Jürgen Dierke

RANS/CAA based prediction of NACA 0012 broadband trailing edge noise and experimental validation

The prediction quality of a fast Computational Aeroacoustics (CAA) approach is studied for noise generated at a NACA 0012 trailing edge. Broadband spectra, the power law underlying their Mach number scaling, and the effect of Reynolds Number on the spectra are juxtaposed against published data from measurements and results from a semi-empirical prediction tool. The CAA method rests on the use of Reynolds Averaged Navier-Stokes (RANS) solutions to describe the turbulent flow problem around the airfoil. Acoustic Perturbation Equations (APE) are solved in the time domain, using a vortex source term, which is a function of turbulent field quantities. The acoustic approach was sccessfully validated in other works by utilizing turbulence data from Large Eddy Simulation (LES) to prescribe the unsteady sound sources. For the fast CAA approach applied in this work unsteady vortex sound sources are determined by a stochastic method, which generates 4D spatio-temporal synthetic turbulence that very accurately accompishes a local realization of all statistical and mean-flow features provided by steady RANS. Based on these prerequisites it becomes now feasible to make an assessment as to which acoustic accuracy can be achieved with such a hybrid RANS / CAA prediction method, bearing in mind the approximative nature and limited turbulence resolution of RANS:

Trailing-Edge Noise Data Quality Assessment for CAA Validation

The paper provides a detailed reexamination of previously published trailing-edge (TE) noise experimental data as acquired in DLRs low-noise open-jet Acoustic Wind Tunnel Braunschweig (AWB). The objective is to set up a parametric reference data base to be later used for CAA validation purposes. A modular plate airfoil with variable chord length was used to investigate Reynolds number effects on both near field quantities and farfield sound. The original data base, including corrected farfield TE noise spectra as well as turbulent boundary-layer (TBL) characteristics, has been extended by unsteady surface pressure measurements in the TE region. Limitations and weaknesses of the applied measurement techniques are pinpointed by comparisons with independent data sets using alternative measurement approaches. Comparisons include also available semi-empirical prediction models for surface pressure spectra as well as for farfield TE sound emission. First RANS-based CAA prediction results using stochastic source models show essential agreement with the measurement data.

CAA-RPM prediction and validation of slat setting influence on broadband high-lift noise generation

Broadband sound generated at the slat of a three-element high-lift wing is simulated with a CAA method. Especially, the effect of different slat and gap settings is studied and the results are validated with measurements. The applied method rests on the use of steady Reynolds Averaged Navier-Stokes (RANS) simulation to prescribe the time-averaged motion of turbulent flow. By means of synthetic turbulence generated with the Random Particle-Mesh (RPM) method the steady one-point statistics (e.g. turbulent kinetic energy) and turbulent length- and time-scales of RANS are translated into fluctuations with statistics that very accurately reproduce the spatial target distributions of RANS. The synthetic fluctuations are used to prescribe sound sources which drive linear acoustic perturbation equations. The whole approach represents a methodology to solve statistical noise theory with state-of-the-art Computational Aeroacoustics (CAA) tools in the timedomain. The comparison with experiments are conducted for four selected settings, chosen from a matrix comprising in total 43 individual slat-gap and overlap combinations. CAA simulations are performed for all matrix positions. The CAA computations are obtained as blind predictions prior to the measurements conducted in the AWB wind tunnel. Good agreement of the noise trends are found between CAA and experiments. The difference in level between the selected configurations is obtained qualitatively and quantitatively by CAA.

Generic Airfoil Trailing-Edge Noise Prediction using Stochastic Sound Sources from Synthetic Turbulence

A Computational Aeroacoustic (CAA) study is conducted for the simulation of broad- band trailing-edge noise radiated from a NACA0012 airfoil and a cambered non-symmetrical DU96 airfoil for different ow conditions. The chosen simulation approach uses a Reynolds Averaged Navier-Stokes (RANS) solution to describe the turbulence around the airfoil. With a stochastic method time resolved synthetic turbulence uctuations are derived which then describe the unsteady sound sources used for acoustic perturbation equations (APE). The simulated broadband noise spectra are evaluated for different sets of input parameters to determine the sensitivity of the used method towards these parameters. Calculated values are compared to available measurement data to validate the underlying assumptions and to give advices on the best practice settings. It is essential for any proper realization of sound sources from a stochastic method that the RANS turbulence statistics are accurately realized. A length scale smoothing procedure is presented which remedies the problem of spurious peaks in the realized turbulence statistics. A study is pursued regarding the tur- bulent energy spectra realized by the stochastic turbulence reconstruction. Two different spectra (Gauss and Liepmann) are analyzed. A positive effect of higher energy levels of the Liepmann spectrum in certain frequency ranges on the simulated sound field can be shown.

Overset DNS with Application to Homogeneous Decaying Turbulence

In this contribution an application of a computational aeroacoustics code as a hybrid Zonal DNS tool is presented. The derivation of the Non-Linear Perturbation Equations (NLPE) extended with viscous terms is shown as well as information related to the numerical method is given. The application of the simulation tool to a generic three-dimensional test case, i.e., the Taylor-Green Vortex (TGV), is presented. This TGV, initially consists of only one length scale, develops into homogeneous decaying turbulence. Results of energy spectrum and grid convergence study are given. It is shown that the observed accuracy of the numerical code matches well with the expected theoretical order of four.

