L26 - Fluid Dynamics 1: Moving in a Fluid

• Nature of Fluids:

  • Fluid is any substance that tends to flow or continuously deform when acted on by a shear force
  • A force that acts parallel to the surface
  • Air & water are fluids that exert force on the human body
  • A time on the wind (tail vs head)
  • To determine if broke record or not
  • Used for designing equipment
• Nature of Fluids - Relative Velocity:

  • Velocity of person in respect/relative to the fluid moving in
  • Is the vector subtraction of the body’s velocity from that of the fluid
  • Imagine the effects of a head wind vs tail wind on the duration of your air travel
  • Persons direction determines if it is head or tail wind
• Nature of Fluids - Relative velocity:

  • Head wind = into the wind (opposite)
  • Cyclist in order to moving must move faster than the wind acting against them
  • Tail wind = with the wind (same) → is an advantage
  • Less effort needed to gain relative velocity
  • Head wind
  • Vhwp = Vp - Vhw
  • Vhwp = -15 m/s - (5 m/s)
  • = 20 m/s
  • Tail wind
  • Vtwp = Vp - Vtw
  • Vtwp = 15 m/s - (5 m/s)
  • = 10 m/s
• Nature of Fluids - Laminar vs Turbulent flow:

  • Laminar flow = smooth layers of fluid molecules flowing parallel to one another
  • Turbulent flow = mixing of adjacent layers of fluid
  • Stability of fluid
• Nature of Fluids - Laminar vs Turbulent flow:

  • Laminar flow is characterised by smooth layers of fluid molecules flowing parallel to one another
  • All particles travelling along own linear path, no interaction with each other; going at same speed etc (eg calm water)
• Nature of Fluids - Laminar vs Turbulent flow:

  • Turbulent flow is characterised by mixing of adjacent layers of fluid
  • Molecules interacting, fluid is disturbed; can see movement of fluid
  • Fluid travels at different speeds
• Nature of Fluids - Laminar vs Turbulent flow:

  • If an object moves with low velocity relative to fluid, the layers of fluid near the surface of the object mix, & the flow becomes turbulent
  • Less disturbance to the fluid
  • If the object moves with high velocity relative to surrounding fluid, the layers of fluid near the surface of the object mix, & the flow becomes turbulent
  • More speed, more disturbance (more turbulence)
• Nature of Fluids - Laminar vs Turbulent flow:

  • Higher speeds cause more turbulence
• Nature of Fluids - Laminar vs Turbulent flow:

  • Turbulence may be affected by - shape, roughness of object & speed of the body
  • Improve smoothness & shape to increase the speed of the body
  • Such as long race suits (more efficient), shave (decrease turbulence & drag); same with swim caps (create less disturbance)
  • Roughness lowers relative velocity
  • Streamline = shape of body hitting the water, increase relative velocity better performance
  • eg roughness of the body, will lower the relative velocity at which turbulence is caused
• Nature of Fluids - Laminar vs Turbulent flow:

  • Roughness of the body, will lower the relative velocity at which turbulence is caused
  • Smoother surface will move faster
• Nature of Fluids - Fluid properties:

  • Density (p) = mass/volume, & the ratio of weight to volume is known as specific weight (y)
  • The denser & heavier the fluid surrounding body, the greater the magnitude of the forces the fluid exerts on the body
• Nature of Fluids - Fluid properties:

  • Density (p) = mass/volume, & the ratio of weight to volume is known as specific weight (y)
  • Viscosity = The property of fluid viscosity involves the internal resistance of a fluid to flow
• Nature of Fluids - Fluid properties:

  • Density (p) is defined as mass/volume, & the ratio of weight to volume is known as specific weight (y)
  • Resistance of fluid affected by density
  • How much mass to the volume of fluid
  • Water more dense (weighs more) than air
  • Specific weight in Newtons, to volume
• Nature of Fluids - Fluid properties:

