A. Tanière

Simulation of a particle-laden turbulent channel flow using an improved stochastic Lagrangian model

The purpose of this paper is to examine the Lagrangian stochastic modeling of the fluid velocity seen by inertial particles in a nonhomogeneous turbulent flow. A new Langevin-type model, compatible with the transport equation of the drift velocity in the limits of low and high particle inertia, is derived. It is also shown that some previously proposed stochastic models are not compatible with this transport equation in the limit of high particle inertia. The drift and diffusion parameters of these stochastic differential equations are then estimated using direct numerical simulation (DNS) data. It is observed that, contrary to the conventional modeling, they are highly space dependent and anisotropic. To investigate the performance of the present stochastic model, a comparison is made with DNS data as well as with two different stochastic models. A good prediction of the first and second order statistical moments of the particle and fluid seen velocities is obtained with the three models considered. Even...

Study on Langevin model parameters of velocity in turbulent shear flows

This paper deals with the stochastic equation used to predict the fluctuating velocity of a fluid particle in a nonhomogeneous turbulent flow, in the frame of probability density function (PDF) approaches. It is shown that a Langevin-type equation is appropriate provided its parameters (drift and diffusion matrices) are suitably specified. By following the approach proposed in the literature for homogeneous turbulent shear flows, these parameters have been identified using data from direct numerical simulations (DNS) of both channel and pipe flows. Using statistics extracted from the computation of the channel flow, it is shown that the drift matrix of the stochastic differential equation can reasonably be assumed to be diagonal but not spherical. This behavior of the drift coefficients is confirmed by the available results for a turbulent pipe flow at low Reynolds number. Concerning the diffusion matrix, it is found that this matrix is anisotropic for low Reynolds number flows, a property which has been ...

Prolate spheroidal particles’ behavior in a vertical wall-bounded turbulent flow

Direct numerical simulations (DNSs) have been performed to examine the inertia, shape, and gravity field effects on the dynamics of ellipsoidal particles within a vertical turbulent channel flow. To investigate the effects induced by the particle inertia and shape, computations have been conducted for three aspect ratios and two response times. The influence of gravity has been examined through a comparison with DNS data provided in earlier studies without gravity. The originality of this study is that the prediction of the hydrodynamic force and pitching torque acting on the non-spherical particles has been carried out with recent expressions valid outside the Stokes flow regime. With the data extracted from the DNS, a statistical analysis of the particle spatial distribution, orientation, and translational and angular velocities is carried out. Results show that the presence of a significant mean relative velocity between the dispersed and continuous phases greatly modifies the dynamics of non-spherical...

Evaluation of a Moments-Based Formulation for the Transport and Deposition of Small Inertia Aerosols

This paper introduces and evaluates a formulation for the modeling of transport and wall deposition of aerosols, written in terms of moments of the particle size distribution (PSD). This formulation allows coupling the moment methods with computational fluid dynamics (CFD) to track the space and time evolution of the PSD of an aerosol undergoing transport, deposition and coagulation. It consists in applying the quadrature method of moments (QMOM) to the diffusion-inertia model of Zaichik et al. [6], associated with the dynamic boundary layer (DBL) approach of Simonin [8] for wall deposition. After presenting the QMOM formulation of the transport equation and of the DBL wall function, the paper presents several test cases in which the method is compared to existing experimental and numerical results. It is shown that the moment formulation of the model does not introduce particular bias compared to its concentration-based formulation. This extension of the diffusion-inertia/DBL approach to the QMOM method hence allows modeling with a good numerical efficiency and at building scale the dynamics of aerosols undergoing transport and modification of their PSD through coagulation and deposition.

Influence of the Gravity Field on the Turbulence Seen by Heavy Discrete Particles in an Inhomogeneous Flow

ABSTRACT The motion of heavy discrete particles in a fully developed horizontal channel flow is investigated by means of direct numerical simulation (DNS). The paper explores the influence of the gravity field on the decorrelation time scales of the fluid seen by the discrete particles comparing to those obtained without external forces (Arcen et al., 2004). As expected, the crossing trajectory effect introduced by the presence of gravity induces a decrease of such time scales in all directions. The data extracted from DNS also enable to test the ability of the famous expressions proposed by Csanady (1963) to model the time scales of the fluid seen in an inhomogeneous flow, under conditions which are somewhat far from his theoretical analysis. It is demonstrated that the time scale decrease is about two times more important in the directions perpendicular to the mean relative velocity, a result which is qualitatively conform to Csanady’s analysis although the mean relative velocity and the turbulent intensity are of the same order.

Particulate pollutant source evaluation using an inverse method under steady-state conditions

ABSTRACT This article presents a method that enables the generation rate from one or /more particle sources to be estimated, using far-field concentration measurements. The method is made up of two distinct steps; a calibration phase, followed by an estimation phase. The calibration phase makes it possible to create a transfer relationship between a known source (“reference source”) and the measurement of the far-field concentration. The second step consists of estimating unknown source generation rates by inverting the transfer relationship and using measurements of far-field concentrations resulting from these unknown sources. In addition, this article presents a technique to improve the positioning of the sensors in the room in which the sources are situated. A numerical study using computational fluid dynamics was first conducted to theoretically validate the estimation method and assist with choosing the sensor positions in the experimental rig. The study established that, with ideal sensors, the difference between the real and estimated generation rates can be accurate to within 0.1%. The method was then deployed on an experimental case. The results confirmed that it is possible to estimate an isolated source. However, the quality of the estimation deteriorated when the source to be estimated was significantly different from the reference source, from an aerodynamic perspective.