[ccordeiro]

Good afternoon to all.
First let me be really clear: I'm really a beginner in CFD simulations and in fact never had any lectures on it. I'm writing my Master thesis and I need to perform some CFD simulations regarding the effect of wind in a suspended bridge. The main goal is to look into turbulence and wake eddies.

Details about the model:
- Main domain size = 400x140m
- Bridge deck domain size = 50x15m
- Bridge deck dimensions = 31x4m

- Base size for mesh in main domain = 3m
- Base size for mesh in bridge domain = 1m
- Volumetric control created for overset mech

- Solvers: implicit unsteady, turbulent, constant density, RANS, SST k-omega,

For this initial solution, body motion is defined as stationary. However further I'll need to simulate it with rotational motion.

I choose a time step of 0,1s with 15 iterations. After 24h running the plots looks like the ones attached, but the simulation is not completed yet.

My question is: with these conditions, there are some methods (rule of thumbs) to estimate if my values for time step and iterations make some sense? I'm afraid these long simulations can take too long and after the result will be trash.

Any feedback will be much appreciated.

[ashokac7]

Originally Posted by ccordeiro

Good afternoon to all.
First let me be really clear: I'm really a beginner in CFD simulations and in fact never had any lectures on it. I'm writing my Master thesis and I need to perform some CFD simulations regarding the effect of wind in a suspended bridge. The main goal is to look into turbulence and wake eddies.

Details about the model:
- Main domain size = 400x140m
- Bridge deck domain size = 50x15m
- Bridge deck dimensions = 31x4m

- Base size for mesh in main domain = 3m
- Base size for mesh in bridge domain = 1m
- Volumetric control created for overset mech

- Solvers: implicit unsteady, turbulent, constant density, RANS, SST k-omega,

For this initial solution, body motion is defined as stationary. However further I'll need to simulate it with rotational motion.

I choose a time step of 0,1s with 15 iterations. After 24h running the plots looks like the ones attached, but the simulation is not completed yet.

My question is: with these conditions, there are some methods (rule of thumbs) to estimate if my values for time step and iterations make some sense? I'm afraid these long simulations can take too long and after the result will be trash.

Any feedback will be much appreciated.

Don't know why you are using polyhedral mesh. For Overset mesh behavior, CCM recommends trimmer, isn't it? Please do make sure. The mesh count will be lot less with trimmer. Again for complex flow behavior, polyhedral performs better than trimmer.

The domain size you are dealing with quite large. So Plot Courant number and try to keep it near 1. Time step size though depend on type of problem, I would recommend using a script for time step such as for some initial hundred odd steps it is small then gradually increase the time step. Like $Time<0.1?0.001: ($Time<0.02?0.01:0.1).

The residual for continuity or momentum should fall by order of 3 or 4 (10^-3 or 10^-4) during each time step. If it does you can increase your time step. Otherwise reduce time step or increase inner iterations. The residual should tend to converge at end of time step which looks better in snapshots you have provided. Hope this helps!!

[ccordeiro]

Thanks for the feedback ashokac7.

Most of the parameters I took from another similar thesis which my supervisor gave me with the remarks the guys made a really good job. I guess in this case we can assume the flow behaviour is complex. Maybe in the screenshots is not visible but I've modelled also the railings since they're relatively close to the deck and the flow has a lot of disturbance there.

Can you formulate a bit more in detail how I can add a script for time step? Or point me out in the right direction.

Seems to me the iterations are quite ok. Especially after 5-6 iterations the values seem to converge. Do you agree or should I try to increase it?

Again, thanks a lot for the valuable inputs.

[ashokac7]

Originally Posted by ccordeiro

Thanks for the feedback ashokac7.

Most of the parameters I took from another similar thesis which my supervisor gave me with the remarks the guys made a really good job. I guess in this case we can assume the flow behavior is complex. Maybe in the screenshots is not visible but I've modeled also the railings since they're relatively close to the deck and the flow has a lot of disturbance there.

Can you formulate a bit more in detail how I can add a script for time step? Or point me out in the right direction.

Seems to me the iterations are quite ok. Especially after 5-6 iterations the values seem to converge. Do you agree or should I try to increase it?

Again, thanks a lot for the valuable inputs.

For the script of time step

$Time<0.1 ? 0.001 : 0.01

This means for time less than 0.1 sec, time step is 0.001 sec (100 time steps with 0.001 s time step) otherwise 0.01 sec (After that it is 0.01 sec). You can add bracket as I have mentioned in previous comment for more gradual increase.

Residuals looks okay to me. Some more details about the problem is needed for further comment though.