- Neubiberg, Bayern, Germany
Steffen Marburg
Universität der Bundeswehr München, Aerospace Sciences, Department Member
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Highly accurate predictions from large-scale numerical simulations are associated with increased computational resources and time expense. Consequently, the data generation process can only be performed for a small sample size, limiting a... more
Highly accurate predictions from large-scale numerical simulations are associated with increased computational resources and time expense. Consequently, the data generation process can only be performed for a small sample size, limiting a detailed investigation of the underlying system. The concept of multi-fidelity modeling allows the combination of data from different models of varying costs and complexities. This study introduces a multi-fidelity model for the acoustic design of a vehicle cabin. Therefore, two models with different fidelity levels are used to solve the Helmholtz equation at specified frequencies with the boundary element method. Gaussian processes (GPs) are trained on each fidelity level with the simulation results to predict the unknown system response. In this way, the multi-fidelity model enables an efficient approximation of the frequency sweep for acoustics in the frequency domain. Additionally, the proposed method inherently considers uncertainties due to the data generation process. To demonstrate the effectiveness of our framework, the multifrequency solution is validated with the high-fidelity (HF) solution at each frequency. The results show that the frequency sweep is efficiently approximated by using only a limited number of HF simulations. Thus, these findings indicate that multi-fidelity GPs can be adopted for fast and, simultaneously, accurate predictions.
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Contactless transportation systems based on near-field acoustic levitation have the benefit of compact design and easy control which are able to meet the cleanliness and precision demands required in precision manufacturing. However, the... more
Contactless transportation systems based on near-field acoustic levitation have the benefit of compact design and easy control which are able to meet the cleanliness and precision demands required in precision manufacturing. However, the problems involved in contactless positioning and transporting cylindrical objects have not yet been addressed. This paper introduces a contactless transportation system for cylindrical objects based on grooved radiators. A groove on the concave surface of the radiator produces an asymmetrical pressure distribution which results in a thrusting force to drive the levitator horizontal movement. The pressure distribution between the levitator and the radiator is acquired by solving the Reynolds equation. The levitation and the thrusting forces are obtained by integrating the pressure and the pressure gradient over the concave surface, respectively. The predicted results of the levitation force agree well with experimental observations from the literature. Parameter studies show that the thrusting force increases and converges to a stable value as the groove depth increases. An optimal value for the groove arc length is found to maximize the thrusting force, and the thrusting force increases as the groove width, the radiator vibration amplitude, and the levitator weight increase.
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Uncertainties during the manufacturing process or due to unexpected load variations in the course of operation can cause damages or faults in structures. These deviations from the original or intended properties can impact the durability... more
Uncertainties during the manufacturing process or due to unexpected load variations in the course of operation can cause damages or faults in structures. These deviations from the original or intended properties can impact the durability and the application for the component. Destructive testing methods are often impractical since the investigated component becomes unfeasible. Instead, a non-destructive testing method, which is based on the change of the dynamic behaviour caused by the faults in the structures, allows examining the produced or built-in component. Experimental investigation of the changes can be employed to detect and quantify the damages to some extent, if the relationship between the properties and the damage is not known. The non-destructive vibrational testing methods, in particular, are based on the relation between the material properties of a structure and its measured vibrational behaviour, e.g. natural frequencies and mode shapes.
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Modal decomposition is used to compute the modal contributions to the sound power radiated from externally excited structures submerged in a heavy fluid. The exterior structural-acoustic problem is discretized using finite elements for... more
Modal decomposition is used to compute the modal contributions to the sound power radiated from externally excited structures submerged in a heavy fluid. The exterior structural-acoustic problem is discretized using finite elements for the structure and boundary elements for the fluid. The structure and fluid domain models are fully coupled. The numerical technique presented here allows the individual contributions of the wet structural modes to the radiated sound power to be observed, thus providing physical insight into the vibro-acoustic problem. Two fluid-loaded structures are examined corresponding to a spherical shell and a cylindrical shell with hemi-spherical end caps. The sphere is excited by a point force and the cylinder is excited by a ring of axial or radial forces acting at one end. In each case, the individual modal contributions to the radiated sound power are presented. The individual modal contributions to the directivity of the radiated sound pressure for the cylindrical shell is also o...
