EURECA 2013 - Calculation of Aerodynamic Drag of Human Being in Various Positions Calculation of Aerodynamic Drag of Human Being in Various Positions Mun Hon Koo*, Abdulkareem Sh. Mahdi Al-Obaidi Department of Mechanical Engineering, school of Engineering, Taylor’s University, Malaysia, *Corresponding author: [email protected]

Abstract— This paper studies the aerodynamic drag of human 2. Methods being in different positions through numerical simulation using CFD with different turbulence models. The investigation 2.1. Theoretical Analysis considers 4 positions namely (standing, sitting, supine and According to Hoerner [4], the total aerodynamic drag of human squatting) which affect aerodynamic drag. Standing has the body can be classified into 2 components which are: highest drag value while supine has the lowest value. The = + numerical simulation was carried out using ANSYS FLUENT 𝐷𝐷𝑡𝑡𝑜𝑜𝑡𝑡𝑎𝑎𝑙𝑙 𝐷𝐷𝑓𝑓𝑟𝑟𝑖𝑖𝑐𝑐𝑡𝑡𝑖𝑖𝑜𝑜𝑛𝑛 𝐷𝐷𝑝𝑝𝑟𝑟𝑒𝑒𝑠𝑠𝑠𝑠𝑢𝑢𝑟𝑟𝑒𝑒 and compared with published experimental results. Where𝐶𝐶 𝐶𝐶 is pressure𝐶𝐶 drag coefficient and is friction Aerodynamic drag studies can be applied into sports field related drag coefficient.𝐷𝐷𝑝𝑝𝑟𝑟𝑒𝑒𝑠𝑠𝑠𝑠𝑢𝑢𝑟𝑟𝑒𝑒 Pressure drag is formed from the𝐷𝐷𝑓𝑓𝑟𝑟𝑖𝑖𝑐𝑐𝑡𝑡𝑖𝑖𝑜𝑜𝑛𝑛 distribution of applications like cycling and running where positions optimising forces 𝐶𝐶 normal .to the human body surface [5]𝐶𝐶. The effects of are carried out to reduce drag and hence to perform better viscosity of the moving fluid (air) may contribute to the rising value during the competition. of pressure drag. Drag that is directly due to wall shear stress can be knows as friction drag as it is formed due to the frictional effect. The Keywords— Turbulence Models, Drag Coefficient, Human Being, friction drag is the component of the wall shear force in the direction Different Positions, CFD toward the flow, and it depends on the body surface area and the magnitude of the wall shear stress. 1. Introduction The performance of human being in various positions is strongly 2.2. Numerical Simulation affected by the resistance they experience in which the resistance The numerical simulation in the present work is carried out using consists of aerodynamic drag. Aerodynamic drag, a resistance force ANSYS FLUENT 14.0. According to Chowdhurry [6], the human that acts upon a body moving through fluid like air and that is body parts can be simplified due to the configuration and size of opposite to the direction of motion of the body [1, 2]. In sports field human body is way too complex. Therefore, a simplified model of that competing with speed, aerodynamic drag is one of the important human body with the average height and frontal area was drawn factors. The lower the value of drag coefficient, the lower the using SolidWorks as shown in Fig. 2 (a). The model is then imported aerodynamic drag of the positions. into the virtual wind tunnel for computational studies as shown in Fig. The main idea of this paper is to investigate the effects of 2 (b) aerodynamic drag of human in different position theoretically and numerically. The total drag is majorly formed by pressure drag and friction drag due to the skin friction of the human body. In speed performing sports like cycling, cyclist often optimize the positions to reduce their drag by conducting experiment using wind tunnel. The measurement of human body flow can be said is time consuming and field tests are very difficult to set up. Even the experts in the related field can only optimize the positions to reduce drag by trial and error Therefore, computational fluid dynamic (CFD) is one of the alternative techniques to be carried out to investigate the aerodynamic drag. However, since the accuracy of numerical 2 (a) simulation method needs to be verified, published experimental data 2 (b) are used to compare and to justify the CFD results. Usually the Fig. 2 (a) Simplified Human Body (b) computational domain and maximum allowable errors in predicting the drag can range boundary condition. between10-12%. The wind tunnel experiment investigation of Steady 3D Reynolds-averaged Navier-Stokes (RANS) is used in various positions of human was published by Schmitt [3]. The results combination with turbulence models such as k-ε and k-ω with the low- can be used to validate the values obtain from CFD simulation. Fig. Reynolds number modeling (LRNM) together is used. The four 1 shows the 4 body positions to be studies during the research. turbulence model to be carried out in the study is standard k- ε model, realizable k-ω model, standard k-ω model and lastly shear stress transport (SST) k-ω model. There is no universally proved that which turbulence model is the most accurate flow for every study case [7]. Only steady flow is performed as the studies of flow of transient can be very complicated and hard to predict. In this study case, the velocity inlet will be set at 10 m/s which is almost equivalent to 38 km/h as this input boundary condition is the normal speed for a casual cycling speed.

