VL X - Boundary layer canonical flow

Whats the Boundary-layer thickness

Where mean velocity reaches 99% of ambient flow

vertical line, very unstable

Whats the displacement thickness

distance from mean velocity profile for velocity profile to be a rectangle

Whats the momentum thickness

Momentum deficit caused by the boundary layer

Formel Prandtl length scale

lm = ky

Prandtl leangth scale for large y+

lm+ = 2ky+

Problem eddy viscosity approach

• RST: scales with y³

• Eddy viscosity scales with y²

Does the RST scale with TKE near the wall

Yes, fairly well

What to hairpin vortecies do?

They lead to lower boundary layer seperation, since they bring down higher momentum fluid

Name three different turbulence modeling CFD

• DNS: Ground truth all scales

• LES: larger scales resolved, smaller modeled

• RANS: largest scale resolved, all turbulent scales modeled

Physical analogy for eddy viscosity

• The collision of molecules for molecular fluctuations, happens between smaller eddies aswell