Chinese name: fluid mechanics mbth: a brief history of fluid mechanics development, subject content, research methods, field observation, laboratory simulation, theoretical analysis, numerical calculation, prospect and development history. Fluid mechanics came into being in the struggle between human beings and nature, and gradually developed in production practice. There is a legend in China that Dayu harnessed water and dredged rivers. Li Bing and his son led the working people to build Dujiangyan in the Qin Dynasty (3rd century BC), which is still playing a role. At about the same time, the Romans built a large-scale water supply pipeline system. Archimedes of ancient Greece first contributed to the formation of fluid mechanics. He established the theory of liquid equilibrium including the buoyancy theorem of objects and the stability of floating bodies, which laid the foundation of hydrostatics. After more than a thousand years, fluid mechanics has not developed significantly. /kloc-in the 0 th and 5 th centuries, Yida Wenxi's works only talked about water waves, pipe flows, hydraulic machinery, and the flight principle of birds. /kloc-in the 0/7th century, Pascal expounded the concept of pressure in still fluid. Fluid mechanics, especially fluid dynamics, as a rigorous science, is gradually formed with the establishment of concepts such as velocity, acceleration, force and flow field in classical mechanics and the establishment of three conservation laws of mass, momentum and energy. /kloc-I Newton, the founder of mechanics in the 0 th and 7 th centuries, studied the resistance of an object moving in a liquid, and obtained that the resistance is directly proportional to the fluid density, the cross-sectional area of the object facing the flow and the square of the moving speed. He also put forward the following hypothesis about the internal friction when viscous fluid moves: the friction stress between two fluid layers is directly proportional to the relative sliding speed of the two layers and inversely proportional to the distance between the two layers (Newton's viscosity law). Later, H. Pitot of France invented the pitot tube for measuring the flow velocity; D'Alembert has done a lot of experiments on the resistance of ships in the canal, and confirmed the square relationship between resistance and object speed. L Euler of Switzerland adopted the concept of continuum, extended the concept of pressure in statics to moving fluid, established Euler equation, and correctly described the motion of inviscid fluid with differential equation. Based on the energy conservation of classical mechanics, Bernoulli studied the flow of water in water supply pipeline, carefully arranged experiments and analyzed them, and obtained the Bernoulli equation, the relationship between velocity, pressure and pipeline elevation under the steady motion of fluid. The establishment of Euler equation and Bernoulli equation marks the establishment of fluid mechanics as a branch discipline, and from then on, the stage of quantitative research on fluid motion by differential equation and experimental measurement began. /kloc-since the 0/8th century, the potential flow theory has made great progress, and many laws have been clarified in water waves, tides, vortex motion, acoustics and so on. J-L Lagrange of France has done a lot of research on non-rotating motion, and H-von Helmholtz of Germany has done a lot of research on vortex motion. In the above research, the viscosity of fluid does not play an important role, that is, inviscid fluid is considered, so this theory can not clarify the role of viscosity in fluid. French C.-L.-M.-H. Naville established the equations of fluid motion considering viscosity in 182 1 and British G. G. Stokes in 1845, respectively, and later named them Naville-Stokes equations, which are the theoretical basis of fluid dynamics. Because Navier-Stokes equation is a set of nonlinear partial differential equations, it is very difficult to study fluid motion by analytical method. In order to simplify the equation, scholars adopted the assumption that the fluid is incompressible and inviscid, but obtained the D 'Alembert paradox that the resistance of the object moving in the fluid is equal to zero. So by the end of 19, although great progress has been made in fluid mechanics by analytical method, it is not easy to popularize production. Parallel to fluid dynamics is hydraulics (see fluid dynamics). This is an empirical science that summarizes some empirical formulas from a large number of experiments to express the relationship between flow parameters to meet the needs of production and engineering. The boundary layer theory unifies the above two methods. It was founded by German L. plante in 1904. From 1904 to 192 1, the prandtl school gradually simplified the N-S equation and established the boundary layer theory from the perspectives of reasoning, mathematical demonstration and experimental measurement, which can actually calculate the flow state in the boundary layer and the viscous force between fluid and solid under simple conditions. At the same time, Planck put forward many new concepts, which are widely used in the design of aircraft and steam turbines. This theory not only defines the application range of ideal fluid, but also can calculate the friction resistance when an object moves. The above two situations have been unified. Bernoulli Theorem The Development of Aircraft and Aerodynamics At the beginning of the 20th century, the appearance of aircraft greatly promoted the development of aerodynamics. With the development of aviation industry, it is expected to reveal the pressure distribution around the aircraft, the stress state and resistance of the aircraft, and promote the development of fluid mechanics in experiment and theoretical analysis. At the beginning of the 20th century, scientists represented by Zhukovsky, Joe Pligin and Plante. Based on the potential flow theory of inviscid incompressible flow, the wing theory is established, and how to lift the wing is expounded, so that air can lift heavy aircraft into the air. The