Balls and Air 1 Balls and Air

Balls and Air 2 Introductory Question You give a left (clockwise) spin to a football. Which way does it deflect? You give a left (clockwise) spin to a football. Which way does it deflect? A. Left B. Right C. It does not deflect

Balls and Air 3 Observations about Balls and Air Air resistance slows a ball down Air resistance slows a ball down The faster a ball moves, the quicker it slows The faster a ball moves, the quicker it slows Some balls have rough surfaces Some balls have rough surfaces Spinning balls curve in flight Spinning balls curve in flight

Balls and Air 4 3 Questions about Balls and Air Why do balls experience air resistance? Why do balls experience air resistance? Why do some balls have fuzz or dimples? Why do some balls have fuzz or dimples? Why do spinning balls curve in flight? Why do spinning balls curve in flight?

Balls and Air 5 Question 1 Why do balls experience air resistance? Why do balls experience air resistance? Can air use viscous forces to slow balls? Can air use viscous forces to slow balls? Can air pressure also cause air resistance? Can air pressure also cause air resistance?

Balls and Air 6 Bending the Flow in a Hose Since water accelerates toward lower pressure, Since water accelerates toward lower pressure, water flow needs a pressure imbalance to bend water flow needs a pressure imbalance to bend The flow naturally develops a pressure gradient The flow naturally develops a pressure gradient higher pressure & lower speed on the outside of the bend higher pressure & lower speed on the outside of the bend lower pressure & higher speed on the inside of the bend lower pressure & higher speed on the inside of the bend

Balls and Air 7 Speeding the Flow in a Nozzle Since water must speed through a narrow nozzle Since water must speed through a narrow nozzle it needs a pressure imbalance to push it forward it needs a pressure imbalance to push it forward The flow naturally develops a pressure gradient The flow naturally develops a pressure gradient lower pressure & higher speed as the neck narrows lower pressure & higher speed as the neck narrows

Balls and Air 8 Viscous Forces and Viscosity Viscous forces Viscous forces oppose relative motion within a fluid oppose relative motion within a fluid and are similar to sliding friction: they waste energy and are similar to sliding friction: they waste energy Fluids are characterized by viscosities Fluids are characterized by viscosities the measure of the strength of the viscous forces the measure of the strength of the viscous forces related to chemical interactions with the fluids related to chemical interactions with the fluids

Balls and Air 9 Water Flow Isn’t Always Smooth We’ve been examining laminar flow We’ve been examining laminar flow in which viscosity dominates flow in which viscosity dominates flow and nearby regions of water remain and nearby regions of water remain nearby nearby Now we’ll also consider turbulent flow Now we’ll also consider turbulent flow in which inertia dominates flow in which inertia dominates flow and nearby regions of water become and nearby regions of water become separated separated

Balls and Air 10 Reynolds Number The flow type depends on the Reynolds number The flow type depends on the Reynolds number Below ~2000 viscosity wins, so flow is laminar Below ~2000 viscosity wins, so flow is laminar Above ~2000 inertia wins, so flow is turbulent Above ~2000 inertia wins, so flow is turbulent

Balls and Air 11 Aerodynamic Forces: Drag Air resistance is also known as “drag” Air resistance is also known as “drag” When a ball moves through air, drag forces arise When a ball moves through air, drag forces arise The air pushes the ball downstream The air pushes the ball downstream and the ball pushes the air upstream and the ball pushes the air upstream Drag forces transfer momentum Drag forces transfer momentum air transfers downstream momentum to ball air transfers downstream momentum to ball ball transfers upstream momentum to air ball transfers upstream momentum to air

Balls and Air 12 Aerodynamic Forces: Lift When a ball deflects passing air, lift forces arise When a ball deflects passing air, lift forces arise the air pushes the ball to one side the air pushes the ball to one side and the ball pushes the air to the other side and the ball pushes the air to the other side Lift forces transfer momentum Lift forces transfer momentum air transfers sideways momentum to ball air transfers sideways momentum to ball ball transfers sideways momentum to air ball transfers sideways momentum to air

Balls and Air 13 Types of Drag & Lift Surface friction causes viscous drag Surface friction causes viscous drag Turbulence causes pressure drag Turbulence causes pressure drag

Balls and Air 14 Perfect Flow Around a Ball Air bends away from ball’s front Air bends away from ball’s front high pressure, slow flow high pressure, slow flow Air bends toward ball’s sides Air bends toward ball’s sides low pressure, fast flow low pressure, fast flow Air bends away from ball’s back Air bends away from ball’s back high pressure, slow flow high pressure, slow flow Pressures balance perfectly, Pressures balance perfectly, so only viscous drag so only viscous drag

