Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 112 (2015 ) 424 429 7th Asia-Pacific Congress on Sports Technology, APCST 2015 Aerodynamic body position of the brakeman of a 2-man bobsleigh Harun Chowdhury a, *, Bavin Loganathan a, Firoz Alam a and Hazim Moria b a School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne,Victoria, 3083, Australia b Mechanical Engineering Department,Yanbu Industrial College, Yanbu Al-Sinaiyah, P.O. Box 30436, KSA Abstract This paper examines the aerodynamic behaviour of a standard 2-man bobsleigh including the crews. The body position of the brakeman was studied over a range of angles from 25 to 90 degrees relative to the wind direction. Wind tunnel experiments were undertaken on a 50% scale model of 2-man bobsleigh and the crews. Aerodynamic drag forces were measured over a range of wind speeds (40 to 120 km/h) in the wind tunnel environment. Additionally, wool tufts and smoke were used to visualise the airflow characteristics around the bobsleigh and crews. The results show that the body inclination angle of the brakeman has significant effect on the overall aerodynamics of bobsleighing and the most aerodynamic body position of the brakeman was found at 55 degree with the minimal drag. The results also indicate that this particular body position of the brakeman can reduce the drag over 10% compared to his body position at 90 degree. However, the brakeman s body position over 35 degree indicates an increase of drag. 2015 2015 The The Authors. Authors. Published Published by Elsevier by Elsevier Ltd. This is Ltd. an open access article under the CC BY-NC-ND license Peer-review (http://creativecommons.org/licenses/by-nc-nd/4.0/). under responsibility of the School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University. Peer-review under responsibility of the the School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University Keywords: bobsleigh; aerodynamic; drag; airflow; experimental; wind tunnel; breakeman; body poisition. 1. Introduction Athletes body position plays an important role in all high speed sports [1]. As the speed of the bobsleighing can go up to 135 km/h [2], the importance of aerodynamics is paramount. Aerodynamic body position of athletes during the performance can reduce drag significantly in various high speed sports like ski jumping and cycling [1]. Similarly, a very small amount of drag reduction in the high speed sports like bobsleighing can determine the winners in competitive events. Table below shows the winning margin in Olympic 2014. * Corresponding author. Tel.: +61 3 99256103; fax: +61 3 99256108. E-mail address: harun.chowdhury@rmit.edu.au 1877-7058 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the the School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University doi:10.1016/j.proeng.2015.07.219
Harun Chowdhury et al. / Procedia Engineering 112 ( 2015 ) 424 429 425 Table 1. Final Results (completion time in seconds) for 3 events of bobsleigh in SOCHI 2014 (Adapted from [3]). Rank Two-Man Two-Woman Four-Man 1 225.39 230.61 220.60 2 226.05 230.71 220.69 3 226.27 231.61 220.99 4 226.3 232.27 221.02 5 226.48 232.29 221.10 6 226.62 232.57 221.42 7 226.79 232.71 221.51 8 226.85 233.2 221.75 9 226.88 233.46 221.76 10 226.96 233.74 221.80 In professional competitions, it can be seen that the brakeman tries to bend his/her body behind the driver to reduce the air resistance. Previous study [4] suggested that the brakeman s body position at 45 degree produces minimal drag. They studied the aerodynamic influence of brakeman for only three angles: 65, 55 and 45 degrees, although it is possible for a brakeman to change the body angle more than the angles they tested. Although some limited studies [4-7] have been reported in open literature, the aerodynamic behaviour of a bobsleigh is not fully understood. Furthermore, no other study has been reported on the optimal body position of the brakeman. Therefore, the objectives of this study is to quantify the drag force at a range of body angles and wind speeds and to investigate the flow behaviour of a bobsleigh using wind tunnel experimentation. Nomenclature A projected frontal area (m 2 ) C D drag coefficient (dimensionless) D aerodynamic drag force (N) Re Reynolds number (dimensionless) V wind speed (m/s) μ dynamic viscosity of air (Pa.s) ρ air density (kg/m 3 ) 2. Methodology 2.1. Experimental models In order to conduct the wind tunnel experiments, a 50% scale model of a standard 2-men bobsleigh that is approved by the Fédération internationale de bobsleigh et de tobogganing (FIBT) was manufactured. Detailed drawing and dimensions were obtained from FIBT official website [8]. Being of half-scale, the bobsleigh model was 1.35 m long and 0.43 m wide. Initially a CAD model of the bobsleigh was created using CATIA as shown in Fig. 1(a). The model was then handcrafted out of high-density polyurethane foam. A wooden rectangular block (about 2 kg) was attached underneath at the CG of the model to increase the weight and to minimise vibration during force measurement. Finally, the model was surface finished with epoxy resin, fibreglass and paint. Fig. 1(b) shows the completed model prepared for wind tunnel testing.
