open TELEMAC-MASCARET The mathematically superior suite of solvers

Hello,

I am working on a system which has only one liquid boundary, on which I have an imposed elevation condition, elevation varying through time (tidal system).
The boundary is very simple, a straight line, and I would like to have the velocities normal to this boundary, during both inflow and outflow. Otherwise, I have a large recirculation pattern with an eddy on the boundary and water flowing in and out at the same time.

In the .conlim file, the boundary points have the 5 4 4 description.
In the .liq file, I only have a list of time and varying elevation (SL(1)).

I tried to use VELOCITY PROFILES=1, which is supposed to give normal velocities, but it doesn't give me normal velocities.

In the case of a North-South boundary, using 5 4 0 in the .conlim file does produce West-East velocities normal to the boundary. But in my case, the boundary is rather NW-SE.

I tried to use 5 1 1 in the .conlim file with VELOCITY PROFILES=2 to impose a SW-NE flow but it doesn't work.

I can't use VELOCITY PROFILES=3 since I don't want to impose the values of the velocities. Neither can I use VELOCITY PROFILES=4 since it is only for imposed flowrate.

Is the VELOCITY PROFILES option disconnected when using a time-varying elevation boundary? Or am I trying to impose an impossible flow pattern? If anyone has some insights about this, it would be very helpful!

Thank you very much,

Caroline

Hello,

The velocity profile option will not work because your velocitues are free. Your problem is not well posed if the velocities are entering the domain, this is why you may have strange patterns of recirculations (they are a solution of Saint-Venant equations, as the velocities are free to have any value). The solution is the Thompson boundary conditions (based on the theory of characteristics):

OPTION FOR LIQUID BOUNDARIES : 2 (if you have only 1 liquid boundary, otherwise 2;2;2, etc. or even 1;2;1, or whatever you want, 2 being : Thompson and 1 being : normal.

Be sure to use version 6.1, it now works in parallel, unlike version 6.0.

With best regards,

Jean-Michel Hervouet

Hi,

Thank you for your answer, I have now tried with Thompson boundary conditions but my velocities are still moving in any directions, and mostly parallel to the boundary.

Another question that might be related: should I used the TREATMENT OF LINEAR SYSTEM=2 ? I noticed previously that it helped to stabilize the velocity patterns.

Thank you very much,
best regards,

Caroline

Hello,

This is strange. Look at the test case called 1_solit, a solitary wave entering in a canal, it can now work with Thompson boundary conditions, just by prescribing the elevation, with free velocity. Another possibility would be to add a high head loss on vertical velocity at the entry, but this is rather artificial. A better solution would consist of putting this boundary more upstream, where there is enough depth to get a small velocity. It can generally work, even without Thompson.
The wave equation (TREATMENT OF LINEAR SYSTEM = 2) is currently fully implicit on depth, hence it is more stable. As this is not mandatory, the implicitation will be relaxed in version 6.2 and then the results will be mostly like primitive equations, but it is a much faster option.

With best regards,

Jean-Michel Hervouet