Search results for \"ohm crisis\". Crisis of resistance The freedom we choose

I=E/R,

I = E / (r + R1).

Resistance Crisis

Resistance Crisis

a decrease in the ball resistance with an increase in the speed of the oncoming flow at Reynolds numbers Re close to the critical value Re. (Resistance crisis) 1.5 * 105. The phenomenon was established in 1912 by A. G. Eiffel and explained in 1914 by L. Prandtl. Since it contradicts the well-known fact that the resistance of a body increases in proportion to the square of the speed, it is also called the Eiffel-Prandtl paradox.
At Re, the laminar boundary layer, which breaks off in the vicinity of the midsection, while the separation zone covers the entire aft part of the ball, which causes significant pressure resistance.
At Re > Re*, the laminar flow regime in the vicinity of the midsection is replaced by turbulent one; compared to laminar, it has a more filled velocity profile and can withstand large positive pressure gradients. As a result, the point 5 of separation of the boundary layer is shifted downstream, the transverse dimensions of the stagnant zone are reduced, and, although this slightly increases, the full ball decreases due to a significant decrease in pressure resistance.
Prandtl confirmed his explanation by the results of an experimental study of the flow around two balls, one of which had a smooth surface, and a thin wire ring was installed on the frontal surface of the other to artificially turbulize the flow. The installation of the ring (turbulator) led to a shift of the flow separation point downstream from the section (φ) ≈ 80(°) with a laminar boundary layer to the section (φ) ≈ 100-120(°) and a decrease in the total resistance of the ball.
K. s. it also occurs when other poorly streamlined bodies with a smooth contour move at subsonic speeds: a circular cylinder, ellipsoids, etc. For well-streamlined bodies (airfoils, etc.), it is practically not observed.

Aviation: Encyclopedia. - M.: Great Russian Encyclopedia. Chief editor G.P. Svishchev. 1994 .

See what the "Crisis of Resistance" is in other dictionaries:

resistance crisis Encyclopedia "Aviation"

resistance crisis Encyclopedia "Aviation"

resistance crisis- Pressure coefficient distribution. drag crisis decrease in ball drag with increasing freestream velocity at Reynolds numbers Re close to the critical value Re* 1.5 105. The phenomenon was established at 19… Encyclopedia "Aviation"

resistance crisis- Pressure coefficient distribution. drag crisis decrease in ball drag with increasing freestream velocity at Reynolds numbers Re close to the critical value Re* 1.5 105. The phenomenon was set to 1... Encyclopedia "Aviation"

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Books

• by Romano Luperini. An autobiographical novel by a modern Italian writer about the life of an intellectual whose personal drama is superimposed on acute historical and social cataclysms. Historical…

Georg Simon Ohm was born into a Protestant family, Johann Wolfgang Ohm and Maria Elisabeth Beck. His father was a plumber and his mother was the daughter of a tailor. The parents did not have an academic education, but this did not prevent the father from self-education. Johann, based on the knowledge he received, independently set about educating his own children. George had a younger brother, Martin, who later became a famous mathematician, and a sister, Elizabeth Barbara. George, along with his brother Martin, by their efforts reached such heights in mathematics, physics, chemistry and philosophy that there was no longer any need for an academic education for boys. However, at the age of 11, Georg enters the Erlangen Gymnasium, where he will study until the age of fifteen. But this stage of learning was not to the boy's liking, consisting, in his own words, only in the development of mechanical memory and the interpretation of texts. The level of education of the Om brothers was so high that Carl Christian von Langsdorf, a professor at the University of Erlangen, compared the boys with the Bernoulli family.

In 1805 Georg Ohm entered the University of Erlagen. Instead of focusing on his studies, he devotes all his time to extracurricular activities. Johann, who noticed that his son was wasting precious years and missing the opportunity to receive a decent education, sent his son to Switzerland in 1806. There, in the town of Gottstadt in the Nidau ​​district, Georg becomes a school mathematics teacher. In 1809, Karl Christian von Langsdorff left his post at the University of Erlangen and moved to the University of Heidelberg. Om also wanted to follow him, but he, having dissuaded the future scientist, advised instead to take up the study of the works of Euler, Laplace and Lacroix. In March 1809, Om leaves his teaching post and moves to Neuchâtel, where he gives private lessons. He devotes his free time to independent study of mathematics. This continues for two whole years, until April 1811, after which Ohm returns to the University of Erlangen.

