Hi Ijaz Fazil Syed Ahmed Kabir, You should use two user-defined functions.

1) DEFINE_ADJUST

for calculating the average value at the second boundary.

2) DEFINE_PROFILE

for defining the value at the objective boundary.

Hi Ijaz Fazil Syed Ahmed Kabir,

I will try to explain what you need to do in a couple of steps:

1. Use DEFINE_ADJUST(temperature, domain)

2. Loop over all face threads in the domain

3. Identify the THREAD_ID of the boundaries of interest, let's assume here Nr. 5 is the boundary of which the average temperature needs to be calculated, and Nr. 8 is the boundary on which you want to apply the Dirichlet BC.

3. Define an if condition like this: if (THREAD_ID(t) == 5) go to step 4

4. Loop over the faces in the face thread - t

5. While doing the loop use a UDMI to save the values of the temperature like this C_UDMI(c0, t0, 0,1,2) = C_T(c0,t0)[0,1,2] -> here the c0 and t0 are special since these define the adjacent cell to the face of interest and help you to get the temperature of the center of the element just right behind the boundary; 0,1,2 are the x,y,z directions respectively and need to be adapted.

6. Once you have saved all the temperatures you can make another loop to sum these and find the average temperature

7. Use DEFINE_PROFILE and the same if condition for boundary Nr. 8: (THREAD_ID(t) == 8) -> go to step 8

8. Loop over the faces in the thread again and set the temperature of the cells right behind boundary Nr.8 to the value found in step 6 -> C_T(c0, t0) = average from step 6

Some additional remarks are important in the case you need to make your UDF work in parallel, for this simply use #if !RP_HOST and #endif.

One other important point would be the storage of the variables since some of them are not stored to optimize the calculation. If this is the case you have two options:

Option 1: Keep temporary solver memory from being freed with the option /solve/set/expert or

Option 2: Use Alloc_Storage_Vars to allocate the variables, to avoid deleting them.

Best of luck!

Antoni Artinov

You can use DEFINE_PROFILE to define a custom boundary profile that varies as a function of spatial coordinates or time. Some of the variables you can customize at a boundary are:

• velocity, pressure, temperature, turbulence kinetic energy, turbulence dissipation rate
• mass flux
• target mass flow rate as a function of physical flow time
• species mass fraction (species transport)
• volume fraction (multiphase models)
• wall thermal conditions (temperature, heat flux, heat generation rate, heat transfer coefficients, and external emissivity, etc.)
• wall roughness conditions
• wall shear and stress conditions
• porosity
• porous resistance direction vector
• wall adhesion contact angle (VOF multiphase model)

In this link you can find the information you need and more:

https://www.afs.enea.it/project/neptunius/docs/fluent/html/udf/node45.htm