Cricket ball swing and airfoils

[mahela007]

I'm a fan of cricket. I've always wondered how bowlers get cricket balls to swing in the air. According to the two articles at the following sites http://web.archive.org/web/20061230003022/www.soest.hawaii.edu/GG/STUDENTS/jfoster/normal.html"
http://content.cricinfo.com/ci/content/story/258645.html"
the swing of the cricket ball through the air is due to the pressure difference caused by air flow over the ball.
Both articles say that the turbulent air flow generated separates from the surface of the ball later than the separation of laminar flow occurs and claim that this creates lower pressure on the side of turbulent flow.. Why is this? I though that, if the flow of air separates from the surface, it creates low pressure because there is a "gap" which contains "nothing" between the air flow and the ball(as with the laminar flow) and if it stays close to the surface, it will not create low pressure.
Furthermore if turbulent air flow creates low pressure then why would an aircraft wing stall? In a stall, the air flow over the wing becomes turbulent and I know, that at that point, the wing stops flying and starts falling. If turbulent air flow creates low pressure like the articles suggest then the wing would simply develop more lift.

(I don't know if it's relevant but I do know that the "Bernoulli explanation " of how airfoils work is incorrect

 

[LightHero]

)The Bernoulli explanation of how airfoils work is actually generally accepted as correct. The issue with the Bernoulli explanation is that it does not fully explain lift in terms of pressure differences, since pressure is only one factor influencing lift. To explain how an aircraft wing stalls, we need to look at the angle of attack (the angle at which the air strikes the wing). When the angle of attack increases, the air flow over the top surface of the wing starts to separate from the surface, creating a turbulent boundary layer. This reduces the lift generated by the wing. So, even though the turbulent air flow creates low pressure, the lift generated by the wing can still be reduced due to the angle of attack. In the case of a cricket ball, the turbulent air flow is generated by the spin imparted on the ball. This spin causes the air flow over one side of the ball to separate from the surface later than on the other side, creating a pressure difference between the two sides and causing the ball to swing in the air.

 

[astrokreng]

and that it is due to the angle of attack and the shape of the wing)

I can provide some insights into the concept of cricket ball swing and airfoils.

Firstly, it is important to understand the concept of air flow over a surface. When air flows over a smooth surface, it creates a boundary layer where the air particles closest to the surface stick to it due to viscosity. This layer of air moves along with the surface, creating a smooth and steady flow known as laminar flow. However, as the air moves further away from the surface, it starts to mix and become more turbulent, creating chaotic and unpredictable flow patterns. This is known as turbulent flow.

Now, in the case of a cricket ball, the rough surface of the ball causes the boundary layer to become turbulent earlier than it would on a smooth surface. This turbulent boundary layer is what creates the pressure difference and subsequently, the swing of the ball. The turbulent air flow on one side of the ball creates a lower pressure region, causing the ball to move towards the higher pressure region on the other side. This is known as the Magnus effect.

To answer your question about why a turbulent boundary layer creates lower pressure, it is because the air particles in turbulent flow have more energy and are moving at a higher speed compared to those in laminar flow. This results in a lower pressure region as the air particles exert less force on the surface.

As for the comparison to aircraft wings stalling, it is important to note that in a stall, the angle of attack of the wing is too high, causing the boundary layer to become completely turbulent. This means that the air particles are no longer sticking to the surface and are instead moving chaotically, resulting in a loss of lift. In this case, the turbulent flow is not creating a lower pressure region, but rather disrupting the flow and causing a loss of lift.

In summary, the concept of cricket ball swing and airfoils is based on the differences in pressure caused by the boundary layer flow. The rough surface of the cricket ball and the angle of attack of an aircraft wing play important roles in determining the type of flow and subsequently, the pressure difference. While the "Bernoulli explanation" may not fully explain the mechanics of airfoils, the concept of pressure difference due to boundary layer flow remains a crucial factor in understanding these phenomena.