2. Historical background

The emergence of hydraulics as a science followed the discovery of certain laws and the elucidation of a number of problems connected with the equilibrium and motion of fluids.

In the second half of the fifteenth century Leonardo da Vinci studied questions of hydraulic engineering. In his paper On the Flow of Water and River Structures he set forth his observations and experience gained in the construction of hydraulic installations in Milan, Florence and other places. In 1612 there appeared Galileo's Discorso intorno alle cose che stanno su Vacqua which contained the first systematic study of the fundamentals of hydrostatics. In 1643, Galileo's pupil Torricelli enunciated the law of free flow of liquids through orifices. In 1650 was discovered the law of the pressure dis­tribution In a liquid, known as Pascal's law. The important law of friction in a moving fluid was formulated, in approximate form, by Isaac Newton; to him also goes the credit of introducing the con­cept of fluid viscosity and of laying the foundations of the theory of hydrodynamic similarity.

These, however, were only isolated laws and problems, and until the mid-eighteenth century there existed no comprehensive science of the behaviour of fluids. The theoretical foundations of fluid mechanics and hydraulics as a science were laid by Daniel Bernoulli and Leonhard Euler in the eighteenth century.

Daniel Bernoulli (1700-1782) belonged to a famous Swiss family from which came eleven celebrated scientists, most of them mathe­maticians or mechanics. As member of the then new Russian Acad­emy of Sciences he spent much of his life in St. Petersburg. Later he was elected to honorary membership of the Academy. Between 1728 and 1778 Bernoulli published 47 papers on mathematics, me­chanics and other subjects in publications of the Russian Academy. In 1738, in his Hydrodynamics Bernoulli formulated the funda­mental law of fluid motion giving the relation between pressure, velocity and head of a fluid. The Bernoulli equation is one of the bedrocks of fluid mechanics in general and hydraulics in particular. The celebrated mathematician, physicist and astronomer Leon-hard Euler (1707-1783), also of Swiss parentage, lived in St. Peters­burg for many years, where he worked at the Russian Academy. In 1755 he developed tlie general differential equations of flow for so-called ideal (nonviscous) fluids, integration of which gives the Bernoulli equation as a partial solution. This marked the beginning of the theoretical method of analysis in fluid mechanics.

Euler is also credited with the general equation of work for all rotodynamic hydraulic machines (turbines, centrifugal pumps and fans) and with laying the foundations of the theory of buoy­ancy.

In his work Euler kept in contact with the great Russian scientist Mikhail Lomonosov and was undoubtedly influenced by him. Lomo-nosov is known to have studied many physical problems having a direct bearing on the flow of liquids and gases and he showed consid­erable interest in engineering hydraulics. The works of Bernoulli, Euler and Lomonosov concluded the first period in the development of hydraulics by laying its foundations as a science.

The second period, embracing the latter part of the eighteenth and most of the nineteenth centuries, is characterised chiefly by the accumulation of experimental data on the flow of fluids in open and closed channels and the determination of correction factors for the Bernoulli equation. As to the theoretical research of that time,/it was based on the ideal fluid concept and could not meet all practical requirements as it failed to take into account such an important property as viscosity.

The second period of hydraulics is associated with the names of such celebrated experimentalists as Antoine Chezy, Henri Darcy and Jean Poiseuille in France, Julius Weisbach and G. Hagen in Germany and many others. Especially thorough and comprehensive in scope were Weisbach's (1806-1871) experiments, and his empiri­cal formulas were in wide use up till the very latest years. Among the outstanding theoreticians in the sphere of fluid mechanics of the period were Lagrange, Helmholtz and Saint-Venant.

In the next period, commencing at the turn of the century, the theoretical basis of hydraulics was further extended by taking into account the viscosity of fluids; the theory of similarity and other theoretical and practical problems were elaborated. This period in the development of hydraulics, as of other engineering disciplines, gained impetus from the rapid expansion of the productive forces and technological progress. It is associated with the names of George Stokes (1819-1903), Osborne Reynolds (1842-1912), Nikolai Joukowski (1847-1921), N. Petrov (1836-1920) and others. Stokes laid the foundations of the theory of fluid flow taking into account viscosity and elaborated other theoretical problems.

