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越圆越光滑就越好? 足球真没你想得这么简单!

2018-07-10 07:41:02    新华网

那么想减少压差阻力,一个最有效的办法是减少物体后方的低压区面积。流体力学实验发现,光滑的球与布满小酒窝的球相比,后者产生的低压区面积更少。球上的无数小酒窝,可以起到让空气紧贴球面的作用,这使得平滑的气流能顺着球体表面延伸到更靠后的位置时才产生分离。

史上最圆足球曾广受诟病

在世界杯的历史上,起初也曾有过一段时间,人们追求制造更圆更光滑的足球,但后来才发现过犹不及。

传统足球是由32块五边形或六边形的皮块组成,而这种蜂窝状的足球自2006年德国世界杯开始有了突破性的变化。当年德国世界杯比赛用球“团队之星”采用了全新的异形拼块和无缝压合技术,史无前例地将足球球面拼块减少到14块,这最大程度地消除了球体表面不规则的凹凸,令球体呈现完美圆弧;到2010年南非世界杯,“普天同庆”仅用8块外表皮组成,并以热黏合技术拼接完成,从而使其较以往更圆,当时堪称历届最圆足球,但就是这个“最圆足球”比赛中却因低进球率,而广受诟病。

《新物理学杂志》发表的一篇论文指出,任何物体在空中运动时,一层相对静止的薄空气会环绕在它周围。当球慢速飞行时,空气从足球表面平滑地流过,形成互不干扰的空气层,这种状态被称为层流,这时空气会在球两侧直线距离最远两个点分开,造成更多空气阻力,加剧球的减速。相反,当球高速运行时,空气就不会分成层,而是乱作一团,在球的表面形成混乱湍流,这时风会沿着球的曲线“包”住球后部,球后方低压区变小,压差阻力变小,球在空中能更久飞行。而飞行的物体周围的空气由湍流变为层流时有一个临界点,不同粗糙度的足球会有不同的临界点。而空气不同流动状态间的转化会使球受到更多空气阻力,造成出乎意料的晃动和下坠。

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