# How to determing convergence in a CFD model [closed]

I am simulating an axial turbine. Under relaxation factors are default.

Residuals are as follows:   And also, mass flow rate, torque (in direction of Y) and inlet and outlet total pressure are as follows(in order to judge on convergence):

Torque (in direction of Y):  Inlet and outlet total pressure:  Mass flow rate in inlet and outlet: Now, I have some question about Residuals:

As you see in first figure, residuals decrease until about iteration of 1500 and then after iteration of 1500, residuals increase. I want to know that this event is logical or not. I already thought that residuals should decrease decidedly because of convergence. But in this simulation, residuals increases and decreases!! What is reason for this event? According above figures, Is solution converged?

• In short: yes, this kind of thing can happen. But without knowing more about your solver, the parameters you chose, or the problem you are solving, it's impossible to diagnose why it is happening. There are a plethora of possible explanations. The most important question you have to ask is: do you know what the answer is supposed to look like, based on empirical observation or analytical solutions?
– Paul
Nov 4 '15 at 22:02
• Thanks. First, my English language is not good. Solver is ansys Fluent. and problem is flow crossing an axial turbine. this turbine is named Wells turbine. Solution is steady. The computation was carried out with changing the rotational speed of the rotor from a high speed to a low speed under a constant axial velocity. I didn't understood following your statement: " There are a plethora of possible explanations. The most important question you have to ask is: do you know what the answer is supposed to look like, based on empirical observation or analytical solutions? " Nov 4 '15 at 22:24