The computation of noise generation and propagation from first principles in a realistic three-dimensional axial fan configuration is a computationally intensive endeavor. As in all computational aeroacoustic (CAA) applications particular care must be taken to ensure adequate spatial and temporal resolution of the small amplitude acoustic waves. The actual wave propagation characteristics, and mesh and time step size requirements of the TRACE code have been established in earlier studies.
Detached Eddy Simulation (DES)
The flow field interaction effects are shown to play a decisive role in the noise generation process. To predict the far-field noise spectra of jet engines a special numerical simulation is necessary. The DES scheme implemented in TRACE has been calibrated and applied to two different test cases that are relevant to turbomachinery flows. Either of them is the rod-airfoil testcase, which aimed at the simulation of broadband noise in the turbomachinery. In order to assess the accuracy of the employed DES model for broadband-noise prediction, simulations of the rod-airfoil test case investigated by Jacob et al. were carried out. To accurately predict the noise radiated by the highly-periodic vortex structures superimposed with more stochastic small-scale vortical fluctuations, a numerical model must be capable of resolving directly at least the larger turbulent structures in the wake of the cylinder and their interaction with the downstream airfoil.
The application of time-linearized RANS methods to the numerical simulation of aerodynamic noise generation and propagation in a modern high-bypass ratio fan is investigated. The convective and viscous fluxes of an existing URANS solver are linearized and the resulting unsteady linear equations are solved in the frequency domain, thereby transforming the problem from unsteady time-integration to a simpler complex linear system. The linear system is solved using a parallel, preconditioned restarted GMRES method. In view of the fact that nonlinear unsteady CFD computations remain costly in terms of CPU time, the results show that using the time-linearized solver for noise prediction constitutes an adequate method to efficiently obtain estimates of noise levels.
UHBR-Fan, Pressure contour of the first harmonic of the sixth mode: left - nonlinear solver, right - time-linearized solver
Coupling to acoustic far-field noise prediction
To evaluate the acoustic characteristics of an engine, typically a hybrid approach is used. Special optimized metheds to predict the wave propation into the far-field are available. The methods includes Boundary Element Methodens (BEM) and integral acoustics methods after Ffowcs Williams & Hawkings or Kirchhoff. TRACE provides suitable surface for all of these applications. Those are as well hard walls or in the flow field located porous surfaces. Either the definition will be made by the user befor a TRACE simulation to reduce the file output or afterwards from the complete 3D flow solution.The definition of a porous surface by the user is made by iso-contour lines and are created with the help of VTK (Visualization Toolkit) by Kitware. The resulting surfaces are unstructured grids, which can also provided with a band structure e.g. to make modal decomposition. The resolution is orientated on the original 3D grid and as well splitted into block to achive an efficient parallelization.