Flinovia—Flow Induced Noise and Vibration Issues and Aspects-III

The Corcos and Chase models for the wavenumber-frequency spectrum of fluctuating pressures in a turbulent boundary layer (TBL) fluid flow are compared. Some of the recent improvements to these basic models (in order to agree better with the measured data) are reviewed. In particular some suggested revisions to Chase’s derivations to improve that model are presented. Also discussed are the coupling of the TBL pressures to structural vibrations, and areas were further research is needed.

Drone propeller noise under static and steady inflow conditions has been studied. A numerical model that couples a blade element momentum theory model with a frequency-domain acoustic model is presented. Experimental acoustic data for a 12-inch drone propeller under static and steady inflow are compared with the model. The model compares well against the experimental data for the fundamental tone and first 2–3 harmonics under steady inflow. Discrepancies at the higher harmonics are likely due to the point-loading assumption used to apply the aerodynamic loads in the acoustic model. Comparisons under static conditions are poor and this is thought to be caused by the limited nature of the aerodynamic model used. The results suggest that an accurate and versatile aerodynamics model is most important for accurate prediction of drone propeller noise over a wide range of operating conditions.

The vibration responses of three flat rectangular plates excited by turbulent boundary layer flow are calculated and compared to measured data. The measurements were made in three different facilities by Wilby at ISVR [1] (high speeds typical of aircraft), Han at Purdue University [2] (moderate speeds typical of automobiles), and Robin at University of Sherbrooke [3] (lowest speeds), spanning 50 years of time. The plates are different sizes, made from different materials, and have different boundary conditions. The boundary layers have different heights and flow speeds. The ratios of plate flexural and convective wavenumbers k_b/k_c over the three cases range from about 0.1 to 2. Plate vibrations are normalized by wall pressure fluctuation autospectra measured by the previous investigators. This wide range of structural and flow conditions and the use of plate vibration spectra normalized by wall pressure autospectra allows for an objective assessment of various TBL wall pressure fluctuation cross-spectral empirical models. Two cross-spectral models are considered: the widely used Corcos model [4] and the less well-known elliptical extension by Mellen [5]. Smolyakov’s empirical models for convection velocity and streamwise and spanwise surface pressure length scales [6] supplement the Corcos and Mellen models. Calculations using the Corcos cross-spectral model overestimate the vibrations by about an order of magnitude at lower speed (and lower k_b/k_c) conditions. Including Smolyakov’s convection velocity and length scale formulations improves accuracy at low frequencies. The Mellen cross-spectral pressure model, supplemented with Smolyakov’s empirical models for convective wave speed and streamwise and spanwise surface pressure length scales, is therefore well suited for calculating plate vibrations due to TBL flows with flow speed/flexural wave speed ratios ranging from 0.1 to 2.

The experimental study of a structure’s response to a turbulent boundary layer (TBL) excitation using wind-tunnel or in-vehicle testing generally requires considerable efforts, including the measurement of both turbulent wall-pressure fluctuations and the structure’s vibration response. As an alternative method to highly demanding testing procedures and numerical simulations, this paper proposes a computationally efficient method to predict vibroacoustic responses of a panel under a TBL excitation. Space-time realizations of a TBL wall pressure field obtained using a spectral synthesis approach are coupled to a deterministic model so as to predict mean quadratic velocity, and radiated sound pressure and power from a panel under a TBL excitation. Each realization of the wall pressure field and obtained vibroacoustic results can be considered as a virtual experiment. The radiated sound pressure as a function of time can be also obtained, and possibly later used for listening and psychoacoustics studies objectives. A summary of existing experimental and numerical methods for obtaining the vibroacoustic response of panels to a TBL excitation is first presented. The proposed method is then detailed. Results obtained using this method are finally compared to results obtained using controlled laboratory experiments and analytical calculations for a low subsonic flow speed.