Teaching activity

Georg Ohm achieved such heights in his private teaching practice that he was able to prepare for the defense of his doctoral degree on his own. On October 25, 1811, at the University of Erlangen, Ohm received the degree of Doctor of Philosophy. Immediately after that, he becomes a lecturer at the university department of mathematics. But he will stay there for only three months, and then, realizing the absence of any prospect, he will leave the university. Om lived in extreme poverty, and the meager salary of a lecturer could not improve his plight. In 1813, responding to the offer of the Bavarian authorities, Ohm became a teacher of mathematics and physics in Bamberg. But, being dissatisfied with this position, George, in order to at least somehow prove himself, starts writing a textbook for the initial geometry course. In 1816, the school was closed, and Om moved to another overcrowded school, all in the same Bamberg.

The following year, in September 1817, Ohm was offered the post of teacher of mathematics and physics at the Jesuit Gymnasium in Cologne. It was impossible to miss such a chance, since this gymnasium was not only better than all the educational institutions in which he taught before, but also had a well-equipped laboratory. Throughout his teaching career, Om never abandoned his self-education, studying the works of French mathematicians: Lagrange, Legendre, Laplace, Biot and Poisson. Later, Ohm will get acquainted with the work of Fourier and Fresnel. And at the same time, having learned about Oersted's theoretical substantiation of the phenomenon of electromagnetism in 1820, George begins to make his own experiments in the school physics laboratory. He does this solely to raise his own level of knowledge. Om is also aware that if he wants to get a job that is really interesting, he will have to work hard on research materials. After all, only relying on something, he could show himself to the world and achieve what he wanted.

Ohm's research

In 1825, Ohm presents an article to the scientific community in which he establishes that the electromagnetic force in a conductor decreases as the length of this conductor increases. The article is based solely on evidence obtained empirically during our own experiments. Two more articles will appear this year. In one of them, the scientist gives a mathematical justification for conductivity in the circuit of an electric circuit, based on the Fourier theory of thermal conductivity. The second article was of extreme importance, since in it Ohm gave an explanation of the results of experiments carried out by other scientists with galvanic current. This very article was the forerunner of what today we call "Ohm's law", published the very next year. In 1827, Ohm published his well-known work "Galvanic circuits, mathematical justification", in which he gives a detailed explanation of the theory of electrical circuits. The book is also valuable in that, instead of proceeding directly to the object of study, Ohm first gives a mathematical confirmation of the theory, which is necessary for further understanding of the subject. This became a very important point, since even the most prominent German physicists needed such an introduction, because this book was that rare case in those days when the approach to physics was directly physical, and not mathematical. According to Ohm's theory, interactions in an electrical circuit occur between "equally charged particles." And, finally, this work clearly illustrated the differences between Ohm's scientific approach and the works of Fourier and Navier.

Later years

In 1826, the Cologne Jesuit Gymnasium granted Ohm leave with half of his salary to continue his scientific research, but, in September 1827, the scientist was forced to resume his teaching duties. During the whole year spent in Berlin, he sincerely believed that his scientific publication would help him get a worthy place in some famous university. However, when this did not happen, he reluctantly returns to his former place of work. But the worst thing in the whole history was that, despite the importance of his work, the scientific world received it more than coolly. Insulted, Om decides to move to Berlin. And in March 1828, he officially leaves his post at the Cologne Jesuit Gymnasium and takes a temporary job as a mathematics teacher in various schools in Berlin. In 1833, the scientist accepts an offer to take a professorship in Nuremberg. But, even having received the coveted position, Om remains dissatisfied. The persistent and hard work of the scientist was finally rewarded in 1842, when he received the Copley medal of the British Royal Society. The very next year he was elected a foreign member of the society. In 1845 Om became a full member of the Bavarian Academy. Four years later, he holds the position of curator of the Physics Museum at the Bavarian Academy in Munich and lectures at the University of Munich. Only in 1852 did Om receive the position he had been striving for all his life: he was appointed head of the department of physics at the University of Munich.

