results in a low-pressure zone at the back of the section, called as wake, with local
fluid recirculation effects.
This dragging effect will be all the greater the larger the wake, so processes such
as thinning the geometry of the section of the bodies, tend to delay the separation
effect and reduce the formed wake and the consequent dragging force. This effect is
also known as form drag because it depends on the shape and orientation of the
body. In this way, a pressure gradient oriented in the direction of flow occurs,
tending to drag the solid body.
Maximum form drag occurs in surfaces at right angles to the fluid flow,
corresponding to bluff bodies matching the maximum pressure that a fluid can exert
in contributing to the total form drag on the body. From Eq. (A2.39), the rate of
moment transfer, or efflux through a control surface, from the fluid to a unit area of
a perpendicular surface of the body is 0.5 qV
2 . This quantity is the total form drag
over the immersed body considering a mean velocity of V/2 that exists if there is a
decrease of velocity from V, under normal flow, to 0 when flow is stopped at the
stagnation point in the body surface. In bluff bodies, the fluid tends to slip around
their sides so that the form drag force in the upstream face is lower than 0.5qV
2
(e.g. Monteith and Unsworth 2013).
The real form drag over a unit area will be given by c d 0.5qV
2
, where c d is the
total drag coefficient which is related to the combined form and skin drag. This
coefficient ranges between 0.4 and 1.2 for spherical and cylindric bodies under
Reynolds numbers between 10
2 and 10
3 . For surfaces parallel to the air stream the
diffusion of momentum in skin friction is analogous to the diffusion of gases
molecules and heat and water vapour. For surfaces perpendicular to airflow, there is
no frictional drag in the direction of flow and the similitude will occur only between
r av and r aH .
A2.5 Topics on Evaporation Physics
A2.5.1 Fundamental Principles
Atmospheric air usually contains vapor from the evaporation and a key concept in
environmental physics is the air relative humidity. It is defined as the ratio of its vapor
pressure, e, and the vapor saturation pressure at the same temperature, e s (T). The
relative humidity of the air (in analogy with saturation deficit) is a measure of the
drying capacity of the air. In a free water surface, the equilibrium condition of vapor
exchanges between that surface and the adjacent air, corresponds to the saturated air
with unitary relative humidity. In a situation of contact between air and a porous
body, such as soil, wood, or a saline solution, a lower air relative humidity exists
under equilibrium of the vapour exchanges (Monteith and Unsworth 1991).
358
Annex A2: Basic Topics on Laws of Motion and Evaporation
fluid recirculation effects.
This dragging effect will be all the greater the larger the wake, so processes such
as thinning the geometry of the section of the bodies, tend to delay the separation
effect and reduce the formed wake and the consequent dragging force. This effect is
also known as form drag because it depends on the shape and orientation of the
body. In this way, a pressure gradient oriented in the direction of flow occurs,
tending to drag the solid body.
Maximum form drag occurs in surfaces at right angles to the fluid flow,
corresponding to bluff bodies matching the maximum pressure that a fluid can exert
in contributing to the total form drag on the body. From Eq. (A2.39), the rate of
moment transfer, or efflux through a control surface, from the fluid to a unit area of
a perpendicular surface of the body is 0.5 qV
2 . This quantity is the total form drag
over the immersed body considering a mean velocity of V/2 that exists if there is a
decrease of velocity from V, under normal flow, to 0 when flow is stopped at the
stagnation point in the body surface. In bluff bodies, the fluid tends to slip around
their sides so that the form drag force in the upstream face is lower than 0.5qV
2
(e.g. Monteith and Unsworth 2013).
The real form drag over a unit area will be given by c d 0.5qV
2
, where c d is the
total drag coefficient which is related to the combined form and skin drag. This
coefficient ranges between 0.4 and 1.2 for spherical and cylindric bodies under
Reynolds numbers between 10
2 and 10
3 . For surfaces parallel to the air stream the
diffusion of momentum in skin friction is analogous to the diffusion of gases
molecules and heat and water vapour. For surfaces perpendicular to airflow, there is
no frictional drag in the direction of flow and the similitude will occur only between
r av and r aH .
A2.5 Topics on Evaporation Physics
A2.5.1 Fundamental Principles
Atmospheric air usually contains vapor from the evaporation and a key concept in
environmental physics is the air relative humidity. It is defined as the ratio of its vapor
pressure, e, and the vapor saturation pressure at the same temperature, e s (T). The
relative humidity of the air (in analogy with saturation deficit) is a measure of the
drying capacity of the air. In a free water surface, the equilibrium condition of vapor
exchanges between that surface and the adjacent air, corresponds to the saturated air
with unitary relative humidity. In a situation of contact between air and a porous
body, such as soil, wood, or a saline solution, a lower air relative humidity exists
under equilibrium of the vapour exchanges (Monteith and Unsworth 1991).
358
Annex A2: Basic Topics on Laws of Motion and Evaporation