A formulation to introduce acoustic waves from a control surface using volumetric source terms is proposed for numerical simulations. A general expression of the source terms is derived from the non-linear Euler equations. The method is validated through three academic configurations: the injection of oblique plane waves and the radiation of a monopole source in two and three dimensions, in uniform flow. The governing equations are solved in a Cartesian grid using a low-dispersion and low-dissipation high order finite-difference numerical scheme. However, the control surface has an arbitrary shape, as demonstrated here with the use of a cylindrical surface. Numerical results show good agreement with analytical solutions in both phase and amplitude. The method is then applied to an open-fan aircraft engine configuration. The source terms are computed from a cylindrical control surface enclosing the rotor, based on data extracted from a previous fluid mechanics simulation. The radiated acoustic field is compared with the one obtained using the Ffowcs Williams-Hawkings integral formulation. The two solutions are again found in good agreement for this more realistic configuration.

A low-order immersed boundary method (IBM) is proposed for the simulation of acoustic scattering by obstacles. The IBM formulation, based on generalized functions, introduces source terms distributed along the obstacle surfaces through a Dirac delta function. For implementation using finite-difference methods, the delta function is smoothed using a Gaussian function. The proposed IBM can accommodate perfectly reflecting, impedance, and non-reflecting boundary conditions (BCs). Analytical investigations demonstrate that, when the delta function is employed, the IBM reproduces the exact reflection and transmission coefficients for oblique wave incidence and recovers the exact scattered field for the canonical problem of acoustic scattering by a cylinder. When a Gaussian function is used instead, the method achieves first-order accuracy for impedance and non-reflecting BCs, and second-order accuracy for perfectly reflecting BCs. Several numerical test cases confirm these theoretical findings.

Acoustic discretion is of prime importance for submarines while operating, in order not to be detected. One of the submarine acoustic sources is the noise induced by water flows passing through singularities in pipes ending at the hull, thus radiating acoustic waves in the vessel surroundings. Predicting the noise induced by singularities in pipes is then fundamental to determine the acoustic signature of an underwater ship. Several Computational Aero-Acoustics methods have been implemented in order to predict the sound induced by singularities such as diaphragms and perforated plates. In this study, the Lattice Boltzmann Method (LBM) coupled to Large Eddy Simulation (LES) is used to simulate the flow passing through a diaphragm and to predict the induced noise. The method is first implemented for an airflow in a rectangular duct, with the final objective of being applied to confined water flows encountered in circular or rectangular ducts.

A new processing method is developed to analyze images from a Fabry-P & eacute;rot interferometer in order to extract point measurements of temperature and velocity within a gas flow, using Rayleigh scattering. Two types of interferograms are generated from a Fabry-P & eacute;rot model combined with a simulated light source. The first type is obtained from a diffuse coherent light source, namely, a laser beam on a diffuser. The interferometer characteristics, defined by only two independent parameters, are retrieved within 0.1% accuracy. The knowledge of these parameters is mandatory to analyze interferograms from Rayleigh scattered light. The second type corresponds to Rayleigh scattered light from a small volume under flow conditions, lighted with a focused laser beam and captured with long exposure time. Several flow parameters are chosen to generate these interferograms. The relative errors on the temperature and velocity estimates are found to be weak. Noise is also added to assess the robustness of the processing method. The error induced by the estimates of the instrument function is found to be of second order compared to the error induced by the image analysis.

Trapped waves in the potential core of high subsonic jet can be described as a system of waves responsible for a series of sharp peaks in the near-field pressure spectrum.In this work, density fluctuations associated with these acoustic waves in a Mach 0.9 jet are measured using Rayleigh scattering in the jet potential core.The set-up is installed in an anechoic wind tunnel, where simultaneous measurements of acoustic near-field pressure and flow density spectra are carried out.The lowest frequency and more prominent peak characteristic of trapped waves is observed at the same frequency in the acoustic and density spectra.The ability of the present setup to detect trapped waves signatures in the density spectrum is demonstrated.

