OpenLB – Open Source Lattice Boltzmann Code Forums on OpenLB General Topics Applying a friction value to non-slip boundary condition

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• #4893
behnamandi
Participant

Hi all,
hope you are all doing well in these tough times.
I have pipe flow with non slip wall boundary condition. currently using the bouzidi for round surfaces. I have a viscous fluid going into the pipe and exiting with a certain laminar reynold number. what Im looking for is the velocity profile near the wall. and for this to be close to the real application, i need to apply some sort of friction factor between the wall and fluid. is there some kind of partial slip boundary condition available in openlb?

As soon as move i away from the wall, the velocity graph starts rising. i was hoping with the friction introduced, my velocity near the wall would take longer time to start increasing/changing.

#4894
sthavishtha
Participant

Dear Behnamandi

The partial slip boundary condition is already available via the function `void addPartialSlipBoundary(...)`, listed in `boundaryCondition2D.h/boundaryCondition2D.hh`. An example of its usage is also available in `examples/laminar/poiseuille2d/` and `examples/laminar/poiseuille3d/` – you will have to use `boundaryType == partialSlip` in those code examples.

Regards
Sthavishtha

#4895
jonathan
Participant

Dear behnamandi,

we don’t currently have (partial-)slip treatment for boundaries other than axis-aligned or 45 deg walls – this is not an option for a pipe geometry.
But even with no-slip walls you actually account for some roughness coefficient known as hydraulically smooth.
You can calculate a present coefficient from the wall shear stress in your simulation.
Usually we can neglect additional friction from the material as viscous effects dominate over the entire diameter in the laminar pipe flow.
Therefore, we only have a very small influence of the relative roughness on the pressure drop.
Roughness gains importance in micro-channels or turbulent flow where the viscous sublayer is much smaller.
In such cases we underestimate the dissipation rate in the viscous sublayer and need modelling of friction effects near the wall.

I agree with you that higher gradients due to partial slip boundaries would increase the wall shear stress and the roughness coefficient.
Maybe you can define a velocity on the first fluid layer beyond the no-slip wall to artificially increase the tangential velocity components for a higher wall shear stress.

Hope this helps!

Jonathan

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