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CONTENTS
Volume 6, Number 6, November 2019
 

Abstract


Key Words


Address


Abstract
This communication investigates exact and distorted similitudes and the related scaling laws for the analysis of both dynamic response and radiated power of rectangular plates. The response of a given panel in similitude from another one is determined from a generalization of the modal approach, allowing the use of mode shapes, natural frequencies and finally radiation functions in order to establish appropriate scaling laws. Analytical models of simply supported rectangular plates are used to produce both original and replica model responses under point mechanical excitation. Emphasis is then especially put on laboratory experiments which are performed on baed simply supported aluminum panels under mechanical excitations. All the six possible scaling directions, i.e., predicting a plate vibroacoustic reponse from another plate, are reported. All obtained results show that structural response or radiated sound power of a given plate can be both recovered with satisfactory accuracy by using the related scaling laws, even if parent models are used.

Key Words
similitude; plates vibration; scaling

Address
Olivier Robin and Alain Berry: Groupe d\'Acoustique de l\'Universite de Sherbrooke, Faculte de Genie, Sherbrooke, J1K 2R1, Canada

Pasquale Margherita, Sergio De Rosa and Francesco Franco: pasta-Lab, Department of Industrial Engineering, Universita degli Studi di Napoli Federico II, Via Claudio 21,
80125 Napoli, Italy

Elena Ciappi:CNR-INM Institute of Marine Engineering, Via di Vallerano 139, 00128, Roma, Italy

Abstract
The present work investigates the effect on the flow-induced vibrations of the lay-up sequence of composite laminated axisymmetric structures, using an hybrid approach based on a wave finite element and a transfer matrix method. The structural vibrations, under deterministic distributed pressure loads, diffuse acoustic field and turbulent boundary layer excitations, are analysed and compared. A multi-scale approach is used for the dynamic analysis of fi nite structures, using an elementary periodic subsystem. Different flow regimes and shell curvatures are analysed and the computational efficiency is also discussed.

Key Words
wave finite element method; flow-induced vibrations; wave propagation; boundary layer excitation

Address
F. Errico: 1.)LTDS, Laboratoire de Tribologie et Dynamique des Systems, Ecole Centrale de Lyon, 36 Avenue Guy de Collongue, 69134, Ecully, France
2.) Pasta-Lab, Laboratory for promoting experiences in aeronautical structures and acoustics, Dipartimento di Ingegneria Industriale, Universita degli Studi di Napoli Federico II, Via Claudio 21, 80125 Napoli, Italy

F. Franco, M. Ichchou, S. De Rosa and G. Petrone: Pasta-Lab, Laboratory for promoting experiences in aeronautical structures and acoustics, Dipartimento di Ingegneria Industriale, Universita degli Studi di Napoli Federico II, Via Claudio 21, 80125 Napoli, Italy

Abstract
Periodic cellular core structures included in sandwich panels possess good stiffness while saving weight and only lately their potential to act as passive vibration filters is increasingly being studied. Classical homogeneous honeycombs show poor vibracoustic performance and only by varying certain geometrical features, a shift and/or variation in bandgap frequency range occurs. This work aims to investigate the vibration filtering properties of the AUXHEX \"hybrid\" core, which is a cellular structure containing cells of different shapes. Numerical simulations are carried out using two different approaches. The first technique used is the harmonic analysis with commercially available software, and the second one, which has been proved to be computationally more efficient, consists in the Wave Finite Element Method (WFEM), which still makes use of finite elements (FEM) packages, but instead of working with large models, it exploits the periodicity of the structure by analysing only the unit cell, thanks to the Floquet-Bloch theorem. Both techniques allow to produce graphs such as frequency response plots (FRF\'s)and dispersion curves, which are powerful tools used to identify the spectral bandgap signature of the considered structure. The hybrid cellular core pattern AUXHEX is analysed and results are discussed, focusing the investigation on the possible spectral bandgap signature heritage that a hybrid core experiences from their \"parents\" homogeneous cell cores.

Key Words
WFEM ; periodic media ; bandgaps ; Transfer matrix method ; cellular structures

Address
S. Del Broccolo: 1.) Department of Applied Mechanics, University of Bourgogne-Franche-Comté - FEMTO-ST – Institute CNRS/UFC/ENSMM/UTBM, 25000 Besançon, France
2.) Bristol Composites Institute (ACCIS), University of Bristol, Queen\'s Building, University Walk, Bristol BS8 1TR, U.K.