correctness of wing theory makes people re-understand the inviscid fluid theory and affirms its great significance in guiding engineering design. The establishment and development of wing theory and boundary layer theory is a great progress in fluid mechanics, which combines inviscid fluid theory with viscous fluid boundary layer theory. With the improvement of steam turbine and the increase of aircraft speed to more than 50 meters per second, the experimental and theoretical research on the effect of air density change has been carried out rapidly since 19 century, which provides theoretical guidance for high-speed flight. After the 1940s, due to the application of jet propulsion and rocket technology, the speed of aircraft exceeded the speed of sound, and then space flight was realized, which made the research on high-speed airflow progress rapidly and formed gas dynamics, physical and chemical fluid dynamics and other sub-disciplines. Formation of Branches and Interdisciplinary Disciplines Since the 1960s, fluid mechanics has begun to cross-permeate with other disciplines, forming new interdisciplinary or marginal disciplines, such as physical and chemical fluid mechanics and magnetohydrodynamics. At first, it was only a qualitative problem, and gradually there was a quantitative study. Biorheology is an example. Based on these theories, in the 1940s, a new theory about detonation waveform in explosives or natural gas became, and the explosion wave theory was developed to study the propagation of shock wave in air or water after nuclear bullets and explosives were detonated. Since then, fluid mechanics has developed many branches, such as hypersonic aerodynamics, supersonic aerodynamics, rarefied aerodynamics, electromagnetic fluid mechanics, computational fluid dynamics, two-phase (gas-liquid or gas-solid) flow and so on. These great advances are inseparable from the adoption of various mathematical analysis methods and the establishment of large-scale precision experimental equipment and instruments. Since the 1950s, with the continuous improvement of electronic computers, problems that are difficult to study by analytical methods can be solved by numerical calculation, and a new branch of computational fluid dynamics has emerged. At the same time, due to the needs of civil and military production, the discipline of fluid dynamics has also made great progress. In 1960s, according to the needs of structural mechanics and solid mechanics, the finite element method for calculating elastic mechanics problems appeared. After more than ten years of development, the finite element analysis, a new calculation method, has been re-applied to fluid mechanics, especially in the problems of low-speed flow and complex fluid boundary shape, and its advantages are more obvious. Since 2 1 century, the problem of using finite element method to study high-speed flow began to appear, and the mutual penetration and integration of finite element method and difference method also appeared. The content of this question basically assumes that the continuum assumes that all substances are composed of molecules, although all molecules are discretely distributed and do irregular thermal motion. However, both theory and experiment show that the statistical average value of fluid molecular micelles doing thermal motion is stable in a small range, so it can be approximately considered that fluid is composed of continuous substances, in which physical quantities such as temperature, density and pressure are scalar fields that are continuously distributed. The purpose of mass conservation is to establish equations describing fluid motion. Euler method describes that the mass flowing into any closed surface in absolute coordinate system is equal to the mass flowing out of this surface, which is an integral equation set. If it is transformed into a differential equation group, the divergence of the product of density and velocity is zero (no stray field). Euler method describes that the rate of change of the follow-up derivative of the fluid micelle mass with time is zero. Momentum Theorem Fluid mechanics belongs to the category of classical mechanics. Therefore, momentum theorem and moment of momentum theorem are applicable to fluid infinitesimal elements. The force of stress tensor on fluid infinitesimal elements mainly includes surface force and volume force. Surface force and volume force are measures of internal force per unit area and volume respectively, so they are bounded. Because we consider fluid infinitesimal when establishing the basic equations of fluid mechanics, the force on the surface of fluid micelle is the second-order infinitesimal size, and the volume force is the third-order infinitesimal size, so when the volume is very small, the role of volume force can be ignored. It is considered that fluid micelle is only subjected to surface force (surface stress). In anisotropic fluids, the stress varies with the position of fluid micelle and the surface normal. Stress is described by the inner product of a second-order tensor and the surface normal, and only three quantities of the second-order stress tensor are independent. Therefore, as long as the stress on three different surfaces of a point is known, the stress distribution of the point can be determined. Viscosity assumes that the fluid has viscosity, and the stress tensor can be derived by using viscosity theorem. Energy conservation is specifically expressed as: the product of volume force per unit time plus work done by surface force and deformation speed of fluid micelle is equal to the increment of internal energy per unit time plus the increment of kinetic energy of fluid micelle. Fluid is a branch of fluid mechanics, which is the general name of gas and liquid. In people's life and production activities, fluids will be encountered anytime and anywhere. Therefore, fluid mechanics is closely related to human daily life and production. Geophysical Fluid Dynamics
