biomechanics 5 fluid dynamics - 2014
TRANSCRIPT
Fluid MechanicsBiomechanics 5
Learning Objectives
Be able to:
Define friction, air resistance and drag
Explain how a streamlined helmet reduces drag with reference to laminar and turbulent air flow
Explain the factors that effect drag and apply them, using examples, to explain how a cyclist, swimmer or skier can reduce the effects of drag
When a solid surface of a body is in contact, whilst in motion, with
a solid surface of another body you will get …………………..
Friction
A force that acts in opposition to the movement of one surface over another
Friction
Good
For sports such as sprinting as prevents feet from slipping and sliding on surface and therefore slowing down running action
Bad
Skiing – want to keep friction to a minimum to ensure maximum forward acceleration. Will wax skis to ensure glide smoothly over snow
Types of Friction
Rolling Friction - Term used to describe the force between surfaces which do not move relative to one another, like a wheel rolling over a surface or a foot driving and pushing without slipping
Sliding Friction –occurs when two surfaces move relative to one another – and is always less that maximum – This is why ABS systems reduce braking force on wheels if sensors detect the beginning of sliding
But…………..A solid moving through a fluid is referred to differently
Fluid Friction
Term applied to objects moving through fluids (gases or liquids)
The force acts in the opposite direction to the direction of motion
Often referred to as DRAG (in water and air) or AIR RESISTANCE in air!
Fluid Friction
Fluid – a material that deforms continuously and permanently under the application of a shearing stress.
Definitions
Air resistance
The force acting in the opposite direction to the motion of a body travelling through air
Depends on shape and surface characteristics of the body, cross-sectional area and velocity of body
Drag The force produced by
the motion of a body in fluid (water or air)
Depends on same points as air resistance but also on the type of fluid.
Water – greater density than air
Water – harder to push through
High values of fluid friction
Occur when any sports person or vehicle is moving through water (swimming)
Or when travelling through air at high speeds (cycling)
Low values of fluid friction
Occur for any sprinter or game player for whom air resistance is usually much less than friction effects and weight. Therefore streamlining seen as less important.
A shot or hammer in flight in which air resistance would be much less than the weight
Exam QuestionReducing drag is especially important in
both cycling and swimming.
Discuss the factors that influence drag in sport and examine strategies that are employed to minimise effects.
Bear this question in mind as we move through the next section – form a list of factors on
board!
Laminar Flow and Drag
Laminar Flow
Layers of fluid flow slide smoothly over one another
Turbulent Flow/Vortex Flow
Boundary layer is composed of vortices that increase surface friction.
Common at rear end of non-streamlined vehicle
Turbulent Laminar
Think bike
helmets
Fluid Mechanics
Turbulent flow causes more friction than Laminar flow.
Less resistance to the movement of fluid if the flow is laminar.
“Fluid” – not water!Can refer to an object
Moving through air
StreamliningStreamlined bodies
incorporate gradual tapering to minimize pressure effect and separation of fluid
The point of a streamlined shape is that the air moves past it in layers
whereas in the case of the non streamlined helmet, vortices are formed where the fluid does not flow smoothly.
When this happens bits of fluid are randomly flung sideways which causes drag.
The drag is caused by bits of fluid being dragged along with the moving object (the cycle helmet)
(a) Normal pressure and friction forces (b) Attached and separated flow around a cylinder (c) Attached flow and pressure recovery along a streamlined body
Figure from Bicycle Science pg. 174
Aerodynamics – which shape causes most drag?
Drag coefficients of various geometriesFigure from Bicycling Science pg. 191
Aerodynamics
Drag coefficients of various geometriesFigure from Bicycling Science pg. 191
Bodies in fluids
Classic aerofoil shape:
streamlined – less friction
therefore less drag.
Fluid Friction or drag
http://www.livescience.com/21761-summer-olympics-science-making-swimmers-faster-video.html
http://www.livescience.com/21921-summer-olympic-science-london-s-pool-making-swimmers-faster-video.html
http://www.bbc.co.uk/learningzone/clips/wind-resistance-in-cycling/2180.html
Factors affecting drag
Shape, surface characteristics and position of the body
Cross-sectional area of the body
Velocity of the body
Streamlining – to go faster!
