vector plus bug
 

Hi
have you ever met a case that is when two very small negative values are plusing, the result turned out to be positive.

No. Give some more background and ask what you really want to ask.

Seems to be an XY problem: http://mywiki.wooledge.org/XyProblem

Thank you for your reply.
In openfoam 1.6-ext. the solver interTrackFoam
it has force calculating
vector totalForce =
interface.totalViscousForce()
+ interface.totalPressureForce();
vector freeSurface::totalPressureForce() const
{
const scalarField& S = aMesh().S();
const vectorField& n = aMesh().faceAreaNormals().internalField();
const scalarField& P = p().boundaryField()[aPatchID()];
vectorField pressureForces = S*P*n;
return gSum(pressureForces);
}
vector freeSurface::totalViscousForce() const
{
const scalarField& S = aMesh().S();
const vectorField& n = aMesh().faceAreaNormals().internalField();
vectorField nGradU =
U().boundaryField()[aPatchID()].snGrad();
vectorField viscousForces =
- muFluidA().value()*S
*(
nGradU
+ (fac::grad(Us())().internalField()&n)
- (n*fac::div(Us())().internalField())
);
return gSum(viscousForces);
}

but in some cases, when these two force is very small, its result turned out to be very big. and sometimes printed out -nan. I do not know why.

Can you give a simple example, so that we can reproduce the issue? It is very difficult to tell what is going on and diagnose the problem.