Atmosphere and water are the two most common fluids. The atmosphere surrounds the whole earth, and the surface of the earth is 70% water. Atmospheric movement, seawater movement (including waves, tides, mesoscale vortex, circulation, etc. ) and even the flow of molten slurry in the deep part of the earth are the research contents of fluid mechanics and belong to the category of geophysical fluid dynamics. hydromechanics
The movement of water in pipes, channels and rivers has been the object of study since ancient times. People also use water to do work, such as ancient water hammers and highly developed hydraulic turbines in modern times. Ships have always been people's means of transportation. The various resistances encountered by ships in water, the stability of ships and the cavitation phenomenon caused by hull and propeller in water have always been the research topics of ship fluid mechanics. These branches that study the laws of water movement are called fluid mechanics. pneumatics
Since the first airplane appeared in the world in the early 20th century, airplanes and other aircrafts have developed rapidly. Space flight, which began in 1950s, extended the range of human activities to other planets and the Milky Way. The vigorous development of aerospace industry is closely related to the development of aerodynamics and gas dynamics, branches of fluid mechanics. These subjects are the most active and fruitful fields in fluid mechanics. Seepage mechanics
The exploitation of oil and natural gas and the development and utilization of groundwater require people to understand the movement of fluid in porous or fractured media, which is the main research object of seepage mechanics, a branch of fluid mechanics. Seepage mechanics also involves the prevention and control of soil salinization, concentration, separation and porous filtration in chemical industry, and the cooling of combustion chamber. Physical and chemical fluid dynamics
Burning coal, oil, natural gas, etc. Heat energy can be obtained to drive machinery or for other purposes. Combustion is inseparable from gas. This is a fluid mechanics problem with chemical reaction and thermal energy change, and it is one of the contents of physical and chemical fluid dynamics. Explosion is a violent instantaneous energy change and transfer process, which involves gas dynamics, thus forming explosion mechanics. Multiphase system, fluid dynamics
Desert migration, river sediment transportation, pipeline pulverized coal transportation, chemical fluidized bed gas catalyst movement, etc. all involve solid particles in fluid or bubbles in liquid. This kind of problem is the research scope of multiphase fluid mechanics. Plasma dynamics and electromagnetic fluid mechanics
Plasma is a collection of free electrons, ions with equal positive charges and neutral particles. Plasma has special motion law under the action of magnetic field. The disciplines that study the laws of plasma motion are called plasma dynamics and electromagnetic fluid mechanics (see electrohydrodynamic and magnetohydrodynamics). They are widely used in controlled thermonuclear reaction, magnetic fluid power generation and cosmic gas movement (see cosmic gas dynamics). Environmental fluid mechanics
Influence of wind on buildings, bridges, cables, etc. Make them bear load and excite vibration; The discharge of waste gas and waste water causes environmental pollution; River bed erosion, migration and coastal erosion; The subject that studies the movement of these fluids and their interactions with humans, animals and plants is called environmental fluid mechanics (including environmental aerodynamics and building aerodynamics). This is a new frontier discipline, involving classical fluid mechanics, meteorology, oceanography and hydraulics, structural dynamics and so on. biorheology
Biorheology studies fluid mechanics problems related to human body or other animals and plants, such as blood flow in blood vessels, physiological fluid movement in heart, lung and kidney (see circulatory system dynamics and respiratory system dynamics), and nutrient solution transportation in plants (see flow in plants). In addition, we also study the flight of birds in the air (see the flight of birds and insects), the swimming of animals (such as dolphins) in the water, and so on. Therefore, fluid mechanics not only contains the basic theory of natural science, but also involves the application of engineering technology science. The above mainly explains the contents and branches of fluid mechanics from the perspective of the research object. In addition, from the perspective of fluid force, it can be divided into hydrostatics, fluid kinematics and fluid dynamics; From the study of different "mechanical models", there are ideal fluid dynamics, viscous fluid dynamics, incompressible fluid dynamics, compressible fluid dynamics and non-Newtonian fluid dynamics. The research methods can be divided into four aspects: field observation, laboratory simulation, theoretical analysis and numerical calculation. Field observation is used to systematically observe the natural flow phenomenon or full-scale flow phenomenon in existing projects, so as to summarize the law of fluid movement and predict the evolution of flow phenomenon. In the past, the weather in observation and forecast was basically like this. However, the occurrence of field flow phenomenon can not be controlled, and the occurrence conditions can hardly be completely repeated, which affects the study of flow phenomenon and law; On-site observation also consumes a lot of material, financial and human resources. Therefore, people set up laboratories to make these phenomena appear under controllable conditions, so as to facilitate observation and research. Laboratory simulation In the laboratory, the flow phenomenon can be repeated many times in much shorter time and much smaller space, and various parameters can be isolated and experimental parameters can be changed systematically. In the laboratory, people can also cause special situations that are rarely encountered in nature (such as high temperature and high pressure), and can show phenomena that were not seen before. Field observation is often the observation of existing things and