[naikpk]

Dear all,

I am trying to work on a FSI problem. I am using ansys transient and fluent modules for a system coupling.

Its a valve opening and closing problem where blood is used as a fluid with properties
rho=1000 kg/m^3
viscosity=0.0043

the properties for valve material are
rho=1000 kg/m^3
E=1.5MPa
poisson`s ratio= 0.49

At inlet I have given a constant velocity of 0.15 m/s and outlet constant pressure of 0 Pa

I have attached the geometry of valve .

When I am trying to run the setup with air as fluid , its running without any error but when I use blood instead of air I get following error

Update for Solution component in system coupling fail:System coupling run completed with errors.One or more elements have become excessively distorted.Try ramping the load up instead of step applying the load (KBC,1).

I am guessing the problem is probably because of high density of fluid which is causing excessive displacement of valve and hence distortion of mesh.

Please if anyone know how to solve this error , it would be of great help

Thank You

[RaiderDoctor]

Hi, I know this is an old post, but I think it's important to provide an answer in case anybody else is struggling with the same issue. Whenever I run a Two-Way FSI problem, there are two main issues that I will continually run into: excessive element distortion in Mechanical, and negative cell volume in Fluent. Each are annoying, but can be fixed with proper understanding of how the mesh moves.

Excessive element distortion is when the load that is being read by Mechanical is so great, that it causes a very large displacement of a small group of nodes. This will lead the mesh to try and compensate by displacing more nodes. The result is a completely unrealistic structure that seems to break the laws of physics (I'll explain how to view this in a second). The solution to this is actually quite simple: lower your step size. By lowering your step size, you are increasing the number of result points, allowing the computer to find a more stable solution. Let's say you have an initial step size of 0.001 s, and then you receive an excessive element distortion error. At this point, lowering your step size to 0.0005 s might help to resolve this problem.

There are limitations, however, sometime the distortion might be occurring for other reasons such as a poor mesh quality. Unfortunately, there is a lot of trial and error associated with this, and no one solution is correct. By the way, to observe deformations, simply add in a "Total Deformation" solution in Mechanical. To do this, navigate to Project-Model-Transient-Solution. Click on Solution. Up at the top, a ribbon should appear with colorful cubes with names like "Deformation", "Strain", "Stress", and the like. Click "Deformation", and in the drop down menu select "Total Deformation". It will now appear underneath the Solution tab on the tree. Right click the "Total Deformation" and select "Evaluate all results". That should help to give you a good idea of what is occurring.

As to the negative cell volume in Fluent, this is a bit trickier. Usually, to help with the mesh distortion, you can try and improve the mesh either by clicking "Improve" on the top ribbon, or entering in the command: /define/dynamic-mesh/actions/remesh-cell-zone. This might help, or your geometry is created in such a way that a negative cell volume is unavoidable. In that case, you must rerun from scratch with a better geometry.

I hope this helps somebody!