Boundary Layer Development Flashcards
What causes a boundary layer?
There must be no discontinuity in velocity, and the no-slip condition, which dictates that the air flow at the boundary must be stationary relative to the boundary.
This implies there must be a region of fluid adjacent to the boundary in which the velocity rises to freestream value.
If there is a relative movement between particles in a boundary layer, what does this imply?
A velocity gradient implies that there must be shear stresses present.
What causes laminar to turbulent transition?
Instabilities in the BL produce breakdown of the laminar structure of the flow and cause turbulence to begin.
How does transition affect the structure of the boundary layer?
After the laminar BL, there is a short transition region above a laminar region. After the transition region there is a large, thickening turbulent region above a smaller laminar sub-layer.
What causes the viscous sub-layer?
The turbulent region is caused by eddies in the flow, which cannot exist at the wall due to the no-slip condition, which instead encourages laminar flow adjacent to the wall.
Why do instabilities occur in the boundary layer?
Re is inversely proportional to viscosity. The viscous forces “damp” disturbances and smooth flow gradients. This means that if the viscous forces are not strong enough to damp out the disturbances, and to keep the laminas sliding past each other, turbulent eddies are created.
What triggers transition?
Re is inversely proportional to viscosity. If the viscous forces are not strong enough to damp out the disturbances, and to keep the laminas sliding past each other, turbulent eddies are created.
How can transition be delayed?
With a favourable pressure gradient the BL will remain laminar for longer.
Where does transition typically occur on an aerofoil?
After the suction peak, shortly after the pressure gradient becomes adverse.
