Fluid mechanics is a branch of mechanics. It mainly studies the static state and motion state of the fluid itself, as well as the interaction and flow rules when there is relative motion between the fluid and the solid boundary wall.
The most studied fluids in fluid mechanics are water and air. Its main basis is Newton's law of motion and the law of conservation of mass. It often also uses knowledge of thermodynamics. Sometimes it also uses the basic laws of macroelectrodynamics, constitutive equations and basic knowledge of physics and chemistry.
When Bernoulli published his monograph in 1738, he first used the term hydrodynamics as the title of the book; around 1880, the term aerodynamics appeared; after 1935, people generalized this term The two aspects of knowledge have established a unified system, collectively called fluid mechanics.
In addition to water and air, fluids also refer to water vapor, lubricating oil, underground oil, sediment-containing river water, blood, metals under ultra-high pressure, and complex compositions produced after combustion as the working medium of the steam turbine. gas, plasma under high temperature conditions, etc.
Fluid mechanics is widely used in the study of meteorology and water conservancy, the design and operation of ships, aircraft, turbine machinery and nuclear power plants, the explosion of flammable gases or explosives, and several astrophysics issues, etc. Knowledge. Many of the concerns of modern science and technology are both guided by fluid mechanics and at the same time promote its continuous development. After 1950, the development of electronic computers gave great impetus to fluid mechanics.
A brief history of the development of fluid mechanics
Fluid mechanics was gradually developed in human beings' struggle with nature and in production practice. In ancient China, there are legends about Dayu controlling water and dredging rivers; the Dujiangyan Irrigation System built by the working people led by Li Bing and his son in the Qin Dynasty is still functioning today; around the same time, the ancient Romans built a large-scale water supply pipeline system and so on.
The first contribution to the formation of the discipline of fluid mechanics was Archimedes of ancient Greece. He established the liquid balance theory including the physical law of buoyancy and the stability of floating bodies, and laid the foundation for hydrostatics. Fundamentals of Mechanics. After more than a thousand years, there was no significant development in fluid mechanics.
It was not until the 15th century that the works of Italian Leonardo da Vinci discussed issues such as water waves, tube flow, hydraulic machinery, and the principles of bird flight; in the 17th century, Pascal clarified the concept of pressure in stationary fluids. However, fluid mechanics, especially fluid dynamics, as a rigorous science, gradually took shape after classical mechanics established concepts such as speed, acceleration, force, and flow fields, as well as the three conservation laws of mass, momentum, and energy. of.
In the 17th century, Newton, the founder of mechanics, studied the resistance experienced by objects moving in fluids and found that the resistance is proportional to the density of the fluid, the cross-sectional area of ??the object facing the flow, and the square of the speed of movement. He also proposed Newton's law of viscosity for internal friction when viscous fluids move. However, Newton had not yet established the theoretical foundation of fluid dynamics, and many of the mechanical models and conclusions he proposed were still quite different from the actual situation.
Later, French Pitot invented the Pitot tube to measure flow velocity; D'Alembert conducted many experimental works on the resistance of ships in the canal and confirmed the square relationship between resistance and the speed of objects; Switzerland's Euler adopted the concept of continuous medium, extended the concept of pressure in statics to moving fluids, established the Euler equation, and correctly described the motion of inviscid fluids with a system of differential equations; Bernoulli derived the energy from classical mechanics Starting from conservation, we studied the flow of water in water supply pipelines, carefully arranged experiments and analyzed them, and obtained the relationship between the flow rate, pressure and pipeline elevation under the steady motion of the fluid - Bernoulli's equation.
The establishment of Euler's equation and Bernoulli's equation marked the establishment of fluid dynamics as a sub-discipline. From then on, the stage of quantitative research on fluid motion using differential equations and experimental measurements began. Since the 18th century, the theory of potential flow has made great progress, and many laws have been clarified in water waves, tides, vortex motion, acoustics, etc.
Lagrange in France did a lot of research on irrotational motion, and Helmholtz in Germany did a lot of research on vortex motion... In the above studies, the viscosity of the fluid does not play an important role, that is, inviscid fluids are considered. This theory, of course, cannot account for the effects of viscosity in fluids.
In the 19th century, engineers were trying to solve many engineering problems, especially those with viscous effects. So they partly used fluid mechanics and partly used semi-empirical formulas that summarized experimental results for research. This formed hydraulics, which is still developing in parallel with fluid mechanics. In 1822, Navier established the basic equation of motion of viscous fluid; in 1845, Stokes derived this equation on a more reasonable basis and demonstrated convincingly the basic concepts of macromechanics involved. This set of equations is the Navier-Stokes equations (NS equations for short) that are still used today, and they are the theoretical basis of fluid dynamics. The Euler equation mentioned above is a special case of the N-S equation when the viscosity is zero.
