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 shaswat February 1, 2012 04:56

help needed in Cfx pre

2 Attachment(s)

I found that

A wall has been placed at portion(s) of an INLET |
| boundary condition (at 11.1% of the faces, 4.3% of the area) |
| to prevent fluid from flowing out of the domain. |
| The boundary condition name is: inlet. |
| The fluid name is: Fluid 1. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead.

so I made a mistake in setting . kindly help me how to solve the problem smoothly

Thank you

Regards

Shashwat

 ghorrocks February 1, 2012 06:08

Your simulation has not converged very well, it might fix itself when you get it to converge. If it remains then you need to move the inlet boundary further upstream.

 cfd_dansir February 3, 2012 16:59

start steady-state and use smaller timesteps

i imagine your pressure function is very dynamic. i am very familiar with this study of blood flowing in Artery.

1. it be wise to let your model "warmup" your pressure loss. start with a steady-state condition based on an average pressure loss to find a close convergence and to test the mesh and other conditions/boundaries/parameters, etc. Experiment and develop appropriate parameters.
1b. start also with more simplier fluid dynamics. e.g. use Newtownian fluid before a non-Newtowian value for viscosity. run the model without the porous media (the stent) before running with it.

ALWAYS PROGRESS SIMPLE TO COMPLEX

2. decrease the size of your timestep. do some hand calculations of the Courant number, based on various points on your pressure curve. you may need to change the timestep accordingly. you can create functions in CEL with if statements, to create little pockets of different values of timesteps. or create table or other input.

3. since the pressure wave is very dynamic, expect to have smaller timesteps and need more inner-loops (coefficient loops) for the 2nd order inflections of the curve; namely the peaks and troughs. much like #2, you can create CEL function for this.

4. extend your mesh adequately length at either the inlet or outlet or both. this is what the previous post means by positioning your inlet.

5. try using smaller section of the pressure-curve to fine tune those aspects of the model. once you have the steadystate version (see #1), you can experiment and develop better parameters.

bottom-line, the blood flows in pulsations, using contractions from the muscles (and arterial walls). therefore, expect yourself to see negative pressure drops or backflow and backpressure and flow through the inlet--- this is expected! consider making it an Opening instead because of this fact.

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