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This is the proceedings of the Second International Workshop on Flow Induced Noise and Vibration (FLINOVIA), which was held in Penn State, USA, in April 2016. 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.

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.

The book 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.

Low Wavenumber Models for Turbulent Boundary Layer Excitation of Structures

When the spatial correlation length of the turbulent boundary layer (TBL) pressure fluctuations is small compared to the structural wavelengths, the vibration response can be determined by forming an equivalent point drive from the effective correlation area. This approach is equivalent to using the zero wavenumber component of the TBL pressure spectrum, so it only works for TBL models that are wavenumber white at low wavenumbers. In this work, a similar simplification is developed for TBL models with a wavenumber-squared dependence, that works for structural modes with a low-pass cutoff wavenumber. This introduces a boundary layer thickness dependence that results in significantly different predictions for structures excited by a developing boundary layer. Based on the analysis, an experimental setup is proposed that may help resolve some of the controversy surrounding the low wavenumber TBL spectrum.
Peter D. Lysak, William K. Bonness, John B. Fahnline

Development of a Generalized Corcos Model for the Prediction of Turbulent Boundary Layer-Induced Noise

The characterization of the wall pressure field generated by a turbulent boundary layer $$\mathrm {\left( TBL\right) }$$ is a challenging problem in different fields. Its description is strictly dependent on the prediction of the noise and vibration induced by the flow-excited structure. $$\mathrm {TBL}$$ characterization often requires specific experimental setups and huge facilities, like wind tunnels, which are quite expensive. Usually, for simple configurations, semi-empirical or empirical models fit to experimental data are used to model the wall pressure characteristics. Many $$\mathrm {TBL}$$ models have been developed since the 1950s. Among others, the Corcos model has been widely used, especially because of its advantageous mathematical features that allow significantly reduced computational effort. However, an obvious problem with the Corcos model is its behaviour for wavenumbers below the convective peak. Within this paper, a Generalized Corcos model is considered for the prediction of $$\mathrm {TBL}$$-induced noise. Being built on a two-dimensional Butterworth filter, such a model is characterized by two more parameters than Corcos, which are the order of the filters along the streamwise and spanwise direction. The applicability of such a model is here investigated considering two representative flow conditions: (1) low-speed flow; (2) high-speed flow. Particular attention is drawn to the effect of the order of the filters. In this way, it will be shown how an accurate description of the wavenumber-frequency spectrum at and below the convective peak can be given by using the Generalized Corcos model.
Anna Caiazzo, Roberto D’Amico, Wim Desmet

Wall Pressure Identification by Using the Force Analysis Technique in Automotive, Naval and Aeronautic Applications

The aim of this research activity is to identify wall pressures which excite the structure, for automotive, aeronautic and naval domains, by using an inverse vibration method, such as FAT (Force Analysis Technique) and/or CFAT (Corrected Force Analysis Technique). The method is based on the local dynamic equilibrium equation of the structure, in which the partial derivatives are approximated by a finite difference scheme. Two schemes are proposed: the first (FAT method) consists in filtering the calculated force distribution by using an adequate spacing, but the associated wavenumber filtering presents a singularity at the flexural wavenumber of the structure, introducing an error around its value. The second uses a corrected finite difference scheme which acts as a complete low-pass wavenumber filter (CFAT method). In order to highlight the relevance of using both methods, results from simulations in the three industrial domains, automotive, aeronautic and naval are then presented where the comparison of FAT and CFAT results gives an interesting indicator to analyze the nature of the excitation. Moreover, for the naval application, a strategy for the identification of the strong fluid-structure coupling due to the pressure radiated by the structure is proposed. It is based on the identification of an effective wavenumber by using CFAT on a preliminary experiment where the structure immersed in the fluid is excited by a shaker. Finally, an experimental validation of the FAT/CFAT identification is shown for the car application. First, it is shown how both techniques identify very well the wall pressure on glass windows when the excitation is acoustic only (reverberant room). Second, the FAT/CFAT methods are applied to a car placed in a wind tunnel, where the analysis of results allows one to extract the whole acoustic component in a frequency range below the critical frequency of glass windows.
Charles Pezerat, Océane Grosset, Justine Carpentier, Jean-Hugh Thomas, Frédéric Ablitzer

