固体材料导热系数的分子动力学模拟

采用分子动力学方法对金属和非金属材料薄膜的导热系数进行数值模拟. 非金属材料氩的粒子间相互作用势采用12/6 L-J势能模型,金属材料钨使用MEAM势能模型. 模拟结果表明,非金属材料氩的导热系数与已有实验结果符合较好,但金属材料钨的导热系数结果较实验结果小1~2个数量级. 研究结果表明,引入自由电子导热模型能较准确预测金属材料钨的导热系数变化规律.     

Lower limb joint kinetics during the first stance phase in athletics sprinting: three elite athlete case studies

Abstract

This study analysed the first stance phase joint kinetics of three elite sprinters to improve the understanding of technique and investigate how individual differences in technique could influence the resulting levels of performance. Force (1000 Hz) and video (200 Hz) data were collected and resultant moments, power and work at the stance leg metatarsal-phalangeal (MTP), ankle, knee and hip joints were calculated. The MTP and ankle joints both exhibited resultant plantarflexor moments throughout stance. Whilst the ankle joint generated up to four times more energy than it absorbed, the MTP joint was primarily an energy absorber. Knee extensor resultant moments and power were produced throughout the majority of stance, and the best-performing sprinter generated double and four times the amount of knee joint energy compared to the other two sprinters. The hip joint extended throughout stance. Positive hip extensor energy was generated during early stance before energy was absorbed at the hip as the resultant moment became flexor-dominant towards toe-off. The generation of energy at the ankle appears to be of greater importance than in later phases of a sprint, whilst knee joint energy generation may be vital for early acceleration and is potentially facilitated by favourable kinematics at touchdown.     

The influence of pitch dimensions on performance during small-sided and conditioned soccer games

Abstract

This study examined the influence of pitch dimensions in small-sided soccer games in shaping opportunities for performers to maintain ball possession, pass to teammates and shoot at goal. Fifteen amateur standard male participants (M = 21.87, σ = 1.96 years) played 5 v 5 small-sided soccer games in three varying pitch conditions (28 m × 14 m, 40 m × 20 m and 52 m × 26 m). Thirty sequences of play in each condition were selected for digitisation using TACTO software, allowing the capture of bi-dimensional displacement coordinate data of all players and the ball. The values of interpersonal distance between all attackers and immediate defenders and the relative distances of defenders to intercept a shot and a pass were computed as dependent variables. Results showed existence of fewer opportunities to maintain ball possession on smaller pitches, compared to medium and larger pitches. Conversely, the different dimensions set to the pitch did not influence opportunities for players to shoot at goal, or to perform passes to other teammates. By examining the specific spatial–temporal relationships of players and key-task constraints, the data from this study explain how effects of manipulating pitch dimensions of small-sided games might enhance opportunities for acquiring specific movement and decision-making skills.     

Dynamics of the martial arts high front kick

Fast unloaded movements (i.e. striking, throwing and kicking) are typically performed in a proximo‐distal sequence, where initially high proximal segments accelerate while distal segments lag behind, after which proximal segments decelerate while distal segments accelerate. The aims of this study were to examine whether proximal segment deceleration is performed actively by antagonist muscles or is a passive consequence of distal segment movement, and whether distal segment acceleration is enhanced by proximal segment deceleration. Seventeen skilled taekwon‐do practitioners were filmed using a high‐speed camera while performing a high front kick. During kicking, EMG recordings were obtained from five major lower extremity muscles. Based on the kinematic data, inverse dynamics computations were performed yielding muscle moments and motion‐dependent moments. The results indicated that thigh deceleration was caused by motion‐dependent moments arising from lower leg motion and not by active deceleration. This was supported by the EMG recordings. Lower leg acceleration was caused partly by a knee extensor muscle moment and partly by a motion‐dependent moment arising from thigh angular velocity. Thus, lower leg acceleration was not enhanced by thigh deceleration. On the contrary, thigh deceleration, although not desirable, is unavoidable because of lower leg acceleration.     

