average pressure vs. pressure

[Eileen Hwuang]

Hello,

I am new to SimVascular and just generated results from a simulation of the descending aorta and select branches (external iliac and uterine arteries). I noticed there are two types of pressure maps: "pressure" and "average pressure" which give opposite-looking maps. Can someone explain what is the difference between these two maps and why they look different?

pressure_maps.png (249.72 KiB) Viewed 553 times

Thanks,
Eileen

[Justin Tran]

Hi Eileen,

Thank you for your interest in SimVascular! We are happy to hear that you got a simulation to run! The naming between pressure and average_pressure can indeed be confusing. You probably noticed that there is also an "average_speed" in the results file as well.

Non-intuitively, both the "average_pressure" and "average_speed" are quantities that are actually related to the mesh adaptor. If you open up the SV Meshing module in the SimVascular GUI, you should notice a tab at the bottom called "Adapt". This option will take the results from a simulation and a given mesh, and adaptively refine the mesh in areas of poor accuracy and adaptively coarsen the mesh in areas where you can coarsen without losing accuracy. Where SimVascular decides to refine or coarsen is determined from the "average_pressure" and "average_speed" fields.

For most purposes, you will be interested in the "pressure" and "velocity" fields, NOT their corresponding "average" fields.

On a side note, from your left figure of the pressure distribution it looks like there is low pressure at your inlet at the top and high pressure at the outlets at the bottom. Usually, we expect the pressure distribution to be high at the inlets and low at the outlets. Can you confirm that the flow is going in the correct direction in that figure?

Hope that helps! Please let us know if you have anymore questions.

[Eileen Hwuang]

Hi Justin,

Thank you for the helpful explanation about "average_pressure" and "average_speed."

I also appreciate the comment on the left figure. I looked more closely at the pressure distribution and realized that since I am modeling pulsatile flow, that particular time point was capturing the short period of flow reversal. Is that supposed to happen? For display purposes, I suppose it makes more sense to choose a different time point showing anterograde flow such as this one:

pressure map_990.JPG (27.56 KiB) Viewed 541 times

Also, the inlet waveform at the aorta is negative based on this tutorial (http://simvascular.github.io/clinicalCase3.html) that said to orient the flow direction into the model opposite of the outward normal vector of the aortic inlet.

inlet_flow.png (14.12 KiB) Viewed 541 times

Please let me know if I am missing anything.

Thanks!
Eileen

[Justin Tran]

Hi Eileen,

Ah yes, that makes sense that the original screenshot was from a period of the heart cycle where there was back flow. That reversal in pressure gradient tends to happen when you have perfectly rigid walls. It looks to me like your flow is heading in the right direction and everything is OK! Thank you for clarifying.