Christian Weckmüller

CFD/CAA Coupling Applied to the DLR UHBR-Fan: Comparison to Experimental Data

A CFD-CAA hybrid method for fan noise prediction is presented and compared to experimental data. The study shows a very good agreement between numerical simulations and experimental measurements regarding the level of the rotor-stator interaction tones generated by a Ultra-High Bypass Ratio Fan designed by DLR. The comparison is done at a single approach condition for which the blade passing frequency (BPF 1) is cut-off. For the BPF 2, the sound pressure level measured in the inlet is in agreement within 1 or 2 dB with the experimental results. The agreement in terms of radial mode amplitudes is also very satisfying. The discrepancies are slightly higher for the BPF 3 and 4. Nevertheless the trends of the results, for instance the fact that the tone is higher at BPF 4 than BPF 3, are all well predicted. Most of the discrepancies are of the order of the amplitude variations measured between the two investigated experimental runs. The boundary layer influence on the sound propagation is shown to increase with the frequency as stated in literature. Some numerical results from the bypass duct are also presented.

Advanced Numerical Methods for the Prediction of Tonal Noise in Turbomachinery—Part II: Time-Linearized Methods

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.

A robust extension to the triple plane pressure mode matching method by filtering convective perturbations

Time-periodic computational fluid dynamics simulations are widely used to investigate turbomachinery components. The triple plane pressure mode matching method 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-sounds are unsteady pressure fluctuations with a convective character. The presented extension to the triple plane pressure 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 triple plane pressure mode matching method; (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.

Advanced Numerical Methods for the Prediction of Tonal Noise in Turbomachinery—Part I: Implicit Runge–Kutta Schemes

This is the first part of a series of two papers on unsteady CFD methods for the numerical simulation of aerodynamic noise generation and propagation. In this part, the stability, accuracy and efficiency of implicit Runge-Kutta schemes for the temporal integration of the compressible Navier-Stokes equations are investigated in the context of a CFD code for turbomachinery applications. Using two model academic problems, the properties of two Explicit first stage, Singly Diagonal Implicit Runge-Kutta (ESDIRK) schemes of second- and third-order accuracy are quantified and compared with more conventional second-order multi-step methods. Finally, to assess the ESDIRK schemes in the context of an industrially relevant configuration, the schemes are applied to predict the tonal noise generation and transmission in a modern high bypass ratio fan stage and comparisons with the corresponding experimental data are provided.

On the Influence of Trailing-Edge Serrations on Open-Rotor Tonal Noise

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.

3D Unsteady RANS Simulation of the Interaction Between Fan Stage, Struts and Bifurcations

Struts and bifurcations installed in the bypass duct of aeroengines can deteriorate the aerodynamic and acoustic performances of the fan and therefore should be accounted for in the design process. This paper compares two 3-D unsteady RANS simulations of a fan model obtained respectively for an isolated and an installed configuration. In the installed case the strut–bifurcation system creates a strong disturbance of the steady flowfield. This involves a deformation of the rotor wakes during their convection, so that their impingement on the stator generates pressure fluctuations acting as sound sources with strong scattered azimuthal mode orders. The scattering is found by analyzing the acoustic field upstream and downstream of the fan stage. The present results are obtained while the rotor is supersonic (Mtip,rel ≈ 1.24). Since all the blades are identical, no frequency scattering (buzz-saw) does happen in the simulations.Copyright

Advanced Numerical Methods for the Prediction of Tonal Noise in Turbomachinery: Part II—Time-Linearized Methods

This is the second part of a series of two papers on unsteady 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 transfered 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 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 progagation. 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.Copyright

Advanced Numerical Methods for the Prediction of Tonal Noise in Turbomachinery: Part I—Implicit Runge Kutta Schemes

This is the first part of a series of two papers on unsteady CFD methods for the numerical simulation of aerodynamic noise generation and propagation. In this part, the stability, accuracy and efficiency of implicit Runge-Kutta schemes for the temporal integration of the compressible Navier-Stokes equations are investigated in the context of a CFD code for turbomachinery applications. Using two model academic problems, the properties of two Explicit first stage, Singly Diagonal Implicit Runge-Kutta (ESDIRK) schemes of second- and third-order accuracy are quantified and compared with more conventional second-order multi-step methods. Finally, to assess the ESDIRK schemes in the context of an industrially relevant configuration, the schemes are applied to predict the tonal noise generation and transmission in a modern high bypass ratio fan stage and comparisons with the corresponding experimental data are provided.Copyright