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Boundary Layers
Hydromechanics VVR090
ppt by Magnus Larson; revised by Rolf L Jan 2014, Feb 2015, Feb 2017
SYNOPSIS
1. Boundar Layer on a Flat Plate
2. Von Karman momentum integral equation
3. Laminar Boundary Layer along a Flat Plate
4. Drag Coefficient for Smooth, Flat Plates
5. Examples/Problems
Figure numbers and Equation numbers refer to
Vennard and Street : Elementary Fluid Mechanics
1. Boundar Layer on a Flat Plate
Boundary layer: the zone in which the velocity profile is
governed by frictional action
V0 = free stream velocity (m/s)
δ = boundary layer (m)
Fig. 13.6
Boundary layer characterized by a Reynolds number:
3900 laminar flow
3900 turbulent flow
oVR
R
R
Viscous sublayer always exists close to the surface.
Mathematical implications of boundary layer:
x = locally tangent to any point along the surface of body
y = locally normal to any point along the surface of body
2. Von Karman momentum integral equation
• Apply momentum equation for control volume ABCD.
• Height of control volume extends beyond the edge of the boundary layer (to
the outer flow).
• At edge of the boundary layer: po(x), Vo(x) (known).
• Small curvature along body.
Conservation of mass:
Conservation of momentum:
2
0
h
oo y h y h
dp dh u dy uv
dx dx
0
0
h
y h
dudy v
dx
Develop conservation of mass equation:
0
h
o o
dv udy
dx
Develop conservation of momentum equation:
2
0
h
oo o o o
dp dh u dy V v
dx dx
Eq. 13.2
Eq. 13.3
Combine conservation of mass and momentum:
2
0 0
h h
oo o
dp d dh u dy V udy
dx dx dx
Euler equation for the outer flow:
0
oo o
o
o oo o
dpV dV
dp dVV
dx dx
Eq. 13.4
2
0 0
h h
oo o o o
dVd du dy V udy V h
dx dx dx
Combining:
Define displacement thickness = δ1
1
0
h
o o o oV V u dy
Flow rate with and without boundary layer
0
o oudy V
1
0
1
h
o o
udy
V
(the distance by which the boundary layer should be displaced to compensate
for the reduction in flow rate on account of boundary layer formation)
Eq. 13.5
Eq. 13.7
In the same manner, define momentum thickness:
2
0
1
h
o o o
u udy
V V
Substitute in and in equation 13.5 and express
o in terms of a local friction coefficient cf:
2
2
fc d
dx
(constant density, no pressure gradients)
21
Eq. 13.7
Eq. 13.11
3. Laminar Boundary Layer along a Flat Plate
Assume parabolic velocity profile:
2
2
2
oV y yu
2
2
2
2
15
2
2 15
22
15
fo
o
oo o
c d
V dx
VdV
dx
Eq. 13.13
2
2
22
15
15
2
30
oo
o
x
VdV
dx
x
V
x R
Integrate the equation:
28
2 15
oo
x
V
R
Substitute in in shear stress expression:/d dx
Eq. 13.16
Local friction coefficient:
8
15f
x
cR
Mean friction coefficient along the plate:
0
1 32
15
x
f f
x
C c dxx R
Eq. 13.17
Eq. 13.18
Relationship between and :
2
30x
RR
= 3900 = 500,000
(transition between laminar and turbulent
conditions in the boundary layer)
R R x
R R x
Eq. 13.19
4. Drag Coefficient for Smooth, Flat Plates
21
2f f oD C V A A: surface area of plate
Fig. 13.9
5. Examples / problems
Example I: Boundary layer and drag on ship model
A ship model 1.5 m long and with a draft of 0.15 m is towed at a
velocity of 0.3 m/s in a basin containing water at 16 C. The model
scale is 1:64.
Assuming that one side of the immersed portion of the hull may
be approximated by a smooth flat plate (1.5 m x 0,15 m), estimate
the frictional drag of the hull and the thickness of the boundary
layer at the stern of the model if the boundary layer is
a) laminar and
b) turbulent.
c) If the measured total drag of the model is 0.178 N, estimate the
total drag of the prototype.