Jens Grabinger

Fluid-Structure-Acoustic Interaction of the Flow Past a Thin Flexible Structure

The acoustic field resulting from the interaction of a thin flexible structure with a turbulent flow was investigated both numerically and experimentally. Two different model configurations were considered: in one a flexible plate acted as a moving wall in a turbulent boundary layer, and in the other the flexible plate was located in the wake of a square cylinder. The fully coupled simulation of the fluid—structure—acoustics interaction was based on a partitioned approach employing two different simulation codes: a finite-volume flow solver of second-order accuracy in space and time and a finite-element structural-mechanics and acoustics solver. A code coupling interface was used for the exchange of data between the different discretizations. The experiments were performed in an acoustic wind tunnel employing microphone measurements of the sound pressure level. Detailed flow measurements were carried out using laser Doppler anemometry and three-component hot-wire anemometry. The flow-induced vibration of the flexible structure was measured with a laser-scanning vibrometer. Experimental and numerical results characterizing the flow field, the structural vibration, and the generated sound are presented.

Numerical Simulation of Fluid-Structure- and Fluid-Structure-Acoustic Interaction Based on a Partitioned Coupling Scheme

A coupling scheme for the simulation of problems involving fluid-structure and fluid-structure-acoustic interactions is described. For the simulation of fluid-structure interactions an implicit coupling between the flow field and the mechanical structure is applied. In order to determine the acoustic radiation due to mechanical vibrations, a direct coupling between the mechanical structure and the acoustic field is employed. At the structure-acoustic interface non-matching grids are realized with mortar elements. The exchange of physical quantities between the structured mesh of the flow computation and the unstructured mesh of the structural computation is performed by MpCCI. Applications to the flow around an oscillating rigid body and the flow over a flexible plate are presented.

Aeroacoustics of Darrieus wind turbine

The objective of this paper is to validate two different numerical methods for noise prediction of the H-Darrieus wind turbine using a complementary approach consisting of experimental measurements and numerical simulations. The acoustic measurements of a model scale rotor were performed in an anechoic wind tunnel. This data is the basis for the validation of the computational aeroacoustic simulations. Thereby, we have applied two different numerical schemes for noise prediction using hybrid methods. As usual in hybrid aeroacoustic approaches, flow field and acoustic calculations are carried out in separate software packages. For both schemes the time-dependent turbulent flow field is solved with Scale-Adaptive Simulation. The two schemes then differ in how the location of the acoustic sources and their propagation is calculated. In the first scheme the acoustic source terms are computed according to Lighthills acoustic analogy which gives source terms located on the original CFD grid. These source terms are projected onto a coarser acoustic grid on which Lighthills inhomogenous wave equation is solved by the Finite Element (FE) method. The second scheme uses the Ffowcs Williams-Hawkings (FW-H) method which is based on a free field Greens function. The scheme uses a porous integration surface and implements an advanced time formulation. Both methodologies are compared with experimental data.

Modeling and Finite Element Formulation for Acoustic Problems Including Rotating Domains

A finite element formulation for the efficient numerical simulation of sound in computational domains, including rotating regions, is presented. The mathematical description is based on an arbitrary Lagrangian–Eulerian framework and results in a convective wave equation for the scalar acoustic potential. Numerically, the capability of nonconforming grids is explored by applying a Nitsche-type mortaring between stationary and rotating regions. The formulation can be applied to classical acoustics (stagnant fluid) as well as moving fluids in the case of aeroacoustics. The validation with the analytical solution of a rotating point source in three dimensions demonstrates the accuracy and robustness of the developed numerical scheme. Additionally, the convergence study shows that the intersection mesh operations needed in the nonconforming setting do not deteriorate the accuracy of the numerical solution.

