Discussion Forum

Hi

I have some problems to follow you on what you mean by "noslip on an interiour boundary".
For me noslip is essentially v=0 on a solid to fluid interface, I do not really see it as a line in the middle of a fluid (what I understand as an interiour boundary of fluid domains) where I would impose a "noslip" conditions.

If its from a multiphysics application node, you can always redefine that the fluid is only in one part, hence create an external boundary for the fluid (onto which one can then define a noslip condition), that is in the same time an interiour boundary for i.e. the temperature

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Good luck
Ivar

Hi - as I've seen the thread is quite new.
I've got a similar problem! In Comsol 3.5 and earlier it works, but I cannot find the feature in Comsol 4.1 --
I'm getting frustrated, because it already took me all day. (And anyway: I think that the old layout was much easier to overlook and to handle, with the symbols for "boundary","domain", etc. to readily click on and the "cards" opening - no need for searching so much --- and equations could be seen BEFORE putting the physics into the model...

Problem:
Imagine a ball is falling in a viscous fluid (2d axisymmetric application) - what I need is a reliable calculation of the force. That's been a problem in the old version. I prefer to set the interior of the ball to be a very viscous fluid (which is used experimentally) and have the other fluid surrounding it. I can calculate the Force from an integral over the ball interior.

In comsol 3.5 that's easy because I can give the "intererior boundaries" some appropriate conditions and solve.
In Comsol 4.1 I cannot do that - and anytime I tried the coupling between the inner and outer domains did not work properly. If I open the old comsol 3.5a model in version 4.1 it has the correct old solution from 3.5a.
I cannot see the difference in the "old imported working model" to one I freshly created in 4.1. AND if I solve the imported model in the new version the solution ist complete rubbish --- please help me out. The interior/exterior coupling seems to be correct, but still the output is pure waste.

And: I really NEED both volumes to be fluid and need the boundary condition in between them. AND I have the same problem in 2D, too.
...

Hi

"Ah - les gouts et les couleurs" which is the best 3.5 and v4 ?
Now that I'm used to 4 it's clearly that one, for me ;)

Back to no-slip on interiour boundaries, there I do not follow, formally a no-slip, for me, is between a solid and a fluid on a fixed/flex wall no ?
There were several choices in 3.5 on the BC levels that were not really physical.
With V4 COMSOL has cleared up the rules.

Are you saying you have no-slip between two fluids ?
One way then could be to use two fluid physics one per item, then the "interiour boundaries" become externals for the physics


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Good luck
Ivar

Hello

I'm having a very similar problem. My "interior boundary" is a solid inside a fluid domain that is so thin that is defined by a parametric curve.

The problem in 2D:
Inside an air volume there is a thin plate that oscillates, acting like a hand-held fan, that induces a fluid flow. This thin plate is built as a parametric curve since it's thickness is negligible.
To simulate the movement two steps are required. First, set the mesh to move according to the plate movement, which works fine with the Moving Mesh. Second, set the Moving Wall boundary condition on the plate so that the plate movement can be translated into fluid movement. I cannot do this second step because the Moving Wall boundary condition doesn't allow me to select the plate---it says it is "not applicable".
If I built the thin plate with a finite thickness (using a Rectangle instead of a Parametric Curve) I can apply the Moving Wall condition to the plate. Unfortunately, the scale is not accurate enough in this last situation.

Do you know how can I apply the Moving Wall boundary condition to a Parametric Curve that represents a thin solid inside a fluid domain?

One way then could be to use two fluid physics one per item, then the "interiour boundaries" become externals for the physics

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Good luck
Ivar

I tried to use this method but I could not figure how to connect the two items (such as defining continuity ) using identity pairs. You may say comsol automatically sets the continuation at interior boundaries. But in my problem for one physics (species transport) I have to define no flux at the interior boundary, and for the other physics (heat transfer) I have continuation at the interior boundary.

It seems that lot of people have this problem. I read somewhere in the manual that we can edit the interior boundary conditions but it did not tell how to do it.

I am having the same problem. I want to edit an interior boundary but could not figure out how to do it.

Hi

by default COMSOL adds continuity on all internal boundaries. This is defined by the last Geometry "Finish" node (default Union). If you select assembly, then all elements not unified within the geometry, and that are in adjacent contact (share a boundary, keeps both adjacent boundaries. but volumes that interpenetrates might remain so ! depends on settings. Check the doc, and do not confuse a CAD assebly and a FEM assembly ;)

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Good luck
Ivar

Hi,

I have also the same problem. I want to know how a flow in water creates a flow in the air, without deformation of the free surface, simply by friction. This is in fact a simple model before adding some convection and diffusion at the interface.

It was modelled as an interior boundary in v3.5 with vertical velocity equal to 0 and horizontal velocity equal to u of the fluid.

When I use the same model in v4, the results are rubish. I thus tried to do two domains with two physics and connect them adding a 'Flow continuity' in each physics. But once again, the dividing boundary cannot be selected in 'Flow continuity' and it is thus 'not applicable' in wall. Am I dong something wrong? Did anybody solve this problem?

Thanks for your help.

Patrice

Hi all

I have same problem, in my problem I have two different regions of fluid one of them should be solved by linear model of flow field solver and the other origin with different solver of flow field. While the wall between these to regions should be interior wall, where the mass flow and heat flux can pass from it.
I need help to link these to flow at their intersection.

Appreciate your guides

Hello,

Does anybody know hoe "dp/dn" varies over a no-slip wall and slip wall?

Thanks