open TELEMAC-MASCARET The mathematically superior suite of solvers

Hi everyone,

I try to understand how is computed the velocity at a boundary where both flow and elevation are prescribed.

Indeed I prescribed both flowrate and (arbitrary but credible) elevation, however the flux value on the boundary does not correspond to the prescribed one. I noticed that the flux may vary when I change elevation or flowrate but I'm not able to force a given flowrate at the boundary.

For a better understanding of my probleme, here is what I try to achieve :

I want simulate a dyke overflow to get water level on the downstream area. The linear flowrate has already been calculated and I have to use this result, i.e. I have to set 50 l/s/m at my boundary (which is the top of the dyke crest).

The elevation is not uniform along the dyke so I cannot prescribe a fixed elevation to get my 50 l/s/m (and I don't want to because I need to simulate a scenario with a uniform 50 l/s/m along the whole dyke crest, even it's not physicaly true).

How can I get 50 l/s/m on my boundary ? And what is the purpose of the (H,Q) boundary if the flowrate (and the velocity) depend on the given H... ?

Thanks a lot for your precious help,

PS: I disabled the free surface gradient compatility, it seems to help a little bit but I would prefere to let it activated

Hi again, I found that disabling continuity correction seems to solve the problem... but I'll be glad to get your explanations !

Hi again,

I am a little bit confused about analysing the flowrate at my boundary :
- My boundary condition is H,Q = 6.50m, 14.25 m³/s
- The graphical printout give me the correct flowrate : FLUX BOUNDARY 1: 14.24971 M3/S
- The control section that I have defined close to my boundary condition gives a variable flowrate between 100 and 0 m³/s (and a volume 4 times higher than expected)
- The integrated value of scalar flowrate along a line close to the cross section gives a flowrate between 70 à 0 m³/s.

All this values are very different. How to assess correctly what flowrate/volume is entering in my model ?

What is the value given by "FLUX BOUNDARY" in the graphical output ?

Thanks in advance,

Thanks in advance

Hello,

Depending on what type of flow you have, you cannot prescribe flowrate and elevation on the same liquid boundary, in particular if subcritical. In that case you have overconstrained conditions and you cannot solve your problem correctly. That may explain you cannot get what you imagine.

If changing your pair of prescribed elevation and flowrate to just prescribed flowrate (5 5 5 --> 4 5 5) and giving an initial elevation of 6.5 m (as in the enclosed steering file), I can get corret value in the listing file of 14.2 m3/s.

The flowrate computed by TELEMAC-2D is exactly what is written in the listing file with "FLUX BOUNDARY".
Locally, if solving is not good, you may have strange values with other methods to compute it.

Hope this helps,

Chi-Tuan

Thank's a lot, I proceeded that way and it is working fine !

I'm still wondering why it is not possible to prescribe both elevation and height at a boundary, for exemple it would be usefull to model a pressurized pipe outlet as a boundary of one model (velocity can then vary for a given height, depending of roughness and hydraulic head...).

Hello,

You can have a look at references concerning the theory of characteristics curves and how to prescribe the right number of boundary conditions depending on the flow regime.

Chi-Tuan