PPT: Boundary Layers | Fluid Mechanics for Mechanical Engineering PDF Download

 Page 1


Introduction Introduction
The drag on a body passing through a fluid may be considered to be made up of
two components: Form drag and Skin friction drag.
Form drag: which is dependent on the pressure forces acting on the body; and
the skin friction drag , which depends on the shearing forces acting between
the body and the fluid.
Page 2


Introduction Introduction
The drag on a body passing through a fluid may be considered to be made up of
two components: Form drag and Skin friction drag.
Form drag: which is dependent on the pressure forces acting on the body; and
the skin friction drag , which depends on the shearing forces acting between
the body and the fluid.
Shear Force and Pressure Force Shear Force and Pressure Force

 Shear forces:
 viscous drag, frictional drag, or skin friction
 caused by shear between the fluid and the 
solid surface
 function of ___________and ______of 
object 
surface area 
length 
flow separation 
U U
U U
Major losses in pipes
Flow expansion 
losses
Projected area

 Pressure forces
 pressure drag or form drag
 caused by _____________from the body
 function of area normal to the flow
Page 3


Introduction Introduction
The drag on a body passing through a fluid may be considered to be made up of
two components: Form drag and Skin friction drag.
Form drag: which is dependent on the pressure forces acting on the body; and
the skin friction drag , which depends on the shearing forces acting between
the body and the fluid.
Shear Force and Pressure Force Shear Force and Pressure Force

 Shear forces:
 viscous drag, frictional drag, or skin friction
 caused by shear between the fluid and the 
solid surface
 function of ___________and ______of 
object 
surface area 
length 
flow separation 
U U
U U
Major losses in pipes
Flow expansion 
losses
Projected area

 Pressure forces
 pressure drag or form drag
 caused by _____________from the body
 function of area normal to the flow
Description of Boundary Layer Description of Boundary Layer
t
w
: wall shear stresses
U
d
In the immediate vicinity of the boundary surface, the velocity of the fluid
increases gradually from zero at boundary surface to the velocity of the
mainstream. This region is known as BOUNDARY LAYER.
Large velocity gradient leading to appreciable shear stress:
0 y
u
y
t µ
=
? ? ?
=
? ?
?
? ?
The nominal thickness of BOUNDARY LAYER is defined as the distance from
the boundary where the velocity of fluid is 99 % of free stream velocity
Page 4


Introduction Introduction
The drag on a body passing through a fluid may be considered to be made up of
two components: Form drag and Skin friction drag.
Form drag: which is dependent on the pressure forces acting on the body; and
the skin friction drag , which depends on the shearing forces acting between
the body and the fluid.
Shear Force and Pressure Force Shear Force and Pressure Force

 Shear forces:
 viscous drag, frictional drag, or skin friction
 caused by shear between the fluid and the 
solid surface
 function of ___________and ______of 
object 
surface area 
length 
flow separation 
U U
U U
Major losses in pipes
Flow expansion 
losses
Projected area

 Pressure forces
 pressure drag or form drag
 caused by _____________from the body
 function of area normal to the flow
Description of Boundary Layer Description of Boundary Layer
t
w
: wall shear stresses
U
d
In the immediate vicinity of the boundary surface, the velocity of the fluid
increases gradually from zero at boundary surface to the velocity of the
mainstream. This region is known as BOUNDARY LAYER.
Large velocity gradient leading to appreciable shear stress:
0 y
u
y
t µ
=
? ? ?
=
? ?
?
? ?
The nominal thickness of BOUNDARY LAYER is defined as the distance from
the boundary where the velocity of fluid is 99 % of free stream velocity
Description of Boundary Layer Description of Boundary Layer
t
w
: wall shear stresses
U
d
shear stress:
u
y
t µ
? ? ?
=
? ?
?
? ?
Shear stress acting at the plate surface
sets up a shear force which opposes
the fluid motion, and fluid close to the
wall is decelerated.
Theoretical understanding on Boundary layer development is very important to 
determine the velocity gradient and  hence shear forces on the surface. 
Consists of two layers:
CLOSE TO BOUNDARY : large velocity gradient, appreciable viscous forces.
OUTSIDE BOUNDARY LAYER: viscous forces are negligible, flow may be
treated as non-viscous or inviscid.
Page 5


Introduction Introduction
The drag on a body passing through a fluid may be considered to be made up of
two components: Form drag and Skin friction drag.
Form drag: which is dependent on the pressure forces acting on the body; and
the skin friction drag , which depends on the shearing forces acting between
the body and the fluid.
Shear Force and Pressure Force Shear Force and Pressure Force

 Shear forces:
 viscous drag, frictional drag, or skin friction
 caused by shear between the fluid and the 
solid surface
 function of ___________and ______of 
object 
surface area 
length 
flow separation 
U U
U U
Major losses in pipes
Flow expansion 
losses
Projected area

 Pressure forces
 pressure drag or form drag
 caused by _____________from the body
 function of area normal to the flow
Description of Boundary Layer Description of Boundary Layer
t
w
: wall shear stresses
U
d
In the immediate vicinity of the boundary surface, the velocity of the fluid
increases gradually from zero at boundary surface to the velocity of the
mainstream. This region is known as BOUNDARY LAYER.
Large velocity gradient leading to appreciable shear stress:
0 y
u
y
t µ
=
? ? ?
=
? ?
?
? ?
The nominal thickness of BOUNDARY LAYER is defined as the distance from
the boundary where the velocity of fluid is 99 % of free stream velocity
Description of Boundary Layer Description of Boundary Layer
t
w
: wall shear stresses
U
d
shear stress:
u
y
t µ
? ? ?
=
? ?
?
? ?
Shear stress acting at the plate surface
sets up a shear force which opposes
the fluid motion, and fluid close to the
wall is decelerated.
Theoretical understanding on Boundary layer development is very important to 
determine the velocity gradient and  hence shear forces on the surface. 
Consists of two layers:
CLOSE TO BOUNDARY : large velocity gradient, appreciable viscous forces.
OUTSIDE BOUNDARY LAYER: viscous forces are negligible, flow may be
treated as non-viscous or inviscid.
Development of Boundary Layer Development of Boundary Layer
In laminar boundary layer the particles are moving along stream lines.
The boundary layer thickness increases as the distance x from leading edge is
increases. This is because of viscous forces that dissipate more and more
energy of fluid stream as the flow proceeds and large group of particles are slow
downed.
The disturbance in fluid flow in boundary layer is amplified and the flow become
unstable and the fluid flow undergoes transition from laminar to turbulent flow.
This regime is called transition regime.