The capabilities of an aeroacoustic wave equation based on Pierce's operator (AWE-PO) for modeling subsonic flow-induced sound and for sound prediction are investigated. The wave equation is applied to an isothermal two-dimensional mixing layer computed by direct numerical simulation. In contrast to a direct numerical simulation, providing the acoustic fluctuations directly, the simulations based on Lighthill's wave equation and the AWE-PO rely on a hybrid workflow to predict the generated sound field. Special attention is put on the interpretation of the right-hand side of both wave equations. Comparing the terms on the right-hand side in Lighthill's theory and AWE-PO suggests a source amplitude for AWE-PO that is 90% smaller. This reduction is attributed to the filtering property of the material derivative. Finally, the results of the acoustic far-field pressure are compared. It is shown that the radiated sound field's directivity, propagation, and convection effects are well captured for both wave equations. The computations using Lighthill's equation and AWE-PO are found to provide acoustic intensities within 1.8 dB from the reference direct numerical simulation. This error is comparable with the errors reported for Lighthill's equation in previous studies.

Various industrial applications involve fluid flow in ducted fans. Turbojet engines or air condition systems are just a few examples. The long-term objective is to reduce the associated noise pollution generated by rotor-stator interaction within a small-scale duct. Modal detection strategies help to understand the characteristics of sound radiation of such systems involving fluid flow. In this work, the authors analyze the modal amplitudes generated by a rotor-stator interaction in the presence of a background mean flow. The inverse approach detecting the modal amplitudes is applied using experimentally as well as numerically determined sound pressure values outside the circular duct. The results are discussed and compared against mode amplitudes estimated by an in-duct microphone array whose data is measured simultaneously with that of the far-field one. Some physical interpretations regarding varying amplitudes are discussed. The approach is finally used to assess the impact of homogeneous and heterogeneous stator vanes on the rotor-stator generated noise.

The propagation of shock waves generated by a transonic flow at the tip of a propeller blade is numerically calculated in order to determine the pressure footprint on an aircraft fuselage. An academic case is first proposed to validate the methodology. An incoming signal is built up as oblique harmonic plane waves. The signal is introduced in the computational domain using a Gaussian volume forcing term in the conservation of mass and energy equations. The Euler equations are solved in two dimensions using finite-difference schemes with low dispersion and dissipation. Selective filtering has also been integrated in the algorithm to remove grid-to-grid oscillations. The numerical solution is compared to an analytical solution based on the tailored Green function. An illustration for a realistic open rotor is then considered. The pressure signal near the blade tip, determined from a preliminary RANS simulation, is introduced using a volume source in a solver of the Euler equations.

This study focuses on the analysis of long-exposure-time interferograms obtained with an Interferometric Rayleigh Scattering (IRS) set-up pointing at a high subsonic isothermal jet flow. The objectives are twofold: retrieve from seeding-free optical measurements the mean characteristics of this flow - in particular the mean velocity profile in the jet shear-layer - and determine higher order statistics for the flow velocity in the shear layer. The results are compared to those obtained on the same test bench by use of hot wire anemometry, and conclusions about the relevancy of the approach together with potential improvements are deduced therefrom.

The turbulent mixing noise radiated by a Mach 0.9 jet is investigated. The focus is put on the proper calculation of acoustic propagation effects by means of adjoint Green’s function that are tailored to the jet mean flow. Tam and Auriault’s statistical mixing noise model is recast for Pierce’s wave equation that is energy preserving. An unconditionally stable formulation to compute propagation effects is thus obtained. Adjoint fields are computed from the direct problem with help of the flow reversal theorem. A finite element solver is used to solve tailored adjoint Green’s functions, and corresponding adjoint fields are displayed. Acoustic predictions are carried out for a wide range of polar angles, and compared to measurements. A particular attention is given to predictions achieved at upstream observer angles. At these angles, the present model describes the physics of upstream travelling guided jet waves. The adjoint method provides a suitable framework to split the generation of sound from its propagation. It is illustrated how tailored adjoint Green’s functions filter the radiating part of Tam and Auriault’s sound source model, by weighting with propagation effects.

