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 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.

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

Streamlining Streamlined 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

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

http://www.bbc.co.uk/learningzone/clips/chris-hoys-track-cycling-basics/5652.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.