L27 - Fluid Dynamics 2: Fluid Forces Affecting Movement

• Fluid flow:

  • Turbulence
  • disturbance, mixed
  • Laminar
  • undisturbed, smooth, parallel
• Fluid properties:

  • Density (m per volume) = p
  • Specific weight (m in newtons; weight vs volume) = y
  • Viscosity (resistance to flow)
  • High viscosity = high resistance
  • Low viscosity in joints - as age get more resistance (can cause arthritis)
• Buoyancy:

  • Force acting upwards on fluid to cause upward movement (to float)
  • Related to volume (V) + its density (Vd) & the specific weight of fluid (Y)
  • CoB = CoV, not CoM
  • Archimedes Principle
  • Fb = Vd * Y
• Drag = forces that slow us down

  • Drag is a force caused by the dynamic action of a fluid that acts in the direction of the free stream fluid flow
  • Drag act in direction of fluid acting against you
  • Generally, drag is a resistance force: a force that slows the motion of a body moving through a fluid
• Drag Equation:

  • FD = ½ * CD * p * Ap * v^2
  • Blasevich simplified equation = ½ * CD * p put together; but use full other one
  • FD = drag force
  • CD = coefficient of drag
  • Given to body based on its property
  • p = fluid density
  • Ap = projected area of the body or surface area of the body oriented perpendicular to the fluid flow
  • Frontal area of body/person
  • v = relative velocity of the body with respect to the fluid
• Drag:

  • FD = (½ * CD* p) * (Ap * v^2)
  • Coefficient of drag unitless number that is an index of a body’s ability to generate fluid resistance
  • Coefficient of drag: not linear, changes
  • Unitless number: given, fixed value based on several parameters of the body
  • From the formula we can see how drag force may be affected by relative movement velocity
• Swimming Techniques:

  • Changed resistance that body can provide during swimming as different body parts (with different coefficients) are creating drag
• Contributors to Drag Force:

  1. Skin/Surface friction = viscous drag
  2. Form drag = resistance from pressure differential bw/ lead and rear
  3. Wave drag = resistance generated by waves, bw/ 2 different fluids (velocity, up & down motion contribute)
• Skin/Surface friction = viscous drag

  • Skin/surface friction derived from the sliding contacts bw/ successive layers of fluid close to the surface of a moving body
  • Outer shell of object; surface of object moving through the fluid
  • Describes forces acting on layer of fluid moving close to the surface
  • Bc/ surface of material cause resistance to flow around the surface
• Skin/Surface friction = viscous drag

  • Boundary layer = layer of fluid immediately adjacent to a body
  • Layer closest to skin - where see most of skin/surface friction acting
• Video:

  • Boundary layer really thin
  • Laminar → transitional → turbulent layers
  • Transitional layer = increase in height
  • Further away are less affected by boundary layer turbulence
  • Turbulent = greatest drag
  • Molecules hit surface & slow down, travel along surface of material, friction bw/ the layers of slow & fast (fast try to pull slow)
  • Layers begin to separate & why get turbulence (friction bw/ molecules)
  • More rough, really slow down molecules, the more turbulence you create at back
  • Difference in speed
  • Turbulence that cause the drag
• Examples of technology to reduce drag:

  • Swimming: magic5 = 3D printed goggles to match face, caps, togs - full length togs like shark skin, forces water to move smoothly
  • Cycling: helmet shape - to reduce turbulence layer, push further back of athlete; carbon fibre material as a 1 piece (becoming similar to swimming)
  • Speed Skating: zones to the suits (different colours are different material types); see mostly white first thing that encounters air flow has lower resistance; crotch reduce friction bw/ legs
• Form Drag:

  • = resistance created by a pressure differential bw/ the lead & rear ends of a body moving through a fluid
  • = the shape of the material (can cause a pressure differential)
  • Pressure difference bw/ front & back side of object - that difference creates the drag
• Form Drag:

  • Whenever a pressure differential exists, a force is directed from the region of high pressure to the region of low pressure
  • High pressure on front, low pressure system on back
  • Created by fluid hitting the object
  • Force pushing object in opposite direction
  • Size/Wider pocket = greater difference = greater drag created
• Contributors to drag force - Form drag, want to:

  • Reduce negative pocket
  • Less of a difference bw/ pos & neg
  • Shape
  • Speed
  • Surface roughness
• Reduce Negative Pocket:

  • Shape
  • Round object: boundary layer separates earlier
  • Tear shape: maintain more laminar flow for longer until tail - reduced drag, have smaller neg pocket
  • Speed
  • Increase speed reduce neg pocket (check)
  • Video: increase speed reduces boundary disturbance
• Reduce Negative Pocket:

  • Surface roughness
  • eg golf ball - to reduce neg pocket
  • Smooth = small boundary layer at front, but at midpoint get separation
  • Dimple: create slightly larger boundary layer at front, eg dimple creates vortex (tumble inside it), this sucks in next layer of molecules (pulls them in towards object), significantly reduce pocket in the back - reduced drag
  • Roughness can alter friction
  • If structured in way can reduce drag
  • Roughness generally normally increases drag
• Negative Pocket:

  • Neg pocket = acts like a suction, ball object backwards - so to move faster want to reduce this
• Form drag: capitalising on negative pocket = drafting

  • Use neg pocket to advantage
  • Neg pocket is a suction, sucks athlete behind in (reduces their energy cost)
  • See often in long distance
  • 2nd athlete in pocket get benefit of laminar flow over disturbed boundary layer goes straight over athlete, neg pocket pulls them in, less turbulence at back
  • Drafting = good technique to reduce energy cost
• Form drag: why a golf ball has dimples

  • Each create slightly different vortexes (hex vs round)
  • Sucking in next layer of fluid
  • Hex slightly better
• Wave drag:

  • = resistance created by the generation of waves at the interface bw/ 2 different fluids, such as air & water
  • Can have in air & fluid
  • Object moving through water will create some wave/wake
  • Wave drag affect by:
  • Velocity = faster move, greater waves generated
  • Up & down motion = create more of a wave, don’t want to bob too much
• 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
  • 1) skin/surface drag, 2) form drag, 3) wave drag
  • Fluid density
  • Only force have is muscle force (no GRF)
  • Technique also important amount of SA in flow (streamline), how hand enters
  • Tight streamline, low disturbance around body = good swimming technique
• Forces While Swimming:

  • Different strokes
  • Change in surface/frontal area of swimming during different phases of the strokes
  • Breaststroke largest area
  • Drag low for flutter kick
  • Butterfly - lowest area of stroke but highest at 1 point = creates more drag
  • Frontal crawl next highest drag
  • In middle is breaststroke frontal crawl & breaststroke has similar drag forces (not expected)