Balls and Air Some balls have textured surfaces to affect the air - PDF document

Ball Sports 1 Ball Sports 2 Observations about Balls and Air  Air resistance slows a ball down  The faster a ball moves, the quicker it slows Balls and Air  Some balls have textured surfaces to affect the air  Spinning balls curve in flight Turn off all electronic devices Ball Sports 3 Ball Sports 4 4 Questions about Balls and Air Question 1 1. Why do balls experience air resistance? Q: Why do balls experience air resistance? 2. How does air flow around a ball? A: Balls interact with and transfer momentum to air 3. Why do some balls have dimples?  When a ball moves through air, drag forces arise 4. Why do spinning balls curve in flight?  Air pushes ball downstream, ball pushes air upstream  Air transfers downstream momentum to ball  When a ball deflects passing air, lift forces arise  Air pushes ball to one side, ball pushes air to other side  Air transfers sideways momentum to ball Ball Sports 5 Ball Sports 6 Types of Aerodynamic Forces Question 2  Surface friction causes viscous drag Q: How does air flow around a ball?  Turbulence causes pressure drag A: That depends on Reynolds number  Deflected flow causes lift  At low Reynolds number, the flow is laminar  Deflected flow also leads to induced drag  Only viscous forces transfer momentum to the ball  The ball experiences only viscous drag  At high Reynolds number, the flow is turbulent  Pressure forces also transfer momentum to the ball  The ball also experiences pressure drag 1

Ball Sports 7 Ball Sports 8 Laminar Flow around a Ball The Onset of Turbulence  Air bends away from ball’s front  Air flowing into the rising pressure behind ball  At front: high pressure, slow flow  accelerates backward (decelerates)  Air bends toward ball’s sides  and converts kinetic energy into pressure potential.  Air flowing nearest the ball’s surface  At side: low pressure, fast flow  Air bends away from ball’s back  also experiences viscous drag forces  and converts kinetic energy into thermal energy.  At back: high pressure, slow flow  If it runs out of total energy, it stops or “stalls”  Pressures on opposite sides balance perfectly  If air nearest the ball stalls, turbulence ensues  Ball experiences only viscous drag Ball Sports 9 Ball Sports 10 Turbulent Flow around Slow Ball Question 3  Air flowing near ball’s surface Q: Why do some balls have dimples?  stalls beyond ball’s sides A: To produce a turbulent boundary layer  and peels main air flow off of ball.  Air affected by ball’s surface is the boundary layer  Big wake forms behind ball  Reynolds # <100,000: laminar boundary layer  Since wake pressure is ambient,  Nearest sublayer is slowed relentlessly by viscous drag  ball experiences unbalanced pressures.  Reynolds # >100,000: turbulent boundary layer  Ball experiences a large pressure drag force  Sublayers tumble and interchange; they help each other  Boundary layer penetrates deeper into rising pressure Ball Sports 11 Ball Sports 12 Turbulent Flow Around Fast Ball Tripping the Boundary Layer  Air flowing near ball’s surface  To reduce pressure drag, some balls have dimples  stalls beyond ball’s sides  Dimples “trips” the boundary layer  and peels main air flow off of ball.  Cause boundary layer to become turbulent.  Boundary layer is turbulent  Turbulent boundary layer resists peeling better  Ball’s main airflow forms smaller turbulent wake.  and retains total energy farther,  Example: Golf balls  so it resists peeling better.  Small wake forms behind ball  Ball experiences a small pressure drag force 2

Ball Sports 13 Ball Sports 14 Question 4 Spinning Balls, Magnus Force Q: Why do spinning balls curve in flight?  Turning surface pushes/pulls on the air flow A: They experience two aerodynamic lift forces  Air on one side makes long bend toward ball  Air on other side makes shorter bend away from ball  Laminar effect: Magnus force  Pressures are unbalanced  Turning surface pushes/pulls on the air flow  The overall air flow is deflected  Air on one side makes longer bend toward the ball  Ball pushes air to one side  Turbulent effect: Wake deflection force  Air pushes ball to other side  Turning surface alters point of flow separation  Ball feels Magnus force  Flow separation and wake are asymmetric Ball Sports 15 Ball Sports 16 Spinning Balls, Wake Force Summary about Balls and Air  Turning surface alters point of flow separation  Balls in air experience aerodynamic forces  Flow separation is delayed on one side  Downstream forces are drag forces  and hastened on the other side,  Sideways pressure forces are lift forces  so wake is asymmetric  Moving particles experience viscous drag forces  The overall air flow is deflected  Moving balls experience pressure drag forces  Ball pushes air to one side  Spinning balls experience Magnus and wake deflection lift forces  Air pushes ball to other side  Ball feels wake deflection force 3