426 Harun Chowdhury et al. / Procedia Engineering 112 ( 2015 ) 424 429 a. b. Fig. 1. (a) CAD model; (b) experimental setup in wind tunnel. Additionally, simplified models of two crews were made with foam material. In order to change the angle of the brakeman s body angle, a rotating mechanism was also devised. Fig. 2 shows the foam models of crews prepared for wind tunnel testing. As the driver s body position was fixed and a small gap was left between the driver s head and the cavity lip, the seating angle of the brakeman (i.e., the inclination of the upper body with respect to the wind direction) was varied from 90 to 25 which covers the all possible angle achieved by a brakeman.. 2.2. Aerodynamic force measurements Fig. 2. Foam models of crews fitted inside the cavity of the bobsleigh (top view). In order to measure the aerodynamic drag acting on a bobsleigh, a mounting system made of a steel sting was developed to hold the bobsleigh on a force sensor in the wind tunnel. A splitter plate and aerodynamic faring were used to eliminate the effect of boundary layer. The distance between the bottom edge of the bobsleigh and splitter plate was 20 mm to simulate the real world situation. The schematic of the experimental setup alone with the mounting systems is shown in Fig. 3(a). A multi-axis force sensor (made by JR3 Inc., USA) alone with a purpose made computer software was used to digitise and record drag force data. Each set of data was recorded for 30 seconds time average with a frequency of 20 Hz ensuring electrical interference and data fluctuation errors are minimal. Multiple data sets were collected at each speed tested and the results were averaged for minimising the further possible errors in the raw experimental data. Fig. 3(b) shows the experimental setup in the wind tunnel. The aerodynamic drag forces were measured over a range of wind speeds from 40 to 120 km/h with an increment of 20 km/h at zero yaw angle using the RMIT Industrial Wind Tunnel. The maximum speed of the tunnel is approximately 145 km/h. The rectangular test section s dimension is 3 m (wide) 2 m (height) 9 m (long). More details about the tunnel can be found in Chowdhury [9]. The repeatability of the measured forces was within ±0.01 N and the wind velocity was less than ±0.5 km/h.
Harun Chowdhury et al. / Procedia Engineering 112 ( 2015 ) 424 429 427 Brakeman Driver Lift Bobsleigh model Drag Body angle of brakeman Wind Direction F airing Splitter plate Wind tunnel floor Load cell Fixed Support Fig. 3. (a) Schematic of the experimental setup; (b) experimental setup inside the wind tunnel test section. The measured aerodynamic drag force (D) on the bobsleigh model was converted to dimensionless parameter: drag coefficient (C D ) defined as: D (1) C D 1 2 V A 2 The full-scale bobsleigh model has a frontal area (A) of 0.3695 m 2, whereas the half-scale bobsleigh model used for the wind tunnel experiments has a projected frontal area of 0.09238 m 2. Additionally, to compare the results with full-scale model, the Reynolds number was defined as: Vl Re (2) 2.3. Flow visualisation Two flow visualisation techniques were employed to understand the flow behaviour around the bobsleigh. These involved the use of wool tufts and smoke. For surface tuft visualisation, the wool tufts were cut 40 mm in length and attached to the bobsleigh model with adhesive tape by providing equal space of 40 mm. The wool tufts were used to examine the airflow for airspeeds ranging from 40 to 120 km/h. However, the smoke visualisation was performed at low wind speed (5 km/h) in order to prevent distortion of the smoke stream. Smoke was used at various areas of the bobsleigh model to illustrate the airflow around critical areas. Photographs and video footage were captured as far away as possible from the bobsleigh model with the intention to not obstruct the airflow around the model. 3. Results and discussion The variation of drag coefficient (C D ) with Reynolds number (Re) is shown in Fig. 4(a). The figure also includes the results obtained by Lewis (2006) [6] for both CFD and experimental values. It can be seen in Fig. 4(a) that the measured drag coefficient (C D ) slightly decreases with increasing Reynolds number (Re). Similar trend is also seen in the work of Lewis [6]. The bobsleigh model without modelling the crew, runners and runner carriers was found to have an average drag coefficient of 0.289 which is consistence with our previous study [7]. This value also compares very well with existing aerodynamic studies of bobsleighs [4, 6]. However, the C D value found in this study is slightly higher. This is believed to be due to high air turbulence intensity of RMIT Wind Tunnel (about 1.8%) [9] and also due to experimental variation. The variation of drag coefficient (C D ) with with body inclination angle of the brakeman is shown in Fig. 4(b). The data indicate that the drag decreases significantly at 55 degree body angle at speeds over 80 km/h. The experimental data also shows that further inclination of the brakeman s body beyond 55 increases drag until it reaches 35 degree. Therefore, it is clear from the experimental results that the brakemen s body position has significant effect on aerodynamic properties of the 2-man bobsleigh and the optimal body angle can produce significant reduction of drag.