Results from an underwater experiment on turbulent boundary layer flows over a plate with a spanwise wavy pattern are presented. The experiments were performed with a towed body measurement system at depths between −90 and −150 m in Sognefjord, Norway. Flow-induced noise on the reverse side of the wavy plate as well as wall pressure fluctuations beneath the turbulent boundary layer were studied for outer flow velocities \(U_\infty \) between 2.2 and 5.5 ms\(^{-1}\), which corresponds to towing speeds between 4 kn and 10 kn, respectively. For the lowest flow velocity within this range a substantial reduction in turbulent wall pressure fluctuations is found in comparison to a flat plate configuration. This reduction is, however, accompanied with an increase of flow-induced noise in the interior. Both effects are even enhanced at a larger pitch angle of the towed body. For higher flow velocities the differences between wavy and flat plate configuration are substantially less pronounced or even absent in some frequency ranges.

Experiments providing high fidelity experimental data are strongly required for physical analysis and numerical validation regarding of modelling and computation of Fluid Induced Vibration for hydrodynamics applications. This requires the development of advanced experiments and methods in very well controlled configurations as in hydrodynamic tunnels to analyse both the structure response and the flow dynamics. The paper presents original Fluid Induced Vibration experiments performed in the hydrodynamic cavitation tunnel of the French Naval Academy along the past few years. The experiments are generally conducted at a relative high Reynolds numbers typically ranging from about one hundred thousand to more than one million. The experiments are conducted on relatively small scale elastic or rigid lifting surfaces or elastic flat plates undergoing various flow conditions as turbulent boundary layer, Laminar Separation Bubble inducing transition, vortex shedding and cavitation. The structural response is analysed through local strains and stresses and vibration measurements for modal analysis. The deformed shape of the structure is obtained from a specific distance laser measurement device. Local stresses are obtained from strain gauges embedded in the structure. The vibration modal response is analysed by means of mono-point and scanning non contact laser Doppler vibrometers. The flow dynamics is examined through instantaneous wall-pressure field measurements using arrays of pressure transducers and through instantaneous velocity field measurement based on Time Resolved-Particle Image Velocimetry measurements and data-driven modal decompositions as Proper Orthogonal Decomposition. High speed cameras and image processing can be also used to analyse both the structure and flow dynamics particularly in cavitating flow. The paper presents the general experimental set-up and methods. Selected results are reported and briefly discussed.

When trying to predict the acoustic experience in aircraft cabins during flight, it is of importance to characterize the possible excitation mechanisms. In the following, we will focus on the fuselage surface pressure fluctuations on the outside of the aircraft hull and to determine its characteristics. The primary focus of this chapter is set on measurement and data processing rather than a deep interpretation of the results found. Flight test measurements using a pressure transducer array installed in window banks at two longitudinal positions on the fuselage are examined. The main focus is set on distinguishing between acoustic and hydrodynamic pressure fluctuations by means of phase propagation velocity. Differentiation between propagation velocities is achieved per-frequency by means of a wavenumber representation of the surface pressure fluctuations: at constant frequency, different propagation velocities will result pressure fluctuations to appear at different locations in the domain. A wavenumber filter can then be easily applied to separate pressure fluctuations with different characteristics. Prior to filtering, a deconvolution scheme is applied to the wavenumber maps in order to attempt to remove the array pattern from the resulting images, thus increasing resolution. The analysis results in two separated frequency spectra for acoustic and hydrodynamic pressure.

The wall pressure measured by an array of flush mounted microphones subject to a flow results from two components. The first one is due to the pressure fluctuations generated by vortices convected by the flow in the turbulent boundary layer developed on the wall, and the second one results from acoustic waves that are emitted by some acoustic sources. Different type of sensor arrays can be used to characterize the first or second components, or even both. In all cases, a major difficulty is to separate as well as possible their contributions to the measurements. Different techniques are reviewed in this chapter to achieve this goal, based on either the measurement device itself or the post processing method. All approaches are implemented in a unique experimental setup, conducted in the frame of the ADAPT Clean Sky 2 project, allowing to objectively highlight their advantages and drawbacks.

