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Cricket ball swing: A preliminary analysis using computational fluid dynamics
Published in Steve Haake, The Engineering of Sport, 2020
J.M.T. Penrose, D.R. Hose, E.A. Trowbridge
Table 2 also shows that the amount of swing increases as the Reynolds Number increases. However, this may not translate to the ball deviating through a greater distance during its flight down the pitch because of the corresponding increase in delivery speed’. Whilst direct comparison between a 2D model and a real 3D cricket ball is obviously difficult, it should be noted that the values in Table 2 for lift and drag coefficients are of the order of those quoted in previous studies1,2,4,7, and exhibit similar trends. The trend whereby the drag decreases with increasing Reynolds Number is unsurprising as it is at this order of Reynolds Number that boundary layer state transitions occur (sometimes known as the ‘drag crisis’).
Mechanical and Aerodynamic Behaviour of Baseballs and Softballs
Published in Franz Konstantin Fuss, Aleksandar Subic, Martin Strangwood, Rabindra Mehta, Routledge Handbook of Sports Technology and Engineering, 2013
Drag is influenced by the size of the wake created by the ball. Consider a ball without rotation traveling through air at a low speed. Air approaching the centre of the ball will stop, forming an upstream stagnation point. At a very low Reynolds number (Re less than four) the air flow around the ball is completely laminar where the flow divides and reconnects on the back side of the ball. The drag is highest in this Reynolds number region. As the Reynolds number is increased, drag decreases before plateauing over the range 103 < Re < 3 × 105. At these speeds (which are representative of play), the laminar boundary layer separates from the ball (at approximately 80 degrees from the upstream stagnation point) creating eddies and a relatively large wake. As the Reynolds number increases further to 3 × 105 < Re < 3 × 106 a so-called ‘drag crisis’ occurs. In the drag crisis region, the boundary layer after the separation point, quickly becomes turbulent, reconnects to the ball surface and separates on the back side of the ball at approximately 120 degrees from the stagnation point (Panton 2005). This reduces the size of the ball’s wake. As a result, drag decreases dramatically, sometimes by as much as 70 per cent. At higher speeds (Re > 3 × 106), the flow becomes turbulent just after the upstream stagnation point and stays turbulent until finally separating just before 120 degrees. The wake in this region is smaller than in the low-speed case (103 < Re < 3 × 105) but larger than in the drag crisis region. Thus, at high speeds (Re > 3 × 106) ball drag increases relative to the drag crisis region but remains less than the low-speed region (103 < Re < 3 × 105).
Effect of surface roughness on vortex-induced vibration response of a circular cylinder
Published in Ships and Offshore Structures, 2018
Yun Gao, Zhi Zong, Li Zou, Zongyu Jiang
As shown in Table 1, early studies were mainly focused on the nearby flow in air around cylinders having different degrees of surface roughness. As the boundary changes from laminar to turbulent, the drag acting on a cylinder decreases abruptly at a critical Reynolds number. This phenomenon is called the ‘drag crisis’. The critical Reynolds number for this ‘drag crisis’ decreases with increased surface roughness. The critical range, the supercritical range, and the upper transitional range will merge into a narrow range when the surface roughness exceeds 3.0 × 10−3. With the rapid development of ocean engineering, research examining the effect of surface roughness on the VIV response of cylinders in water has received considerable attention. Kiu et al. (2011) studied the effects of the surface roughness of a cylinder in water, and found that as the roughness increases, the maximum VIV amplitude and the mean drag coefficient decrease, tending toward constant values. Compared with those for a smooth cylinder, the Strouhal numbers of rough cylinders are larger.
The aerodynamic roughness of textile materials
Published in The Journal of The Textile Institute, 2019
X. Y. Hsu, J. J. Miau, J. H. Tsai, Z. X. Tsai, Y. H. Lai, Y. S. Ciou, P. T. Shen, P. C. Chuang, C. M. Wu
In terms of flow around a circular cylinder, within a certain range of Reynolds numbers, which is named the critical regime (Achenbach, 1971; Bearman, 1969; Farell & Blessman, 1983; Nakamura & Tomonari, 1982; Schewe, 1983), the flow phenomenon of boundary-layer transition from laminar to turbulent states can be strongly coupled with the development of boundary-layer separation. In this regime, flow is very sensitive to subtle disturbances, such as surface roughness, free stream turbulence, aspect ratio and flow oscillations. An important feature is that a significant reduction in the drag coefficient can be identified, which is called the drag crisis phenomenon (Wieselsberger, 1922). Physically, in the critical regime, the boundary layer developed on the cylinder is laminar at separation, but soon transitions to turbulence. In the turbulent state, the separated shear layer can entrain external fluid more efficiently than the laminar state, therefore reattaches to the surface of the cylinder within a short distance, then separates again further downstream. The region between laminar separation and turbulent reattachment is called a ‘separation bubble’ (Roshko, 1961; Tani, 1964). The bubble initially forms on one side of the cylinder bi-stably, which is named the one-bubble state. The appearance of a single bubble actually produces asymmetry in the pressure distribution around the cylinder, which causes a large lift coefficient (Bearman, 1969; Farell & Blessman, 1983; Schewe, 1983). As the Reynolds number increases, a second laminar-turbulent separation bubble forms on the other side of the cylinder. Two separation bubbles form on both sides so that the wake region narrows. Consequently, the time-mean drag and lift coefficients are comparatively low. In the stable two-bubble state, the minimum drag coefficient can be as low as around 0.23 (Roshko, 1993), compared to a drag coefficient of more than one in the sub-critical regime.