Welke daalhouding is het meest aerodynamisch? Bert Blocken a,b

a Faculteit Bouwkunde, Technische Universiteit Eindhoven, Nederland b Departement Burgerlijke Bouwkunde, KU Leuven, België

Welke daalhouding is het meest aerodynamisch?

Bert Blocken a,b, Thijs van Druenen a, Yasin Toparlar a, Thomas Andrianne c, Thierry Marchal d

a Department of the Built Environment, Eindhoven University of Technology, The Netherlands b Department of Civil Engineering, KU Leuven, Belgium

c Department of Aerospace and Mechanical Engineering, University of Liège, Belgium d ANSYS International

Different professional cyclists use very different hill descent positions

indicating there is no consensus on which is best. Which hill descent

position is most aerodynamic?

. Why this study?

• Tour de France, Edition 2016, Stage 8, Descent of the Peyresourde

. Why this study?

• Tour de France, Edition 2016, Stage 8, Descent of the Peyresourde

. Why this study?

• Tour de France, Edition 2016, Stage 8, Descent of the Peyresourde

• Top cyclist Chris Froome (team Sky) assumed a very particular position, won the

race, and took the prestigious yellow jersey.

Different hill descent positions

• Different professional cyclists use very different hill descent positions.

Different hill descent positions

• Different professional cyclists use very different hill descent positions.

• There is at present (2017) no consensus in the peloton on which position is really

superior.

Different hill descent positions

• Different professional cyclists use very different hill descent positions.

• There is at present (2017) no consensus in the peloton on which position is really

superior.

• Most cyclists did not test many different positions, for example in wind tunnels, to find

which position would give them the largest advantage.

Different hill descent positions

• Different professional cyclists use very different hill descent positions.

• There is at present (2017) no consensus in the peloton on which position is really

superior.

• Most cyclists did not test many different positions, for example in wind tunnels, to find

which position would give them the largest advantage.

• This project: two different and independently applied research methods, wind-

tunnel testing and CFD simulations, to investigate which cyclist hill descent position is

aerodynamically superior.

Different hill descent positions

• Different professional cyclists use very different hill descent positions.

• There is at present (2017) no consensus in the peloton on which position is really

superior.

• Most cyclists did not test many different positions, for example in wind tunnels, to find

which position would give them the largest advantage.

• This project: two different and independently applied research methods, wind-

tunnel testing and CFD simulations, to investigate which cyclist hill descent position is

aerodynamically superior.

• Comments on safety and power generation.

Different hill descent positions

• Different professional cyclists use very different hill descent positions.

• There is at present (2017) no consensus in the peloton on which position is really

superior.

• Most cyclists did not test many different positions, for example in wind tunnels, to find

which position would give them the largest advantage.

• This project: two different and independently applied research methods, wind-

tunnel testing and CFD simulations, to investigate which cyclist hill descent position is

aerodynamically superior.

• Comments on safety and power generation.

• Some of the results are counter-intuitive.

Wind tunnel tests

• 4 models, scale: ¼

Wind tunnel tests

Wind tunnel tests

Wind tunnel tests

Wind tunnel tests

Wind tunnel tests

Wind tunnel tests

• 4 models, scale: ¼ • Cyclist model on force balance • Wind speed U = 54 km/h x 4 = 216 km/h, turbulence intensity TI = 0.2%

Wind tunnel tests

• 4 models, scale: ¼ • Cyclist model on force balance • Wind speed U = 54 km/h x 4 = 216 km/h, turbulence intensity TI = 0.2%

• Dimensions in mm • Elevated platform to reduce boundary layer height • Cyclist full-scale height and weight: 189 cm & 84 kg • Blockage ratio < 3 % but corrections needed • Wheels fixed • Boundary-layer height (model-scale): 6 cm

Wind tunnel tests

• Results in CdA (m²):

Wind tunnel tests

FD = Drag force (N)

A = Frontal area (m²)

CD = Drag coefficient (-)

= Air density (kg/m³)

U = Relative air speed (m/s)

• Results in CdA (m²):

Wind tunnel tests

CFD simulations: first: for the four geometries tested in the wind tunnel

• Uniform inlet velocity: U = 15 m/s (= 54 km/h) • Uniform inlet turbulence intensity: TI = 0.2%

Boundary conditions

Computational grids

20 micrometer cells

Computational grids

• Total cell count: 32 x106 - 38 x106 • yP = 20 μm (micrometer)

y* 1 • Prismatic cells near walls • Cell size away from surfaces: tetrahedral cells

CFD simulations: results

CFD simulations: results

CFD simulations: results

Results in terms of drag area

FD = Drag force (N)

A = Frontal area (m²)

CD = Drag coefficient (-)

= Air density (kg/m³)

U = Relative air speed (m/s)

Results in terms of drag area

CFD simulations: More geometries

CFD simulations: More geometries

CFD simulations: More geometries

CFD simulations: More geometries

CFD simulations: More geometries

CFD simulations: More geometries

CFD simulations: More geometries

CFD simulations: Overall ranking – regular road race

CFD simulations: Overall ranking – individual time trial

CFD simulations: Overall ranking – individual time trial

CFD simulations: Overall ranking – individual time trial

CFD simulations: Overall ranking – individual time trial

CFD simulations: time differences

For a descent of D = 5 km with an average speed of 72 km/h (= 20 m/s)

Linked In post:

Which position is most aerodynamic?

A B C

D E C

Which position is most aerodynamic?

A B C

D E C

D E C

• Aerodynamic drag can be analyzed with wind-tunnel tests and CFD simulations

• Large differences in aerodynamic drag between different positions.

• Infamous “Froome position” of the Tour 2016 is not faster, not safer and does not allow more power generation than other positions.