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© 2014 Sheffield Hallam University All Rights Reserved
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About Us
The Centre for Sports Engineering Research is internationally renowned for its applied research and consultancy, with over 230
years of cumulative experience. We are the largest research centre in the world that focuses on sports engineering.
Our expertise spans several disciplines. At the core is mechanical engineering, and engineering design, which is enhanced by
physics, biomechanics, biomedical engineering, mathematics, and computer science. Through our work we are helping to develop a
deeper understanding of the complex sporting environment. We apply this knowledge to generate innovations for business, sports,
healthcare and governing bodies.
We undertake projects for a diverse range of sports companies and organisations, including the English Institute of Sport, UK Sport,
Adidas, the International Tennis Federation, Sport England, Prince, Puma, Ping and Oxylane.
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About This Exhibition
This exhibition presents a series of images that show research and consultancy work conducted with The Centre for Sports
Engineering Research using a method called Computational Fluid Dynamics (CFD). This is a powerful numerical technique used in
the understanding and study of fluid motion.
The application of CFD can be summarised as follows. A discrete finite mesh is created around an object of interest. This creates a
flow space surrounding the object, through which the motion of fluid is prescribed, effectively creating a numerical wind tunnel.
However CFD differs from traditional wind tunnels in that there are no constraining walls (unless otherwise imposed), allowing the
creation of real world environments. To resolve the flow field around the object a series of equations are solved at all nodal points
within the mesh. This determines fields of velocity, pressure, and other variables as appropriate that may include temperature, and
turbulence.
Thus a complex description of the flow field around an object can be formed, including phenomena that are invisible to experiment,
and visualised in stunning detail and colour. For this reason CFD has gained a reputation for perhaps being the most colourful
technique in engineering.
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Tangled
There is little understanding of the detailed flow behaviour of the feathers, and the
simulation of a feather is difficult to perform accurately. These complex keratin based
structures comprise a lightweight but extremely strong rachis (shaft) and vanes of complex
interlocking barbs that flex and deform. A traditional badminton shuttle has 16 of these
complex structures, and the aerodynamic interaction between the feathers results in some
complex yet beautiful flow structures. This image reveals a section of the tangled turbulent
vortex core structures that form behind a feather shuttle. Two distinct zones of structures are
observed to exist. A central core of structures form as air flows, inside, through the shuttles
cage of feather rachis. This is surrounded by a ring of vortices that form as air passes over and
between the feather vanes.
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Splash
Sometimes the simplest shape is the most difficult to simulate correctly. Spherical bluff
bodies, balls, are central to many sports. What distinguishes one ball from another is its size,
surface finish, seams, or surface pattern, all of which play an important role in the behaviour
of the ball. This is because the majority of ball sports are played at speeds where an
aerodynamic phenomena called transition exists. This is the point where the air flow around
an object switches from a laminar to turbulent behaviour. Much time is spent attempting to
influence transition when designing sports balls, particularly footballs, through seam size and
surface finish. This is because transition causes the flight behaviour of a projectile to alter
radically. So it is also important to understand it and simulate it correctly. Yet it is the
accurate prediction and simulation of transition that still remains the greatest challenge in
computational fluid dynamics, CFD. This image shows the prediction of the formation and
shedding of vortices over the surface of an adidas Brazuca football. The front portion of the
ball is pictured passing through a pressure iso-surface, creating the impression of a splash.
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Shedding
Cylindrical shapes are another classical bluff body form that, like the sphere, are
commonplace throughout sport. Cylindrical forms and cross sections are found in many items
of sports equipment, such as golf club shafts, baseball bats, rackets, etc. The human form can
also be easily simplified in representation into a collection of cylinders. Simulation of a
cylindrical object poses the same challenges as any other bluff body, namely the accurate
prediction of flow transition, though perhaps these forms are slightly easier to deal with than
a sphere. This image shows the formation of large scale turbulent structures behind the
barrel of a baseball bat. The structures are coloured by directional vorticity in relation to the
bats main axis. Blue structures move and rotate in a -ve direction whilst red structures depict
+ve direction of movement. This reveals the complicated roll up and movement within these
classical wakes. The underlying mesh upon which the calculations are performed can also be
observed.
