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Stefan Schoder

Publications and source records attributed to Stefan Schoder.

26 records · Page 2Linked to original sources

Helmholtz's decomposition for Aeroacoustics using a standard Flow Solver

This paper is a short guideline to the decomposition of a compressible velocity into vortical and compressible structures using standard flow solvers. In particular, this is a fast solution to get an idea of the compressible fields inside your simulation, respectively acoustics for low Mach number isothermal flows. The details of the implementation are presented and the algorithm is applied to an overflown cavity with a lip. The results of the flow solver show clearly visible to acoustic radiation for the application. Finally and as previous studies showed, the use of the scalar potential formulation of Helmholtz's decomposition is valid for convex propagation domains.

physics.flu-dyn↗

Offline coupling of segregated multi-physical simulations with consistent boundary conditions and source terms based on scattered data

This article presents the openCFS submodule scattered data reader for coupling multi-physical simulations performed in different simulation programs. For instance, by considering a forward-coupling of a surface vibration simulation (mechanical system) to an acoustic propagation simulation using time-dependent acoustic absorbing material as a noise mitigation measure. The nearest-neighbor search of the target and source points from the interpolation is performed using the FLANN or the CGAL library. In doing so, the coupled field (e.g., surface velocity) is interpolated from a source representation consisting of field values physically stored and organized in a file directory to a target representation being the quadrature points in the case of the finite element method. A test case of the functionality is presented in the "testsuite" module of the openCFS software called "Abc2dcsvt". This scattered data reader module was successfully applied in numerous studies on flow-induced sound generation. Within this short article, the functionality, and usability of this module are described.

cs.CE↗

Implementation of an aeroacoustic simulation pipeline using openCFS-Acoustics and openCFS-Data applied to human phonation

The human phonation process be modeled using the Finite Element Method (FEM) which provides a detailed representation of the voice production process. A software implementation in C++ using FEM (openCFS) has been used to simulate the phonation process. The FEM model consists of a 3D mesh of the upper human airways. The simVoice model provides an accurate representation of the phonation process and was valid in several publications. In this article, we show how to set up the model using openCFS and openCFS-Data.

cs.SD↗

A Validated Finite Element Model for Room Acoustic Treatments with Edge Absorbers

Porous acoustic absorbers have excellent properties in the low-frequency range when positioned in room edges, therefore they are a common method for reducing low-frequency reverberation. However, standard room acoustic simulation methods such as ray tracing and mirror sources are invalid for low frequencies in general which is a consequence of using geometrical methods, yielding a lack of simulation tools for these so-called edge absorbers. In this article, a validated finite element simulation model is presented, which is able to predict the effect of an edge absorber on the acoustic field. With this model, the interaction mechanisms between room and absorber can be studied by high-resolved acoustic field visualizations in both room and absorber. The finite element model is validated against transfer function data computed from impulse response measurements in a reverberation chamber in style of ISO 354. The absorber made of Basotect is modeled using the Johnson-Champoux-Allard-Lafarge model, which is fitted to impedance tube measurements using the four-microphone transfer matrix method. It is shown that the finite element simulation model is able to predict the influence of different edge absorber configurations on the measured transfer functions to a high degree of accuracy. The evaluated third-octave band error exhibits deviations of 3.25dB to 4.11dB computed from third-octave band averaged spectra.

physics.class-ph↗

Anisotropic Minimum Dissipation Subgrid-Scale Model in Hybrid Aeroacoustic simulations of Human Phonation

This article deals with large-eddy simulations of 3D incompressible laryngeal flow followed by acoustic simulations of human phonation of five cardinal english vowels /u, i, \textipa{A}, o, æ/. The flow and aeroacoustic simulations were performed in OpenFOAM and in-house code openCFS, respectively. Given the large variety of scales in the flow and acoustics, the simulation is separated into two steps: (1) computing the flow in the larynx using the finite volume method on a fine 2.2M grid followed by (2) computing the sound sources separately and wave propagation to the radiation zone around the mouth using the finite element method on a coarse 33k acoustic grid. The numerical results showed that the anisotropic minimum dissipation model, which is not well known since it is not available in common CFD software, predicted stronger sound pressure levels at higher harmonics and especially at first two formants than the wall-adapting local eddy-viscosity model. We implemented the model as a new open library in OpenFOAM and deployed the model on turbulent flow in the larynx with positive impact on the quality of simulated vowels. Numerical simulations are in very good agreement with positions of formants from measurements.