Shape, surface characteristics and position of the body
Cross-sectional area of the body
•The more streamlined and aerodynamic, the less drag
•Speed skiers keep air resistance to a minimum by crouching down low – small cross-sectional area
•Also wear helmets that extend to shoulders – more streamlined position
•Suits and boots also streamlined
Tips to reduce drag in Cycling
http://www.bikeradar.com/gear/article/technique-lose-that-drag-875/
TIP: get out of the wind using other riders wherever possible
TIP: keep clothing zipped up, relatively snug and adjust it to the wind and heat
TIP: lower spoke count drops drag, and lighter wheels will make climbing easier too
TIP: consider aero bars to make your ride more comfortable and faster
TIP: get low when you hit a head wind
1 BODY POSITION: 1 to 6 minutes. - Cost from £20 - Moving the torso into a flat position, producing a lower head and flatter arms, significantly reduces frontal area without buying much more than a new stem.
2 AEROBARS: 30sec to 2 minutes - Cost from £50 - Assuming the arms are narrowed, torso position is easier to maintain and drag from the bars is reduced.
3 AERO HELMET: 30 to 120 seconds - Cost from £70 - Reducing vents and smoothing airflow behind the rider's head is a significant drag reducer that can give you more than a second per pound spent.
4 SKINSUIT: 30 to 60 seconds - Cost from £30 - Flapping pockets, rough material and bad seam placement make run of the mill clothing un-aero. Skin-tight suits work, though the exact figures are kept secret by the likes of Nike, Pearl Izumi and Descente.
5 FRONT WHEEL: 30sec to 60 seconds - Cost from £200 - Keeping air close to the rim as opposed to air swirling around a box-shaped rim reduces drag. As does a reduced spoke count to ideally 12 to 18.
6 AERO FRAME: 30sec to 2 minutes - Cost from £500 - Taking round tubes and giving them an aero profile reduces drag. Reducing the seat tube or wrapping it around the rear wheel, or just behind the front wheel, also works
7 FRONTMOUNTED AERO BOTTLE: 30sec to 60 seconds - Cost from £15 - Keeping the rider's arms on the bars and not reaching for a bottle keeps drag low and allows power to be applied constantly. Bottles behind the saddle will slow you down by 30-40 seconds.
8 REAR DISC WHEEL: 15 to 30 seconds - Cost from £500 - The effect is roughly half that of a front wheel due to the frame shielding the wheel (Martin & Cobb). It may make you more aerodynamic, but high winds can make handling tricky.
9 OVERSHOES:10 to 20 seconds - Cost from £15 - You can get some cheap speed with tight rubberized Lycra shoe covers that take straps, vents and buckles out of the wind.
10 CONCEALED CABLES: 10-20 seconds approx. - Cost from none - Merely routing cables through bars and into the appropriate frame hold can reduce drag.
http://www.bikeradar.com/gear/article/know-how-beating-the-wind-12090/
http://www.sciencelearn.org.nz/Science-Stories/Cycling-Aerodynamics/Sci-Media/Video/Testing-aerodynamics-of-elite-cyclists
Exam Question
Reducing drag is especially important in both cycling and swimming.
Discuss the factors that influence drag in sport and examine strategies that are employed to minimise effects.
In pairs – formulate an answer
All together (Kerry typing) we will formulate a class answer.
Discuss the factors that influence drag in sport and examine strategies that are employed to minimize its effects. [6]
Fluid friction and air resistance are the two forces acting against moving objects that slow down. They do this because they act in the opposite direction to the movement. The faster an object moves, the more resistance it will encounter.
In both swimming and cycling, we refer to this resistance as drag.
Drag is affected by the shape of the object and the way in which water (in the case of swimming) and air (in the case of cycling) flows past it. Examples should be provided.
Streamlining is an effective way of reducing drag and aiding a smoother flow of air past an object. This smooth flow involves fluid/air flowing in layers known as laminar flow.
In cycling, streamlining can be achieved in a number of ways. Cyclists adopt a low crouch position (using drop handlebars to reduce their frontal cross-section area) and often wear tight fitting ‘skinsuits’. Advances in bike design such as oval-shaped frame tubes and disc wheels have helped reduce drag. Additionally, helmets have been designed to have a more aerodynamic shape.
In swimming, an efficient technique will lead to more streamlined shape in the water. For example, an effective flutter kick will help raise the legs in the water and reduce the frontal cross section. Other strategies employed by swimmers include shaving (to reduce frictional drag), the use of swim caps and more recently, the use of specially designed fully body suits known as ‘fastskins’. The most popular version, the Speedo LZR, compresses the body into a more aerodynamic shape and claims to reduce skin friction drag by as much as 24 percent.
Diagrams may be beneficial in helping to explain this response.