Visualization of the shear layer through a lobe mixer

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Broadband noise

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. One of them is the rod-airfoil test case, 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 able to simulate at least the larger turbulent structures in the wake of the cylinder and their interaction with the downstream airfoil.

A further validation of the method is the flow through a lobe mixer. The flow in mixer nozzles is characterized by shear layers with strong velocity gradients between bypass and core flow, giving rise to strong turbulent structures in both circumferential and radial directions, which interact with the tonal noise field on the one hand and cause broadband noise on the other.

Tonal noise

Up to now the dominant noise source of an airplane are still the propulsive systems. Propulsive noise itself has traditionally been dominated by jet mixing noise. In particular, the introduction of the bypass engine concept has, however, drastically reduced this noise source. For modern high-bypass-ratio turbofan engines, fan noise radiating forward through the inlet and aft through the fan exhaust duct is now the dominant source of noise during both takeoff and landing.
Fan noise comprises both tonal and broadband components and arises due to a variety of mechanisms:

rotating shock structures

the interaction of inlet flow disturbances (ingested atmospheric turbulence, asymmetric inflow, boundary-layer turbulence) with the rotor blades

rotor/stator interaction

secondary flow phenomena such as that in the tip clearance gap

For subsonic blade tip speeds the radiated acoustic field is typically dominated by discrete tones at the blade passing frequency (BPF) and harmonics thereof. In such cases the interaction of the rotor-wakes with the downstream stators and the interaction of the rotor itself with inflow disturbances are the dominant noise source mechanisms.

LES of a turbulent flow in a low-speed axial fan: iso-surfaces of lambda-2

UHBR-Fan, Pressure contour of the first harmonic of the sixth mode: left - nonlinear solver, right - time-linearized solver

Counter-rotating open rotor with a enclosing surface for acoustic far-field noise prediction

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LinearTRACE

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. If you consider 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.

Boundary Conditions

An equally important aspect of any CAA simulation is the boundary conditions, and in particular the radiation boundary conditions required along the external boundaries of the computational domain. Such boundary conditions should ideally be transparent to outgoing vertical-, acoustic- and entropy waves, while permitting incoming disturbances and prescribed external physical boundary conditions. Accurate non-reflecting boundary conditions are implemented in the TRACE code. In particular, an implementation was sought and developed specifically for use with the implicit dual-time stepping solver algorithm of TRACE.

Coupling to acoustic far-field noise prediction

To evaluate the acoustic characteristics of an engine, typically, a hybrid approach is used. Special optimized methods to predict the wave propagation into the far-field are available. The methods include Boundary Element Methods (BEM) and integral acoustics methods after Ffowcs Williams & Hawkings or Kirchhoff. TRACE provides suitable surface for all of these applications.

Those are as well solid walls or in the flow field located porous surfaces. Either the definition will be made by the user before a TRACE simulation to reduce the file output or afterwards from the complete 3D flow solution.

Kontakt

Dr. Graham Ashcroft Institute of Propulsion Technology Numerical Methods Köln Tel.: +49 2203 601-2283