2.1. THE PHYSICAL ENVIRONMENT
downstream of a body, thereby reducing form drag (e.g. Koehl 1977a, 1986,
Vogel 1984,Koehl and Alberte 1988, Carrington 1990). Vogel (1984) has proposed an index, the "figure of merit" B, to describe the relative reduction
in drag experienced by flexible structures as they reconfigure as flow velocity increases , where B is the slope of a log-log plot of speed-specific drag
D/ U
2 as a function of velocity; the greater the absolute value of the negative
slope, the greater the relative drag reduction experienced with an increase
in velocity. In addition, if the flexibility of a blade-like sessile organism permits it to be pushed down close to the substratum, the underside of the
blade encounters slower flow than the upper surface, thereby reducing the
shear and thus the skin friction drag on that surface (Koehl 1986). Although
shape can affect the hydrodynamic forces which flexible organisms experience (e.g. Koehl 1977a, Koehl and Alberte 1988, Johnson and Koehl 1994),
Carrington (1990) found that a variety of very flexible blade-like, branching, and bushy seaweeds converged onto similar drag coefficients when
subjected to high water velocities at which they all were compacted into
similar streamlined bundles.
Benthic organisms in a water current can also experience lift, the hydrodynamic force acting at right angles to drag. When water speeds up locally as
it moves over and around an obstacle, such as an organism, the local pressure
on the organism is lower where the flow along its surface is faster; a pressure difference across the body of an organism can thus develop. Organisms
protruding above the substratum are pulled up by lift, and organisms that
present an asymmetric shape to the oncoming current are pulled laterally if
the water speeds up more to move around one side of the organism relative
to the other (Denny 1988, Vogel 1994). Lift (L) is given by
19
(2.8)
where C L is the coefficient oflift (which depends on shape) and S is a relevant
area (usually planiform area normal to the direction in which the lift acts) .
Thus, as with drag, lift increases as an organism grows, not only because
its S increases, but more importantly because it encounters higher water
velocities. Even a symmetric structure, such as the cylindrical branch of an
animal colony, can experience transient lateral lift, alternating from side to
side as vortices are shed in the wake behind it (explained in Denny 1988).
If such structures are flexible, they wobble side-to-side as water flows past .
Furthermore, if flexible organisms are pushed over by drag, and pulled back
up by lift, they can flutter like a flag; the wake behind a fluttering organism
can be bigger and the drag force higher than on a body of similar shape and
orientation that does not flutter (Koehl and Alberte 1988). When ambient
currents encounter a branching structure, such as a coral colony, some of
the water flows between the branches, but most of it is diverted above the
colony. The lower pressure that occurs above a coral colony as water speeds
up to flow over it not only subjects the colony to lift, but also can draw the
slowly-moving water between the branches up and out of the colony,thereby
reducing the stagnation of flow that can occur in the middle of colonies as
they grow larger (Chamberlain and Graus 1977).
WAVES. Sessileorganisms exposed to wavesexperience back-and-forth water motion. Since the velocity changes with time, the instantaneous lift and
drag that the organisms experience (which are proportional to the square
downstream of a body, thereby reducing form drag (e.g. Koehl 1977a, 1986,
Vogel 1984,Koehl and Alberte 1988, Carrington 1990). Vogel (1984) has proposed an index, the "figure of merit" B, to describe the relative reduction
in drag experienced by flexible structures as they reconfigure as flow velocity increases , where B is the slope of a log-log plot of speed-specific drag
D/ U
2 as a function of velocity; the greater the absolute value of the negative
slope, the greater the relative drag reduction experienced with an increase
in velocity. In addition, if the flexibility of a blade-like sessile organism permits it to be pushed down close to the substratum, the underside of the
blade encounters slower flow than the upper surface, thereby reducing the
shear and thus the skin friction drag on that surface (Koehl 1986). Although
shape can affect the hydrodynamic forces which flexible organisms experience (e.g. Koehl 1977a, Koehl and Alberte 1988, Johnson and Koehl 1994),
Carrington (1990) found that a variety of very flexible blade-like, branching, and bushy seaweeds converged onto similar drag coefficients when
subjected to high water velocities at which they all were compacted into
similar streamlined bundles.
Benthic organisms in a water current can also experience lift, the hydrodynamic force acting at right angles to drag. When water speeds up locally as
it moves over and around an obstacle, such as an organism, the local pressure
on the organism is lower where the flow along its surface is faster; a pressure difference across the body of an organism can thus develop. Organisms
protruding above the substratum are pulled up by lift, and organisms that
present an asymmetric shape to the oncoming current are pulled laterally if
the water speeds up more to move around one side of the organism relative
to the other (Denny 1988, Vogel 1994). Lift (L) is given by
19
(2.8)
where C L is the coefficient oflift (which depends on shape) and S is a relevant
area (usually planiform area normal to the direction in which the lift acts) .
Thus, as with drag, lift increases as an organism grows, not only because
its S increases, but more importantly because it encounters higher water
velocities. Even a symmetric structure, such as the cylindrical branch of an
animal colony, can experience transient lateral lift, alternating from side to
side as vortices are shed in the wake behind it (explained in Denny 1988).
If such structures are flexible, they wobble side-to-side as water flows past .
Furthermore, if flexible organisms are pushed over by drag, and pulled back
up by lift, they can flutter like a flag; the wake behind a fluttering organism
can be bigger and the drag force higher than on a body of similar shape and
orientation that does not flutter (Koehl and Alberte 1988). When ambient
currents encounter a branching structure, such as a coral colony, some of
the water flows between the branches, but most of it is diverted above the
colony. The lower pressure that occurs above a coral colony as water speeds
up to flow over it not only subjects the colony to lift, but also can draw the
slowly-moving water between the branches up and out of the colony,thereby
reducing the stagnation of flow that can occur in the middle of colonies as
they grow larger (Chamberlain and Graus 1977).
WAVES. Sessileorganisms exposed to wavesexperience back-and-forth water motion. Since the velocity changes with time, the instantaneous lift and
drag that the organisms experience (which are proportional to the square