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International audience; An iterative technique, taken from the field of optics, is used to obtain tailored Green’s functions suitable for the evaluation, in the nearfield, of pressurefluctuations generated by turbulent flow in the vicinity of solid boundaries. Comparisons are made with the analytical solution for the solid sphere, and with results obtained using conventional boundary element method(BEM) for the case of a thick semi-infinite plate. A divergence issue in the case of the solid sphere is resolved by the introduction of a relaxation factor. The performance of the iterative approach is found to be comparable to that of conventional BEM, except at irregular frequencies, where the bandwidth of the error is slightly larger than that of the conventional BEM. The main advantage of the iterative approach is a significantly reduced computational cost, which allows for higher surface mesh densities and a broader useful frequency range.

The purpose of this paper is to study the effect of the high-voltage power supply waveform and the presence of a dielectric barrier on the collection of submicron particles in a wire-to-cylinder electrostatic precipitator. The experiments are carried out on two reactors (with or without dielectric barrier on the surface of the cylindrical collector electrode) with the same active section and volume. The results show that the highest collection efficiency is obtained with the negative dc corona. At equivalent electric power consumption, the Dielectric Barrier Discharge (DBD) is as effective as the positive dc corona and the ac corona. The efficiency can reach 99% with both studied reactors, if the necessary power is provided.

The aim of the present work has been to quantify the efficiency of Dielectric Barrier Discharges (DBDs) for the collection of submicron particles. The experiments were performed with incense smoke particles having a mean size of about 0.3 mum. An aerosol spectrometer was employed for characterizing the size distribution of these particles at the outlet of Wire-to-Cylinder (WC) and Plate-to-Plate (PP) DBD reactors. The collection efficiency was estimated for various applied voltages (6 - 26 kV, 1 - 2000 Hz), and airflow rates of 1.6 - 24 L/min. The discharge mode was diffuse, in the case of the axisymmetric configuration, and filamentary, in the case of the planar configuration. When flow rate increases, the discharge current and the electric power decrease in both cases. This effect is less pronounced with the planar configuration. Results obtained with the aerosol spectrometer show that the particle collection efficiency of both reactors is higher at high applied voltage and low flow rate; it decreases at high frequency because of particle oscillations, and at low frequency due to the intermittent nature of the discharge. The frequency range for which the collection efficiency is higher than 90% is wider in the case of WC reactor.

This paper concerns the experimental study of the control of a turbulent plane mixing layer by a single array of synthetic (loudspeakers) jets deployed through circular orifices. The displacement of the mixing layer towards the wall partially mimics the flow control scenario of the reattachment of a massively separated flow with the jets positioned after the separation location. The experimental setup is the mixing layer downstream of a splitter plate positioned at a small distance away from a flat plate. PIV observations show that the synthetic jet abruptly destroys the large scale coherent structures. There is a large modification to the turbulence field due to the distortion in the mean velocity field. An overall increase in the mixing of the flow between the flat plate and the mixing layer which cause the deviation of the flow towards the wall surface. Lastly, a parametric study of the synthetic jet trajectory is realized by varying the velocity of the air stream that generates the mixing layer. The ratio of the peak jet velocity to the lower velocity of the mixing layer should not be overly excessive such that the jet penetrates the mixing layer completely.

Laboratory studies and in-field observations have shown that the charge-decay characteristics of the granular materials at the surface of the grounded roll electrode significantly influence the outcome of the electrostatic separation process. This paper validates an indirect method of charge-decay characterization, based on the measurement of the electrical potential at the surface of a monolayer of granular insulating material. The study was performed on three materials - polyvinyl chloride, polyethylene, and rubber - extracted from chopped electric wire wastes. The granules (characteristic size in the range 1-4 mm) were disposed on the surface of a grounded plate electrode (layer area: 100 mm times 100 mm; electrode area: 200 mm times 200 mm). A wire-type corona electrode, energized from a dc high-voltage supply, was employed for charging the granules. The potential due to the charge at the surface of the granular layer was measured with the capacitive probe of an electrostatic voltmeter connected to a personal computer. Data acquisition and processing were done using the LabView environment. The influence of particles characteristics and of ambient factors was studied. The findings enabled a more accurate modeling of discharging phenomena that affect the performances of electrostatic separators. The method can be easily adopted in electrostatic discharge studies for material characterization.