Death and legacy

George Ohm's heart stopped in Munich in 1854. He was buried in the Old South Cemetery in Munich. Little is known about the cause of his death. The name of this scientist entered the terminology of electricity in the name "Ohm's law." In addition, the unit of measurement of resistance in the International System of Units (SI), denoted by the Greek letter "Ω", bears his name.

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The dot-com crisis is an economic bubble and a period of stock market speculation and the rapid development of the Internet in 1997-2001, accompanied by a rapid increase in the use of the latter by business and consumers. Then there were many network companies, a significant part of which failed. The bankruptcy of startups such as Go.com, Webvan, Pets.com, E-toys.com and Kozmo.com cost investors $2.4 billion. Others, like Cisco and Qualcomm, lost a large share of their market capitalization but recovered and surpassed their peaks of that period.

The dot-com bubble: how was it?

The second half of the 1990s was marked by the explosion of a new type of economy, in which stock markets, under the influence of venture capital and IPO-funded companies in the Internet sector and related fields, experienced high growth rates. The name “dotcom” that characterized many of them refers to commercial websites. It was born as a term for companies with Internet domain names ending in .com. The large volumes of stock exchange transactions were fueled by the fact that this was a new industry with high potential and the complexity of assessing market participants. They were caused by the high demand for shares in this sector from investors looking for new investment objects, which also led to the revaluation of many companies in this industry. At its peak, even those enterprises that were not profitable became participants in the stock exchange and were extremely highly quoted, given that their performance in most cases was extremely negative.

Back in 1996, Alan Greenspan, then chairman of the Fed, warned against "irrational exuberance" when prudent capital investment was replaced by impulsive investment. 2000, the Nasdaq tech stock index peaked at over 5,000 points, the day after the tech stock fire sell-off marked the end of the "new economy" growth.

Irrational investment

The invention of the Internet led to one of the biggest economic shocks in history. The World Wide Web of Computers dates back to early research efforts in the 1960s, but it was not until the creation of the World Wide Web in the 1990s that it began to be widely distributed and commercialized.

Once investors and speculators realized that the Internet had created an entirely new and untapped international market, IPOs from Internet companies quickly followed each other.

One of the features of the dot-com crisis is that sometimes the valuation of these enterprises was based only on the concept outlined on one piece of paper. The excitement about the commercial possibilities of the Internet was so great that every idea that seemed viable could easily receive millions of dollars of funding.

The basic tenets of investment theory regarding understanding when a business will turn a profit, if at all, have in many cases been ignored because investors feared missing out on the next big hit. They were willing to invest large sums in companies that did not have a clear business plan. This was rationalized by the so-called. dot-com theory: for an Internet enterprise to survive and grow, a rapid expansion of the customer base was required, which in most cases meant huge initial costs. The truth of this statement is proven by Google and Amazon, two extremely successful companies that took several years to show any profit.

Irrational expenses

Many of the new companies spent the money they received thoughtlessly. Stock options made employees and executives millionaires on the day of the IPO, and businesses themselves often spent money on luxury business facilities, because the credibility of the "new economy" was extremely high. In 1999, there were 457 initial public offerings in the US, most of which were organized by Internet and technology companies. Of these, 117 managed to double their value during the first day of trading.

Communications companies such as mobile network operators and ISPs began investing heavily in network infrastructure as they wanted to be able to grow with the needs of the new economy. Huge loans were required to be able to invest in new network technologies and acquire wireless network licenses, which also contributed to the onset of the dot-com crisis.

