Best Approach for Introducing Optional Vector in ComputeResidual for Scalar Solvers

[emaberman]

Hi everyone,

I'm currently working on a modification to the ComputeResidual function in scalar_diffusion.hpp to introduce an optional vector that enhances the stability and ensures positivity of the Jacobian in turbulent solvers.

The current function call in the solver class is:

auto residual = conv_numerics->ComputeResidual(config);

This function currently returns a struct:

struct ResidualType {
    const Vector_t residual;
    const Matrix_t jacobian_i;
    const Matrix_t jacobian_j;
};

To achieve the desired modification, I need to compute an additional vector in the numerics class, which needs to be returned to the solver class. My question is:

What would be the best approach to include this additional vector? Should I modify the existing ResidualType struct to include the optional vector, or should I create a new type of struct for this specific case?
Will returning an extra vector from the residuals have any performance consequences when it is not used?
I want to ensure that the solution is both backward compatible and efficient. Any insights or recommendations would be greatly appreciated!

Thanks in advance for your help!

[pcarruscag]

Modify that struct, in general, first make it work, then worry about performance.