Fluid Mechanics

Question 1

Fluid Mechanics

Answer 1

the study of forces that fluids (air, water) exert on objects moving through them
- includes: buoyant and dynamic forces

Question 2

buoyant force

Answer 2

due to immersion in fluid

-tendency to be lifted up

Question 3

dynamic force

Answer 3

due to relative movement in fluid

• lift force
• drag force

Question 4

An object submersed in water will experience…

Answer 4

• pressure from the weight of the water above
• pressure from the water beside
• pressure from the water below to hold up the object and the water above the object
• submerging the object deeper in the water the force (pressure) above and below increase

Question 5

Pressure on submerged objects in water

Answer 5

• the force below the object will always be higher than the force above it
  R(lower) - R(upper) = buoyant force
  -the difference between the force above and the force below is related to the weight of the volume of fluid displaced, therefore the buoyant force will always be acting up because more will have been displaced at the bottom than at the top

Question 6

determining if an object will float

Answer 6

R(lower) - R(upper) = buoyant force
Weight of object (W = mg)
- buoyant force = mg, object will be in static equilibrium
- buoyant force > mg, object will accelerate upwards
- buoyant force < mg, object will accelerate downwards

Question 7

Floating/ sinking due to density

Answer 7

density = mass / volume
P = m/v
if something is more dense than water it will sink and vice versa
- muscle/bone are more dense than water (will sink)
fat is less dense (will float)

Question 8

Point of application? can’t think of a better word

Answer 8

• force due to gravity and buoyant forces do not always act through the same point
• Fg acts through CofG (related to body orientation and distribution of mass, density of tissues), Fb acts through centre of volume

Question 9

Air and Buoyant forces

Answer 9

yes, air exerts a buoyant force but because our densities are so different we don’t notice it

Question 10

Dynamic fluid force

Answer 10

is proportional to:

• density of the fluid
• surface area of the object immersed in the fluid
• relative velocity of the object to the fluid (squared)
  
  • relative velocity is the difference between the objects velocity and the fluids velocity and is the most important factor in determining dynamic fluid force

Question 11

Drag Force

Answer 11

a component of dynamic fluid force that opposes (parallel but opposite direction) the relative motion of the object with respect to the fluid

Question 12

Types of Drag force

Answer 12

Surface drag
- the sum of the friction forces acting between the fluid molecules and the surface of the object
Form drag
- a result of how fluid molecules travel around an object due to the objects shape

Question 13

Surface drag

Answer 13

also called skin friction or viscous drag

• as a molecule slides past the surface of an object, the friction between the surface of the object and the molecules in the fluid creates a layer of molecules called a boundary layer (laminar layer)
• the surface drag is due to the boundary layer of molecules that come in contact with the object and “rub up against” it

Question 14

Surface drag in sports (swimming)

Answer 14

swimmers shave their body to reduce drag and they wear swim caps and special types of suits to reduce drag on their bodies in the water

Question 15

Surface drag is affected by…

Answer 15

1) coefficient of drag (similar to friction)
- coefficient of drag is affected by the roughness of the surface and viscosity of the fluid
- shaving, bodysuits
2) density of the fluid
3) cross sectional area of the object
4) relative velocity (squared)
- how quickly the object and the surface are moving

Question 16

Form Drag

Answer 16

• fluid traveling around any fairly smooth object will travel in a path called laminar flow
• laminar flow is when the fluid molecules diverge at the point of contact of the object and then converge after the object has passed (a wake)
• the amount of divergence depends on the shape of the object and the convergence is a result of the natural flow of fluid to rebalance pressure
  
  • rebalance because the laminar flow causes a vacuum behind the object and a high pressure area at the point of contact that opposses forward motion

Question 17

the shape of an object can influence the form drag on it…

Answer 17

• it minimizes the change in direction of the molecules first contact with the object
• takes up space where the turbulent flow or vacuum would occur behind the object
• football shaped object does the best

Question 18

Form drag is affected by…

Answer 18

1) shape of the object
2) drag coefficient
3) density of the fluid
4) cross sectional area of the object
5) relative velocity (squared)

Question 19

Drag and sport

Answer 19

• make body or clothing smoother
• streamline the shape of the body or equipment
  
  • i.e. track cycling helmets are designed to nullify the vacuum created behind the athlete
• tailgating
• body position

Question 20

Golf and Drag

Answer 20

the dimples on a golf ball create tiny pockets of turbulence which then pull the ball along and keep the air passing by closer to it, thereby minimizing the turbulent wake behind it by changing the laminar flow

Question 21

Lift force

Answer 21

• is the dynamic fluid force component that acts perpendicular to the relative motion of the object with respect to the fluid
• used to change the direction of the relative motion of the object through the fluid
• determined by the direction of flow of the fluid

Question 22

Lift force is caused by…

Answer 22

• lateral deflection of fluid molecules as they pass the object
  
  • object exerts lateral force on molecules as molecules exert an equal but opposite lateral force on the object causing lift
  • Lift is proportional to the lateral acceleration of the fluid molecules amd the mass of the molecules that are deflected
  • the shape and orientation of the object can modify the effect of the lift force

Question 23

Spoilers and lift force

Answer 23

the spoilers on cars actually push it down so that the tires can grip the road better

Question 24

Spin and the Magnus effect

Answer 24

• the magnus effect explains the flights of spinning balls in sports
• it can be described using Bernoulli’s principle
  
  • faster moving fluids exert less pressure than do slower moving fluids
  • if a ball is spinning in the same direction as the fluid there will be a decrease in pressure
  • If a ball is spinning in the opposite direction as the fluid there will be an increase in pressure

Question 25

How does the spinning of a ball change the relative velocity of the fluid to the object?

Answer 25

Topspin:
- the molecules on the top will slow down due to friction
- those on the bottom will not slow down
- therefore there will be greater pressure on the top of the ball compared to the bottom of the abll
the ball will have a net downward force

backspin:

• the molecules on the bottom will slow down due to friction
• those on the top will not slow down
• therefore there will be greater pressure on the bottom of the ball compared to the top of the ball
• the ball will have a net upward force