Learning Forum

[manelaero2020]

Hi everybody ; 

I am simulating airflow in a pipeline with fluent. When I use coupled scheme for velocity-pressure, there is a considerable reverse flow ( order of 90%) at the outlets, and the computation diverges. Still, the problem disappears when I use simplec scheme. 

Have you an idea why coupled scheme causes this instability? is this scheme not suitable for flow in pipelines? 

Thank you a lot 

[Rob]

Without seeing the mesh and set up it's difficult to diagnose. I use both PBCS and SIMPLE in most models and haven't observed problems linked to the scheme that couldn't be explained by something else: most models I see are on support so there's usually something wrong. 

[manelaero2020]

Thank you Rob. 

The computation converged with no problem when I used simplec scheme. I have imposed mass flow inlet BC and pressure outlet for outlets. With the coupled scheme, computation converges when I block artificially reverse flow. I don't have mesh actually to show you, sorry. As a solution to the problem, I have refined mesh near outlets and extended outlets but it doesn't work. The mesh is tetrahedral with inflation layers.  The initialization is standard, computation is steady, flow is incompressible, and a turbulence model is K-Omega SST . All spatial schemes are second order. 

[Rob]

I need to see some pictures. Blocking reverse flow isn't usually recommended, and I wouldn't expect much difference between PBCS and SIMPLEC. Did you use the same initialisation values in both cases? Is the flow compressible? 

[manelaero2020]

Hi @Rob 

Sorry for the late reply :), I don't have mesh actually to show you some pictures. 

The flow is incompressible, I have used ' standard initialization' for both cases ( SIMPLEC AND COUPLED schemes) 

Maybe mesh or geometry is the reason  because I have used coupled scheme for other problems of airflow in pipes 

and solution has converged ( without preventing reverse flow option)

Have a nice day 

[Rob]

It could be several reasons, the solver settings tend not to be the cause: it's usually geometry, mesh or bc's. Without more information there's not a lot we can suggest.