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Colloque avec actes et comité de lecture. Internationale.; International audience; Heated and unheated coaxial jets are computed for a range of realistic physical parameters. The motivation is the investigation of the physical mechanisms responsible of the sound generation and the temperature effects. Comparison of the LES flow characteristics, the aerodynamic forces and the noise radiated are reported.

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This paper presents a new PDF model for turbulent fluid-particles interactions in the near wall region. Such models have already been presented by Swailes & Reeks [1] or more recently by Van Dijk & Swailes [2]. The PDF approach allows calculating directly the spatial evolution of the wall normal particles velocity distribution. As shown by Swailes & Reeks [1] this approach as many advantages. First, all the statistical moments used (concentration, mean velocity, fluctuating velocities) are contained in the PDF. Thus all the equations for those moments are coupled together within the PDF equation. Second, the boundary conditions can be easily specified, even for the most complex cases as partial absorption. The present study proposes to simplify the fluid particle interaction term of the PDF equation with a new closure model. This simplification is inspired by the BGK models. The paper presents first the model and its numerical implementation. It is evaluated in a wall bounded homogenous turbulence as described by Swailes & Reeks [1]. The results of the present model are then compared to those obtained by Swailes & Reeks [1]. The model is also applied to a channel flow boundary layer configuration. Those simulations illustrate the mechanisms responsible for particles dispersion in the boundary layer turbulence.

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At the solid-liquid interface, a charge zone called the electrical double layer (EDL) appears. It is constituted of two zones of opposite sign: one in the solid and another one in the liquid. When a liquid flows through a pipe, an axial streaming current is generated. This current is due to the convection of the charges coming from the EDL. The physicochemical reaction at the solid-liquid interface is one of the most important parameters which control the diffuse layer development inside the liquid and, consequently, the space charge density. In this paper, we present an analysis for the physicochemical reaction in the case of liquid containing additives or impurities partially dissociated into positive and negative ions. The addition of impurities, in our case, is associated with changing both the polarity and the value of the streaming current. Thus, from this difference in streaming current, we will be able to identify the reagent of the oil with the solid material. Furthermore, the effect of streaming potential on the EDL has been undertaken. Moreover, the fully developed space charge density at the wall is calculated with the help of streaming electrification experiments which are conducted in the case of nonfully developed EDL. This procedure can be conducted for the investigation of flow electrification in transformers with oil/metal configurations.

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In this work we propose some model problems (computed by DNS) to test the performance and utility–in terms of the physical insight which is provided–of an energy corollary proposed by Doak 8 and Jenvey. 14 The energy corollary, which is based on Helmholtz decompositions of both linear momentum and velocity, aspires to provide a clearer interpretational framework for studying the local physical mechanisms which underpin the production of sound energy by turbulence. In this paper we compute the response of the two-dimensional Euler equations to a wavepacket body-force excitation whose amplitude and vorticity-level can be controlled, and we assess the resulting flow fields using the said framework. We look at linear and non-linear, irrotational and rotational scenarios, and we present the flux and source terms of the energy corollaries. Initial results show how it is possible to identify the different flow processes (source mechanisms) which lead to the transport of fluctuation energy, as well as the physical mechanisms by which that fluctuation energy is transported away from the source region as propagating sound energy. I. Introduction The question ‘How does turbulent fluid motion generate sound ?’ is only meaningful if we know what we mean by ‘sound’ within the confines of the turbulent field. A sound wave is generally defined as a smallamplitude, energy-conserving, irrotational fluctuation characterised by propagation at the speed of sound; and so the notion of a ‘sound-field’ is, at the heart of a turbulent flow, meaningless. This is probably the single greatest obstacle blocking our efforts to understand the dynamics by which subsonic turbulent jets drive sound waves in the farfield. Rayleigh’s three classes of fluctuating motion—vortical, entropic and acoustic—constitute an unambiguous system of distinct kinds of fluid motion. These are uncoupled in the simplest, uniform, linear flow systems; and they can be shown to interact quadratically in another class of slighthly more complex flows, such that these quadratic interactions constitute further sources of excitation for each of the modes. 5 This classification provides an interpretational framework which is invaluable for the obtention of a mechanistic understanding of the different flow process which underpin energy exchange in compressible turbulent flows. However, the three kinds of motion can not be so easily defined in the general case, and so, in the kinds of turbulent flows which are of practical interest, we struggle to arrive at a phenomenological understanding of the said flow processes. In this work we consider a framework which was considered by Doak 8 and Jenvey 14 to be useful for precisely this: both proposed the Helmholtz decomposition, applied, respectively, to the momentum and velocity variables, as a means by which to distinguish vortical motion (which they considered synonymous with turbulence) from irrotational motion (which they considered synonymous with acoustic or entropic fields). Such decompositions are not readily applicable experimentally, and so they have seen little use in aeroacoustic analysis. In this work we apply the decomposition to a number of model problems (computed by DNS), where we can carefully control the level of complexity: from potential flows with linear (smallamplitude) perturbations to rotational flows were the dynamics become non-linear. In the simplest flows there is no ambiguity regarding the quantities we study and the decompositions we apply–the sound-production problem can here be described analytically; the degree of ambiguity increases with increasing flow complexity. Our objective is to assess the Helmholtz decompositions and the energy corollaries which result

