An Aerodynamic Study of Bicycle Wheel Performance - CD

Transcription

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


Benefits to cyclists from Wind Tunnels




Typical test for Zipp, 85h at $850/h,
conducted 3 or 4 times per year
Advertiseme
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‟
How much does it matter?
Tour de France 2008
Stage 20 Individual Time Trial
1
2
3.0%
3
4
From Greenwell et.al.
 Wheel drag is responsible for 10% to
15% of total aerodynamic drag

5
Finish Position
6
Rider makes up the majority of overall
aerodynamic drag
7
8
9
10
11
12
13

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%
14
15
0
1
2
3
4
5
Percentage Time Difference
Q
Q
Q
Q
Q
IronManTM Lake Placid Triathlon 2008
Male 45-49 Age Group
1
2
3.3%
3
4
5
6
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!‟.
Finish Position

7
8
9
10
11
12
13
14
15
0
1
2
3
4
5
Percentage Time Difference
6
7
8
Scope


Wheels* (700c)



Forks



12
Zipp 1080
11
10
9
8
Zipp 404
7
6
5
4
3
2
1
0
Reynolds Full Carbon Aero
Blackwell Time Bandit (slotted)
Frame (partial)


Zipp 404
Zipp 1080
13
Based on 2005 Razor Elite
RANS calculations run for



Reynolds Carbon

Isolated front wheel
Rotating
Ground contact
Zipp 1080
2 speeds (20mph & 30mph)
10 yaw angles (0o thru 20o)
120 total cases
Blackwell Bandit

Zipp 404
Wheel only
Study is limited to
Rim Depth [cm]

*Continental Podium 19mm tubular tire
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


Set yaw angle for specific
case


Set forward axial speed


20mph or 30mph
Matches constant direction
of travel of bicycle
Wheel, hub & spokes


0o thru 20o
Ground Plane


20mph or 30mph
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,


For unsteady case,


Use Moving Reference Frame
Use rotational mesh motion
Fork & Frame

No-slip surface in relative
reference frame
Postprocessing the Results
For the wheel…
Resolved Forces
 Drag, Vertical & Side

Pressure & Viscous
Top View
Turning Moment
Wind Velocity
(effective)
Axial Drag Force
Wheel components
 Circumferential variations
Turning Moments
 Based on side forces

Side (Lift) Force
Side View
Vertical Force
For the fork…
Resolved Forces
 Drag & Side
Bike Velocity
(relative)
Wind Velocity
(effective)
Axial Drag Force
Showing Flow Field Features
 Use streamlines
Direction of Wheel Rotation
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 Forces
Zipp 404
Zipp 1080
Drag Force vs. Yaw Angle
Drag Force vs. Yaw Angle
1.5
1.5
30mph, Blackwell Fork
30mph, Reynolds Fork
30mph, wheel only
20mph, wheel only
1.2
0.9
1.2
0.9
0.6
0.6
0.6
0.6
0.5
0.3
0.4
0
30mph, wheel only
20mph, wheel only
0.3
0.2
Drag Force [N]
Drag Force [N]
0.5
0.3
0.4
0
0.3
0.2
0.1
0.1
0
2
4
6
8
10
12
14
16
18
20
0
2
8
10
12
14
Drag force varies significantly with yaw angle


6
Yaw Angle [degrees]
Yaw Angle [degrees]

4
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
16
18
20
Zipp 404
Circumferential Variation, Drag Force
Zipp 1080
Direction of Flow
No Fork
Reynolds Carbon
Blackwell Bandit
Side Forces
Zipp 404
Zipp 1080
Side Force vs. Yaw Angle
Side Force vs. Yaw Angle
6
30mph, wheel only
20mph, wheel only
12
16
6
4
4
Side Force [N]
Side Force [N]
20
8
12
8
8
2
2
0
0
0
2
4
6
8
10
12
14
16
18
4
30mph, wheel only
20mph, wheel only
0
20
0
0
2
4
Yaw Angle [degrees]


6
8
10
12
Dependence on yaw from wind tunnel studies is generally linear
Fork has only small influence on wheel side force

14
Yaw Angle [degrees]
Side force varies significantly with yaw angle

4
Note: Side force scales are different for each wheel
16
18
20
Turning Moment
Zipp 404
Zipp 1080
Turning Moment vs. Yaw Angle
0.35
Turning Moment vs. Yaw Angle
30mph, wheel only
20mph, wheel only
0.3
0
-0.2
Moment [N·m]
Moment [N·m]
0.25
0.2
0.15
-0.4
-0.6
0.1
0.05
30mph, wheel only
20mph, wheel only
-0.8
0
-1
0
2
4
6
8
10
12
14
16
18
20
0
2
4
Yaw Angle [degrees]

8
10
12
Turning moment varies significantly with yaw angle


6
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
14
Yaw Angle [degrees]
16
18
20
Zipp 404
Circumferential Variation, Side Force
Zipp 1080
Direction of Flow
No Fork
Reynolds Carbon
Blackwell Bandit
Flow Structures, Effect of Yaw
Suction Side
Zipp 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 Side
Zipp 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
Zipp 404
Zipp 1080
Drag Force vs Yaw Angle
1
1
0.9
0.9
Drag Force [N]
Drag Force [N]
Drag Force vs Yaw Angle
0.4
0.3
30mph, Blackwell
20mph, Blackwell
30mph, Reynolds
20mph, Reynolds
0.2
0.4
0.3
30mph, Blackwell
20mph, Blackwell
30mph, Reynolds
20mph, Reynolds
0.2
0.1
0.1
0
2
4
6
8
10
12
14
16
18
20
0
2
Yaw Angle [degrees]

6
8
10
12
Yaw Angle [degrees]
Significant differences seen between forks



4
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
14
16
18
20
Drag Force Profiles on Forks
Zipp 1080
Blackwell Bandit
Reynolds Carbon
Zipp 404
Flow Structures, Slotted Fork
Suction Side
Zipp 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 Side
Zipp 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
20mph
Need for Automated Postprocessing
Zipp 404
Zipp 1080
wheel only
wheel only
Reynolds
Carbon
Reynolds
Carbon
Blackwell
Bandit
Blackwell
Bandit
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


Run FieldView Parallel

30mph
wheel only
wheel only

Reynolds
Carbon
Blackwell
Bandit
Blackwell
Bandit
5X speed-up on 8 processor system
Operate concurrently using Batch-only licensing

Reynolds
Carbon
Customizable environment, one-time investment
Create spreadsheet-ready files, figures of merit &
animations all at the same time
10 yaw angles for each folder,
120 sim files + 120 FV-UNS files
~500 GB
(approx 2400 files in total)
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



Use Zipp404/Blackwell combination to
obtain baseline drag


Wind (usually relatively) calm
Course is flat
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



Wheel/Component
interactions




Shedding frequency
Force fluctuations
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 - CD

Transcription

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