This work aims to predict the transfer function of a given modal content inside a circular duct with a bellmouth inlet in the presence of a mean flow. The transfer function is the relation in amplitude and phase between a given mode inside the duct and an observer located in the far-field. The numerical solution is obtained by finite element simulation in which the mean flow is input data. Verification is provided by comparison to the analytical solution of an unbaffled circular duct with uniform flow. Influence from various parameters such as the geometry and mean Mach number on the radiated pressure field is investigated. The analytical solution is a good approximation for finding the radiated principal lobe, and the inlet geometry is found to be more important than other parameters such as mean flow when static inlet configuration is studied.

In the context of management of the radioactive waste in deep geological formations, the effect of temperature (20-80 degrees C) on U(VI) adsorption by Callovo-Oxfordian claystone (COx) was studied. A step-by-step approach was followed, starting with the single mineral, illite, followed by an increase in the complexity of the system, through the analysis of the clay fraction and the natural samples of the Callovo-Oxfordian formation. Depending on the study conditions, and the speciation of U(VI) in solution (hydrolysed species, carbonate species and presence of ternary U(VI)-Ca(Mg)-carbonate complexes), the temperature effect was either negligible, or positive (where the increase in temperature favours retention). The most important positive effect was observed for the U(VI)/COx system in the presence of ternary complexes. The data were modelled considering an existing sorption model at 20 degrees C and the thermodynamic data available to describe the evolution of the speciation of U(VI) in solution in function of temperature. The enthalpy values associated with the surface complexes were fitted from the experimental data following a stepwise approach based on the van't Hoff equation.

In the context of the radioactive waste management in deep geological formations, U(VI) retention by intact Callovo-Oxfordian claystone (COx) was studied by percolation-type experiments at 20 and 80 degrees C. The experimental results were confronted with modelling prediction based on a published adsorption model developed from dispersed media in the 20-80 degrees C temperature range. For the experiments at 20 degrees C, the adsorption model allowed to explain the results for the intact system; the retention was weak (R-d similar to 10 L.kg(-1)) and the analysis of the COx phases at the end of the experiment confirmed a retention of U by the clay fraction. The adsorption model in temperature also explained the observed trend of increasing retention with increasing temperature. However, it underestimated the temperature effect on the adsorption of U(VI) by the COx clay fraction, and other phases contributed to the retention. Solid-state analysis of the percolation-doped samples indicated a reactivity in the order pyrite>clay>calcite phases. The transposition of the knowledge at 20 degrees C from the dispersed system to the intact medium was therefore not possible at 80 degrees C for the studied U(VI)/COx system.

Drafting is a strategy in marathon running races that reduces the drag force acting on a designated runner. Drafting involves a formation of athletes, called pacers, running in front of (and sometimes behind) the designated runner. The present experiments evaluated complex formations involving up to seven pacers with the aim of enhancing the performances of an elite marathoner. Wind tunnel measurements with 1/10 scale articulated runner manikins were carried out. First, simple formations with one or two pacers were examined and the results were compared with previous investigations. Second, the runner formations of the Nike Breaking2 and INEOS 1:59 Challenge events whose goal was to break the 2 h marathon barrier were studied. The main finding was the identification of three complex formations of six and seven pacers that allow a drag-force reduction of about 60% with respect to a solo runner and an estimated time saving of 262 s. These results point to how it may be possible to run the fastest marathon ever.

Drag coefficient vs Reynolds number for a smooth and a rough cylinder

Tam and Auriault's statistical mixing noise model has been reformulated by the authors so to be able to compute propagation effects with help of Pierce's wave equation, that is fairly accurate and acoustic preserving. This study presents predictions computed with this model for the sound emitted by two Mach 0.9 round jets. One is isolated, the other one is installed beneath a flat plate. Tailored adjoint Green's functions are computed using the finite element solver Actran TM. The methodology is able to retrieve acoustic measurements within 2 dB for a range of Strouhal numbers of more than two orders of magnitude. For an observer located close to the jet axis, at polar angle θ smaller than θ=50°, poorer predictions are obtained however. It is found that Tam and Auriault's mixing noise model designed for the radiation of the turbulence fine scales does not radiate isotropically but peaks instead around θ=45°. Tailored adjoint Green's functions computed for the isolated and installed configuration demonstrate the ability of the proposed methodology to account for propagation effects due to the flow and the presence of surfaces.