In first step, you need to make sure about your grid indepency. As you know, there is a optimum mesh size and division for each problem. After finding a certain mesh size, it's time to choose a turbulency model based on your problem condition.

Hi,

Could you please explain the problem a bit more, are you using axisymmetrical model ?

Also, what are the other boundary conditions that is need used and what are the outputs that you are monitoring ?

Regards,

Rajesh.

Hi Nikita,

The issue could be many things.

1. Validation: Have you validated the hydrodynamics for a plain pipe to ensure the turbulence model you are using is predicting correct velocities especially near the wall. You can check and compare your velocity profile with analytical results, esp the law of the wall: u+=y+.

2. Grid convergence: As Meisam mentioned you need to make sure your results are grid independent.

3. Convergence: Generally residuals do not give a good indication of the convergence of the solution. As Rajesh mentioned make sure you are monitoring shear stress at wall (for hydrodynamics) or temperature (for heat transfer) when you solve the equations in Fluent.

Once you are sure the above issues are resolved for a plain pipe, then introduce roughness and see the impact. Generally, a pipe is considered rough based on the roughness height and the viscous sub-layer height. Make sure your pipe is indeed rough for a given flow condition.

Hope this helps.

Keep safe!