Question

When the flow is not aligned with the grid lines, the diffusion produced by the upwind scheme is ____________

(a) false advection

(b) false convection

(c) anti-diffusion

(d) false diffusion

The question was posed to me by my college professor while I was bunking the class.

This question is from Convection-Diffusion Problems topic in section Convection-Diffusion Problems of Computational Fluid Dynamics

Answer 1

The correct answer is:

(d) false diffusion

Explanation:

In convection-diffusion problems, when the flow is not aligned with the grid lines, an upwind scheme (which is a numerical method used for solving advection-diffusion equations) can introduce an effect called false diffusion. This occurs because the upwind scheme can produce numerical diffusion even when the physical diffusion in the problem is small or negligible.

False diffusion happens when the numerical method diffuses the solution incorrectly, leading to additional, unphysical spreading of the scalar (such as temperature, concentration, etc.) in regions where there is no real diffusion. This can occur when the flow direction does not align well with the grid, causing the numerical solution to be artificially smeared in the direction of the flow.