Quantitative reconstruction of ultrasound fields in optically transparent isotropic solids

This contribution reports on a method of utilizing light refractive tomography (LRT) to reconstruct ultrasound dilatation fields in optically transparent isotropic solids. Furthermore, it proposes the description of ultrasound in solids using a normal stress component, which shares the same unit with sound pressure and can be approximately derived from dilatation. After giving the theoretical support for this novel application, we reconstruct the distribution of normal stress σzz in a cross section within poly(methyl methacrylate) (PMMA). Because of the lack of techniques for verifying the measurement results directly, systematic errors of the LRT measurements in PMMA are checked by comparing the results from LRT and hydrophone measurements in water. The small differences relative to the maximum pressure (e.g., <;2% at the center) indicate the reliability of the corresponding LRT measurements in PMMA. With the aid of numerical simulations, the uncertainty of the reconstructed dilatation is determined to be about 5% near the rotation axis. We also succeed in reconstructing ultrasound fields throughout water and PMMA in time and in three spatial dimensions. The reconstructed fields match the corresponding numerical simulation quite well; e.g., the simulation predicts most of the phenomena emerging in the reconstructed images. The reliability of the measurement is then verified by calculating two quantities; each of them is computed with two different methods. Small differences between results are achieved in both cases (i.e., -5.1% and -1.3%).

Simulation and Experiments of the Fluid-Structure-Acoustic Interaction of a Flexible Structure in the Wake of a Square Cylinder

In the present paper the fluid-structure-acoustic interaction of a thin flexible structure in the wake of a wall-mounted square cylinder is investigated both numerically and experimentally. The simulation is based on a partitioned approach employing two different simulation codes: a finite-volume flow solver of second order accuracy in space and time and a finite-element structural mechanics and acoustics solver. A code coupling interface is used for the exchange of data between the different discretizations. The experiments are performed in an acoustics wind tunnel employing microphone measurements of the sound pressure level. Detailed flow measurements are carried out using laser-Doppler anemometry and 3D-hot-wire anemometry. The flow induced vibration of the flexible structure is measured with a laser-scanning vibrometer. Experimental and numerical results characterizing the flow field, the structural vibration and the generated sound are presented.

CAA of an Air-Cooling System for Electronic Devices

This paper presents the workflow and the results of fluid dynamics and aeroacoustic simulations for an air-cooling system as used in electronic devices. The setup represents a generic electronic device with several electronic assemblies with forced convection cooling by two axial fans. The aeroacoustic performance is computed using a hybrid method. In a first step, two unsteady CFD simulations using the Unsteady Reynolds-Averaged Navier-Stokes simulation with Shear Stress Transport (URANS-SST) turbulence model and the Scale Adaptive Simulation with Shear Stress Transport (SAS-SST) models were performed. Based on the unsteady flow results, the acoustic source terms were calculated using Lighthill’s acoustic analogy. Propagation of the flow-induced sound was computed using the Finite Element Method. Finally, the results of the acoustic simulation are compared with measurements and show good agreement.

Simulation of Broadband Noise Behind a Backward Facing Step

The internal flow and the sound field over a backward-facing step in the transitional regime (ReD = 6000) was predicted based on direct numerical simulations (DNS) for the flow field. The flow predictions were carried out with the help of a finite-volume NavierStokes solver equipped with a co-visualization facility which allows one to investigate the flow dynamics at high temporal resolution. The sound field was investigated with the Finite Element method using the acoustic analogy approach. To analyse the broadband character of the sound radiation a special absorbing layer (Perfect Matched Layer) technique was implemented in the time domain to achieve optimal free radiation conditions. The results show clearly the broadband characteristics of the radiated acoustic field.

Fluid‐structure interaction and computational aeroacoustics of the flow past a thin flexible structure

In many technical applications the interaction between a fluid flow and a thin flexible structure leads to the generation of acoustic noise which is caused by flow induced structural vibrations. Examples for such applications are coverings and panelings of cars and airplanes. In many cases the generated noise is unwanted so that noise reduction is a topic of major interest. In the present work we investigate the acoustic field resulting from the interaction of a thin flexible structure with a turbulent flow field by means of numerical simulation. Two different model configurations are considered: one is the flow over a flexible plate, in the second case the flexible plate is located in the wake of a square cylinder. The major aim of this work is to provide a better understanding of the noise generation processes in these flow cases. The numerical methodology applied is utilized for a decomposition of the acoustic field into one part generated by the structural vibrations and another part which is due to s...