Secrets of Swing

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2 Responses

  1. John Leney says:

    Hmm…not sure about the convection/lack of convection argument. Its effects would surely be minimal on a projectile with the mass and velocity of a cricket ball over such a short distance. More likely I think is that increased cloud cover – and therefore worse light – affects the batsman’s ability to pick up the flight of the ball early. Essentially it’s an optical illusion.

    Another observation: I’ve played some indoor cricket with those moulded, rubberised balls – a seam, but no shine/scuffing – and they swing pretty wildly (me bowling, not batting). This would indicate to me that the main determinant on whether or not a ball will swing is the condition of the seam and the way in which the bowler lets the ball go: “aiming” the seam and providing flight-stability by applying back-spin.

    Another optical illusion is provided by what the ball does off the pitch – the impression of a curving trajectory will be enhanced if the ball goes off the seam in the same direction.

    Pretty much all of this I’m sure you’ve already thought of, but I think the optical illusion aspect has not been considered overmuch

  2. Tim Sanders says:

    Hello Simon, that’s fascinating stuff. One point I like to make about cricket and aerodynamics is that both are governed by Laws which are at times counter-intuitive… Anyway, two things about your article and one suggestion for a further line of enquiry:

    Thanks for the explanation about why cloud cover is important – it’s the still atmospheric conditions that help a stable airflow form around the ball, and of course that includes the effect of warm air rising from the pitch. It makes perfect sense and I hadn’t come across that before.

    One possible correction, though I’m not an expert: You write that “The ridge on the seam ‘trips’ the air coming towards it creating turbulence in a thin layer around the ball on the rougher side.” From reading RD Mehta’s article on the subject (‘Fluid Mechanics Of Cricket-Ball Swing’), the seam indeed “trips” the airflow and causes a micro-turbulent layer – but for conventional swing this is on the shiny side. One counter-intuitive thing about aerodynamics (and the reason why golf balls are dimpled rather than smooth) is that a microturbulent “boundary layer” actually gives lower drag than a smooth laminar flow. So this causes faster-moving air on the shiny side; Bernoulli’s law tells us that the faster the flow, the lower the sideways pressure, so for conventional swing the ball is pushed towards the shiny side, with the angled seam acting as a “spoiler” to create microturbulence.

    Reverse swing: As the ball gets older and the seam gets flatter, it needs the additional roughness of the other side to trip the airflow into microturbulence, and so the ball goes the other way around. It’s possible that the additional time needed for the airflow to form, without a proud seam as a ‘spoiler’, is the reason why reverse swing can happen alarmingly late for the poor chap with the bat…?

    A further line of enquiry related to the swing bowler’s art: does backspin help the ball to “kiss the turf” ? You’ll know better than I that backspin is vital for a stable seam position. (‘Conservation of angular momentum’ is the relevant law of physics – in layperson’s terms, the reason why a moving bicycle stays upright, but it you stop with your feet still in the toeclips, you’re in trouble). Given that topspin gives “dip”, the same laws of forces on a spinning ball (Magnus effect) should mean that backspin causes the opposite: the trajectory of the ball levels off as it approaches the ground, and give the seam more time to grab as the ball skids on. Mike Selvey once said in a conversation on The Guardian website, that he imagines the seam as a “buzzsaw” when this happens. What I don’t know is whether the revs of backspin on the ball from a swing bowler are sufficient to generate the upward force to cause this levelling off ? Maybe with your contacts….

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