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-   -   Steady Laminar Backwards Facing Step (https://www.cfd-online.com/Forums/main/199942-steady-laminar-backwards-facing-step.html)

IbizaFR March 20, 2018 03:46
Steady Laminar Backwards Facing Step
 
Hello!

I am writing code to solve 2D Steady Laminar Backwards Facing Step problem for Reynolds number up to 400 through finite differences with dimensionless 2D Navier-Stokes equations. The boundary conditions are the following:

Inlet: Parabolic profile for u, v=0, pressure calculated by the pseudo-compressibility continuity equation.

Walls: u=v=0, pressure calculated by the pseudo-compressibility continuity equation.

Outlet: θu/θx=0, θv/θx=0, p=0.

I am running the code but after the step the flow has an upwards direction where it would be expected going downwards with recirculation area.

Any help?

Thank you very much.

FMDenaro March 20, 2018 03:59
Quote:
Originally Posted by IbizaFR (Post 685826)
Hello!

I am writing Matlab code to solve 2D Steady Laminar Backwards Facing Step problem for Reynolds number up to 400 through finite differences with dimensionless 2D Navier-Stokes equations. The boundary conditions are the following:

Inlet: Parabolic profile for u, v=0, pressure calculated by the pseudo-compressibility continuity equation.

Walls: u=v=0, pressure calculated by the pseudo-compressibility continuity equation.

Outlet: θu/θx=0, θv/θx=0, p=0.

I am running the code but after the step the flow has an upwards direction where it would be expected going downwards with recirculation area.

Any help?

Thank you very much.

I suggest to debug the code to find some bugs... have a look to the pressure equation that must have suitable Neumann BC.s

IbizaFR March 20, 2018 04:03
Thank you FMDenaro.

Are the boundary conditions that I am using correct except for the pressure boundary conditions?

FMDenaro March 20, 2018 04:06
Quote:
Originally Posted by IbizaFR (Post 685830)
Thank you FMDenaro.

Are the boundary conditions that I am using correct except for the pressure boundary conditions?

I suggest not to use the constant pressure at the exit but the condition derived from the fact that you supposed du/dx=0 in the continuity equation evaluated at the exit. You will also easily see that you get v=0 along the outflow.

IbizaFR March 20, 2018 04:17
Thank you. I will check it.

FMDenaro March 20, 2018 04:47
Are you using a second order central discretization on staggered grid?

IbizaFR March 20, 2018 04:53
I am working with finite differences and in some cases with second order central discretization and in another cases with forward or backward discretization.

FMDenaro March 20, 2018 05:21
what about the colocation of the variables on the grid?

IbizaFR March 20, 2018 06:40
Quote:
Originally Posted by FMDenaro (Post 685848)
what about the colocation of the variables on the grid?
I am using finite differences with the same arrangement for all variables u,v,p.

selig5576 March 21, 2018 00:12
Bfs
 
In your pressure solver are you sure your treating the outlet boundary correctly? For a collocated grid and using the classical Rhie-Chow method your PPE will have \frac{\partial^{2} P}{\partial x^{2}} = 0. at the outlet.

IbizaFR March 22, 2018 15:09
Quote:
Originally Posted by IbizaFR (Post 685826)
Hello!

I am writing code to solve 2D Steady Laminar Backwards Facing Step problem for Reynolds number up to 400 through finite differences with dimensionless 2D Navier-Stokes equations. The boundary conditions are the following:

Inlet: Parabolic profile for u, v=0, pressure calculated by the pseudo-compressibility continuity equation.

Walls: u=v=0, pressure calculated by the pseudo-compressibility continuity equation.

Outlet: θu/θx=0, θv/θx=0, p=0.

I am running the code but after the step the flow has an upwards direction where it would be expected going downwards with recirculation area.

Any help?

Thank you very much.
Hello!

I wrote SIMPLE algorithm for the same problem but the velocity field is not the expected.
Is there any advice about the correct boundary conditions of the problem?

Thank you.


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