Environmental Engineering Reference
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Inertia forces
faster moving
air toward
outside wall
R
FIGURE 3.3 
Secondary motion in a curved tube with radius of curvature R .
Flow in curved tubes has been studied experimentally, 79 theoretically, 77,80 and numerically. 81-83
Experimental 84,85 and computational 86,87 studies of low in curved airway conigurations have also
been performed.
3.6.4  F low in  b iFurcations and  b rancHing  n etworks
Because the lungs are a system of branching airways, much work has been done to characterize the
airlow patterns within both single bifurcations and branching networks. 84,85,88-91 Different types of
bifurcations have been considered, incorporating two-dimensional, three-dimensional, symmetric,
and asymmetric geometries. In general, experimental studies have indicated that a parabolic veloc-
ity proile will be skewed as it passes through a bifurcation, in a manner similar as in a curved
tube. 92 Figure 3.4 provides a qualitative example of these skewed velocity proiles.
M-shaped profile
x
y
Inner
walls
R 1
Parabolic velocity profile
Outer
walls
x
y
R 2
Profile skewed to
outer wall
FIGURE 3.4  Qualitative description of velocity proiles in a bifurcation. Flow passing through a bifurcation
behaves in a manner similar to low passing through a curved tube. The radii of curvature R 1 and R 2 for the two
daughter branches are shown. In the x plane (the plane of the bifurcation), the velocity proile becomes skewed
toward the outer wall. In the y plane (which is perpendicular to the plane of the bifurcation), an M-shaped
proile is observed.
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