A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Sir Charles Parsons in 1884; because the turbine generates rotary motion, it is suited to be used to drive an electrical generator—about 85% of all electricity generation in the United States in the year 2014 was by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency from the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible expansion process; the first device that may be classified as a reaction steam turbine was little more than a toy, the classic Aeolipile, described in the 1st century by Hero of Alexandria in Roman Egypt. In 1551, Taqi al-Din in Ottoman Egypt described a steam turbine with the practical application of rotating a spit. Steam turbines were described by the Italian Giovanni Branca and John Wilkins in England.
The devices described by Taqi al-Din and Wilkins are today known as steam jacks. In 1672 an impulse steam turbine driven car was designed by Ferdinand Verbiest. A more modern version of this car was produced some time in the late 18th century by an unknown German mechanic. In 1775 at Soho James Watt designed a reaction turbine, put to work there. In 1827 the Frenchmen Real and Pichon constructed a compound impulse turbine; the modern steam turbine was invented in 1884 by Sir Charles Parsons, whose first model was connected to a dynamo that generated 7.5 kW of electricity. The invention of Parsons' steam turbine made cheap and plentiful electricity possible and revolutionized marine transport and naval warfare. Parsons' design was a reaction type, his patent was the turbine scaled-up shortly after by an American, George Westinghouse. The Parsons turbine turned out to be easy to scale up. Parsons had the satisfaction of seeing his invention adopted for all major world power stations, the size of generators had increased from his first 7.5 kW set up to units of 50,000 kW capacity.
Within Parson's lifetime, the generating capacity of a unit was scaled up by about 10,000 times, the total output from turbo-generators constructed by his firm C. A. Parsons and Company and by their licensees, for land purposes alone, had exceeded thirty million horse-power. A number of other variations of turbines have been developed that work with steam; the de Laval turbine accelerated the steam to full speed before running it against a turbine blade. De Laval's impulse turbine does not need to be pressure-proof, it can operate with any pressure of steam, but is less efficient. Auguste Rateau developed a pressure compounded impulse turbine using the de Laval principle as early as 1896, obtained a US patent in 1903, applied the turbine to a French torpedo boat in 1904, he taught at the École des mines de Saint-Étienne for a decade until 1897, founded a successful company, incorporated into the Alstom firm after his death. One of the founders of the modern theory of steam and gas turbines was Aurel Stodola, a Slovak physicist and engineer and professor at the Swiss Polytechnical Institute in Zurich.
His work Die Dampfturbinen und ihre Aussichten als Wärmekraftmaschinen was published in Berlin in 1903. A further book Dampf und Gas-Turbinen was published in 1922; the Brown-Curtis turbine, an impulse type, developed and patented by the U. S. company International Curtis Marine Turbine Company, was developed in the 1900s in conjunction with John Brown & Company. It was used in John Brown-engined merchant ships and warships, including liners and Royal Navy warships; the present-day manufacturing industry for steam turbines is dominated by Chinese power equipment makers. Harbin Electric, Shanghai Electric, Dongfang Electric, the top three power equipment makers in China, collectively hold a majority stake in the worldwide market share for steam turbines in 2009-10 according to Platts. Other manufacturers with minor market share include Bharat Heavy Electricals Limited, Alstom, General Electric, Doosan Škoda Power, Mitsubishi Heavy Industries, Toshiba; the consulting firm Frost & Sullivan projects that manufacturing of steam turbines will become more consolidated by 2020 as Chinese power manufacturers win increasing business outside of China.
