# Diffusion control

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 February 17, 2002, 13:25 Diffusion control #1 Julie Guest   Posts: n/a Hi! Who knows how can I control the diffusion processes in the problem (e.g. I need to rise the diffusion rate)? Regards, Julie

 February 18, 2002, 03:36 Re: Diffusion control #2 stephen Guest   Posts: n/a Increase the molecular viscosity? Increase the turbulence levels (turbulent viscosity)? Run on a tetrahedral grid (artificial viscosity)? Solve using Upwind Differencing (numerical viscosity)? Why would you want to do that? Stephen

 February 18, 2002, 04:36 Re: to Stephen #3 Julie Guest   Posts: n/a I have some questions about your advises: 1) You tell me to increase the turbulence levels. There are some problems: (a) the flow is laminar (Re=974) and (b) if even I'll use a turbulent model, how can I set turbulence level that differs from the calculated one? 2) I am using UD scheme. What's its effect comparing to MARS? The reason I want to increase the diffusion rate is that I deal with simulation of counterflow jet. The flow is laminar and I can't obtain the steady combustion process. If the residual is small enough, than the obtained temperature field is very close to the initial one. Regards, Julie

 February 18, 2002, 06:28 Re: to Stephen #4 stephen Guest   Posts: n/a 1a) Generally more turbulence leads to an increased turbulent viscosity and therefore more diffusion. 1b) You define turbulence properties at inlet (or pressure) boundaries. By increasing the value of k and decreasing the value of epsilon you increase the value of the turbulent viscosity = CMU*RHO*K*K/EPSILON 2) The UD scheme is first order, MARS is formally second order. First order schemes are generally more diffusive (they increase the effective viscosity of the solution) and are therefore less accurate. I never use UD differencing for U V and W for this reason.

 February 19, 2002, 08:54 Re: to Stephen #5 Julie Polyakh Guest   Posts: n/a Hi! Would you, please, give me some reference literature on this topic? Thank you, Julie

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