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Research Papers: Flows in Complex Systems

Computational Fluid Dynamics Study of the Effect of Leg Position on Cyclist Aerodynamic Drag

[+] Author and Article Information
Martin D. Griffith

Fluids Laboratory for Aeronautical
and Industrial Research (FLAIR),
Department of Mechanical
and Aerospace Engineering,
Monash University,
Melbourne 3800, Australia
e-mail: martin.d.griffith@gmail.com

Timothy Crouch, Mark C. Thompson, David Burton, John Sheridan

Fluids Laboratory for Aeronautical
and Industrial Research (FLAIR),
Department of Mechanical
and Aerospace Engineering,
Monash University,
Melbourne 3800, Australia

Nicholas A. T. Brown

Australian Institute of Sport,
Belconnen, Canberra 2617, Australia

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received June 19, 2013; final manuscript received April 3, 2014; published online July 24, 2014. Assoc. Editor: Mark F. Tachie.

J. Fluids Eng 136(10), 101105 (Jul 24, 2014) (9 pages) Paper No: FE-13-1381; doi: 10.1115/1.4027428 History: Received June 19, 2013; Revised April 03, 2014

An experimental and numerical analysis of cycling aerodynamics is presented. The cyclist is modeled experimentally by a mannequin at static crank angle; numerically, the cyclist is modeled using a computer aided design (CAD) reproduction of the geometry. Wind tunnel observation of the flow reveals a large variation of drag force and associated downstream flow structure with crank angle; at a crank angle of 15 deg, where the two thighs of the rider are aligned, a minimum in drag is observed. At a crank angle of 75 deg, where one leg is at full extension and the other is raised close to the torso, a maximum in drag is observed. Simulation of the flow using computational fluid dynamics (CFD) reproduces the observed variation of drag with crank angle, but underpredicts the experimental drag measurements by approximately 15%, probably at least partially due to simplification of the geometry of the cyclist and bicycle. Inspection of the wake flow for the two sets of results reveals a good match in the downstream flow structure. Numerical simulation also reveals the transient nature of the entire flow field in greater detail. In particular, it shows how the flow separates from the body of the cyclist, which can be related to changes in the overall drag.

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References

Figures

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Fig. 1

A picture of the posable experimental mannequin at a crank angle of θ = 0 deg

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Fig. 2

A sketch of the geometry used in the numerical model, along with the computational domain

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Fig. 3

Plot of CDA against crank angle for both wind tunnel measurements and ANSYS numerical simulations (points). Also plotted are the angles of each of the thighs from the horizontal through the crank angle cycle (lines).

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Fig. 4

Contours of streamwise vorticity with vectors of cross-stream velocity for a cyclist at crank angle of 15 deg (left) and 75 deg (right), at cross sections x = −0.04 m (top), 0.32 m (middle), and 0.60 m (bottom). In each set of three images, results are shown for experimental (left), numerical steady-state (middle), and numerical transient average (right) results. Contours vary across the range -100 s-1≤ωx≤100 s-1, from blue (negative) to white (zero) to red (positive) (see color online).

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Fig. 5

At left, contours of streamwise velocity in a plane located at x = 0.60 m for the 15 deg (top) and 75 deg (bottom) cases. Middle, the same contours but for an instantaneous field. At right are the corresponding instantaneous streamwise vorticity contours for the same contour color levels as in Fig. 4.

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Fig. 6

Contours of time-averaged streamwise velocity taken from the transient sas-SST numerical simulations in a plane located at z = 0 m, for the 15 deg (left) and 75 deg (right) cases. Contours vary across the same scale as shown for the velocity plots in Fig. 5.

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Fig. 7

Contribution to total drag area CDA from various sections of the geometry. The vertical dashed line at θ = 180 deg represents the halfway point of the crank cycle.

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Fig. 8

Left and right perspective views for an instantaneous snap shot of isosurfaces for a single value of the q-criterion, colored by the cross-stream velocity (blue-negative, red-positive (see color online)). At top is shown the case for crank angle of 15 deg and at bottom for 75 deg.

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Fig. 9

Left and right perspective views of isosurfaces for a single value of the q-criterion, colored by streamwise vorticity (blue-negative, red-positive (see color online)), but calculated on the velocity field transient average. At top is shown the case for crank angle of 15 deg and at bottom for 75 deg.

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