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-   -   UDS - Convective Flux (https://www.cfd-online.com/Forums/fluent/32264-uds-convective-flux.html)

 Victor October 6, 2003 05:28

UDS - Convective Flux

Hello!

I´m involved in a CFD-project which is to be solved with the help of UDS. Since I´m a novice when it comes to UDF- and UDS-programming I have started with very simple and easy problems in order to get a hang of it. The problem is that I immediately ran into a problem and it concerns the convective flux-term in the transport equation.

The problem setup is a simple duct flow where I want to simulate the scalar as temperature and then compare the result with the correct one given by FLUENT by activating the energy equation. The boundary conditions are known velocities at the inlet and known temperatures at the inlet and at the walls.

When I activate the UDS-panel I make sure to select mass flow rate in the Flux Function list but when I examine the results from the simulation Scalar-0 displays a temperature field that is equal to the correct result if a diffusive problem was considered i.e. no velocities in the domain!

I would be most grateful for any comments or suggestions on the matter!

/Victor

 Greg Perkins October 7, 2003 16:55

Re: UDS - Convective Flux

if you define you scalar as temperature then you will need to modify the treatment of the convective flux uds. Why?

well FLUENT assumes the flux is rho*u*scalar but for the energy equation the flux must be rho*u*(enthalpy) = rho*u*(cp*T) for a simple constant specific heat situation and I've neglected kinetic energy etc.

So you need to change the treatment of the convective flux that's why you're only seeing the influence of the diffusive component since cp is about 1000 - ie your convective flux is too small by a factor of around 1000 or so.

Greg

 Victor October 8, 2003 07:26

Re: UDS - Convective Flux

Thanks Greg!

The results look so much better now!

/Victor

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