Basic Fluid Dynamics

1. Real fluids have viscosity and "stick" to the surfaces over which they flow, for water "particles" at the interface between the fluid and the surface there is no relative motion - the "no slip condition".
2. Fluid flow may be described using the concept of streamlines - describing the path of water particles.
3. Streamlines area vectors (having magnitude and direction). The velocity of streamlines increases as the distance from a surface over which the fluid flows until it approaches the " free-stream" velocity. The rate of increase is parabolic and the region over which the velocity of the fluid increases from zero at the surface to the free-stream value is called the boundary layer.
4. Boundary layers may be laminar or turbulent - streamlines are parallel and chaotic respectively.
5. Shear stress in the boundary layer accounts for the origin on frictional drag.
6. The magnitude of frictional drag is proportional to fluid density, wetted surface area, velocity squared and the drag coefficient. Among other things the drag coefficient depends on the shape and surface texture of the object.
7. The frictional drag coefficient is higher for turbulent boundary layer flow than laminar boundary layer flow.
8. Inertial forces in a fluid promote turbulence. Large size and/or high speed increase inertial forces. Viscous forces dominate for small, slow objects.
9. The Reynolds number, R_e is the ratio between the inertial and viscous forces in a fluid. High R_e (10³), are associated with turbulent boundary layers and turbulent frictional coefficients.
10. Pressure (form) drag is a shape dependent drag force. Bluff objects(spheres, cubes, etc.) have high form drag. For sleak, thin objects, form drag is low.
11. Fast, pelagic forms are streamlined, minimizing the total of form and friction drag.
12. Fineness ratio is defined as body length divided by maximum body depth. A plot of drag coefficient against fineness ratio gives a minimum at a fineness ratio of 4.5.
13. Selective pressure "should" produce adaptations that minimize the energy cost of aquatic locomotion. The energy of locomotion is associated with overcoming drag. Adaptations that reduce drag may be morphological, behavioural and ecological.

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