Fraunhofer-Forschungsgruppe Hydroakustik FHAK

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Advanced views of cavitation noise

1992 , Baiter, H.-J.

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Sound radiation of a cantilever plate - comparisons of BEM results with experimental data

1991 , Estorff, O. von , Homm, A. , Bartels, F.

The sound radiation of a vibrating cantilever plate is investigated by a combined Finite Element / Boundary Element approach. First, in a FE-analysis the mode shapes of the plate are determined. Second, for three selected frequencies the acoustic pressure distribution around the plate is calculated using an indirect boundary element formulation. The results are compared with experimental data in order to show the accuracy and applicability of the numerical approach.

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Geräuschabstrahlung von Schiffspropellern im Nachstrom

1991 , Bartels, F.

The hydrodynamic and acoustic behavior of a ship propeller is determined by its geometry, load and speed, location and by the non-uniform wake. Acoustical predictions are based either on experience with reliable propulsion systems or with restrictions on model tests. The contribution deals with results of experimental investigations of model propellers in non-uniform wake flows, carried out with a special designed facility for propeller acoustics, which operates on an natural lake. The origins of noise with its spectral properties as tonals, frequency distributions and noise modulation by the wake are discussed. Cavitation, at higher speeds unavoidable, is the predominant underwater noise source of a ship. Types of cavitation as bubble or sheet cause very different noise levels as far as uniform flow is concerned, but in non-uniform wake flows for conditions as the same propeller load and speed, the differences in noise levels diminish. Propeller vibrations are another noise source, wh ich have special acoustical line-spectra caused by the directivity pattern of excited propeller eigenfrequencies.

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Richtcharakteristik einer einseitig eingespannten ebenen Platte

1991 , Homm, A. , Ganser, A. , Bartels, F.

Experiments on vibration and sound radition of a cantilever aluminium plate were carried out. For 3 eigenfrequencies the mode shapes were visualized by the Chladni method and the directivity patterns of the radiated sound were determined by the reciprocity method. The directivity patterns could be measured for nearly the whole of all directions in a 2 degree step resolution are represented in a 3-dimensional hidden line technique.

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Publication

Geräuschabstrahlung von Schiffspropellern im Nachstrom

1991 , Bartels, F.

The hydrodynamic and acoustic behavior of a ship propeller is determined by its geometry, load and speed, location and by the non-uniform wake. Acoustical predictions are based either on experience with reliable propulsion systems or with restrictions on model tests. The contribution deals with results of experimental investigations of model propellers in non-uniform wake flows, carried out with a special designed facility for propeller acoustics, which operates on an natural lake. The origins of noise with its spectral properties as tonals, frequency distributions and noise modulation by the wake are discussed. Cavitation, at higher speeds unavoidable, is the predominant underwater noise source of a ship. Types of cavitation as bubble or sheet cause very different noise levels as far as uniform flow is concerned, but in non-uniform wake flows for conditions as the same propeller load and speed, the differences in noise levels diminish. Propeller vibrations are another noise source, wh ich have special acoustical line-spectra caused by the directivity pattern of excited propeller eigenfrequencies.

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Determination of the sound speed in materials by measurements at thin plate samples in free-field water

1991 , Brebeck, D. , Tilmann, P.

In order to determine the sound speed of a layer of an unknown material by measurements in the free-field the advantages of the velocity transfer coefficient of the layer is investigated in comparison to the more usual reflection coefficient and transmission coefficient. A few test measurements are presented which show in principal the feasibility of the sound determination using the velocity transfer coefficient.

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Geräuschabstrahlung von tructure via propeller shaft and thrust bearing

1991 , Bartels, F.

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Numerical and experimental study of noise generated by a vibrating plate

1991 , Estorff, O. von , Homm, A. , Bartels, F.

Applying an indirect Boundary Element Method the sound radiation of a vibrating cantilever plate is investigated. The numerically obtained results are compared with measured data in order to show the accuracy and applicability of the numerical approach. The study clearly enhances the confidence in the computational solution of noise problems which are of particular importance in connection with the qualification of large spacecraft structures like HERMES.

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Finite element calculation of vibration modes of clamped propeller blades

1991 , Homm, A. , Bartels, F.

The study presents results of numerical FE- calculations for modern skew propeller blades and their experimental validation. It is restricted to single clamped blades, an assumption easy to implement in the theoretical calculation but difficult to achieve in an experiment. Assumptions for the FE- calculations and the incomplete rigid clamped boundary condition in the experiment yield differences between experiment and theory, mainly in the eigenfrequencies. The fluid coupling to the structure is limited to the inviscid, incompressible case or k 1 << 1. In the limits of these assumptions the results of both, theory and experiments agree fairly.

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Hydrodynamic noise research with a planing boat under acoustical free field conditions

1991 , Bartels, F.

A planing boat has been designed as a tool for research of hydrodynamic noise sources under acoustical free field conditions. The facility is used primarily for investigations of propeller cavitation noise in non-uniform wake flows. The objective of these tests is to disclose the influence of propeller design and wake fields upon cavitation noise. The operational test conditions are the same as for prototype surface ships. Compared to full scale the advance speed or Mach number are the same, but the Reynolds number is lower by the scale factor. The boat is equipped with several devices for observing and recording cavitation and noise. A small acoustic range with a suspended stationary hydrophone is used for recording the radiated far field noise of the test objects.

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