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Both Boeing and Airbus have trumpeted the efficiency of their newest aircraft, the 787 and A350 respectively. Their clever designs and lightweight composites certainly make a difference. But a group of researchers at Stanford University, led by Ilan Kroo, has suggested that airlines could take a more naturalistic approach to cutting jet-fuel use, and it would not require them to buy new aircraft.The answer, says Dr. Kroo, lies with birds. Since 1914, and a seminal paper by a German researcher called Carl Wieselsberger, scientists have known that birds flying in formation—a V-shape, echelon or otherwise—expend less energy. The air flowing over a bird’s wings curls upwards behind the wingtips, a phenomenon known as upwash. Other birds flying in the upwash experience reduced drag, and spend less energy propelling themselves. Peter Lissaman, an aeronautics expert who was formerly at Caltech and the University of Southern California, has suggested that a formation of 25 birds might enjoy a range increase of 71%.When applied to aircraft, the principles are not substantially different. Dr. Kroo and his team modeled what would happen if three passenger jets departing from Los Angeles, San Francisco and Las Vegas were to rendezvous over Utah, assume an inverted V-formation, occasionally swap places so all could have a turn in the most favourable positions, and proceed to London. They found that the aircraft consumed as much as 15% less fuel (with a concomitant reduction in carbon-dioxide output). Nitrogen-oxide emissions during the cruising portions of the flight fell by around a quarter.There are, of course, kinks to be worked out. One consideration is safety, or at least the perception of it. Would passengers feel comfortable travelling in convoy’ Dr. Kroo points out that the aircraft could he separated by several nautical miles, and would not be in the unnervingly easy groupings favoured by display teams like the Red Arrows. A passenger peering out of the window might not even see the other planes. Whether the separation distances involved would satisfy air-traffic-control regulations is another matter, although a working group at the International Civil Aviation Organization has included the possibility of formation flying in a blueprint for new operational guidelines.It remains to be seen how weather conditions affect the air flows that make formation flight more efficient. In zones of increased turbulence, the planes’ wakes will decay more quickly and the effect will diminish. Dr. Kmo says this is one of the areas his team will investigate further. It might also he hard for airlines to coordinate the departure times and destinations of passenger aircraft in a way that would allow them to gain from formation flight. Cargo aircraft, in contrast, might be easier to reschedule, as might routine military flights.As it happens, American’s armed forces are on the ease already. Earlier this year the country’s Defense Advanced Research Projects Agency announced plans to pay Boeing to investigate formation flight, though the programme has yet to begin. There are reports that some military aircraft flew in formation when they were low on fuel during the Second World War, hut Dr. Lissaman says they are apocryphal. "My father was an RAF pilot and my cousin the skipper of a Lancaster lost over Berlin," he adds. So he should know. Which of the following best defines the word "kinks" ( Line 1, Paragraph 4)（）
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并发使得处理机的利用率得到提高，其主要原因是处理机与IO可以同时为多个进程服务，也即处理机与IO设备真正地并行。但是处理机的利用率提高并不是简单地将二个进程的处理机利用率相加，而是遵循一定的规律。现在有一个计算机系统采用多道程序技术实现了并发，调度算法采用时间片轮转，时间片很小可以不计，忽略系统的开销，请分析以下问题： 假设每个进程的处理机的利用率为u1=20%。 假设为这个系统增加了16MB内存，系统有足够的并发度，此时处理机的利用率最大为多少系统的吞吐量比(2)增加了多少
During the past generation, the American middle-class family that once could count on hard work and fair pay to keep itself financially secure has been transformed by economic risk and new realities. Now a pink slip, a bad diagnosis, or a disappearing spouse can reduce a family from solidly middle class to newly poor in a few months.In just one generation, millions of mothers have gone to work, transforming basic family economics. Scholars, policymakers, and critics of all stripes have debated the social implications of these changes, but few have looked at the side effect: family risk has risen as well. Today’s families have budgeted to the limits of their new two-paycheck status. As a result, they have lost the parachute they once has in times of financial setback-- a back-up earner (usually Mom) who could go into the workforce if the primary earner got laid off or fell sick. This "added-worker effect" could support the safety net offered by unemployment insurance or disability insurance to help families weather bad times. But today, a disruption to family fortunes can no longer be made up with extra income from an otherwise-stay-at-home partner.During the same period, families have been asked to absorb much more risk in their retirement income. Steelworkers, airline employees, and now those in the auto industry are joining millions of families who must worry about interest rates, stock market fluctuation, and the harsh reality that they may outlive their retirement money. For much of the past year, President Bush campaigned to move Social Security to a savings-account model, with retirees trading much or all of their guaranteed payments for payments depending on investment returns. For younger families, the picture is not any better. Both the absolute cost of healthcare and the share of it borne by families have risen--and newly fashionable health-savings plans are spreading from legislative halls to Wal-Mart workers, with much higher deductibles and a large new dose of investment risk for families’ future healthcare. Even demographics are working against the middle class family, as the odds of having a weak elderly parent-- and all he attendant need for physical and financial assistance -- have jumped eightfold in just one generation.From the middle-class family perspective, much of this, understandably, looks far less like an opportunity to exercise more financial responsibility, and a good deal more like a frightening acceleration of the wholesale shift of financial risk onto their already overburdened shoulders. The financial fallout has begun, and the political fallout may not be far behind. Which of the following is the best title for this text（）
