Khác biệt giữa bản sửa đổi của “Động lực học chất lưu”

Nội dung được xóa Nội dung được thêm vào
Cheers!-bot (thảo luận | đóng góp)
n →‎Các phương pháp nghiên cứu chuyển động của chất lỏng: clean up, replaced: {{Commonscat → {{thể loại Commons using AWB
Re-translate the whole original paper from English into Vietnamese
Dòng 1:
<p role="presentation">{{Continuum mechanics|fluid}}</p><figure role="presentation">[./Tập_tin:Teardrop_shape.svg [[Tập_tin:Teardrop_shape.svg|liên_kết=|300x300px]]]<figcaption>Typical [[aerodynamic]] teardrop shape, assuming a [[viscous]] medium passing from left to right, the diagram shows the pressure distribution as the thickness of the black line and shows the velocity in the [[boundary layer]] as the violet triangles. The green [[Vortex generator|vortex generators]] prompt the transition to [[Turbulent_flow]] and prevent back-flow also called [[Flow_separation]] from the high pressure region in the back. The surface in front is as smooth as possible or even employs [[Dermal denticle|shark-like skin]], as any turbulence here reduce the energy of the airflow. The truncation on the right, known as a [[Kammback]], also prevents backflow from the high pressure region in the back across the [[Spoiler (aeronautics)|spoilers]] to the convergent part.</figcaption></figure>In [[Physics]], '''fluid dynamics''' is a subdiscipline of [[Fluid_mechanics]] that deals with '''fluid flow'''—the [[natural science]] of [[Fluid|fluids]] ([[Liquid|liquids]] and [[Gas|gases]]) in motion. It has several subdisciplines itself, including [[Aerodynamics]] (the study of air and other gases in motion) and '''hydrodynamics''' (the study of liquids in motion). Fluid dynamics has a wide range of applications, including calculating [[Force|forces]] and [[Moment (physics)|moments]] on [[Aircraft]], determining the [[Mass_flow_rate]] of [[Petroleum]] through pipelines, predicting [[Weather]] patterns, understanding [[Nebula|nebulae]] in [[Interstellar_space]] and modelling [[Nuclear weapon design|fission weapon]] detonation. Some of its principles are even used in [[Traffic engineering (transportation)|traffic engineering]], where traffic is treated as a continuous fluid, and crowd dynamics.
{{chú thích trong bài}}{{wiki hoá}}
'''Thủy động lực học''' nghiên cứu các quy luật đặc trưng [[chuyển động]] của [[chất lỏng]] và nghiên cứu ứng dụng quy luật đó vào thực tế kỹ thuật và đời sống. Để đơn giản trong việc nghiên cứu, ta coi chất lỏng không có [[độ nhớt|tính nhớt]] khi chuyển động (chất lỏng lý tưởng), sau đó xét đến tính nhớt của chất lỏng thực, phải đưa thêm vào các hệ số cần thiết để hiệu chỉnh sự sai lệch do bỏ qua tác dụng của [[lực nhớt]].
 
Fluid dynamics offers a systematic structure—which underlies these [[Practical_disciplines]]—that embraces empirical and semi-empirical laws derived from [[Flow_measurement]] and used to solve practical problems. The solution to a fluid dynamics problem typically involves calculating various properties of the fluid, such as [[Flow_velocity]], [[Pressure]], [[Density]], and [[Temperature]], as functions of space and time.
==Các yếu tố chuyển động, các phương pháp nghiên cứu thủy động lực học==
===Các yếu tố chuyển động===
Chất lỏng chuyển động là một môi trường liên tục do vô số phần tử chất lỏng chuyển động tạo nên, mỗi phần tử được đặc trưng bởi các đại lượng cơ bản của sự chuyển động, gọi là yếu tố chuyển động. Ví dụ như vận tốc <math>u</math>, áp suất thủy động <math>p</math>, khối lượng riêng ρ… Do chất lỏng là một môi trường liên tục nên các yếu tố chuyển động đều là hàm số liên tục của tọa độ không gian và thời gian. u = u(x,y,z,t); p = p(x,y,z,t); ρ = ρ(x,y,z,t)... Trong [[thủy lực học|thủy lực]], ta thường xét đến <math>u</math> và <math>p</math>, còn ρ coi như không đổi vì ta coi chất lỏng như không nén được.
 
