### Wind tunnel modelling, BCs and convergence issues

Posted:

**Wed Apr 17, 2019 8:56 am**Hello,

I am working on a wind tunnel for which I set some Inlet, Outlet, Wall and Symmetry BCs. To avoid modelling the fan, I would like to assign an Inlet BC with a negative volumic flow rate to my outlet (I am not sure it is the more relevant solution).

Besides, I have assigned the same value of volumic flow rate at the inlet because my real system is closed, it seems to me a good way to preserve continuity.

After starting steady calculation, I have an error that I can't figure out.

I don't know if the problem comes from the BC definition or something else so I tried another modelling where I changed the geometry to have both Inlet and Outlet almost at the same location (Outer annular duct, see attached for more info). That way allows me to assign the same velocity (less physically representative of the fan but better than nothing) to each BC (from 1 to 10 m/s).

This solution worked and seems to converge after thousands of iterations but the residuals are way too high !

For the two calculations, I followed the advice from Antech:

viewtopic.php?f=2&t=2445&p=13435&hilit= ... nce#p13435

- SIMPLEC algorithm

- Least squares method over partial extended cell neighbourhood

- Reference time-step = 1e-3 or 1e-4 s; Max CFL = 5; Max Fourier = 10;

Min time-step factor = 1e-3; Max time-step factor = 100; time-step variation = 0.1

- First computation with Upwind convective scheme for all variables, Restart SOLU with 0.8

blend for Velocity and Upwind for other variables, Restart SOLU for Velocity and Upwind for

other variables

1st questions: Can you please tell me what is wrong with my first modelling? Is this a BC problem or something else?

2nd questions: Is there any way to obtain lower values of residuals? Maybe a thinner mesh (combined with a Low Reynolds model?)? Can you give me (if a general definition exists) a limit range not to exceed to be sure that the solution is accurate enough?

For now I use Code_Saturne 5.0.9 on Windows10 x64, only with the GUI.

The final purpose will be to add an obstacle of more or less complex geometry.

I hope I am clear enough, please find attached details of the two computations (listing, mesh, xml, etc...). Please tell me if you need other file/data/information to understand my issue.

Thank you.

Best regards,

Luc

I am working on a wind tunnel for which I set some Inlet, Outlet, Wall and Symmetry BCs. To avoid modelling the fan, I would like to assign an Inlet BC with a negative volumic flow rate to my outlet (I am not sure it is the more relevant solution).

Besides, I have assigned the same value of volumic flow rate at the inlet because my real system is closed, it seems to me a good way to preserve continuity.

After starting steady calculation, I have an error that I can't figure out.

I don't know if the problem comes from the BC definition or something else so I tried another modelling where I changed the geometry to have both Inlet and Outlet almost at the same location (Outer annular duct, see attached for more info). That way allows me to assign the same velocity (less physically representative of the fan but better than nothing) to each BC (from 1 to 10 m/s).

This solution worked and seems to converge after thousands of iterations but the residuals are way too high !

For the two calculations, I followed the advice from Antech:

viewtopic.php?f=2&t=2445&p=13435&hilit= ... nce#p13435

- SIMPLEC algorithm

- Least squares method over partial extended cell neighbourhood

- Reference time-step = 1e-3 or 1e-4 s; Max CFL = 5; Max Fourier = 10;

Min time-step factor = 1e-3; Max time-step factor = 100; time-step variation = 0.1

- First computation with Upwind convective scheme for all variables, Restart SOLU with 0.8

blend for Velocity and Upwind for other variables, Restart SOLU for Velocity and Upwind for

other variables

1st questions: Can you please tell me what is wrong with my first modelling? Is this a BC problem or something else?

2nd questions: Is there any way to obtain lower values of residuals? Maybe a thinner mesh (combined with a Low Reynolds model?)? Can you give me (if a general definition exists) a limit range not to exceed to be sure that the solution is accurate enough?

For now I use Code_Saturne 5.0.9 on Windows10 x64, only with the GUI.

The final purpose will be to add an obstacle of more or less complex geometry.

I hope I am clear enough, please find attached details of the two computations (listing, mesh, xml, etc...). Please tell me if you need other file/data/information to understand my issue.

Thank you.

Best regards,

Luc