1,721,125 research outputs found
Reacting Flows and the Interaction between Turbulence and Chemistry
No full textThis contribution summarizes the state of the art concerning the simulation with computational fluid dynamics of reacting flows with a particular focus on the interaction between turbulent mixing and chemistry. When dealing with nonisothermal gas-phase reactions (e.g., combustion) and rapid reactions (e.g., acid-base) in the liquid phase, the calculation of the chemical source term becomes challenging. The different computational methods proposed in the last 3 decades to deal with this problem are discussed and their application in the context of direct numerical simulation, Reynolds-averaged Navier-Stokes equation, and large-eddy simulation approaches are illustrated
Effect of the conditional scalar dissipation rate in the conditional moment closure
Full text linkIn the context of modeling turbulent scalar mixing using probability density function (PDF) methods, the treatment of molecular mixing is of paramount importance. The conditional moment closure (CMC) offers a high-fidelity description for molecular mixing in nonpremixed flows. Recent work has demonstrated that first-order CMC can be implemented numerically using the moments of the conditioning variable and first-order joint moments of the scalar of interest. When solving the CMC using, for example, quadrature-based moment methods (QBMM), a functional form must be chosen for the conditional scalar dissipation rate (CSDR) of the conditioning variable. In prior work, the CSDR was chosen to produce a β-PDF for the conditioning variable (mixture fraction) at steady state. This choice has the advantage that the system of moment equations used in QBMM-CMC can be written in closed form. In this work, an alternative choice for the CSDR is investigated, namely, the amplitude mapping closure (AMC). With the AMC, the moment equations can be closed using the quadrature method of moments incorporated into a realizable ordinary differential equation solver. Results are compared with the β-CSDR closure for binary, passive scalar mixing in homogeneous single- and disperse-phase turbulent flows. It is also demonstrated that the moment formulation of CMC provides a straightforward method for modeling the effect of differential diffusion in the context of CMC.This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Fox, Rodney O. "Effect of the conditional scalar dissipation rate in the conditional moment closure." Physics of Fluids 32, no. 11 (2020): 115118 and may be found at DOI: 10.1063/5.0030092. Posted with permission.</p
Multivariate Quadrature-Based Moments Methods for turbulent polydisperse gas-liquid systems
No full textThe Conditional Quadrature Method of Moments (CQMOM) and the Direct Quadrature Method of Moments (DQMOM) are compared with Direct Simulation Monte Carlo (DSMC) for the description of gas bubble coalescence, breakage and mass transfer with the surrounding continuous liquid phase. CQMOM and DQMOM are both moment methods based on the idea of overcoming the closure problem by using a quadrature approximation. The methods are compared and performances evaluated for spatially homogeneous and inhomogeneous systems. Eventually CQMOM and DQMOM are implemented in a commercial CFD code to simulate a realistic two-dimensional bubble column. Particular attention is paid to the impossibility of conserving moments with DQMOM in the presence of numerical diffusion. To cure this problem a fully-conservative DQMOM formulation is presented and tested. The relationship
between the two methods is investigated, showing that under particular conditions CQMOM is identical to DQMOM. The different methods are employed under a number of different conditions including very fast chemical reactions, in order to highlight if the problem of bubble coalescence, breakage and mass transfer really needs a bivariate population balance to be tackled and what is the optimal number of nodes for the quadrature approximation
The particle–fluid–particle pressure tensor for ideal-fluid–particle flow
Full text linkStarting from the coupled Boltzmann–Enskog (BE) kinetic equations for a two-particle system consisting of hard spheres, a hyperbolic two-fluid model for binary, hard-sphere mixtures was derived in Fox (2019, J. Fluid Mech. 877, 282). In addition to spatial transport, the BE kinetic equations account for particle–particle collisions, using an elastic hard-sphere collision model, and the Archimedes (buoyancy) force due to spatial gradients of the pressure in each phase, as well as other forces involving spatial gradients. The ideal-fluid–particle limit of this model is found by letting one of the particle diameters go to zero while the other remains finite. The resulting two-fluid model has closed terms for the spatial fluxes and momentum exchange due to the excluded volume occupied by the particles, e.g. a momentum-exchange term Ffp that depends on gradients of the fluid density ρf, fluid velocity uf and fluid pressure pf. In Zhang et al. (2006, Phy. Rev. Lett. 97, 048301), the corresponding unclosed momentum-exchange term depends on the divergence of an unknown particle–fluid–particle (pfp) stress (or pressure) tensor. Here, it is shown that the pfp-pressure tensor pfp can be found in closed form from the expression for Ffp derived in Fox (2019, J. Fluid Mech. 877, 282). Remarkably, using this expression for pfp ensures that the two-fluid model for ideal-fluid–particle flow is well posed for all fluid-to-particle material-density ratios Z=ρf/ρp.