Steam turbines are made in a variety of sizes ranging from small <0.75 kW units used as mechanical drives for pumps and other shaft driven equipment, to 1.5 GW turbines used to generate electricity. There are several classifications for modern steam turbines. Turbine blades are of two basic types and nozzles. Blades move due to the impact of steam on them and their profiles do not converge; this results in a steam velocity drop and no pressure drop as steam moves through the blades. A turbine composed of blades alternating with fixed nozzles is called an impulse turbine, Curtis turbine, Rateau turbine, or Brown-Curtis turbine. Nozzles appear similar to blades; this results in a steam pressure velocity increase as steam moves through the nozzles. Nozzles move due to both the impact of steam on them and the reaction due to the high-velocity steam at the exit. A turbine composed of moving nozzles alternating with fixed nozzles is called a reaction turbine or Parsons turbine. Except for low-power applications, turbine blades are arranged in multiple stages in series, called c
A wind turbine, or alternatively referred to as a wind energy converter, is a device that converts the wind's kinetic energy into electrical energy. Wind turbines are manufactured in a wide range of horizontal axis; the smallest turbines are used for applications such as battery charging for auxiliary power for boats or caravans or to power traffic warning signs. Larger turbines can be used for making contributions to a domestic power supply while selling unused power back to the utility supplier via the electrical grid. Arrays of large turbines, known as wind farms, are becoming an important source of intermittent renewable energy and are used by many countries as part of a strategy to reduce their reliance on fossil fuels. One assessment claimed that, as of 2009, wind had the "lowest relative greenhouse gas emissions, the least water consumption demands and... the most favourable social impacts" compared to photovoltaic, geothermal and gas. The windwheel of Hero of Alexandria marks one of the first recorded instances of wind powering a machine in history.
However, the first known practical wind power plants were built in Sistan, an Eastern province of Persia, from the 7th century. These "Panemone" were vertical axle windmills, which had long vertical drive shafts with rectangular blades. Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind grain or draw up water, were used in the gristmilling and sugarcane industries. Wind power first appeared in Europe during the Middle Ages; the first historical records of their use in England date to the 11th or 12th centuries and there are reports of German crusaders taking their windmill-making skills to Syria around 1190. By the 14th century, Dutch windmills were in use to drain areas of the Rhine delta. Advanced wind turbines were described by Croatian inventor Fausto Veranzio. In his book Machinae Novae he described vertical axis wind turbines with V-shaped blades; the first electricity-generating wind turbine was a battery charging machine installed in July 1887 by Scottish academic James Blyth to light his holiday home in Marykirk, Scotland.
Some months American inventor Charles F. Brush was able to build the first automatically operated wind turbine after consulting local University professors and colleagues Jacob S. Gibbs and Brinsley Coleberd and getting the blueprints peer-reviewed for electricity production in Cleveland, Ohio. Although Blyth's turbine was considered uneconomical in the United Kingdom, electricity generation by wind turbines was more cost effective in countries with scattered populations. In Denmark by 1900, there were about 2500 windmills for mechanical loads such as pumps and mills, producing an estimated combined peak power of about 30 MW; the largest machines were on 24-meter towers with four-bladed 23-meter diameter rotors. By 1908, there were 72 wind-driven electric generators operating in the United States from 5 kW to 25 kW. Around the time of World War I, American windmill makers were producing 100,000 farm windmills each year for water-pumping. By the 1930s, wind generators for electricity were common on farms in the United States where distribution systems had not yet been installed.
In this period, high-tensile steel was cheap, the generators were placed atop prefabricated open steel lattice towers. A forerunner of modern horizontal-axis wind generators was in service at Yalta, USSR in 1931; this was a 100 kW generator on a 30-meter tower, connected to the local 6.3 kV distribution system. It was reported to have an annual capacity factor of 32 percent, not much different from current wind machines. In the autumn of 1941, the first megawatt-class wind turbine was synchronized to a utility grid in Vermont; the Smith–Putnam wind turbine only ran for 1,100 hours before suffering a critical failure. The unit was not repaired, because of a shortage of materials during the war; the first utility grid-connected wind turbine to operate in the UK was built by John Brown & Company in 1951 in the Orkney Islands. Despite these diverse developments, developments in fossil fuel systems entirely eliminated any wind turbine systems larger than supermicro size. In the early 1970s, anti-nuclear protests in Denmark spurred artisan mechanics to develop microturbines of 22 kW.