From 1904 to 1921, the Prandtl school gradually simplified the N-S equation, and established the boundary layer theory from various perspectives such as reasoning, mathematical demonstration, and experimental measurement, which can actually calculate simple situations. The flow state in the boundary layer and the viscous force between the fluid and the solid. At the same time, Planck proposed many new concepts, which were widely used in the design of aircraft and steam turbines. This theory not only clarifies the scope of application of ideal fluids, but also calculates the frictional resistance encountered when an object moves. The above two situations are unified.
At the beginning of the 20th century, the emergence of airplanes greatly promoted the development of aerodynamics. The development of the aviation industry is expected to reveal the pressure distribution around the aircraft, the stress condition and resistance of the aircraft, etc., which promotes the development of fluid mechanics in experimental and theoretical analysis. At the beginning of the 20th century, scientists represented by Zhukovsky, Chaplekin, Planck, etc. pioneered the wing theory based on the potential flow theory of inviscid incompressible fluids and clarified how the wing can Being lifted, the air can lift a heavy aircraft into the sky. The correctness of the airfoil theory enables people to re-understand the theory of inviscid fluids and affirms its great significance in guiding engineering design.
The establishment and development of airfoil theory and boundary layer theory is a major progress in fluid mechanics, which perfectly combines the theory of inviscid fluids with the boundary layer theory of viscous fluids. With the improvement of steam turbines and the increase in aircraft flight speed to more than 50 meters per second, experimental and theoretical research on the effects of air density changes that began in the 19th century has been rapidly expanded, providing 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 space flight was realized. The research on high-speed gas flow progressed rapidly, forming sub-disciplines such as gas dynamics and physical-chemical fluid dynamics. .
Based on these theories, in the 1940s, new theories were formed about the detonation waves that occurred in media such as explosives or natural gas. In order to study the shock waves in the air after the detonation of atomic bombs and explosives, or propagation in water, developing the blast wave theory. Since then, fluid mechanics has developed many branches, such as hypersonic aerodynamics, supersonic aerodynamics, rarefied aerodynamics, electromagnetic fluid dynamics, computational fluid dynamics, two-phase (gas-liquid or gas-solid) flow, etc.
These tremendous progress are inseparable from the use of various mathematical analysis methods and the establishment of large-scale, sophisticated experimental equipment and instruments and other research methods. Since the 1950s, electronic computers have been continuously improved, making it possible to use numerical calculation methods to study topics that were originally difficult to study using analytical methods, and a new branch of computational fluid dynamics emerged. At the same time, due to the needs of civilian and military production, subjects such as liquid dynamics have also made great progress.
In the 1960s, according to the needs of structural mechanics and solid mechanics, the finite element method appeared to calculate elastic mechanics problems. After more than ten years of development, the new calculation method of finite element analysis has begun to be applied in fluid mechanics. Especially in problems with low-speed flow and complex fluid boundary shapes, its advantages are even more significant.
In recent years, the finite element method has been used to study high-speed flow problems, and the mutual penetration and integration of the finite element method and the difference method have also appeared.
Since the 1960s, fluid mechanics has begun to interpenetrate fluid mechanics and other disciplines, forming new interdisciplinary or edge disciplines, such as physical-chemical fluid dynamics, magnetohydrodynamics, etc.; Originally a problem that was basically described qualitatively, it has gradually been studied quantitatively. Biorheology is an example.
Research content of fluid mechanics
Fluid is the general term for gases and liquids. Fluids can be encountered anytime and anywhere in people's daily life and production activities, so fluid mechanics is closely related to human daily life and production undertakings. Atmosphere and water are the two most common fluids. The atmosphere surrounds the entire earth, and 70% of the earth's surface is water. Atmospheric movement, seawater movement (including waves, tides, mesoscale vortices, circulation, etc.) and even the flow of magma deep in the earth are all research contents of fluid mechanics.
At the beginning of the 20th century, after the appearance of the first airplane in the world, airplanes and other various aircraft developed rapidly. Spaceflight that began in the 1950s expanded the scope of human activities to other planets and galaxies. The vigorous development of the aerospace industry is closely linked to the development of aerodynamics and gas dynamics, a branch of fluid mechanics. These subjects are the most active and fruitful areas in fluid mechanics.
The extraction of oil and natural gas and the development and utilization of groundwater require people to understand the movement of fluids in porous or interstitial media, which is the main object of research on seepage mechanics, one of the branches of fluid mechanics. Seepage mechanics also involves technical issues such as the prevention and control of soil salinization, concentration, separation and porous filtration in the chemical industry, and the cooling of combustion chambers.
