[palofpinaki]

I am performing 2-way coupled FSI for blood flow in axisymmetric abdominal aortic aneurysm. I am using k-omega turbulence model for analysis. But the following error is showing up everytime:

One or more elements have become highly distorted. Excessive distortion of elements is usually a symptom indicating the need for corrective action elsewhere. Try ramping the load up instead of step applying the load (KBC,1). You may need to improve your mesh to obtain elements with better aspect ratios. Also consider the behavior of materials, contact pairs, and/or constraint equations. If this message appears in the first iteration of first substep,be sure to run shape checking of elements.

If anybody has an idea regarding how to deal with the above error, please give suggestions.

Thanks in advance!!

[palofpinaki]

I have changed the fluid mesh....but the same error occurs again.

[Lance]

some hints:

improve the fluid AND solid mesh
ramp up the pressure, do not apply a step
use better initial conditions

[palofpinaki]

How should I change the mesh....should I make it more fine or coarse??(Sorry for the silly ques....but I really don't understand...I am just a beginner!!)

On a different note, I tried to modify the standard k-omega turbulence model in ANSYS CFX using the advanced controls to make it more suitable for low Re flow. In wilcox's book "Turbulence modelling for CFD", a method is given according to which modifications are made to turbulent viscosity expression, beta star coefficient in k-equation and alpha coeffcient in Omega equation. I tried using CEL expression. All the expressions are functions of turbulent Reynolds number(which is a function of k and omega). But it gives an error saying that the alpha and beta star coefficients can only be constants or they can be expressed in the form of expressions which amount to a constant.

So I just can't figure out how do I implement the necessary modifications to the k-omega model to make it suitable for low Re flows in ANSYS....if anybody has tried it before or has any idea.....please feel free to give suggestions.....
Thanx!!

[Lance]

Improve the mesh quality, i.e. skewness, aspect ratio, etc. This could mean more cells, it depends on how you mesh/geometry look like.

Do not change the turbulence model parameters unless you really know what you are doing.

[palofpinaki]

Hey Lance...

Thanx for ur suggestion......but I am aware of turbulence....in my problem the Reynolds no is quite low of the order of 2500....so I am pretty sure...I have to modify the coefficients to make it capture the physics well. But I am not sure how....I have adressed my problem in the previous mail....if u have any idea....feel free to help!!!

Thanx

[ghorrocks]

Does the turbulence transition model help? It is designed to operate in Re numbers in this range, but it aimed at external airfoil flows.

[palofpinaki]

Yes...you guessed it right..... I need to capture the transition to turbulence as the flow isn't laminar for one whole cardiac cycle.....please post any suggestions that might be helpful.... I just wanna know how can I modify the standard k-omega turbulence model in ANSYS CFX to capture transition(you can refer to my previous mails on this thread).....I am little short of time...please include details if possible...

Thanx a lot!!

[ghorrocks]

I just wanna know how can I modify the standard k-omega turbulence model in ANSYS CFX to capture transition

Ha ha ha! And I guess tomorrow you want to invent cold fusion.....

You cannot "simply" modify the k-omega model to capture transition! If it was simple then it would have already be done. The turbulence transition model was developed to do this, but it is definitely not simple and it was developed for external aerodynamic flows so you will have to be careful in applying to your flow.