6
Marine Mammal Physiology: Requisites for Ocean Living
The structure of the boundary layer, that is, how much of it is laminar and how much is
turbulent, largely determines how much drag is produced.
Another important feature of the boundary layer is flow separation, which occurs when
the flows within are decelerated to a near zero speed when approaching an area of greater
pressure, as usually occurs along the posterior half of a tapered body. If these adverse pressure gradients are strong enough, the boundary layer flows may reverse direction relative
to the streamlined flows above the layer. The interaction between the two creates a large
zone of turbulent flow that keeps the streamline flow away from the surface— effectively
“separating” the streamlined flow away from the body (Figure 1.1e). The creation of both the
turbulent wake and turbulent zone underneath the separated flows lead to more drag. Note
that turbulent boundary layers are less prone to adverse pressure gradients than laminar
layers and thus less conducive to flow separation and the ensuing drastic increase in drag.
“Tripping” or inducing the boundary layer to transition from laminar to turbulent, for example, by dimples (as on a golf ball) or sharp protrusions (by barnacles), can help reduce drag.
Note also that in some forms of undulatory swimming where more adverse pressure gradients appear along the body, boundary layer control becomes crucial for efficient propulsion.
1.2.2 Resistive forces
In order to move through the water, marine mammals must apply a force to part the fluid
around their body. In reaction to this force, the fluid applies a force (drag) onto the animal
(i.e., Newton’s third law of motion). From the point of view of the swimmer, drag is an
energy-dissipating force because it transfers kinetic energy away from the swimmer and
into the water. The animal never recovers this energy—instead, it gets dissipated into heat.
There are different types of drag depending on the mechanism that generates the
resistive force. The two most important are friction drag, due to the shear force (or sliding
“friction”) among the sub-layers of the boundary layer; and pressure drag, due to the existence of the turbulent wake and/or zone under separated flows (when present). Separated
flows sustain pressures that are low enough to effectively “suck” the body back into it,
thus resulting in drag. Generally, laminar boundary layers produce less friction drag than
turbulent boundary layers do. Turbulent wakes of smaller width (“girth”) (Figure 1.1c)
produce less pressure drag. With non-streamlined or bluff bodies, pressure drag dominates over friction drag, and the opposite occurs with streamlined bodies.
Another source of drag appears when a body is accelerated or decelerated, namely,
the acceleration reaction, which arises because of the necessity to accelerate—along with
the animal’s body—a fluid mass roughly equivalent to that of the fluid displaced by the
body (Daniel 1984). This added mass is important, for example, with sea lions because
of their accelerating–decelerating swimming style (Feldkamp 1987a; Stelle et al. 2000).
Added mass drag can be reduced by body streamlining, as spheres tow-along far
more added mass than javelins. For fusiform body shapes exhibited by marine mammals,
the added mass is approximately 5%–10% that of the displaced water (Williams 1987).
Some marine mammals use winglike flippers and flukes to generate lift and produce
thrust. Although lift is produced, these surfaces simultaneously generate a fourth type of
drag called induced drag. Induced drag appears because winglike structures produce a largescale vortex at the tip of the wing that extends far behind. Like the other vortices and eddies
produced in boundary layers and in turbulent wakes, tip vortices are tornado-like structures
of spinning fluid whose rotation is driven by the wing’s own kinetic energy. As a general rule,
induced drag is minimal when the wing is tapered, as is the case with most flukes and flippers. Induced drag is a major energy sink with aircraft, relative to the friction and pressure
Marine Mammal Physiology: Requisites for Ocean Living
The structure of the boundary layer, that is, how much of it is laminar and how much is
turbulent, largely determines how much drag is produced.
Another important feature of the boundary layer is flow separation, which occurs when
the flows within are decelerated to a near zero speed when approaching an area of greater
pressure, as usually occurs along the posterior half of a tapered body. If these adverse pressure gradients are strong enough, the boundary layer flows may reverse direction relative
to the streamlined flows above the layer. The interaction between the two creates a large
zone of turbulent flow that keeps the streamline flow away from the surface— effectively
“separating” the streamlined flow away from the body (Figure 1.1e). The creation of both the
turbulent wake and turbulent zone underneath the separated flows lead to more drag. Note
that turbulent boundary layers are less prone to adverse pressure gradients than laminar
layers and thus less conducive to flow separation and the ensuing drastic increase in drag.
“Tripping” or inducing the boundary layer to transition from laminar to turbulent, for example, by dimples (as on a golf ball) or sharp protrusions (by barnacles), can help reduce drag.
Note also that in some forms of undulatory swimming where more adverse pressure gradients appear along the body, boundary layer control becomes crucial for efficient propulsion.
1.2.2 Resistive forces
In order to move through the water, marine mammals must apply a force to part the fluid
around their body. In reaction to this force, the fluid applies a force (drag) onto the animal
(i.e., Newton’s third law of motion). From the point of view of the swimmer, drag is an
energy-dissipating force because it transfers kinetic energy away from the swimmer and
into the water. The animal never recovers this energy—instead, it gets dissipated into heat.
There are different types of drag depending on the mechanism that generates the
resistive force. The two most important are friction drag, due to the shear force (or sliding
“friction”) among the sub-layers of the boundary layer; and pressure drag, due to the existence of the turbulent wake and/or zone under separated flows (when present). Separated
flows sustain pressures that are low enough to effectively “suck” the body back into it,
thus resulting in drag. Generally, laminar boundary layers produce less friction drag than
turbulent boundary layers do. Turbulent wakes of smaller width (“girth”) (Figure 1.1c)
produce less pressure drag. With non-streamlined or bluff bodies, pressure drag dominates over friction drag, and the opposite occurs with streamlined bodies.
Another source of drag appears when a body is accelerated or decelerated, namely,
the acceleration reaction, which arises because of the necessity to accelerate—along with
the animal’s body—a fluid mass roughly equivalent to that of the fluid displaced by the
body (Daniel 1984). This added mass is important, for example, with sea lions because
of their accelerating–decelerating swimming style (Feldkamp 1987a; Stelle et al. 2000).
Added mass drag can be reduced by body streamlining, as spheres tow-along far
more added mass than javelins. For fusiform body shapes exhibited by marine mammals,
the added mass is approximately 5%–10% that of the displaced water (Williams 1987).
Some marine mammals use winglike flippers and flukes to generate lift and produce
thrust. Although lift is produced, these surfaces simultaneously generate a fourth type of
drag called induced drag. Induced drag appears because winglike structures produce a largescale vortex at the tip of the wing that extends far behind. Like the other vortices and eddies
produced in boundary layers and in turbulent wakes, tip vortices are tornado-like structures
of spinning fluid whose rotation is driven by the wing’s own kinetic energy. As a general rule,
induced drag is minimal when the wing is tapered, as is the case with most flukes and flippers. Induced drag is a major energy sink with aircraft, relative to the friction and pressure