Summary Fig. 3 Time histories of pressure fluctuation and re- sulting sound pressure levels versus frequency Fig. 4 Estimated noise waveform and spectrum The paper presented a numerical method of how aeroacoustic noise from a wing is estimated. In a CFD simulation, an un- steady flow was first simulated to find pressure fluctuations on the wing surface. With a CAA simulation, noise was then es- timated by solving the wave equation with the fluctuating pressure as a source. It was found that the estimated noise has large components in the low frequency region and perceptible components in the middle frequency region. data at a point 4.0 m downstream and data on upper and 58 degree downward from the trailing lower surfaces of the wing edge of the main wing. ted in blue and green, respectively. The av- the calculation domain. A snapshot of the erage of the pressure is smaller on the up- sound pressure wave radiating from the per surface than on the lower surface. This wing is shown in Fig. 2, where high and difference corresponds to lift. The pressure low pressure is represented in red and blue. fluctuation amplitude is larger on the up- The radiation pattern is to some extent like per surface than on the lower surface. This that of the dipole radiation. The phases of is because of the turbulent flow generated the waves radiating towards the upper and due to the flow separation above the upper lower directions are opposite to each other. surface. The frequency of the fluctuation is This is, however, not obvious because the about 25 Hz. The spectrum of the pressure wing shape is not symmetric. on the upper surface has a broad peak at this frequency.

1 Simulated flow and pressure around a wing with a slat. A flow is represented by stream lines. Pressure is denoted in color. Red and blue indicate high and low pressure, respectively. 2 Pressure wave radiating from the wing.