Question

What is the problem of numerical diffusivity?

(a) The simulated model has a higher diffusivity than the actual flow

(b) The simulated model has a lower diffusivity than the actual flow

(c) The simulated model has a different diffusivity than the actual flow

(d) The simulated model has a zero diffusivity

This question was addressed to me by my college professor while I was bunking the class.

The doubt is from Convection-Diffusion Problems in chapter Convection-Diffusion Problems of Computational Fluid Dynamics

Answer 1

The correct answer is:

(a) The simulated model has a higher diffusivity than the actual flow

Explanation:

Numerical diffusivity refers to the artificial diffusion introduced into a computational model as a result of numerical methods, especially when solving convection-diffusion problems. It is a consequence of the discretization scheme used to approximate the differential equations that govern fluid flow and heat transfer.

• Numerical Diffusivity: In computational fluid dynamics (CFD), numerical methods often introduce a form of artificial diffusion to stabilize the solution, particularly in convection-dominated problems. This artificial diffusion can cause the simulated model to have a higher diffusivity than the actual physical system, leading to smearing or spreading of the transported quantities (like temperature, concentration, etc.) more than in reality.

Thus, the simulated model exhibits a higher diffusivity than the true physical flow, which can distort the results, especially in problems where accurate representation of sharp gradients (such as in boundary layers) is important.

Correct Answer: (a) The simulated model has a higher diffusivity than the actual flow