Moving
Dolphins: How to Watch the Water
Rohr believed that
bioluminescence could solve a very large question:
do whales and dolphins have frictionless skin and can we imitate it?
Laminar flow creates much less
shear stress than turbulent flow at the same velocity because there is no
swirling or random motion. So the
intensity of light triggered by a swimming dolphin should reveal the way water
flows over their bodies. When Rohr
put a dolphin in plankton rich water he found just what he expected.
The areas of the body that lit up the brightest were those behind the
blowhole and flukes, where there was probably a turbulent wake.
The tail half of the body was brighter than the head due to the motion
the tail made as it swam. The
darkest area was the melon, where the dolphin uses echolocation to find its way,
which is an area designed to prevent turbulence.
One way to inperpret these light patterns is that the fluid starts out moving along the body as a laminar flow. As it rushes along the body it becomes increasingly turbulent. It is difficult to calibrate light to a value for turbulence and this is where tests and calculations take over.