.This article is publihsed as Fox, Rodney O. "The particle–fluid–particle pressure tensor for ideal-fluid–particle flow." Journal of Fluid Mechanics 1010 (2025): A8.
doi: https://doi.org/10.1017/jfm.2025.333
A Kinetic-Based Model for High-Speed, Monodisperse, Fluid–Particle Flows
Full text linkThree-dimensional (3-D) hyperbolic conservation equations for fully compressible, monodisperse, fluid–particle flows with added mass and fluid-phase pseudoturbulence are proposed. A particle-phase kinetic model is developed that accounts for collisional and frictional terms, as well as added mass and internal energy. Transport equations for 3-D velocity moments up to second order (or total kinetic energy) are closed using a Maxwellian distribution. The resulting two-fluid model is well posed for any fluid–particle material density ratio. The numerical methods associated with the hyperbolic system of equations are designed to fulfill the main features of a compressible two-phase flow solver: capturing sharp particle fronts, preserving contact discontinuities, and ensuring stability in all flow regimes. This is done by employing a combination of an AUSM+up scheme for the particle phase, and a HLLC scheme for the fluid phase. Stability is obtained by keeping the discrete consistency between spatial fluxes and buoyancy-like terms implying derivatives. Test cases involving a high-speed fluid interacting with heavy/light particles are used to demonstrate that the qualitative behavior of the flow dynamics is captured correctly by the model.This is a preprint from Boniou, Victor and Fox, Rodney O. and Laurent, Frédérique, A Kinetic-Based Model for High-Speed, Monodisperse, Fluid–Particle Flows.
doi: http://dx.doi.org/10.2139/ssrn.4388742
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Modeling of nanoparticles precipitation in a Confined Impinging Jets Reactor by means of Computational Fluid Dynamics
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A Lagrangian probability-density-function model for turbulent particle-laden channel flow in the dense regime
Full text linkModeling particle-laden turbulent flows at high volume fractions requires accounting for the coupling between phases. The latter is often a sensitive point, and proper closure of the exchange and production terms due to the presence of particles is not straightforward. In the present work, a Lagrangian probability-density-function model developed for homogeneous cluster-induced turbulence is extended to a channel flow. The key features are consistent two-way coupling and the decomposition of the particle velocity into spatially correlated and uncorrelated components, which is crucial for dense flows and which allows dealing with collisions from a statistical point of view. A numerical scheme for the coupled solution of the stochastic differential equations for the particles and a Reynolds-stress model for the fluid is developed. Tests with tracer particles without two-way coupling are done to assess the validity and the consistency of the numerical scheme. Finally, two sets of numerical simulations with particles with different diameters in a turbulent channel flow at a shear Reynolds ofRe tau = 300 are reported. The effect of two-way coupling by varying the mass loading of the dispersed phase in the mass-loading rangephi = 0-2 is analyzed, and the results are compared to previous Eulerian-Lagrangian and Eulerian-Eulerian direct-numerical simulation (DNS) studies. Mean velocities and turbulent kinetic energy show good agreement with DNS, especially regarding the trend with respect to mass loading. Consistent with prior work, increased mass loading causes a drastic reduction of turbulent kinetic energy in the rangephi = 0-2
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