Dynamic Measurements and Drag Crisis Hysteresis in Garment Aerodynamics

The advantage of tripping boundary layer turbulence around bluff bodies in order to reduce the aerodynamic drag and stabilize the flow has been known for many decades and applied in a number of engineering fields. However, its use in the sport technology area is quite recent. The first practical application of modeled surfaces used to reduce drag has been in golf balls, with the introduction of a dimpled surface. This solution allowed the ball to experience a drag value that is nearly half of the drag value of the same ball with a smooth surface. More recently, with the introduction of the NIKE swift suit in 2002 the focus of sports companies and athletes moved to the so called “low drag suits”. Recent published research proved that low drag suits with panels with different types of roughness clearly improve the athletes’ performances. Currently, most of the research carried out on textiles aerodynamics relies on static measurements on cylindrical models and limited research considers a possible hysteresis in the drag crisis phenomenon for low drag suits applications. The hysteresis phenomenon in the drag crisis is however a well-known phenomena and previous authors have been addressing it. In the present work, the instant drag on a cylinder model mounted in the wind tunnel and covered with different fabrics has been measured and compared with static measurements made on the same model. The experiments were carried out with a sampling frequency of 200 Hz using an AMTI BP-600400 HF with a natural frequency of 470 Hz and thus capable of measuring dynamic forces. Results show that a hysteresis process exists and that dynamic measurements could be a good alternative to static measurements requiring less testing time and giving more accurate results.