Flow-Induced Noise of Shedding Partial Cavitation on a Hydrofoil

The cavitation dynamics on a NACA0015 hydrofoil can have different cavitation shedding behaviors depending upon the attack angle and flow conditions, as discussed in Arndt et al. (Instability of partial cavitation: a numerical/experimental approach, 2000) [1] and Kjeldsen et al. (J Fluids Eng 122(3):481–487, 2000) [8]. Shedding can exhibit varying shedding frequencies (Strouhal numbers) measured based on measurements of the resulting surface pressure and body forces. Moreover, the transition from one regime of shedding to another can be abrupt with changes in cavitation number. In this work, we present an analysis of the acoustic signatures measured by a hydrophone for different shedding conditions using a Morse wavelet analysis. Using measurements of the cavity dynamics based on X-ray densitometry and high-speed video observations (Ganesh et al., 31st symposium on naval hydrodynamics, ONR 2016) [6], we present an explanation for the observed acoustic behavior.
Harish Ganesh, Juliana Wu, Steven Ceccio

Sound Sources of Radial Compressors—A Numerical Study on the Outlet Side

As follow-up work of an experimental investigation performed by Raitor and Neise (J Sound Vib 314:738–756, 2008) [1] from 2001–2006, a numerical study using the unsteady RANS code for turbomachinery applications from the DLR (TRACE, Ashcroft et al., J Turbomach 136:021002, 2013 [2]) was performed by the University of Aachen (RWTH) and thoroughly analysed in collaboration with the Technical University of Berlin (TUB). Raitor and Neise were discriminating dominating noise source mechanisms in different frequency ranges both on the suction and the pressure side of the investigated radial compressor setups. Their elaborated work was focussed on the suction side of the experiment. In contrast to the suction side, where Raitor and Neise installed a multitude of microphones that allowed for a sound field decomposition into circumferential modes as well as—utilising the ISO 5136 methodology—the determination of the sound power, the pressure side was equipped with just a few microphones providing only spectral but no spatial information nor the sound power of the excited compressor sound field. In order to gain more insight into the sound propagation to the pressure side of radial compressors, the dominant tonal components of the conducted numerical study were acoustically analysed in terms of their radial mode constituents and the downstream transmitted sound power. The ongoing research is a first step towards the deeper physical understanding of the dominant constituents of sound fields propagating into the downstream ducts of radial compressors.
Lars Enghardt, Armin Faßbender, Jakob Hurst

Noise and Noise Reduction in Supersonic Jets

This paper describes a novel noise reduction method for supersonic jets. It involves the injection of relatively small amounts of air into the diverging section of a convergent-divergent nozzle. The air is injected through a series of injectors that are aligned axially. The injected air diverts the jet core flow and forms “fluid inserts.” These inserts have the same effect as hard-walled corrugated seals, but can be modified by changes to the injector pressure ratios. The inserts change the effective area ratio of the nozzle and can enable the jet to operate closer to an on-design condition. This has the effect of weakening the shock cell structures in the jet and reducing the broadband shock-associated noise. In addition, streamwise vortices are generated that break up the large-scale turbulent structures in the jet and result in a reduction in the supersonic mixing noise in the peak noise radiation direction. Experiments are described that examine the levels of noise reduction achieved by the fluid inserts. The effects of the number and azimuthal distribution of the inserts are examined. The effect of a change of experimental scale from small to moderate model sizes is also given. Noise reductions in the peak noise direction of up to 6 dB are obtained and broadband shock-associated noise is reduced. Ongoing plans and open questions are also discussed.
Philip J. Morris, Dennis K. McLaughlin

