The critical Reynolds number

The effect of dimpling the surface of the ball is to energise the flow and trigger turbulence

The dimples correspondence is interesting but incomplete, how could there not be any mention of Reynolds?
 
The key point is that the flow around a smooth sphere the size of a golf ball travelling through stationary air at the speeds experienced when driven from the tee is nominally laminar and just below the critical Reynolds number. The effect of dimpling the surface of the ball is to energise the flow and trigger turbulence. A feature of turbulent boundary layers is that they are more stable than laminar layers under conditions of an adverse pressure gradient, and consequently the turbulent flow remains attached to the rear half of the ball considerably longer resulting in a significantly narrower wake. Although the skin friction acting on the turbulent ball is higher than that of a laminar one, the form or pressure drag resulting from the wake represents the bigger contribution to the overall drag. The net effect is that the dimpled ball has a significantly lower total drag than the smooth ball, and will “carry” much further, typically 30%. So, roughening the surface of an object to make it travel further is counter-intuitive and only applies to certain flow conditions.
 
At the risk of expanding the debate, this insight into flow around spheres also explains why a cricket ball when bowled under certain conditions can be made to swing (lateral deviation). A fast/medium paced bowler will also be delivering a cricket ball at speeds nearing the critical Reynolds number and if he/she can project it in such a way that the seam remains at a constant angle to the direction of flight (about 20 degrees according to most coaching manuals) with the polished side leading, the ball will swing in the direction that the seam is pointing in. So, in the case of an outswinger bowled to a right handed batsmen, the bowler will align the seam with first slip and the polished side to the right. If the seam remains in this alignment whilst in flight, the seam will lead on the left hand side of the ball (when viewed from above) and trigger turbulence, with the flow on the right hand (mainly polished) side remaining laminar. This will result in an off-axis wake pushing the ball to the left from the bowler’s perspective. Much is discussed about the effect of the make and construction of the ball, air conditions, balls swinging more on certain grounds, etc, but the key point is the skill of the bowler and the “knack” of releasing the ball in a certain way to maintain a constant in flight seam position.
 
To me, the above encapsulates engineering and the role of the engineer. It is through our knowledge and wider understanding of the underlying phenomena and requirements that we are able to better optimise and formulate appropriate solutions. Without such insights, we could have situations where there could, for example, be widespread and inappropriate application of dimples to all fluid flows based on an incomplete understanding.
 
John Hunt, Leatherhead, Surrey

Next letter: HS2 line defects