Abstract
A wind tunnel test was conducted that measured surface fluctuating pressures aft of a ramp at transonic speeds. Dynamic pressure test data was used to perform a study to determine best locations for streamwise sensor pairs for shocked and unshocked runs based on minimizing the error in root-mean-square acceleration response of the panel. For unshocked conditions, the upstream sensor is best placed at least 6.5 ramp heights downstream of the ramp, and the downstream sensor should be within 2 ramp heights from the upstream sensor. For shocked conditions, the upstream sensor should be between 1 and 7 ramp heights downstream of the shock, with the downstream sensor 2 to 3 ramp heights of the upstream sensor. The shock was found to prevent the passage coherent flow structures; therefore, it may be desired to use the shock to define the boundary of subzones for the purpose of loads definition. These recommendations should be generally applicable to a range of expansion corner geometries in transonic flow provided similar flow structures exist. The recommendations for shocked runs is more limited, relying on data from a single dataset with the shock located near the forward end of the region of interest.

Key Words
wind tunnel testing; shock; vibroacoustics; transonic; dynamic pressure; random; acceleration response

Address
Michael Y. Yang and Michael T. Palodichuk: ATA Engineering, Inc. 13290 Evening Creek Drive South, Suite 250, San Diego, California, U.S.A.

Abstract
The structural vibrations of a flat plate induced by fluctuating wall pressures within wall-bounded transonic jet flow downstream of a high-aspect ratio rectangular nozzle are simulated. The wall pressures are calculated using Hybrid RANS/LES CFD, where LES models the large-scale turbulence in the shear layers downstream of the nozzle. The structural vibrations are computed using modes from a finite element model and a time-domain forced response calculation methodology. At low flow speeds, the convecting turbulence in the shear layers loads the plate in a manner similar to that of turbulent boundary layer flow. However, at high nozzle pressure ratio discharge conditions the flow over the panel becomes transonic, and the shear layer turbulence scatters from shock cells just downstream of the nozzle, generating backward traveling low frequency surface pressure loads that also drive the plate. The structural mode shapes and subsonic and transonic surface pressure fields are transformed to wavenumber space to better understand the nature of the loading distributions and individual modal responses. Modes with wavenumber distributions which align well with those of the pressure field respond strongly. Negative wavenumber loading components are clearly visible in the transforms of the supersonic flow wall pressures near the nozzle, indicating backward propagating pressure fields. In those cases the modal joint acceptances include significant contributions from negative wavenumber terms.

Key Words
wavenumber analysis; transonic jet; wall pressure fluctuations; structural vibration; nozzle

Address
Stephen A. Hambric, Matthew D. Shaw and Robert L. Campbell: Applied Research Lab, Penn State University, PO Box 30, State College, PA 16804, U.S.A.

Abstract
The effect of multiple process parameters on a set of continuous response variables is, especially in experimental designs, difficult and intricate to determine. Due to the complexity in aeroacoustic and vibroacoustic studies, the often-performed simple one-factor-at-a-time method turns out to be the least effective approach. In contrast, the statistical Design of Experiments is a technique used with the objective to maximize the obtained information while keeping the experimental effort at a minimum. The presented work aims at giving insights on Design of Experiments applied to aeroacoustic and vibroacoustic problems while comparing different experimental designs and approximation models. For this purpose, an experimental rig of a ducted low-pressure fan is developed that allows gathering data of both, aerodynamic and aeroacoustic nature while analysing three independent process parameters. The experimental designs used to sample the design space are a Central Composite design and a Box-Behnken design, both used to model a response surface regression, and Latin Hypercube sampling to model an Artificial Neural network. The results indicate that Latin Hypercube sampling extracts information that is more diverse and, in combination with an Artificial Neural network, outperforms the quadratic response surface regressions. It is shown that the Latin Hypercube sampling, initially developed for computer-aided experiments, can also be used as an experimental design. To further increase the benefit of the presented approach, spectral information of every experimental test point is extracted and Artificial Neural networks are chosen for modelling the spectral information since they show to be the most universal approximators.

Key Words
artificial neural networks, design of experiments; Latin hypercube sampling; aeroacoustics, aerodynamics, spectral analysis

Address
Till M. Biedermann and Frank Kameier: Institute of Sound and Vibration Engineering ISAVE, University of Applied Sciences Duesseldorf, Muensterstrasse 156, D-40476 Dusseldorf, Germany

Marius Reich and Mario Adam: Centre of Innovative Energy Systems ZIES, University of Applied Sciences Duesseldorf, Muensterstrasse 156, D-40476 Dusseldorf, Germany

C. O. Paschereit: Institute of Fluid Dynamics and Technical Acoustics ISTA, Technical University Berlin,
Mueller-Breslau-Strasse 8, D-10623, Berlin, Germany


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