Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
Figure 5.7 Separation points (S) on a smooth ball for boundary layer flow that is: (a) laminar; (b) turbulent. T indicates
transition from laminar to turbulent boundary flow.
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An object moving from a region of low pressure to one of high pressure, experiences
a drag force, which will be discussed below. The boundary layer separation, which
leads to the formation of the wake, occurs far more readily if the fluid flow in the
boundary layer is laminar (Figure 5.7(a)) than if the flow is turbulent (Figure 5.7(b)).
This is because kinetic energy is more evenly distributed across a turbulent boundary
layer, enabling the fluid particles near the boundary to better resist the increasing
pressure. Figure 5.7 shows the difference in the separation point (S) positions and the
size of the wake between laminar and turbulent boundary layers on a ball. The
change from laminar to turbulent boundary layer flow will occur, for a given object
and conditions, at a speed related to the critical Reynolds number. The change
occurs at the transition point (T, Figure 5.7(b)), and the relationship between the
transition and separation points is very important. If separation occurs before its
transition to turbulent flow, a large wake is formed (Figure 5.7(a)), whereas transition
to turbulent flow upstream of the separation point results in a smaller wake (Figure
5.7(b)).