[P_Anu9]

I am trying to solve porous media non-equilibrium condition using two energy model in FLUENT. For that i need the values of both solid and fluid from previous time step but when I searched in udf manual there is no mention of using two temperatures together in a code.
It just creates two different solid and fluid zone and solves separately for each zone.
How can i access temperature of solid zone from previous time step for calculation in current time step in fluid zone and vice-versa?

The coupling term in both the solid and fluid energy equation is :
G(Ts-Tf) for fluid and G(Tf-Ts ) for solid. which needs to be given through source term but i am only able to give only single temperature as C_T(c,t) and other as constant. how can i give both as variable(or at least feed value from other zone and previous time step)?
Can anyone help?

[Martin_Sz]

You're right, the FLUENT UDF manual might not explicitly mention using two temperatures together in a single User Defined Function (UDF). However, there are ways to achieve the non-equilibrium heat transfer with a two-energy model using UDFs. Here's how you can approach it:

**1. Define UDFs for Source Terms:**

Create separate UDFs for the source terms in both the solid and fluid energy equations. In these UDFs, you can access the following information:

* **Cell-centered Variables:** You can use `C_CENTROID(c,t)` to get the current time step values for various variables at the cell centroid, including fluid temperature (`C_T(c,t)`) and other relevant properties.
* **Custom User Defined Memory (C_UDM):** This allows you to store and access data from previous time steps. You can define separate UDFMs for solid and fluid temperatures from the previous time step (e.g., `C_UDM(c,T_SOLID_PREV)` and `C_UDM(c,T_FLUID_PREV)`) and update them within the UDFs.

**2. Calculation Steps within UDFs:**

Within your UDFs, follow these steps:

* **Calculate Coupling Term:** Calculate the heat transfer term `G(Ts-Tf)` for the fluid source term and `G(Tf-Ts)` for the solid source term. You can access the current fluid temperature `C_T(c,t)` and store the previously saved solid temperature from UDM (`C_UDM(c,T_SOLID_PREV)`) for the fluid source term calculation. Similarly, use the current solid temperature from UDM (`C_UDM(c,T_FLUID_PREV)`) for the fluid temperature in the solid source term calculation.
* **Update UDM:** After calculating the source term, update the UDM for the current time step's solid and fluid temperatures:
* `C_UDM(c,T_SOLID_CURRENT) = C_T(c,t)` (for fluid UDF)
* `C_UDM(c,T_FLUID_CURRENT) = C_T(c,t)` (for solid UDF)

**3. Linking UDFs in FLUENT:**

In FLUENT, define source terms in the energy equations for both solid and fluid phases. Link the corresponding UDFs you created (one for the fluid source term and another for the solid source term) to these source terms.

**4. Initialization:**

For the first time step, you might need to provide initial values for the UDM of both solid and fluid temperatures. You can set them to the initial temperature condition or any other appropriate value.

**Additional Notes:**

* Make sure your UDFs are compiled correctly and loaded into FLUENT.
* Double-check the UDF syntax and logic to ensure proper calculation and update of the UDM variables.
* Consider using DEFINE_PROFILE for defining the heat transfer coefficient `G` if it's a function of other variables.

By implementing these steps, you should be able to access and utilize both solid and fluid temperatures from the previous time step within your UDFs for calculating the source terms in the two-energy model for non-equilibrium heat transfer in porous media.