6 Life in a Fluid Medium

How is seawater different than Air?More viscous

More dense (density increases linearly with salinity)

Lower levels of O2 in water

How is living in seawater different than living in air??O2 can be obtained from solution

More supportive medium than air (no need for skeletons)

Movement is much more difficult (viscous)

NOTE: freezing temp of seawater is –1.9 C

Streamline

Cylinder (in cross section)

CONSIDER FLUID MOVING IN STREAMLINES: Water flow can be visualized as streamlines.Particles entrained in flow move with streamlines and do not cross.

Inertial and viscous forces compete.

Reynolds Number, Re: measure of relative importanceof viscous and inertial forces in fluid

Re Vl

Note that we are always working with seawater, so weConsider no variation in density) or viscosity)Therefore we conclude that Re increases with velocity (V) and size of object (l).

We can make a calculation of Re if an object is moving in water or stationary, with the water movingpast the object.

V

V

l

l

ANIMAL AND VELOCITY Re

Large whale swimming at 10 m/s 300,000,000

Tuna swimming at 10 m/s 30,000,000

Copepod swimming at 20 cm/s 30,000

Sea urchin sperm at 0.2 mm/s

0.03

Reynolds numbers for a range of swimmingorganisms and sperm

Reynolds number implications • Re > 1000 : inertial forces predominate• Re < 1 : viscous forces predominate

World of very small size and velocity is a viscous world; takes continuous work to move an object. Particles will stop moving when no work exerted (e.g., ciliate can stop instantaneously and reverse direction by simply stopping waving of external cilia).

World of large size and high velocity is an inertial world; if work is done, object will tend to continue to move in fluid (e.g., supertanker at full speed will continue to move several km after propulsive power shut off).

Drag

• Water moving past an object creates drag.• At high Reynolds number, the pressure difference up and

downstream explains the pressure drag. Streamlining and placing the long axis of a structure parallel to the flow will both reduce pressure drag.

• At low Reynolds number, the interaction of the surface with the flow creates skin friction.

Drag and fish form. The left hand fish is streamlinedand creates relatively little pressure drag while swimming.the right hand fish is more disk shaped and vortices arecreated behind the fish, which creates a pressure differenceand, therefore, increased pressure drag. This disk shape,however, allows the fish to rapidly turn.