CPCE CPCE Mesh Generation & Boundary Conditions 3

Question 1

What does 'wall function' approach mean in turbulent CFD simulations?

Accepted Answer

Empirical semi-analytical functions that bridge the wall viscous sublayer and the fully turbulent region, allowing coarser near-wall meshes

Answer

Wall functions use empirical profiles (log-law of the wall) to model the near-wall region without resolving the viscous sublayer, enabling coarser meshes (y+ 30–300) near solid boundaries.

Question 2

In CFD mesh generation, what is the difference between 'refinement' and 'coarsening'?

Accepted Answer

Refinement decreases cell size (adds cells) in a region; coarsening increases cell size (removes cells) in a region

Answer

Refinement subdivides cells into smaller ones to improve resolution in critical areas, while coarsening merges cells into larger ones to reduce computational cost in regions of low interest.

Question 3

What is a 'mass flow inlet' boundary condition and when is it preferred over a velocity inlet?

Accepted Answer

It specifies the mass flow rate directly; preferred when the mass flux is known and must be precisely controlled

Answer

A mass flow inlet prescribes the mass flow rate entering the domain, ensuring exact conservation of mass flux; it is preferred over velocity inlet when the mass flow rate is the known design parameter.

Question 4

What is 'overset' (or Chimera) meshing and when is it used in CFD?

Accepted Answer

A mesh that overlaps multiple zones, allowing relative motion between components without remeshing

Answer

Overset (Chimera) meshing uses overlapping mesh zones where data is interpolated between them, enabling simulation of moving bodies (e.g., rotating propellers, pitching airfoils) without remeshing.

Question 5

What is the 'outflow' boundary condition in CFD and what assumption does it make?

Accepted Answer

It extrapolates interior values to the outlet boundary; assumes fully developed flow with zero normal gradients

Answer

The outflow boundary condition extrapolates all flow variables from interior cells to the outlet, assuming the flow is fully developed (zero streamwise gradients) at the exit plane.

(CPCE) Certified Professional CFD Engineer Practice Test

(CPCE) Certified Professional CFD Engineer Practice Test

CPCE CPCE Mesh Generation & Boundary Conditions 3