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在不久的未來，也許除了“頭燈白”與“尾燈紅”之外，夜間的高速公路的車流中可能還將迎來一種新的色彩：“等離子紫”。據(jù)了解，這種新色彩將來自美國(guó)一家創(chuàng)業(yè)公司帶來的新型節(jié)能設(shè)備：“等離子體激光器”。該系統(tǒng)可以通過發(fā)射等離子體平順大型卡車的尾部湍流。計(jì)算機(jī)模擬與風(fēng)洞試驗(yàn)結(jié)果均表明，這種主動(dòng)氣流控制系統(tǒng)可以控制卡車周圍的風(fēng)力流動(dòng)，從而將其風(fēng)阻降低近五分之一，也就是說可以協(xié)助卡車實(shí)現(xiàn)最高10%燃料經(jīng)濟(jì)性提升。

等離子體，也就是鮮為人知的“物質(zhì)第四態(tài)”，是由空氣中的高壓電場(chǎng)產(chǎn)生的帶電原子離子云。這種電火焰現(xiàn)象常見于一些家用空氣凈化器中，另外圣艾爾摩之火等自然現(xiàn)象以及有時(shí)掛在飛機(jī)翅膀上幽靈般的紫色火焰也是這回事。當(dāng)卡車安裝了這款等離子體激光器時(shí)，系統(tǒng)表面電極產(chǎn)生的強(qiáng)電場(chǎng)可向周圍發(fā)射等離子流，引導(dǎo)拖掛貨車等大型8級(jí)卡車周邊的氣體流動(dòng)，從而平順湍流氣流，減少尾流阻力。

研究人員發(fā)現(xiàn)，卡車大約65%的燃料消耗均用在了克服行車阻力上，而美國(guó)的卡車數(shù)量非常龐大，全國(guó)1.33億輛卡車每年大約將消耗600億加侖（2,270億升）燃料。因此，哪怕僅有一小部分卡車采用了這種創(chuàng)新的氣流技術(shù)，全美卡車車隊(duì)的整體節(jié)油成果也將非常顯著。

等離子體激光器

Plasma Stream Technologies（等離子體流技術(shù)）公司是一家位于愛荷華州Bettendorf市的小型創(chuàng)業(yè)公司。這家公司的終極目標(biāo)是打造一系列適用于卡車的“介質(zhì)阻擋等離子體激光器”。公司聯(lián)合創(chuàng)始人兼投資人Pranay Bajjuri表示，截止目前，我們的概念已經(jīng)通過了超級(jí)計(jì)算機(jī)模擬和風(fēng)洞測(cè)試的驗(yàn)證，預(yù)計(jì)真實(shí)道路工況測(cè)試將安排在今年3月進(jìn)行。他表示，如果一切順利，公司將繼續(xù)推進(jìn)該概念的商業(yè)化，樂觀情況下，最終成品將于2018年初投入市場(chǎng)。

Bajjuri的本職工作是在一家重型設(shè)備制造商擔(dān)任電氣工程師。他介紹說，幾年前，他和幾位搭檔在尋找技術(shù)的商業(yè)應(yīng)用機(jī)會(huì)時(shí)，發(fā)現(xiàn)了等離子體激光器在優(yōu)化交通運(yùn)輸設(shè)備空氣動(dòng)力學(xué)性能方面的潛力，比如飛機(jī)機(jī)翼、風(fēng)機(jī)葉片，甚至渦輪發(fā)動(dòng)機(jī)的壓縮機(jī)葉片等。他們還了解到，通用汽車（General Motors）等廠商也一直在研究如何利用基于等離子體的主動(dòng)氣流控制設(shè)備，優(yōu)化公路車輛的空氣動(dòng)力學(xué)性能，而且已經(jīng)在實(shí)驗(yàn)室內(nèi)取得了不錯(cuò)的成果。

“等離子體激光器的出現(xiàn)已經(jīng)大約有15年了，但卻沒有什么相關(guān)商業(yè)化產(chǎn)品。”Bajjuri說，“我們希望成為首家將這種技術(shù)推入大規(guī)模商業(yè)市場(chǎng)的公司。”

Plasma Stream Technologies公司的團(tuán)隊(duì)意識(shí)到，這些等離子體激光器能夠替代很長(zhǎng)時(shí)間以來一些卡車公司為了降低風(fēng)阻而采用的被動(dòng)氣流控制設(shè)備，比如導(dǎo)流板、渦流發(fā)生器及錐形車尾等。

