Why is it difficult to calculate drag forces in swimming?
Flow about a swimmer is unsteady, and a swimmer's acceleration and deceleration significantly affect drag and somewhat lift.
Free surface effects complicate the flow by allowing wave drag.
Flow around a swimmer may become compressible because of local density changes caused by air bubble entrainment.
- Water exerts a resistive pressure (always perpendicular to the body surface) on each individual "local" part (head, arms, legs, etc.) of a swimmer's body.
- Pressure and other forces are calculated by dividing the pressure and shear stresses by the areas upon which they act.
- One axis is aligned with the direction in which the local body part is moving. The force component in this direction is drag, and this always points in the direction opposite of the motion.
Active Drag- encountered when a swimmer is "actively" stroking and kicking
CBS Video: Anderson Cooper vs. Michael Phelps
Question: Why is it faster to swim completely submerged rather than on the surface of the water?
Answer: Turbulence (wave-making) at the surface of the water increases drag. Trained swimmers, such as Michael Phelps, know that swimming under the water is faster than swimming on the surface. The more efficient transfer of momentum to water creates more forward thrust (according to Newton's Third Law). Less energy is used when there is less splashing water.
MAD System- Machine used to measure active drag force
This system allows the swimmer to push off from fixed pads attached to a rod with the swimmer swimming at a constant speed. Many pads are located under the water 1.35 meters apart with force transducers attached to each pad. The swimmers used a pull buoy, obviously enabling the swimmers to only use their arms.
Swimmers are asked to swim at varying speeds for a constant duration for each. Average (mean) drag force and mean swimming speed are then calculated with the equation: D= Avn.
Types of Drag:
Form drag- caused by the shape and position of the swimmer
Skin friction drag- caused by friction between water and the swimmer
Wave drag- caused by the formation of waves
Spray drag- caused by the formation of spray (turbulence while kicking)
Interference drag- caused by two body parts being close to each other
Parasitic drag- equal to total drag - induced drag
Pressure drag- equal to total drag - skin fraction drag
Eddy resistance- another term for form drag (ship-building industry)
Drag Equation for Fluid Dynamics:
- FD is the force of drag, which is by definition the force component in the direction of the flow velocity
- ρ is the mass density of the fluid (water, in this case)
- u is the velocity of the object (example: hand speed)
- A is the area being used (example: hand surface area)
- CD is the drag coefficient — a dimensionless constant, used to show the resistance of an object in a fluid environment, such as air or water (lower drag coefficient indicates less hydrodynamic drag)
[The drag and lift coefficients are determined from the pitch and sweepback angles of the hand.]
These are drag coefficients for several well-known shapes, which are often used in applying drag coefficients to more complex objects, such as models of the human body being studied in fluid mechanics equations. Scientists often used these more simple objects to calculate turbulent flow, as well.
Drag increases with area, density, and speed.
Since increases in temperature decrease density, temperature increases will also reduce drag.
The following graph compares swimmer Inge de Bruin (previous world record holder in 50 meter and 100 meter freestyle, and 100 meter butterfly) with other swimmers measured on same system. Inge has smaller drag force for any given speed.
This shows that drag increases proportional to the square of velocity.
(As speed increases, drag also increases.)