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Dimples
The drag acting on a sphere is dominated by the separation of flow from the rear of the body.
Patterned surfaces can be used to control or delay this separation. This is achieved by causing
the boundary layer formed, as flow moves over the sphere, to transition from laminar to
turbulent flow. Transition reduces the size of the separation, and thus wake behind the
sphere, reducing the total drag force experienced. Golf balls use dimples arranged in carefully
designed patterns and sizes to achieve this transition. This image shows a 184 dimple golf
ball, coloured by predicted skin friction coefficient. Areas of high friction are coloured
red/yellow, and areas of low friction are in the blue shades. It is possible to observe in part
how the dimples cause flow transition over the ball. Flow moving over a dimple separates
from the leading edge of that dimple indicated by pockets of low skin friction. The flow then
reattaches and impinges on the back face of the dimple where friction rises again. This
motion causes transition of the boundary layer over the surface of the ball delaying flow
separation to the leeward side of the ball, reducing drag.
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Does my bum look big in this?
Lying face down on a thin sled travelling at 140 kph along an ice covered track, a skeleton
athlete attempts to adopt a position streamlined to the oncoming flow. However regardless
of this the curvature of the human body still creates complicated swirls and separations that
are unique to each persons shape. There are however certain features that can be observed
that are common to all, such as depicted here. This image shows an oil flow visualisation of
air movement over the buttocks of a skeleton athlete. As the flow moves over the buttocks it
separates away from the body forming a wake as it contains insufficient momentum to cling
to the surface. A classic swirling pattern and motion is observed the size of which is dictated
by the size and shape of the buttocks.
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Flaming Wake
Golf ball manufacturers develop balls with elaborate dimple patterns in an attempt to control
and delay the separation of flow. Balls are designed with anywhere up to 500 dimples, that
have been optimised in shape, diameter, depth, and layout, in an attempt to create a ball
that has optimum performance in lift, drag, and accuracy. Some manufacturers have looked
to NASA space shuttle technology to help in achieving this optimum distribution and design.
This image shows the wake that forms from the rear side of a 184 dimple golf ball in flight. A
relatively low number of dimples for a golf ball, with the majority of balls having 300+. The
wake can be seen to separate away from the rear of the ball, at first in streams caused by the
affect of the dimples, before finally collapsing and breaking down in to an unsteady swirl of
vortices.
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Cycle Helmet
Cycle helmets not only protect the head but play an important role in the aerodynamic
performance of a cyclist. A cyclists body accounts for around 70% of the total drag force
experienced, and significant drag savings can be made through prudent choice of helmet.
Helmets with a classic smooth teardrop shape are typically favoured for time trial events
when attempting to optimise speed. However the aerodynamic gains these helmets afford is
at the expense of cooling and comfort as they reduce airflow to the scalp. Helmets designed
to maximise cooling with large air vents, and open structures, typically result in higher drag
force. This is because these features stir up the passing flow, and suffer from abrupt flow
separations. However they maximise cooling and comfort which is important for distance
riding. This image shows the swirling flow behind the cycle helmet of a triathlete, designed
for comfort. Flow is seen to channel along the sides of the helmet before rolling up into a
series of vortices that merge to form an extensive vortex core that clings along the cyclists
back.
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Driving for Distance
The aerodynamic performance of golf drivers has become increasingly important for
manufacturers. Enabling their customers to achieve that few extra percent in swing speed
enables a well struck ball to travel those few extra yards. Optimising the shape of a driver
head, essentially a bluff body with a flat face, to create an aerodynamic advantage is a
challenge. Not only does the club experience a rapid acceleration during the swing, at the
same time the face angle of the club relative to the direction of travel also changes. This
complicates the aerodynamic optimisation of the head. This image shows an oil flow
visualisation revealing some of the flow structures that can occur over the surface of a driver
head during the swing. Dependent upon club orientation a large vortex rolls up behind the
hozel, where the shaft meets the head. At the same time a series of interacting shedding
vortices appear along the edge of the crown.