Fig.1 (a) Standing (b) Sitting (c) Squatting (d) Supine. [4]

99 EURECA 2013 - Calculation of Aerodynamic Drag of Human Being in Various Positions 3. Results limitations when solving the same condition study; therefore, it may produce the different final values. 3.1 Comparison of Introductory Test The value of drag coefficient is dimensionless. Fig. 3 shows the values of drag coefficient of various positions using various Table 1. Percentage Error between Published Experimental turbulence models. Data and Numerical Simulation Data

Percentage of Error (%) standard realizable standard SST Positions k-ϵ k-ϵ k-ω k-ω Standing 3.3 0.008 11.1 3.3 Sitting 5.1 2.5 3.8 0.05 Supine 44.4 40 66.7 40 Squatting 48.9 40 35.6 37.8 5. Conclusion and Recommendations In conclusion, the standing position gives the highest value of drag coefficient while supine gives the lowest drag coefficient. To improve the accuracy, the modeling of the human body cannot be too simplified as the sometimes the real body do not generate the same projected frontal area as the simplified model. The input method and meshing may affect the accuracy too. Further study on these options need to be carried out more critically. However, although the accuracy of the wind tunnel and CFD are not totally the same, CFD still can be used to study in high speed application sports like running, swimming, etc. to optimize the body configuration as it gives detailed flow for every single study. In the other hand, the results presented in the current study are part of the ongoing study.

Investigation on apply velocities are considered such as average of Fig. 3 Drag coefficient of Various Positions with Turbulence Models. walking speed, running, skiing and etc. Angle of attack is also one of 4. Discussion the parametric to be considered in the overall studies. Figure 3 shows that the supine position gives the lowest drag Acknowledgment coefficient while the standing position gives the highest. The drag Appreciation to Taylor’s University School of Engineering for the coefficient of various positions from lowest to highest value in funding and equipment support of this project. ascending: supine, squatting, sitting and standing. It also shows that the lesser projected frontal area can greatly reduce the drag References coefficient. Since the input boundaries of velocity inlet, air viscosity, 1. Defraeye, T., Blocken, B., Koninckx, E., Hespel, P., and air density and flow direction are the same for 4 positions; the only Carmeliet, J. (2010). Aerodynamic study of different cyclist factor to affect the changes in drag coefficient is the frontal area. In positions: CFD analysis and full-scale wind-tunnel tests. Journal others words, a less frontal area means the body expose lesser of Biomechanics, 43(7), 1262-1268. external flow to the wind. As the standing position has the highest 2. Anderson, John D. Jr. (2000). Introduction to Flight. Fourth projected frontal area, it exerts more forces due to the pressure and Edition, McGraw Hill Higher Education, Boston, Massachusetts, skin friction drag on the frontal area on the human opposite the wind USA. direction. While the supine position has the lowest projected frontal 3. Schmitt, T. J. (1954). Wind Tunnel Investigation of Air Loads on area because the exposed area in this position is less and thus the skin Human Being.Virginia, USA. friction drag is less as well. 4. Hoerner, S. F. (1965). Fluid Dynamic Drag. Midland Park, New In other hand, the research also carries out using different Hersey, USA. turbulence models. For squatting and supine positions, the values between the four turbulence models are very close. However, it has 5. Cengel, Y.; Cimbala, J.; Turner, R. (2012). Fundamentals of huge differences between the experimental data and numerical Thermal Fluid Sciences. simulation data. This may due to the simplified human body model 6. Chowdhury, H.; Alam, F. and Subic, A. (2010). An Experimental as the projected frontal area is not the same as the real human sample Methodology for a Full Scale Bicycle Aerodynamics Study. carry out by Schmitt [3]. The geometry of the upper body of the Inter-national Conference on Mechanical, Industrial and Energy, simplified human body is too wide and therefore it has higher drag Khulna, Bangladesh. coefficient. The difference of percentage errors for supine and 7. Defraeye, T., Blocken, B., Koninckx, E., Hespel, P., and squatting is relatively high, from 35% to 67%. For standing and Carmeliet, J. (2010). Computational fluid dynamics analysis of sitting positions, the values between the experimental data and cyclist aerodynamics: Performance of different turbulence- numerical simulation data are very close, the maximum error modeling and boundary layer modeling approaches. Journal of differences is only approximately 11%, which is still in the accepted Biomechanics, 43(12), 2281-2287. range. The difference values of each turbulence model may occur due to the input method of the computational domain and boundary condition at the set-up. Each turbulence model has their own

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