2014-07-08-Speed-on-Ice

Speed on Ice, Bobsleigh and Luge Aerodynamics, Dr. Norbert Grün, 11. Tagung “Fahrzeug-Aerodynamik”, München, 8.-9.7.2014 Page 1 of 34
Dr. Norbert Grün
11. Tagung „Fahrzeug-Aerodynamik“, München, 8.-9.7.2014

SCOPE.
German Bobsleigh and
Luge Federation
Institute for Research and
Development of Sports
Equipment, Berlin
Technology Partnership,
Aerodynamics & Materials
Cooperation for the development of new equipment for the 2014 Winter Olympics.

APPROACH.
Confirmation & Detail Optimization
in the Wind Tunnel
Analysis and Brainstorming
using CFD

THE RULES (CONCERNING GEOMETRY).
• All dimensions of cowling, frame and
runners prescribed in narrow margins.
• Top and rear of the bobs must be open.
• No transparent materials.
• No active components on bob and crew.
• No additions (vortex generators).
• No holes (except for axles and brake).
• No fairings of axles and runners.
• The entire cowling must be convex
(except at axle holes and bumpers).

Slope angle 
Bank angle ß
AERODYNAMICS AND RUNTIMES.
FORCES.
Weight mg
Aerodynamic Drag
FD = ½v²  CD  A
Aerodynamic
Quality
Frontal
Area
FF =  (FN – FL + FZ)
Shape and Material
of the Runners
Centrifugal
Force
Aerodynamic Lift

AERODYNAMIC DRAG VS. ICE FRICTION.
(Friction = force between runners and ice with =0.010 and CL=0 on a 0°-slope with nz=1)

BALLPARK FIGURES OF DRAG COEFFICIENTS.
CDA  0.14 m²
A  0.43 m²
CD  0.32
CDA  0.15 m²
A  0.50 m²
CD  0.30
CDA  0.04 m²
A  0.12 m²
CD  0.33

EQUATION OF MOTION.
    2
2
1
coscossin v
R
m
ACC
m
g
dt
dv
LD 









cvkv  2
or
with
 







R
m
ACC
m
k LD


2
1
  coscossin  gc
The equation of motion reads
FDS FFF
dt
mvd

)( Aerodynamic Drag   2
2
vACF DD


sin mgFSDownhill-slope force
Friction runners/ice   






R
v
mvACmgF LF
2
2
2
coscos


To calculate the runtime, the equation of motion has to be integrated along the race line,
starting with the initial velocity v1 (t=t1 ) reached after the push start at s1=50m.
VerticalDrop
Time
H
0
0 T
L
If the track details (x,y,z,R,,ß) along
the racing line are not available for
integration, a simplified estimation to
analyze the influence of aerodynamic
properties can be made by developing
the track into a straight slope (i.e. ß=0°,
R ∞,ignoring increased friction between
runners and ice due to the centrifugal force).

the last 10m before the finish
Time[s]
Bobsleigh mass m = 630kg
Initial velocity v1 (t1=5 s)=35km/h
Friction coefficient  = 0,014
Length [m]
SIMPLIFIED RUNTIME CALCULATION.
Lift changes, modifying the
normal force, have only a
marginal influence on runtimes.
CD A= -10%
t  - 0,15 s
Example: KÖNIGSSEE Track length L = 1240 m
Vertical drop H = 110 m
Average slope  = 5,1°
Straight slope
(no centrifugal effects))
v(t)

t  -0,15 s
t  -0,20 s
t  -0,25 s
m = 630 kg
 7
Downhill-Slope Force 5°
Aerodynamic Drag 100km/h
IMPACT OF A DRAG REDUCTION OF CD*A = -10%
m = 110 kg
 6
m = 390 kg
 5
Runtime Reductions
at 5° average slope (Königssee)

SIMULATION RESULTS.
CFD MODEL.
1400 mm
600 mm
R100 mm
Luge athlete
completely scanned
Track walls extending along the entire simulation volume
Athlete models
morphed to fit the real
(laser scanned) postures.

