Hydrodynamics Flashcards
Hydrodynamics
The part of fluid dynamics concerned with dynamics in liquid, rather than gaseous, mediums
Form drag (profile drag)
Major contributor to overall drag during most human and projectile motions; significant in swimming
The form of the object, frontal surface area and velocity affect drag forces
Surface drag - also called skin friction or viscous drag
Layer of fluid particles adjacent to surface slowed because of shear stress
Next adjacent layer affected by particles adjacent to the surface
Wave drag
Acts at the interface of two fluids, e.g. water and air
As swimmer moves, a wave is created in more dense fluid (the water)
Reaction force directed toward swimmer
Magnitude increases with up and down movement of swimmer and with velocity
Wave drag is the largest component of drag acting on a swimmer at fast swimming speeds
Waves in swimming…
build up at the front of the body during swimming. These waves oppose the forward movement of the swimmer. Other waves also build up around the swimmer according to pressure differentials.
How might we reduce wave drag during front crawl swimming?
- minimize up and down motion
- keep lead arm outstretched for longer
- keep head down, body lower in water
- stay underwater as long as legally possible at start and turns
- small amplitude flutter kick
- minimize skin and form drag so that water isn’t pushed in front of swimmer
Lift in swimming
The hand of a swimmer can imitate an aerofoil and create lift forces
To maximize lift, some lateral movement of hand is required relative to direction of movement
Lateral movement may also allow hand to find ‘clean’ water free of turbulence, but also create drag
Vortex formation?
Work done on vortices (created as water flows around limbs) may provide some propulsion since vortices carry momentum
Water in vortex remains ‘stationary’ whilst hand applies force
More research required to show influence of vortices on swimming propulsion
Pressure gradient and Bernoulli
Oar-like pull (compared to curvilinear pull) describes the ‘overtaking’ of rotational movements of the hand and forearm with respect to the shoulder
Causes gradual increase of hydrodynamic reaction force
Avoids changes in intra-cycle velocity that affect efficiency
In swimming, it also produces greater pressure in hand and firearms to be used for propulsion
The rotation of the arm during the stroke would be such that the tangential (across the arm) velocity near the hand would be greater than at the elbow
According to Bernoulli, this would create less pressure at the back of the hand than the elbow
Water would then flow toward the hand
The faster fluid flow could decrease pressure on the back of the hand, and therefore increase the pressure differential
Therefore, greater resulting propulsive force would occur
Summary - swimming
Forward velocity in swimming is affected by propulsion and drag
Hydrodynamic body positioning can minimize drag
Propulsion can be improved by the use of a curved hand path to maximize lift in addition to drag forces
There is some thought that vortices may be useful in swimming propulsion, although more research is required
There is some evidence that greater propulsion can be derived from increasing pressure gradients across the arm and hand
This is maximized by allowing proximo-distal water flow around the dorsal aspect of the arm and hand utilizing Bernoulli’s principle: best for sprint swimming?
More research require to determine how best to maximize pressure gradients
Uses in other sports?
Design of rowing blades/oars
Design of boat/yacht hulls
Efficiency of water polo treading
Synchronized swimming?
Summary
Principles of fluid dynamics are central to many sports
An understanding of form (shape), surface and wave drag is important
Lift also can be manipulated to improve performance in many sports
A thorough knowledge of hydrodynamics can improve a coachs ability to improve performance in many sports including swimming