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Squeak and rattle noise in a vehicle’s interior is perceived as an annoying sound by customers. Since persistent noise (e.g. engine, wind or drive train noise) has been reduced continuously during the last decades, the elimination of... more
Squeak and rattle noise in a vehicle’s interior is perceived as an annoying sound by customers. Since persistent noise (e.g. engine, wind or drive train noise) has been reduced continuously during the last decades, the elimination of sounds, which have their origin in the vehicle’s interior components, is getting more important. Therefore, noise prediction based on simulation models is useful, since design changes can be realized at lower costs in early virtual development phases. For this task, linear simulation methods are state of the art for the identification of noise risk, but in general without knowing if a sound is audible or not. First approaches have been developed based on the Harmonic Balance Method to predict squeak noise and assess their audibility. This paper presents vibroacoustic measurements at a door trim panel for squeaking and non-squeaking configurations. Vibrations are excited harmonically by a force controlled low noise shaker. The system response is measured in a semi-anechoic chamber by acceleration sensors and audibility is assessed. Additionally, a 3D finite element model is built and the Harmonic Balance Method using a dry friction law is applied to predict the acoustic behavior. Finally, the simulation results are compared to the measurements. A good agreement between simulation and experiment can be observed.
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Abstract Owing to their distinct non-contact and oil-free characteristics, squeeze film air bearings have been introduced to satisfy ultra-precision, low wear, and ultra-clean requirements. This paper proposes an analytical model of a... more
Abstract Owing to their distinct non-contact and oil-free characteristics, squeeze film air bearings have been introduced to satisfy ultra-precision, low wear, and ultra-clean requirements. This paper proposes an analytical model of a three-pad squeeze film bearing to study its static and dynamic performance. The bearing force is calculated by integrating the pressure distribution over the bearing surface, which is governed by the Reynolds equation. The stable equilibrium position of the rotor is obtained by the Newton-Raphson method. A numerical method to acquire the bearing dynamic coefficients is originally proposed by considering the vibration of its pad. These dynamic coefficients are determined by solving the perturbation equations derived from the combination of the Reynolds equation and the modified film thickness and pressure. The predicted static and dynamic results show good agreement with experimental results. The parameter study shows that the variation in eccentricity with respect to the rotational speed can be controlled by reasonably adjusting the vibration amplitude or the nominal clearance of the bearing. In addition, the results indicate that the direct stiffness and damping coefficients are increased by decreasing the rotation speed or increasing the vibration amplitude of the bearing pad.
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Squeak noise is a phenomenon which is often categorized as BSR, i.e. buzz, squeak and rattle. It arises from stick-slip movement between parts in contact. In a vehicle cabin, this noise is undesirable and has to be prevented. Simulation... more
Squeak noise is a phenomenon which is often categorized as BSR, i.e. buzz, squeak and rattle. It arises from stick-slip movement between parts in contact. In a vehicle cabin, this noise is undesirable and has to be prevented. Simulation methods are used to find critical contact conditions in the construction at an early stage of development. For accurate predictions, these methods have to take nonlinear behavior into account. In this paper, the Harmonic Balance Method is applied to a three-dimensional finite element model in order to calculate vibrations of systems with dry friction contacts. The system’s sensitivity to different contact formulations is investigated. For a reference, a test rig has been built. It consists of a beam with harmonic excitation. The beam is clamped at one end. At the other end it is in dry friction contact with a plate. Different excitation levels, frequencies, normal contact forces and material pairs are evaluated. The system’s response in time and frequency domain is compared to corresponding simulation results. The contact definition is updated to achieve accuracy in calculated dynamic responses. Results of calculation can be used to estimate the radiated noise, e.g. by determining the Equivalent Radiated Power.
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Composite materials offer a high freedom of design with regard to stiffness, strength and damping. In contrast to efficient anisotropic but linear material models, these composites often tend to react nonlinearly. Commonly, such nonlinear... more
Composite materials offer a high freedom of design with regard to stiffness, strength and damping. In contrast to efficient anisotropic but linear material models, these composites often tend to react nonlinearly. Commonly, such nonlinear material damping models imply frequency and temperature dependency. In addition, some materials show a substantial amplitude sensitivity of the damping. Within this study, metal–plastic composites with highly dissipating shear sensitive cores have been used to experimentally determine the damping values with varying amplitudes. The results show a significance of this parameter already for small deflection within the geometrically linear range. The derived nonlinearity is further described by an exponential approach and parametrized by a regression analysis. Furthermore, the amplitude sensitivity is retraced to the contributions of the layered material by a detailed numerical analysis of the stress states. Therefrom, the mean strain energy density per material is derived as an amplitude criterion for the nonlinear damping model. The resulting model can be further applied to the finite element analysis to improve the determination of vibrations as well as structure borne sound of such acoustically improved materials.