SIMULATION RESULTS.
PowerFLOW SETUP.
27 Mio. Voxels
6 Mio. Surfels
3 mm smallest voxel
Flow Conditions
Velocity 100 km/h
Static pressure 100000 Pa
Temperature 0° C
Density 1,276 kg/m³
Kinematic viscosity 1,5 10-5 m²/s
Boundary Conditions
Bobsleigh & athletes „Standard Wall“
Ground & track walls „Sliding Wall“

SIMULATION RESULTS.
CENTERPLANE.
Total Pressure
Velocity
Static Pressure

SIMULATION RESULTS.
TOTAL PRESSURE DISTRIBUTION IN SLICES.
RED: no loss
BLUE: high loss
Attached
boundary layer
Vortex cores

SIMULATION RESULTS.
TOTAL PRESSURE ISOSURFACES.
Cpt = 0
Cpt = 0.95
Vortex cores

SIMULATION RESULTS.
SKIN FRICTION.
Reverse Flow

SIMULATION RESULTS.
AERODYNAMIC FORCE VECTOR.
CL / CD  1 / 4
CD,Friction / CD  20%

SIMULATION RESULTS.
DRAG CONTRIBUTIONS.
61%
23%
Bobsleigh (cowling+runners) 84%
6%
1%
6%
3%
All athletes together 16%

SIMULATION RESULTS.
INFLUENCE OF TRACK WALLS ON DRAG & LIFT.
CDA=+10%
19%
81%
21%
79%
Abs
(=)
Track walls increase
drag and downforce.
Aerodynamic friction drag
is unaffected by the walls.

SIMULATION RESULTS.
PUSHING THE LIMIT.
AX = + 3%
CD = -16%
---------------------
CDAX = -14%
Closed cowling:
Does not comply
with the rules!

SIMULATION RESULTS.
THE OLYMPIC CHAMPION (FELIX LOCH).
Drag
distribution
and force axis
CL / CD  1 / 2
CD,Friction / CD  33%
Reverse flow
Near surface velocity Isosurface Cpt = +0.5
Isosurface 2 = -10
Static pressure

WINDTUNNEL MEASUREMENTS.
EXPERIMENTAL SETUP.
• For technical and safety reasons
(when measuring with athletes) the
sports equipment can not be measured
with moving ground and track walls.
• Stationary walls would distort the flow
field due to the (velocity dependent)
boundary layer development and
potential inlet effects.
• The bobsleigh is mounted on a plate
(over the centerbelt) whose pillars are
connected to the wind tunnel balance.
• Luge and skeleton are mounted directly
on one side of the balance behind the
boundary layer suction.

LiftAreaCLA[m²]
DragAreaCDA[m²]
SIMULATION RESULTS.
INFLUENCE OF TRACK WALLS ON TRENDS.
-14.0%
-14,7%
-0,040m²
-0,057m²

COMPARISON OF A MOVING GROUND
WITH A STATIONARY TABLE MOUNT.
Bobsleigh on a moving ground
Bobsleigh on a stationary table
 In the wind tunnel the
drag of the empty table
can be subtracted from
the measured combined
drag force.
2. The drag of the table itself
is not affected by the
presence of a bobsleigh.
1. The drag of the bobsleigh
on the table is identical to
the drag on a moving
ground.

BOBSLEIGH OPTIMIZATION.
The small drag forces (ca. 100N at 140km/h) in relation to
the weight (ca. 3800N) only require to fix the bobsleigh
against getting out of place during mounting or modifying.
He (190cm, 110kg)
is really inside there.

LUGE MEASUREMENTS WITH ATHLETES.
Luge mounted
directly on the
balance pad.
Rolling road NOT used
for safety reasons.

DragRatio„LugeA/LugeB“
Velocity [km/h]
LUGE MEASUREMENT SCATTER.
„A“better„B“better
Ratio „Luge A / Luge B“ and
behaviour over velocity
influenced by posture changings.

LUGE MEASUREMENT USING A DUMMY.
Guarantees reproducible results,
unaffected by changes of the athlete‘s
posture between or during measurements.
Measurement with
moving ground

STRUT INTERFERENCE.
Absolute
differences
due to the
presence
of the strut:
CDA = -2.3%
CLA = +5.7%
Trend
predicition
unaffected!

SUMMARY & CONCLUSION.
• The aerodynamic characteristics of sports equipment for bobsleigh,
luge and skeleton competitions has been investigated and optimized.
• A simplified runtime estimation shows that drag reductions which are
feasible within the constraints imposed by the rules, may still lead to
noticeable runtime improvements.
• Lift is of marginal importance for runtimes.
• CFD has been used to gain insight and develop ideas before going to
the wind tunnel for confirmation and detail optimization.
• The moving ground and walls of a track can not be represented in the
wind tunnel. However, simulation results prove that this does not invalidate
the experimental rating of optimization measures.
• For unambiguous results it is recommended to use dummies instead
of athletes who may change their posture between different measurements.
and the result was …

… 4 x GOLD MEDAL.
Natalie Geisenberger
Women‘s Single Luge
Felix Loch
Men‘s Single Luge
Tobias Wendl + Tobias Arlt
Men‘s Double Luge
Geisenberger / Loch / Wendl + Arlt
Team Relay Competition

Dr. Norbert Grün
11. Tagung „Fahrzeug-Aerodynamik“, München, 8.-9.7.2014

2014-07-08-Speed-on-Ice

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