【视频】高手在民间，懂CFD才能拿百万大奖-CFD界我有一帘幽梦
撰稿：赵一铭
编辑：CFD界
更高，更快，更强！
CFD来助力！
2017年5月5日至28日，第100届环意大利自行车公开赛火爆燃起，各国选手以50-70km/s的速度在意国狂奔。为了获得更快的速度！更低的阻力！自行车厂商更是寻求风洞和CFD来助力！
众所周知的是，如果运动员跟在一个车后，阻力会大幅降低。日本基于这种玩法，甚至发明了自行车凯林赛（如下图）。
自行车凯林赛，在头车之前有一个摩托车领跑可降低阻力。图片来源于Google
在电脑游戏中，基于这种减阻的原理，著名动作游戏极品飞车中也植入了“滑流”增加后方的机车速度。
但是，Blocken andToparlar在2015年的研究中发现：
即使运动员在骑行中，身后跟着一辆车，同样也会减少风阻！
极品飞车党崩溃了：“感情老子打算跟着敌方滑流，没想到还给敌方减少了阻力桂花蛋 ？”
不过放心，EA应该不会在极品飞车中植入了相关的CFD算法，至少在2015年前不会！
这种事情在常规的自行车比赛中也常有发生，在自行车赛场中，通常选手后面会跟随其他选手或者采集录像的摩托车之类。
恰巧的是，自行车赛场上经常发生事故很大一部分的事故发生在自行车和摩托车之间。
那么这种自行车摩托车耦合的鬼事情...是否对最终百万奖金花落谁家有影响呢侠隐记？
奇文继续吼住全场：
“Aerodynamic benefit for a cyclist by a following motorcycle”
Bert Blocken et al.Journal of Wind Engineering and Industrial Aerodynamics(2016)
Abstract:Compressible turbulence shapes the structure of the interstellar medium of our Galaxy and likely plays an important role also during structure formation in the early Universe. The density PDF and the power spectrum of such compressible, supersonic turbulence are the key ingredients for theories of star formation. However, both the PDF and the spectrum are still a matter of debate, because theoretical predictions are limited and simulations of supersonic turbulence require enormous resolutions to capture the inertial-range scaling. To advance our limited knowledge of compressible turbulence, we here present and analyse the world's largest simulations of supersonic turbulence. We compare hydrodynamic models with numerical resolutions of 256^3-4096^3 mesh points and with two distinct driving mechanisms, solenoidal (divergence-free) driving and compressive (curl-free) driving. We find convergence of the density PDF, with compressive driving exhibiting a much wider and more intermittent density distribution than solenoidal driving. Analysing the power spectrum of the turbulence, we find a pure velocity scaling close to Burgers turbulence with P(v)~k^(-2) for both driving modes in our hydrodynamical simulations with Mach = 17. The spectrum of the density-weighted velocity rho^(1/3)v, however, does not provide the previously suggested universal scaling for supersonic turbulence. We find that the power spectrum P(rho^(1/3)v) scales with wavenumber as k^(-1.74) for solenoidal driving, close to incompressible Kolmogorov turbulence老表你好嘢 , k^(-5/3), but is significantly steeper with k^(-2.10) for compressive driving. We show that this is consistent with a recent theoretical model for compressible turbulence that predicts P(rho^(1/3)v)~k^(-19/9) in the presence of a strong div(v) component as is produced by compressive driving and remains remarkably constant throughout the supersonic turbulent cascade.In recent years, many accidents have occurred between cyclists and in-race motorcycles, even yielding fatal injuries. The accidents and the potential aerodynamics issues have impelled the present authors to perform dedicated wind-tunnel measurements and Computational Fluid Dynamics (CFD) simulations to assess cyclist drag reduction when followed by one, two or three motorcycles. The 3D steady-state Reynolds-Averaged Navier–Stokes simulations with the standardk–εmodel are validated by the wind-tunnel tests. The cyclist drag reduction goes up to 8.7% for a single trailing motorcycle and to 13.9% for three trailing motorcycles at a distance of 0.25m behind the cyclist. This distance is not uncommon in elite races, as evidenced by the many recent accidents. The effect by a single following motorcycle at realistic short distancesd=0.25m (8.7%),d=0.5m (6.4%) andd=1m (3.8%) is larger than that by a following car at realistic short distanced=5m (1.4%). Therefore it could be argued that in-race motorcycles are not only more dangerous but also aerodynamically more influential. This study reinforces the necessity for the International Cycling Union to change the rules concerning in-race motorcycles, not only to avoid accidents but also to avoid unwanted aerodynamic benefits.
这篇文章使用CFD并结合风洞来研究自行车运动员群体的空气动力学行为。结果表明如果运动员后方如跟随有其他运动员，则前排的运动员收到的风阻的略微降低。同时，两个运动员之间的尾随距离越近，阻力降低的越大。
首先看模型和网格：
模拟为稳态，网格数量最大3400万，湍流模型选用kEpsilon模型梦落繁花蓝雪，压力速度耦合采用SIMPLEC，求解器当然是大名鼎鼎的ANSYS Fluent 15.0。
直接看结果，作者明确表示：
如果自行车选手和后面的摩托车距离保持0.5m，则会提前108秒完成比赛！
如果自行车选手和后面的摩托车距离保持2.5m，则会提前20秒完成比赛！
如果自行车选手和后面的摩托车距离保持10m，则会提前1秒完成比赛！
不同摩托车数量，以及不同摩托车和选手的距离，对自行车选手带来的风阻减少百分数
看这个图更直观：
不同全程自行车比赛中苏比萨雷塔 ，不同自行车和后部摩托车距离对应的时间减少量
是非成败转头空
奖金取决于摩托车城市判官 啊风笛舞曲！
所以，很明确了武道丹尊 ，自行车比赛的车距虽然会可能增加事故的危险性暴君的圈宠 ，但是却和潜在的空气动力学息息相关周鸿鸭 。
然而，有趣的问题发生了。
本来落后的选手废了吃奶的力气跟了上来，没想到给前方的选手减阻！又被拉下了？
这不厚道啊！
因此在最后，文中作者的表示：建议国际自行车联合会严格限制运动员之间的距离（>10m）广外外校，不仅仅使得比赛更公平学乐云教学 ，还可以减少事故淮南一中网站。
奇葩手动减阻自行车，高手在民间，视频来源于Youtube
原作者表示：本文献给比利时自行车赛选手Antoine Demoitie除虫大将军，其在2016年3月28日的自行车大赛中和摩托车发生碰撞事故遇难。
拓展阅读：
CFD名人堂 | A. D. Gosman
CFD名人堂 | Brian Spalding
商软和开源的对决？齐慧娟 以人民的名义
被逼迫的创新：重叠网格
我能想到最浪漫的事，就是跟你慢慢收敛