Transcript Aula Teórica 17

Aula Teórica 24
Introdução à Camada Limite.
Potential flow
• No Friction; flow in front of
the object is symmetric of
the flow in the back and
there is no pressure
resistance, i.e. there is no
drag.
• If the flow did not follow the surface of the object, the velocity
would not decrease and consequently the pressure could not
increase.
• This is what happens when BL separation occurs.
BL Separation
http://www.thermopedia.com/content/595/?
tid=104&sn=1159
Boundary
Layer
Separation
• The concavity of the velocity profile depends
on the sign of the pressure gradient: is
negative when the pressure decays and
positive otherwise.
Favourable pressure gradient
• There is no force that can invert the flow, i.e. there is
no separation of the boundary layer.
• Acceleration is the result of the pressure gradient
and of the divergence od the diffusive flux (negative)
Null pressure gradient
Positive pressure gradient
Shear stress is maximum at
the inflexion point,
decreasing towards the
wall. Faster fluid drags
slower fluid.
Adverse pressure gradient
• In case of the adverse pressure gradient
pressure force decreases the velocity and can
invert the sense of the flow.
• For layers close to the wall the forward shear
stress (above) is larger that the backward
shear (below) and thus friction contribute to
keep the forward movement, i.e. reduces the
probability of flow separation.
Turbulent BL separate
latter than laminar BL
BL separate latter in
fusiform (aerodynamic)
geometries.
Is this profile
ok?
http://www.aerospaceweb.org/question/aer
odynamics/q0215.shtml
Cd for circular Cylinders
Shape and friction resitance
Flow around a cylinder for different
values of Re
aerofoils
http://www.comsol.com/showroom/animations/963/
• Sebenta de Mecânica dos Fluidos