Fluid Mechanics

Improving Movement Through Air and Fluid

Air resistance is the force that opposes the horizontal motion of a body whilst moving through the air. The body can be an implement such as a football, or a person who may be running or jumping. Once a body is under the influence of air resistance, it will usually cause a deceleration but in most situations the effect it has on an object can be ignored because there is a negligible effect on its momentum or flight. Certain objects will resist the effects of air more than others, a shot putt for example is too heavy and smooth to be significantly affected by air resistance; whereas a shuttle-cock being lighter and less aerodynamic slows significantly. In events such as cycling the faster a performer is travelling the greater the effect air resistance has. The amount of frontal cross sectional area also impacts on the degree that air resistance affects motion. For example, racing bikes promote a crouched position that reduces cross-sectional area and a skier adopts a schuss position to reduce when skiing in downhill races. Cyclists such as Bradley Wiggins, use wind tunnel testing to evaluate different riding positions (to reduce frontal cross-sectional surface area) and how his equipment enabled him to be more streamlined and reduced drag. The smoothness of the surface area of the athlete can determine the effects of air resistance, the rougher the surface, the greater the effect of air resistance. Golf balls have dimples and this also counteracts the effects air resistance has on the ball.

Aerodynamics is the study of an objects motion whilst travelling through the air. The motion of air around an object is often referred to as a flow field and understanding this motion enables the calculation of forces and moments acting on objects. This helps to understand why a boomerang can swerve in the air and travel back to the thrower or why an aeroplane can fly. Two of the most important terms used when studying the flight of an object through the air (e.g. a discus or a table tennis ball) are drag and lift. An object is said to be aerodynamic when its shape minimises the drag caused by the air as it passes over and behind it in flight. This helps an object to flow through the air minimising the resistance and allows an object to maintain momentum.

Drag

Drag is a force that resists or opposes the motion of an object through air or a fluid. Drag is highly dependant on the velocity that an object is travelling and only when objects are moving rapidly does drag play a significant role in sport.

• Surface drag is also known as skin friction. As air or fluid molecules slide over the surface of an object friction slows them and this impacts on the object because the velocity is slowed. The roughness of a surface increases drag, for example a tennis ball has a rougher surface, compared to a golf ball. This is also discussed at the start of the technology chapter, with regards to the banned swimwear used in the 2010 World Championships.
• Form drag (sometimes called shape or profile drag). This occurs when the molecules in the air come into contact with the object and slow its velocity. When a ski jumper travels down the take-off ramp air particles hit their body exerting a force that resists their motion. As they accelerate down the ramp more and more particles collide with their body so that drag increases. The greater the frontal cross-sectional surface area the more air particles will collide with the object and the greater the force impeding its flight. Having a curved or pointed leading edge or front allows the air particles to be deflected less and pass more easily around it, reducing the drag force. If the tail or trailing surface of the object doesn’t gradually reduce, turbulent flow occurs. The air particles do not pass smoothly or ‘stick’ to the surface and if there are no air particles a ‘suction vacuum’ will be created, greatly reducing momentum. Having an object with a gradual decrease in cross-sectional area allows the flow to remain ‘laminar’, which means it passes over and behind an object smoothly so that the vacuum isn’t created and there is less drag. This is why streamlining works to reduce drag. When a cyclist in an upright position moves at a high velocity, the air particles hit their body and this causes a force that resists momentum. If the cyclist leans forward less of their body ‘hits’ the air particles which reduces drag. Having a curved helmet also helps as this enables the air to pass over more effectively.

Streamlining

Streamlining works to reduce the drag forces experienced by an object passing through the air or a fluid. It involves making the overall shape effective in moving through the fluid (including air) with the minimum resistance. A crouched position is adopted by cyclists, skaters and skiers and equipment is designed so that there is less drag because less turbulence is created behind them. Reducing the frontal cross-sectional surface area also makes a person more streamlined.

If a discus is thrown effectively, it will be released from the hand so it is flying at the correct angle. The angle at which it is travelling in relation to the ground is called the angle of attack. In the case of the discus if the angle of attack is perfect, air will be hitting the underside as it flies and this will create lift, which is a force that counteracts gravity’s action in pulling the discus back down to earth. The Bernoulli principle explains how this occurs. Air that has to travel further over the top surface of an object will also be faster. Therefore, because air travelling on the underside of the object is travelling less distance and slower, there will be a pressure difference between the air passing across the two surfaces. Faster moving air exerts less pressure than slower moving air so it will be lower on top of the object. The Bernoulli force is formed from a high to low pressure which causes a lift force upwards.

Aerofoils

An aerofoil is an object where the air flow travels further over the top of the projectile than the air passing underneath; this air travels faster over the top and creates a low pressure. With low pressure above and high pressure below the projectile, the projectile is forced upwards causing it to stay in the air for longer, therefore take a non-parabolic flight path.

In sports where the projectile is a ball such as tennis, performers who are able to apply spin can be at a massive advantage. A footballer applies spin on a ball to curl it around the wall and into the top corner, without enough spin the ball will end up in the crowd having flown wide of the goal. Similarly, a tennis player will play a topspin lob so that it passes over their opponent and bounces in at the back of the court and without enough spin it will land out of the court so that they lose the point. The main types of spin are:

• Topspin; this will shorten the flight path and make ball dip in flight.
• Backspin; this will lengthen the flight path and make ball appear to hang.
• Sidespin; this will make ball swerve and is often referred to as a hook or slice.

To generate spin the tennis player applies an eccentric force; one that is off-centre and outside the centre of mass of the ball. This gives the ball angular motion so that it is rotating as it travels. A principle called the Magnus Effect is responsible for the ball curving in flight after a player has generated the spin. In the topspin lob example mentioned above, as the ball rotates the upper surface of the ball has a forward velocity, whereas the lower surface has a backward velocity. As the ball travels through the air it collides with air particles in the atmosphere. As the air molecules hit the upper surface they slow down because the surface and airflow are opposing each other. A boundary layer of air collects on the surface of the ball and the collisions with the air decelerate, creating an area of high pressure. Molecules that hit the lower side of the ball travel faster over the surface of the ball because the boundary layer of air is travelling in the same direction as the air particles. This creates an area of low pressure and due to the Magnus force, the ball moves in the direction of the lower pressure (from high to low). This is similar to the Bernoulli principle where the faster moving air exerts a lower pressure, which in the case of topspin, is on the lower surface of the ball and therefore force is exerted on the top of the ball in a downwards direction; in backspin the lift force acts underneath the ball causing the ball to rise; in side spin the ball hooks and swerves as the force is applied to the side of the ball