Pressure values negative

[Nishanth Surianarayanan]

Hi Developers,

1. If inlet flow condition and RCR BC's are sufficient to run a simulation and since we are not specifying pressure values anywhere, Is pressure undetermined upto a constant?

2. In my simulation all the pressure values are negative and seem to be higher at the outlets than the inlet. What can be the probable causes? My inlet values are prescribed properly as attached in the picture. I am using around 2.7 million elements and have tried time step of 0.001 and 0.0005.

simvascular doubt1.PNG (166.2 KiB) Viewed 246 times

3. I determined Rp,C and Rd values based on the pulse pressure method devised by N. Stergiopulos ( https://www.ncbi.nlm.nih.gov/pubmed/9950841 ) and the procedure published by Prof. Shawn Shadden titled "Quantification of Hemodynamics in Abdominal Aortic Aneurysms During Rest and Exercise Using MRI and CFD" ( https://www.ncbi.nlm.nih.gov/pubmed/20143263 ). For this I solved the 3-element windkessel model equation using Matlab to determine Capacitance. Is the equation to be solved the same as the equation shown in the attached picture? I have also attached the values of the boundary conditions. Do they look like the usual values? (I have some really small values of capacitance O(10^-8) for lumbar arteries)

simvascular doubt.PNG (10.1 KiB) Viewed 246 times

simvascular doubt2.PNG (13.94 KiB) Viewed 246 times

simvascular doubt3.PNG (28.27 KiB) Viewed 246 times

4. This method of evaluating parameters is different from what you suggest for users. We can use this method in SimVascular right?

5. I understand that it can take around 6 cycles for the pressure to stabilize. But will the pressure values be random (non-physiological) in the first few cardiac cycles?

6. Can you please tell me a procedure as to how to systematically vary grid size, time step size and step construction values?

7. Also the geometry of some vessels are very thin at some places. Will this cause any error?

Please let me know if you need any further information to answer my questions!

Thanks a lot for your support!

Best,
Nishanth

[Weiguang Yang]

Theoretically, the absolute pressure is undetermined up to a constant. The pressure map doesn't look right. You can use a coarse mesh (<1 M elements) to run 1-2 cycles and check if the inflow waveform is correctly imposed on the inlet. If there is flow streaming from the inlet to outlets, the pressure should not be -100 mmHg under current RCR values. You can check velocity vectors in paraview. You can take a look at the residuals in histor.dat file to see if the solutions satisfy the equations.

Initial pressure and velocity fields are usually set to 0 but the pressures approach physiological values fairly quickly (it won't take several cycles before you see something normal). There is no unique way to determine boundary condition values. It is totally fine to use methods different from the tutorial as loon as the final results make sense.

The RCR values seems OK. Usually capacity is small ~1e-5-1e-6. If the outlet pressure pulse for the lumbar arteries is too large for you, you may need to increase it.

You can keep step construction values unchanged (usually 5-8 for pulsatile flow with rigid walls) with residual control. So the Newton iteration is not executed once the residual reach the threshold value set by the user. If the mesh size is reduced by 50%, you can also reduce your time step by 50%. Sometimes the simulations won't blow up even if the step size is not reduced by 50%.