60
2 Water at Rest and in Motion
Table 2.2: Measured lift coefficients for marine organisms (adapted from
Denny, 1988)
Organism
Re
Cz
Limpets
5(7) x 10 4 0.28-0.67
Barnacles
10 5
0.5
Snails
10 4
0.67
Coral Acropora reticulata
2 x 10 6 0.71-1.02
Kelp
7 x 10 4
1.5
In the case of lift forces, the relevant area is that projected perpendicular to
the direction of flow, Sp, often called the platform area. Similarly to drag force,
the dependence of the lift force on the shape of the object is included through
an empirically determined lift coefficient, Cz• Thus we have:
(2.75)
Lift force data are far less consistent than drag force data and the scatter (from
o to 1.4) is attributed to various causes, such as the effect of the end gaps and
free stream turbulence. Following Denny (1988), the values of lift coefficients
for some marine organisms have been collected in Table 2.2. We also note here
that Wilson and Reid (1963) showed that the Cz value for a pipeline seated on
the sea floor reaches a value of 4.50.
In real situations in the ocean, both lift and drag components produce the
resulting force on an immersed body. The same force is imposed on animals
moving in calm water. Locomotion is an energetically costly activity that
may comprise a significant component of an animal's overall energy budget or
may require a powerful engine to set a man-made object (i. e. ship) in motion.
Therefore, in nature as well as in human activity there are some shapes adopted
and strategies developed which minimize energy expenditure. These natural
and human-designed body shapes, producing lift in water with minimum drag,
are called hydrofoils, in contrast to such shapes in air called aerofoils. To
minimize drag, a hydrofoil has to be a streamlined body (Fig. 2.27).
A measure of a hydrofoils usefulness is the ratio of lift to drag forces:
Lift
Drag
~ CzPw Spu6
1CdPw Su 6
(2.76)
2 Water at Rest and in Motion
Table 2.2: Measured lift coefficients for marine organisms (adapted from
Denny, 1988)
Organism
Re
Cz
Limpets
5(7) x 10 4 0.28-0.67
Barnacles
10 5
0.5
Snails
10 4
0.67
Coral Acropora reticulata
2 x 10 6 0.71-1.02
Kelp
7 x 10 4
1.5
In the case of lift forces, the relevant area is that projected perpendicular to
the direction of flow, Sp, often called the platform area. Similarly to drag force,
the dependence of the lift force on the shape of the object is included through
an empirically determined lift coefficient, Cz• Thus we have:
(2.75)
Lift force data are far less consistent than drag force data and the scatter (from
o to 1.4) is attributed to various causes, such as the effect of the end gaps and
free stream turbulence. Following Denny (1988), the values of lift coefficients
for some marine organisms have been collected in Table 2.2. We also note here
that Wilson and Reid (1963) showed that the Cz value for a pipeline seated on
the sea floor reaches a value of 4.50.
In real situations in the ocean, both lift and drag components produce the
resulting force on an immersed body. The same force is imposed on animals
moving in calm water. Locomotion is an energetically costly activity that
may comprise a significant component of an animal's overall energy budget or
may require a powerful engine to set a man-made object (i. e. ship) in motion.
Therefore, in nature as well as in human activity there are some shapes adopted
and strategies developed which minimize energy expenditure. These natural
and human-designed body shapes, producing lift in water with minimum drag,
are called hydrofoils, in contrast to such shapes in air called aerofoils. To
minimize drag, a hydrofoil has to be a streamlined body (Fig. 2.27).
A measure of a hydrofoils usefulness is the ratio of lift to drag forces:
Lift
Drag
~ CzPw Spu6
1CdPw Su 6
(2.76)