A numerical and experimental investigation into the flow and noise produced by a blunt-edged flat plate with a Reynolds number based on chord of 6.8 million and a Mach number of 0.0053 is presented. The flat plate had a 4:1 aspect ratio elliptic leading edge and a square trailing edge with a thickness-to-chord ratio of 0.0054. Experimental measurements were performed in a reverberant water tunnel. Pressure sensors were flush mounted on the top, bottom and rear faces of the blunt edge at the mid-span plane. Further, a hydrophone was mounted in a flooded cavity in the tunnel wall beneath a polyurethane diaphragm. An analytical model for trailing edge scattering was extended to account for near-field effects and to consider reflection of pressure waves by the tunnel walls. A large eddy simulation was also conducted, with hydrodynamic pressures on the surface of the plate extracted and combined with the analytical scattering model to predict the pressure fluctuations on the wall of the water tunnel. Numerical predictions are found to agree well with the experimental measurements.

Mitigating flow-induced noise is currently a major goal in the acoustic design of automotive and aircraft cabins. In order to avoid the introduction of active or massive components, structures made up of backed or unbacked micro-perforated panels (MPP) are potential lightweight solutions that could enhance the absorption and decrease the transmission of flow-induced noise. The present work describes experimental and modelling studies that examine the effect of MPPs, either flush-mounted or in a recessed configuration, on the wall-pressure fluctuations induced by a low-speed turbulent boundary layer (TBL) of air. The first part of this paper focuses on the vibro-acoustic properties of flush-mounted MPP-Cavity-Panel partitions forced by an aero-acoustic excitation. Parametric studies are carried out that show the influence of the MPP and of the excitation parameters on the absorption and transmission loss of the partition. In a second part, experimental studies and Lattice-Boltzmann simulations are presented that examine the effect of a recessed MPP, located at the floor of a shallow cavity, on the attenuation of the tunnel-cavity resonances and of the broadband noise components under a low-speed TBL.

Cylindrical shells are often employed as simplified models to represent the physics of underwater vehicles. When a vehicle is moving underwater, the flow is likely to create a turbulent boundary layer (TBL) over the shell surface. The TBL induces vibrations on the shell, which results in the radiation of noise. Semi-analytical formulations are nowadays widely used to predict the vibro-acoustics of structures and re-design them to mitigate the flow-induced noise. However, most of the methods available in literature tackle the case of flat plates and only few results can be found for cylindrical shells. In this chapter, we give an overview of a method for calculating both the response of a fluid loaded cylindrical shell and the outward emitted noise, with different degrees of complexity. The considered test cases involve various phenomena and underlying physics. Aspects such as the role of propagative Bloch-Floquet waves in noise radiation by periodically stiffened structures, or the influence of mechanical coupling between the shell and non-axisymmetric inner structures are addressed. An explanation is also given for the very different behavior of the radiated acoustic pressure in the near and far fields.

We present an analytical framework that can be used for quantitatively estimating the effect of wall roughness on turbulent boundary-layer noise. For this purpose we extend the viscous-elastic analogy for boundary-layer noise (the well-known analogy between the motion of a viscoelastic medium and a viscous fluid) that was proposed in our previous work. The refined model accounts for the elastic (impedance) properties of the underlying surface with some fine-scale morphological features (natural roughness or special engineering coating). Our analysis is restricted to significantly-subsonic flows. The speed of the flow is also assumed to be much less that the speed of elastic waves (both longitudinal and shear) in the material of the underlying surface.

The acoustic comfort level in modern car design mainly depends on the aerodynamic noise generated at high speed. This noise is characterized by a mid- and high- frequency spectrum and mainly radiates inside the passenger cabin through the different car glass surfaces. Predicting such noise can be done by coupling a Computational Fluid Dynamics (CFD) code solving the compressible turbulent flow with a vibro-acoustic simulation software. The simulation of the flow around the vehicle gives access to the wall pressure applied on the car glass surfaces, while the vibro-acoustic model takes this aerodynamic loading as excitation to predict the internal noise. In this paper, the vibro-acoustic frequency response of an existing Finite Element (FE) model is extended using a Statistical Energy Analysis (SEA) approach called Virtual SEA. In this approach, that simulates an experimental SEA process, the necessary information required to build the SEA matrix are extracted from the FE models. The evaluation of the injected power corresponding to the wall pressure field, which excites the Virtual SEA model, relies on the decomposition of the glass surfaces wall pressure into a turbulent and an acoustic contribution. Finally, the efficiency of the proposed methodology is demonstrated on an industrial car model, where numerical acoustic responses predicted by the Actran software are compared to experimental data measured in wind tunnel over a large frequency range.

