I need to use a User Defined Scalar in turbulent flow. However I could not find where I could define turbulent diffusion coefficient for UDS as we usually did, for example, TKE Prandtl number.

I have thought an idea to define diffusion coefficient in the Properties of Materials. But I guessed it was for physical properties instead of properties related to turbulence.

Thanks a lot for your help!

I think using DEFINE_DIFFUSIVITY

If your idea is correct, transport of User Defined Scalar in turbulent flow is not same treated as other scalars such as temperature.

Because the transport of User Defined Scalar in turbulent flow is actually treated in laminar flow, I have to use DEFINE_DIFFUSIVITY to define turbulent diffusion coefficient for UDS by myself.

Is my understanding right?

Yes, thats right. Although turbulent diffusion tends to overwhelm laminar diffusion so you can neglect it in high Re flows.

Fluent uses the follwing turbulent difusion coefficients

Species transport model (includes laminar+turbulent diffusion)

Diff_coeff=D_lam*C_R(c,t)+C_MU_T(c,t)/0.7 with D_lam = laminar coefficient

Non premixed combustion (full turbulent)

Diff_coeff=C_MU_T(c,t)/0.85

RoM

[pramodnarayan]

Hi ROM,

I know this is an old post, but I have a question on UDS transport due to diffusion. Since you have been active in answering questions about species, Please help me on this.

I have a fully turbulent flow coming into a tank filled with water and air. I have modeled it as a Multiphase flow using VOF approach. I using UDS to find the Residence time distribution curve. Since there is buoyancy effect, I could not use species transport as it was not including the effect of buoyancy in the simulation.

My question is, How is turbulent diffusion included in the UDS equation? Since mine is a fully turbulent flow( Re ~10e5), do I have to define a UDF for defining the turbulent diffusivity. The manual and the theory guide does not talk about this clearly.

Thank you
Pramod