“阿维塔风阻”风波，喧嚣数月后以“阿维塔直播公测”的自证手段，暂告一段落。

　　5月9日当晚，阿维塔在央视记者见证、公证机构监督下，通过中国汽研重庆风洞实验室直播测试（包括多种工况），从低风阻轮毂+电子后视镜的工况，最后到偏置5°+普通后视镜+格栅打开+空悬提高的工况，测试结果从0.217Cd逐步变为0.2971Cd。

　　从公关角度，这是一次完整且有力的案例，起码让一个原本小众的工程问题走进公众视野，开始有人认识到，风阻系数是一个“很脆弱”的参数，它和外形条件密切相关。改变了外形，哪怕只是很微小的改变（比如打开格栅，意味着增加了风道，改变了气流），也能影响结果。

　　鉴于工况是很难穷尽的，阿维塔在社交场域的回应点比较聪明，“自证清白”虽属被迫，但抢“议题设置权”这一点，做得还是挺标准的。从9日晚上直播测试之后，舆论明显反转。质疑主机厂的声音明显减少，

　　穿透了公关表象，这件事多少有点“技术娱乐化”的倾向。关于风阻这一频频在新能源汽车提及的重要词频，我们还是需要明确几个基本事实。

　　首先，CFD（计算流体力学）模拟和缩比模型测试，可以有效降低测试成本。但专业工程师很清楚，汽车风阻本身是无法精确计算的，任何模拟和小尺寸模型吹风洞，都是妥协行为。想精确标定，必须吹全尺寸模型或者上实车测试。

　　其次，风阻这个参数目前没有国标，自然也没有标准测试方法，只有行业推荐标准（非强制）。这样一来，测试过程大家各行其是，导致测试结果的人为痕迹比较重。一些舆论的认知水平还停留在“特调车”上。其实根本不用，就用量产车，只须微调工况，就可引导结果。

　　再次，国内公开接受委托，对汽车风阻测试的低速风洞实验室，绝大多数只接受车企委托，且能给出测试报告。某种程度，双方更像是一种合伙伙伴关系。

　　总之，风阻这个参数本身，远没有舆论场上的争论那么高调。

　　不过回到技术上，控制风阻本身是非常有意义的，起码能降低能耗、提升安全（增加下压力，对抗地面效应）。

　　有人认为小米Su7的风阻系数0.195是在160公里时速下测出来的，而非通常车企采用的120公里时速，这其实透露了风阻系数的工程难度。理论上，这个无量纲参数是常数，和速度没关系，只与外形和表面材质有关。

　　而实际上，工程问题要考虑的影响因素比公众认为的更复杂。除了形状（9日直播测试的所有工况本质上都是改变形状），影响风阻最主要的因素，其实是雷诺数（Re）。

　　雷诺数的意思，是惯性力和粘性力的比值。只有抛开牛顿力学，才能窥见这一混沌世界。雷诺数小（小于1）的时候，粘性力占主导地位，流体流动稳定，即“层流”。你可以将流体想象为一片一片叠在一起流动的。当雷诺数大（大于10^3）的时候，惯性力开始占据主导地位，流动不稳定起来，容易形成“湍流”，流体开始涡旋式前进。当然，还有居中（1~10^3）的时候，惯性力和粘性力接近。

　　对于汽车在空气流体里的相对运动，高速行驶（超过80公里时速），虽然雷诺数都很大（10^7以上），意味着湍流早已成为主导因素。边界层（汽车表皮附近的空气）附着力提高，压差阻力降低，风阻（不是风阻系数）有突降。比如高尔夫球，就用表面凹坑制造湍流，减少风阻。

　　而从120公里时速升到160公里时速，边界层厚度减少，空气粘性影响区变小，减少了粘性导致的能量损失。这才是速度提升导致风阻系数减少的真正原因。可惜摩擦阻力影响很小，只有百分之几的样子，远不及阻力的陡然上升（空气阻力与速度平方成正比）。

　　如果想在固定外形下，测出更好的风阻系数，应该怎么办？雷诺数公式告诉我们，应该致力于改变边界条件，比如提升流体密度、降低温度、提升压力。这就意味着把表面做成坑坑洼洼的，尽量在低海拔地区、冬季来测试。

　　如果从工程角度，降风阻系数降到一定程度，再往下压的代价很高（比如压降车内空间高度），要做很多仿真和测试，远抵不过测试时玩的一点小花招。而且，这些招数比那个博主玩的要高明一些。属于控制变量的一部分，不是作弊（换轮毂、摆角度等）。

　　从技术再回到舆论，吃瓜群众围观可以，不宜过于沉浸，认真你就输了。技术不能过度娱乐化，回归技术本身，厂商们将精力和费用投入到更能提高产品点的风向，本也就是必行之事。

The controversy surrounding the "Avita wind resistance" has come to a temporary end after months of hype, using the self verification method of "Avita live beta testing".

