Sjoerd Rienstra: Mathematician in Aeroacoustics Legacy at NLR Harry Brouwer
Sjoerd and NLR (NLR = Nederlands Lucht- en Ruimtevaartcentrum Netherlands Aerospace Centre) Worked at NLR from 1979 to 1986 Short period, long time ago. Left any traces? NLR Advisory Committee Aerospace Vehicles: 2014 - present 2
Some recent and not so recent aeroacoustics research at NLR Lined duct acoustics (1983 2013) Diffraction of propulsion noise by the fuselage (1986 present) Analytic description of fan tones (1986 present) Long range noise propagation (1996 2014) In-duct acoustic beamforming (2007 present) 3
Lined duct acoustics First report on subject of duct acoustics at NLR: S.W. Rienstra: A Study On The Transmission Of Sound Through Flow Ducts Of Varying Cross Section, 1983, NLR TR 83128 Computer program: LINDA (LINed Duct Acoustics), ~1987 4
Lined duct acoustics Validation of LINDA by Edward Rademaker, in NLR s Spinning Mode Synthesizer: 5
Lined duct acoustics LINDA: Used to compute optimal impedance, per mode: Served as reference and benchmark in various projects, e.g. to support later developments with CFD/CAA. 6
Lined duct acoustics Later developments: ~ 1994: LINDA for an annular duct (application to exhaust): ALIDA Annular LIned Duct Acoustics, by Pieter Sijtsma 2013: Thesis Martien Oppeneer and paper: Lined APU exhaust duct. 7
Diffraction of propulsion noise by the fuselage Research question: if we know the noise field radiated by an engine, what are the noise levels on the fuselage? (... and inside the cabin) 22 nd AIAA/CEAS Aeroacoustics Conference, Lyon, 2016 Paper 2016-2878 J. Gaffney, A. McAlpine, M. Kingan: Sound radiation of fan tones from an installed turbofan aero-engine: fuselage boundary-layer refraction effects. Paper 2016-2741 H.H. Brouwer: The Scattering of Open Rotor Tones by a Cylindrical Fuselage and its Boundary Layer. 8
Diffraction of propulsion noise by the fuselage Start at NLR: Continued by Brian Williams (master thesis)... 9
Diffraction of propulsion noise by the fuselage... and others at NLR: adding a boundary layer: DifRef extension to non-circular cross-sections: ScatRef 10
Diffraction of propulsion noise by the fuselage Latest extension: Coupling to comprehensive source descriptions: Open rotors (single and contra-rotating propellers) Fan tone (Wiener-Hopf description) 11
Diffraction of propulsion noise by the fuselage Some results, CROR 1,1 interaction tone: Sound pressure on fuselage wall: no boundary layer with boundary layer 12
Diffraction of propulsion noise by the fuselage Sound pressure in the far field, cross section at x = 0: 13
Analytic description of fan tones Needed: description of radiated fan tones as input for diffraction computation. Schematically: 14
Analytic description of fan tones Solved a long time ago, by application of the Wiener-Hopf method, e.g.: G.F. Homicz and J. Lordi. A note on the radiative directivity patterns of duct acoustic modes. Journal of Sound and Vibration, 41:283 290, 1975. R. M. Munt. Acoustic radiation from a circular cylinder in a subsonic stream. Journal of the Institute of Mathematics and Applications 16, 1-10. 1975.... and at NLR: 15
Analytic description of fan tones Computer codes based on this paper still available and used at NLR. 16
Analytic description of fan tones Solution in terms of Fourier transform, i.e. instead of x. Inverse Fourier transform: p x = e iαx p α dα In p α we have a factor K + α, given by: log K + α = 1 2πi C + log i πj m H m 2 z α dz For the numerical evaluation we need 2 suitable integration contours, 1 in the z plane, and 1 in the plane. 17
Analytic description of fan tones Contour in the z plane used by Sjoerd: (In practice A has a much large value than suggested here) Now fairly straightforward to do integration for areas I and II. Which contour for III? (No practical use, just out of curiosity) 18
Analytic description of fan tones Suitable contour in the α plane : adds t = β2 α M ω 1 x input mode, required to make the solution vanish at large distances 19
Long range noise propagation 1996: bankruptcy of Fokker Aircraft (NIVR) Redirect research from aircraft technology to noise impact Self-imposed constraint: Stay away from politics but stick to asymptotics Problem attacked by Johan Schulten: long range propagation in a nonuniform atmosphere. Ray tracing: not new, but not yet existing at NLR. T z 0 V x z 0 20
Long range noise propagation Ray tracing theory: Expansion in 1/ω. In lowest order q 2 = 1 w q 2 (q is the slowness vector, direction of ray). c Eikonal equation standard ray tracing Next order: relation for the amplitude of the ray. First version by Johan Schulten (1997), finalized by Harry Brouwer (2014). Complicated: solved only after frequent consultation of favourite handbook: 21
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Long range noise propagation For example: in a flow c 2 ρ p, but: c 2 Dρ Dt + u ρ 0 = Dp Dt + u p 0 Relation for the amplitude of the ray P: dp dt = P 2q w w q c c q 2q z γg 2c + c z + q q w z Published in Brouwer, H.H., A ray acoustics model for the propagation of aircraft noise through the atmosphere, International Journal of Aeroacoustics 13 (5+6), p. 363, 2014 Reviewed by NLR s Advisory Committee (= Sjoerd). Sjoerd s own derivation: 23
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In-duct beamforming Beamforming is locating noise sources by combining the signals measured by a (large) number of microphones. Initiated by Pieter Sijtsma: Using a ring of microphones installed in duct to locate sources on fan blades or stator vanes. 25
In-duct beamforming Required: the transfer function between possible source locations and the microphone positions: Green s functions. Pieter s first applications: duct walls with acoustic lining reflections can be ignored use of freefield Green s functions gives good results. Experiments in present EU projects ENOVAL and TurboNoiseBB: Hard walls in inlet and interstage section. Green s functions required for lined and hard-walled ducts... 26
In-duct beamforming 27
In-duct beamforming Example: hard-walled duct, no hub: G m = ±i 2π μ=1 J m γ mμ r J m γ mμ r 0 2 J m γ ± mμ αmμ β 2 ωm 1 m2 2 γ mμ e iα ± mμ (x x0 ) Zeros in denominator: resonances. Correspond to modes on the verge of cut-on. 28
In-duct beamforming Example: single source at y = 0.32, z = 0. 3500 Hz Far from resonance 2497.6 Hz At resonance 2497.6 Hz Discarding resonant mode 29
In-duct beamforming With deconvolution: 2497.6 Hz Discarding resonant mode With deconvolution 30
These examples show that analytical solutions of simplified configurations: Are indispensable to understand the physics of the problem Provide quick estimates of relevant quantities Can serve as benchmarks for high-fidelity numerical methods Enable simulations during development of advanced data post-processing methods... 31
CEAS Aeroacoustics Specialists' Committee 32
CEAS Aeroacoustics Specialists' Committee Sjoerd and the CEAS-ASC: Member since 1996 Chair 2013 2014 Organizer of the 5th CEAS-ASC Workshop (Eindhoven, 2001) Designed and maintains the web page Editor of the yearly Highlights in 1997, 2007. Chair of the Education subcommittee (students award AIAA/CEAS Aeroacoustics Conferences). 33
To conclude on Sjoerd s contribution to aeroacoustics: From De Avonden, by Gerard Reve: Het is niet onopgemerkt gebleven (It has not gone unnoticed) 34
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