open TELEMAC-MASCARET The mathematically superior suite of solvers

Hello everybody,
Because I'm not familiar with TELEMAC-2D I try to understand the effects of the different Boundary conditions. Therefore I created a very simple mesh for a straight canal. Now i wanted to compare the impacts of different boundary conditions on the results around the boundaries itself.
Therefore I created a stage-discharge file which shall ensure that 3900 m³/s flows out of the domain, no matter how high the free surface elevation is.
When I now start the simulation the outflow at the outflow-boundary in the first 30 Minutes is way less than the desired 3900 m³/s and starts then to cenverge to this value. Thus the domain will be filled with water during that time. This is an effect which I want to avoid.
Does anyboday have an idea what I did wrong or how I can avoid this effect?

The stage-discharge file is attached.

Thanks for your help.

Cheers Sarah

Hi Sarah,

you can use your result as initial conditions (restart).

If you start your simulation with resting water in the canal it will take some time to reach a steady state condition. If you use results from a previous computation (moving water, etc.) as initial condition you can earlier reach the steady state. However, if you change your boundary conditions it will always take some time to get to the new steady state.

Hope this helps.

Best regards

Leo

Okay, but when I use the results of my first computation then the free surface elevation is on a level much higher than the 91.8, which I wanted to have as initial elevation due to the filling of the domain.

Cheers
Sarah

I don't understand your model setup. You have a canal with an inflow and outflow boundary?

Usually you give the discharge at the inflow boundary and set the water depth at the outflow boundary.

Best regards,

Leo

Right. A straight canal with an inflow of 3900 m³/s and an outflow. I computed the system with a prescribed discharged of -3900m³/s at the outflow and now i want to try what is the effect if I use a stage discharge curve instead at the outflow boundary which also says that 3900 m³/s flows out of the domain without any regards of the free surface elevation. As initial elevation I use 91.8 m ASL.
Is it obligatory to prescribe an elevation in the system. Can't I use the stage-discharge curve downstream?

Cheers Sarah

Hi
I think there is a misunderstanding.
Even if you choose a stage discharge curve in your boundary, this is the water level which is imposed according to the local discharge and not the opposite!

hope this helps

Okay, I see. But isn't it possible to hold the discharge constant with a stage-discharge-curve in any way?

This has no sense!
If you want a constant discharge, you could impose this discharge has a negative value on the boundary.
I don't understand the reason of using a stage discharge curve to have a fix discharge...

hope this helps

okay. thank you!

Hello,

A constant discharge is simply a matter of giving a keyword:

PRESCRIBED DISCHARGES : ..., with as many numbers as liquid boundaries, separated by a semicolumn. For example if the discharge at the entrance is 100 m3/s and the entrance is boundary 2, you have :

PRESCRIBED DISCHARGES : 0.;100.

+100. is an entering discharge, -100. is an exiting discharge.

the figures given for liquid boundaries which are not of type 4 5 5 (given discharge and 4 is here free depth) are not considered (like the 0. in my example).

If you want to have a stage-discharge curve, with elevation depending on the exiting discharge, it is better to set up first a steady state flow, and then to do a restart from this steady state.

With best regards,

Jean-Michel Hervouet