On the evening of May 9th, Avita, witnessed by CCTV reporters and supervised by notary institutions, conducted live testing at the Chongqing Wind Tunnel Laboratory of China Automotive Research Institute (including various working conditions). From the working condition of low wind resistance wheel hub+electronic rearview mirror, to the working condition of offset 5 °+ordinary rearview mirror+grille opening+suspension lifting, the test results gradually changed from 0.2117Cd to 0.2971Cd.
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From a public relations perspective, this is a complete and powerful case that at least brought a previously niche engineering problem into the public eye, and some people began to realize that drag coefficient is a "very fragile" parameter closely related to external conditions. Changing the appearance, even a small change (such as opening the grille, which means adding air ducts and changing the airflow), can still affect the results.

Given that the working conditions are difficult to exhaust, Avita's response points in the social arena are quite clever. Although "self proving innocence" is forced, he still does a fairly standard job in seizing the "issue setting power". After the live test on the evening of the 9th, public opinion has clearly reversed. The voices questioning the host factory have significantly decreased,

Penetrating the surface of public relations, this matter has a tendency towards "technological entertainment" to some extent. We still need to clarify a few basic facts about wind resistance, an important term frequently mentioned in new energy vehicles.

Firstly, CFD (Computational Fluid Dynamics) simulation and scaled model testing can effectively reduce testing costs. But professional engineers are well aware that the wind resistance of a car itself cannot be accurately calculated, and any simulation or small-scale model of blowing holes is a compromise. To achieve precise calibration, it is necessary to blow out a full-size model or test it on a real vehicle.

Secondly, there is currently no national standard for the parameter of wind resistance, nor is there a standard testing method. Only industry recommended standards (not mandatory) are available. In this way, everyone goes their own way during the testing process, resulting in a heavy human influence on the test results. The cognitive level of some public opinion still remains at the level of "special tuned vehicles". Actually, it's not necessary at all. Just adjust the operating conditions to guide the results.

Once again, the majority of low-speed wind tunnel laboratories in China that openly accept commissions for automotive drag testing only accept commissions from automotive companies and can provide test reports. To some extent, the two parties are more like a partnership.

In short, the parameter of wind resistance itself is far less high-profile than the debates in the public opinion field.

However, technically speaking, controlling wind resistance itself is very meaningful, as it can at least reduce energy consumption and improve safety (increase downforce, counteract ground effects).

Some people believe that the drag coefficient of 0.195 for Xiaomi Su7 was measured at a speed of 160 kilometers per hour, rather than the usual 120 kilometers per hour used by car companies, which actually reveals the engineering difficulty of the drag coefficient. In theory, this dimensionless parameter is a constant that is not related to velocity, but only to the shape and surface material.

However, in reality, the influencing factors to be considered in engineering problems are more complex than the public perceives. Apart from the shape (all the conditions tested in the live broadcast on the 9th essentially involve changing the shape), the most significant factor affecting wind resistance is actually the Reynolds number (Re).

The meaning of Reynolds number is the ratio of inertial force to viscous force. Only by setting aside Newtonian mechanics can we glimpse this chaotic world. When the Reynolds number is small (less than 1), viscous forces dominate and the fluid flow is stable, known as "laminar flow". You can imagine a fluid flowing in layers. When the Reynolds number is large (greater than 10 ^ 3), inertial forces begin to dominate, the flow becomes unstable, and it is easy to form "turbulence", causing the fluid to vortex forward. Of course, when it is centered (1-10 ^ 3), the inertial force and viscous force are close.

For the relative motion of cars in air and fluid, high-speed driving (over 80 kilometers per hour), although the Reynolds number is high (above 10 ^ 7), means that turbulence has already become the dominant factor. The adhesion of the boundary layer (air near the car skin) increases, the pressure difference resistance decreases, and the wind resistance (not the wind resistance coefficient) suddenly decreases. For example, golf balls use surface pits to create turbulence and reduce wind resistance.

As the speed increased from 120 kilometers per hour to 160 kilometers per hour, the thickness of the boundary layer decreased, and the area affected by air viscosity became smaller, reducing the energy loss caused by viscosity. This is the real reason why the increase in speed leads to a decrease in drag coefficient. Unfortunately, the impact of frictional resistance is very small, only a few percent, far less than the sudden increase in resistance (air resistance is proportional to the square of velocity).

What should I do if I want to measure a better drag coefficient under a fixed shape? The Reynolds number formula tells us that efforts should be made to change boundary conditions, such as increasing fluid density, reducing temperature, and increasing pressure. This means making the surface bumpy and testing as much as possible in low altitude areas and winter.

From an engineering perspective, if the drag coefficient is reduced to a certain extent, the cost of further pressure reduction (such as lowering the height of the vehicle interior space) is very high, requiring a lot of simulation and testing, which is far less than a little trick played during testing. Moreover, these tricks are more clever than those played by that blogger. Belonging to a part of controlling variables, not cheating (changing wheel hubs, swing angles, etc.).

Returning from technology to public opinion, it's okay for onlookers to watch, but it's not advisable to get too immersed. If you're serious, you'll lose. Technology should not be overly entertainment oriented. Returning to the technology itself, manufacturers must invest their energy and costs in directions that can improve product quality.