Reynolds is credited with establishing the criteria of similarity, which made it possible to summarise and systematise the vast amount of experimental data that had been accumulated by then. He was also the first to study turbulent flow, the most complex type of fluid motion.

In Russia, Professor N. P. Petrov's classical experiments proved the validity of Newton's law of friction in a fluid, till then treated as hypothetical. This served as the basis for his theory of machine lubrication, which played an important part in the. further investi­gation of that question.

Of tremendous importance for the development of hydraulics were the works of the great Russian scientist Nikolai Joukowski. In the first period of his versatile and extremely fruitful studies (the nineteen-eighties and nineties), before he took up aerodynamics, Joukowski published several papers on hydraulics which brought him world-wide fame.

His major work in the field is his investigation of so-called water hammer in pipelines, a cause of many breakdowns. Not only did he develop the theory of the complex phenomenon caused by the sudden closure of a valve, turbine gate or faucet, but he also carried out many experiments at the Moscow waterworks which confirmed his theory and ensured its practical application. The treatise was soon translated into many languages and Joukowski's water-hammer theory is today included in all textbooks on hydraulics.

Joukowski laid the foundations of the theory of flow of ground water (the theory of percolation), also in connection with the needs of the Moscow waterworks. He developed the basic equations of ground-water flow and obtained results of practical importance. J( kowski studied problems of liquid flow through orifices, the theory of lubrication, velocity distribution in water mains, the reaction of fluid jets and vibration of fluids, and he established the analogy between wave formation on a liquid surface and pressure jump in air at supersonic speeds.

Joukowski's interest in hydraulics, one of his pupils writes, lasted till the end of his life. Joukowski can be regarded as the founder of the school in hydraulics which combines theoretical and experimen­tal findings and carries research to the stage where it yields practical results.

This combination of the methods of theoretical fluid mechanics and experimental hydraulics and the growing tendency towards the merging of the two formerly isolated disciplines, with their entirely different methods, is one of the characteristic features of modern hydraulics.

In speaking of contemporary hydraulics, the names of Ludwig Prandtl, Theodor von Karman and Johann Nikuradse should be mentioned first. Prandtl and Karman, who are famous for their re­searches in fluid mechanics and aerodynamics, contributed sub­stantially to the theory of turbulent flow, while Nikuradse, who worked in contact with them, carried out many laboratory expe­riments on flow in pipes, the results of which are widely known.

The construction of huge hydroelectric plants, canals and pipe­lines and the development of hydraulic machinery confronted Soviet scientists and hydraulic engineers with more and more new prob­lems which had to be studied and solved for practical needs.

An important part in the development of the theory and calcu­lation of hydraulic structures was played by the works of Academi­cian N. N. Pavlovsky, one of the founders of the Soviet school of hydraulics. His numerous works in various spheres of hydraulics, especially on flow in open channels and the theory of seepage, repre­sent an outstanding contribution to science.

Credit for the development of the Soviet school of hydraulics also goes to Academician L. S. Leibenzon and his pupils. Their works deal mainly with the hydraulics of highly viscous fluids, petroleum hydraulics and the theory of percolation (ground-water hy­draulics).

Major contributions in other spheres of general hydraulics and its specialised branches have been made by the eminent Soviet scientists-Academicians S. A. Chaplygin, A. N. Kolmogorov and S. A. Khristianovich, Professors A. N. Akhutin, I. L Agroskin, M. A. Velikanov, L. G. Loitayansky, M. D. Chertousov, I. A. Charny and others.

Soviet scientists and engineers have achieved marked success in hydraulic engineering. Soviet hydroturbine construction owes much to Professors I.N. Voznesensky, N. N. Kovalyov and J. I. Kukolevsky. The great advances in this branch of Soviet engineering are reflected in the design and manufacture of turbines for the world's biggest hydroelectric stations.

The successful launchings of manned spaceships present another spectacular example of the great achievements of Soviet science. Their orbiting required powerful multistage rockets, in the con­struction of which hydraulic engineers took part together with scien­tists and engineers of other fields.