11.4 Fluid Mechanics Flashcards
(21 cards)
What are fluid mechanics?
- The study of an object/human that travels through any liquid or gas
e.g. swimmers travel through water
Tennis ball/javelin travels through the air- both of these are slowed down by friction
What are the two dynamic fluid forces?
- Drag
- Lift
these two forces effect variety of sports e.g. cycling,sprinting, swimming
any projectile such as a ball & javelin experience drag & lift force
What is a drag force?
- A drag force slows something down
- Drag is the resistence force cause by the motion of a body travelling through a fluid
- Acts in opposite direction to the motion & therefore has a negative effect on velocity
- Produced from air resistance & friction
What are the two different types of drag?
- Surface drag
- Form drag
What is surface drag?
- Friction between the surface of an object & fluid environment
- Swimmers wear specialised clothing/shave their body hair to reduce surface drag
sometimes referred to as ‘skin drag’.
What is form drag?
- Relates to the impact of the fluid environment on an object
- The forces affecting the leading edge of an object increase form drag & the forces affecting the trailing edge reduce form drag
- Form drag relates to streamlining
Sometimes reffered to as ‘shape’ drag
How does streamlining work for a swimmer?
- Streamlining in swimming is key to success
- Swimmer has to make themselves as thin & straight as possible
- Therefore they decrease the form drag
What is a slipstream?
- A large form of drag also offers less drag force for anything following it
- In cycling, this is referred to as a slipstream
- In cycling another cyclist will use another riders slipstream (drafting)
How is a slipstream used in cycling?
- As wind hits the first rider, it goes around the sides
- The cyclist behind gets the benefits of the air pocket
- For it to work the second cyclist has to be v close to the bikes in front (around 15-30cm)
- It can save up to 30% more energy!
What are the main ways to reduce drag?
- The velocity of a moving body
- The cross sectional area of the moving body
- The shape & surface characterisitcs of a moving body
How can the velocity of a moving body reduce drag?
- The greater the velocity through a liquid the greater the drag
- e.g. a racing car will experience greater air resistance which increases drag
- In quick sports its important to reduce this drag- we do this by making an object as streamline as possible
How can the cross-sectional area of the moving body reduce drag?
- Cross sectional size/area of moving object can reduce/increase drag
- Large area increases drag
e.g. in cycling they crouch down low
How can the shape & surface characteristics of a moving body reduce drag?
- A more streamlined aerodynamic shape reduces drag
- Sport scientists are regularly trialling drag-resistant clothing to achieve ‘marginal gains’ in speed to give competive edge over competition.
- Speed skier: fitted suit,boots streamlined,helmet extends to their shoulders
- Cycling:Lycra/clothing makes athlete more aerodynamic, helmets are stramlined to reduce effect of drag
- Elite swimmers: shave body hair, wear half body suit, make sure they have streamlined body position
- Badminton Shuttle: Shap & surface of a badminton shuttle resuslt in larger drag force from air resistance, unusual shape & v light,therefore larger drag force from air resistance means it loses speed quickly
What is a lift force?
- Causes a body to move perpendicular to the direction of travel
What is the angle of attack?
- The tilt of a projectile relative to air flow
Define the Bernoulli principle
- Where air molecules exert less pressure the faster they travel & more pressure when they travel slower
Explain the Bernoulli principle in terms of a discuss.
- When a discuss is thrown it has an upward lift force
- A lift force allows the discuss to stay in the air for longer
- Therefore, increasing the horizontal displacement
- The more lift a projectile has during flight, the longer it will stay in the air
- Lift is achieved when 2 different air pressures act on an object
- Air that travels faster has a lower pressure than the air travelling slower
- Of a discuss low pressure on top, high pressure on bottom
When do you talk about the Bernoulli principle?
- ONLY TALK ABOUT BERNOULLI PRINCIPLE EFFECT IF OBJECT IS NOT ROUND
- For something that is round this wouldnt be relevant so the air pressure wld be equal & doesnt travel as far because it doesnt have the upward lift
Why is the angle of attack important for a discus?
- The angle of attack changes flow of air round that discus
- Air travels over the top of the discus, has to travels further so is at a faster velocity, creating low pressure
- The air below the discus has less distance to cover to travels at a slower velocity creating higher pressure.
- Higher pressure creates the upwards lift force & allows discus to stay in air for longer
- If angle of attack is too great the lift is reduced & drag increases causing discus to stall
- The discus will begin to fall back down
How does the downward lift link with the Bernoulli principle?
- A lift force doesn’t always need to be upwards
- Bernoulli principle can also be used to describe downward lift
- This is required by speed skiers, cyclists & racing cars
- The car, bike and skis need to be pushed into the ground- the higher pressure pushes it into the ground meaning it can maintain it grip and maintain its peed means higher chance of success
- The cars in racing have spoilers that are angled
- The lift force is pushed in a downward direction, to push car onto the track
- This is because the air over the top of the car travels a shorter distance than underneath- due to the angle of the spoiler
- The tyres on the road gives the race car give a greater friction force allowing them to build up speed be tighter around the corner etc
Downward force also in a sprint start
What is the ideal angle of attack for a discuss?
- Anything between 25-40 degrees
