Advanced ANSYS CFD[Fluent] Aeroacoustics

•Acoustics Tools in ANSYS CFD
–Direct Computational Aeroacoustics (CAA)
– Ffowcs Williams-Hawkings Model
–Broadband Noise Models
– FFT Signal Postprocessing
– Coupling with Acoustics Codes

Direct Aeroacoustic Predictions
• Time accurate compressible flow solution
• All acoustic wavelengths of interest are spatially and temporally sufficiently resolved from source to receiver location – 15-20 grid points per wavelength for second-order schemes – The same temporal resolution is required for high fidelity CAA solutions  This implies a time step corresponding to CFL=1  Larger time steps will still propagate information at approximately the speed of sound, but phase errors will build up. May be an acceptable approach for ‘dirty’ industrial acoustics if only the lower frequency modes need to be captured. – Underresolved waves dissipate quickly – Requires higher order discretization schemes to propagate waves with fidelity into farfield
• Non-reflective outflow boundary conditions, NRBCs – Artificially reflected sound should not interfere with primary sound field

Acoustic Analogy Methods
• Magnitude of acoustic pressure fluctuations are very small compared to hydrodynamic pressure fluctuations
• Acoustic radiation contains only tiny fraction of energy of the primary flow
• The classical CFD methods are not suited to cope well with the needed level of accuracy

Broadband Noise Models
• CAA and Acoustic Analogy methods are very time consuming
• For design purposes, one often does not need detailed acoustic information
• Can the required information be extracted from steady RANS calculations?
• Broadband noise models were developed to partially, or qualitatively, answer such questions, and are potentially useful to screen noisy designs

Acoustic Postprocessing
FFT Signal Postprocessing
• Results  Plots  FFT… – General FFT utility • Process receiver (FW-H) data directly or read any FLUENT xy-file • OASP reported in console window • Y-axis functions: Power Spectral Density (PSD), Magnitude, Sound Pressure Level (SPL), Sound Amplitude, A-, B-, C-Weighted • X-axis functions: Frequency, Strouhal Number, Fourier Mode, Octave and 1/3Octave Band

Coupling with Third Party codes
• ANSYS FLUENT – Virtual.Lab (revision 9) coupling implemented through file exchange – Virtual.Lab imports standard FLUENT acoustic source files, for stationary and rotating source surfaces – Virtual.Lab requires files in CGNS format – Virtual.Lab uses imported dipole and quadrupole (optional) source information for BEM/FEM analysis

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