L9 - Ball aerodynamics

Question 1

Aerodynamics is useful to which 3 groups of people? Give an example of why for each.

Answer 1

Designers (optimising equipment)
Governing bodies (controlling pace of game for spectator enjoyment)
Players (decieving other players)

Question 2

Clarify the direction of each of the main forces acting on a ball.

Answer 2

Drag (opposite to flight direction)

Lift (perpendicular to flight direction)
Forces (orthogonal to lift and drag

Question 3

The coefficients of each of the main forces acting on the ball depend (via the __________________) on what 3 parameters?

Answer 3

Question 4

Give the equation for the non-dimensional spin parameter

Answer 4

Question 5

The magnus effect:

Air is dragged around the ball with the spin.
For the retreating side, air velocity is ______ so __________________.
For the advancing side, air velocity is ______ so _________________.

This _______________ pushes the wake sideways.
F_m opposes this action.

The trajectory of the ball then depends on the ___________ and is achieved through ________ ______________.

Answer 5

The magnus effect:

Air is dragged around the ball with the spin.
For the retreating side, air velocity is increased so separation is delayed.
For the advancing side, air velocity is decreased so separation is enhanced.

This assymetric separation pushes the wake sideways.
Fm opposes this action.

The trajectory of the ball then depends on the spin axis and rate and is achieved through hitting the ball off-centre.

Question 6

The reverse magnus effect occurs at ___ velocities and therefore only affects _____ balls as f=______

Air is dragged around the ball with the spin.
For the retreating side, air velocity is _____________ relative to the surface to be _______ so ________________.
For the advancing side, air velocity is ____ enough to _________________________ so ________________.

This __________________ pushes the wake sideways in the ________ direction to _________.
Fm opposes this action.

Answer 6

The reverse magnus effect occurs at low velocities and therefore only affects light balls as f=1/2mv2

Air is dragged around the ball with the spin.
For the retreating side, air velocity is slow enough relative to the surface to be laminar so separation is early.
For the advancing side, air velocity is fast enough to induce turbulence in the boundary layer so separation is delayed.

This assymetric separation pushes the wake sideways in the opposite direction to the spin.
Fm opposes this action.

Question 7

A golf shot can be optimised for high ____ and minimal ____ through use of a ball with as many _______ on as possible, and the application of ________, to make use of the magnus effect in the ________ direction.

Answer 7

A golf shot can be optimised for high lift and minimal drag through use of a ball with as many dimples on as possible, and the application of backspin, to make use of the magnus effect in the vertical direction.

Question 8

Dimples increase ____________, keeping the flow in the ______________ regime.
________ occurs at a low Re, and there is no increase in ________________ after transition, because unlike pimples, dimples don’t _______________________ much.

Answer 8

Dimples increase surface roughness, keeping the flow in the super-critical regime.
Transition occurs at a low Re, and there is no increase in drag coefficient after transition, because unlike pimples, dimples don’t thicken the boundary layer much.

Question 9

Golf ball manufacturers have to consider both the effects of ____ and the __________ for different designs. For example, a beginners design, concentrates the dimples _______________; meaning _______________only really acts vertically. (reducing ____________ but also ___________).

Answer 9

Golf ball manufacturers have to consider both the effects of seams and the market demand for different designs. For example, a beginners design, concentrates the dimples around the centreline; meaning the magnus effect only really acts vertically. (reducing hooking and slicing but also flight distance).

Question 10

Why is tennis hard.

Answer 10

Fuzzy dunlop.

Question 11

Tennis: Felt ______ at high _____, increases __________ drag and can _______________ drag. Its complexity called for a new drag term. Name and define this.

State a consequence of this drag on the approach taken by sports engineers involved in tennis.

Answer 11

flattening
speeds

skin friction

increase pressure

Fuzz drag - the extra contribution to drag due to individual Complexity of problem means that wind tunnel tests > CFD

Question 12

Pop quiz The ITF ball introduced a larger ball in ____, to slow down the game for _______________ because _______________________________.

Answer 12

1999
the spectator’s enjoyment
excessive achievable speeds meant serves were dominating gameplay

Question 13

Cricket:

Assymetrical airflow is achieved through ___________________ (2).

Which side is rough in this picture?
Which side is experiencing low pressure in this picture?
This pressure difference is caused by what and causes what?

Answer 13

alignment of the seam and polishing on one side of the ball.

The side which is leading the sideways motion.
The side which is leading the sideways motion.
The difference in separation points.
The side force.

Question 14

When using first class cricket balls with quarter seams, this can happen. How? and at what kind of speed?

Answer 14

The flow can become turbulent around both sides of the ball, because these balls are much more rough.

The seam thickens the boundary layer on the rough side so it separates more quickly, reversing the pressure difference and thus swing direction.

65 mph with the rough side facing forwards
85 mph with the smooth side leading

Question 15

Describe a knuckleball. How fast would this be?

Research has shown that the separation point ‘_____ ______’ when it occurs at the seam. If the ball is rotating slowly, the seam will _______ with the ____________ on either side of the ball a number of times causing ________________________, a little like fluctuating cricket ball swing. It is this effect that causes the _____ flight.

Answer 15

comparatively low speed pitch (typically, 65 mph), where the ball is thrown with little or no spin.

Research has shown that the separation point ‘jumps around’ when it occurs at the seam. If the ball is rotating slowly, the seam will coincide with the separation points on either side of the ball a number of times causing fluctuating pressure difference, a little like fluctuating cricket ball swing. It is this effect that causes the erratic flight.

Question 16

The “curveball” occurs when the ball is thrown with ___ about a _____ axis, inducing both ______ and ________ and causing the ball both to curve and to ____ suddenly in flight, due to the ______________.

Answer 16

The “curveball” occurs when the ball is thrown with spin about a tilted axis, inducing both side-spin and top-spin and causing the ball both to curve and to drop suddenly in flight, due to the Magnus Effect.

Question 17

Football:
Kicking action can apply spin in both the horizontal and vertical axes. ___________________ on ball create surface roughness which reduces the ___________ and raises the ___________ drag coefficient.

As diameter, density and viscosity can be assumed relatively constant, flow around the ball depends on the balls velocity along the flight path. The ________ greatly increases the drag force and results in a change in the balls ___________.

Answer 17

Panels and seams on ball create surface roughness which reduces the critical Re number and raises the trans-critical drag coefficient.

The transition from turbulent to laminar flow greatly increases the drag force and results in a change in the balls flight path.

Question 18

What are the equations for the drag force and magnus force on a football during a flight path. How might these forces be obtained?

Answer 18

The flight of a ball can be calculated using trajectory simulation programs which calculate the changes in the ball’s velocity and the corresponding forces (gravity, drag and magnus) at various points in space. These forces are found from the following equations.

Question 19

It is also possible to compare the effect of drag and Magnus, by calculating ____. Using the aforementioned equations, it is clear that this simply cancels down to what?

Answer 19

the ratio of the two forces for a particular ball velocity
C_D /C_M.

See lecture notes for additional questions.