Balls and Air 15 The Onset of Turbulence Air flowing into the rising pressure behind ball Air flowing into the rising pressure behind ball accelerates backward (decelerates) accelerates backward (decelerates) and it loses speed and kinetic energy and it loses speed and kinetic energy Air flowing near the ball’s surface Air flowing near the ball’s surface experiences viscous drag, experiences viscous drag, accelerates backward even more, accelerates backward even more, and rapidly loses speed and kinetic energy and rapidly loses speed and kinetic energy If this surface flow stops, turbulence ensues If this surface flow stops, turbulence ensues

Balls and Air 16 Imperfect Flow & a Slow Ball Pressure rises in front Pressure rises in front Pressure drops on side Pressure drops on side Flow detaches just beyond sides Flow detaches just beyond sides Big wake forms behind ball Big wake forms behind ball Wake pressure is nearly ambient Wake pressure is nearly ambient Ball experiences imbalanced pressure & big pressure drag Ball experiences imbalanced pressure & big pressure drag

Balls and Air 17 Question 2 Why do some balls have fuzz or dimples? Why do some balls have fuzz or dimples?

Balls and Air 18 Boundary Layer Flow near the surface forms a “boundary layer” Flow near the surface forms a “boundary layer” At low Reynolds number (<100,000) At low Reynolds number (<100,000) the boundary layer is laminar the boundary layer is laminar closest layer is slowed relentlessly by viscous drag closest layer is slowed relentlessly by viscous drag At high Reynolds number (>100,000) At high Reynolds number (>100,000) boundary layer itself is turbulent boundary layer itself is turbulent tumbling continually renews closest layer’s energy tumbling continually renews closest layer’s energy boundary layer penetrates deeper into rising pressure boundary layer penetrates deeper into rising pressure

Balls and Air 19 Imperfect Flow & a Fast Ball Pressure rises in front Pressure rises in front Pressure drops on side Pressure drops on side Flow detaches near back of ball Flow detaches near back of ball Small wake forms behind ball Small wake forms behind ball Wake pressure is nearly ambient Wake pressure is nearly ambient Ball experiences imbalanced pressure & small pressure Ball experiences imbalanced pressure & small pressure drag drag

Balls and Air 20 Tripping the Boundary Layer To reduce pressure drag, some balls have fuzz To reduce pressure drag, some balls have fuzz Fuzz “trips” the boundary layer Fuzz “trips” the boundary layer and initiates turbulence, and initiates turbulence, which delays flow separation at the back of the ball which delays flow separation at the back of the ball and shrinks the turbulent wake and shrinks the turbulent wake Examples: Tennis balls and Golf balls Examples: Tennis balls and Golf balls

Balls and Air 21 Question 3 Why do spinning balls curve in flight? Why do spinning balls curve in flight?

Balls and Air 22 Spinning Balls, Magnus Force Turning surface pushes/pulls on the air flow Turning surface pushes/pulls on the air flow Air on one side undergoes long bend toward ball Air on one side undergoes long bend toward ball Air on other side undergoes shorter bend away Air on other side undergoes shorter bend away Pressures are unbalanced Pressures are unbalanced The overall air flow is deflected The overall air flow is deflected Ball pushes air to one side Ball pushes air to one side Air pushes ball to other side Air pushes ball to other side Ball feels Magnus lift force Ball feels Magnus lift force

Balls and Air 23 Spinning Balls, Wake Force Turning surface alters point of flow separation Turning surface alters point of flow separation Flow separation is delayed on one side Flow separation is delayed on one side and hastened on the other side and hastened on the other side so wake is asymmetric so wake is asymmetric The overall air flow is deflected The overall air flow is deflected Ball pushes air to one side Ball pushes air to one side Air pushes ball to other side Air pushes ball to other side Ball feels Wake lift force Ball feels Wake lift force

Balls and Air 24 Introductory Question You give a left (clockwise) spin to a football. Which way does it deflect? You give a left (clockwise) spin to a football. Which way does it deflect? A. Left B. Right C. It does not deflect

Balls and Air 25 Summary about Balls and Air The air pressures around these objects are not uniform and result in drag and lift The air pressures around these objects are not uniform and result in drag and lift Balls experience mostly pressure drag Balls experience mostly pressure drag Spinning balls experience Magnus and Wake Deflection lift forces Spinning balls experience Magnus and Wake Deflection lift forces