Tonal noise of contra-rotating open rotors can be divided into self and interaction noise. The latter can be subdivided into potential field interaction noise, noise due to the interaction of the front-rotor wakes with the aft-rotor blades and tip vortex interaction noise. The excited tonal sound field is closely related to the properties of the impinging wakes provided that the tip vortex interaction is not the dominant source of interaction noise. Designing the trailing edge of the front-rotor blades with serrations will enhance the mixing in the wakes which should have a beneficial effect on the interaction tones. This process is investigated with 3D RANS calculations of 3 serration designs. Through assessment of the wake properties one configuration is down selected for evaluation of the far-field sound levels. These are computed using a 3D URANS approach coupled to an integral method that implements the FW-H acoustic analogy. The effect observed with the serrations on the far-field directivity is twofold: at low frequencies the sound energy is redistributed in the polar arc; at higher frequencies a systematic reduction of the sound emission is observed.
This is the second part of a series of two papers on unsteady computational fluid dynamics (CFD) methods for the numerical simulation of aerodynamic noise generation and propagation. It focuses on the application of linearized RANS methods to turbomachinery noise problems. The convective and viscous fluxes of an existing URANS solver are linearized and the resulting unsteady linear equations are transferred into the frequency domain, thereby simplifying the solution problem from unsteady time-integration to a complex linear system. The linear system is solved using a parallel, preconditioned general minimized residual (GMRES) method with restarts. In order to prescribe disturbances due to rotor stator interaction, a so-called gust boundary condition is implemented. Using this inhomogeneous boundary condition, one can compute the generation of the acoustic modes and their near field propagation. The application of the time-linearized methods to a modern high-bypass ratio fan is investigated. The tonal fan noise predicted by the time-linearized solver is compared to numerical results presented in the first part and to measurements.
Time-periodic CFD simulations are widely used to investigate turbomachinery components. The triple-plane pressure mode matching method (TPP) developed by Ovenden and Rienstra extracts the acoustic part in such simulations. Experience shows that this method is subject to significant errors when the amplitude of pseudo-sound is high compared to sound. Pseudo-sound are unsteady pressure fluctuations with a convective character. The presented extension to the TPP improves the splitting between acoustics and the rest of the unsteady flow field. The method is simple: i) the acoustic eigenmodes are analytically determined for a uniform mean flow as in the original TPP; ii) the suggested model for convective pressure perturbations uses the convective wavenumber as axial wavenumber and the same orthogonal radial shape functions as for the acoustic modes. The reliability is demonstrated on the simulation data of a low-pressure fan. As acoustic and convective perturbations are separated, the accuracy of the results increases close to sources, allowing a reduction of the computational costs by shortening the simulation domain. The extended method is as robust as the original one-giving the same results for the acoustic modes in absence of convective perturbations.
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