Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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below the critical Reynolds number, it is possible for roughness elements on one part of
the ball to stimulate transition of the boundary layer to turbulent flow, while the
boundary layer flow on the remaining smoother portion of the ball remains laminar.
This is very important, for example, in cricket ball swing, in which the asymmetrical
disposition of the ball’s seam accounts for the lateral movement of the ball known as
swing. The seam promotes turbulence in the boundary layer on the ‘rough’ side of the
ball, on which the seam is upstream of the separation point, as in Figure 5.7(b), while
separation occurs on the other (‘smooth’) side of the ball, as in Figure 5.7(a). The
asymmetrical wake causes the ball to swing towards the side to which the seam points.
For reverse swing to occur, the ball must be released above the critical speed for the
smooth side of the ball, which can only be done by bowlers who can achieve such
speeds. The boundary layer becomes turbulent on both hemispheres before separation.
On the rough side, the turbulent boundary layer thickens more rapidly and separates
earlier than on the smooth side. The result is the reversal of the directions of wake
displacement and, therefore, swing.
Skin friction drag
Skin friction drag is the force caused by friction between the molecules of fluid and a
solid boundary. It is only important for streamlined bodies for which separation – and
pressure drag – has been minimised. Unlike pressure drag, skin friction drag is reduced
by having a laminar as opposed to a turbulent boundary layer. This occurs because
the rate of shear at the solid boundary is greater for turbulent flow. Reduction of
skin friction drag is important for racing cars, racing motor cycles, gliders, hulls of
boats, skiers and ski-jumpers and, perhaps, swimmers. It is minimised by reducing the
roughness of the surfaces in contact with the fluid.
Wave drag
Wave drag occurs only in sports in which an object moves through both water and air.
As the object moves through the water, the pressure differences at its boundary cause
the water level to rise and fall and waves are generated. The energy of the waves is
provided by the object, which experiences a resistance to its motion. The greater the
speed of the body, the larger the wave drag, which is important in most aquatic sports.
Wave drag also depends on the wave patterns generated and the dimensions of the
object. The drag is often expressed as a function of the Froude number. Speed boats
and racing yachts are designed to plane – to ride high in the water – at their highest
speeds so that wave drag – and pressure drag – are then very small. In swimming the
wave drag is small compared with the pressure drag, unless the swimmer’s speed is
above about 1.6 m/s, when a bow wave is formed.
Other forms of drag
Spray-making drag occurs in some water sports because of the energy involved in
generating spray. It is usually negligible, except perhaps during high-speed turns in
surfing and windsurfing. Induced drag arises from a three-dimensional object that is
generating lift. It can be minimised by having a large aspect ratio – the ratio of the