Question

The central difference approximation goes wrong when _____________

(a) Peclet number is negative

(b) Peclet number is positive

(c) Peclet number is low

(d) Peclet number is high

This question was addressed to me in unit test.

My question comes from Convection-Diffusion Problems in chapter Convection-Diffusion Problems of Computational Fluid Dynamics

Answer 1

Right answer is (d) Peclet number is high

For explanation: When the Peclet number is low in the positive or negative direction, the central differencing scheme is valid. If the Peclet number goes beyond a certain value both in the positive and negative direction, this approximation gives unphysical answers.