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16 Feathers
Traditional feather shuttles consist of 16 feathers that have been trimmed to shape, inserted
into a cork base, stitched, and lacquered. The finest quality feather shuttles use goose feather
taken from a single wing only, as the curvature of the feather rachis, and vane, is dependant
upon whether it came from the left or right wing. This is important as the curvature of the
feather, and the overlapping manner in which they are inserted into the base, induce spin in
flight. Constructing a shuttlecock in this manner ensures that it will always spin in the same
direction (counter clockwise from the servers view point) which is important as the spin of
the shuttle causes the shuttle to drift sideways towards the end of its flight. This image shows
the influence that a single feather has on the direction of flow behind a shuttle. Post
simulation flow ribbons are released from a single feather and their movement is observed.
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Not all balls are round
Rugby balls are what are classically referred to as prolate spheroids. In flight they spin in a
manner that is specific to the type of kick or pass which is played. The spin helps with both
the distance achieved and the accuracy of the pass. As rugby balls are typically formed from
four moulded panels, the seams of the ball play less of a role in transition, than for example
a football. The surface roughness of the ball is of greater importance in this case, not only
for the aerodynamics but also the handling and grip on the ball. This image shows the
movement of particles released over the surface of a rugby ball, as observed during a stage
of a spiral kick. It can be seen how the particles form into two tight swirls indicative of the
formation points of vortex core structures stretching back from the rear of the ball in flight.
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Rolling Vortices
Helmets have a huge role to play in the aerodynamic performance of skeleton athletes. The
helmet is the first part of the athlete to hit the oncoming air flow and presents significant
frontal area. When optimising a shape to reduce drag one of the primary goals is typically to
reduce the frontal area as much as possible. The shape of the helmet also plays a significant
role in the aerodynamic drag experienced by the athlete’s body and sled. As the air passes
over the surface of the helmet it directs flow and eventually separates away forming a wake.
The aerodynamics of the helmet directly influence the aerodynamic performance of anything
that lies behind, therefore it must be designed carefully. This image shows a volume
rendering of large scale vortex cores structures rolling up behind a skeleton athlete’s helmet.
These pass and move over the shoulders and back of the athlete, however as every athlete is
a different shape their influence and behaviour on the body differs for each person.
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Head Down
Wheelchair racers need to overcome two predominant resistive sources when competing on
the track: rolling resistance, and aerodynamic drag. The aerodynamic drag of the wheel chair
and athlete can account for up to three quarters of the total resistive force experienced.
Athletes will attempt to reduce their frontal area as much as possible through positioning and
tucking. This is a trade off against efficiency if the athlete is pushing, however significant
aerodynamic gains are achievable when coasting. Aerodynamically optimised helmets, as
with cycling can help. Streamlining of the chair with aerodynamically optimised frames
sections, aero wheels, and farings where permitted are also implemented. This image shows
a wheelchair athlete in a tuck position. The image shows a vertical slice taken through the
central plane of the flow field revealing velocity contours, whilst the surface of the athlete
and wheelchair are coloured by skin friction coefficient.
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Drafting
Downhill skateboarding is an extreme gravity sport of increasing popularity. Yet it is still very
much a minority sport and unsurprisingly has not been the subject of any significant research
to date. Indeed, the engineering aspects of skateboarding in general are a very under
researched area, despite the high popularity of the sport amongst teenagers. Aerodynamics
play a significant role in this gravity sport, as speeds in excess of 75 mph are reached in
competition. Skaters try to adopted aerodynamic tuck positions and draft to minimize
aerodynamic drag as much as possible. The tucks are similar in fashion to those adopted by
speed skiers, and the skaters are starting to use a range of aero helmets very much modelled
on those as worn by speed skiers. This images shows flow lines passing over two drafting
skateboarders. The flow is observed to pass smoothly over the skaters helmets, but swirls
strongly behind their legs.
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Jump
Water ski jumping is one of the oldest disciplines in water skiing. The first jump was
performed by Ralph Samuelson of Minnesota (USA) in 1925, three years after he had
invented water skiing. Samuelson jumped 18 m off the end of a greased ramp. Today water
ski jumping is an international sport with elite male athletes jumping distances in excess of 70
m. Similarities can be drawn between water ski jumping and Nordic ski jumping. Water ski
jumpers try to manipulate their skis into a V flight style as used by their winter counterparts.