Some countries are more populous; some have more crime. But in no other country are crime fighters quite so knowledgeable about citizens as in Britain. On January 4th a boastful Home Office detailed the triumphs of the world’s biggest forensic DNA database, which holds samples from more than 5% of the entire population of England and Wales. Recent changes to the rules governing the database mean that it may eventually hold profiles from more than a fifth of all adults. Once a country starts storing DNA samples from criminals it is hard to resist the urge to expand the collection. When the National DNA Database (NDNAD) was set up in 1995, samples could only be taken from those charged with "recordable" offences. If a suspect was not tried, or was freed, the sample had to be destroyed and the profile removed from the database. That law was abandoned in 2001 , after two men who had been convicted of murder and rape had their cases overturned on appeal -the DNA evidence against them related to crimes they had not been convicted of, and so ought to have been removed from the database. The change has led to the retention of around 200,000 samples that would previously have been destroyed. Some 7,591 of these were subsequently matched with samples from crime scenes, including those from 88 murders and 116 rapes. And since April 2004, police have been able to take and keep samples from anyone arrested for a recordable offence, even if charges do not ensue. The main reason the NDNAD is larger than databases in other countries is that Britain was first to start using DNA as an investigative tool. So not only has it had time to collect more DNA samples, but it has also had longer to appreciate the sheer power of a large database. "Every other country that does data basing will get to where Britain is now," says Chris Asplen, a consultant to law enforcement agencies and governments on DNA technology. The increased use of DNA evidence has given rise to intriguing new courtroom defences. DNA tests are now so sensitive that they can detect if a person has sneezed or sweated near an object. John Swain, a barrister with a background in biochemistry, recently defended a man charged with armed robbery. The defendant’s DNA was on the gun that was used, but the defences argued that he might just have been near it after he had been to the gym, and that an errant bead of sweat could account for the presence of his DNA on a weapon he had never handled. He was declared not guilty. The text mainly tells us that
During the past generation, the American middle-class family that once could count on hard work and fair pay to keep itself financially secure has been transformed by economic risk and new realities. Now a pink slip, a bad diagnosis, or a disappearing spouse can reduce a family from solidly middle class to newly poor in a few months.In just one generation, millions of mothers have gone to work, transforming basic family economics. Scholars, policymakers, and critics of all stripes have debated the social implications of these changes, but few have looked at the side effect: family risk has risen as well. Today’s families have budgeted to the limits of their new two-paycheck status. As a result, they have lost the parachute they once has in times of financial setback-- a back-up earner (usually Mom) who could go into the workforce if the primary earner got laid off or fell sick. This "added-worker effect" could support the safety net offered by unemployment insurance or disability insurance to help families weather bad times. But today, a disruption to family fortunes can no longer be made up with extra income from an otherwise-stay-at-home partner.During the same period, families have been asked to absorb much more risk in their retirement income. Steelworkers, airline employees, and now those in the auto industry are joining millions of families who must worry about interest rates, stock market fluctuation, and the harsh reality that they may outlive their retirement money. For much of the past year, President Bush campaigned to move Social Security to a savings-account model, with retirees trading much or all of their guaranteed payments for payments depending on investment returns. For younger families, the picture is not any better. Both the absolute cost of healthcare and the share of it borne by families have risen--and newly fashionable health-savings plans are spreading from legislative halls to Wal-Mart workers, with much higher deductibles and a large new dose of investment risk for families’ future healthcare. Even demographics are working against the middle class family, as the odds of having a weak elderly parent-- and all he attendant need for physical and financial assistance -- have jumped eightfold in just one generation.From the middle-class family perspective, much of this, understandably, looks far less like an opportunity to exercise more financial responsibility, and a good deal more like a frightening acceleration of the wholesale shift of financial risk onto their already overburdened shoulders. The financial fallout has begun, and the political fallout may not be far behind. According to the author, health-savings plans will（）
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