Fraunhofer-Forschungsgruppe Hydroakustik FHAK

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  • Publication
    Advanced views of cavitation noise
    ( 1992)
    Baiter, H.-J.
  • Publication
    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.
  • Publication
    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.
  • Publication
    Recent development in measuring material parameters by a tube facility
    ( 1991)
    Brebeck, D.
    Paper describes a tube facility for investigations in underwater acoustics. The purpose is the measurement of physical parameters of materials. The device can be driven in the pulse mode and in the standing wave mode. The pulse mode delivers the echo reduction of test material at normal incidence. The standing wave mode is used to measure and calculate the input impedance of the material in magnitude and phase.
  • Publication
    Cavitation noise scaling. Status based on NATO RSG. Part 1. Survey of the problems in cavitation noise scaling and prediction
    ( 1991)
    Baiter, H.-J.
    Paper presents a status report on cavitation noise scaling based on work done by a relevant Nato Research Study Group (RSG). The RSG 3 was expected to find out how to improve reliability in predicting cavitation noise from model testing, and how to equalize this reliability for the various model test facilities in use.
  • Publication
    A novel tube device for measuring hydroacoustic properties of homogeneous and layered materials
    ( 1991)
    Brebeck, D.
    Paper describes a tube facility for investigations in underwater acoustic. The purpose is the measurement of physical parameters of materials. The device can be driven in the pulse mode and in the standing wave mode. The pulse mode delivers the echo reduction of a test material at normal incidence. The standing wave mode is used to measure and calculate the input impedance of the material in magnitude and phase.
  • Publication
    Excitation of the ship structure via propeller shaft and thrust bearing
    ( 1991)
    Wittek, G.
    This paper demonstrates by model measurements the influence of in-plane forces (exciting the ship structure ) on structureborne noise and the adjacent underwater radiated noise. The results were compared with the structureborne noise and the radiated underwater radiated noise due to normal excitation of the ship's hull. Concerning underwater radiated noise the in-plane excitation of the ship structure is of minor importance and can be abandoned with respect to the normal excitation.
  • Publication
    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.
  • 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.
  • Publication
    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.