Organizing owners into associations and co-operatives lead to the lobbying of the government and utilities and provided incentives for larger turbines throughout the 1980s and later. Local activists in Germany, nascent turbine manufacturers in Spain, large investors in the United States in the early 1990s lobbied for policies that stimulated the industry in those countries. Wind Power Density is a quantitative measure of wind energy available at any location, it is the mean annual power available per square meter of swept area of a turbine, is calculated for different heights above ground. Calculation of wind power density includes the effect of air density. Wind turbines are classified by the wind speed they are designed for, from class I to class III, with A to C referring to the turbulence intensity of the wind. Conservation of mass requires that the amount of air exiting a turbine must be equal. Accordingly, Betz's law gives the maximal achievable extraction of wind power by a wind turbine as 16/27 of the total kinetic energy of the air flowing through the turbine.
The maximum theoretical power output of a wind machine is thus 16/27 times the kinetic energy of the air passing through the effective disk area of the machine. If the effective area of the disk is A, the wind velocity v, the maximum theoretical power output P is: P = 16
In physics, a force is said to do work if, when acting, there is a displacement of the point of application in the direction of the force. For example, when a ball is held above the ground and dropped, the work done on the ball as it falls is equal to the weight of the ball multiplied by the distance to the ground; when the force is constant and the angle between the force and the displacement is θ the work done is given by W = Fs cos θ. Work transfers energy from one form to another. According to Jammer, the term work was introduced in 1826 by the French mathematician Gaspard-Gustave Coriolis as "weight lifted through a height", based on the use of early steam engines to lift buckets of water out of flooded ore mines. According to Rene Dugas, French engineer and historian, it is to Solomon of Caux "that we owe the term work in the sense that it is used in mechanics now"; the SI unit of work is the joule. The SI unit of work is the joule, defined as the work expended by a force of one newton through a displacement of one metre.
The dimensionally equivalent newton-metre is sometimes used as the measuring unit for work, but this can be confused with the unit newton-metre, the measurement unit of torque. Usage of N⋅m is discouraged by the SI authority, since it can lead to confusion as to whether the quantity expressed in newton metres is a torque measurement, or a measurement of work. Non-SI units of work include the newton-metre, the foot-pound, the foot-poundal, the kilowatt hour, the litre-atmosphere, the horsepower-hour. Due to work having the same physical dimension as heat measurement units reserved for heat or energy content, such as therm, BTU and Calorie, are utilized as a measuring unit; the work W done by a constant force of magnitude F on a point that moves a displacement s in a straight line in the direction of the force is the product W = F s. For example, if a force of 10 newtons acts along a point that travels 2 metres W = F s = = 20 J; this is the work done lifting a 1 kg object from ground level to over a person's head against the force of gravity.
The work is doubled either by lifting twice the weight the same distance or by lifting the same weight twice the distance. Work is related to energy; the work-energy principle states that an increase in the kinetic energy of a rigid body is caused by an equal amount of positive work done on the body by the resultant force acting on that body. Conversely, a decrease in kinetic energy is caused by an equal amount of negative work done by the resultant force. From Newton's second law, it can be shown that work on a free, rigid body, is equal to the change in kinetic energy K E of the linear velocity and angular velocity of that body, W = Δ K E; the work of forces generated by a potential function is known as potential energy and the forces are said to be conservative. Therefore, work on an object, displaced in a conservative force field, without change in velocity or rotation, is equal to minus the change of potential energy P E of the object, W = − Δ P E; these formulas show that work is the energy associated with the action of a force, so work subsequently possesses the physical dimensions, units, of energy.
The work/energy principles discussed here are identical to Electric work/energy principles. Constraint forces limit the movement of components in a system, such as constraining an object to a surface. Constraint forces restrict the velocity in the direction of the constraint to zero, which means the constraint forces do not perform work on the system. For a mechanical system, constraint forces eliminate movement in directions that characterize the constraint, thus constraint forces do not perform work on the system, because the component of velocity along the constraint force at each point of application is zero. For example, in a pulley system like the Atwood machine, the internal forces on the rope and at the supporting pulley do no work on the system; therefore work need only be computed for the gravity forces acting on the bodies. For example, the centripetal force exerted inwards by a string on a ball in uniform circular motion sideways constrains the ball to circular motion restricting its movement away from the center of the circle.
