Aerodynamic and ventilation performance of sports helmets

• Main Author: Koh, Ryan Han Wei
• Other Authors: Leong Kah Fai
• Format: Final Year Project
• Language: English
• Published: 2016
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/67690
• Institution: Nanyang Technological University

Bicycle helmets today have far surpassed the basic requirements for safety and protection of the rider. In the commercial world, companies now compete to improve both the comfort and performance of their helmets, particularly through the optimisation of ventilation and drag capabilities. The first objective of this project was to establish and simulate links between ventilation and drag from past literature. The findings had shown that there was an inverse relationship between ventilation and drag. Additionally, critical market research had been conducted on helmets available in the market today to study their design features and understand why the bicycle helmets were designed in that manner. An example of one of the helmets reviewed was the POC super ventilating helmet [1]. The review of this helmet showed that goggle and chimney vents, when introduced, resulted in a large increase in air flow through the helmet. The shortcomings of past work discussed in the critical literature review had also been used to improve this project’s experiment methodology. Past wind tunnel experiments by Alam in 2005 [2] and 2010 [3] showed that the use the thermocouple was an inaccurate measurement tool when measuring temperature dissipation, due to its inability to simultaneously take temperature readings from the entire test area. Therefore, in this project, a thermal camera was used as it could measure the heat dissipation of the entire desired experimental area simultaneously. The second objective of this project was to develop an experimental setup to carry out ventilation and drag data collection. A wind tunnel was selected as it provided an environment in which variables such as wind speed could be controlled and iii variables such as drag could be measured with a load cell without interference. A hot air gun was selected as the heat source as it could simulate internal heat generated by the human body through exercise by heating the underside of the mannequin head. The mannequin head was heated to 42℃ and allowed to cool to 30℃ to emulate the conditions a cyclist would undergo when cycling. An existing test jig, which served the purpose of holding the mannequin head, load cell and thermal camera in the wind tunnel, was modified to increase the time efficiency and user friendliness of the experiments. The head mounting device was redesigned and fabricated via additive manufacturing to feature a close fit sliding grove, thus simplifying the mounting/dismounting process and cutting the time taken by approximately 7-8 minutes. The existing jig made use of an L-bracket to hold the thermal camera under the mannequin head to record the heat dissipation data. As the position of the L-bracket was fixed, this inhibited the access available to the hot air gun when heating. Therefore, an L-bracket mounting was designed to allow the L-bracket to be easily removed and hence created space for the hot air gun to carry out even heating of the mannequin head for more consistent results. The wind tunnel tests were conducted on three bicycle helmets, the POC Octal, Specialized Prevail and Specialized Evade. To study the effects of the vent geometry on ventilation and drag, the helmet vents were selectively blocked in five configurations – Fully covered, Front free, Top free, Back free and Fully free. The wind speed of the wind tunnel was kept constant at 10m/s as it is the approximate average cycling speed of a cyclist. Multiple tests were repeated for consistency of results. iv The evaluation of experimental results concluded that when large unobstructed vents were present, the cooling rate of that section of the helmet was increased. This conclusion was supported by evidence in the form of heat maps taken via the thermal camera. An indicative perception test was also carried out to determine if the wind tunnel data could be translated into real life applications. The results of the perception test strongly supported the findings of the ventilation test. In terms of drag performance, helmets with larger vent surface areas suffered drag penalties. Therefore, this suggested an adverse relationship between vent surface area and drag performance. Finally, the results of this project highlighted two potential areas for future studies. Firstly, the study of the vent surface area as a percentage of the total helmet surface area and the effects different percentages have on the ventilation and drag performances. Secondly, a validity study of the design tool proposed in this project, the Comfort Coefficient. The Comfort Coefficient was conceptualised to study the combined effects of ventilation and drag collectively and could be a potential parameter for bicycle helmet designers to consider in the future if validated.