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As an example for pipe flows, with the Reynolds number based on the pipe diameter: =. Here l is the turbulence or eddy length scale, given below, and c μ is a k – ε model parameter whose value is typically given as 0.09;
The laminar finite rate model computes the chemical source terms using the Arrhenius expressions and ignores turbulence fluctuations. This model provides with the exact solution for laminar flames but gives inaccurate solution for turbulent flames, in which turbulence highly affects the chemistry reaction rates, due to highly non-linear Arrhenius chemical kinetics.
The characteristic which is responsible for the enhanced mixing and increased rates of mass, momentum and energy transports in a flow is called "diffusivity". [16] Turbulent diffusion is usually described by a turbulent diffusion coefficient. This turbulent diffusion coefficient is defined in a phenomenological sense, by analogy with the ...
Turbulent diffusion is the transport of mass, heat, or momentum within a system due to random and chaotic time dependent motions. [1] It occurs when turbulent fluid systems reach critical conditions in response to shear flow , which results from a combination of steep concentration gradients, density gradients, and high velocities.
where ε is the average rate of dissipation of turbulence kinetic energy per unit mass, and; ν is the kinematic viscosity of the fluid.; Typical values of the Kolmogorov length scale, for atmospheric motion in which the large eddies have length scales on the order of kilometers, range from 0.1 to 10 millimeters; for smaller flows such as in laboratory systems, η may be much smaller.
One class of models, closely related to the concept of turbulent viscosity, are the k-epsilon turbulence models, based upon coupled transport equations for the turbulent energy density (similar to the turbulent pressure, i.e. the trace of the Reynolds stress) and the turbulent dissipation rate .
Reynolds Stress equation models rely on the Reynolds Stress Transport equation. The equation for the transport of kinematic Reynolds stress = ′ ′ = / is [3] = + + + Rate of change of + Transport of by convection = Transport of by diffusion + Rate of production of + Transport of due to turbulent pressure-strain interactions + Transport of due to rotation + Rate of dissipation of .
In fluid dynamics, eddy diffusion, eddy dispersion, or turbulent diffusion is a process by which fluid substances mix together due to eddy motion. These eddies can vary widely in size, from subtropical ocean gyres down to the small Kolmogorov microscales , and occur as a result of turbulence (or turbulent flow).