Numerical and Experimental Investigation of the Flow-Induced Noise of a Wall-Mounted Airfoil

A numerical and experimental investigation into the flow field around a finite wall-mounted airfoil is presented. Measurements were performed in an open-jet anechoic wind tunnel for a finite wall-mounted NACA 0012 airfoil with an aspect ratio of one. The airfoil was tested at zero degree angle of attack, with a Mach number of 0.06 and Reynolds number based on chord of 274,000. The measurements include single hotwire anemometry in the near-wake of the airfoil at a number of locations in the mid-span and tip regions. A large eddy simulation (LES) of flow past the airfoil was performed, and good agreement with measurements was obtained. Based on Lighthill’s acoustic analogy, flow-induced noise sources were then extracted from the LES data. Sound radiation to the far-field and the incident acoustic pressure on the airfoil were both predicted using a near-field formulation for the aeroacoustic pressure. The boundary element method (BEM) was then used to predict the scattering of the incident pressure field by the airfoil as well as the total far-field acoustic pressure.
Paul Croaker, Danielle Moreau, Manuj Awasthi, Mahmoud Karimi, Con Doolan, Nicole Kessissoglou

Turbulence Ingestion Noise from an Open Rotor with Different Inflows

Measurements have been performed on a scaled version of a Sevik rotor ingesting both a planar turbulent wake and a turbulent boundary layer flow. In both cases, detailed measurements were made of the inflow turbulence, including three-component turbulence profiles and the full cross-sectional 4-dimensional space-time correlation function. Far-field sound measurements were also made of the turbulence ingestion noise for a comprehensive range of rotor advance ratios varying from zero to high thrust, for rotor yaw angles out of the plane of the wake from −15 to 15°, and for a range of wake strike positions on the rotor disk. Probes mounted on two of the rotor blades were used to measure upwash fluctuations seen in the rotating frame, as well as blade-to-blade coherence spectra. Comparisons have been made with predictions of the far-field sound levels based on the measured inflow turbulence for both configurations and good results were obtained in all cases.
W. Nathan Alexander, William J. Devenport, Nicholas J. Molinaro, N. Agastya Balantrapu, Christopher Hickling, Stewart A. L. Glegg, Jack Pectol

Numerical and Experimental Assessment of the Linflap Technology for Regional Aircraft Noise Reduction

Within Green Regional Aircraft (GRA), a JTI Integrated Technology Development (ITD) program, an acoustically treated flap (called lined-flap), has been assessed. The design of such a lined-flap, conceived as a low-noise high-lift device, has been optimized through a suitable evolutionary algorithm that refers to an acoustic finite element (FE) model. An original turbulent empirical model has been implemented to estimate the noise, generated by the trailing edge and scattered by the wing body. A semiempirical model has instead supported the design process, relating the acoustic impedance to structural and materials properties. The capability of the proposed system has been finally checked within devoted wind tunnel test experiments.
Mattia Barbarino, Ignazio Dimino, Antonio Concilio

Measurement, Prediction, and Reduction of High-Frequency Aerodynamic Noise Generated and Radiated from Surfaces of Various Textures

We examine the acoustic characteristics of wind tunnels with and without acoustic foam liners, illustrate how the aeroacoustics of the liner can control the sound in the facility, and provide analytical evidence on the existence of the acoustic impedance of the liner in altering the dipole strengths of the wall’s roughness sources that produce sound in the facility.
Paul R. Donavan, Wlliam K. Blake

Accelerated Acoustic Boundary Element Method and the Noise Generation of an Idealized School of Fish