This force does zero work. Another example is a book on a table. If external forces are applied to the book so that it slides on the table the force exerted by the table constrains the book from moving downwards; the force exerted by the table supports the book and is perpendicular to its movement which means that this constraint force does not perform work. The magnetic force on a charged particle is F = qv × B, where q is the charge, v is the velocity of the particle, B is the mag
World War II
World War II known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from over 30 countries; the major participants threw their entire economic and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China, it included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, the only use of nuclear weapons in war. Japan, which aimed to dominate Asia and the Pacific, was at war with China by 1937, though neither side had declared war on the other. World War II is said to have begun on 1 September 1939, with the invasion of Poland by Germany and subsequent declarations of war on Germany by France and the United Kingdom.
From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, formed the Axis alliance with Italy and Japan. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union partitioned and annexed territories of their European neighbours, Finland and the Baltic states. Following the onset of campaigns in North Africa and East Africa, the fall of France in mid 1940, the war continued between the European Axis powers and the British Empire. War in the Balkans, the aerial Battle of Britain, the Blitz, the long Battle of the Atlantic followed. On 22 June 1941, the European Axis powers launched an invasion of the Soviet Union, opening the largest land theatre of war in history; this Eastern Front trapped most crucially the German Wehrmacht, into a war of attrition. In December 1941, Japan launched a surprise attack on the United States as well as European colonies in the Pacific. Following an immediate U. S. declaration of war against Japan, supported by one from Great Britain, the European Axis powers declared war on the U.
S. in solidarity with their Japanese ally. Rapid Japanese conquests over much of the Western Pacific ensued, perceived by many in Asia as liberation from Western dominance and resulting in the support of several armies from defeated territories; the Axis advance in the Pacific halted in 1942. Key setbacks in 1943, which included a series of German defeats on the Eastern Front, the Allied invasions of Sicily and Italy, Allied victories in the Pacific, cost the Axis its initiative and forced it into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained its territorial losses and turned toward Germany and its allies. During 1944 and 1945 the Japanese suffered major reverses in mainland Asia in Central China, South China and Burma, while the Allies crippled the Japanese Navy and captured key Western Pacific islands; the war in Europe concluded with an invasion of Germany by the Western Allies and the Soviet Union, culminating in the capture of Berlin by Soviet troops, the suicide of Adolf Hitler and the German unconditional surrender on 8 May 1945.
Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender under its terms, the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki on 6 and 9 August respectively. With an invasion of the Japanese archipelago imminent, the possibility of additional atomic bombings, the Soviet entry into the war against Japan and its invasion of Manchuria, Japan announced its intention to surrender on 15 August 1945, cementing total victory in Asia for the Allies. Tribunals were set up by fiat by the Allies and war crimes trials were conducted in the wake of the war both against the Germans and the Japanese. World War II changed the political social structure of the globe; the United Nations was established to foster international co-operation and prevent future conflicts. The Soviet Union and United States emerged as rival superpowers, setting the stage for the nearly half-century long Cold War. In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia.
Most countries whose industries had been damaged moved towards economic expansion. Political integration in Europe, emerged as an effort to end pre-war enmities and create a common identity; the start of the war in Europe is held to be 1 September 1939, beginning with the German invasion of Poland. The dates for the beginning of war in the Pacific include the start of the Second Sino-Japanese War on 7 July 1937, or the Japanese invasion of Manchuria on 19 September 1931. Others follow the British historian A. J. P. Taylor, who held that the Sino-Japanese War and war in Europe and its colonies occurred and the two wars merged in 1941; this article uses the conventional dating. Other starting dates sometimes used for World War II include the Italian invasion of Abyssinia on 3 October 1935; the British historian Antony Beevor views the beginning of World War II as the Battles of Khalkhin Gol fought between Japan and the fo
French Academy of Sciences
The French Academy of Sciences is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research. It was at the forefront of scientific developments in Europe in the 17th and 18th centuries, is one of the earliest Academies of Sciences. Headed by Sébastien Candel, it is one of the five Academies of the Institut de France; the Academy of Sciences traces its origin to Colbert's plan to create a general academy. He chose a small group of scholars who met on 22 December 1666 in the King's library, thereafter held twice-weekly working meetings there; the first 30 years of the Academy's existence were informal, since no statutes had as yet been laid down for the institution. In contrast to its British counterpart, the Academy was founded as an organ of government; the Academy was expected to remain apolitical, to avoid discussion of religious and social issues. On 20 January 1699, Louis XIV gave the Company its first rules.
