[Vignesh2508]

Guys,

In an interview i was asked this question. How do you decide on the number of boundary layers to have in an external aerodynamics case?

My answer was that first one has to calculate the boundary layer height. After determining that and deciding on the y+ value, calculate the first cell height. From there adjust the number of layers to match the boundary layer height with a growth of 1.2 - 1.5 (user's wish).

Question:

1. Is there a single expression used to determine the boundary layer height? Are there different expressions for different cases?
2. The velocity increases more than freestream in case of say an airfoil. So do we have to change the boundary layer a posteriori?

[mazhar16823]

Originally Posted by Vignesh2508

Guys,

In an interview i was asked this question. How do you decide on the number of boundary layers to have in an external aerodynamics case?

My answer was that first one has to calculate the boundary layer height. After determining that and deciding on the y+ value, calculate the first cell height. From there adjust the number of layers to match the boundary layer height with a growth of 1.2 - 1.5 (user's wish).

Question:

1. Is there a single expression used to determine the boundary layer height? Are there different expressions for different cases?
2. The velocity increases more than freestream in case of say an airfoil. So do we have to change the boundary layer a posteriori?

Well. You should've a target value for y+ in the start. The number of boundary layers depend whether you want to fully capture the boundary layer or in other words boundary layer thickness.
To determine how many BL's I should keep to fully capture the BL, I would use the turbulent boundary layer for flate plate equations i.e. first determining chord_based Re number, skin coefficient, wall shear stress, friction velocity,and then wall distance (Y) for the first layer. Then, I will use thickness ratio either 1.2 or 1.3 to see how many boundary layers I need to reach the total boundary layer thickness.