Hello Vinayak Gautam ,

You should be able to modify the value of the Cs constant using a... DEFINE_PROFILE UDF. Nevertheless, if it works, you will have to check that it will still give physical values...

Philippe Béard Thank you so much for the reply sir. It means a lot. Will give it a try.

To bypass or customize the roughness constant (Cs) in ANSYS Fluent, you can use a User-Defined Function (UDF). Fluent's standard wall roughness model imposes a limit on the roughness constant, but a UDF allows you to implement a custom roughness model, overriding the default behavior.

Here's how you can proceed:

Step-by-Step Process to Implement a UDF for Wall Roughness in ANSYS Fluent

1. **Write the UDF Code**:

The UDF will define a custom roughness function that calculates the roughness effects based on your specific model or requirements. Below is an example UDF code to modify the roughness constant for specific wall zones.

```c

#include "udf.h"

/* Example UDF to override the roughness constant */

DEFINE_PROFILE(custom_roughness, t, i)

{

face_t f;

real roughness_constant = 0.5; /* Set your desired roughness constant */

begin_f_loop(f,t)

{

/* Set the roughness constant (Cs) to a custom value */

F_PROFILE(f, t, i) = roughness_constant;

}

end_f_loop(f,t)

}

```

- `DEFINE_PROFILE` is used here to apply a profile for the roughness constant to the wall boundary.

- `t` represents the thread corresponding to the boundary where the UDF is applied.

- `F_PROFILE` sets the value of the roughness constant for each face on the boundary.

2. **Compile or Interpret the UDF**:

After writing the UDF, you need to compile or interpret it in Fluent. If the UDF involves complex functions or external libraries, it's better to compile the UDF; otherwise, you can interpret it.

- **Interpreting the UDF**:

1. Open Fluent.

2. Navigate to `Define > User-Defined > Functions > Interpreted`.

3. Load your UDF source file.

- **Compiling the UDF**:

1. Navigate to `Define > User-Defined > Functions > Compiled`.

2. Add your UDF source file.

3. Click `Build` to compile the UDF.

3. **Apply the UDF to Wall Zones**:

Once the UDF is interpreted or compiled successfully:

- Go to `Boundary Conditions`.

- Select the wall boundary where you want to apply the custom roughness.

- Set the UDF as the profile for the roughness constant.

4. **Run the Simulation**:

After applying the UDF, you can run the simulation. Fluent will now use the custom-defined roughness constant instead of the built-in limit.

Customizing the UDF

You can modify the UDF to:

- Make the roughness constant depend on other flow parameters (e.g., Reynolds number, wall distance).

- Apply different roughness constants to different walls or regions.

Here’s an advanced example where the roughness constant varies based on the wall shear stress:

```c

#include "udf.h"

DEFINE_PROFILE(dynamic_roughness, t, i)

{

face_t f;

real shear_stress;

real roughness_constant;

begin_f_loop(f,t)

{

shear_stress = F_WALL_SHEAR(f,t);

/* Custom formula for roughness constant based on shear stress */

roughness_constant = 0.5 + 0.01 * shear_stress;

/* Set the roughness constant */

F_PROFILE(f, t, i) = roughness_constant;

}

end_f_loop(f,t)

}

```

This UDF dynamically adjusts the roughness constant based on wall shear stress.

By using UDFs in ANSYS Fluent, you can bypass the default limitations on the roughness constant (Cs) and apply a custom model for wall roughness.

Gopal Sharma Thank you so much sir for your comprehensive reply. I deeply appreciate it _/\_