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Hello,

I have some problems of running a simple flume test. In my case, the inlet boundary is Free H, prescribed UV and outlet is prescribed H. Instead of giving a prescribed UV at inlet points, I modified 'bor3d.f' in oder to get a correct velocity vertical profile as experiment. UBORL is calculated by a log equation.
In the result, the velocity vertical profile was correct. However the water surface at inlet increased a lot which is not my expectation. Actually, I want inlet surface keeping constant elevation during the whole simulation. I attached a short video of my result and my code. Do I miss something in programming?



Best Regards
Yue

Sorry for the failure of uploading the viedo.

I attached some pictures instead.

Hello,
I think this is a normal behaviour that you have such a water surface displacement since you prescribe an inflow to a water body initially at rest.
The pictures indicate that you didn't reach a steady state, so the water level at the inlet will probably descrease further with additional simulation time.

Then it would be a matter of calibration to adjust the roughness or turbluence model in order to match the measured water level at the inlet.

However you can also fix the water level at the inlet by prescribing a constant water depth or water level but then you will probably overconstrain the system, in your case of a subcritical flow regime.

By the way, nice pictures!

Best regards,
Clemens

Thank you very much, Clemens. I aslo checked 3-hour result but it still showed unsteady flow. Due to very small time step, it is really painful to make simulation longer.

Actually, I have tried to fix the water level at inlet but you are right, system was over constrained. Result was worse.

Anyway, I need to find a good way to avoid shock wave going through the domain.

Thank you again for the reply.

Cheers
Yue

Hello,

You can prescribe boundary conditions that start like the initial conditions and then linearly evolve with time for some time, before reaching the value that you want, this to avoid violent unsteady flow at the beginning.

Another question is why do you need to have a very small time-step, did you try a larger one ?

With best regards,

Jean-Michel Hervouet

Hello, Jean-Michel

Thank you very much for your suggestion. I will definitely try it.

I am doing some simulations for turbulence model and sediment transport, the mesh of the test domain is very fine therefore I have very small time-step.

Best Regards
Yue

Hi Yue,

you can take a large time-step to get your initial state and make a restart with a fine time-step afterwards.

Best Regards

Leo