In the framework of finite elements based methods, this work proposes two numerical approaches to deal with the vibrations and noise induced by a random excitation on periodic and homogeneous structural systems. First, a 1D Wave Finite Element scheme is developed to deal with flat, curved and tapered finite structures. A single substructure is modelled using finite elements and one-dimensional periodic links among nodes are applied to get the set of waves propagating along the periodicity direction. The set of waves is then used to calculate the Green transfer functions between a set of target degrees of freedom and a subset representing the loaded ones. A 2D approach is also developed in combination with a wavenumber-space load synthesis to simulate the sound transmission of infinite flat, curved and axisymmetric structures: both homogenised and complex periodic models are analysed. The proposed numerical approaches are validated with analytical, numerical and experimental results and under different load conditions. From the experimental point of view, the approach is validated comparing results in terms of transmission loss evaluated on aircraft fuselage panels under diffuse acoustic field excitation.

A hybrid uncorrelated wall plane wave (UWPW) and finite element method (FEM) technique is introduced to the predict vibroacoustic response of a panel under turbulent boundary layer (TBL) excitation. The spectrum of the wall pressure fluctuations is evaluated from the TBL parameters and by using semi-empirical models from literature. TBL parameters can be estimated by different means, using theoretical formula, Reynolds-averaged Navier Stokes (RANS) simulations or experimental data. The wall pressure field (WPF) underneath the TBL is then synthesized by realisations of uncorrelated wall plane waves. The FEM is employed to compute the structural and acoustic responses of the panel for each realisation of uncorrelated wall plane waves. The responses are then obtained from an ensemble average of the different realisations. Selection criteria for cut-off wavenumber, mesh size and number of realisation are discussed. Two simply-supported baffled panels under TBL excitation are examined. Numerical results are compared with analytical results using the sensitivity functions of the panels, showing excellent agreement.

A novel method for reduction in the airfoil tonal noise using flow-induced vibrations is explored by using a flush-mounted elastic panel over the suction surface of a NACA 0012 airfoil at low Reynolds number of \(5\times 10^4\). The fundamental aim of this approach is to reduce the airfoil tonal noise while maintaining laminar boundary layer over the airfoil with minimum or no penalty on the aerodynamic performance of the airfoil. Direct aeroacoustics simulation using conservation element and solution element method along with linear stability analysis is employed to study the aeroacoustic structural interaction between the flow field and elastic panel. Panel parameters are carefully selected to ensure that the natural frequency of panel in the presence of flow field coincides with the first dominant frequency of naturally evolving boundary layer disturbance on the airfoil suction surface. To gain further insight on the sensitivity of panel parameters on its vibration behavior and magnitude of reduction in tonal noise, a parametric study is also carried out. Contributions of panel density and thickness are found to be dominant in noise reduction. A maximum sound pressure level reduction of 2.74 dB is achieved for the current flow conditions through the proposed strategy.

Turbulence generated broadband noise is an important source of noise that can be found in many applications. The characteristics of noise are defined by a combination of turbulence statistics, flow and geometry, imposing challenges in developing efficient and robust prediction methods. Direct simulations of flow governing equations are currently too costly for design optimisation. An efficient strategy is to solve governing equations such as the full or linearized Euler equations with a synthetic turbulence model, which reproduces main turbulence statistics, therefore capturing the physics of noise generation and sound propagation. In this paper, synthetic turbulence methods for aeroacoustics, especially those suitable for the broadband noise simulations of leading edge noise problems, are introduced. Examples are provided, together with progresses made in tackling the challenges of computational aeroacoustics for transonic flows and far-field noise computation.