Advice about BCs
 

Hello,

I am working with a simulation to build upon an actual model.
This model is a pipe with perforations. The fluid flow at the inlet is also time varying. My inlet is currently set to laminar inflow rate in m^3/s. I can then add a step function to make this time varying. My question is what is the best way to set the hole BC and the output from the main tube. I know the approximate flow ratio from the total hole/perforation flow rate to that of the outlet of the tube. What type of boundary conditions should be used on these two outlet considering both SS and transient solutions are requied. Note that I have tried setting, presure, normal stress and flow rates. All of the above work but I am not sure is they are physics sound or if there are any types pof BCs that should be set here.

Any advice or opinion much appreciated.

It depends on what you know at the outlet. Atmosferic pressure?

As previously stated, I know nothing about the outlets other than the flow ratio between the tube outlet and the total hole outlet.

Hi Mel, could you give a little more information on the application? Am I right in thinking you are not explicitly modelling the perforations and you are wanting to use a representitive boundary condition?

If you're not individually modelling the perforation holes and you're looking to effectively represent the tube wall with a BC, then I'd consider modelling the tube wall as a porous region (in a subdomain). You can model the pressure drop across the tube wall using the KPerm, KLoss and Porosity terms.

If it is possible, then I'd assign the same pressure to the tube external wall as to that of the tube outlet. I'm not sure if this is actually possible without trying - pretty sure the option is there in CFX (what software are you using?)

When post processing the results, you can then look at the ratio of the tube outlet mass flow to the total perforation mass flow and see how far away you are from your steady test data. You can then adjust the pressure boundary conditions until you find an agreement.

You can then move on to the transient case.

Good luck

I am using COMSOL Multiphysics.

My model geometry consists of a tube/pipe and a small portion of each hole (length of each hole is equal to the thickness of the tube material).

What should be set for the BCs at both the outlet and each hole? Should I be playing around with pressure, normal force,or is there any other option? Or should I be deriving an equation?

you'll have to forgive me if I've still not grasped the scenario. Please feel free to attach a simple sketch if I'm way off track with this.

So my understanding is that you have a tube with an INLET and OUTLET but which also has holes (and hence additional OUTLETS) on the walls. Lets call these holes 'perf holes'. You mention the INLET flow rate and the ratio between flow out of the perf OUTLETS and tube OUTLET is known. So the objective must be to find what fluid pressure the perf OUTLET and tube OUTLET are exposed to, which of course could be different.

I'd therefore initially assign the same pressure boundary condition to both the tube and the perf OUTLETS. The value you choose should not actually matter, given you've prescribed an INLET flow rate. Upon post processing you can hence calculate your perf OUTLETS to tube OUTLET flow ratio. Depending on how close this ratio is with your steady test data, you can adjust either the perf OUTLET or tube OUTLET pressure and re-solve. Just as before, when post-processing, compare the ratio and if needed, perform subsequent analyses until a tight agreement between the CFD and TEST results is achieved.

When you're at this stage you can then set-up using a transient inlet flow rate.

Does this make any sense?

1. When people take precious time to help you with your issue, make sure you do all you can to clarify your setup.

2. Where exactly do you state you know nothing about the outlets?

3. From your description I still see no reason why there wouldnt be atmosferic pressure at your outlet.