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Dear Users,

I have some doubts in terms of choosing right boundary conditions at the open boundary. In case of a tidal simulation I used 566 and velocities are read from the tidal velocity file. But in a case when I do not have a tide but just a wind forcing I am not sure if the same boundary conditions is a right choice. I have tried 544 but unfortunately it gave me very high velocities at the open boundary even without wind force. Also 544 and Thompson produced good velocities but unfortunately very high surface elevations. I have a stratified channel with one open boundary and wind forcing constant in time and space. When I choose 566 at the open boundary the velocities look fine to me. I am just not sure when prescribed velocity is 0 at the open boundary then does it mean that it is like the closed boundary even I can see that water is leaving and coming into domain and at the same time prescribed velocities at the open boundary is 0? It is probably just my misunderstanding and velocity stays as 0 just at the first time step only. I have done some simulations with various wind forcing in stratified closed domain and boundary conditions were clear there. It is when I opened a boundary the velocity problem occured and in some cases when velocity was fine the surface had very high elevations. Please see few pics with 544 boundary where velocity is high and with 566 where it looks fine, but I am not sure if it is right option to use if I do not know what velocity values should be at an open boundary, but not 0.

I can attached my files but it is just a clarification that I need on what exactly 566 does to velocities at an open boundary as in cli file it is set as 0 (UBOR and VBOR the second and third columns have 0 values) but I still have results for mass balance were it is seen that water travels in both ways.
Please advise. Thank you in advance.

Kind Regards!

Violeta

Dear Violeta,

We are currently working on a similar case and find that we are facing an open problem. Thompson's theory is based on the 2-dimensional theory of characteristics and gives depth-averaged velocities which are then imposed as constants over verticals in 3D (option 1), or we consider (option 2) that there is a logarithmic profile and it is tuned to give the correct depth-averaged velocity as prescribed by Thompson. When we have flottability effects, or wind, none of the two options is realistic, e.g. we can have velocities only at the free surface and working on a depth-averaged velocity has no real meaning. This is probably why we experience problems and we try to go back to solutions without Thompson, imposing elevation or velocities depending on the boundary.

With best regards,

Jean-Michel Hervouet

Thank you very much Jean-Michel for you explanations. I am happy to use the boundary conditions 566 without Thompson in this case, as so far it gave me the best results ( velocities are in normal range and surface does not oscillate). My concern is that when I prescribe 566 at an open boundary in boundary conditions file but without entering any specific values for velocities (automatically 0 in cli files) then it does not stop the water flow in and out, so velocities are not 0 then. I am not sure about this. As I wished to have the open boundary there but if velocities are 0 so it is like artificially "closed boundary". Thank you.

Kind Regards!

Violeta

Hello,

I looked at the implementation and actually it should not behave like this, unless you have removed the diffusion step, which applies the Dirichlet values of velocities. Your 5 6 6 should be equivalent to 5 0 0. In the case 566 and if you do not have used the keyword PRESCRIBED VELOCITIES, subroutine bord3d.f does nothing and then subroutinse wave_equation and diff3d and airwik2 will set the requested boundary conditions, unless I miss something. So if you don't find out an explanation I would need your case to study it more carefully.

With best regards,

Jean-Michel Hervouet

Dear Jean-Michel,

Thank you very much for your explanations. I am relieved. In a case when 566 is used, I have not entered PRESCRIBED VELOCITIES into cas file and did not cancel the diffusion steps. So it is fine then. I was just afraid that values from the boundary conditions file dictate 0 velocities even I did not enter it in the cas file. I am delighted that it works well. So the case with the wind forcing on a stratified tank with one open boundary (566 +Thompson or 555+Thompson as a boundary conditions) is working well, I think. The surface elevation and velocities are in normal range for 3 months sumulation run. I probably should not call it a tank experiment, as it has one open boundary.

However, when I look back to the real fjord bathymetry case with wind forcing on it, the surface elevation is getting higher after 3 months run. I have tried boundary cond 544/or566+Thompson, it works fine but at the end of the third month the elevation is nearly 1.8m. I would expect some elevation due to the wind activities depending on its direction but not as high. This was the case with constant wind speed 10m/s. I thought that it is because of the unrealistic physical scenario, but the case for 6 months run with the real wind data gave me high elevation as well, up to 4.5m. In a case when OPTION FOR LIQUID BOUNDARY = 1 ( with 566 or 544 in cli file) gave errors with high velocities and depths at the very first steps. I have attached the plots of surface elevations for a "tank case" for 3 months run and one month result for a case with the real bathymetry. Even when I reused the steering file from the tank experiments to the real fjord bathymetry case, it still gave high elevation as well.
The closed fjord or tank wind cases are fine. I have attached the real fjord bathymetry files. Sorry to be a pain. Thank you very much. I appreciate your help.

Kind Regards!

Violeta

Hello,

At least the case 5 4 4 without Thompson should give correct elevations, but your prescribed elevations, e.g. if they are measurements, should take into account the wind. If you combine measurements without wind (like tide alone) at the boundaries + wind I do not think it can work, because the boundaries will not take into account the effect of wind on the boundary, which may depend on geography outside your domain.

It is probable also that Thompson applied on all elevations at liquid boundaries will give eventually a shift, because nowhere a real elevation is really enforced.

Regards,

JMH

Thank you Jean-Michel for your explanations and patience. The tidal case was clear to me, but I need to work more on wind alone case. I will keep experimenting. Thank you for directions. Sure I will get good results at the end:). I will come back if problem still persists.

Kind Regards!

Violeta