Coherent flow noise beneath a flat plate in a water tunnel experiment
Introduction
Flow induced noise originating from a moving body which is surrounded by a turbulent boundary layer flow is not only of large relevance for many applications, but also of fundamental scientific interest [1], [2]. Propagating away from the moving body this flow induced sound may be perceived as unwanted noise in the far field. Examples arise from aircraft noise [3], ship noise [4], and wind turbine noise [5]. If there are fluid filled spaces inside the body, pressure fluctuations can generate additional noise therein. This is known, for instance, as cabin noise of aircrafts [6] and cars [7] as well as in sonar applications [8]. In the latter case flow induced noise is generated by the motion of a sonar antenna through quiescent water and contributes to sonar-self-noise [9].
The fluctuations of velocity and pressure generated inside an (incompressible) turbulent boundary layer flow act on an underlying mechanical wall structure [10]. The fluctuations behave as a random field with a small coherence area and a slow propagation component in the flow direction. Wall pressure fluctuations beneath a turbulent boundary layer are of particular interest due to their dominant role in turbulent sound generation and have therefore been subjected to numerous experimental and theoretical investigations in the past [11], [12], [13], [14], [15], [16], [17], [18], [19]. Various semi-empirical models of the temporal and the space–time correlations behaviour of wall pressure fluctuations have been developed in order to understand the underlying physical processes but also to cope with applicational needs [20], [21], [22], [23], [24], [25].
Inside of a fluid-filled (moving) body pressure fluctuations can only propagate as sound waves and the wall structure plays a crucial role in the generation of this interior flow noise [26]. In principle turbulent pressure fluctuations can be transmitted locally through a wall structure into the fluid on the reverse side, but typically the mechanical wall structure responses to excitations from the turbulent boundary layer. This response reflects the modal character of the mechanical wall structure [27].
The vibroacoustical response to turbulent boundary layer excitation has been studied in detail, for instance, for flat plates (see e.g. [1], [28], [29]). Here, the Finite-Element Method [30], [31], [32] has played a crucial role. A particular focus of recent work in vibroacoustic response to turbulent excitations is given to the improvement of predictive methodology [33], [34], [35], [36]. Fluid–structure interaction between boundary layer flow and wall structure can furthermore be considered to be of importance for the generation of flow-induced noise [37].
In this work the vibroacoustic response of a flat plate to a turbulent boundary layer excitation and the hydroacoustic pressure field in the vicinity of the vibrating plate are investigated. The plate is entirely immersed in water and flow-induced noise is measured on the reverse side of the plate, i.e. in the (quiescent) interior of a model surrounded by the turbulent boundary layer flow. In order to understand the relevant sources of flow-induced noise in the experiment particular focus is given to the coherence between wall pressure fluctuations, plate vibrations, and interior flow noise. Vibroacoustic properties and hydroacoustic response of the plate are determined by a numerical simulation based on the Finite-Element method.
Section snippets
Experimental setup
The experiments are performed with a streamlined model in the HYKAT cavitation tunnel at HSVA, Hamburg. The model has a symmetrical outer shape in flow direction and is made of coated wood. Inbetween a curved nose and tail the model contains a flat plate region which is inclined in vertical direction. The shape of the model is based on a towed body which was designed for flow noise measurements at open sea [38]. A schematic drawing of the starboard side of the towed body is depicted in Fig. 1
Numerical analysis of plate vibrations
In order to understand the vibroacoustic properties and hydroacoustic response of the plate immersed in water a numerical analysis based on the Finite-Element Method (FEM) has been performed. This includes an investigation of the eigenmodes and eigenfrequencies as well as a numerical response analysis. Here, a spatially localised pressure perturbation is applied to the plate in order to mimic the excitation from turbulent wall pressure fluctuations within the coherence area. This approach is
Coherence of interior flow noise
In this section the spectral and coherence properties of flow noise in the interior of the streamline model inside the water tunnel are investigated. Power spectral density (PSD) of wall pressure fluctuations (+) and interior flow noise (solid line) measured at positions FMH and Hyd1 are depicted in Fig. 6(a). The flow speed in the water tunnel was U=7 m/s and a bandwidth of was used. Below f=10 Hz disturbances which originate from the experimental setup contaminates the spectra and
Structural vibrations and flow noise
In this section the behaviour for structural vibrations and its origin as a source for interior flow noise in the experiment is investigated. Experimentally plate vibrations and spatial coherence are measured with three accelerometer positioned as represented in Fig. 2. The accelerometer ACC3 is located on the up–down symmetry axis at a sufficient distance from the hydrophone array while both the accelerometers ACC1 and ACC2 are located off-axis on either side of the symmetry axis. The power
Conclusion
Experiments on flow noise generation in the interior of a streamlined model surrounded by a turbulent boundary layer have been performed in the HYKAT cavitation tunnel at HSVA, Hamburg. Flow noise near the wall is found to be substantially generated by evanescent eigenmodes of the plate which are excited by the wall pressure fluctuations beneath the turbulent boundary layer on the reverse side of the plate. Evidence is provided that the non-local, modal response of the plate to spatially
Acknowledgements
We thank H. Bretschneider from HSVA, Hamburg, and R. Kühl and S. Osburg from FWG for excellent technical support. Discussions with V. Nejedl from FWG are gratefully acknowledged.
References (41)
- et al.
Estimating turbulent-boundary-layer wall-pressure spectra from CFD RANS solutions
Journal of Fluids and Structures
(2007) - et al.
Wall pressure fluctuation spectra due to boundary-layer transition
Journal of Sound and Vibration
(2009) Modelling the wavevector–frequency spectrum of turbulent boundary layer wall pressure
Journal of Sound and Vibration
(1980)A comparison of models for the wavenumber–frequency spectrum of turbulent boundary layer pressures
Journal of Sound and Vibration
(1997)- et al.
Comparison of semi-empirical models for turbulent boundary layer wall pressure spectra
Journal of Sound and Vibration
(2009) - et al.
Hydrodynamic and hydroelastic analysis of a plate excited by the turbulent boundary layer
Journal of Fluids and Structures
(2009) - et al.
Application of the spectral finite element method to turbulent boundary layer induced vibrations of plates
Journal of Sound and Vibration
(2003) - et al.
Vibration of plates with clamped and free edges excited by low-speed turbulent boundary layer
Journal of Fluids and Structures
(2004) - et al.
Equivalent ‘rain on the roof’ loads for random spatially correlated excitations in the mid-high frequency range
Journal of Sound and Vibration
(2009) - et al.
Exact and numerical response of a plate under turbulent boundary layer excitation
Journal of Fluids and Structures
(2008)
Prediction of the response of a thin structure to a turbulent boundary-layer-induced random pressure field
Journal of Sound and Vibration
Numerical approximations on the predictive response of plates under stochastic and convective loads
Journal of Fluids and Structures
Mechanics of Flow-Induced Sound and Vibration
Aeroacoustic research in Europethe CEAS-ASC report on 2010 highlights
Journal of Sound and Vibration
Mechanics of Underwater Noise
Wind Turbine Noise
Principles of Underwater Sound
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