The Academy was installed in the Louvre in Paris. Following this reform, the Academy began publishing a volume each year with information on all the work done by its members and obituaries for members who had died; this reform codified the method by which members of the Academy could receive pensions for their work. On 8 August 1793, the National Convention abolished all the academies. On 22 August 1795, a National Institute of Sciences and Arts was put in place, bringing together the old academies of the sciences and arts, among them the Académie française and the Académie des sciences. All the old members of the abolished Académie were formally re-elected and retook their ancient seats. Among the exceptions was Dominique, comte de Cassini, who refused to take his seat. Membership in the Academy was not restricted to scientists: in 1798 Napoleon Bonaparte was elected a member of the Academy and three years a president in connection with his Egyptian expedition, which had a scientific component.
In 1816, the again renamed "Royal Academy of Sciences" became autonomous, while forming part of the Institute of France. In the Second Republic, the name returned to Académie des sciences. During this period, the Academy was funded by and accountable to the Ministry of Public Instruction; the Academy came to control French patent laws in the course of the eighteenth century, acting as the liaison of artisans' knowledge to the public domain. As a result, academicians dominated technological activities in France; the Academy proceedings were published under the name Comptes rendus de l'Académie des Sciences. The Comptes rendus is now a journal series with seven titles; the publications can be found on site of the French National Library. In 1818 the French Academy of Sciences launched a competition to explain the properties of light; the civil engineer Augustin-Jean Fresnel entered this competition by submitting a new wave theory of light. Siméon Denis Poisson, one of the members of the judging committee, studied Fresnel's theory in detail.
Being a supporter of the particle-theory of light, he looked for a way to disprove it. Poisson thought that he had found a flaw when he demonstrate that Fresnel's theory predicts that an on-axis bright spot would exist in the shadow of a circular obstacle, where there should be complete darkness according to the particle-theory of light; the Poisson spot is not observed in every-day situations, so it was only natural for Poisson to interpret it as an absurd result and that it should disprove Fresnel's theory. However, the head of the committee, Dominique-François-Jean Arago, who incidentally became Prime Minister of France, decided to perform the experiment in more detail, he molded a 2-mm metallic disk to a glass plate with wax. To everyone's surprise he succeeded in observing the predicted spot, which convinced most scientists of the wave-nature of light. For three centuries women were not allowed as members of the Academy; this meant that many women scientists were excluded, including two-time Nobel Prize winner Marie Curie, Nobel winner Irène Joliot-Curie, mathematician Sophie Germain, many other deserving women scientists.
The first woman admitted as a correspondent member was a student of Curie's, Marguerite Perey, in 1962. The first female full member was Yvonne Choquet-Bruhat in 1979. Today the Academy is one of five academies comprising the Institut de France, its members are elected for life. There are 150 full members, 300 corresponding members, 120 foreign associates, they are divided into two scientific groups: the Mathematical and Physical sciences and their applications and the Chemical, Biological and Medical sciences and their applications. Each year, the Academy of Sciences distributes about 80 prizes; these include: the Grande Médaille, awarded annually, in rotation, in the relevant disciplines of each division of the Academy, to a French or foreign scholar who has contributed to the development of science in a decisive way. The Lalande Prize, awarded from 1802 through 1970, for outstanding achievement in astronomy the Valz Prize, awarded from 1877 through 1970, to honor advances in astronomy the Richard Lounsbery Award, jointly with the National Academy of Sciences the Prix Jacques Herbrand, for mathematics and physics the Prix Paul Pascal, for chemistry the Louis Bachelier Prize for major contributions to mathematical modeling in finance the Prix Michel Montpetit for computer science and applied mathematics, awarded since 1977 the Leconte Prize, awarded annually since 1886, to recognize important discoveries in
A Pelton wheel is an impulse-type water turbine invented by Lester Allan Pelton in the 1870s. The Pelton wheel extracts energy from the impulse of moving water, as opposed to water's dead weight like the traditional overshot water wheel. Many earlier variations of impulse turbines existed, but they were less efficient than Pelton's design. Water leaving those wheels still had high speed, carrying away much of the dynamic energy brought to the wheels. Pelton's paddle geometry was designed so that when the rim ran at half the speed of the water jet, the water left the wheel with little speed. Lester Allan Pelton was born in Vermillion, Ohio in 1829. In 1850, he travelled overland to California. Pelton worked by selling fish. In 1860, he moved to a center of placer mining activity. At this time many mining operations were powered by steam engines which consumed vast amounts of wood as their fuel; some water wheels were used in the larger rivers, but they were ineffective in the smaller streams that were found near the mines.