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A perturbation method is used to solve an unsteady one-dimensional heat conduction problem in a cylinder. A simple second order explicit solution is obtained. It is shown that this solution is accurate even for high values of the Biot number in a region surrounding the center of the cylinder.

The use of plasma actuators for airflow control cou ld be restricted to specific atmospheric conditions. Pressure, temperature and relative humi dity could affect the effectiveness of surface plasma discharge by disturbing the plasma regime. In the present study, the effects of humid ambient air are investigated. A typical dielectric barrier discharge actuator is operated in relative humidity (RH) ranging from 40% up to 98%. The current behaviour of the plasma discharge is analyzed and time-averaged and time-resolved measurements of the produced electric wind are proposed. Two plasma discharge modes are observed, regardless of the relative humidity. A brief statistical analysis of the curre nt discharge indicates that the number of negative current peaks is reduced when RH is increased. Positive current peaks also encounter a decrease in number for RH from 40% up to 85%. At higher RH, the number of positive current peaks drastically increases, presumably due to cond ensation over the dielectric material. Nevertheless, the present results demonstrate that DBD actuators can be operated at high relative humidity. Measurements of the induced electric wind clearly indicate a reduction of the momentum transfer effectiveness for increasing RH levels. Indeed, beyond 70% of RH, the electric wind production is weaker but the actuator conserves it capability to transfer momentum at the applied electrical frequency. Moreover, the detrimental effects of high RH on the actuator effectiveness can easily be compensate d by increasing the applied high-voltage.

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Since large eddy simulation methods (LES) are too CPU-demanding for many complex industrial applications, a multitude of unsteady low-cost strategies have gained prominence over the last decade Frohlich and von Terzi (2008); Sagaut et al (2006). Some of these models can be described as seamless hybrid RANS-LES models since the transition between RANS and LES occurs in a continuous manner so that there is no need to define explicit boundaries between RANS and LES regions. In statistically homogeneous flows, such a model can be seen as an LES with a filter width Δ S continuously going to infinity - a limit that corresponds formally to a RANS approach. However, the majority of flows of practical relevance are inhomogeneous, and in that case such models suffer from an important conceptual weakness due to the fact that the LES yields spatially filtered fields and the RANS yields long-time averaged fields.

The trajectories of the insulating particles in roll-type corona-electrostatic separators depend on the configuration of the electrode system, the applied high voltage, the roll speed, the size of the particles and the relative humidity of the ambient air. The aim of this work was to point out how numerical modeling can be of use in the study of the effects of these factors. Particle charging is modeled using the Pauthenier's equation for spheres in uniform electric field. The equation of particle discharging was obtained after an experimental study of the surface potential decay of a granular layer of insulating material in contact with an electrode. The trajectories are computed based on the balance equation of the electrical and mechanical forces that act on such charged particles. The numerical results are in good agreement with the experimental findings.

