open TELEMAC-MASCARET The mathematically superior suite of solvers

Hi guys,

I had some problems with my modeled bridges in a telemac2d simulation and so I decided to investigate my problem in detail, based on the provided telemac example (v632 -> exmaples/telemac2d/bridge).
I did only two changes for the input files: Steady state flow right from the beginning with 50 m3/s and increasing the "Hau" value from the file "fo1_bridge.txt" to 15m. This should always provide a free surface flow and the tubes never switch to a pressurized flow. I expected a free surface which is almost parallel to the bottom slope (normal flow conditions) but it is way off this behavior. As you can see in the provided images there is a ~41cm drop of the water surface at the location of the bridge. So, I am little confused if this example really shows the right way how to model a bridge in telemac2d. How, for example, did you determine the width of the tubes (each 2.5 m in the example) whereas we have a river bed width of more than 50 m?
Probably my french is just too bad to understand the input files of the example:) but I would really appreciate it if you have some hints for me to understand/solve this problem.

Doeby

PS: This is the link to my previous post:
www.opentelemac.org/index.php/kunena/16-...-ascii-file-ok#14654

Hello Doeby

There are lots of points to clarify here:
- the width of the tube is given by the value of Lrg in the ascii file, so if you want to introduce N tubes of Lrg=2.5m, you will have a total width (theoretically) of 2.5xN. But this is not the biggest issue here.
- in your example, you are expecting to retrieve a nice parallel free surface (as for normal flow) but this is simply not possible. In deed, first of all, the bridge is numerically a discontinuity in the mesh (island) and it is not possible to recover the true (physical) flow through it. We are using (in Telemac) a weir law to compute the flow through the tube (which, I agree, could be improved, since it is a semi-emprical law). the main idea is that with this modelization we can not retreive th "true physical" flow.
- In your previous post, you sent the graph of the drop of the free surface across the bridge, but this is unfair because you should consider the total head which includes the free surface and velocity and which will show you the head loss in the tube. These head losses are also responsible for the change in the flow behavior while they are not considered (at least in the same way) for free surface flow.
- The computed flowrate accross the tube can be considered in two ways depending on the value of the keyword TYPE of SOURCE. The default value (1, linear function) is not mass conservative but more stable. If you want to use 2 (Dirac function), this will be more mass conservative and could raise slightly the downstream free surface.
- You can have a glance on subroutine buse.f (/sources/telemac2d) to understand how tubes are modelized. You will also remark that even velocity is modified (see also subroutine prosou.f) in a way that can't give the same value as a "normal" free surface flow.
- the empirical bridge modelization (like the one implemented in Telemac) can only give global behaving of the flow. By the way, their use is recommanded only for cases where mesh size is bigger than the size of the bridge. If you are interested by an accurate LOCAL flow in the vicinity of the bridge, see next point
- the best way for me to consider physically local flow close to the bridge is to really represent bridge peers in the mesh and let the water flowing normally between them. This will cause severe refinement of the mesh in this area, but if you are running in parallel, this will not be so prohibitive. Then, you can add tubes in case of overtopping. You can calibrate the head loss induced by loaded tube afterward. But at least, your model will perform nicely for non overtopping scenarios.

I hope that this helps
with my best regards.
Riadh

Hi Riadh,

thanks for your answer. The reason why I am so fussy with these bridges is because I have to redo a simulation which was previously done by a commercial software (they just use the normal river bed mesh if no overtopping occurs). I will consider your solution and try to get a better result.


I really appreciate your help and the work done so far by the telemac-team!

Doeby

It is our pleasure !
We are happy to give you satisfaction.
Good luck
Riadh