Pelton worked on a design for a water wheel that would work with the small flow found in these streams. By the mid 1870's, Pelton had developed a wooden prototype of his new wheel. In 1876, he approached the Miners Foundry in Nevada City to build the first commercial models in iron; the first Pelton Wheel was installed at the Mayflower Mine in Nevada City in 1878.. The efficiency advantages of Pelton's invention were recognized and his product was soon in high demand. By the mid-1880s, the Miners Foundry could not meet the demand, in 1888, Pelton sold the rights to his name and the patents to his invention to the Pelton Water Wheel Company in San Francisco; the company established a factory at 121/123 Main Street in San Francisco. The Pelton Water Wheel Company manufactured a large number of Pelton Wheels in San Francisco which were shipped around the world. In 1892, the Company added a branch on the east coast at 143 Liberty Street in New York City. By 1900, over 11,000 turbines were in use. In 1914, the company moved manufacturing to new, larger premises at 612 Alabama Street in San Francisco.
In 1956, the company was acquired by the Baldwin-Lima-Hamilton Company, which ended manufacture of Pelton Wheels. Nozzles direct forceful, high-speed streams of water against a series of spoon-shaped buckets known as impulse blades, which are mounted around the outer rim of a drive wheel—also called a runner; as the water jet hits the blades, the direction of water velocity is changed to follow the contours of the blades. The impulse energy of the water jet exerts torque on the bucket-and-wheel system, spinning the wheel. In the process, the water jet's momentum is transferred to the wheel and hence to a turbine. Thus, "impulse" energy does work on the turbine. Maximum power and efficiency are achieved when the velocity of the water jet is twice the velocity of the rotating buckets. A small percentage of the water jet's original kinetic energy will remain in the water, which causes the bucket to be emptied at the same rate it is filled, thereby allows the high-pressure input flow to continue uninterrupted and without waste of energy.
Two buckets are mounted side-by-side on the wheel, with the water jet split into two equal streams. Because water is nearly incompressible all of the available energy is extracted in the first stage of the hydraulic turbine. Therefore, Pelton wheels have only one turbine stage, unlike gas turbines that operate with compressible fluid. Pelton wheels are the preferred turbine for hydro-power where the available water source has high hydraulic head at low flow rates. Pelton wheels are made in all sizes. There exist; the largest units – the Bieudron Hydroelectric Power Station at the Grande Dixence Dam complex in Switzerland – are over 400 megawatts. The smallest Pelton wheels are only a few inches across, can be used to tap power from mountain streams having flows of a few gallons per minute; some of these systems use household plumbing fixtures for water delivery. These small units are recommended for use with 30 metres or more of head, in order to generate significant power levels. Depending on water flow and design, Pelton wheels operate best with heads from 15–1,800 metres, although there is no theoretical limit.
