Fluid Mechanics and Dynamic Fluid Force
Fluid mechanics is the study of how an object or the human body moves through a fluid environment – either a liquid (e.g. a swimmer moving through water) or a gas (e.g. a sprinter or ball moving through air).
- Fluid dynamics, the branch of fluid mechanics concerned specifically with moving fluids, identifies two key dynamic fluid forces that act on a body travelling through air or water: drag, which opposes motion, and lift, which acts perpendicular to the direction of travel. Both forces influence a wide range of sports, including swimming, cycling, sprinting and any thrown projectile such as a discus or javelin
Drag Force: Surface and Form Drag
Drag is a resistance force that acts in the opposite direction to a body's motion through a fluid, produced by a combination of friction and air/water resistance, and always has a negative effect on velocity.
- Surface drag ('skin drag') is friction between the outer surface of a moving body and the fluid around it – e.g. elite swimmers shave body hair and wear smooth, close-fitting swimwear specifically to reduce this surface friction against the water
- Form drag ('shape drag') results from the impact of the fluid on the overall shape of a moving body, and is reduced through streamlining – shaping the body to be as thin and straight as possible in the direction of travel, minimising the fluid's resistance. A large form drag on the object at the front of a group also creates a pocket of calmer, less turbulent air/water immediately behind it
Slipstreaming (Drafting)
Slipstreaming, or drafting, is when a performer takes advantage of the low-drag pocket of air created directly behind another moving competitor, most commonly seen in cycling.
- As air strikes the lead cyclist, it flows around their body rather than the rider immediately behind, creating a pocket of reduced air resistance that the trailing cyclist can exploit – provided they stay very close (roughly 15-30cm) to the wheel in front, this can save the following rider up to around 30% of the energy they would otherwise need to expend
Factors Affecting Drag
- Velocity – the faster a body moves through a fluid, the greater the drag force acting against it. This is why very fast sports (e.g. sprinting, cycling, motor racing) place such heavy emphasis on minimising drag through streamlined body positioning
- Cross-sectional area – a larger area facing the direction of travel increases drag, while reducing it lowers drag. Cyclists crouch low over the handlebars (rather than sitting upright) and speed skiers adopt a low tucked stance specifically to reduce their cross-sectional area and travel faster
- Shape and surface characteristics – a smooth, aerodynamic shape reduces drag, which is why specialist streamlined equipment and clothing (e.g. aerodynamic cycling helmets and ridged clothing, streamlined ski helmets extending to the shoulders, form-fitting suits) are used to gain small ('marginal') performance advantages. In contrast, an irregular shape with a large surface area relative to its weight – such as a badminton shuttlecock, which is light and feathered – experiences a much larger drag force and loses speed very quickly in flight
The Bernoulli Principle
The Bernoulli principle states that faster-moving air exerts lower pressure, while slower-moving air exerts higher pressure. This pressure difference is what generates a lift force – a force acting perpendicular to a body's direction of travel – on projectiles such as the discus, and it depends on the angle of attack: the tilt of the projectile relative to the oncoming airflow.
- Greater lift keeps a projectile in the air for longer, increasing the horizontal distance it can travel before landing
Upward Lift Force: The Discus
When a discus is released with an appropriate angle of attack, the air passing over its curved top surface is forced to travel a longer distance than the air passing beneath it, so the air above must move faster.
- Under the Bernoulli principle, this faster-moving air above the discus creates lower pressure, while the slower-moving air underneath creates relatively higher pressure. This higher pressure beneath the discus generates an upward lift force, extending its time in the air and increasing the horizontal distance thrown
- The optimum angle of attack for a discus is generally between 25° and 40° – too great an angle of attack instead reduces lift and increases drag, causing the discus to stall and lose distance
Downward Lift Force: Skiers, Cyclists and Racing Cars
The Bernoulli principle can also be applied to generate a downward lift force, which is useful whenever a performer or vehicle needs to be pressed more firmly into the ground or track to increase friction/grip, rather than lifted into the air.
- In a Formula 1 car, the rear spoiler is angled so that air passing over the top of the car travels a shorter distance (and therefore more slowly) than the air passing underneath. This creates higher pressure above the car and lower pressure below, producing a downward lift force that pushes the car into the track, increasing tyre grip at high speed and through corners
- A cyclist crouched low over the handlebars creates a similar effect: air passing over their back travels a shorter, slower path than the air passing beneath, generating higher pressure above and a resulting downward lift force that helps the tyres grip the road
- Speed skiers rely on the same principle – generating downward lift helps keep their skis firmly in contact with the snow, and the resulting increase in friction actually helps melt a thin layer of the snow surface, producing a faster, lower-friction surface to ski on