在不久的未来，也许除了“头灯白”与“尾灯红”之外，夜间的高速公路的车流中可能还将迎来一种新的色彩：“等离子紫”。据了解，这种新色彩将来自美国一家创业公司带来的新型节能设备：“等离子体激光器”。该系统可以通过发射等离子体平顺大型卡车的尾部湍流。计算机模拟与风洞试验结果均表明，这种主动气流控制系统可以控制卡车周围的风力流动，从而将其风阻降低近五分之一，也就是说可以协助卡车实现最高10%燃料经济性提升。

等离子体，也就是鲜为人知的“物质第四态”，是由空气中的高压电场产生的带电原子离子云。这种电火焰现象常见于一些家用空气净化器中，另外圣艾尔摩之火等自然现象以及有时挂在飞机翅膀上幽灵般的紫色火焰也是这回事。当卡车安装了这款等离子体激光器时，系统表面电极产生的强电场可向周围发射等离子流，引导拖挂货车等大型8级卡车周边的气体流动，从而平顺湍流气流，减少尾流阻力。

研究人员发现，卡车大约65%的燃料消耗均用在了克服行车阻力上，而美国的卡车数量非常庞大，全国1.33亿辆卡车每年大约将消耗600亿加仑（2,270亿升）燃料。因此，哪怕仅有一小部分卡车采用了这种创新的气流技术，全美卡车车队的整体节油成果也将非常显著。

等离子体激光器

Plasma Stream Technologies（等离子体流技术）公司是一家位于爱荷华州Bettendorf市的小型创业公司。这家公司的终极目标是打造一系列适用于卡车的“介质阻挡等离子体激光器”。公司联合创始人兼投资人Pranay Bajjuri表示，截止目前，我们的概念已经通过了超级计算机模拟和风洞测试的验证，预计真实道路工况测试将安排在今年3月进行。他表示，如果一切顺利，公司将继续推进该概念的商业化，乐观情况下，最终成品将于2018年初投入市场。

Bajjuri的本职工作是在一家重型设备制造商担任电气工程师。他介绍说，几年前，他和几位搭档在寻找技术的商业应用机会时，发现了等离子体激光器在优化交通运输设备空气动力学性能方面的潜力，比如飞机机翼、风机叶片，甚至涡轮发动机的压缩机叶片等。他们还了解到，通用汽车（General Motors）等厂商也一直在研究如何利用基于等离子体的主动气流控制设备，优化公路车辆的空气动力学性能，而且已经在实验室内取得了不错的成果。

“等离子体激光器的出现已经大约有15年了，但却没有什么相关商业化产品。”Bajjuri说，“我们希望成为首家将这种技术推入大规模商业市场的公司。”

Plasma Stream Technologies公司的团队意识到，这些等离子体激光器能够替代很长时间以来一些卡车公司为了降低风阻而采用的被动气流控制设备，比如导流板、涡流发生器及锥形车尾等。

结构简单

Plasma Stream Technologies公司推出的这款设备名为“eTail”，其结构非常简单，并没有任何活动部件。基本来说，eTail的主要结构即为两个由Teflon（聚四氟乙烯）绝缘体隔开的铜电极。整个装置的宽度仅为102mm，需要安装在车辆尾部的顶边与两条侧边上。当车速超过72km/h时，eTail的表面电极将激活其上方的空气，产生带电粒子，进而产生“风”。Bajjuri解释道，这种“风”将改变“车辆尾部的高动能湍流，并将其重新引导至车辆后方的低动能区域”，从而最大限度地降低拖曳力损失。

尽管eTail系统的工作电压较高，但由于其对电流的要求不高，因此还是相当安全的。Bajjuri介绍说，“系统所采用的带状电极，平均功率仅为每米1瓦。”他补充说，该系统可在各种天气条件下工作，包括雨、雪、霾等。此外，如果外加一层网格为电极进行除冰，系统甚至还可以在寒冷的冬季中保持正常工作。

Plasma Stream Technologies公司团队领导人Thomas Corke表示，设备原型是基于圣母大学一组空气动力学工程师与研究人员的等离子体激光器专利技术设计而成。“一般来说，地面车辆通常通过改变尾流特性，来平顺车辆尾部的气流。”他介绍说，“效果最佳的状态，是让气流沿水平面向上12度的方向流动。”

eTail系统的流量控制装置采用了一款紧凑的轻量化电源，可通过iPad控制。Corke表示，由于“采用了直流脉冲电源，系统实际采用间隔供电方式，占空比仅为0.001%”，因此整体功耗很低。

在2015年底进行的委托研究中，Corke的团队利用CFD技术模拟了等离子体激光器在降低8级半拖挂卡车风阻方面的性能。模拟中的虚拟卡车模型采用了与之前圣母大学风洞测试中选择的同款卡车，但比例缩小为原尺寸的十二分之一。这里的计算机模拟主要是为了确定车尾坡度的最佳几何形状（即设备的斜率和宽度），以及等离子体激光器的安装位置，从而在最大程度上降低半挂车的风阻。与传统的锥形车尾设计不同，eTail 并不会影响车辆后门的开合与上下货。

Plasma Stream Technologies公司的最终设计方案的角度是11度，最终达成了高速行驶风阻降低22.7%的目标，取得了11%的燃料经济性提升。这款等离子体设备安装在拖车尾部，与传统的锥形车尾设计（安装尺寸为1.4m）相比，系统的整体尺寸较小，宽度仅为102mm。Bajjuri表示，该产品的成本应在2,000美元左右，预计可每年为柴油车型节约8,000美元的油费。

这种系统可安装在半挂车的多个位置，包括卡车与拖车之间的缝隙、牵引车的后视镜及前方格栅区域等，从多个方向优化车辆的空气动力学表现，Plasma Stream Technologies公司认为，该技术可同时适用于公交车、火车和赛车等应用。

“Headlight white” and “stoplight red,” the characteristic colors of the highway at night, could soon get a companion: “plasma purple”—that is, if a new fuel-saving, aerodynamic technology catches on big with big-rig trucks. The electric plasmas emitted by the devices, which glow purple-violet in the dark, can smooth the turbulent wakes that sweep off the rear edges of truck trailers travelling at highway speeds. Simulations and wind tunnel tests indicate that these novel active-flow control systems could reduce the aerodynamic drag generated by semi-trailers by almost one-fifth, thus lowering fuel consumption by up to 10%.

