The Drag Crisis Phenomenon on an Elite Road Cyclist—A Preliminary Numerical Simulations Analysis in the Aero Position at Different Speeds

Abstract

The drag crisis phenomenon is the drop of drag coefficient (Cd) with increasing Reynolds number (Re) or speed. The aim of this study was to assess the hypothetical drag crisis phenomenon in a sports setting, assessing it in a bicycle–cyclist system. A male elite-level cyclist was recruited for this research and his competition bicycle, helmet, suit, and shoes were used. A three-dimensional (3D) geometry was obtained with a 3D scan with the subject in a static aero position. A domain with 7 m of length, 2.5 m of width and 2.5 m of height was created around the cyclist. The domain was meshed with 42 million elements. Numerical simulations by computer fluid dynamics (CFD) fluent numerical code were conducted at speeds between 1 m/s and 22 m/s, with increments of 1 m/s. The drag coefficient ranged between 0.60 and 0.95 across different speeds and Re. The highest value was observed at 2 m/s (Cd = 0.95) and Re of 3.21 × 105, whereas the lower Cd was noted at 9 m/s (Cd = 0.60) and 9.63 × 105. A drag crisis was noted between 3 m/s and 9 m/s. Pressure Cd ranged from 0.35 to 0.52 and the lowest value was observed at 3 m/s and the highest at 2 m/s. The viscous drag coefficient ranged between 0.15 and 0.43 and presented a trend decreasing from 4 m/s to 22 m/s. Coaches, cyclists, researchers, and support staff must consider that Cd varies with speed and Re, and the bicycle–cyclist dimensions, shape, or form may affect drag and performance estimations. As a conclusion, this preliminary work noted a drag crisis between 3 m/s and 9 m/s in a cyclist in the aero position.

Assessment of Able-Bodied and Amputee Cyclists’ Aerodynamics by Computational Fluid Dynamics

“…AC d values are in accordance to literature for velocities faster than 5 m/s (0.37 m 2 ≤ AC d ≤ 0.42 m 2 ) for able-bodied cyclists (Grappe et al, 1997;Candau et al, 1999;Defraeye et al, 2010b). In other studies, the AC d ranged between 0.261 and 0.332 m 2 for able-bodied cyclists in the upright, dropped and time-trial position (Zdravkovic et al, 1996;Grappe et al, 1997;Candau et al, 1999;Defraeye et al, 2010a;Beaumont et al, 2018;Forte et al, 2020d). The AC d variations are affected by Cd fluctuations at different speeds (Schlichting and Gersten, 2016;Forte et al, 2020d).…”

“…In other studies, the AC d ranged between 0.261 and 0.332 m 2 for able-bodied cyclists in the upright, dropped and time-trial position (Zdravkovic et al, 1996;Grappe et al, 1997;Candau et al, 1999;Defraeye et al, 2010a;Beaumont et al, 2018;Forte et al, 2020d). The AC d variations are affected by Cd fluctuations at different speeds (Schlichting and Gersten, 2016;Forte et al, 2020d). It is expected that Cd varies about 30% on an able-bodied cyclist between 1 and 22 m/s (Forte et al, 2020d).…”

“…The AC d variations are affected by Cd fluctuations at different speeds ( Schlichting and Gersten, 2016 ; Forte et al, 2020d ). It is expected that Cd varies about 30% on an able-bodied cyclist between 1 and 22 m/s ( Forte et al, 2020d ). That can be explained by the drag crisis phenomenon.…”

“…That can be explained by the drag crisis phenomenon. The drag crisis consists in a drop of Cd values due the fluid flow transition from laminar to turbulent ( Schlichting and Gersten, 2016 ; Forte et al, 2020d ). Typically, in cycling this phenomenon may occur at Re between 3.21 × 10 5 and 9.63 × 10 5 and, speeds between 3 and 9 m/s.…”

“…The Reynolds-Averaged Navier–Stokes (RANS) equations were solved in Fluent CFD code (Ansys Fluent 16.0, Ansys Inc., Canonsburg, PA, United States) by the finite volume approach with the Realizable k-epsilon turbulence model. The RANS model was used based on previous cycling studies ( Forte et al, 2020b , d ). Moreover, the Realizable k-epsilon showed higher computation economy in comparison to Standard k-epsilon, RST and RNG k-epsilon models ( Defraeye et al, 2010a , b ; Forte et al, 2018a ; Forte et al, 2020b ).…”