[FAQ] Gap calculation in node node contact.

[zhouxinwei]

Many of our users are asking what will happen if the two nodes overlap (i.e. sum to two radii is bigger than the center-to-center distance) at time zero.

In MARS, when a node-node contact is created, a variable called Distance At Contact (dac) is calculated as:

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    // tk1, tk2 are the contact thickness set via input for node list 1 and node list 2, respectively
    dac = max(n1->getRad(), tk1) +  max(n2->getRad(), tk2);
    dst = n1->calcDist(n2); // distance between two nodes (center to center)
    if (!adc) // adc is false by default
        dac = min(dac, dst);


So, if the two nodes overlap at time zero, dac is set to the current distance (dac == dst holds).

When MARS evaluates the forces within the contact object, it first checks if nodes are close enough:

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    // dst: distance between two nodes (center to center)
    real gap = dst - dac;
    if (gap > cdd) { // cdd: contact detection distance via input
        remove();
        return;
    } else if (gap >= 0.) {
        csv[0] = 0.;
        return;
    }


From the second code segment, the contact gap is checked against dac. Therefore, if the two nodes overlap at time zero, in the first step, dst == dac holds, and the contact is inactive.

This logic is implemented to prevent a model from exploding at time zero.