An Aerodynamic Study of Bicycle Wheel Performance using CFD Matthew N. Godo, Ph.D. FieldView Product Manager STAR European Conference © 2010 Intelligent Light Background Wind Tunnel testing used extensively in cycling for over 20 years Typical test for Zipp, 85h at $850/h, conducted 3 or 4 times per year Advertiseme Benefits to cyclists from Wind Tunnels nt ca 2007 Improved knowledge of positioning Significant improvement in the performance of equipment (helmets, clothing, frames, wheels, spokes,…) Enhanced awareness of the role of aerodynamics in the community Current status Still considerable variation in design UCI rule changes & enforcement can be rapid & unpredictable Wind Tunnel reaching its limit today Interpretation of results „controversial‟ STAR European Conference © 2010 Intelligent Light Tour de France 2008 Stage 20 Individual Time Trial 1 How much does it matter? 2 3 3.0% 4 From Greenwell et.al. 5 6 n o 7 i t Wheel drag is responsible for 10% to i s o 8 P h s 15% of total aerodynamic drag i 9 n i F 10 Rider makes up the majority of overall 11 aerodynamic drag 12 13 Improvements in wheel design can 14 reduce drag between wheels by as 15 0 1 2 3 4 5 Percentage Time Difference much as 25% IronManTM Lake Placid Triathlon 2008 Male 45-49 Age Group Q 1 2 Overall reduction in drag can be on the Q 3.3% Q 3 order of 2% to 3% Q 4 Q 5 6 n o 7 i t i s o 8 P In the ‟05 Tour of Germany, Ullrich‟s Xentis front wheel h s i 9 was mistakenly fitted backward for the Stage 8 time n i trial. Although he won the trial, he finished second F 10 overall to Levi Leipheimer, behind by a final margin of 11 31 seconds. If the wheel had been the right way round, 12 might Ullrich have won stage 8 by a greater margin, 13 perhaps enough to win the race overall? Wheel 14 manufacturer Xentis says „Yes!‟. 15 0 1 2 3 4 5 6 7 8 Percentage Time Difference STAR European Conference © 2010 Intelligent Light Scope Zipp 404 Zipp 1080 Study is limited to Isolated front wheel 13 Rotating 12 Zipp 1080 11 Ground contact 10 9 ] Wheel onlyWheel m 8 c Zipp 404 [ h 7 Wheels* (700c) t p e 6 D m i 5 Zipp 404 R 4 Zipp 1080 3 2 1 Forks 0 Reynolds Full Carbon Aero Blackwell Time Bandit (slotted) Frame (partial) Carbon Reynolds Based on 2005 Razor Elite RANS calculations run for 2 speeds (20mph & 30mph) 10 yaw angles (0o thru 20o) 120 total cases Bandit Blackwell *Continental Podium 19mm tubular tire STAR European Conference © 2010 Intelligent Light Methodology Overview STARCCM+ v4.06.011 Meshing models Polyhedral Mesh, prism layers Physics models Steady, incompressible, segregated solver RANS Turbulence K-Omega model SST Mentor All defaults applied Low Re Damping Modification turned ON Force Report convergence after 600 iter Low y+ wall treatment FieldView 12.2.1 (Intelligent Light) FV-UNS exported from STARCCM+ Parallel export compatible with FV STAR European Conference © 2010 Intelligent Light Boundary Conditions Surround Boundary Set upstream flow speed 20mph or 30mph Set yaw angle for specific case 0o thru 20o Ground Plane Set forward axial speed 20mph or 30mph Matches constant direction of travel of bicycle Wheel, hub & spokes Set rotational speed to match forward axial speed Wheel contact Rotational speed matches ground plane axial velocity STAR European Conference © 2010 Intelligent Light Boundary Conditions (cont’d) Nonconformal interface applied to inner region of wheel Permits accurate ground contact Allows for easy spoke count changes For steady case, Use Moving Reference Frame For unsteady case, Use rotational mesh motion Fork & Frame No-slip surface in relative reference frame STAR European Conference © 2010 Intelligent Light Postprocessing the Results For the wheel… Resolved Forces Top View Turning Moment Wind Velocity Drag, Vertical & Side (effective) Pressure & Viscous Axial Drag Force Wheel components Bike Velocity Side (Lift) Force (relative) Circumferential variations Turning Moments Based on side forces Vertical Force Side View For the fork… Wind Velocity (effective) Resolved Forces Axial Drag Force Drag & Side Showing Flow Field Features Direction of Wheel Rotation Use streamlines STAR European Conference © 2010 Intelligent Light CFD Results vs Wind Tunnel Data Wind Tunnel Protocols vary widely! “Wheel-only” studies mount wheel to floor with upright supports Tests start at 30 degrees yaw, angle gradually reduced Rotational wheel speed independently adjusted at each yaw Ground plane boundary condition differs (wind tunnel floor doesn‟t move) Results are often „normalized‟ STAR European Conference © 2010 Intelligent Light Drag Forces Zipp 404 Zipp 1080 Drag Force vs. Yaw Angle Drag Force vs. Yaw Angle 1.5 1.5 30mph, Blackwell Fork 1.2 30mph, Reynolds Fork 1.2 30mph, wheel only 