# ViscoBoundFactor

Hello!

Can anyone explain the visco bound factor and show in which equation it appears, in DualSPHYsics? I cannot figure it out from the documentation.

My understanding is that it acts as some kind of roughness factor for fixed, moving and floating objects - I just want to understand how it acts physically / appears mathematically.

Kind regards

## Comments

Hi @Asalih3d,

Have you looked here ?

Also, how to use it and calibrate it is describe here:

Barreiro A, Domínguez JM, Crespo AJC, González-Jorge H, Roca D, Gómez-Gesteira M. 2014. Integration of UAV photogrammetry and SPH modelling of fluids to study runoff on real terrains. PLoS ONE, 9(11): e111031. doi:10.1371/journal.pone.0111031.

Best regards

## DualSPHysics/DualSPHysics

https://github.com/DualSPHysics/DualSPHysics/wiki/3.-SPH-formulation#321-artificial-viscosityHi, thanks for the answer @TPouzol

I have read through the paper and I understand that in their approach they had a Manning number to go after, which seems like a good way to do it for different materials. To be completely sure you linked this equation;

My understanding is that "alpha" is the ViscoBoundFactor - is this correct?

And does this means that this factor is only available for artificial viscocity? If I change it while using LES SPS then it should have no effect what so ever?

Kind regards

Hi,

Yeah the coeffcient is only available for artificial viscosity.

Alpha represent the energy dissipation when 2 part are interacting. But there is 2 different alphas: 1) alphaFF for fluid fluid ineraction, 2) alphaBF for boudary fuid ineraction.

In .xml, you provide alphaFF

<parameter key="Visco" value="0.025" comment="Viscosity value" />

But not alphaBF directly, rather you give a coefficient such as AlphaBF = ViscoBoundFactor * alphaFF

<parameter key="ViscoBoundFactor" value="3.5" comment="Multiply viscosity value with boundary (default=1)" />

hope I'm clear

Kind regards

Thanks for clarifying @TPouzol. If this is the case I fear that there unfortunately is a bug in the code, I am using the following parameters

And I get the result:

Which is clearly wrong since the top wall (yellow) is moving with 1 m/s in x-direction. If I change the visco bound factor to 1 I see;

Which is much more accurate. Nothing is changed between the two cases except this ViscoBoundFactor. Just tagging @Alex as well. It seems to me this term should have any effect on LES approach, but maybe there is something missing. Both pictures were taken at the same time step, time step 27.

Kind regards