Combustion is inseparable from gas. This is a fluid dynamics problem involving chemical reactions and thermal energy changes. It is one of the contents of physical-chemical fluid dynamics. Explosion is a violent instantaneous energy change and transfer process involving gas dynamics, thus forming explosion mechanics.
Desert migration, river sediment movement, pulverized coal transportation in pipelines, movement of gas catalysts in chemical industry, etc. all involve problems such as solid particles in the fluid or bubbles in the liquid. Such problems are Scope of Multiphase Fluid Dynamics Research.
Plasma is a collection of free electrons, ions with equal positive charge, and neutral particles. Plasma has special movement rules under the influence of magnetic field. The disciplines that study the motion laws of plasma are called plasma dynamics and electromagnetic hydrodynamics. They are widely used in controlled thermonuclear reactions, magnetic fluid power generation, and cosmic gas movement.
The effect of wind on buildings, bridges, cables, etc. causes them to bear loads and stimulate vibrations; the discharge of waste gas and waste water causes environmental pollution; river bed erosion and migration and coastal erosion; study of the movement of these fluids themselves and The subject of its interaction with humans, animals and plants is called environmental fluid mechanics (including environmental aerodynamics and architectural aerodynamics). This is an emerging edge discipline involving classical fluid mechanics, meteorology, oceanography and hydraulics, structural dynamics, etc.
Biorheology studies related fluid mechanics problems in the human body or other animals and plants, such as the flow of blood in blood vessels, physiological fluid movement in the heart, lungs, and kidneys, and the transportation of nutrient solutions in plants. In addition, the flight of birds in the air, the swimming of animals in the water, etc. are also studied.
Therefore, fluid mechanics includes both the basic theory of natural science and its application in engineering and technical science. 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", it can be divided into ideal fluid dynamics, viscous fluid dynamics, Incompressible fluid dynamics, compressible fluid dynamics and non-Newtonian fluid dynamics, etc.
Research methods of fluid mechanics
Research on fluid mechanics can be divided into four aspects: on-site observation, laboratory simulation, theoretical analysis, and numerical calculation:
On-site observation is a systematic observation of flow phenomena inherent in nature or full-scale flow phenomena of existing projects, using various instruments to summarize the laws of fluid movement and predict the evolution of flow phenomena. Weather observations and forecasts were basically carried out in this way in the past.
However, the occurrence of on-site flow phenomena is often uncontrollable, and the occurrence conditions are almost impossible to completely repeat, which affects the research on flow phenomena and laws; on-site observations also cost a lot of material, financial and human resources. Therefore, people set up laboratories so that these phenomena can occur under controllable conditions for observation and research.
Like physics, chemistry and other disciplines, fluid mechanics is inseparable from experiments, especially the study of new fluid motion phenomena. Experiments can show the characteristics of motion and its main trends, help form concepts, and test the correctness of theories. Every major development in the history of fluid mechanics over the past two hundred years is inseparable from experiments.
Model experiments play an important role in fluid mechanics. The model mentioned here refers to changing (enlarging or reducing) the scale of the research object according to theoretical guidance so that experiments can be arranged. Some flow phenomena are difficult to solve by theoretical calculations, and some are impossible to conduct prototype experiments (the cost is too high or the scale is too large). In this case, based on the data obtained from the model experiment, the prototype data can be obtained using a simple algorithm such as converting the unit system.
On-site observations are often observations of existing things and existing projects, while laboratory simulations can be carried out on things that have not yet appeared and phenomena that have not occurred (such as projects to be designed, machinery, etc.) Observe and improve. Therefore, laboratory simulation is an important method for studying fluid mechanics.
Theoretical analysis is based on the universal laws of fluid movement, such as conservation of mass, conservation of momentum, conservation of energy, etc., using mathematical analysis methods to study the movement of fluids, explain known phenomena, and predict possible results. The steps of theoretical analysis are roughly as follows:
The first is to establish a "mechanical model", that is, to analyze the various contradictions in the actual fluid mechanics problem and grasp the main aspects, simplify the problem and establish a reflective problem The essential "mechanical model". The most commonly used basic models in fluid mechanics are: continuum medium, Newtonian fluid, incompressible fluid, ideal fluid, planar flow, etc.
The second step is to use mathematical language to express the laws of mass conservation, momentum conservation, and energy conservation based on the characteristics of fluid motion, thereby obtaining the continuity equation, momentum equation, and energy equation. In addition, some relational expressions (such as state equations) or other equations related to flow parameters must be added. Together these equations are called the fundamental equations of fluid mechanics.
After finding the solutions to the system of equations, explain the physical meaning and flow mechanism of these solutions based on specific flows. 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.
A series of quantitative studies from basic concepts to basic equations all involve deep mathematical problems, so the development of fluid mechanics is based on the development of mathematics. In turn, those fluid mechanics theories that have been tested by experiments and engineering practices have tested and enriched mathematical theories. Some of the unsolved problems they have raised are also good topics for mathematical research and development of mathematical theories. According to the current level of mathematics development, there are many problems that will be difficult to solve from a purely mathematical perspective in the next few decades.