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A particle accelerated computational fluid dynamics/boundary element method technique to predict the sound pressure field produced by low Mach number flow past a rigid body is presented. An incompressible computational fluid dynamics... more
A particle accelerated computational fluid dynamics/boundary element method technique to predict the sound pressure field produced by low Mach number flow past a rigid body is presented. An incompressible computational fluid dynamics solver is used to calculate the transient hydrodynamic flowfield. A near-field formulation based on Lighthill’s analogy is coupled with a particle condensation technique to predict the incident acoustic field and its normal derivative on the body. The near-field formulation involves singular surface and volume integrals, which are regularized via singularity subtraction. A particle condensation technique is applied to accelerate the incident field computations and reduce the amount of data that must be stored during the computational fluid dynamics analysis. The incident field is then combined with a boundary element method model of the body, and the scattered sound pressure field is obtained by solving the Burton–Miller boundary integral equations. The accuracy and computati...
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In this paper, an algorithm is derived to solve a problem of inverse acoustics. It considers the damped acoustic boundary value problem, i.e. the Helmholtz equation and admittance boundary condition, in order to approximate the boundary... more
In this paper, an algorithm is derived to solve a problem of inverse acoustics. It considers the damped acoustic boundary value problem, i.e. the Helmholtz equation and admittance boundary condition, in order to approximate the boundary admittance of interior domains. The algorithm is implemented by using a finite element method and tested for two-dimensional cavities with arbitrary shapes. The admittance condition is reconstructed based on sound pressure measurements. The solution of the arising nonlinear system of equations is obtained by applying the Newton method following a presetting method for finding reasonable initial boundary admittance values. A residual norm accounts for the objective function. Its first- and second-order sensitivities are determined analytically by using a modal decomposition in order to avoid direct inversion of the system matrix. The experiment is simulated by taking sound pressure data of the forward solution as inputs for the inverse problem. Test examples show that very few measurement points are necessary to reproduce piecewise constant boundary admittance values very accurately. Then, the admittance boundary condition is applied to reproduce the sound pressure distribution in the cavity. Again, it becomes obvious that only a few measurement points are required to reconstruct the sound pressure field.
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In this paper, the application of generalized polynomial chaos expansion in stochastic structural modal analysis including uncertain parameters is investigated. We review the theory of polynomial chaos and relating error analysis. A... more
In this paper, the application of generalized polynomial chaos expansion in stochastic structural modal analysis including uncertain parameters is investigated. We review the theory of polynomial chaos and relating error analysis. A general formulation for the representation of modal problems by the polynomial chaos expansion is derived. It shows how the modal frequencies and modal shapes are influenced by the parameter uncertainties. The key issues that arise in the polynomial chaos simulation of modal analysis are discussed for two examples: a discrete 2-DOF system and continuous model of a microsensor. In both cases, the polynomial chaos expansion is used for the approximation of uncertain parameters, eigenfrequencies and eigenvectors. We emphasize the accuracy and time efficiency of the method in estimation of the stochastic modal responses in comparison with the sampling techniques, such as the Monte Carlo simulation.
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This paper presents a method to identify the surface areas of a vibrating structure that contribute to the radiated sound power. The surface contributions of the structure are based on the acoustic radiation modes and are computed for all... more
This paper presents a method to identify the surface areas of a vibrating structure that contribute to the radiated sound power. The surface contributions of the structure are based on the acoustic radiation modes and are computed for all boundaries of the acoustic domain. The surface contributions are compared to the acoustic intensity, which is a common measure for near-field acoustic energy. Sound intensity usually has positive and negative values that correspond to energy sources and sinks on the surface of the radiating structure. Sound from source and sink areas partially cancel each other and only a fraction of the near-field acoustic energy reaches the far-field. In contrast to the sound intensity, the surface contributions are always positive and no cancelation effects exist. The technique presented here provides a method to localize the relevant radiating surface areas on a vibrating structure. To illustrate the method, the radiated sound power from a baffled square plate is presented.