Before the twentieth century, ''hydrodynamics'' was synonymous with fluid dynamics. This is still reflected in names of some fluid dynamics topics, like [[Magnetohydrodynamics]] and [[Hydrodynamic_stability]], both of which can also be applied to gases.<ref>{{Cite book|title=The Dawn of Fluid Dynamics: A Discipline Between Science and Technology|first=Michael|last=Eckert|publisher=Wiley|year=2006|isbn=3-527-40513-5|page=ix}}</ref>
* Khi coi chất lỏng là lý tưởng (không có tính nhớt) [[áp suất thủy động]] hướng theo pháp tuyến của mặt tác dụng, còn trong chất lỏng thực, áp suất thủy động cũng hướng vào mặt tiếp xúc nhưng xiên góc với phương pháp tuyến, vì nó là tổng hợp của [[ứng suất pháp tuyến]] và [[ứng suất tiếp tuyến]] do lực nhớt gây ra.
==Equations of fluid dynamics==
The foundational axioms of fluid dynamics are the [[Conservation law (physics)|conservation laws]], specifically, [[Conservation_of_mass]], [[Conservation of momentum|conservation of linear momentum]] (also known as [[Newton's laws of motion|Newton's Second Law of Motion]]), and [[Conservation_of_energy]] (also known as [[First_Law_of_Thermodynamics]]). These are based on [[Classical_mechanics]] and are modified in [[Quantum_mechanics]] and [[General_relativity]]. They are expressed using the [[Reynolds transport theorem|Reynolds Transport Theorem]].
 
In addition to the above, fluids are assumed to obey the ''continuum assumption''. Fluids are composed of molecules that collide with one another and solid objects. However, the continuum assumption considers fluids as continuous, rather than discrete. Consequently, properties such as density, pressure, temperature, and flow velocity are assumed well-defined at [[Infinitesimal|infinitesimally]] small points, and are assumed to vary continuously from one point to another. The fact that the fluid is made up of discrete molecules is ignored.
* Tại một vị trí nhất định trong lòng chất lỏng chuyển động, ở một thời điểm nhất định, vận tốc của một phần tử chất lỏng đo được gọi là vận tốc tức thời, kí hiệu <math>u</math>. Riêng đối với dòng chảy rối, vận tốc điểm tức thời này luôn thay đổi về hướng và trị số nên ta thường thay bằng giá trị trung bình trong một thời gian <math>T</math> nhất định gọi là vận tốc trung bình thời gian:
:<math>
\bar{u}=1/T \int_{0}^T u (t) dt.
</math>
 
For fluids that are sufficiently dense to be a continuum, do not contain ionized species, and have flow velocities small in relation to the speed of light, the momentum equations for [[Newtonian fluid|Newtonian fluids]] are the [[Navier–Stokes_equations]]—which is a [[Non-linear]] set of [[Differential_equations]] that describes the flow of a fluid whose stress depends linearly on flow velocity gradients and pressure. The unsimplified equations do not have a general [[Solution in closed form|closed-form solution]], so they are primarily of use in [[Computational_Fluid_Dynamics]]. The equations can be simplified in a number of ways, all of which make them easier to solve. Some of them allow appropriate fluid dynamics problems be solved in closed form.{{citation needed|date=May 2014}}
Tuy vậy, trừ trường hợp đặc biệt của chuyển động, nói chung người ta thường lấy giá trị vận tốc điểm trung bình thời gian, thay cho vận tốc tức thời, nhưng vẫn dùng kí hiệu <math>u</math>. Còn trong kỹ thuật, người ta thường dùng khái niệm vận tốc trung bình của toàn dòng chảy qua một mặt cắt ngang, thẳng góc với một trục dòng chảy, gọi là vận tốc trung bình mặt cắt của dòng chảy, kí hiệu <math>v</math>.
 
