The Experts below are selected from a list of 327 Experts worldwide ranked by ideXlab platform
Redha Taiar - One of the best experts on this subject based on the ideXlab platform.
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Numerical investigation of the early Flight Phase in ski-jumping
Journal of biomechanics, 2017Co-Authors: Nicolas Gardan, Alexandre Schneider, Guillaume Polidori, H. Trenchard, Jean-marc Seigneur, Fabien Beaumont, François Fourchet, Redha TaiarAbstract:The purpose of this study is to develop a numerical methodology based on real data from wind tunnel experiments to investigate the effect of the ski jumper's posture and speed on aerodynamic forces in a wide range of angles of attack. To improve our knowledge of the aerodynamic behavior of the ski jumper and his equipment during the early Flight Phase of the ski jump, we applied CFD methodology to evaluate the influence of angle of attack (α=14°, 21.5°, 29°, 36.5° and 44°) and speed (u=23, 26 and 29m/s) on aerodynamic forces in the situation of stable attitude of the ski jumper's body and skis. The standard k-ω turbulence model was used to investigate both the influence of the ski jumper's posture and speed on aerodynamic performance during the early Flight Phase. Numerical results show that the ski jumper's speed has very little impact on the lift and drag coefficients. Conversely, the lift and drag forces acting on the ski jumper's body during the early Flight Phase of the jump are strongly influenced by the variations of the angle of attack. The present results suggest that the greater the ski jumper's angle of inclination, with respect to the relative flow, the greater the pressure difference between the lower and upper parts of the skier. Further studies will focus on the dependency of the parameters with both the angle of attack α and the body-ski angle β as control variables. It will be possible to test and optimize different ski jumping styles in different ski jumping hills and investigate different environmental conditions such as temperature, altitude or crosswinds.
Hermann Schwameder - One of the best experts on this subject based on the ideXlab platform.
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Characteristics of the early Flight Phase in the Olympic ski jumping competition
Journal of biomechanics, 2005Co-Authors: Mikko Virmavirta, Juha Isolehto, Paavo V. Komi, Gert-peter Brüggemann, Erich Müller, Hermann SchwamederAbstract:Abstract Early Flight Phase (approximately 40 m) of the athletes participating in the final round of the individual large hill ski jumping competition in Salt Lake City Olympics was filmed with two high-speed pan & tilt video cameras. The results showed that jumpers’ steady Flight position was almost completed within 0.5 s. The most significant correlation with the length of the jump was found in the angle between the skis and body ( r = . 714 , p ⩽ . 001 at 1.1 s after the take-off). This particular Phase seemed to be important because the ski angle of attack was also related to the jumping distance at the same Phase. Although the more upright ski position relative to Flight path resulted in longer jumping distance, the winner of the competition had significantly lower ski position as compared to the other good jumpers. This may be due to the high altitude (>2000 m) of the ski jumping stadium in this competition. Because of the low air density, the aerodynamic forces were also low and this probably caused less skilful jumpers to lean too much forward at this Phase. Maintenance of speed seemed to be emphasized in this particular competition.
Nicolas Gardan - One of the best experts on this subject based on the ideXlab platform.
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Numerical investigation of the early Flight Phase in ski-jumping
Journal of biomechanics, 2017Co-Authors: Nicolas Gardan, Alexandre Schneider, Guillaume Polidori, H. Trenchard, Jean-marc Seigneur, Fabien Beaumont, François Fourchet, Redha TaiarAbstract:The purpose of this study is to develop a numerical methodology based on real data from wind tunnel experiments to investigate the effect of the ski jumper's posture and speed on aerodynamic forces in a wide range of angles of attack. To improve our knowledge of the aerodynamic behavior of the ski jumper and his equipment during the early Flight Phase of the ski jump, we applied CFD methodology to evaluate the influence of angle of attack (α=14°, 21.5°, 29°, 36.5° and 44°) and speed (u=23, 26 and 29m/s) on aerodynamic forces in the situation of stable attitude of the ski jumper's body and skis. The standard k-ω turbulence model was used to investigate both the influence of the ski jumper's posture and speed on aerodynamic performance during the early Flight Phase. Numerical results show that the ski jumper's speed has very little impact on the lift and drag coefficients. Conversely, the lift and drag forces acting on the ski jumper's body during the early Flight Phase of the jump are strongly influenced by the variations of the angle of attack. The present results suggest that the greater the ski jumper's angle of inclination, with respect to the relative flow, the greater the pressure difference between the lower and upper parts of the skier. Further studies will focus on the dependency of the parameters with both the angle of attack α and the body-ski angle β as control variables. It will be possible to test and optimize different ski jumping styles in different ski jumping hills and investigate different environmental conditions such as temperature, altitude or crosswinds.
