Drag is a force which resists the propulsive force and can occur as friction drag, pressure (or form) drag, and gravitational wave drag.
The drag force is proportional to velocity, v; density, rho; wetted surface area, Sw; and a variety of other factors which are, collectively, the drag coefficient, CD.Drag, therefore, is determined by the following:
and, this drag is the sum of Pressure Drag, CDP and Friction Drag, CDF so:
Pressure drag results from the large eddies or swirls of air caused by an objects that presents are large surface area to the direction of flow. Fluid particles form vortices that have a lower pressure relative to the side facing the flow. This creates a suction force that opposes the direction of propulsion. The energy required to move fluid particles through such a convoluted path is energy lost from the forward movement of the object.
Friction drag results from the friction between fluid and the object and is highest when the most surface is exposed to the fluid.
The amount to which these two types of drag forces contribute to the total drag, CD depends on the ratio of the length of the object to its depth:
As it turns out, there is an optimum fineness ratio of 5; however, there is little penalty to having a fineness ratio between 3 and 6 due to the flat bottom of the total drag curve.
Gravitational Wave DragGravitational wave drag results from the transverse waves created by a swimming animal (bow waves and stern waves). Deeply submerged there is no effect of the wave; however, as the animal gets closer to the surface it creates a wave above it (as you can see if you've ever been to a whale tank at the aquarium). The cost of creating this wave, lifting the water, is taken into account by the drag augmentation factor, X. For example, if the drag on the animal increases by 100% due to gravitational wave factor we multiply our energy equation by a factor of 2, X = 2.
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