an aerodynamic study of bicycle wheel performance using cfd · an aerodynamic study of bicycle...

STAR European Conference © 2010 Intelligent Light

An Aerodynamic Study of Bicycle

Wheel Performance using CFD

Matthew N. Godo, Ph.D.FieldView Product Manager


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

Benefits to cyclists from Wind Tunnels

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‟

Advertisement ca 2007


How much does it matter?

From Greenwell et.al.

Wheel drag is responsible for 10% to

15% of total aerodynamic drag

Rider makes up the majority of overall

aerodynamic drag

Improvements in wheel design can

reduce drag between wheels by as

much as 25%

Overall reduction in drag can be on the

order of 2% to 3%

0 1 2 3 4 5

Percentage Time Difference

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

Fin

ish

Po

sit

ion

Tour de France 2008

Stage 20 Individual Time Trial

3.0%

0 1 2 3 4 5 6 7 8

Percentage Time Difference

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

Fin

ish

Po

sit

ion

IronManTM Lake Placid Triathlon 2008

Male 45-49 Age Group

3.3%

QQQQQ

In the ‟05 Tour of Germany, Ullrich‟s Xentis front wheel was mistakenly fitted backward for the Stage 8 time trial. Although he won the trial, he finished second overall to Levi Leipheimer, behind by a final margin of 31 seconds. If the wheel had been the right way round, might Ullrich have won stage 8 by a greater margin, perhaps enough to win the race overall? Wheel manufacturer Xentis says „Yes!‟.


Scope Study is limited to

Isolated front wheel

Rotating

Ground contact

Wheels* (700c)

Zipp 404

Zipp 1080

Forks

Reynolds Full Carbon Aero

Blackwell Time Bandit (slotted)

Frame (partial)

Based on 2005 Razor Elite

RANS calculations run for

2 speeds (20mph & 30mph)

10 yaw angles (0o thru 20o)

120 total cases

Zipp 404 Zipp 1080

Wh

ee

l o

nly

Re

yn

old

s C

arb

on

Bla

ck

we

ll B

an

dit

*Continental Podium 19mm tubular tire

0

1

2

3

4

5

6

7

8

9

10

11

12

13

Rim

Dep

th [

cm

]

Zipp 404

Zipp 1080


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


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


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


Postprocessing the Results

Side View

Axial Drag Force

Vertical Force

Direction of Wheel Rotation

Wind Velocity

(effective)

Top View

Axial Drag Force

Side (Lift) Force

Bike Velocity

(relative)

Wind Velocity

(effective)

Turning Moment

For the wheel…

Resolved Forces

Drag, Vertical & Side

Pressure & Viscous

Wheel components

Circumferential variations

Turning Moments

Based on side forces

For the fork…

Resolved Forces

Drag & Side

Showing Flow Field Features

Use streamlines


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‟


Drag ForcesZipp 404 Zipp 1080

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

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

0.1

0.2

0.3

0.4

0.5

0.6

Dra

g F

orc

e [

N]

0

0.3

0.6

0.9

1.2

1.5

30mph, Blackwell Fork

30mph, Reynolds Fork

30mph, wheel only



20mph, wheel only

Drag Force vs. Yaw Angle

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

0.1

0.2

0.3

0.4

0.5

0.6

Dra

g F

orc

e [

N]

0

0.3

0.6

0.9

1.2

1.5



30mph, wheel only



20mph, wheel only

Drag Force vs. Yaw Angle


Circumferential Variation, Drag Force

D i r e c t i o n o f F l o w

Zip

p 4

04

Zip

p 1

08

0

No Fork Reynolds Carbon Blackwell Bandit


Side Forces

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

0

2

4

6

Sid

e F

orc

e [

N]

0

2

4

6

8



30mph, wheel only



20mph, wheel only

Side Force vs. Yaw Angle

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

0

4

8

12

Sid

e F

orc

e [

N]

0

4

8

12

16

20



30mph, wheel only



20mph, wheel only

Side Force vs. Yaw Angle

Zipp 404 Zipp 1080

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


Turning Moment

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Mo

men

t [N

·m]



30mph, wheel only



20mph, wheel only

Turning Moment vs. Yaw Angle

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

-1

-0.8

-0.6

-0.4

-0.2

0

Mo

men

t [N

·m]



30mph, wheel only



20mph, wheel only

Turning Moment vs. Yaw Angle

Zipp 404 Zipp 1080

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


Circumferential Variation, Side ForceZ

ipp

40

4Z

ipp

10

80

No Fork Reynolds Carbon Blackwell Bandit

D i r e c t i o n o f F l o w


Flow Structures, Effect of Yaw

Suction SideZipp 404 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


Flow Structures, Effect of Yaw

Pressure SideZipp 404 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


Forces on Fork only

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

0.1

0.2

0.3

0.4

0.9

1

Dra

g F

orc

e [

N]

30mph, Blackwell

20mph, Blackwell

30mph, Reynolds

20mph, Reynolds

Drag Force vs Yaw Angle

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

0.1

0.2

0.3

0.4

0.9

1

Dra

g F

orc

e [

N]

30mph, Blackwell

20mph, Blackwell

30mph, Reynolds

20mph, Reynolds

Drag Force vs Yaw Angle

Zipp 404 Zipp 1080

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


Drag Force Profiles on ForksZipp 404 Zipp 1080

Re

yn

old

s C

arb

on

Bla

ck

we

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an

dit


Flow Structures, Slotted Fork

Suction SideZipp 404 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


Flow Structures, Slotted Fork

Pressure SideZipp 404 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


Need for Automated Postprocessing2

0m

ph

30

mp

h

Zipp 404 Zipp 1080

wheel only

wheel onlywheel only

wheel only

Reynolds

Carbon

Reynolds

Carbon

Reynolds

Carbon

Reynolds

Carbon

Blackwell

Bandit

Blackwell

Bandit

Blackwell

Bandit

Blackwell

Bandit

10 yaw angles for each folder,

120 sim files + 120 FV-UNS files

~500 GB (approx 2400 files in total)

Production Challenge

18 months from concept to shelf

Only a few weeks available to make design changes

FieldView Automation Methodology

Use FVX high level programming language

Customizable environment, one-time investment

Run FieldView Parallel

5X speed-up on 8 processor system

Operate concurrently using Batch-only licensing

Create spreadsheet-ready files, figures of merit & animations all at the same time


How much does it matter?

45-49M Age Group Eagleman ‘09 Triathlon

(World Championship Qualifier)

Pick target speed (23mph) & wattage

(275W) (use data from Cobb et.al. to get

total drag)

Estimate time spent at different yaw angles

Wind (usually relatively) calm

Course is flat

Use Zipp404/Blackwell combination to

obtain baseline drag

Linear interpolation 20mph & 30mph

Exchange front wheel & fork

Calculate wheel drag for yaw angles

Add to baseline drag

Recalculate speed, same wattage

Compare seconds saved


Future Work

Examine transient effects

Shedding frequency

Force fluctuations

Wheel/Component

interactions

Front fork can choke flow

Calipers can cause

significant disruption

Effect of downtube position

relative to wheel/faired to

wheel unknown

Automate postprocessing

Cheaper compute

Faster solvers

More & more data

Transient will add to this!“Rarely can one‟s bike set-up compensate as profoundly as improving the human on it.” Maffetone, P., Inside Triathlon, 1995, 10(3), p 20.

an aerodynamic study of bicycle wheel performance using cfd · an aerodynamic study of bicycle...

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