A transient, two-dimensional acoustic boundary element solver is developed using double-layer potentials accelerated by the fast multiple method for application to multibody, external field problems. The formulation is validated numerically against canonical radiation and scattering configurations of single and multiple bodies, and special attention is given to assessing model error. The acoustic framework is applied to model the vortex sound generation of schooling fish encountering 2S and 2P classes of vortex streets. Vortex streets of fixed identity are moved rectilinearly in a quiescent fluid past representative schools of two-dimensional fish, which are composed of four stationary NACA0012 airfoils arranged in a diamond pattern. The induced velocity on the fish-like bodies determines the time-dependent input boundary condition for the acoustic method to compute the sound observed in the acoustic far field. The resulting vortex noise is examined as a function of Strouhal number, where a maximum acoustic intensity is found for $$St \approx 0.2$$, and an acoustic intensity plateau is observed for swimmers in the range of $$0.3< St < 0.4$$. In the absence of background mean flow effects, numerical results further suggest that the value of Strouhal number can shift the acoustic directivity of an idealized school in a vortex wake to radiate noise in either upstream or downstream directions, which may have implications for the the study of predator-prey acoustic field interactions and the design of quiet bio-inspired underwater devices.
Nathan Wagenhoffer, Keith W. Moored, Justin W. Jaworski

Simultaneous Finite Element Computation of Direct and Diffracted Flow Noise in Domains with Static and Moving Walls

Curle’s acoustic analogy allows one to compute aerodynamic noise due to flow motion in the presence of rigid bodies. However, the strength of the dipolar term in the analogy depends on the values of the total flow pressure on the body’s surface. At low Mach numbers, that pressure cannot be obtained from the computational fluid dynamics (CFD) simulation of an incompressible flow, because the acoustic component cannot be captured. To circumvent this problem, and still being able to separate the flow and body noise contributions at a far-field point, an alternative approach was recently proposed which does not rely on an integral formulation. Rather, the acoustic pressure is split into incident and diffracted components giving rise to two differential acoustic problems that are solved together with the flow dynamics, in a single finite element computational run. In this work, we will revisit the acoustics of that approach and show how it can be extended to predict the flow noise generated in domains with moving walls.
Oriol Guasch, Arnau Pont, Joan Baiges, Ramon Codina

Panel Vibrations Induced by Supersonic Wall-Bounded Jet Flow from an Upstream High Aspect Ratio Rectangular Nozzle

The panel vibrations induced by fluctuating wall pressures within wall-bounded jet flow downstream of a high aspect ratio rectangular nozzle are simulated. The wall pressures are calculated using a Hybrid RANS/LES method, where LES models the large-scale turbulence in the shear layers downstream of the nozzle. The convecting turbulence in the shear layers loads the structure in a manner similar to that of turbulent boundary layer flow. However, at supersonic discharge conditions the shear layer turbulence also scatters from shock cells, generating backward-traveling surface pressure loads that drive the structure at low frequencies. The panel is rectangular with clamped edges along the sides oriented in the flow direction and free edges at the nozzle discharge and downstream edge. The panel modes of vibration are simulated with Finite Element Analysis. The structural vibration time histories are simulated by Fourier transforming the loading to the complex frequency domain, combining with the structural frequency response functions and inverse transforming the response back to the time domain. Simulated wall pressures and structural vibration agree well with measurements at on-design and underexpanded (about 50% higher pressure ratio) nozzle operating conditions. Filtering the negative wavenumber components from the loading and recomputing the structural response shows that the backward-traveling loading is responsible for about 12% of the overall structural vibration at on-design conditions and 25% of the response at underexpanded conditions.
Stephen A. Hambric, Matthew D. Shaw, Robert L. Campbell

Determination of the Acoustic and Hydrodynamic Contributions to the Vibrational Response of an Air-Conveying Rectangular Duct

This paper focuses on the vibratory response of a rectangular duct of finite length excited by an internal turbulent flow. The wall pressure distribution is decomposed into a hydrodynamic and acoustic contribution. Two configurations are investigated: (i) a straight duct with no singularity, in which duct acoustic modes are excited by the TBL and (ii) a straight duct with a diaphragm inserted upstream generating a localized acoustic source. The acoustic contribution is either measured via cross-spectra-based methods or calculated using Computational Fluid Dynamics and aeroacoustic analogies. Semi-analytical predictions are compared with experimental results. It is concluded that in both scenarios, the acoustic contribution is largely dominant.
Florian Hugues, Emmanuel Perrey-Debain, Nicolas Dauchez, Nicolas Papaxanthos

