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Time step size changes drag coefficient?

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Old   August 27, 2020, 08:45
Default Time step size changes drag coefficient?
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Hello everyone,

im simulating an Open-Channel problem and im trying to get Drag values of bridge inside that open channel. The simulations works and im getting results for the Drag coefficient. The global courant number of the problem is under 1 with a time step size of 0.005 s and the calculated drag-coefficient is about 0.9.

But when I change the time-step-size to 0.0025 s the drag-coefficient jumps up to about 1.1 and will stay there.

When i change the time-step-size to 0.00005 s the drag-coefficient jumps up to about 30.

When i look at the calculated drag forces the same thing will happen, so i guess its not a problem of reference values or something, but a problem inside the simulation. But i cant figure out what causes the problem, since the courant number is below 1 in every of the simulations.

Can anyone help me with this problem or does anyone at least have an idea what causes this behaviour of the simulation.

Thank you very much
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