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Unexpeted drop in UDS-value

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Old   May 20, 2020, 10:06
Default Unexpeted drop in UDS-value
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Anne-Sophie
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Hi,
I have a problem regarding the behavior of a UDS in a transient model.
I am modelling air flowing through a potato pile. The pile is modeled as a porous zone (fluid: air, solid: potato). In order to model the moisture content of the potatoes, I defined a user defined scalar (units: kg water/kg potato). Water in the air is modeled via species transport. I implemented source terms for the solid (potato) part in this way that the uds (potato moisture content) will drop in time due to respiration and evaporation. Water diffusivity in the potato is defined as a UDF and implemented in the material-properties for potato as UDS-diffusivity.
After convergence of the flow field (in a steady state run), I did a transient run for the energy and mass equations with adaptive time steps, after patching the initial UDS-value.
After only a couple iterations, the UDS-value drops immensely and unrealistically. I checked the values of the sources, but none of them explains the immense drop in the UDS-value. I tried varying the UDS-diffusivity, but this does not help either.
I would like to ask advice on this problem, since I really donít know what to try next. Does anyone have an idea of what could cause this drop in UDS-value?


Thanks in advance!
Kind regards,
Anne-Sophie

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Old   May 20, 2020, 16:16
Default Uds
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There could be at least two reasons - one is UDS diffusivity and second is boundary conditions at the openings, such as, inlet and outlet. Default conditions might be 0 flux while a more appropriate condition is 0 value. By default, Fluent does not take turbulence diffusion of UDS into account. That means, it can get collected in a small region. So, you should always use a UDF or expression to define diffusivity in a manner that includes turbulent viscosity.
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Old   May 25, 2020, 09:42
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Hi Vinerm,

Thanks for your answer!
I am using a 0 value boundary condition, so that is not the problem, I guess. For the UDS diffusivity I am using an equation from a paper, but I doubt if this equation includes the turbulent viscosity. In the equation UDS diffusivity is expressed as a function of the potato moisture content (=the UDS) and the temperature of the potato. I wrote a UDF to hook this equation as the UDS diffusivity.


What would you advise me to do? Can I include the turbulent viscosity in this equation and in the UDF in some way?

Thank you for your time!
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Anne-Sophie
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Old   May 25, 2020, 11:41
Default UDS Diffusion
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Fluent does not turbulent diffusion of UDS into account. So, if the flow is turbulent, then you must add C_MU_T/Schmidt to the the diffusion coefficient. Schmidt number is usually around 0.85.
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