Review of Efficient Methods for the Computation of Transmission Loss of Plates with Inhomogeneous Material Properties and Curvature Under Turbulent Boundary Layer Excitation

The excitation of structures by turbulent boundary layer requires the consideration of the cross-spectral density for all degrees of freedom excited by the pressure fluctuations. This leads to large and computationally expensive matrices for the calculation of the structural response or even radiation into the fluid. This paper deals with the comparison of different methods to reduce the size of the problem. One option is the decomposition of the cross-spectral matrix into major principal components in order to represent the random matrix by few deterministic load cases. The second option is the conversion of the random equations of motion into modal space. A detailed investigation of the matrix coefficients under diffuse acoustic field and turbulent boundary layer excitation shows in which case the off-diagonal coefficients may be neglected. The paper concludes with an evaluation of the precision and a discussion about the computational expense of all applied methods as far as the advantages and disadvantages of each method.
Alexander Peiffer, Uwe Christian Mueller

Numerical Study of Nonlinear Fluid–Structure Interaction of an Excited Panel in Viscous Flow

Vibration of flexible panel induced by flow and acoustic processes in a duct can be used for silencer design, but it may conversely generate noise if structural instability is induced. Therefore, a complete understanding of fluid–structure interaction is important for effective noise reduction. A new time-domain numerical methodology has been developed for the calculation of the nonlinear fluid–structure interaction of an excited panel in internal viscous flow. This paper reports its validation with two experiments. The first aims to validate that the methodology is able to capture flow-induced structural instability and its acoustic radiation. The second one is to show that the methodology captures the aeroacoustic–structural interaction in a low-frequency silencer and its response correctly. The importance of inclusion of viscous effect in both cases is also discussed.
Harris K. H. Fan, Garret C. Y. Lam, Randolph C. K. Leung

Exact Geometric Similitude Laws for Flat Plate Vibrations Induced by a Turbulent Boundary Layer

Similitude laws for the vibration response of simply supported plates under random excitations are derived and tested numerically and experimentally for the case of a turbulent boundary layer. Analytical calculations show that under the assumption of proportional sides, perfect similitude in terms of vibration response scaling can be achieved between plates of variable thicknesses. It is also highlighted that even if the similitude conditions are not all satisfied (i.e., a complete scaling of all the involved parameters, from panel dimensions to flow speed), an approximation can be made in the mid-high-frequency domain that leads to satisfactorily scaled results. Based on the analytical study, a series of tests is performed in an anechoic wind tunnel on three-scaled simply supported panels at different flow velocities. Applying the proposed procedure to this set of vibration measurements leads to satisfactory scaling of results between each other.
Olivier Robin, Francesco Franco, Elena Ciappi, Sergio De Rosa, Alain Berry

Flow Noise Estimation with the Vibroelastic Analogy: Effect of Material Properties

Quantifying flow noise reduction caused by different surface coatings is a ubiquitous topic in aero- and hydroacoustics and is still an area of active research. Methods for doing so commonly involve huge costs in computational power (coupled CFD and FE simulations) and infrastructure (experimental studies). These high costs often make assessment, prototyping or rigorous sensitivity analysis a challenging undertaking. A self-consistent analytical framework is proposed for rapid estimation of relative flow noise intensity for a significantly subsonic turbulent flow over an elastic boundary (such as a turbulent boundary layer). The method utilises the well-known analogy between motion of viscous flow and wave propagation in soft elastic media (materials with low shear moduli). Together with the turbulent flow represented by an ensemble of viscous (shear or vortex) waves, the problem is transformed into that of elastic wave propagation within a multi-layered medium. The resulting noise within the fluid layer is determined by quantifying the transformation of shear waves into longitudinal waves at the flow boundary. This framework extends on previous work that examined the half-space idealisation by including the complex wave propagation within multi-layered coatings and by including the effect of the stream-wise convective velocity. The technique is demonstrated by assessing the influence of different homogenous and voided materials on flow noise from turbulent flow. Last, it is shown how these effects could be quantified by measuring the noise in the interior space of a body in flow.
Ian MacGillivray, Alex Skvortsov, Paul Dylejko

