Fraunhofer-Forschungsgruppe Hydroakustik FHAK

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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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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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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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Model tests on noise characteristics of propeller cavitation in wake flow

1989 , Bartels, F.

The paper describes results of propeller cavitation noise experiments in non-uniform wake fields. Two propellers different in design with respect to the dominant type of cavitation have been investigated. The experiments were carried out with a planing boat, designed for cavitation noise research. The boat operates on a lake under acoustical free field conditions. The objective of these tests is to disclose the influence of propeller design and wake fields upon cavition noise.

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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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Geräuschabstrahlung von tructure via propeller shaft and thrust bearing

1991 , Bartels, F.

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Acoustical farfield directivity and vibration pattern of propeller eigenmodes in air

1991 , Blöchl, B. , Bartels, F.

The paper describes experimental results of an investigation about the propeller vibration and the directivity of sound radiation for some selected resonance frequencies. in an anechoic chamber the spatial distribution of the amplitudes and phase angles of the sound radiation has been measured by means of the reciprocity method. The results indicate that with increasing numbers of propeller blades the spatial variation of the sound amplitude decreases. The origin of the complex nature of the acoustic directivity characteristics in amplitude and phase is the vibration pattern of the propeller. The eigenvalues and the nodal lines of the eigenmodes have been evaluated by holography or by scanning the propeller surface with two acceleration pick-ups, determining the lines of phase shift. The results reveal, that for several eigenvalues the vibration amplitudes of the blades differ from each other and no vibration symmetry exist. The spatial structure of the amplitude and phase variation ca uses a very special spectral noise signature of a rotating and vibrating ship propeller.

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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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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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Directivity pattern of a cantilever plate. Comparison of numerical results with experimental data

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

For a cantilever aluminium plate, the vibration modes were investigated experimentally by means of the Chladni method and numerically with the FE-method (NASTRAN). The sound radiation of three selected eigenfrequencies was determined by the reciprocity method and was calculated with a coupled FEM/BEM-approach. The comparison of the experimental and numerical results shows excellent agreement.

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