physics.flu-dyn↗

Machine-learning applied to classify flow-induced sound parameters from simulated human voice

Disorders of voice production have severe effects on the quality of life of the affected individuals. A simulation approach is used to investigate the cause-effect chain in voice production showing typical characteristics of voice such as sub-glottal pressure and of functional voice disorders as glottal closure insufficiency and left-right asymmetry. Therewith, 24 different voice configurations are simulated in a parameter study using a previously published hybrid aeroacoustic simulation model. Based on these 24 simulation configurations, selected acoustic parameters (HNR, CPP, ...) at simulation evaluation points are correlated with these simulation configuration details to derive characteristic insight in the flow-induced sound generation of human phonation based on simulation results. Recently, several institutions studied experimental data, of flow and acoustic properties and correlated it with healthy and disordered voice signals. Upon this, the study is a next step towards a detailed dataset definition, the dataset is small, but the definition of relevant characteristics are precise based on the existing simulation methodology of simVoice. The small datasets are studied by correlation analysis, and a Support Vector Machine classifier with RBF kernel is used to classify the representations. With the use of Linear Discriminant Analysis the dimensions of the individual studies are visualized. This allows to draw correlations and determine the most important features evaluated from the acoustic signals in front of the mouth. The GC type can be best discriminated based on CPP and boxplot visualizations. Furthermore and using the LDA-dimensionality-reduced feature space, one can best classify subglottal pressure with 91.7\% accuracy, independent of healthy or disordered voice simulation parameters.

cs.SD↗

Aeroacoustic source term filtering based on Helmholtz decomposition

Hybrid aeroacoustic methods seek for computational efficiency and robust noise prediction. Using already existing aeroacoustic wave equations, we propose a general hybrid aeroacoustic method, based on compressible source data. The main differences to current state of the art aeroacoustic analogies are that an additional decomposition is used to compute the aeroacoustic source terms and their application is extended to the source formulation, based on compressible flow data. By applying the Helmholtz-Hodge decomposition on arbitrary domains, we extract the incompressible projection (non-radiating base flow) of a compressible flow simulation. This method maintains the favorable properties of the hybrid aeroacoustic method while incorporating compressible effects on the base flow. The capabilities are illustrated for the aeroacoustic benchmark case, "cavity with a lip", involving acoustic feedback. The investigation is based on the equation of vortex sound, to incorporate convective effects during wave propagation.

physics.flu-dyn↗

Numerical investigation of a deep cavity with an overhanging lip considering aeroacoustic feedback mechanism

In modern transport systems, passengers' comfort is greatly influenced by flow-induced noise. In this study we investigate a generic deep cavity with an overhanging lip, mimicking a door gap in a vehicle, that is overflowed by air at two different free stream velocities, 26.8m/s and 50m/s. The turbulent boundary layer and the acoustic waves interact with the cavity's geometry and form a strong feedback mechanism. In the present work, we focus on the details of the compressible turbulent flow structures and their variations concerning the velocity, the boundary layer as well as the domain dimensionality for a later acoustic simulation within a hybrid aeroacoustic workflow. Furthermore, we verify the feasibility of reducing the acoustic computational domain from 3D to 2D for this application by conducting a coherence study of acoustically active flow structures in the spanwise direction. The role of the three-dimensional Taylor-Görtler vortices from the recirculation regarding the vortex formation and the vortex-edge interaction was also evaluated. Remarkably for the lower approaching velocity (26.8m/s), we found a special vortex-edge interaction, namely an alternating sequence of complete clipping and a subsequent partial escape. Lastly, we assigned previous unknown peaks in the pressure spectrum to their corresponding mechanisms.

physics.flu-dyn↗