The specific speed η s parameter of a particular turbine's size. Compared to other turbine designs, the low specific speed of the Pelton wheel, implies that the geometry is inherently a "low gear" design, thus it is most suitable to being fed by a hydro source with a low ratio of flow to pressure. The specific speed is the main criterion for matching a specific hydro-electric site with the optimal turbine type, it allows a new turbine design to be scaled from an existing design of known performance. Η s = n P / ρ 5 / 4, where: n = Frequency of rotati
A fluid flowing past the surface of a body exerts a force on it. Lift is the component of this force, perpendicular to the oncoming flow direction, it contrasts with the drag force, the component of the force parallel to the flow direction. Lift conventionally acts in an upward direction in order to counter the force of gravity, but it can act in any direction at right angles to the flow. If the surrounding fluid is air, the force is called an aerodynamic force. In water or any other liquid, it is called a hydrodynamic force. Dynamic lift is distinguished from other kinds of lift in fluids. Aerostatic lift or buoyancy, in which an internal fluid is lighter than the surrounding fluid, does not require movement and is used by balloons, dirigibles and submarines. Planing lift, in which only the lower portion of the body is immersed in a liquid flow, is used by motorboats and water-skis. A fluid flowing past the surface of a body exerts a force on it, it makes no difference whether the fluid is flowing past a stationary body or the body is moving through a stationary volume of fluid.
Lift is the component of this force, perpendicular to the oncoming flow direction. Lift is always accompanied by a drag force, the component of the surface force parallel to the flow direction. Lift is associated with the wings of fixed-wing aircraft, although it is more generated by many other streamlined bodies such as propellers, helicopter rotors, racing car wings, maritime sails, wind turbines in air, by sailboat keels, ship's rudders, hydrofoils in water. Lift is exploited by flying and gliding animals by birds and insects, in the plant world by the seeds of certain trees. While the common meaning of the word "lift" assumes that lift opposes weight, lift can be in any direction with respect to gravity, since it is defined with respect to the direction of flow rather than to the direction of gravity; when an aircraft is cruising in straight and level flight, most of the lift opposes gravity. However, when an aircraft is climbing, descending, or banking in a turn the lift is tilted with respect to the vertical.
Lift may act as downforce in some aerobatic manoeuvres, or on the wing on a racing car. Lift may be horizontal, for instance on a sailing ship; the lift discussed in this article is in relation to airfoils, although marine hydrofoils and propellers share the same physical principles and work in the same way, despite differences between air and water such as density and viscosity. An airfoil is a streamlined shape, capable of generating more lift than drag. A flat plate can generate lift, but not as much as a streamlined airfoil, with somewhat higher drag. There are several ways to explain; some are more physically rigorous than others. For example, there are explanations based directly on Newton’s laws of motion and explanations based on Bernoulli’s principle. Either can be used to explain lift. An airfoil generates lift by exerting a downward force on the air. According to Newton's third law, the air must exert an equal and opposite force on the airfoil, lift; the airflow changes direction as it passes the airfoil and follows a path, curved downward.
According to Newton's second law, this change in flow direction requires a downward force applied to the air by the airfoil. Newton's third law requires the air to exert an upward force on the airfoil. In the case of an airplane wing, the wing exerts a downward force on the air and the air exerts an upward force on the wing; the downward turning of the flow is not produced by the lower surface of the airfoil, the air flow above the airfoil accounts for much of the downward-turning action. Bernoulli's principle states that there is a direct mathematical relationship between the pressure of a fluid and the speed of that fluid, so if one knows the speed at all points within the airflow one can calculate the pressure, vice versa. For any airfoil generating lift, there must be a pressure imbalance, i.e. lower average air pressure on the top than on the bottom. Bernoulli's principle states that this pressure difference must be accompanied by a speed difference. Starting with the flow pattern observed in both theory and experiments, the increased flow speed over the upper surface can be explained in terms of streamtube pinching and conservation of mass.
For incompressible flow, the rate of volume flow must be constant within each streamtube since matter is not created or destroyed. If a streamtube becomes narrower, the flow speed must increase in the narrower region to maintain the constant flow rate, to satisfy the principle of conservation of mass; the upper streamtubes constrict as they flow around the airfoil. Conservation of mass says; the lower streamtubes expand and their flowrate slows. From Bernoulli's principle, the pressure on the upper surface where the flow is moving faster is lower than the pressure on the lower surface where it is moving slower; this pressure difference creates a net aerodynamic force. As explained below under a more comprehensive physical explanation, producing a lift force requires maintaining pressure differences in both the vertical and horizontal directions, thus requires both downward turning of the flow and changes in flow speed consistent with Bernoulli's principle; the simplified explanations given above are therefore incomplete because they define lift in terms of only one o