Characteristics of the fluctuating pressure field beneath the turbulent boundary layer attached to the hull of a high speed vessel

The fluctuating pressure field generated by the turbulent boundary layer is one of the main sources of excitation and radiated noise for aircraft as well as for marine vehicles when for instance the induced vibrations in the hull plate of high-speed vessels or in the sonar dome are considered. In this frame, the hydrodynamic load acting on an elastic structure is usually described by the wave-number frequency spectrum that is a quantity difficult to obtain by direct measurements and it is usually provided by mathematical models deduced from the behaviour of the normalised cross spectrum. However also this last analysis is not easy to perform because the finite size of the pressure sensors implies a poor spatial resolution and signal attenuation at high frequency. On the other side, the background noise of the facilities often causes contamination of the data at low frequency. The wall pressure fluctuations spectra, when considering a body completely immersed in a fluid, depend on Reynolds number while, in the case of a surface piercing body such as a ship, a contribution from the Froude number must be taken into account hence introducing a new variable in its description. In view of the evaluation of the noise radiated on board of civil high speed vessels generated by flow exciting vibrations, an experimental campaign aimed to measure the pressure fluctuations beneath the turbulent boundary layer attached to the hull of a ship model was performed in a towing tank. The chance to use this kind of facility, characterised by a very low background noise, allows also the description of the low frequency part of the pressure spectra. An array of pressure transducers, flush mounted on a ship hull, were used to derive general laws for the streamwise cross spectral density behaviour at model scale based on the Corcos model. A detailed discussion of the scaling laws for the power spectral density in the different frequency range is also provide

INSEAN second year report

The main objective for the second year report was to provide the model scale pressure spectra produced by the turbulent boundary layer measured on the hull of the MISTRAL and of the JUMBO CAT. The acquired data were used to understand the principal characteristics of the TBL (turbulent boundary layer) induced noise. At the same time the final design of the experimental set up relative to SRF (surface random flow) excitation was completed with the help of numerical simulations

A Method to Measure Standard Costs of Juvenile Justice Systems: the example of Italy A Method to Measure Standard Costs of Juvenile Justice Systems: the example of Italy

Abstract: This study proposes a standard costs method to evaluate the total direct costs of a juvenile justice system and applies it to the Italian case

CHARACTERISATION OF THE TURBULENT BOUNDARY-LAYER WALL-PRESSURE SPECTRUM FOR A SHIP HULL

For a new generation of fast ships, the hydrodynamic noise sources represent an important item to be considered for a correct evaluation of the noise transmitted on board and its reduction is necessary to improve the comfort of passengers and crew. In particular, the pressure fluctuations induced on the ship hull by the turbulent boundary layer are investigated in this paper. The full scale characterization of the pressure fluctuations would imply expensive and cumbersome measurements on the real ship during navigation or, on the other hand, direct numerical simulations of the fluid flow around the hull that are not yet possible with available computational resources. In this paper a procedure able to estimate the full scale pressure spectra based on a combination of model scale measurements and direct numerical simulations is presented. The key of the method is the selection of scaling laws for the pressure spectra obtained from model scale measurements, that in this way can be recast as a universal curve.The scaling factors are defined as suitable combinations of the mean flow local parameters. Once a scaling law for all the frequency regions of the spectrum is established, the universal curve is simply re-scaled by the mean flow parameters obtained by direct numerical simulations of the boundary layer around the hull at full size to get the final spectra. Numerical simulations are made using 3D N-S averaged equations with taking into account an unsteady behaviour of the free surface around the hull. The advantage of this procedure is that only mean properties of the flow are requested, thus implying a dramatic decrement of time computation. In particular, the present procedure is applied to estimate the characteristics of the wall pressure spectrum over the hull of an high speed vessel

Dimensionless Representations of the Interaction Between Turbulent Boundary Layer and Elastic Plates

