CFD

Question 1

Describe what meshing requirement all CFD would need to produce the best results.

Answer 1

Resolve all scales of eddies.

Question 2

Describe what RANS is good at modelling.

Answer 2

Attached turbulent BLs, or those with mild separations.

Question 3

Describe what RANS is bad at modelling.

Answer 3

Large separation and recirculation, such as wakes.
Wall boundaries.

Question 4

Describe why the k-epsilon model is bad at modelling wall conditions.

Answer 4

It uses no-slip and zero-gradient boundary conditions at walls. They use a ‘production term’ which dictates that where there’s a velocity gradient, there is Ek.
At walls there are the highest gradients but zero Ek.

Question 5

What do k and epsilon represent?

Answer 5

Turbulent kinetic energy and dissipation rate.

Question 6

What does the k-epsilon model use to model wall boundary conditions.

Answer 6

They use wall functions like y+.

Question 7

Describe high y+.

Answer 7

Near wall cells are in the turbulent part of the BL.
No attempt to resolve the BL in detail.
Model used instead.

Question 8

Describe low y+.

Answer 8

The cells lie inside the viscous regime.
The grid is resolving the BL in detail.
Requires fine mesh at wall.