  • The denser & heavier the fluid medium surrounding a body, the greater the magnitude of the forces the fluid exerts on the body
  • More dense more forces on body
  • In water more dense is less efficient
  • Dead sea has greater density (easier for people to float) = 1240 kg/m^2; so difficult for people to dive in it & for things to live in it
• Nature of Fluids - Fluid properties:

  • Viscosity = The property of fluid viscosity involves the internal resistance of a fluid to flow
  • The greater the extent to which a fluid resists flow under an applied force, the more viscous the fluid is
  • Gas - increases viscosity with increase temperature
  • Liquid - decrease viscosity with increase temperature
• Nature of Fluids - Fluid properties:

  • Viscosity = The property of fluid viscosity involves the internal resistance of a fluid to flow
  • Resistance of a fluid to flow
  • High resistance = highly viscous
  • More forces act on body when more viscous & makes it more difficult
• Nature of Fluids - Fluid properties:

  • The extent to which a fluid resists flow under an applied force, the more viscous the fluid is
  • Temperature affects viscosity depending on nature of fluid
  • Gas - increases viscosity with increase temperature
  • Molecules become more actively greater rate/frequency
  • So this increases viscosity/more resistant
  • eg when get off plane in hotter climate (harder to breath)
  • Liquid - decrease viscosity with increase temperature
  • eg oil when cooking; as oil heats up runs more → decrease resistance to flow
• Forces acting on a body related to fluid:

  • Buoyancy - fluid once acting upward is equal to the weight of the fluid displaced by the body
  • Archimedes principle
  • Physical law stating that the buoyant force acting on a body is equal to the weight of the fluid displaced by the body
  • Density & viscosity important as forces that act on body when in fluid
  • Buoyancy also related to shape of object
• Forces acting on a body related to fluid - Buoyancy:

  • Fb = p * g * V

  Or

  • Fb - Vd * Y
• Forces acting on a body related to fluid

  • Fb = p * g * V or Fb - Vd * Y
  • Buoyancy force related to:
  • p = row/density of fluid
  • g = gravity
  • V = volume of object
  • Related to fluid & volume of object
  • Centre of buoyancy not the same as CoG; so CoB is more important when in water
  • Same weight:steel bowl has greater volume therefore greater buoyancy compared to a steel ball the same weight/mass
  • Volume is important in terms of buoyancy
  • Y = specific weight = p * g
  • Vd = volume density
• Forces acting on a body related to fluid:

  • Buoyancy forces act through our centre of volume as opposed to our centre of mass or gravity
  • Think about what this might mean for a streamlined swimmer
• Forces acting on a body related to fluid:

  • Buoyancy forces for a streamlined swimmer
  • If you’re buoyant can be more streamlined
  • Work to maintain streamline so if CoB/CoM = more work needed
  • Kick maintains streamline
  • Arthoregmic measures:
  • Swimmers with larger CSA tend to be better performers (larger chests)
  • Makes you higher in water (=better)
  • Foot size also important; bigger foot size correlated with an advantage
  • Larger arm length, gives more pull in water
• Forces acting on a body related to fluid:

  • Where is buoyancy going to be based on volume
  • More mass down lower, near hips (larger muscles); greater volume = chest as is by lungs
  • Why if you lay your legs sink
  • Higher CoB - feel like a sinker + weather having lungs full or not
  • More air in lungs – more filled, more volume, means more buoyancy
• Forces acting on a body related to fluid:

  • How can we use this knowledge to help us swim?
  • Fluid flow - laminar vs turbulence
  • Trying to reduce the disturbance
  • eg by swimwear, shaving
  • Help keep laminar flow
  • Relative velocity
  • Moving as fast as can relative to fluid flow
  • Fluid properties
  • Affect density
  • Better if water warmer (lower viscosity), but harder to dissipate heat
  • Buoyancy
• Technological Doping:

  • Suits banded
  • Big dip in records with suits
  • Unfair for those without the technology