Fuselage Excitation During Cruise Flight Conditions: From Flight Test to Numerical Prediction

In the context of aircraft cabin interior noise, the fuselage structural excitation by turbulent boundary layer (TBL) flows is an important noise source for aircraft manufacturers to deal with. During cruise flight, it is the dominant source of cabin noise for state-of-the-art aircraft. Aircraft at cruise conditions is flying at high Mach numbers, typically between Ma = 0.78...0.85, dependent on the type and mission of the aircraft. The vortices within the TBL cause pressure fluctuations on the fuselage and therefore, its structure receives energy and starts to vibrate. In this paper, methods to estimate the aerodynamic TBL sources by numerical and semi-empirical tools are presented. Besides, also the induced vibration of real aircraft structures is calculated with existing industrial tools. Furthermore, measured flight test data used for validation are presented and finally measurements and predictions are compared.
Alexander Klabes, Sören Callsen, Michaela Herr, Christina Appel

Turbulent Flow Noise Generation Under Sea Conditions

Flow-induced noise contributes to the self-noise level of a hydroacoustic antenna that is either attached to or towed behind a moving platform at sea. It is induced in the interior of the antenna by hull vibrations excited by an outer turbulent boundary layer. Two different hull configurations were studied in a research cruise with an underwater towed body measurement system in Sognefjord, Norway. While the hydrophones were embedded into the hull structure in one of the flat plate configurations, they were separated from the hull by a water layer in the other. Material properties and hydrophone positions with respect to the flow were very similar in both configurations. By means of wavenumber–frequency analysis, the (flow-induced) spectral noise level is determined for towing speeds ranging from 4 to 12 kn. The noise level at the embedded hydrophones is systematically higher for all speeds than that at the separated hydrophones.
Jan Abshagen, Dennis Küter, Volkmar Nejedl

Measurement Techniques of the Sensitivity Functions to Characterize the Vibration Response of Panels Under Turbulent Boundary Layer Excitation

This study aims at developing an experimental method for characterizing the vibro-acoustic behavior of panels excited by random pressure fields. Although the method would be theoretically applicable to any stationary in time and spatially homogeneous random process, and for points belonging to the acoustic medium or to the panel, the turbulent boundary layer excitation is considered in this study while considering the vibration response exclusively. The interest of industrials toward this excitation has grown over the years. The main reasons being that the associated test means (i.e., wind tunnel or in situ measurements) are hard to control and very expensive. They are also subjected to large variabilities between laboratories, which makes it hard to attest the validity of the measuring technique. The proposed method allows to experimentally characterize a panel under such an excitation by separating the contribution of the excitation from the vibration behavior of the panel.
Christophe Marchetto, Laurent Maxit, Olivier Robin, Alain Berry

Inference of Random Excitations from Contactless Vibration Measurements on a Panel or Membrane Using the Virtual Fields Method

This paper aims at identifying random excitations acting on thin, plane structures from their measured vibration response. For random pressure fields such as the diffuse acoustic field (DAF) and turbulent boundary layer (TBL), two quantities of interest are to be determined, namely the wall pressure auto-spectral and cross-spectral density functions. These quantities are reconstructed using the virtual fields method, an identification technique based on the principle of virtual work. Numerical identification results for the auto-spectral and cross-spectral density functions are presented for both a plate and a membrane submitted to DAF and TBL excitations. Experimental identification results are then presented for the pressure auto-spectrum applied to an aluminum panel under DAF excitation from vibration response measurements that were obtained using deflectometry, a full-field optical measurement technique.
Patrick O’Donoughue, Olivier Robin, Alain Berry