Second-Order Temporal Discretization Except for Turbulence Model Equations

[finzeo]

Hi everyone,

I would like to use, where possible, a second-order temporal discretization for all relevant equations in an incompressible case with the DDES k-omega SST turbulence model. I understand that, in the case of this model, there is a restriction against using second-order time discretization for the k and omega equations. However, I would like to at least enable second-order time discretization for the other equations. I am using version 8.0.

I have tried various ways to achieve this, but I always encounter some failure or a warning indicating that some variables are being treated as first-order. Below is my latest attempt. The following code was added to cs_user_parameters.c:

Code: Select all

cs_time_scheme_t *t_sc = cs_get_glob_time_scheme();
//t_sc->time_order = 2; // it gets error for iturb=60 (k-omega SST)
t_sc->istmpf = 2;
t_sc->isno2t = 1;
t_sc->thetcp = 1/2;
int key_t_ext_id_t = cs_field_key_id("time_extrapolated");
cs_field_set_key_int(cs_field_by_name("density"), key_t_ext_id_t, 1);

I receive the following warning message:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@
@ @@ WARNING: ADVANCED MODIFICATION FOR
@ ========,
@ velocity OF THE VARIABLE
@ THETAV.
@
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@

@
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@
@ @@ WARNING: ADVANCED MODIFICATION FOR
@ ========,
@ k OF THE VARIABLE
@ THETAV.
@
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@

@
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@
@ @@ WARNING: ADVANCED MODIFICATION FOR
@ ========,
@ omega OF THE VARIABLE
@ THETAV.
@
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@

@
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@
@ @@ WARNING : WHEN READING INPUT DATA
@ =========
@ CHOICE OF TIME-SCHEME
@
@ TIME-SCHEME FOR VELOCITY IS SECOND ORDER
@ (THETAV = 0.50E+00)
@ SOME TERMS ARE HOWEVER FIRST ORDER IN TIME WITH
@ THE FOLLOWING SETTINGS:
@
@ parameters ISTMPF ISNO2T ISTO2T IROEXT IVIEXT ICPEXT
@ Values entered 2 1 0 1 0 0
@
@ computation will go on.
@
@ Check the input data.
@
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

I am also attaching the setup.log file.

What is still set as first-order time discretization that I could change to second-order? How do I change it in the code?

Thank you in advance.

[Yvan Fournier]

Hello,

I know the handling of "per variable" time scheme is a complicated mess, with some options that have been available for years but never really tested, and others which may be missing but would make sense.

I am not too well versed in this part of the code, but will ask some colleagues if there is a solution to this, or if you can just ignore those warnings (which, if I understand, do not prevent the computation from running ?).

Best regards,

Yvan

[finzeo]

Yvan,

Yes, with the code I showed, the simulation runs despite the warnings.
This is interesting because using second-order in time allows for larger time steps (under a certain accuracy criterion), thereby reducing the need for extensive cluster resources over time.

[Yvan Fournier]

Hello,

Any detailed feedback on this is very welcome

Best regards,

Yvan

[finzeo]

Update: I managed to prevent the program from aborting when setting time_order=2 by editing the varpos.f90 file. By activating time_order=2, everything implemented for second-order time discretization executes accordingly. Analyzing cs_turbulence_kw.c, I noticed there's no code to run in second order, but at least everything else is running in second order in time and it works.

In fact, I modified the cs_turbulence_kw.c code to incorporate a theta=0.5 in the corresponding expressions, but I did not obtain positive results (it diverges if I do this; if I don't edit it, it does not diverge). My strategy is basically to leave the explicit terms as they are and multiply the implicit terms by a factor of theta=0.5. Do you see anything I might be forgetting? I attach this source file. Any suggestion to advance this is appreciated.

[Yvan Fournier]

Hello,

I asked a colleague, who told me trying to force 2nd order this way is risky and might ne behave well, and that a PhD student might be looking into this soon, but have no more details.

I'll keep you updated if I get more info on what can be done.

Best regards,

Yvan

[finzeo]

Thanks, Yvan.

Similarly, if I manage to make any progress, I'll post it here.