## Fluids

#### What is the minimum value of Under Relaxation factor?

• Anadi_Mondal
Subscriber
Hello I am trying to simulate annular flow using EWF and DPM. For a momentum under a relaxation factor of 0.0001, the residual of the continuity equation is in order of e-5. Can you tell me whether using a momentum factor of 0.0001 is logical or not? I notice a sharp decrease of residual after decreasing momentum factor. This is why I reduced the momentum factor to 0.0001 to reduce continuity residual. Can I trust this simulated result?
Or, is there any restriction on reducing under relaxation factor?
Regards Anadi

• Kishan Konannavar
Ansys Employee
Hello
I have not come across that low a value of URF. Even though there seems to be no low limit as such ( 32.3.3. Setting Under-Relaxation Factors (ansys.com) ), but if you are going that low (which does not much sense), then maybe you would want to check other solver settings if they are correctly specified.
Thank you
Regards
Kishan
• Anadi_Mondal
Subscriber

Thank you for your reply!
I read the 'Setting Under-Relaxation Factors' paragraph from the Ansys user guide again. Here it's written that ' it's good practice to start calculation using default under-relaxation factors and ONLY need to change when residual begin to increase'
Moreover, it's also written that ' For most flows, the default under-relaxation factors don't require modification. If divergent or unstable behavior is observed, it's allowed to reduce pressure, momentum,k, and epsilon to 0.2,0.5,0.5,0.5 respectively. Should I be strict with these values as the lowest relaxation factors?
According to Ansys's user manual explanation, I should decrease the relaxation factor ONLY when divergence is noticed. It's a transient case and I am using the PISO solver. If I use the default relaxation factor(pressure, momentum,k, and epsilon are 0.3,0.7,0.7,0.7 respectively), the residual of the continuity equation is steady at around 5.7E-3. Then the ONLY way to reduce the continuity residual is to decrease the momentum relaxation factor or increase the pressure relaxation factor. If the pressure relaxation factor is 0.9 and the momentum relaxation factor is 0.05, the continuity residual ranges between 1E-4 and 7.7E-5. Here my solution is not diverging. But as the residual is the order of E-3 only with default relaxation factors, I am reducing momentum relaxation factors to decrease the residual further. Is there any problem with reducing the momentum relaxation factor to 0.05 to decrease continuity residual?
Regards Anadi

• Anadi_Mondal
Subscriber

Thank you for your reply!
I just read the 'setting Under-Relaxation Factors' paragraph from the Ansys user guide again. Here it's written that ' it's a good practice to start calculation using default under-relaxation factors and ONLY need to change when residual begin to increase'
Moreover, it's also written that ' For most flows, the default under-relaxation factors don't require modification. If divergent or unstable behavior is observed, I can reduce pressure, momentum, k, and epsilon to 0.2,0.5,0.5,0.5 respectively'. Should I be strict with these values as the lowest under-relaxation factors?
According to Ansys's explanation, I should decrease the relaxation factor ONLY when divergence is noticed. It's a transient case and I am using a PISO solver. If I use the default relaxation factors (pressure, momentum, k, and epsilon are 0.3,0.7,0.7,0.7 respectively), the residual of the continuity equation is steady at around 5.7E-3. Then the ONLY way to reduce the continuity residual is to increase the pressure relaxation factor or decrease the momentum relaxation factor. If the pressure relaxation factor is 0.9 and the momentum relaxation factor is 0.05, the continuity residual ranges between 1E-4 to 7.7E-5. Here my solution is not DIVERGING but residual is the only order of E-3, therefore I am reducing momentum relaxation factors to decrease residual further. Is there any problem with reducing the momentum relaxation factor to 0.05 to decrease continuity residual?
Regards Anadi
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