Turbulence

The nature of turbulent flow is irregular with rapid fluctuations in velocity, density, temperature, and composition. This fluctuating nature makes turbulent flow highly diffusive resulting in enhanced transport of momentum, mass, and energy.

The basic physics of these transport mechanisms is the same as for laminar flow and the motion of both laminar and turbulent flow is described by the classical Navier-Stokes equations. However, in most practical turbulent flows, the task of resolving the turbulent fluctuations would require extremely fine grids and high time resolution. Even for modern computers this would be an impracticable task.

To overcome this obstacle, a turbulence model may be used. In Kameleon FireEx the well-known k-ε turbulence model is employed. The k-ε model is, by far, the most widely used turbulence model in engineering applications. It is thorougly validated and has proven to give good results for a wide range of turbulent flows. In the k-ε model two transport equations are solved, one for the turbulence kinetic energy, k, and one for the dissipation of turbulence kinetic energy, ε.

The effect of turbulence on the fluid flow is taken into account by introducing a turbulent viscosity as a function of k and ε. The variables k and ε are also used to model the effect of turbulence on combustion, cf. the description of the EDC model below.

kfx log in

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