How .com companies became dot-bombs

In 2000, the Nasdaq Composite, an index of technology stocks traded on Wall Street, peaked at 5,046.86, doubling its value a year earlier. The next day, stock prices began to fall and the dot-com bubble burst. One of the direct reasons for this was the completion of the antitrust case against Microsoft, which in April 2000 was declared a monopoly. The market expected this, and in the 10 days after March 10, the Nasdaq index lost 10%. The day after the release of the official results of the investigation, the technology index experienced a large intraday drop, but returned back. However, this was not a sign of recovery. The Nasdaq began a free fall when investors realized that many of the money-losing new companies really were. Within a year of the dot-com crisis hit, most of the venture capital firms that backed Internet startups lost all their money and went bankrupt when new funding dried up. Some investors have begun to call the once-stellar companies “dot bombs” as they managed to destroy billions of dollars in a very short time.

On October 9, 2002, the Nasdaq hit a low of 1114.11. It was a whopping 78% loss of the index from its peak 2.5 years prior. In addition to many tech start-ups, many communications companies also ran into trouble as they had to pay back the billions in loans they had taken out to invest in network infrastructure, the payback of which was now suddenly delayed much longer than anticipated.

History of Napster

In terms of legal issues, Microsoft wasn't the only dot-com to face trial. Another well-known technology company of that era was founded in 1999 and was called Napster. She was developing an application that shared digital music on a p2p network. Napster was founded by 20-year-old Sean Parker and two of his friends, and the company quickly gained popularity. But due to copyright infringement, it almost immediately came under fire from the music industry and eventually ceased to exist.

hacker multimillionaire

Kim Schmitz perhaps best illustrates the actions of sole proprietors in dealing with the dot-com crisis. This German hacker went on to become a multimillionaire starting various Internet companies in the 1990s and eventually changed his last name to Dotcom, hinting at what made him rich. In early 2000, just before the collapse of the new economy, he sold TÜV Rheinland 80% of his shares in DataProtect, which he founded, which provided data protection services. The company went bankrupt less than a year later. In the 1990s, he was the centerpiece of a series of convictions for insider trading and embezzlement related to his technology ventures.

In 1999, he had a customized Mercedes-Benz that, among many other electronic gadgets, had a high-speed wireless Internet connection that was unique at the time. With this car, he participated in the European Gumball Rally. when many people in expensive cars compete on public roads. When Kimble (his nickname at the time) had a flat tire, a new tire was delivered to him by jet plane from Germany.

He survived the aftermath of the dot-com crash and continued to launch new startups. In 2012, he was arrested again on charges that he illegally distributed copyrighted content through his company Mega. He currently lives in New Zealand in his $30 million home and is awaiting extradition to the US.

Have investors learned their lesson?

Some companies that were launched during the dot-com bubble have survived to become tech giants like Google and Amazon. However, most failed. Some risk-taking entrepreneurs were active in the industry and eventually created new companies, such as the aforementioned Kim Schmitz and Napster's Sean Parker, who became Facebook's founding president.

After the dot-com crisis, investors became wary of investing in risky ventures and returned to evaluating realistic plans. However, there have been a number of high-profile IPOs in recent years. When LinkedIn, the social network for professionals, went public on May 19, 2011, its stock instantly more than doubled, reminiscent of what happened in 1999. The company itself warned investors not to be too optimistic. Today, IPOs are carried out by companies that have been in business for several years and have good prospects for profit, if not already profitable. Another IPO, held in 2012, was expected for many years. Facebook's IPO was the largest among tech companies and set a record for $16 billion in trading volume and capital raised.

Finally

The dot-com bubble of the 1990s and early 2000s was characterized by new technology that created a new market with many potential products and services, and highly opportunistic investors and entrepreneurs blinded by early successes. Since the crash, companies and markets have become much more cautious when it comes to investing in new technologies. However, the current popularity of mobile devices such as smartphones and tablets, their near-limitless possibilities, and several successful IPOs are opening the door to a generation of companies looking to capitalize on this new market. The question is, will investors and entrepreneurs be wiser this time around not to create a second dot-com bubble?