Amir Degani - One of the best experts on this subject based on the ideXlab platform.
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A Flight-Phase terrain following control strategy for stable and robust hopping of a one-legged robot under large terrain variations
Bioinspiration & biomimetics, 2017Co-Authors: Natan Shemer, Amir DeganiAbstract:This work demonstrates a simple, once per step, Flight-control method for robots running on a planar unknown rough-terrain environment. The robot used to exemplify these control strategies is the ParkourBot, a spring loaded inverted pendulum (SLIP)-based robot. The SLIP model is widely used for the description of humans and animals running motion and has been the basis for many robots. A known control scheme for increasing robustness of the conservative, SLIP model is the swing leg retraction (SLR) method. Despite of the SLR's popularity, it is not intended to be used on the more realistic, non-conservative damped SLIP model. On the damped SLIP model, the SLR controller failed to provide adequate results, therefore, we have derived a new simple, Flight-Phase control method called polynomial energy insertion (PEI). The new PEI method is based on the dead-beat solution of the damped simplified instantaneous SLIP (iSLIP) model, which assumes an infinitely stiff spring. Unlike the SLR which, starting from apex, changes the leg angle monotonically during Flight, the PEI requires the leg length (hence, energy insertion) to change monotonically throughout the Flight Phase. Interestingly, the leg angle remains nearly constant. In simulations and experiments, we have compared the newly developed PEI to the previous SLR method. We have found that since the SLR does not control the horizontal velocity, it looses its stability under rough terrain. The PEI method was able to control the horizontal velocity and height from ground and hence showed great improvement in robustness to rough terrain. Moreover, in both simulations and experiments the PEI methods showed an increase in the mean jumps to failure of more than 30% compared to SLR-based controllers.
Mikko Virmavirta - One of the best experts on this subject based on the ideXlab platform.
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Characteristics of the early Flight Phase in the Olympic ski jumping competition
Journal of biomechanics, 2005Co-Authors: Mikko Virmavirta, Juha Isolehto, Paavo V. Komi, Gert-peter Brüggemann, Erich Müller, Hermann SchwamederAbstract:Abstract Early Flight Phase (approximately 40 m) of the athletes participating in the final round of the individual large hill ski jumping competition in Salt Lake City Olympics was filmed with two high-speed pan & tilt video cameras. The results showed that jumpers’ steady Flight position was almost completed within 0.5 s. The most significant correlation with the length of the jump was found in the angle between the skis and body ( r = . 714 , p ⩽ . 001 at 1.1 s after the take-off). This particular Phase seemed to be important because the ski angle of attack was also related to the jumping distance at the same Phase. Although the more upright ski position relative to Flight path resulted in longer jumping distance, the winner of the competition had significantly lower ski position as compared to the other good jumpers. This may be due to the high altitude (>2000 m) of the ski jumping stadium in this competition. Because of the low air density, the aerodynamic forces were also low and this probably caused less skilful jumpers to lean too much forward at this Phase. Maintenance of speed seemed to be emphasized in this particular competition.
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Techniques Used by Olympic Ski Jumpers in the Transition From Takeoff to Early Flight
Journal of Applied Biomechanics, 1995Co-Authors: Anton Arndt, Mikko Virmavirta, Gert-peter Brüggemann, Paavo V. KomiAbstract:This study was concerned with identifying important Flight characteristics of the ski jump at the end of the early Flight Phase and describing how these characteristics developed from the run-in through the takeoff and during the early Flight Phase. The K90 individual competition of the 1994 Olympic Winter Games was analyzed. The 2-D data (takeoff) were collected by a high-speed video camera, and the 3-D analysis (early Flight) used an algorithm whereby two cameras followed the jumpers through the early Flight Phase. Center of mass (CM) velocities at takeoff and after early Flight and CM heights at these positions had no significant linear correlations with total distance. Only small differences in these parameters were distinguished between better and poorer performers. Significant differences between jumpers were identified in angular parameters at takeoff and in early Flight. A combination of five defined Flight angles yielded an R2 value of .84. It was found that the complex movement sequences involve...