Oceanic Physical-biological Process - Drag

Drag

Drag is the tendency of an object to move in the direction of the flow. The magnitude of drag depends on the current velocity, the shape and size of the organism and the density of the fluid. Drag is a dissipative process which generally results in the generation of heat.

In sea water, drag can be decomposed into two different forms: skin friction and pressure drag.

1. Skin friction: just like other frictional forces, skin friction is a consequence of the relative movement between the surface of the organisms and its fluid environment. Under conditions of low Re, where viscous forces dominate, the skin friction is apparent and is more important, although it is also present under high Re conditions.
2. Pressure drag: pressure drag is a result of the pressure difference in front of, and behind, an organism. Incidentally, the shape that has the lowest pressure drag coefficient is a hollow hemisphere oriented in the direction of fluid flow. In the oceanic environment plants and settled animals have bodies that are soft and flexible in order to minimize the effects of pressure drag.

Besides being soft and flexible, organisms have other methods to minimize drag.

• Smooth skin: dolphins have little tear drops in their skin which traps some water so water flows over the water that is trapped. The skin feels soft and flaky and sheds every two hours. This helps dolphins swim through the sea water at high speed.
• Shark skin: the surface of shark skin is covered with tiny 'teeth' or dermal denticles. The shape and positioning of these denticles varies across the shark's body, altering the flow of water in a way to minimize form drag.
• Barracuda skin: Barracuda have hundreds of skin conduits which force the fluid flows to follow the parallel tubes and become laminar. Again, this arrangement decreases water drag.