Appropriate Boundary Conditions-Moving Wall Pipe

ashtonJ · April 4, 2014, 02:08

Dear all,
I have a question regarding appropriate boundary conditions for modelling fluid flow in a pipe with moving boundaries. As shown in below image, region II (middle section of the pipe) starts to narrow from its initial form (solid blue line with number 1) to number 2 (dashed red line), number 3 (dashed green line) and finally number 4(dashed black line). Then the region II starts to return back to its initial form (from number 4 to number 1).

The axial velocity profile is measured experimentally in region II as illustrated in above image. As seen, there is a negative axial velocity between time 0.2 – 0.4 (s), which shows that during this time, there is a back flow in region II.
Now, I am trying to model this pipe using ANSYS CFX. However, I am confused what boundary conditions. Should I use the measured axial velocity as inlet or outlet boundary conditions? For example, if use the velocity as inlet B.C., then at time 0.2-0.4, the flow does not enter the pipe (region I) and there would be zero flow which does not make sense.
Could anyone please help me out with this.

Regards,
AshtonJ

brunoc · April 4, 2014, 08:59

Do you know what the velocity profile vs. time is in the inlet of region 1? Does that become negative too? If it does, use an opening with a prescribed velocity, and set the profile by using a 1D function of velocity vs. time.

Set the interfaces between region 2 and regions 1 and 3 as GGI, make sure that the mesh movement isn't copied through the interface and change the interface options on the faces on regions 1 and 3 so that faces without intersection are treated as no-slip walls.

ashtonJ · April 4, 2014, 20:17

Thank you brunoc.
This model is a simplified model of a myocardial bridge coronary artery. I have no idea about the velocity at the inlet of Region 1, the only known boundary condition is the velocity within Region 2.
Can I use this velocity at the intersection between Regions 2 and 3, as outlet boundary condition? In this case, do I need to define GGI on the interface between regions 2 and 3?
I have done experimental investigation in this artery and have observed a recirculated flow in Region I at the time corresponding to the negative velocity. I want to see the same recirculation in Region I by CFD.

April 4, 2014, 02:08	Appropriate Boundary Conditions-Moving Wall Pipe	#1
ashtonJ Senior Member Ashkan Javadzadegan Join Date: Sep 2010 Posts: 255 Rep Power: 16	Dear all, I have a question regarding appropriate boundary conditions for modelling fluid flow in a pipe with moving boundaries. As shown in below image, region II (middle section of the pipe) starts to narrow from its initial form (solid blue line with number 1) to number 2 (dashed red line), number 3 (dashed green line) and finally number 4(dashed black line). Then the region II starts to return back to its initial form (from number 4 to number 1). The axial velocity profile is measured experimentally in region II as illustrated in above image. As seen, there is a negative axial velocity between time 0.2 – 0.4 (s), which shows that during this time, there is a back flow in region II. Now, I am trying to model this pipe using ANSYS CFX. However, I am confused what boundary conditions. Should I use the measured axial velocity as inlet or outlet boundary conditions? For example, if use the velocity as inlet B.C., then at time 0.2-0.4, the flow does not enter the pipe (region I) and there would be zero flow which does not make sense. Could anyone please help me out with this. Regards, AshtonJ

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April 4, 2014, 08:59		#2
brunoc Senior Member Bruno Join Date: Mar 2009 Location: Brazil Posts: 277 Rep Power: 21	Do you know what the velocity profile vs. time is in the inlet of region 1? Does that become negative too? If it does, use an opening with a prescribed velocity, and set the profile by using a 1D function of velocity vs. time. Set the interfaces between region 2 and regions 1 and 3 as GGI, make sure that the mesh movement isn't copied through the interface and change the interface options on the faces on regions 1 and 3 so that faces without intersection are treated as no-slip walls.

April 4, 2014, 20:17		#3
ashtonJ Senior Member Ashkan Javadzadegan Join Date: Sep 2010 Posts: 255 Rep Power: 16	Thank you brunoc. This model is a simplified model of a myocardial bridge coronary artery. I have no idea about the velocity at the inlet of Region 1, the only known boundary condition is the velocity within Region 2. Can I use this velocity at the intersection between Regions 2 and 3, as outlet boundary condition? In this case, do I need to define GGI on the interface between regions 2 and 3? I have done experimental investigation in this artery and have observed a recirculated flow in Region I at the time corresponding to the negative velocity. I want to see the same recirculation in Region I by CFD.