428 Harun Chowdhury et al. / Procedia Engineering 112 ( 2015 ) 424 429 Drag coefficient (C D ) 0.41 0.39 0.37 0.35 0.33 0.31 0.29 0.27 This study Lewis (2006) [Experimental] Lewis (2006) [CFD] 0.25 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 Reynolds number (Re 10 ) Drag coefficient (C D ) 0.32 0.3 0.28 0.26 0.24 0.22 120 km/h 100 km/h 80 km/h 0.2 15 35 55 75 95 Body angle (Degree) Fig. 4. (a) The variation of drag coefficient (C D) with Reynolds number (Re); (b) The variation of drag coefficient (C D) with body inclination angle of the brakeman. Fig. 5 shows the percentage reduction of drag from the baseline (i.e., brakeman s body position is 90 ). It clearly shows that maximum drag reduction is found over 10% from the baseline. Brakeman s body position over 35 indicates an increase of drag compared to the baseline. 12% 10% % decrease of drag from baseline 8% 6% 4% 2% 0% -2% -4% -6% 75 65 55 45 35 25 Fig. 5. Percentage reduction of drag at different body angles compared to brakeman s position at 90. Smoke flow visualisation on the external structure of the different regions of the bobsleigh body and the flow around the crews are shown in Fig. 6(a). As the smoke stream was only effective at visualising the flow a low Reynolds number (i.e. at 5 km/h), it did not permit analysis of the bobsleigh flow at higher Reynolds numbers. However, the effectiveness of the wool-tufts at the higher wind speeds tested was not compromised, which allowed higher Reynolds number flow visualisation. Fig. 6(b) shows side-view flow visualisation with wool-tufts at 100 km/h.
Harun Chowdhury et al. / Procedia Engineering 112 ( 2015 ) 424 429 429 a. b. Fig. 6. (a) Smoke flow visualisation; (b) flow visualisation test with wool tufts. It is immediately apparent that the wool-tufts respond better to unsteady flow by means of oscillation. It can also be seen that the flow downstream of the front bumper is deflected upwards before following the contour of the bobsleigh model. Additionally, the air flow which passes over the rear cowling is partly directed into the bobsleigh cavity and travels over the crews. At 100 km/h, the wool-tufts directly behind the driver and brakeman show significant oscillation. Wool-tufts tests were carried out for different brakeman body positions to notice the airflow pattern around the driver and brakeman surrounding region. It was noted that at 55 inclination seating angle of the brakeman produced less fluctuations on the wool-tufts than any other body position tested. The reason for this reduction in oscillation of the wool-tufts is due to the more streamline flow over the crews. This particular seating position of the brakeman produced less wake region near the bobsleigh cavity and consequently resulted in the least drag. 4. Conclusions The results show that the body inclination angle of the brakeman has significant effect on the overall aerodynamics of bobsleighing and the most aerodynamic body position of the brakeman was found at 55 with the minimal drag. The results also indicate that this particular body position of the brakeman can reduce the drag over 10% compared to his body position at 90 degree. However, the brakeman s body position over 35 degree indicates an increase of drag. References [1] H. Chowdhury, F. Alam, Bicycle aerodynamics: an experimental evaluation methodology, Sports Engineering. 15(2) (2012) 73-80. [2] M. Denny, Gliding for Gold: The Physics of Winter Sports, The John Hopkins University Press, USA, 2011. [3] Official website of the Olympic Movement: olympic.org. Obtained from http://www.olympic.org/olympic-results/sochi-2014/bobsleigh/ [assessed on 14 May 2015]. [4] P. Dabnichki, E. Avital, Influence of the position of crew members on aerodynamics performance of two-man bobsleigh, Journal of Biomechanics. 39(5) (2006) 2733-2742. [5] H. Kummer, A basic guide to bobsledding, Griffin Publishing Group, California, USA, 2002. [6] O. Lewis, Aerodynamic analysis of a 2-man bobsleigh (Master thesis), Delft University of Technology, Netherlands, 2006. [7] H. Chowdhury, F. Alam, S. Arena, I. Mustary, An experimental study of airflow behaviour around a standard 2-man bobsleigh, Procedia Engineering. 60 (2013).479-484. [8] Fédération internationale de bobsleigh et de tobogganing (FIBT), Bobsleigh Drawings 2007. Obtained from http://www.fibt.com/uploads/media/bob_drawings_all_2007.pdf [assessed on 15 March 2013]. [9] H. Chowdhury, Aerodynamics of sports fabrics and garments (PhD Thesis), RMIT University, Australia, 2012.