10
Marine Mammal Physiology: Requisites for Ocean Living
(i.e., are more symmetrical)—because they are used to produce forward-directed propulsion, which involves the use of both downward- and upward-directed lift.
The basic design of a lift-producing winglike surface hinges on minimizing drag
and maximizing lift. In general, both lift and drag increase with increasing hydrofoil
angle of attack (AOA). AOA characterizes the angle between the direction of the flow
relative to the surface of the foil, and orientation of its mean chord line (Figure 1.4a).
In order to produce the most lift at the lowest energetic (drag) cost, marine mammals
must optimize the lift-to-drag ratio. At low AOA, drag is the result of friction, pressure,
and induced drag. In this regime, the “best” AOA for optimal lift-to-drag is at about 15°,
depending on the morphological design of the hydrofoil. This is why dolphins, which
use flukes as hydrofoils to produce lift, appear to swim almost effortlessly by barely
moving their flukes at low AOA.
When a wing is tilted beyond a certain “critical” AOA of ~18° (depending on specific
wing design), drag increases and lift decreases resulting in stall. During stall the boundary
layer has separated all the way to the wing’s leading edge, leaving a large zone of turbulent
air—and lots of pressure drag—over the wing (Figure 1.4b). The ratio of lift to drag decreases
significantly and this diminishes the flipper or fluke’s ability to produce the beneficial force
(lift) for control and propulsion. In the average wing example above, 15° is not that far from
18° and so, from the point of view of design, one needs wing profile that keeps the “best”
AOA as far away from the “critical” AOA. Because wing friction drag dominates pressure
drag at low AOA, it is advantageous to have a wing over which the boundary layer is largely
laminar. Marine mammals achieve this by having flexible flippers and flukes (Fish et al.
2006) and/or integration with structures that further control the flow, exemplified by the
tubercles on humpback whale flippers (Miklosovic et al. 2004; Fish and Lauder 2006).
Fluke or flipper (hydrofoil) thrust along the line of the body arises from the vertical
motions of these appendages. In combination with the forward movement of the body,
this motion effectively changes the direction of the flow relative to the hydrofoil (and corresponding AOA), in a manner to tilt forward the lift force relative to the animal’s body,
and with enough magnitude to cancel the rearward action of the drag (likewise tilted) (see
p. 268 in Vogel 2003). It is here that the flukes’ flexibility becomes advantageous, as it puts
the AOA closer to the “best” AOA and also further away from the critical regime of stall.
Many marine mammals that have flippers use them primarily for maneuvering, that is, to
effect turns and rolls. Here, the flippers may be rotated to adjust AOA and the magnitude
of lift along the vertical.
(a)
(b)
Flow separation
Figure 1.4 (a) The effect of the angle of attack (AOA) on hydrodynamic performance of h ydrofoillike marine mammal appendages. (b) The effect of appendage flexibility on hydrodynamic
performance.
Marine Mammal Physiology: Requisites for Ocean Living
(i.e., are more symmetrical)—because they are used to produce forward-directed propulsion, which involves the use of both downward- and upward-directed lift.
The basic design of a lift-producing winglike surface hinges on minimizing drag
and maximizing lift. In general, both lift and drag increase with increasing hydrofoil
angle of attack (AOA). AOA characterizes the angle between the direction of the flow
relative to the surface of the foil, and orientation of its mean chord line (Figure 1.4a).
In order to produce the most lift at the lowest energetic (drag) cost, marine mammals
must optimize the lift-to-drag ratio. At low AOA, drag is the result of friction, pressure,
and induced drag. In this regime, the “best” AOA for optimal lift-to-drag is at about 15°,
depending on the morphological design of the hydrofoil. This is why dolphins, which
use flukes as hydrofoils to produce lift, appear to swim almost effortlessly by barely
moving their flukes at low AOA.
When a wing is tilted beyond a certain “critical” AOA of ~18° (depending on specific
wing design), drag increases and lift decreases resulting in stall. During stall the boundary
layer has separated all the way to the wing’s leading edge, leaving a large zone of turbulent
air—and lots of pressure drag—over the wing (Figure 1.4b). The ratio of lift to drag decreases
significantly and this diminishes the flipper or fluke’s ability to produce the beneficial force
(lift) for control and propulsion. In the average wing example above, 15° is not that far from
18° and so, from the point of view of design, one needs wing profile that keeps the “best”
AOA as far away from the “critical” AOA. Because wing friction drag dominates pressure
drag at low AOA, it is advantageous to have a wing over which the boundary layer is largely
laminar. Marine mammals achieve this by having flexible flippers and flukes (Fish et al.
2006) and/or integration with structures that further control the flow, exemplified by the
tubercles on humpback whale flippers (Miklosovic et al. 2004; Fish and Lauder 2006).
Fluke or flipper (hydrofoil) thrust along the line of the body arises from the vertical
motions of these appendages. In combination with the forward movement of the body,
this motion effectively changes the direction of the flow relative to the hydrofoil (and corresponding AOA), in a manner to tilt forward the lift force relative to the animal’s body,
and with enough magnitude to cancel the rearward action of the drag (likewise tilted) (see
p. 268 in Vogel 2003). It is here that the flukes’ flexibility becomes advantageous, as it puts
the AOA closer to the “best” AOA and also further away from the critical regime of stall.
Many marine mammals that have flippers use them primarily for maneuvering, that is, to
effect turns and rolls. Here, the flippers may be rotated to adjust AOA and the magnitude
of lift along the vertical.
(a)
(b)
Flow separation
Figure 1.4 (a) The effect of the angle of attack (AOA) on hydrodynamic performance of h ydrofoillike marine mammal appendages. (b) The effect of appendage flexibility on hydrodynamic
performance.