Drag Coefficient Calculation

Using SimScale
1

Hello everyone!

As you guys probably know, calculating the drag coefficient in SimScale gives you a graph of drag coefficient with respect to time so I was wondering if there was a way to calculate a discrete value for it.

Thanks! :slight_smile:

2

Hey,

Are you talking about transient analysis? Thatā€™s the only case where Cd is given with respect to time.

For steady-state, ā€œtimeā€ doesnā€™t mean time in seconds. It means iterations (simulation is an iterative process).
The only ā€œrealā€ Cd would be the coefficient in a fully converged result set.

For example, in this steady-state analysis, the Cd values in the ellipse have no meaning. The value that really matters is the last one:

image
image1479Ɨ828 49.4 KB

3 Likes
3

Hey @Ricardopg,

Thanks a lot for your response!

Would that mean in steady-state simulations, the last value is the only one that is relevant for other results as well such as velocity and pressure?

4

No problem!

Yes, in steady-state simulations, the relevant results for all parameters are the ones from last iteration (given that theyā€™re converged).

5

Hi, this may seem a bit late, but how do you know they are converged?

6

Perhaps, the following information may help:

CFD simulations use an iterative process to improve a solution until a limit or stable state or ā€œconvergenceā€ is reached. In other words, convergence occurs when a solution no longer varies significantly. For this particular case of determining the drag coefficient, the drag coefficient reaches a stable state at or a limit value of around 0.2 (the black arrow shows this ā€œconvergedā€ value of drag coefficient to be between 0 and 0.5).

This documentation page gives more examples on how to check convergence of a CFD simulation: How to Check Convergence of a CFD Simulation? | SimScale

1 Like
7

Here are two examples for judging steady-state convergence in CFD:

  • ā€œApplying a residual level for each transport equation, below which the equation is deemed to be converged. When all equations satisfy their respective residual controls, the simulation then stopsā€ ā†’ convergence". This means that when your residuals (not just one) dont change significantly (this significantly is a convergence criterion number) over consecutive iterations.

  • ā€œConvergence can also be determined by monitoring any suitable metric, including objective measurements from the simulation, e.g. a force coeļ¬ƒcient, or drag coefficient. When the metric no longer changes signiļ¬cantly over subsequent steps, the simulation is stopped.ā€

Adapted from: https://doc.cfd.direct/notes/cfd-general-principles/steady-state-convergence