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Hello,

I am trying to model the effect of the presence of blocks in a river on downstream flow.
I have tried several turbulence models : constant viscosity and the k-epsilon model but the results are quite different.
Away from the blocs (represented as cylinders), there are no significant differences in velocities and water heighs but the flow in the wake is really different according to the model used. Indeed, with the "constant viscosity" model, we can observe alternated eddies. On the contrary, with the k-epsilon model, there are no clear eddies and the wake seems less turbulent.
My problem is : I wonder which one of those two models represents better the wake behind an obstacle.
Could anyone advise me?

Thank you

This is actually a very philosophical question. We indeed find von Karman eddies when using a constant and low viscosity. In this case it is something like large eddy simulation in some sense : we see the eddies forming, the initial symmetry of the problem is broken. The k-epsilon model gives Reynolds averaged equations so that in theory we should not see eddies. If we compute a Reynolds number using the turbulent viscosity given by k-epsilon, it may place us in a range that does not give eddies behind a bridge pier.
Another problem is that generally in numerical models the k-epsilon turbulence has not enough time or distance to settle, the turbulence remains what you put as boundary conditions. The last problem is that in free surface flows the mesh size is generally too large to really get correct velocity gradients. So what should we do ? If you are interested in head loss caused by bridge piers, it is better to have a good turbulence model so that the velocity profile is correct, which gives the correct value of velocity upstream the pier (the one that gives the head loss in Hydraulic formulas).

Hope this helps,

JMH

Thanks for your answer.

In fact, what I am interested in are the velocities values behind the obstacle, in the wake. (The objective of my study is to reduce velocities behind the boulders in order to improve fish habitat.) I need to determine if the velocities downstream the block are acceptable, and it seems to be difficult in presence of von Karman eddies.