However this is hampered by the dynamics of hitting a water ski jump ramp, and the lack of
articulation in the ski bindings that prevent the jumper leaning out over their skis. Water
jump skis commonly lack a constant curvature (rocker) from the tip to tail, however the front
third of the skis are angled upwards and turned out. This design feature of the ski is known as
the Stokes tip. This image shows the path of the wash behind the jumper mid jump. Flow
lines released from the skis reveal the large rolling structures that form from the Stokes tips.
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Smoke Flow
Smoke visualisation is commonly used in wind tunnel analysis to understand and visualise the
aerodynamic forces that are measured. The smoke can reveal flow separations from surfaces
where the boundary layer contains insufficient energy to remain attached, and also reveal
some of the complex flow structures that can occur as vortices form. A similar approach can
be used in Computational Fluid Dynamics to reveal the same structures using volume
rendering methods. However unlike traditional wind tunnels, with simulation it is possible to
pick and choose between the flow structures you wish to observe and pull out the smallest of
details. This can be extremely useful when designing for and analysing the aerodynamics of a
bluff body such as a golf club. This image shows the use of volume rendering to visualise the
large scale wake structures that form around a golf driver head and shaft. Large scale swirling
vortices can be seen to originate from both the leading edge of the club crown and the club
sole. Whilst rapidly shed repeating structures can be seen originating from the shaft.
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Pushing Optimisation
The use of simulation in the recreational sports equipment industry is becoming more
common. Where once sports equipment was largely designed based upon accumulated
experience and intuition, simulation is now being regularly incorporated into the design loop
to further enhance products. One sector that has particularly embraced this is the cycling
industry, as the general public expect, and demand, more and more of their bicycles. This
image shows an aerodynamic study of a full triathlon bicycle and rider. The analysis not only
revealed the complex flow phenomena that occur around the bicycle and rider, but also
provided a comprehensive aerodynamic force breakdown of the entire system that informed
a redesign of the frame. Visualisations show how the rear half of the bicycle sits in the wake
of the rider. The rider’s legs are seen to have a significant influence upon the seat post, down
tube, chain stay, seat stay posts, and rear wheel.
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Speed
Sometimes you don’t want to go fast to win a race, sometimes you just want to break a
World Record. This was the case when CSER were commissioned to design a gravity powered
sled, to help celebrity biker Guy Martin set a World Record as part of the Channel 4 series
Speed. Using non-contact laser scanning techniques an accurate model of Martin was created
around which the record attempt sled could be designed. The faring for the sled deflecting
the oncoming air flow around Martin was designed using Computational Fluid Dynamics to
understand the aerodynamics of the problem. Early in January 2014 the record attempt was
made at the Grandvalira speed ski slope in Andorra. Martin dropped 360 ft over the 984 ft
run, travelling down a slope steeper than the main climb route on Everest. The speed record
was beaten by 30 kph and a new World Record of 134.37 kph (83.49 mph) was set. This
image shows the wake structures around the record breaking sled.
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Another fine mesh
The accuracy of any simulation is not only dependent upon the numerical methods used but
is heavily dependent upon the geometric representation of the subject of study. Accurately
capturing the complex curvatures of the human body, or the fine details of a feather shuttle
are a challenge. However the use of non contact laser scanning methods allows this to be
achieved, and permits the bespoke design and optimisation of equipment for a specific
athlete. Non contact laser scanners project a laser stripe on to the surface of an object, and
then capture point readings along this stripe with a camera, as the stripe is swept over the
surface of interest. Typically millions of data points are captured during a scan which describe
the geometry. These points can then be wrapped using software to create a triangular mesh
that creates a surfaced model of the geometry, ready for use in simulation. This image shows
a triangulated mesh of an athlete scanned on a bicycle.
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Centre for Sports Engineering Research
Academy of Sport and Physical Activity
Sheffield Hallam University
Collegiate Hall
Collegiate Crescent
Sheffield, S10 2BQ
Email: [email protected]
Web: www.shu.ac.uk/research/cser
© 2014 Sheffield Hallam University All Rights Reserved
