[fstra]

Hi,

I am having the following issue for a while, hope somebody here can help.

I am using Ansys 16.2, and employing open-channel-flow-BC to simulate the generation of a periodic wave at the inlet. The computational domain is 30m long and water depth is 5 m.
The simulation is correct for the case of large waves (such as H=2 m and L=12m), with dt=0.005s and refined mesh at the interface with size of 0.25 m.
However, when trying to simulate smaller wave, I get waves with a strong decay effect after less than 1 s of simulation. This would happen for: H=0.2m, L=3m, dt=0.001s and reduced mesh sized of 0.005 m.
In both cases, turbulence model was set to relizable k-eps.
It is worth noting that such a behavior does not happen when viscosity effects are neglected. Thus, it can probably be a problem related to the turbulence model.

If any of you could help indicating the most suitable turbulence model I would really appreciate.

Thanks in advance.

[FMDenaro]

Originally Posted by fstra

Hi,

I am having the following issue for a while, hope somebody here can help.

I am using Ansys 16.2, and employing open-channel-flow-BC to simulate the generation of a periodic wave at the inlet. The computational domain is 30m long and water depth is 5 m.
The simulation is correct for the case of large waves (such as H=2 m and L=12m), with dt=0.005s and refined mesh at the interface with size of 0.25 m.
However, when trying to simulate smaller wave, I get waves with a strong decay effect after less than 1 s of simulation. This would happen for: H=0.2m, L=3m, dt=0.001s and reduced mesh sized of 0.005 m.
In both cases, turbulence model was set to relizable k-eps.
It is worth noting that such a behavior does not happen when viscosity effects are neglected. Thus, it can probably be a problem related to the turbulence model.

If any of you could help indicating the most suitable turbulence model I would really appreciate.

Thanks in advance.

I am not sure to understand the details of your flow problem...but if you need to simulate the details of the onset of an unsteady phenomenon you definitely cannot use RANS/URANS ...
I can suggest to see if you have enough computational power to do an LES.
However, it would be better if you provide more details.

[fstra]

Thank you for your answer.

I have to simulata transient flow. I am not interested in the detail on the interface, but to have a fully-developed sea wave field.
I have understand the problem is related to the turbulence model: for a small wave, viscosity effects are significantly higher and the wave decays. My question is whether there is a suitable turbulence model to avoid this, or else how can I modify the parameters of the turbulence model in order to have a reduced effect on the viscosity at the water/air interface.

[FMDenaro]

Your problem is critical in terms of modelling... URANS implies by definition an effect of the model on all wavenumbers of the flow.
Again, I suggest to see if you have enough computational power to do an LES. Fluent allows you to use the dynamic Smagorinsky model. The key is to produce a quite fine grid for your problem

[fstra]

I was hoping there would be a simpler way to achieve a reasonable solution.
Last hope: if the problem is related to the predominance of viscosity effect at the interface, is there a way to impose a sheer-stress free condition at the free-surface with VOF? Maybe when defining the phases..? Do you have any suggestions with this regard?
Thank you in advance

[FMDenaro]

The physical viscosity effect is overwhelmed by the modelling effect of the RANS formulation.