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Hi,

I´m simulating energy extraction from a channel and comparing my results to an analytical model. In general there is good agreement except in the area where energy extraction occurs. There I get some fluctuations in the velocities at the entrance and exit of the "farm". I´ve checked my Courant numbers and they are all below 0.5.

I´ve attached a couple of figures with the results.

Any help will be really appreciated.

Apologies, I failed to include more details of the example.

I've used the DRAGFO subroutine to include energy extraction. The farm is located at the centre of the channel, from X coordinates 4900 to 5100m, and occupies the whole section of the channel. Mesh is refined in the area where extraction occurs.

Thanks,

Alberto

Hello,

This could be due to features of the local mesh, but looking at your subroutine dragfo would be also useful.

Regards,

JMH

Hi Jean Michel,

Thanks for your answer. Sure, I can provide you the dragfo subroutine used. I would prefer not posting it in the forum, can I send it to your email if that is ok?

Thanks,

Alberto

Hello,

Sorry for the late answer, yes you can send it on my E-mail.

JMH

Hello Alberto,

I think I have a hint explaining oscillations. The headloss is a function of the square of the velocity, but in theory this velocity is taken upstream in an undisturbed zone. As we use here as an approximation the local velocity, assuming that it is close to the upstream velocity, which may be not your case. To be quite correct we should thus take the velocity upstream but then we would have an explicit formula, which is to be avoided. However in dragfo.f you can take an upstream point for computing UNORM. We did not do that as a standard coding because it is very case dependent, and in parallel it would also require a combination of the functions global_to_local_point and p_dmax, but well it would be closer to the real theory and would give a more physical meaning the the drag coefficients.

With best regards,

Jean-Michel Hervouet

Hi Jean-Michel,

I appreciate that you looked into this and answered in such short period of time. When you say that we should use the upstream velocity, do you mean upstream from where energy extraction occurs? And how far? Or if our area of energy extraction contain several elements in the direction of flow, you mean that we should use the velocity of one, two or several elements upstream which might be still located within the tidal farm?

Thanks for your time!

Alberto

Hello,

Yes, in theory the drag force on an obstacle is computed with a velocity taken "infinitely" upwind, assuming that the flow is constant everywhere, which is not the case in practical cases. So we need a velocity upwind, possibly not perturbed by the turbine, but not too far because the flow is not constant everywhere. For our goal let's say that 3 elements is OK, we just do not want the fluctuations due to energy extraction to have an influence on this velocity (which may trigger oscillations as we suspect).

Regards,

JMH