L28 - Fluid Dynamics 3: Fluid Forces That Swing Cricket Ball

• Why is swimming such an inefficient form of motion?

  • All 3 forms of drag must be dealt with in swimming which occurs within a dense more viscous fluid
  • Surface friction
  • Form drag
  • Wave drag
• Lift Force:

  • While drag forces act in the direction of the free stream fluid flow, another force, known as lift, is generated perpendicular to the fluid flow
  • Drag = force against the object
  • Lift acts at 90° to onset of free flow/drag force (90° direction)
  • The factors affecting the magnitude of lift are basically the same factors that affect the magnitude of drag
• Lift Force:

  • FL = ½ * CL* p * Ap* vr^2
  • FL = lift force
  • CL = coefficient of lift
  • p = fluid density
  • Ap = surface area against which lift is generated
  • v = relative velocity of a body with respect to a fluid
• Bernoulli Principle =

  • Regions of relative high velocity fluid flow are associated with regions of relative low pressure
  • Regions of relative low velocity flow are associated with regions of relative high pressure
  • Whenever a pressure differential exists, a force is directed from the region of high pressure to the region of low pressure
• Bernoulli Principle:

  • P + (½ * p * v^2) + (p * g * h) = constant
  • P = pressure
  • ½ * p * v^2 = Kinetic energy associated with movement of object
  • p * g * h = Potential energy associated with object
  • ΔP = 0.5 * p * (v_lower^2 - v_upper^2)
  • Side differentials
  • Lift = Δ P * wing_area
  • Change in pressure, calculated above * by the SA = the lift
• Bernoulli Principle:

  • Relationship bw/ velocity & pressure
  • Take advantage of principle when want flight
  • Can cause it to go upwards to hang in air for longer
  • Low pressure on top help lift it up
  • Move faster over top compared to bottom reaching end surface sooner (flow doesn’t take same amount of time)
• Bernoulli Principle:

  • Bc/ of its shape generates the lift for a plane to fly
  • Greater difference in pressure more up/lift force
  • Take advantage of this at take off with increase velocity - move faster greater difference
  • Why get more push when used in stroke in swimming
• Lift Force:

  1. Foil shape
  2. Angle of attack
  3. Spin
• Lift Force - Foil shape:

  • Creates fluid separation with higher velocity - low pressure over the top & lower velocity - higher pressure over the bottom
  • Separation of layers specifically
  • Lift in water = a force to push forward with (propulsion)
  • Low pressure at front (top of hand), high pressure at back (palm of hand)
• Lift Force - Angle of attack:

  • The angle of orientation of the projectile with respect to the fluid flow
  • The optimum angle of attack for maximising range is the angle at which the lift/drag ratio is maximum
• Examples of Angle of Attack:

  • Taking advantage of lift
  • Head wind is your friend/helpful
  • More fluid flow (one time when head wind is good) to create more hang time
  • Plane: good to take off into wind (head wind), but not fly in head wind
  • Ski jumping: advantage by body position, to create fluid flow over body, in a V position large surface area (to create a giant wing to generate a larger area of)
• Examples of Angle of Attack:

  • More SA, more pressure points have (can distribute)
  • Larger SA gives more lift (eg flying squirrels)
  • Skis are giant, to give advantage, & stability in air
  • The lighter you are greater advantage that lift force has to hold you in air
  • Greater SA & being lighter = bigger lift
• Generating lift on a non-foil shaped object - Spin:

  • Spinning objects also generate lift
  • When an object in a fluid medium spins, the boundary layer of fluid molecules adjacent to the object spins with it
• Generating lift on a non-foil shaped object - Spin:

  • Ideal lift but foil shape but with ball (not foil shape) can generate spin ball to generate lift
  • Higher velocity on top lower velocity on bottom
  • if high velocity on top then is low pressure on top so will have more flight time as pressure moves from high to low
  • Create higher velocity with greater change in pressure
  • Speed is generating a lift/bend in the ball = change in direction of the ball
• Generating lift to bend the flight path:

  • The pressure differential creates what is called the Magnus force, a lift force directed from the high pressure region to the low pressure region
  • Magnus force = lift force created by spin
• Create spin in desired bend/direction
• Another way curve a ball:

  • How a curveball curves in baseball
  • Encounters seen becomes turbulent speeds more time on ball so pushed in opposite direction can be done by also angling a cricket ball
  • Can be done by angling a cricket ball
  • Turbulent along sleep deflecting to left & it goes to the right
• Another way curve a ball:

  • Get affect by rough vs smooth surface side
  • Curveball in baseball by magnus effect
  • Seam creates turbulence making ball spin, spins towards rough side (causes greater swing so harder to hit)
  • Seam will cause bend but to bend it more need more roughness on one side
• Swing a cricket ball:

  • Create turbulence to cause pressure difference
  • Other ways to create swing:
  • Add seams
  • Add polish
• Swing a cricket ball:

  • The conventional way to swing a ball is to use the seam, angles away from the intended direction of travel so the shiny side of the ball faces the batsmen
  • Ball tampering may maximise this effect
  • Polishing the leading side
  • Using Vaseline or mints to aid polishing
  • Tampering by making more rougher on one side eg with sandpaper (is banned)
• Swing a cricket ball:

  • Reverse swing - This happens when the seam is angled for conventional swing but this time the rough side faces the batsman
  • If the ball is bowled fast enough (above 85 mph), the laminar layer goes into a turbulent state before it reaches the seam
  • Ball tampering - making the rough side rougher, or picking the seams
• Swing a cricket ball:

  • Such as with half tennis ball:
  • Rough side creates more turbulent air flow which cause it to spin opposite way
  • Rough side turbulence in its direction (similar to dimples in golf ball)
  • More swing by making rough side rougher
  • Some pick seam to raise it up to make more rough on one side, biting on one side, scratching rough side, scratching rough side