Biomedical Engineering Reference
In-Depth Information
where
p
is the perimeter of the cross section (wetted perimeter). We can replace
the unknown wall shear stress by a dimensionless unknown, the friction factor
f
defined as
τ
w
f
=
1
2
(2.49)
2
ρ
U
Combining (2.48) and (2.49) yields
pL
æ
1
2
ö
2
D =
P f
ρ
U
(2.50)
ç
÷
S
è
ø
Equation (2.50) may be transformed into a standard form by introducing the
notion of a hydraulic diameter.
2.2.6.2
HydraulicDiameter
Tubes and ducts are compared through their hydraulic diameter defined as
S
D
=
4
(2.51)
H
p
where
S
is the cross section and
p
is the wetted perimeter. Some values of the hy-
draulic diameter are indicated in Figure 2.25.
Figure 2.25
Hydraulicdiameterofdifferenttubes.
