8
Marine Mammal Physiology: Requisites for Ocean Living
whenever portions of the body break the water surface, such as with a seal’s head or a
cetacean’s blowhole. Ventilation drag arises because of the mass of water that is elevated
on the leading side of the exposed body feature; and also because of the depressed wake
that follows behind (Figure 1.2a). “Drag” appears because its takes a force to push water
upward above the water surface as well as downward below that surface. Ventilation
drag can be minimized through streamlining and reduction of the surface-breaking
body feature.
Wave drag—a combined form of “wave-making resistance” or “area blockage” drag—
arises from the necessity to move the dorsal portion of the freestream over the smaller
volume in comparison to the same at depth (Figure 1.2b). As with moving water through
a funnel, the fluid has to accelerate through the narrower section in order to pass the
same amount of fluid mass per unit time as through the wider section (by Venturi effect).
Furthermore, increasing the speed of flow means higher friction drag and (possibly)
pressure drag. This is the effect routinely observed in aircraft wind tunnel testing. When
the body is close enough to the surface, the accelerating fluid begins forming a wave at
the surface above the body. At a depth less than three times its maximum girth, area
blockage and wave-making drag effects increase the overall drag. At a depth of half the
animal’s girth, this increase in drag can be up to ~5 times that experienced at depth.
Beyond that point, drag decreases somewhat, to ~3 times the “at depth” value when the
dorsal portion of the body is about to break the surface (Hertel 1966). These physical
principles suggest that with all else being equal, foraging at depth is always more efficient than at the surface (at least from a drag minimization perspective).
Note that added mass, ventilation, and induced would exist even in a zero viscosity world because these resistive forces arise from the water’s own mass. This contrasts
with wave (area blockage), pressure and friction drag, which need the boundary layer
(and thus viscosity) to exist. Minimizing the latter requires morphology (specifically a
forward–backward body taper) that controls the boundary layer and size of the turbulent wake.
1.2.3 How to limit drag: Forward and backward taper
As a general rule and during non-foraging travel, the most important contribution to
marine mammal body drag is the sum total of friction and pressure drag. In this context, the body fineness ratio (body length/body diameter) is a crucial determinant of drag
(Figure 1.3). With the so-called bluff shapes where (lateral) body girth far exceeds body
length in the direction of flow (i.e., very low fineness ratio), pressure drag is far more
important than friction drag. Here, the oncoming flow at maximum girth is tripped and
separated into a sizable turbulent wake behind the body (Figure 1.3a).
Animals that exhibit long-distance migration, on the other hand, are never bluff
but rather streamlined into a fusiform shape. Recall that the size of the turbulent
wake behind a fusiform body depends on the width of the boundary layer posteriorly (Figure 1.1); therefore, a more tapered posterior region yields a smaller turbulent
wake and thus smaller pressure drag (Figure 1.3b). Additionally, to reduce friction
drag, one also needs high taper over the anterior portion of the body to get the longest
extension of the laminar boundary layer. It would thus follow that marine mammals
ought to be shaped like javelins with fineness ratios exceeding 20; however, most species have a fineness ratio less than 8 (Fish and Rohr 1999; Ahlborn et al. 2009). Friction
drag increases with the body length, thus fineness ratios exceeding 20 would entail
Marine Mammal Physiology: Requisites for Ocean Living
whenever portions of the body break the water surface, such as with a seal’s head or a
cetacean’s blowhole. Ventilation drag arises because of the mass of water that is elevated
on the leading side of the exposed body feature; and also because of the depressed wake
that follows behind (Figure 1.2a). “Drag” appears because its takes a force to push water
upward above the water surface as well as downward below that surface. Ventilation
drag can be minimized through streamlining and reduction of the surface-breaking
body feature.
Wave drag—a combined form of “wave-making resistance” or “area blockage” drag—
arises from the necessity to move the dorsal portion of the freestream over the smaller
volume in comparison to the same at depth (Figure 1.2b). As with moving water through
a funnel, the fluid has to accelerate through the narrower section in order to pass the
same amount of fluid mass per unit time as through the wider section (by Venturi effect).
Furthermore, increasing the speed of flow means higher friction drag and (possibly)
pressure drag. This is the effect routinely observed in aircraft wind tunnel testing. When
the body is close enough to the surface, the accelerating fluid begins forming a wave at
the surface above the body. At a depth less than three times its maximum girth, area
blockage and wave-making drag effects increase the overall drag. At a depth of half the
animal’s girth, this increase in drag can be up to ~5 times that experienced at depth.
Beyond that point, drag decreases somewhat, to ~3 times the “at depth” value when the
dorsal portion of the body is about to break the surface (Hertel 1966). These physical
principles suggest that with all else being equal, foraging at depth is always more efficient than at the surface (at least from a drag minimization perspective).
Note that added mass, ventilation, and induced would exist even in a zero viscosity world because these resistive forces arise from the water’s own mass. This contrasts
with wave (area blockage), pressure and friction drag, which need the boundary layer
(and thus viscosity) to exist. Minimizing the latter requires morphology (specifically a
forward–backward body taper) that controls the boundary layer and size of the turbulent wake.
1.2.3 How to limit drag: Forward and backward taper
As a general rule and during non-foraging travel, the most important contribution to
marine mammal body drag is the sum total of friction and pressure drag. In this context, the body fineness ratio (body length/body diameter) is a crucial determinant of drag
(Figure 1.3). With the so-called bluff shapes where (lateral) body girth far exceeds body
length in the direction of flow (i.e., very low fineness ratio), pressure drag is far more
important than friction drag. Here, the oncoming flow at maximum girth is tripped and
separated into a sizable turbulent wake behind the body (Figure 1.3a).
Animals that exhibit long-distance migration, on the other hand, are never bluff
but rather streamlined into a fusiform shape. Recall that the size of the turbulent
wake behind a fusiform body depends on the width of the boundary layer posteriorly (Figure 1.1); therefore, a more tapered posterior region yields a smaller turbulent
wake and thus smaller pressure drag (Figure 1.3b). Additionally, to reduce friction
drag, one also needs high taper over the anterior portion of the body to get the longest
extension of the laminar boundary layer. It would thus follow that marine mammals
ought to be shaped like javelins with fineness ratios exceeding 20; however, most species have a fineness ratio less than 8 (Fish and Rohr 1999; Ahlborn et al. 2009). Friction
drag increases with the body length, thus fineness ratios exceeding 20 would entail