Bicycle helmets present complex geometries intended for protection and ventilation. Models often seem driven by esthetic considerations rather than by designs for optimal ventilation. To quantify the amount of flow entering a helmet, four bicycle helmets were investigated using flow visualization in a water tunnel, at representative riding speed for two helmet inclinations. Design features such as location of vents, visors and wall thickness of openings were examined as potential factors influencing ventilation efficiency. Cross-sections of streamtubes available for ventilation ahead of a helmet were evaluated. Results revealed that several helmet openings were seldom ventilated making them largely ineffective for forced convection under the given conditions. Velocities of the flow entering helmet vents were estimated to decrease by a factor > 3 relative to the freestream and varied from one vent to another. The general trend observed for velocities was described by a simple analytical model based on channel flow.

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