Manipulation of the boundary layer transition to enhance the aerodynamic performance of athletes

Every second counts. For an athlete racing against the clock, even hundredths of a second can make the difference between a podium and a top 10 finish. For a speed skater, more than 80% of the resistance to motion comes from the aerodynamic drag on the body.The speed at which the skater is racing, the fluid density and the characteristic dimensions of the human body, which can be approximated as a combination of multiple circular cylinders, define Reynolds numbers that can be in the critical range. The critical Reynolds number range is characterized by an important reduction of the drag coefficient due to the transition of the flow from laminar to turbulent in the boundary layer. Depending on the body shape and proportion, the velocity, the wind turbulence and the surface roughness of the fabric covering the athlete, the drag area coefficient can be reduced to a minimum value.This experimental research was performed in a wind tunnel. Measurement of drag and manipulation of the state of the flow were carried out around life-size mannequins for three different positions of a speed skater: sidepush, gliding and cross-over leg. Innovative technology using surface pressure measurement through fabrics with a variety of surface roughness allowed a comprehensive study of the flow to capture the state of the boundary layer in the critical Reynolds number range at the surface of the model. Local measurements indicated the contribution of each part of the body to the total drag and the parts that were Reynolds number dependent.The experimental results have shown that it was possible to reduce the drag of a speed skater by 15% using appropriate surface roughness for the fabrics but moreover they indicated clearly the importance of correctly simulating parameters that affect drag reduction. The development of the flow around non-circular cross-sectional shape and the flow interaction between different parts of the body dominated the location of the transition, corresponding to a different position than for a single bidimensional circular cylinder. Manipulation of drag reduction in laminar flow led to an error of 5 to 30% in the targeted range of speeds compared to the results obtained with a representative wind turbulent flow.