INTRODUCTION: The purpose of this study was to examine the effect of airflow condition around the jumper's back on the aerodynamic forces in ski-jumping take-off, and to propose an appropriate index to evaluate the take-off motion with an aerodynamic analysis. METHOD: A wind-tunnel test and the measurements in actual jumping were carried out. In the wind-tunnel lest, the lift and drag forces exerted on a human model were measured with different trunk angles of attack (t-AOA). The flow-direction, the flow-separation and the turbulence around the mode! were visualized with surface tuft method. We extracted the characteristics of airflow condition that have a strong influence on the aerodynamic forces. Then, we attempted to quantify the characteristics by flow-direction and propose a new index to indicate the aerodynamic effect in the take-off motion. To examine the validity of the analysis and the proposed index, this technique was applied to actual jumping trials. With the tuft method the airflow on the jumper's back in a take-off phase was visualized in 9 jumping trials(HS=100m). The relationship between the index and the jumping distance was analyzed. RESULTS: In the wind-tunnel test the lift force decreased and the following changes were observed when the t-AOA exceeded approximately 30 degrees. On the jumper's back the Separation of airflow started, and the flow-direction changed from parallel to diverging along the rear surface of the jumper's trunk. We defined the angle of this divergence as the "angle of outflow", that was the arithmetic mean of the rms values of 18 tufts. The Separation and the wake vortices were formed behind the model's body. The angle of outflow showed a sharp increase as the t-AOA exceeded 30 degrees. In the actual jumping experiment, the same change in the airflow along the jumper's back as in the wind-tunnel test was observed. Obvious negative correlation (r= -0.77) was obtained between the angle of outflow and the jumping distance (Fig.1). DISCUSSION: The generation of the vortices in the downstream could be caused by the change of airflow condition along the jumper's back with the increase of t-AOA. Consequently, the drag force increases and the lift force decreases leading to the aerodynamically disadvantageous conditions for the jumping distance. CONCLUSION; Aerodynamic forces around jumper's model were measured in a wind-tunnel test, and their characteristics were analyzed. According to the results, a new aerodynamic index was proposed to evaluate the take-off motion. The validity of the index was verified in the measurement of actual jumping.
© Copyright 2007 4th International Congress on Science and Skiing. Julkaistu Tekijä University of Salzburg. Kaikki oikeudet pidätetään. / Kääntänyt https://www.DeepL.com/Translator