In fluid mechanics theory, the method of simplifying the physical properties of fluid is used to establish a theoretical model of a specific fluid, and methods such as reducing independent variables and unknown functions are used to simplify mathematical problems, which is successful to a certain extent. , and solved many practical problems.
For a specific field, after considering the specific physical properties and the specific environment of movement, abstraction by grasping the main factors and ignoring secondary factors is also simplification. Establishing a specific mechanical theoretical model can overcome the problem. Mathematical difficulties, further in-depth study of the equilibrium and motion properties of fluids.
Beginning in the 1950s, when designing rocket engines to carry artificial satellites to space, theoretical research coupled with experiments relied on the introduction and simplification of one-dimensional steady flow to provide timely guidance for the design. Fluid mechanics conclusions.
In addition, various small perturbation simplifications are often used in fluid mechanics to change differential equations and boundary conditions from nonlinear to linear. Acoustics is the earliest discipline in fluid mechanics to achieve significant achievements by using small perturbation methods. The so-called small disturbance in acoustics means that when sound propagates in a fluid, the state of the fluid (pressure, density, fluid particle velocity) is very different from when the sound is not transmitted. Although linearized water wave theory and thin airfoil theory are somewhat rough due to simplification, they are all examples of relatively good use of the small perturbation method.
Every reasonable simplification has its mechanical results, but it also always has its limitations. For example, you cannot discuss the propagation of sound if you ignore changes in density; you cannot discuss the resistance and certain other effects associated with viscosity if you ignore it. Mastering reasonable simplification methods, correctly interpreting the rules or conclusions drawn after simplification, comprehensively and fully understanding the scope of application of the simplified model, and correctly estimating the deviation it brings from reality are the essence of theoretical and experimental work in fluid mechanics.
The basic equations of fluid mechanics are very complex, especially when considering viscous effects. Without a computer, we can only calculate relatively simple situations or simplified Euler equations or N-S equations. . In the 1930s and 1940s, for complex and particularly important fluid mechanics problems, manpower was organized to spend months or even years doing numerical calculations. For example, the inviscid flow field around a cone flying at supersonic speeds has been calculated since 1943. The years are counted until 1947.
The development of mathematics, the continuous advancement of computers, and the invention of various calculation methods in fluid mechanics have made it possible to obtain numerical solutions to many complex fluid mechanics problems that could not be solved by theoretical analysis. It also promoted the development of fluid mechanics calculation methods and formed "computational fluid dynamics".
Since the 1960s, electronic computers have often been used to perform numerical simulations on aircraft and other topics involving fluid motion, which can complement physical experiments. Numerical simulation and experimental simulation work together to speed up scientific and technological research and engineering design and save money. Numerical calculation methods have developed rapidly recently and their importance is increasing day by day.
When solving fluid mechanics problems, on-site observations, laboratory simulations, theoretical analysis and numerical calculations complement each other. Experiments require theoretical guidance to draw regular conclusions from scattered, seemingly unconnected phenomena and experimental data. On the contrary, theoretical analysis and numerical calculations also rely on on-site observations and laboratory simulations to provide physical patterns or data to establish mechanical and mathematical models of flow; finally, experiments must be relied on to test the perfection of these models and models. In addition, actual flows are often extremely complex (such as turbulent flow), and theoretical analysis and numerical calculations will encounter huge mathematical and computational difficulties. Specific results cannot be obtained and can only be studied through on-site observations and laboratory simulations.
Prospects of Fluid Mechanics
Over the past two thousand years from Archimedes to the present, especially since the 20th century, fluid mechanics has developed into a part of the basic scientific system. It is also widely used in industry, agriculture, transportation, astronomy, earth science, biology, medicine, etc.
In the future, on the one hand, people will conduct applied research on fluid mechanics based on the needs of engineering technology, and on the other hand, they will conduct more in-depth basic research to explore the complex flow laws and mechanisms of fluids.
The latter aspect mainly includes: through theoretical and experimental research on turbulence, understanding its structure and establishing calculation models; multi-phase flow; interaction between fluids and structures; boundary layer flow and separation; issues such as biogeology and environmental fluid flow; related issues Various experimental equipment and instruments, etc.
Other branches of mechanics
Statics, dynamics, fluid mechanics, analytical mechanics, kinematics, solid mechanics, material mechanics, composite material mechanics, rheology, structural mechanics, Elastic mechanics, plastic mechanics, explosive mechanics, magnetohydrodynamics, aerodynamics, rational mechanics, physical mechanics, celestial mechanics, biomechanics, computational mechanics