Question

Which of these terms represent the flow of fluid into and out of the observation model?

(a) \(\frac{\partial(\rho\Phi)}{\partial t}\)

(b) \(div(\rho\Phi\vec{u})\)

(c) div(ΓgradΦ)

(d) ΓgradΦ

The question was asked at a job interview.

This is a very interesting question from General Transport Equation in section Governing Equations of Fluid Dynamics of Computational Fluid Dynamics

Answer 1

The correct answer is (b) \(div(\rho\Phi\vec{u})\)

Easy explanation: \(div(\rho\Phi\vec{u})\) is the convective term of the transport equation. Convection is the flow of fluid into and out of the model of observation.