Why is there reverse flow?
 

Hi,

I am simulating a axial turbine and number of cell are about 3000000.

Under relaxation factors are as follows:

Pressure: 0.1

momentum:0.1

Turbulent kinetic energy: 0.5

Turbulent dissipation rate: 0.5


Turbulence model is K-E relizable. The convergence of solution is monitored by checking the residuals of the numerically solved governing equations. Moreover, in order to residual is reduced in order to allow the monitored quantities to stagnate at consistent values. It seems that convergence is ok but there is reverse flow!! why?

residuals :

http://upsara.com/images/h7yl_residuals.jpg


torque coefficient:

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

total pressure at the inlet and outlet boundaries:

http://upsara.com/images/m5ym_inlet_...t_pressure.jpg

Mass flow rate:


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

I am grateful that guide me about this case.

I usually got the reverse flow

(1) when the grid quality is not so good,
(2) when the outlet is not so far downstream,
(3) when using coupled method for velocity-pressure coupling

Sometimes, I just got it for few iterations and later, it stops. When it doesn't stop until many iterations, I regenerated the grid again.

Thanks for your answer.

I think that grid quality is good.

Simple method is used for pressure-velocity coupling.

Destance of outlet is 8 times length of cord. I obtained this destance in papers.

How can i understand that mesh is good or not?

I saw you got reverse flow on many faces. You should check your grid again. You can check the skewness, orthogonality for mesh quality.

Mesh is structure completely. And also, maximum of skewness is 0.8.

Is this value 0.8 good? Mesh is generated in Gambit.

Now when I see messages of Fluent, there is the reverse flow on pressure outlet(outlet boundary).