The study of the interaction between elastic structures and turbulent boundary layer still presents some uncertainties. This is true even assuming a oneway coupling and stationary turbulent boundary layer (TBL) over smooth and flat plates in subsonic flow. The reasons are mainly related to (i) the limitation for the direct numerical simulations of Reynolds number value of the Navier-Stokes equations, (ii) the high frequency structural and acoustic numerical and modelling difficulties and (iii) the lack of experimental data representative of all frequencywavenumber pressure fluctuation regions, needed for the direct validation of the semi-empirical pressure models. In fact, when the pressure convective terms are the dominant sources of vibrations and radiated noise, consolidated and almost case independent formulations exist; on the contrary, when the subconvective terms are of concern, the definition of models seems to be strongly dependent on the flow conditions and the characteristics of the fluid-structure interaction. In order to find a general procedure for the estimation of the response of elastic thin panels to TBL excitation, some scaling laws derived using dimensional analysis and energetic considerations are proposed. These dimensionless relations contain a combination of both flow and structural parameters yielding to simple analytical expressions relating a dimensionless structural response metric and a dimensionless frequency. The found scaling expressions are validated with wall pressure fluctuations and vibrational response data acquired in wind tunnels and towing tank for the case of thin flat plates made of homogeneous isotropic and composite materials

A Method to Measure Standard Costs of Juvenile Justice Systems: the example of Italy

This study proposes a standard costs method to evaluate the total direct costs of a juvenile justice system and applies it to the Italian case

DISTORTED SIMILITUDES FOR THE FREQUENCY RESPONSE OF COMPOSITE PLATES

This work presents an investigation on the definition and applicability of distorted similitudes and the related scaling laws for the analysis of the dynamic forced response of rectangular composite plates. Such responses are determined from a generalization of the modal approach, which allows the use of the mode shapes and natural frequencies in order to establish the proper scaling laws. The cases, herein discussed, are exclusively theoretical and form the basis for the application of numerical methods such as the Finite Element Method. Analytical models of simply supported rectangular plates are used to produce both the original and distorted model responses. The results show that even in the case of distorted models it is possible to reproduce with reasonable accuracy the response of the original plate

DE 1.1 Report on literature review

The purpose of this document is to provide a literature review of the state of the art of numerical models and the definition of eventually necessary adaptation, for Thin Line Arrays, focusing on specific characteristics for small and non constant diameters

Analysis of distorted similitudes for the frequency response of composite plates

This work presents an investigation on the definition and applicability of distorted similitudes and the related scaling laws for the analysis of the dynamic forced response of rectangular composite plates. Such responses are determined from a generalization of the modal approach, which allows the use of the mode shapes and natural frequencies in order to establish the proper scaling laws. The cases, herein discussed, are exclusively theoretical and form the basis for the application of numerical methods such as the Finite Element Method. Analytical models of simply supported rectangular plates are used to produce both the original and distorted model responses. The results show that even in the case of distorted models it is possible to reproduce with reasonable accuracy the response of the original plate

First international symposium on Flow Induced Noise and Vibration Issues and Aspects

Flow induced vibration and noise (FIVN) remains a critical research topic. Even after over 50 years of intensive research, accurate and cost-effective FIVN simulation and measurement techniques remain elusive. This book gathers the latest research from some of the most prominent experts in the field. It describes methods for characterizing wall pressure fluctuations, including subsonic and supersonic turbulent boundary layer flows over smooth and rough surfaces using computational methods like Large Eddy Simulation; for inferring wall pressure fluctuations using inverse techniques based on panel vibrations or holographic pressure sensor arrays; for calculating the resulting structural vibrations and radiated sound using traditional finite element methods, as well as advanced methods like Energy Finite Elements; for using scaling approaches to universally collapse flow-excited vibration and noise spectra; and for computing time histories of structural response, including alternating stresses. This book presents the proceedings of the First International Workshop on Flow Induced Noise and Vibration (FLINOVIA), which was held in Rome, Italy, in November 2013. The authors’ backgrounds represent a mix of academia, government, and industry, and several papers include applications to important problems for underwater vehicles, aerospace structures and commercial transportation. The book offers a valuable reference guide for all those working in the area of flow induced vibration and noise