Hi,

I am simulating an axial turbine. I am simulating an axial turbine by using Fluent. The convergence of solution is monitored by checking the residuals of the numerically solved governing equations. Moreover, in order to judge the convergence, the behaviour of other quantities, such as the total pressure at the inlet and outlet boundaries, and torque coefficient generated by the rotor, are also monitored. Here, the default convergence criterion of each residual is reduced in order to allow the monitored quantities to stagnate at consistent values.

Under relaxation factors are default. Solver is ansys Fluent.  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.

Residuals are as follows:

http://upsara.com/images/ak6c_residuals3.jpg

http://upsara.com/images/nlfk_residuals4.jpg

http://upsara.com/images/qif_residuals5.jpg

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):

http://upsara.com/images/8q40_y_momment3.jpg

http://upsara.com/images/zf53_y_momment4.jpg

Inlet and outlet total pressure:

http://upsara.com/images/89os_inlet_and_outlet_pressure3.jpg

http://upsara.com/images/uep0_inlet_and_outlet_pressure4.jpg

Mass flow rate in inlet and outlet:

http://upsara.com/images/5ep1_mass_flow_rate3.jpg

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?

I am grateful that guide me about these cases.

Thanks.