Oliver Kornow

Spectral broadening of jet engine turbine tones

The process of turbulent scattering is studied for the generic experiment conducted by Candel et al.1 applying an analytic weak scattering model and CAA computations. For the analytic weak scattering model an approximate form of the Lilley equation is used. The source terms of this equation are in terms of the turbulence and the incident acoustic field. In the CAA simulations the wave equation proposed by Pierce for sound in fluids with unsteady inhomogeneous flow is integrated. The unsteady turbulent base-flow is modeled using a stochastic method to generate turbulence with locally varying turbulence features as provided by time-averaged RANS. To study the spectral broadening effect analytically and computationally, the experimental set-up of Candel is considered, which involves an omnidirectional sound source, located on the axis of a round jet. The analytical predictions show very good agreement with the general trends as measured by Candel for an observer position normal to the jet axis. The computations reveal a spectral shape, which is in good agreement with those found in the experiments.

A CAA Based Approach to Tone Haystacking

The far-field noise spectra of jet engines show for certain jet configurations and turbine tones a characteristic spectral broadening effect, causing a reduction of tone peaks in favor of a more distributed spectral hump around each tone frequency. This haystacking effect likely occurs due to the interaction of the turbine tones with the unsteady turbulent jet shear layer. A better understanding of this effect may help to utilize it for noise reduction purposes. Furthermore, the effect is of interest for the measurement of tone sources in an open acoustic wind tunnel test section, since the tone will be scattered in the open jet shear layer. A correction for this measured broadening effect is desirable. A non-empirical computational approach to predict tone haystacking as a function of Reynolds number/jet shear layer characteristics is currently missing. This paper reports about ongoing work to utilize Computational Aeroacoustics (CAA) methods for the prediction of haystacking. In a first step CAA techniques are applied to simulate the propagation of tones through the time averaged steady exhaust of a jet engine. To simulate the haystacking effect with CAA, the unsteady turbulent base-flow is modeled with a 4D synthetic turbulence method. The employed RPM approach generates turbulence with all local statistical features as predicted by time-averaged RANS. To study the spectral broadening effect computationally, the experimental set-up of Candel is considered first, which involves an omnidirectional sound source located on the axis of a round jet. The analytical predictions show very good agreement with the general trends as measured by Candel for an observer position normal to the jet axis. The computations reveal a spectral shape, which is in good agreement with those found in the experiments. In a next step the methodology is combined with the exhaust problem to simulate sound propagation through the unsteady turbulent exhaust.

Acoustic Mode Decomposition of Compressor Noise under Consideration of Radial Flow Profiles

§To explore the sound generating mechanisms of turbomachinery like turbines or compressors in rig tests, the sound field radiated in the inlet or outlet duct is acquired by means of a sensor array and fitted to a theoretical model of the modal sound field. For this purpose, an analytical model with a uniform radial flow profile is usually deployed. This paper presents the derivation of an alternative sound field model under consideration of a realistic radial flow profile. The new model, which has to be solved numerically, was fitted against experimental data acquired in the inlet of a three-stage low pressure compressor, yielding the amplitudes of all radiated modes. The results are compared with the outcome of the classical radial mode decomposition approach basing on the analytical model.

Evaluation of the RPM Approach for the Simulation of Broadband Combustion Noise

The derivation and validation of a broadband combustion noise model is presented. The random particle-mesh approach for combustion noise is a hybrid computational fluid dynamics/computational aeroacoustics method and reliesonthestochastic reconstructionofcombustionnoisesources inthe timedomain.Thestochastic reconstruction of unsteady sound sources based on statistical turbulence quantities from a reacting Reynolds-averaged Navier– Stokes simulation is realized with the random particle-mesh method. In the present paper, the modeled combustion noise sources are derived for the use in conjunction with the linearized Euler equations for the computation of the acoustic propagation. Two open, nonpremixed, turbulent jet flames (DLR-A and DLR-B), which differ in their fuel outletvelocityandtheirrespectiveReynoldsnumber,areusedforthevalidationoftheparticle-meshforcombustion noise approach. Results of the reacting flow computations and the subsequent acoustic simulations are compared with measurements. Excellent agreement is found between the computed narrow band sound spectra and the experimental data.

Broadband combustion noise simulation of open non-premixed turbulent jet flames

Numerical broadband combustion noise simulations of open non-premixed turbulent jet flames applying the Random Particle-Mesh for Combustion Noise (RPM-CN) approach are presented. The RPM-CN approach is a hybrid Computational Fluid Dynamics/Computational Aeroacoustics (CFD/CAA) method for the numerical simulation of turbulent combustion noise, based on a stochastic source reconstruction in the time domain. The combustion noise sources are modeled on the basis of statistical turbulence quantities, for example achieved by a Reynolds averaged Navier-Stokes (RANS) simulation, using the Random Particle-Mesh (RPM) method. RPM generates a statistically stationary fluctuating sound source that satisfies prescribed one- and two-point statistics which implicitly specify the acoustic spectrum. Subsequently, the propagation of the combustion noise is computed by the numerical solution of the Linearized Euler Equations (LEE). The numerical approach is applied to the DLR-A, the DLR-B and the H3 flames. The open non-premixed turbulent jet flames differ in the mean jet exit velocity, therefore in their respective Reynolds number, and in the fuel composition. Computed radial profiles of the reacting flow field are compared to experimental data and discussed. Computed sound pressure level spectra of the DLR-A and DLR-B flames and acoustic intensity level spectra of the H3 flame at different microphone locations are presented and compared to measurements.