projects, while laboratory simulation can observe things and phenomena that have not yet appeared (such as projects and machinery to be designed) and improve them. Therefore, laboratory simulation is an important method to study fluid mechanics. However, in order to make the experimental data consistent with the field observation results, the flow similarity condition must be completely satisfied (see similarity law). However, for the scale model, some similar criteria, such as Reynolds number and Froude number, are difficult to meet at the same time, and it is also difficult to achieve large Reynolds numbers in some engineering problems. Therefore, in the laboratory, it is usually aimed at specific problems, try to meet some main similar conditions and parameters, and then verify or correct the experimental results through field observation. According to the general laws of fluid motion such as mass conservation, momentum conservation and energy conservation, theoretical analysis is carried out. Through mathematical analysis, we study fluid motion, explain known phenomena and predict possible results. The steps of theoretical analysis are roughly as follows: ① The general method of establishing a "mechanical model" is to analyze various contradictions, grasp the main aspects, simplify the problem and establish a "mechanical model" that reflects the essence of the problem. The most commonly used basic models in fluid mechanics are continuum (see continuum hypothesis), Newtonian fluid, incompressible fluid, ideal fluid (see viscous fluid), plane flow and so on. (2) Establishing control equations According to the characteristics of fluid motion, the laws of mass conservation, momentum conservation and energy conservation are expressed in mathematical language, so as to obtain continuity equations, momentum equations and energy equations. In addition, some relations (such as state equation) or other equations related to flow parameters should be added. These equations are collectively called the basic equations of fluid mechanics. Fluid motion is often limited in space and time, so boundary conditions and initial conditions should be given. The mathematical model of the whole flow problem is to establish closed equations that flow parameters must meet, and give appropriate boundary conditions and initial conditions. (3) solving equations; Under the given boundary conditions and initial conditions, the equations are solved by mathematical methods. Because these equations are nonlinear partial differential equations, it is difficult to get analytical solutions and must be simplified, which is one of the reasons for establishing the above mechanical model. After years of efforts, mechanics has created many mathematical methods or skills to solve these equations (mainly simplified equations) and obtained some analytical solutions. (4) After analyzing and explaining the solutions, the physical meaning and flow mechanism of these solutions are explained in combination with specific processes. These theoretical results are usually compared with experimental results to determine the accuracy of the obtained solutions and the applicable scope of the mechanical model. Numerical calculation uses numerical method to solve the equations of the simplified model or the closed basic equations of fluid mechanics. With the appearance and development of electronic computers, it is possible to obtain numerical solutions to many complex hydrodynamic problems that cannot be solved by theoretical analysis. Numerical methods can partially or completely replace some experiments and save experimental expenses. In recent years, numerical calculation methods have developed rapidly, and their importance is increasing day by day. The relationship between field observation, laboratory simulation, theoretical analysis and numerical calculation in solving hydrodynamic problems is complementary. Experiments need the guidance of theory to draw regular conclusions from scattered and seemingly unrelated phenomena and experimental data. On the contrary, theoretical analysis and numerical calculation also rely on field observation and laboratory simulation, give physical models or data, and establish mechanical and mathematical models of flow; Finally, it is necessary to test the perfection of these models and modes by experiments. In addition, the actual flow is often extremely complex (such as turbulence), and theoretical analysis and numerical calculation will encounter great mathematical and computational difficulties, and no specific results can be obtained, so we can only study it through field observation and laboratory simulation. Looking forward to more than 2000 years from Archimedes to now, especially since the 20th century, fluid mechanics has developed into a part of the basic scientific system, and has been widely used in industries, agriculture, transportation, astronomy, geosciences, biology, medicine and other fields. In the future, on the one hand, people will study the applicability of fluid mechanics according to the needs of engineering technology, on the other hand, they will carry out more in-depth basic research to explore the complex flow law and mechanism of fluid. On the other hand, it mainly includes: through theoretical and experimental research on turbulence, we can understand its structure and establish a calculation model; Multiphase flow; Interaction between fluid and structure; Boundary layer flow and separation: biogeography and environmental fluid flow; About all kinds of experimental equipment and instruments. The research fields of fluid mechanics include: theoretical fluid mechanics, fluid mechanics, aerodynamics, aerodynamics, suspension mechanics, theory of turbulence viscous fluid mechanics, multiphase fluid mechanics, seepage mechanics, physical and chemical fluid mechanics, plasma dynamics, electromagnetic fluid mechanics, non-Newtonian fluid mechanics, fluid mechanics, rotation and stratification, fluid mechanics, radiation fluid mechanics, fluid mechanics, fluid mechanics calculation, fluid mechanics, environmental fluid mechanics, micro-fluid.