Introdução à Camada Limite. 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/aerodynamics/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