5
Chapter one: Hydrodynamics
rotation in each layer, and (2) the molecular forces at the body’s surface which prevent the fluid
from moving past it—a phenomenon called the no-slip condition (Figure 1.1d). The body’s surface atoms attract, and come into contact with, the nearest atoms of the liquid; the latter then
slow down the motions of the water molecules above it, which in turn slow down those of the
layer further up; and so on. The boundary layer is a fluid structure that exists wherever there
is flow past a solid surface, such as above the seafloor: there too, the water near the seafloor’s
surface is always moving at a slower speed than water above it.
The boundary layer is more dynamic and has more structure than the streamlined
flows above it. It begins as laminar and its thickness increases posteriorly. But a laminar boundary layer is an unstable structure that is easily disrupted when encountering
surface roughness and random perturbations from the flows above it. The result is often
a laminar boundary layer changing into a turbulent boundary layer (Figure 1.1b). On a
well-streamlined, fusiform animal, and depending on the swim speed, this laminar-toturbulent transition occurs approximately halfway along the body (Fish and Rohr 1999).
Where the transition occurs is dynamic and difficult to predict because it depends on
many factors, as well as on the body (and limb) postures adopted at any point in time.
(c)
Turbulent wake
(a)
Body
Boundary
layer
Speed vectors
(d)
Streamlines
Layer
Body
Streamlines
(e)
E ddies and vortices
Line of
zero flow
velocity
Transition
(b)
Laminar
boundary layer
Laminar flow in
boundary layer
Streamlines
Eddies in
boundary
layer
Turbulent
boundary layer
Figure 1.1 Fluid motions around a swimming marine mammal (the motions are shown relative to
the body): (a) Flow streamlines showing the path of the fluid particles; (b) laminar and turbulent
boundary layers generated along the surface of the body; (c) boundary layer aft of the body, terminating into the turbulent wake; (d) close-up view of the boundary layer, showing the mean speeds
of the fluid particles within it (again relative to the animal): increasing from near-zero just above the
body surface, up to the speed of the streamline nearest to the top of the layer; (e) boundary layer
structure at the threshold of the separated flow genesis.
Chapter one: Hydrodynamics
rotation in each layer, and (2) the molecular forces at the body’s surface which prevent the fluid
from moving past it—a phenomenon called the no-slip condition (Figure 1.1d). The body’s surface atoms attract, and come into contact with, the nearest atoms of the liquid; the latter then
slow down the motions of the water molecules above it, which in turn slow down those of the
layer further up; and so on. The boundary layer is a fluid structure that exists wherever there
is flow past a solid surface, such as above the seafloor: there too, the water near the seafloor’s
surface is always moving at a slower speed than water above it.
The boundary layer is more dynamic and has more structure than the streamlined
flows above it. It begins as laminar and its thickness increases posteriorly. But a laminar boundary layer is an unstable structure that is easily disrupted when encountering
surface roughness and random perturbations from the flows above it. The result is often
a laminar boundary layer changing into a turbulent boundary layer (Figure 1.1b). On a
well-streamlined, fusiform animal, and depending on the swim speed, this laminar-toturbulent transition occurs approximately halfway along the body (Fish and Rohr 1999).
Where the transition occurs is dynamic and difficult to predict because it depends on
many factors, as well as on the body (and limb) postures adopted at any point in time.
(c)
Turbulent wake
(a)
Body
Boundary
layer
Speed vectors
(d)
Streamlines
Layer
Body
Streamlines
(e)
E ddies and vortices
Line of
zero flow
velocity
Transition
(b)
Laminar
boundary layer
Laminar flow in
boundary layer
Streamlines
Eddies in
boundary
layer
Turbulent
boundary layer
Figure 1.1 Fluid motions around a swimming marine mammal (the motions are shown relative to
the body): (a) Flow streamlines showing the path of the fluid particles; (b) laminar and turbulent
boundary layers generated along the surface of the body; (c) boundary layer aft of the body, terminating into the turbulent wake; (d) close-up view of the boundary layer, showing the mean speeds
of the fluid particles within it (again relative to the animal): increasing from near-zero just above the
body surface, up to the speed of the streamline nearest to the top of the layer; (e) boundary layer
structure at the threshold of the separated flow genesis.