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Hi all !

I am modeling the flow leaving a dam, through the spillway, the dissipation pool and the downstream reach.
I face some stability problems on the spillway, which has an irregular surface (excavated in rock), even though my settings allow a rather small courant number (0,7 max).
The Froude number along the spillway is of 1,5 - 3.

I add that these instabilities does not seem to be induced by my US boundary, where I forced the flow to be subcritical.

My first guess is that the real flow conditions might be more supercritical than what Telemac can compute. Does anyone have ran stable computations for F>3 ?

Thank you in advance for your help!
PL

Hi,

I think that the problem which could generate your instabilities is not only the high Froude number but mainly the steep bathymetry gradient at the spillway location.
You can stabilize the model by using the following options
Discretisation in space= 12;11
Treatment of linear system = 2 (wave equation option)
free surface gradient compatibility = 0.9
Mass lamping for H = 1.
Implicitation for depth = 0.5
implictation for velocity = 0.5

I hope that this helps

Sincerely

Riadh

Hi,

Thank you for your comments.
I have the same settings excepted:
- Discretisation in space= 11;11
- Implicitation h and v= 0.55

I will make a try with your suggestion.

PL

Hi,

I have ran my case with your recommendations, and the results are more stable on the spillway. Thank you for that !
But I still have instabilities downstream, especially by the hydraulic jump, where i have a maximum froude number of ca 4. I also reduced my time step to obtain a maximum courant number of about 0,1 at the hydraulic jump location, which seems to be a satisfying value...

Does anyone have any other tips ?

Thank you in advance,
PL

Hi all !

I have another question regarding this topic: can anyone give some explanations on how does telemac solve hydraulic jumps, especially its location, its length and the energy dissipation ?

Thank you very much in advance !
PL

Hi
you could find all explanations in the Jean-Michel Hervouet book : Hydrodynamics of free surface flows

Regards