Specifying Initial Conditions

Provides a system for patient-specific cardiovascular modeling and simulation.
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Kimberly Stevens
Posts: 8
Joined: Mon Dec 03, 2018 12:06 pm

Specifying Initial Conditions

Post by Kimberly Stevens » Tue May 14, 2019 12:40 pm

Hello,

I was wondering how initial conditions are specified in SimVascular. It seems to me there are three possible ways to specify initial conditions, and I was wondering exactly how they each work, and how they differ from each other.

1.) Specify an initial pressure and x, y, and z velocities under Basic Parameters in the Simulation tab of the GUI.
2.) Specify an IC file, which is just the final restart.x.0 file from a previous simulation.
3.) After creating the data files, you can replace the restart.0.1 with the restart.x.0 file from a previous simulation, renaming it restart.0.1 and placing it in the Simulation folder where the original restart.0.1 file was located.

Is this correct? My additional questions regarding pertaining to each of those approaches are below.
1.) Does this set a uniform pressure and velocity field throughout the volume? It seems like whatever you put here is overridden almost instantly, and seems fairly pointless. For example, I would have assumed that if you knew the pressures were going to be on the order of 100 mmHg you could specify an initial pressure of 100 mmHg (in the proper units, of course) and that would save some time in the simulation so that the solver didn't have to start with 0 pressure and build up to the final pressure. However when I have tried this, the pressure starts high, then almost immediately drops back down to a much lower value, then slowly builds back up to 100 mmHg.
2.) How does the restart file specify the initial conditions if the mesh used in the original simulation is different than the mesh used in the current simulation? Does it interpolate from one mesh to the other? Or does it average over different faces? I've used this approach to specify initial conditions with meshes of various resolution (though the same model) and most of the time it works fine, but recently I had a case where I could use the output of a simulation with a more coarse mesh but it would not work for a finer mesh, though I know I've done this in the past. And if you made a modification to the model (such as adding an additional outlet), could you use the restart file from the simulation run prior to the modification as the IC file for the simulation after the modification?
3.) Is there any difference between approach 2 and 3?

Thank you,
Kimberly

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Weiguang Yang
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Joined: Mon Apr 07, 2008 2:17 pm

Re: Specifying Initial Conditions

Post by Weiguang Yang » Wed May 15, 2019 11:44 am

If you use (1) to do the initialization, the pressure and velocities in the domain will be uniform. Supplying a restart file, the initial pressure and velocity are the same as the pressure and velocity in the restart file. Generally, for rigid simulations, initial values are not important. The results should converge quickly via a few iterations regardless of initial pressure/velocity values. Since the first cycle usually is not used, specifying a non zero pressure (say mean pressure) may not save too much time.

If the mesh is changed, restart files from previous simulations cannot be used directly. To my knowledge, the solver doesn't know if a restart file is given by a different mesh and doesn't do the interpolation automatically. If the array size is <= the size in your previous restart file, the simulation may start but incorrect initial values are used for each node. During mesh adaptation, solutions on the previous mesh are projected to an adapted mesh generating an adapted mesh file and an interpolated restart file. This interpolated restart file works with the adapted mesh.

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Kimberly Stevens
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Joined: Mon Dec 03, 2018 12:06 pm

Re: Specifying Initial Conditions

Post by Kimberly Stevens » Thu May 16, 2019 7:12 am

Thank you for the clear explanation.

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