Plasmas, the lesser-known “fourth state of matter,” are clouds of charged atomic ions created in the air by high-voltage electric fields. The glowing electric-flame phenomena are commonly seen in household air purifiers and more rarely in natural phenomena such as St. Elmo’s Fire, the ghostly purple haze sometimes seen hanging off airplane wings. For application to trucks, strong electric fields emitted by surface electrodes push the plasmas around, producing breezes that can redirect critical airflows at the trailing edges of “bluff-bodied” vehicles such as Class 8 cargo trucks, helping to smooth out the turbulent, high-drag wakes.

Somewhere around 65% of the fuel that a truck burns is wasted in overcoming highway drag, researchers say. And with about 133 million American trucks consuming 60 billion gallons of fuel annually, if only a fraction were to adopt this promising new slipstreaming technology, the effect would soon start to become significant.

Plasma actuators

That’s the ultimate target for Plasma Stream Technologies, the small start-up firm based in Bettendorf, Iowa, that is developing the “dielectric-barrier plasma actuator” devices for trucks. Up to now its concept has been tested solely in supercomputer simulations and sub-scale wind tunnels, but real-world confirmation should come from road tests planned for this March, according to Pranay Bajjuri, a cofounder and investor. Favorable results would mean that the company could proceed toward full commercialization in the hope that retrofit products could hit the market in early 2018, he noted.

(Go to https://www.youtube.com/watch?v=8HAa-bI6170 for a company video on the technology and its development.)

Bajjuri, whose day job is as an electrical engineer at a heavy-equipment maker, explained that a few years ago he and his partners had searched for promising technology to commercialize and discovered the potential benefits of plasma actuators in improving aerodynamics in applications such as airplane wings, wind turbine blades, and even compressor blades in turbine engines. They also learned that General Motors and other organizations had investigated using plasma-based active airflow control devices for highway vehicles and had achieved favorable results in the lab.

“Plasma actuators have been around for about 15 years, but there’s still no commercial product yet,” Bajjuri said. “We’re trying to be the first to bring them to the commercial market in volume.”

The Plasma Stream team realized that these plasma actuators could replace the many passive flow-control add-ons that trucking firms have employed over the years to reduce vehicle drag, including air deflectors, vortex generators, and aft boat tails.

Simple electrode strips

The simple retrofit device, called eTail, has no moving parts. It’s basically two long, high-voltage electrode strips made of copper separated by an insulator such as Teflon (polytetrafluoroethylene), he explained. The 4-inch-wide (102-mm) units are fitted to the top and side trailing edges of semi-trailers. When the juice switches on at speeds of 45 mph (72 km/h) and up, the surface electrodes energize the air above, producing charged ions that the field then sends in one direction as wind. The wind alters “the turbulent, high-energy airflow at the edges and directs it to a low-energy region” right behind the vehicle, minimizing drag losses, Bajjuri explained.

Even though the strap-on eTail system runs on high-voltage current, operations are quite safe because it requires low amperages. “The average power of the electrode strips is only about 1 watt per meter,” he stated. He added that the technology works in all weather conditions—rain, snow and dust—and with an added grid overlay, it might even be possible to heat up the electrodes for deicing in winter.

The company’s prototypes are based on licensed plasma actuator technology developed by a group of aerodynamics engineers and researchers at the University of Notre Dame, said Thomas Corke, who leads the team. “For a ground vehicle, the general approach is to shape the wake, to smooth out the airflow coming off the rear,” he said. “The optimum is to turn the airstream about 12 degrees from the horizontal.”

The flow-control devices are powered by a compact, lightweight power supply also of UND design that can be operated with an Apple iPad. Low power is achieved, Corke said, because “the DC power is pulsed, so the device is powered only for brief intervals,” giving it a duty cycle of 0.001%.

In contract research conducted in late 2015 at Notre Dame, Corke’s team ran CFD simulations to investigate the use of plasma actuators to cut drag on Class 8 truck semi-trailers. The virtual truck’s shape resembled a 1/12th-scale model that had been previously evaluated in wind tunnel tests at the university. The computer simulations sought to determine an optimum arrangement of an aft ramp geometry (the slope and width of the device) and plasma actuators that would in combination, yield a drag reduction comparable to the passive boat-tail retrofit devices currently used on semi-trailers. And unlike conventional boat-tail units, the eTail does not impede easy access via roll-up doors and swing doors.

Plasma Stream’s final design, which features an 11-degree angle, attains this goal, achieving a predicted 22.7% drag reduction at highway speeds that would yield an 11% fuel savings. The design requires only a 4-in extension around three sides of a trailer which makes it considerably smaller than boat-tails, which add about 4.5 ft (1.4 m) to the trailer length. The retrofit product is expected to cost around $2,000, Bajjuri said, and could save somewhere around $8,000 per year in diesel fuel.

The new technology can be installed at various locations around a semi-truck to streamline the aerodynamics including near the gap between tractor and trailer, on the rearview mirrors, and in the front grille area of tractors. The company thinks that the technology could also work on buses, locomotives and racecars.

Author: Steven Ashley
Source: SAE Truck & Off-highway Engineering Magazine