Add distal pressure for resistance BC

[natalia-rubio]

Problem

Currently, the Neumann resistance BC only allows a user to specify a resistance, not a distal pressure.

Solution

There should be an option to specify a distal pressure such that the BC enforces the equation: boundary_pressure = resistance * flow + distal_pressure

Here is a link to the resistance BC with distal pressure, as implemented in the 0D solver.
https://simvascular.github.io/svZeroDSolver/class_resistance_b_c.html

Additional context

No response

Code of Conduct

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[ktbolt]

@natalia-rubio For the svSolver the distal pressure represents the terminal pressure (resistance of the capillary bed I guess), this is typically set to 0.0.

From the 0D solver docs it is not clear how Pd value is used, don't understand what C^e is supposed to be.

svFSI and thus svMultiPhysics has never had a resistance distal pressure term probably because it is never used in 3D simulations.

[alisonmarsden]

This is actually used! Zinan had a case she discussed this morning with me that needed a distal pressure defined with a resistance BC! It provides a constant pressure offset, for example if your circuit is emptying into a constant pressure resevoir like the atrium.

[ktbolt]

@alisonmarsden Well let's add functionality for this then.

[ktbolt]

I've added a pressure member to ResistanceBoundaryCondition that is set using the Distal_pressure XML parameter. The ResistanceBoundaryCondition class for an equation is now named resistance_bc.

In main.cpp I see that the equation resistance_bc is used like so

// Resistance term for coupled Neumann BC tangent contribution
res(i) = eq.gam * dt * eq.bc[iBc].resistance_bc.resistance;

I was wondering what eq.gam is but not to worry because it is has comment in ComMod.h

    /// @brief \f$\gamma\f$
    double gam = 0.0;

[aabrown100-git]

If I remember correctly, gamma is one of the parameters of the generalized alpha method.

[ktbolt]

@aabrown100-git Yes, thanks! The gam, af, am parameters are used in the generalized alpha method.