Question

Which of these models of fluid flow give complete partial differential equations directly?

(a) Finite control volume moving along with the flow

(b) Finite control volume fixed in space

(c) Infinitesimally small fluid element fixed in space

(d) Infinitesimally small fluid moving along with the flow

I had been asked this question in an internship interview.

Asked question is from Partial Differential Equation topic in portion Mathematical Behaviour of Partial Differential Equations of Computational Fluid Dynamics

Answer 1

Right option is (c) Infinitesimally small fluid element fixed in space

Best explanation: Infinitesimally small fluid element gives partial differential equations. When they are fixed in space, the equations are directly in partial differential form and there will be no need for changing a substantial derivative into partial differentials.