結(jié)構(gòu)簡(jiǎn)單

Plasma Stream Technologies公司推出的這款設(shè)備名為“eTail”，其結(jié)構(gòu)非常簡(jiǎn)單，并沒有任何活動(dòng)部件。基本來說，eTail的主要結(jié)構(gòu)即為兩個(gè)由Teflon（聚四氟乙烯）絕緣體隔開的銅電極。整個(gè)裝置的寬度僅為102mm，需要安裝在車輛尾部的頂邊與兩條側(cè)邊上。當(dāng)車速超過72km/h時(shí)，eTail的表面電極將激活其上方的空氣，產(chǎn)生帶電粒子，進(jìn)而產(chǎn)生“風(fēng)”。Bajjuri解釋道，這種“風(fēng)”將改變“車輛尾部的高動(dòng)能湍流，并將其重新引導(dǎo)至車輛后方的低動(dòng)能區(qū)域”，從而最大限度地降低拖曳力損失。

盡管eTail系統(tǒng)的工作電壓較高，但由于其對(duì)電流的要求不高，因此還是相當(dāng)安全的。Bajjuri介紹說，“系統(tǒng)所采用的帶狀電極，平均功率僅為每米1瓦。”他補(bǔ)充說，該系統(tǒng)可在各種天氣條件下工作，包括雨、雪、霾等。此外，如果外加一層網(wǎng)格為電極進(jìn)行除冰，系統(tǒng)甚至還可以在寒冷的冬季中保持正常工作。

Plasma Stream Technologies公司團(tuán)隊(duì)領(lǐng)導(dǎo)人Thomas Corke表示，設(shè)備原型是基于圣母大學(xué)一組空氣動(dòng)力學(xué)工程師與研究人員的等離子體激光器專利技術(shù)設(shè)計(jì)而成。“一般來說，地面車輛通常通過改變尾流特性，來平順車輛尾部的氣流。”他介紹說，“效果最佳的狀態(tài)，是讓氣流沿水平面向上12度的方向流動(dòng)。”

eTail系統(tǒng)的流量控制裝置采用了一款緊湊的輕量化電源，可通過iPad控制。Corke表示，由于“采用了直流脈沖電源，系統(tǒng)實(shí)際采用間隔供電方式，占空比僅為0.001%”，因此整體功耗很低。

在2015年底進(jìn)行的委托研究中，Corke的團(tuán)隊(duì)利用CFD技術(shù)模擬了等離子體激光器在降低8級(jí)半拖掛卡車風(fēng)阻方面的性能。模擬中的虛擬卡車模型采用了與之前圣母大學(xué)風(fēng)洞測(cè)試中選擇的同款卡車，但比例縮小為原尺寸的十二分之一。這里的計(jì)算機(jī)模擬主要是為了確定車尾坡度的最佳幾何形狀（即設(shè)備的斜率和寬度），以及等離子體激光器的安裝位置，從而在最大程度上降低半掛車的風(fēng)阻。與傳統(tǒng)的錐形車尾設(shè)計(jì)不同，eTail 并不會(huì)影響車輛后門的開合與上下貨。

Plasma Stream Technologies公司的最終設(shè)計(jì)方案的角度是11度，最終達(dá)成了高速行駛風(fēng)阻降低22.7%的目標(biāo)，取得了11%的燃料經(jīng)濟(jì)性提升。這款等離子體設(shè)備安裝在拖車尾部，與傳統(tǒng)的錐形車尾設(shè)計(jì)（安裝尺寸為1.4m）相比，系統(tǒng)的整體尺寸較小，寬度僅為102mm。Bajjuri表示，該產(chǎn)品的成本應(yīng)在2,000美元左右，預(yù)計(jì)可每年為柴油車型節(jié)約8,000美元的油費(fèi)。

這種系統(tǒng)可安裝在半掛車的多個(gè)位置，包括卡車與拖車之間的縫隙、牽引車的后視鏡及前方格柵區(qū)域等，從多個(gè)方向優(yōu)化車輛的空氣動(dòng)力學(xué)表現(xiàn)，Plasma Stream Technologies公司認(rèn)為，該技術(shù)可同時(shí)適用于公交車、火車和賽車等應(yīng)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