Turbulent flow computational fluid dynamics

The Prandtl mixing length concept is useful for shear flows parallel to walls, but is inadequate for more general three-dimensional flows. A more complicated semiempirical model commonly used in numerical computations, and found in most commercial software for computational fluid dynamics (CFD see the following subsection), is the A — model described by Launder and Spaulding (Lectures in Mathematical Models of Turbulence, Academic, London, 1972). In this model the eddy viscosity is assumed proportional to the ratio /cVe. 

Particle trajectories can be calculated by utilizing the modern CFD (computational fluid dynamics) methods. In these calculations, the flow field is determined with numerical means, and particle motion is modeled by combining a deterministic component with a stochastic component caused by the air turbulence. This technique is probably an effective means for solving particle collection in complicated cleaning systems. Computers and computational techniques are being developed at a fast pace, and one can expect that practical computer programs for solving particle collection in electrostatic precipitators will become available in the future. 

Computational fluid dynamics (CFD) is the numerical analysis of systems involving transport processes and solution by computer simulation. An early application of CFD (FLUENT) to predict flow within cooling crystallizers was made by Brown and Boysan (1987). Elementary equations that describe the conservation of mass, momentum and energy for fluid flow or heat transfer are solved for a number of sub regions of the flow field (Versteeg and Malalase-kera, 1995). Various commercial concerns provide ready-to-use CFD codes to perform this task and usually offer a choice of solution methods, model equations (for example turbulence models of turbulent flow) and visualization tools, as reviewed by Zauner (1999) below. 

Chemical engineers, however, have to find practical ways for dealing with turbulent flows in flow devices of complex geometry. It is their job to exploit practical tools and find practical solutions, as spatial variations in turbulence properties usually are highly relevant to the operations carried out in their process equipment. Very often, the effects of turbulent fluctuations and their spatial variations on these operations are even crucial. The classical toolbox of chemical engineers falls short in dealing with these fluctuations and its effects. Computational Fluid Dynamics (CFD) techniques offer a promising alternative approach. 

Kollmann, W. (1990). The PDF approach to turbulent flow. Theoretical and Computational Fluid Dynamics 1, 249-285. 

In the recent years, the advance of computer power has allowed numerical solutions for the differential equations that describe fluid motion. The use of computational fluid dynamics (CFD) is beginning to give a better understanding of the strongly swirling turbulent flow inside hydrocyclones and, consequently, of their performance [46-50]. 

Computational fluid dynamics were used to describe the flow which undergoes a fast transition from laminar (at the fluid outlets) to turbulent (in the large mixing chamber) [41]. Using the commercial tool FLUENT, the following different turbulence models were applied a ke model, an RNC-ki model and a Reynolds-stress model. For the last model, each stream is solved by a separate equation for the two first models, two-equation models are applied. To have the simulations at... 

The interaction of dispersing clouds with vapor fences is a complex physical process. When a flow meets an obstruction, turbulence levels are increased downstream because of vorticities introduced into the flow field, and increased velocity gradients are induced by flow momentum losses. Detailed modeling of such a process is very difficult and requires a combination of small-scale experiments and computational fluid dynamics. 

P. Moin. Large eddy simulation of multi-phase turbulent flows in realistic combustors. Prog. Comput. Fluid Dynamics, 4 237-240, 2004. 

Simonin, O., Modelling turbulent reactive dispersed two-phase flows in industrial equipments. Proc. Third World Conf. Applied Computational Fluid Dynamics, May 19-23, Freiburg, Germany, Workshop E, 17.9,1996. 

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