Figure 7. Smoke flow around the scale football model showing: (a) laminar flow at Re=90,000; and (b) turbulent flow at Re=130,000.
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Also shown on Figure 6 are the results of other aerodynamic studies on spheres. Achenbach (1974) found that an increase in surface roughness had the effect of moving the transition from laminar to turbulent behaviour to a lower Reynolds number. A similar effect has been found for sports balls that have surface roughness (Mehta and Pallis, 2001). It is well known that dimples on golf balls help to reduce drag by moving the transition point to lower Reynolds number so that the flow around a golf ball is turbulent for more of its flight. Judging by Figure 6, the seam pattern on a football, seems to have a similar effect, but not to such a great extent. If a football were completely smooth, according to Achenbach’s (1972) data, it would have a higher drag coefficient for most of the range of Reynolds numbers, seen in play.