因此,代码fvc::flux(phi, alpha1, alphaScheme)即为$\nabla\cdot\left(\alpha \mathbf{U}\right)$
https://github.com/OpenFOAM/OpenFOAM-dev/blob/master/src/finiteVolume/finiteVolume/convectionSchemes/gaussConvectionScheme/gaussConvectionScheme.C
https://github.com/OpenFOAM/OpenFOAM-dev/blob/master/src/finiteVolume/finiteVolume/fvc/fvcFluxTemplates.C