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In this paper, a parametric study of a single dielectric barrier discharge (SDBD) plasma actuator has been performed in order to enhance its electromechanical performances. Three input parameters have been investigated: the dielectric thickness, the waveform of the applied high voltage and the voltage slew rate. The goal is to increase the velocity of the discharge-induced electric wind and/or to reduce the electrical power consumption of the discharge. First, electrical measurements with high voltages ranging from 5 to 30 kV have been conducted, with a frequency set here to 1 kHz. This consisted of current recordings and electrical power consumption computations. Secondly, the velocity of the produced electric wind has been measured with a Pitot tube probe. From these velocity profiles, the maximum velocity, the produced mass flow rate and the electromechanical efficiency of the discharge have been determined. It highlights that the way to enhance the electromechanical performances of such a surface discharge consists in using a thick dielectric (at least a few millimeters) and a sine high voltage (several tens of kV) at a moderate frequency value.

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International audience; A report is given of the 13th ERCOFTAC SIG 15 Workshop on Refined Turbulence Modelling, which was held at Graz University of Technology on 25th and 26th September, 2008.

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This paper is aimed at characterizing two possible designs of star-connected three-phase capacitors developed for utilization with the protection schemes of industrial installations. A superficial charge simulation program was employed for estimating the capacitances of one design. The results pointed out the influence of various geometrical parameters on the magnitude of the mutual capacitance between the armatures of any two of the three elementary capacitors of the three-phase unit. The experimental results confirmed the theoretical predictions and validated the advantageous features of a second design, which ensured balanced mutual capacitances between the three phases.

A charge zone known as the electrical double layer exists at a solid-liquid interface. The liquid flow induces a phenomenon called flow electrification: it generates a streaming current (caused by charge convection) and a rise in the solid's potential (if it is insulated from the ground). These potentials may reach values high enough to produce electrical discharges and cause accidents. Although this phenomenon was identified a long time ago, its physical description remains unknown (i.e., production and displacement of charges, equilibrium, etc.). We have modeled flow electrification phenomena occurring when transformer oil flows through a rectangular pressboard duct. The results of a parametric study made with this model are presented in this paper. The duct's geometry and the materials were selected to compare some of the numerical results to experimental ones. The facility used to obtain these experimental results was developed some years ago as a part of the research program of Electricite de France and the University of Poitiers. Another facility will be designed in the near future with the aim of reproducing surface electrical discharges. The results of this parametric study will be useful for its design.

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This paper is concerned with the transition of the laminar flow in a duct of square cross-section. Like in the similar case of the pipe flow, the motion is linearly stable for all Reynolds numbers, rendering this flow a suitable candidate for a study of the 'bypass' path to turbulence. It has already been shown \citep{Biau_JFM_2008} that the classical linear optimal perturbation problem, yielding optimal disturbances in the form of longitudinal vortices, fails to provide an 'optimal' path to turbulence, i.e. optimal perturbations do not elicit a significant nonlinear response from the flow. Previous simulations have also indicated that a pair of travelling waves generates immediately, by nonlinear quadratic interactions, an unstable mean flow distortion, responsible for rapid breakdown. By the use of functions quantifying the sensitivity of the motion to deviations in the base flow, the 'optimal' travelling wave associated to its specific defect is found by a variational approach. This optimal solution is then integrated in time and shown to display a qualitative similarity to the so-called 'minimal defect', for the same parameters. Finally, numerical simulations of a 'edge state' are conducted, to identify an unstable solution which mediates laminar-turbulent transition and relate it to results of the optimisation procedure.

Even if the phenomenon of flow electrification has been observed for several decades, the physicochemical process appearing at the solid/liquid interface creating the double layer is not yet totally understood. In particular, returns from experiments (Cabaleiro, 2008) made with oil and pressboard seems to show that the wall current density at the interface for a diffuse layer under development is not only a function of the chemical behaviour of the interface but also of the flow wall shearing stress. The present work concerns analyze of experiments made with heptane flows through a stainless steel capillary of varying length. In that case also, even if the wall material is conductive and not porous, divergences appear with the classical physicochemical model predictions, while a model taking into account the effect of the wall shearing stress on the interfacial process seems to have a much better agreements with the experiments, especially, for high laminar Reynolds numbers.

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