To analyze aerodynamic resistance, it is necessary to define overall drag F, on an
isolated obstacle (e.g., area of a flat leaf) with an area A, as follows (Thom 1975):
F ¼ qu
2 AC M
ð2:26Þ
where q is the air density and C M is a dimensionless proportionality constant
denoted the drag coefficient, a function of the orientation of the obstacle and airspeed, u. The overall drag is equal to the rate of change of linear momentum in the
airflow, and therefore, the C M value is a measure of the efficiency of the obstacle in
absorbing momentum from the atmospheric flow. The C M parameter may thus be
defined as the momentum transfer coefficient. This drag, also termed as form drag,
is a force applied to bodies immersed in moving fluids, additional to skin friction, in
the flow direction because of the fluid acceleration and is dependent on the shape
and orientation of bodies (Annex 2).
The momentum concentration Q M, in an incident air flow, is given by:
Q M ¼ qu
ð2:27Þ
with units as (ML
−3 ) (LT
−1 ) = MLT
−1 /L
3 , (linear momentum/volume). Using
Eq. (2.25) to calculate aerodynamic drag r M , for linear momentum transfer between
the incident air and the surface of an individual element where the momentum
concentration is zero, gives
r M ¼
quA
F
ð2:28Þ
where F/A, corresponding to the overall drag applied per unit area of the obstacle, is
representative of the momentum flux over the obstacle. From Eq. (2.26) comes
r M ¼
1
uC M
ð2:29Þ
Drag force per unit of the horizontal area of the vegetative canopy can be
expressed in a way equivalent to Eq. (2.26) giving
s ¼ qu
2 C aM
ð2:30Þ
where C aM is the drag coefficient per unit horizontal area for the entire canopy.
Equations (2.10), (2.16), and (2.30) can be written as
C aM ¼
u Ã
u z
ð Þ
&
' 2
¼
K
2
ln z À d
ð
Þ=z OM
ð
Þ
2
ð2:31Þ
22
2 Aerodynamic Characterization of the Surface Layer
isolated obstacle (e.g., area of a flat leaf) with an area A, as follows (Thom 1975):
F ¼ qu
2 AC M
ð2:26Þ
where q is the air density and C M is a dimensionless proportionality constant
denoted the drag coefficient, a function of the orientation of the obstacle and airspeed, u. The overall drag is equal to the rate of change of linear momentum in the
airflow, and therefore, the C M value is a measure of the efficiency of the obstacle in
absorbing momentum from the atmospheric flow. The C M parameter may thus be
defined as the momentum transfer coefficient. This drag, also termed as form drag,
is a force applied to bodies immersed in moving fluids, additional to skin friction, in
the flow direction because of the fluid acceleration and is dependent on the shape
and orientation of bodies (Annex 2).
The momentum concentration Q M, in an incident air flow, is given by:
Q M ¼ qu
ð2:27Þ
with units as (ML
−3 ) (LT
−1 ) = MLT
−1 /L
3 , (linear momentum/volume). Using
Eq. (2.25) to calculate aerodynamic drag r M , for linear momentum transfer between
the incident air and the surface of an individual element where the momentum
concentration is zero, gives
r M ¼
quA
F
ð2:28Þ
where F/A, corresponding to the overall drag applied per unit area of the obstacle, is
representative of the momentum flux over the obstacle. From Eq. (2.26) comes
r M ¼
1
uC M
ð2:29Þ
Drag force per unit of the horizontal area of the vegetative canopy can be
expressed in a way equivalent to Eq. (2.26) giving
s ¼ qu
2 C aM
ð2:30Þ
where C aM is the drag coefficient per unit horizontal area for the entire canopy.
Equations (2.10), (2.16), and (2.30) can be written as
C aM ¼
u Ã
u z
ð Þ
&
' 2
¼
K
2
ln z À d
ð
Þ=z OM
ð
Þ
2
ð2:31Þ
22
2 Aerodynamic Characterization of the Surface Layer