20mph, Blackwell Fork 20mph, Reynolds Fork 0.9 0.9 20mph, wheel only 0.6 0.6 0.6 0.6 0.5 0.5 ] ] 0.3 0.3 N N [ [ e e 0.4 0.4 c c r r o o 0 0 30mph, Blackwell Fork F F 30mph, Reynolds Fork g g 0.3 0.3 a a 30mph, wheel only r r 20mph, Blackwell Fork D D 20mph, Reynolds Fork 0.2 0.2 20mph, wheel only 0.1 0.1 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 Yaw Angle [degrees] Yaw Angle [degrees] Drag force varies significantly with yaw angle 10 to 15 degrees yaw is considered a design target by manufacturers Significant differences seen comparing wheel/fork combinations Blackwell slotted fork w/ Zipp 1080 shows considerable promise STAR European Conference © 2010 Intelligent Light Circumferential Variation, Drag Force Zipp 404 Zipp D i r e c t i o n o f F l o w Zipp 1080 Zipp No Fork Reynolds Carbon Blackwell Bandit STAR European Conference © 2010 Intelligent Light Side Forces Zipp 404 Zipp 1080 Side Force vs. Yaw Angle Side Force vs. Yaw Angle 6 8 20 30mph, Blackwell Fork 30mph, Reynolds Fork 30mph, wheel only 20mph, Blackwell Fork 20mph, Reynolds Fork 12 16 20mph, wheel only 6 4 ] ] N N [ [ 12 e e c c 8 r 4 r o o F F e e d d 8 i i S 2 S 4 2 30mph, Blackwell Fork 30mph, Reynolds Fork 4 30mph, wheel only 20mph, Blackwell Fork 20mph, Reynolds Fork 20mph, wheel only 0 0 0 0 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 Yaw Angle [degrees] Yaw Angle [degrees] Side force varies significantly with yaw angle Dependence on yaw from wind tunnel studies is generally linear Fork has only small influence on wheel side force Note: Side force scales are different for each wheel STAR European Conference © 2010 Intelligent Light Turning Moment Zipp 404 Zipp 1080 Turning Moment vs. Yaw Angle Turning Moment vs. Yaw Angle 0.35 30mph, Blackwell Fork 0 30mph, Reynolds Fork 30mph, wheel only 0.3 20mph, Blackwell Fork 20mph, Reynolds Fork 20mph, wheel only -0.2 0.25 ] ] m · m 0.2 · N -0.4 [ N [ t t n n e 0.15 e m m o o -0.6 M M 0.1 30mph, Blackwell Fork 30mph, Reynolds Fork 30mph, wheel only 0.05 -0.8 20mph, Blackwell Fork 20mph, Reynolds Fork 20mph, wheel only 0 -1 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 Yaw Angle [degrees] Yaw Angle [degrees] Turning moment varies significantly with yaw angle Direction of moment for Zipp 404 acts opposite to Zipp 1080 Significant differences seen comparing wheel/fork combinations Both forks dampen moment on Zipp 404, Blackwell fork amplifies moment on Zipp 1080 STAR European Conference © 2010 Intelligent Light Circumferential Variation, Side Force Zipp 404 Zipp D i r e c t i o n o f F l o w Zipp 1080 Zipp No Fork Reynolds Carbon Blackwell Bandit STAR European Conference © 2010 Intelligent Light Flow Structures, Effect of Yaw Zipp 404 Suction Side Zipp 1080 At low yaw, Strong recirculation observed at top, outer edge of wheel Weaker recirculation observed at bottom half, inner edge of wheel As yaw angle increases, Top recirculation extends along front of wheel, combines with inner wheel recirculation STAR European Conference © 2010 Intelligent Light Flow Structures, Effect of Yaw Zipp 404 Pressure Side Zipp 1080 At low yaw, Backflow zone is created between fork and top of wheel As yaw angle increases, Inner wheel recirculation being driven from pressure side STAR European Conference © 2010 Intelligent Light Forces on Fork only Zipp 404 Zipp 1080 Drag Force vs Yaw Angle Drag Force vs Yaw Angle 1 1 0.9 0.9 ] ] N N [ [ 0.4 0.4 e e c c r r o o F F g g a a r r 0.3 0.3 D D 30mph, Blackwell 30mph, Blackwell 20mph, Blackwell 20mph, Blackwell 30mph, Reynolds 30mph, Reynolds 0.2 20mph, Reynolds 0.2 20mph, Reynolds 0.1 0.1 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 Yaw Angle [degrees] Yaw Angle [degrees] Significant differences seen between forks Blackwell Bandit slotted fork has much higher drag force (>2X) Choice of wheel does not significantly affect fork drag Dependence on yaw angle is very low STAR European Conference © 2010 Intelligent Light Drag Force Profiles on Forks Zipp 404 Zipp 1080 Reynolds Carbon Reynolds Blackwell Bandit Blackwell STAR European Conference © 2010 Intelligent Light Flow Structures, Slotted Fork Zipp 404 Suction Side Zipp 1080 Flow is drawn into fork slots for all yaw angles, both wheels Flow is pulled away from the wheel rim & tire At higher yaw angles, Flow gets trapped behind fork Strong recirculation pulls flow upward STAR European Conference © 2010 Intelligent Light Flow Structures, Slotted Fork Zipp 404 Pressure Side Zipp 1080 Flow is drawn into fork slots for all yaw angles, both wheels, AGAIN! Even on the pressure side, flow is pulled away from the wheel rim & tire Pressure side flow at high yaw does not predominantly cross the center axis STAR European Conference © 2010 Intelligent