15th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 05-08 July, 2010 - 1 - Wake characteristics of time trial helmets using PIV-3C technique Vincent Chabroux 1 , Marcellin Nsi Mba 2 , Patrick Sainton 3 , Daniel Favier 4 1-4: Institute of Movement Sciences, ISM - Wind Tunnel Platform UMR6233 CNRS and University of Mediterranee, CP 918, F-13288 Marseille Cedex 09, France 1: chabroux@crans.org , 2: nsi-mba@univmed.fr, 3: patrick.sainton@univmed.fr , 4: daniel.favier@univmed.fr _______________________________________________________________________________________ Abstract The paper focuses on stereoscopic PIV-3C measurements performed to investigate the vortical flow generated in the wake of helmets dedicated to Time Trial (TT) stages. The objective is to characterize the wake flow of TT helmet shapes and to provide the characteristics of the wake generated by different streamlined helmet geometries. The study specifically aims to provide detailed flow databases on helmet wakes in order to improve our understanding to identify efficient factors that may provide an efficient drag force reduction. Experiments are carried out in the ISM wind tunnel at a freestream velocity close to 14 ms -1 . The use of a laser light technique does not allow testing on human and PIV measurements are thus performed on a rigid articulated mannequin (full scale 1/1) installed on the bicycle. The ensemble mannequin-bicycle is mounted on a force measurement platform in the wind tunnel. The posture of the articulated mannequin is specified by different geometrical parameters (hands, elbows and arms positions; head, trunk and back inclinations, …) which are deduced from previous tests conducted on real cyclists operating in identical flow conditions within the test section. Such mannequin-bicycle flow configurations are thus representative of real ones and take into account the aerodynamic interactions occuring between all parts of the cyclist’s body, the bicycle and the flow environment. For a given helmet geometry installed on the mannequin-bicycle flow configuration, the helmet wake is surveyed. Velocity fields are determined in Z- planes normal to the uniform free-stream and at different distance Z from the helmet trailing edge, ranging from Z=70mm to Z=370mm in the downstream wake. The wake characteristics (height hs, width ls, surface S) as well as the axial velocity deficit (DW), are then used to quantify the momentum (Qs=S.DW 2 ) contained in the wake and thus to determine the drag force contribution (Fs=k.Qs) produced by the helmet geometry. Such wake data allow to discriminate the streamlined geometries and to define helmet characteristics suited for future TT helmets design. Main conclusions deduced from present results indicate that : (1) external helmet dimensions must be minimized, (2) the trailing edge must not be smoothed, and (3) contour discontinuities on the rear part must be banned. _________________________________________________________________________________________________________________________________________ 1. Introduction During current TT stages the high speed levels performed by professional cyclists are shown to be dependent on various factors which non exhaustively consist of mechanics, material, aerodynamics, sport skills, sport coaching … Over the last 25 years, many studies have been dedicated to investigate the specific role played by the aerodynamic drag force resistance on the cyclist’s performance. A few examples are including Gross et al. 1983, Kyle and Burke 1984, Dal Monte 1987, McLean et al. 1994, Burns and Sullivan 1995, Grappe et al. 1997, Belluye and Cid 2001, Lukes et al. 2005, Chabroux et al. 2006, Barelle et al. 2007, Alam et al. 2007, 2008, Blair and Sidelko 2008, Garcia-Lopez 2008. Results from such several studies have shown that the areodynamic force contribution represents at least 90% of the global force resistance and power developed by the cyclist (Grappe et al. Grappe et al. 1997, Belluye and Cid 2001). The cyclist’s body and his posture on the bicycle are clearly shown to contribute to the largest part (nearly 70%) of the aerodynamic drag force (Kyle and Burke 1984, Chabroux et al. 2009, Chabroux 2010, Defraeye et al. 2010). A few recent studies (Alam et al. 2007, 2008, Blair and Sidelko 2008, Chabroux 2010, Chabroux et al. 2010) have also shown that