Pulsatile flow and B.C.
 

Dear All

I am simulating pulsatile flow in a coronary artery. I have the velocity information at the inlet (a time dependent velocity distribution) but I have not any information at the outlet. First, I considered zero static pressure at the outlet ( reference pressure is 1 atm) but during solution, a note is given by CFX solver that some fluid want to flow into the domain and solution is not converged and CFX suggests me to change the B.C to opening. When I apply opening boundary condition, solution is easily converged but I don’t know using opening B.C is correct physically.

I would be too pleased if anyone let me know what type of boundary condition at outlet gives more physical results.

Thank you

ashtonJ, you can find some basic information typing "Using Inlets, Outlets and Openings" on CFX manual.

Willian

Dear Willian,
Opening Boundary condition is generally given for cases the exit can have either forward or backward flow. in Outlet back flow is not allowed hence you are getting some error. The flow might be able to get out barely with very less pressure or the pressure loss through the artery is dropping to very low values causing a back flow ( I am not sure but this is what seems from your description ) How many iterations have been giving this error? if the error/warning has just started, ignore it for some iterations ( I would say 10-20 ) It is possible that it ( error ) might go away as solution converges.
Hope this helps.
-Dinesh

sorry for the wrong reference, it should have been addressed to ashtonj

Dear Dinesh

Thank you very much for your kind answer. I have this error from first of the simulation. When I apply the opening B.C., my problem is solved but I am not sure that this assumption for outlet B.C. is correct. Also, when I use outlet boundary condition and impose a value (not zero) for pressure, the solution is converged. I just want to know what mathematical operation opening B.C. does over fluid variables (like velocity and pressure) at outlet.

Thanks a lot

Dear Asthon,
I am not sure about the mathematical operations on fluid variables, but my guess is that instead of providing an outlet pressure a pseudo cell layer is created at the exit which mirrors the values of the last layer meshed by you.
What I would like to know is what is the physical condition at the exit that you are trying to model, I mean what is present at the exit of your artery, since you are assuming atmospheric pressure I assume you are trying to model the flow when an artery is cut ( In that case opening condition is fine ) . Only when the exit condition which is being modeled is known will one be able to tell whether an opening condition is fine or not.
Also how many iterations have been completed ?

Dear Dinesh

I want to simulate a whole coronary artery which has many branches. First of all, I want to simulate a coronary artery without branches (a curved tube with varying diameter along the artery). I know the velocity distribution at inlet which is a time dependent wave form. I don’t have any information at outlet and I am trying to suppose a valid B.C. at outlet. When I solve the problem for two cardiac cycle (2 seconds), the solution is solved only until 1.2 second, after this time a notice is shown that 99 percent of outlet is covered by wall.

Thank you

Dear Ashton,
I have a doubt. Aren't arteries supposed to be flexible in size and if yes is this the case physically if the size is varying then the simulation would be incorrect.
As far as the error is concerned, until the issue of the boundary condition is solved nothing can be said about these errors.
What is the exact BC at the exit when this error is given, is it opening or outlet ?
Also I am not good at the physics of blood flow, but if back flow is not possible then i guess opening boundary condition would be incorrect.
I am not too confident with outlet pressure equal to atmospheric provided either. Where does this artery supposed to end ? first figure that out and then maybe you can google and see what actually happens physically at this location ( if you already don know it ) , Once you know the physics then giving the correct BC becomes easy.
-Dinesh

Dear Dinesh

I have this error when I use outlet B.C. with zero pressure(my refernce pressure is 1 atm). Oultlet of the artery is connected to another artery and I want to predict the fluid variables at outlet and also at where there is a constriction as I mentioned before,

I really appreciate your kind help

Dear Ashton,
If you want to simulate the situation at exit, then i guess giving outlet pressure is definitely inappropriate especially since yours is an unsteady state simulation where the pressure very well might be fluctuating at the exit. I don't have in depth knowledge about opening boundary condition. Just read through CFX help and manual for this opening condition, everything has been explained in detail about any kind of boundary condition.
-Dinesh

Dear Danesh

I am really thankful for your kind attention to my problem.

Ashtonj

No probs,
Do share what you find out in this regard, i would love to know what is the exact BC which should be given in such cases.
-Dinesh

Dear friend
I am also doing blood flow analysis
I need your kind help .I need the velocity waveform for my input. I saw You have created velocity waveform . I would like to how did you get the velocity with respect to time . Please help

Thank you

Regards

Govind