Boundary Layer Flow 11 - Boundary Layer Flow Equations

Question 1

Boundary layer concept

Answer 1

At high Re, viscous effects are confined to a thin layer close to solid surface - the boundary layer

Question 2

Difference between laminar and turbulent boundary layers

Answer 2

Laminar is thinner, turbulent is thicker

Question 3

Boundary layer properties

Answer 3

Boundary layer is driven by main free stream’s flow
Outside the BL, the flow is inertia dominated and can be assumed to be inviscid
Within the BL, viscous is important but not dominant because flow speed is still high in most areas of BL
Thickness of BL approaches 0 as viscosity goes to 0 or Re becomes infinitely large

Question 4

Difference between inside and outside the boundary layer

Answer 4

Inside - convection and diffusion are of the same order of magnitude
Outside - convection dominant, diffusion is relatively weak

Question 5

Order of magnitude of a non-dimensional quantity

Answer 5

1

Question 6

Significance of boundary layer equations

Answer 6

Pressure does not change across the BL
Pressure in the BL is dictated by free stream flow
In the x-direction, flow is dominated by convection, therefore downstream will not affect upstream
Diffusion only occurs in wall normal direction
Nonlinear term remains difficult to solve in general
Isolate viscous effects to a small region so that inviscid theory can be used for outer region
Solve entire flow field iteratively if necessary between outer inviscid flow and inner BL flow
Simple laminar flow BL can be solved analytically, turbulent BL flow is more complex
BL equations hold for flows in a thin BL with high Re only
Not valid for BL after flow separation from surface or over highly curved surface