Evaluation of the RPM-CN approach for broadband combustion noise prediction

The derivation and validation of a broadband combustion noise model is presented. The applied RPM-CN approach, a hybrid CFD/CAA approach relies on the stochastic reconstruction of combustion noise sources in the time domain. The stochastic reconstruction is conducted by the RPM method out of statistical turbulence quantities which can be delivered by a reacting RANS simulation. In the present work, the modeled combustion noise sources are derived for the use in conjunction with the LEE for the computation of the acoustic propagation. The DLR-A and the DLR-B ames, both non-premixed open jet ames which dierentiate in the fuel outlet velocity and the respective Reynolds number, are used for the validation of the RPM-CN approach. Results of the reacting ow computations and the subsequent acoustic predictions are compared to measurements and discussed. The reliability and accuracy of the RPM-CN approach will be demonstrated by a good agreement of the computed sound pressure level spectra with the experimental data.

Numerical Simulation of Broadband Combustion Noise With the RPM-CN Approach

A new numerical approach called RPM-CN approach is applied to predict broadband combustion noise. This highly efficient hybrid CFD/CAA approach can rely on a reactive RANS simulation. The RPM method is used to reconstruct stochastic broadband combustion noise sources in the time domain based on statistical turbulence quantities. Subsequently, the propagation of the combustion noise is computed by solving the acoustic perturbation equations (APE-4). The accuracy of the RPM-CN approach will be demonstrated by a good agreement of the simulation results with acoustic measurements of the DLR-A flame. The high efficiency and therefore low computational costs enable the usage of this numerical approach in the design process.Copyright

Numerical RANS/URANS simulation of combustion noise

In the present work, numerical simulation tools for two different combustion noise source mechanisms are presented. The generation and propagation of entropy noise is computed directly using a compressible CFD approach in combination with appropriate acoustic boundary conditions. The EntropyWave Generator (EWG) experiment is taken for validation of the proposed approach and for evaluating the acoustic sources of entropy noise. Simulation results of pressure fluctuations and their spectra for a defined standard test configuration as well as for different operating points of the EWG agree very well with the respective experimental data. Furthermore, a new numerical approach called RPM-CN approach was developed to predict broadband combustion noise. This highly efficient hybrid CFD/CAA approach can rely on a reactive RANS simulation. The RPM method is used to reconstruct stochastic broadband combustion noise sources in the time domain based on statistical turbulence quantities. Subsequently, the propagation of the combustion noise is computed by solving the acoustic perturbation equations (APE-4). The accuracy of the RPM-CN approach will be demonstrated by a good agreement of the simulation results with acoustic measurements of the DLR-A flame. The high efficiency and therefore low computational costs enable the usage of this numerical approach in the design process.

Numerical Simulation of Combustion Induced Noise using LES and Computational Aeroacoustics

Many technical combustion devices are susceptible to thermoacoustic instabilities. In this work, the noise emission by a turbulent jet flame is analyzed by means of a hybrid LES/CAA (Large Eddy Simulation/Computational Aero Acoustics) approach as a first step towards a numerical investigation of combustion instability. The hybrid LES/CAA approach is based on a LES of the reactive flow utilizing a low Mach number formulation. Within the CAA part of the simulations, linearized Euler equations (LEE) are solved. A simplified formulation to describe the thermoacoustic sound sources is extracted from the reactive LES. For the present study, the CFD code FASTEST is coupled with the aeroacoustic simulation tool PIANO. The two solvers are combined to a single tool for the description of the acoustics of reacting flows. Both codes make use of geometry flexible grids enabling the simulation of complex geometries commonly used within technical combustion systems.Copyright

Broadband Simulation Of Flow-Induced Noise GenerationOn Orifice Plates In Air Conditioning Ducts

In this study the acoustic sources generated by turbulent flow inside internal ducts are studied experimentally and numerically. The noise is generated due to the flow passing orifices as typical components used in the aircraft climate control system. The noise generation was simulated using a two step approach. In the first step the steady incompressible flow problem is simulated using the RANS approach. In the second step the acoustic field is computed using acoustic perturbation equations. The turbulent flow fluctuations are induced via a broadband turbulence model whereby the turbulent vorticities are interpreted as sources producing broadband sound by interaction with the orifices.