Boundary Layer Flashcards
Boundary Layer
A thin layer, a region where shear stress dominates and is rotational with non-negligible viscous forces.
Outer Flow
Inviscid and/or irrotational region of flow.
Boundary layer edge
the edge around and defines boundary layer.
No-split Condition
The speed of the fluid at the wall/surface is 0, important for the development of velocity profile close to the surface and skin friction drag forces.
Laminar flow in boundary layer
No fluctuations, predictable, limited mixing, no vortices, lower velocity close to the wall, lower shear stress.
Separates easily due to low momentum close to wall.
Turbulent flow in Boundary layer
High fluctuation, chaotic, increased mixing, vortices, higher velocity close to the wall, higher shear stress.
Resist to flow separation due to high momentum close to wall.
Displacement Thickness
Imaginary increase in thickness of the wall seen by the outer flow due to the reduce āuā velocity within the boundary layer. // the distance by which the solid boundary would have to be displaced to maintain the same mass flow rate in an imaginary frictionless flow.
Momentum Thickness
Distance by which the solid boundary would have to be displaced to maintain the same momentum transport rate at the actual mass flow rate in an imaginary frictionless flow.
Zero Pressure Gradient
dP/dx = 0; V1=V1; P1=P1
Favourable Pressure Gradient
dP/dx < 0; V2 > V1; P2 < P1
Adverse Pressure Gradient
dP/dx > 0; V1 > V2; P2> P1
Nozzle (BL growth and Separation)
Decreasing pressure and area. Increasing velocity and favourable gradient.
Throat (BL growth and Separation)
Constant pressure and area, velocity constant and zero pressure gradient.
Diffuser (BL growth and Separation)
Increasing pressure and area, decreasing velocity and adverse pressure gradient.
Separation Region (BL theory is not valid)
A region of recirculating flow:
1. Separation due to abrupt geometry change.
2. Separation due to adverse pressure gradient.