Overset DNS with Application to Sound Source Prediction

In this contribution, we present an application of a computational aeroacoustics code as a hybrid Zonal DNS tool. The extension of the Non-Linear Perturbation Equations (NLPE) with viscous terms is presented as well as information related to the numerical method. The applicability of the simulation tool is illustrated with two testcases, i.e., a 2D circular cylinder in a uniform flow at moderate Reynolds numbers and a 3D decaying flow initialised with Taylor-Green vortices. Both testcases provide results which match well with data reported in literature. The cylinder testcase verifies that the viscous terms are indeed correctly implemented (at least in 2D) and the Taylor-Green vortex case illustrates that the numerical scheme introduced minimal numerical dissipation.

Direct Combustion Noise Simulation of a Lean Premixed Swirl Flame using Stochastic Sound Sources

A lean, swirl-stabilized gas turbine model combustor is simulated with a stochastic approach for combustion noise prediction. The employed hybrid and particle based method, FRPM-CN (Fast Random Particle Method for Combustion Noise Prediction) reconstructs temperature variance based direct combustion noise sources from local CFD-RANS turbulence and flow field statistics. Those monopole sound sources are used as right hand side forcing of the Linearized Euler Equations. First, findings from steady state CFD simulations are validated with experimental results. It is shown that the employed RANS models accurately reproduce the experimental flow field and combustion. Turbulence is treated with a two equation model and a global reaction mechanism is utilized for combustion. Subsequently, the specifications of the CCA (Computational Combustion Acoustics) setup is introduced and selected pressure spectra of the acoustics simulations are compared to experimental results, showing that FRPM-CN is able to deliver absolute combustion noise levels for the investigated burner at low computational costs.

Towards CAA based Acoustic Wind Tunnel Corrections for Realistic Shear Layers

The free shear layers induced by the nozzle in aeroacoustic open jet wind tunnels perform a significant influence on sound wave propagation, causing direction alteration and amplitude reduction (due to velocity gradient and turbulence), as well as spectral redistribution (due to turbulence). The effects need to be seriously considered in measurement data correction, in order to eliminate the misunderstanding or incorrect conclusions drawn from the experiments conducted in such a wind tunnel. A commonly used correction method derived by Amiet for refraction effect has been utilized for many years, since it provides a simplified but practical approach for shear layer correction. However, the zero-thickness shear layer assumption taken by Amiet may cause deviation in application while the real shear layers are finite in thickness and spread as the flow goes downstream. At the same time, the aerodynamic model located in the test section could also affect the shear layer shape. Hence, it will be helpful if the various situations are taken into account and systematically evaluated for the correction method. In this paper an investigation is carried out using Computational Aeroacoustics (CAA) methods to simulate sound propagation through free shear layers representative for the jets of acoustic wind tunnels of rectangular cross section. The analysis is carried out for 1kHz and 10kHz while considering the shear layer thickness, flow curvature due to test model lift and reflection from the nozzle surface. The results are compared with the theoretical correction based on Amiet’s approach. While the effects of shear layer thickness for plane shear layers even with streamwise spreading seems comparatively small, the curvature effects have a larger impact. The presence of the actual nozzle rim also has a considerable influence on the overall sound field. Moreover, 3D simulations of the sound propagation through the shear layers of DLR’s acoustic wind tunnel AWB with rectangular jet cross section show significant effect around the intersection corner of two shear layers.

Efficient Combustion Noise Simulation of a Gas Turbine Model Combustor Based on Stochastic Sound Sources

A gas turbine model combustor is simulated with a hybrid, stochastic and particle-based method for combustion noise prediction with full 3D sound source modeling and sound propagation. Alongside, an incompressible LES simulation of the burner is considered for the investigation of the performance of the hybrid approach. The highly efficient time-domain method consists of a stochastic sound source reconstruction algorithm, the Fast Random Particle Method (FRPM) and sound wave propagation via Linearized Euler Equations (LEEs). In the context of this work, the method is adapted and tested for Combustion Noise (CN) prediction. Monopole sound sources are reconstructed by using an estimation of turbulence statistics from reacting CFD-RANS simulations. First, steady state and unsteady CFD calculations of flow field and combustion of the model combustor are evaluated and compared to experimental results. Two equation modeling for turbulence and the EDM (Eddy Dissipation Model) with FRC (Finite Rate Chemistry) for combustion are employed. In a second step, the acoustics simulation setup for the model combustor is introduced. Selected results are presented and FRPM-CN pressure spectra are compared to experimental levels.Copyright

Broadband Combustion Noise Prediction With the Fast Random Particle Method

A new highly efficient, hybrid CFD/CAA approach for broadband combustion noise modeling is introduced. The inherent sound source generation mechanism is based on turbulent flow field statistics, which are determined from reacting RANS calculations. The generated sources form the right-hand side of the linearized Euler equations for the calculation of sound fields. The stochastic time-domain source reconstruction algorithm is briefly described with emphasis on two different ways of spatial discretization, RPM (Random Particle Method) and the newly developed FRPM (Fast RPM). The application of mainly the latter technique to combustion noise (CN) prediction and several methodical progressions are presented in the paper.(F)RPM-CN is verified in terms of its ability to accurately reproduce prescribed turbulence-induced one- and two-point statistics for a generic test and the DLR-A jet flame validation case. Former works on RPM-CN have been revised and as a consequence methodical improvements are introduced along with the progression to FRPM-CN: A canonical CAA setup for the applications DLR-A, -B and H3 flame is used. Furthermore, a second order Langevin decorrelation model is introduced for FRPM-CN, to avoid spurious high frequency noise. A new calibration parameter set for reacting jet noise prediction with (F)RPM-CN is proposed. The analysis shows the universality of the data set for 2D jet flame applications and furthermore the method’s accountance for Reynolds scalability. In this context, a Mach number scaling law is used to conserve Strouhal similarity of the jet flame spectra. Finally, the numerical results are compared to suitable similarity spectra.Copyright