## Fluids

#### exclude mesh motion in boundary conditions (Fluent)

• danbence
Subscriber
This option is available in Rigid Motion and UDF. I have a requirement for excluding the mesh motion in a deforming boundary. As this does not seem to be directly available, is it somehow possible to patch the boundary velocities of the deforming surface to zero ? In a UDF ?nThe reason I want this is because I have a rigid body boundary moving very close and parallel to a stationary surface. I do not wish to expend the cost of tiny cells in order to resolve the gap between the boundaries, so want to approximate the situation by having them touch. I think this is analogous to the approximation of turbomachinery touching the shroud. However, turbomachinery has the luxury of not needing dynamic meshing.n
• Rob
Ansys Employee
Please post some images on what you're trying to do. n
• YasserSelima
Subscriber
Define wall function and return U+ = 0 .. hopefully this worksn
• danbence
Subscriber
Hi,nThere is a diagram.n There is a stationary flat surface and a UDF deforming object scraping along the stationary surface (with a small air gap). I am currently getting this to work with the stationary surface and the small air gap in one static zone and interfacing this to a dynamic zone that has the UDF deforming object. I have to expend many cells resolving the small air gap. However I am not interested in the flow in this gap.n What would be ideal is to have a single zone with the UDF deforming object attached to the stationary surface and the stationary surface is set to “deforming”. However, unless I can somehow exclude mesh motion in boundary conditions for the deforming surface, then the physics will be incorrect (ie the stationary surface will be dragged along with the UDF object)n I believe what I want to do is analogous to approximating a turbomachinery fan by attaching it to the shroud and defining relative motion between the shroud and the fan. In this case the mesh is untouched, but appropriate shear stresses/motions are imparted to the fluid close to the surfaces.n Thanks.nn
• danbence
Subscriber
Thanks for the suggestion.nWon't a custom wall function just vary the velocity profile relative to the wall velocity ? If so, I can?t see this would help, as it is the wall velocity that I want to set to zero in the absolute reference frame.n Also, I don?t think custom wall functions are available with the turbulence model I have chosen (LES)n n n
• Rob
Ansys Employee
What's controlling/defining the motion? It may be possible to move the hole where the deforming body is and connect it directly to the stationary surface. If you have heat transfer this may get very messy. Alternatively have a look at overset. n
• danbence
Subscriber
Hi Rob.nThe motion is controlled by a math function in a DEFINE_GRID_MOTION udf.n I was encouraged  when I read up on overset mesh functionality, but that was dashed when I discovered it is not available with LES. n