Lower extremity power during the squat jump with various barbell loads

The purpose of this study was twofold: (1) to determine the barbell load that maximised the system power as well as the ankle, knee, and hip joint powers during the squat jump, and (2) to compare the system powers computed from two different methods: the centre of mass (COM) method and the barbell method. Seven male throwers were recruited in this study. The system power (COM method) and the ankle, knee, and hip joint powers were determined with the load incrementally set at 0%, 10%, 30%, 50%, 70%, and 90% of one repetition maximum. The largest system power was observed at the load of 30% (p < 0.008) while the largest ankle and knee powers were observed at 70% and 0% (p < 0.05). The barbell method overestimated the system power (p < 0.001) when compared to the COM method. It was concluded that the barbell method could influence load optimisation in squat jump. The optimal barbell load which maximised the system power did not maximise the ankle, knee, or hip power simultaneously.     

Comparison of support leg kinetics between side-step and cross-step cutting techniques

The purpose of this study was to investigate differences in the support leg joint moment and moment power between side-step (SS) and cross-step (CS) cutting techniques with a prescribed 90° cutting angle. Ground reaction forces (1,000 Hz) and three-dimensional kinematics (250 Hz) of SS and CS cutting techniques were collected from 20 male college athletes. Normalised peak knee extension moment was larger in the SS technique than in the CS technique (0.40 ± 0.10 in SS; 0.26 ± 0.08 in CS). In the SS technique, the knee extensors ( ? 0.10 ± 0.06 in SS; ? 0.02 ± 0.04 in CS) and ankle plantarflexors ( ? 0.12 ± 0.05 in SS; ? 0.07 ± 0.03 in CS) did significantly more negative work (normalised). The direction change angle (40.5 ± 8.7° in SS; 33.0 ± 6.8° in CS) and the decrease in horizontal velocity of the centre of mass ( ? 0.63 ± 0.23 m/s in SS; ? 0.31 ± 0.23 m/s in CS) were significantly larger in the SS technique. These results suggest that the SS technique is an effective means of changing running direction at the expense of velocity of the centre of mass and that the CS technique is better for minimising the reduction in horizontal velocity of the centre of mass.     

Mechanical power output in rowing should not be determined from oar forces and oar motion alone

Mechanical power output is a key performance-determining variable in many cyclic sports. In rowing, instantaneous power output is commonly determined as the dot product of handle force moment and oar angular velocity. The aim of this study was to show that this commonly used proxy is theoretically flawed and to provide an indication of the magnitude of the error. To obtain a consistent dataset, simulations were performed using a previously proposed forward dynamical model. Inputs were previously recorded rower kinematics and horizontal oar angle, at 20 and 32 strokes?min^?1. From simulation outputs, true power output and power output according to the common proxy were calculated. The error when using the common proxy was quantified as the difference between the average power output according to the proxy and the true average power output (P?_residual), and as the ratio of this difference to the true average power output (ratio_res./rower). At stroke rate 20, P?_residual was 27.4 W and ratio_res./rower was 0.143; at stroke rate 32, P?_residual was 44.3 W and ratio_res./rower was 0.142. Power output in rowing appears to be underestimated when calculated according to the common proxy. Simulations suggest this error to be at least 10% of the true power output.     

铁饼飞行中的空气动力学原理与关键技术分析

铁饼投掷的运动成绩是由运动员的身体素质、专项技术能力和铁饼在空中飞行时空气动力等因素所决定的。将空气动力学的基础知识应用到投掷铁饼出手动作“控饼”技术的教学与训练中 ,从而获得理想的出手初速度、适宜的出手角度和铁饼飞行中的合理倾角 ,以达到提高教学与训练质量和运动技术水平的目的。     

纳米流体液滴撞击固体壁面的实验和模拟研究

液滴撞击固体壁面现象常见于动力机械、喷雾冷却和薄膜材料沉积制备等工业生产中。将石墨烯和碳纳米管作为分散颗粒,应用超声波分散技术制备均匀性与稳定性良好的树脂基纳米流体。利用高速摄像技术,研究纳米流体液滴撞击固体壁面的动力学行为。基于有限元法,通过修正的幂律黏度模型耦合纳米流体的剪切变稀特性,采用水平集方法捕捉相界面的移动,建立纳米流体液滴撞击固体壁面的数值模型。模拟结果与实验结果较好一致,验证了数值模型的正确性与准确性。研究发现实验配制的纳米流体表现出非牛顿剪切变稀特性,纳米颗粒的加入抑制了液滴撞击固体壁面过程中的铺展和回缩行为。随着幂律指数m的减小,液滴撞击固体壁面后的铺展范围变大。随着表面张力的增大,液滴的无量纲直径在铺展阶段无明显变化,但在回缩阶段逐渐减小。