[ODE] Approximate Hydrodynamic Forces

[Ander Taylor]

Hey David,

This is what I got now. I works great.

Could you have a look and let me know if you I should be doing anything 
different?

Cheers,

Ander

note: I have added this to the .Net wrapper that I am using

void BoxGeom::ApplyHydrodynamicForces(dReal linear_viscosity, dReal 
angular_viscosity, dReal density, Vector3 flow)
    {
        const dReal *lvel = dBodyGetLinearVel(this->_body->Id);
        const dReal *avel = dBodyGetAngularVel(this->_body->Id);
        const dReal *R = dBodyGetRotation(this->_body->Id);
        dReal ss[3];
        dGeomBoxGetLengths(this->_id, ss);

        dReal AreaX = ss[1] * ss[2];
        dReal AreaY = ss[0] * ss[2];
        dReal AreaZ = ss[0] * ss[1];

        dReal nx = (R[0] * (lvel[0] - flow.x) + R[4] * (lvel[1] - flow.y) + 
R[8] * (lvel[2]) - flow.z) * AreaX;
        dReal ny = (R[1] * (lvel[0] - flow.x) + R[5] * (lvel[1] - flow.y) + 
R[9] * (lvel[2] - flow.z)) * AreaY;
        dReal nz = (R[2] * (lvel[0] - flow.x) + R[6] * (lvel[1] - flow.y) + 
R[10] * (lvel[2] - flow.z)) * AreaZ;

        dReal temp = -nx*linear_viscosity;
        dBodyAddForce(this->_body->Id, temp*R[0], temp*R[4], temp*R[8]);

        temp = -ny*linear_viscosity;
        dBodyAddForce(this->_body->Id, temp*R[1], temp*R[5], temp*R[9]);

        temp = -nz*linear_viscosity;
        dBodyAddForce(this->_body->Id, temp*R[2], temp*R[6], temp*R[10]);

        nx = (R[0] * avel[0] + R[4] * avel[1] + R[8] * avel[2]) * AreaZ;
        ny = (R[1] * avel[0] + R[5] * avel[1] + R[9] * avel[2]) * AreaX;
        nz = (R[2] * avel[0] + R[6] * avel[1] + R[10] * avel[2]) * AreaY;

        temp = -nx * angular_viscosity;
        dBodyAddTorque(this->_body->Id, temp * R[0], temp * R[4], temp * 
R[8]);

        temp = -ny * angular_viscosity;
        dBodyAddTorque(this->_body->Id, temp * R[1], temp * R[5], temp * 
R[9]);

        temp = -nz * angular_viscosity;
        dBodyAddTorque(this->_body->Id, temp * R[2], temp * R[6], temp * 
R[10]);

        dMass m;
        dBodyGetMass(this->_body->Id, &m);
        dReal gravity[3];
        dWorldGetGravity(this->_body->get_WorldId(), gravity);

        temp = -density * ss[0] * ss[1] * ss[2];
        dBodyAddForce(this->_body->Id, temp*gravity[0], temp*gravity[1], 
temp*gravity[2]);

    }

}

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