In addition to the mass, momentum, and energy conservation equations, a [[Thermodynamics|thermodynamical]] equation of state giving the pressure as a function of other thermodynamic variables for the fluid is required to completely specify the problem. An example of this would be the [[Ideal gas law|perfect gas equation of state]]:
===Các phương pháp nghiên cứu chuyển động của chất lỏng===
<dd><math>p= \frac{\rho R_u T}{M}</math></dd><p>where ''p'' is [[pressure]], ρ is [[density]], ''R<sub href="Thể loại:Cơ học môi trường liên tục">u</sub>'' is the [[gas constant]], ''M'' is [[molar mass]] and ''T'' is [[temperature]].</p>
Có hai phương pháp nghiên cứu: phương pháp [[Lagrange]] và phương pháp [[Leonhard Euler|Euler]].
 
=== Conservation laws ===
*Phương pháp Lagrange:
Three conservation laws are used to solve fluid dynamics problems, and may be written in [[integral]] or [[Differential (infinitesimal)|differential]] form. Mathematical formulations of these conservation laws may be interpreted by considering the concept of a ''control volume''. A control volume is a specified volume in space through which air can flow in and out. Integral formulations of the conservation laws consider the change in mass, momentum, or energy within the control volume. Differential formulations of the conservation laws apply [[Stokes' theorem]] to yield an expression which may be interpreted as the integral form of the law applied to an infinitesimal volume at a point within the flow.
Khảo sát chuyển động của từng phần tử chất lỏng riêng biệt, như nghiên cứu chuyển động của điểm vật rắn trong cơ lý thuyết. Nếu dùng một hệ tọa độ vuông góc bất kỳ <math>Oxyz</math> trong môi trường chất lỏng, thì một phần tử chất lỏng ở thời điểm ban đầu t0 tại vị trí <math>M_0</math>, tọa độ <math>(a,b,c)</math> sẽ chuyển đến vị trí <math>M</math> có tọa độ <math>(x,y,z)</math> ở thời điểm <math>t</math>, ta có:
* [[Continuity equation#Fluid dynamics|Mass continuity]] (conservation of mass): The rate of change of fluid mass inside a control volume must be equal to the net rate of fluid flow into the volume. Physically, this statement requires that mass is neither created nor destroyed in the control volume,<ref>Anderson, J.D., ''Fundamentals of Aerodynamics'', 4th Ed., McGraw–Hill, 2007.</ref> and can be translated into the integral form of the continuity equation:
:: <math>{\partial \over \partial t} \iiint_V \rho \, dV = - \, {} </math> {{oiint|preintegral=|intsubscpt=<math>{\scriptstyle S}</math>|integrand=<math>{}\,\rho\mathbf{u}\cdot d\mathbf{S}</math>}}
 
: Above, <math>\rho</math> is the fluid density, '''u''' is the [[flow velocity]] vector, and ''t'' is time. The left-hand side of the above expression contains a triple integral over the control volume, whereas the right-hand side contains a surface integral over the surface of the control volume. The differential form of the continuity equation is, by the [[divergence theorem]]:
:<math>x = f_1(a,b,c,t)</math>
:<math>y = f_2(a,b,c,t)</math>
:<math>z = f_3(a,b,c,t)</math>
 
:: <math>\ {\partial \rho \over \partial t} + \nabla \cdot (\rho \mathbf{u}) = 0 </math>
Các tọa độ <math>a,b,c</math> của phần tử chất lỏng ở thời điểm ban đầu và thời gian <math>t</math>, gọi là biến số Lagrange. Có thể biết được chuyển động của phần tử và quỹ đạo của nó nếu biết được <math>x,y,z</math> và <math>t</math> (xem 3). Trong thực tế, ngoài một số trường hợp như nghiên cứu chuyển động của sóng bề mặt hay chuyển động biến dạng của phần tử chất lỏng, người ta ít quan tâm đến chuyển động riêng biệt của từng phần tử chất lỏng, hơn nữa nó dẫn đến những tính toán phức tạp nên trong thủy lực học, phương pháp Lagrange được dùng ít hơn phương pháp Euler.
* [[Momentum|Conservation of momentum]]: This equation applies [[Newton's second law of motion]] to the control volume, requiring that any change in momentum of the air within a control volume be due to the net flow of air into the volume and the action of external forces on the air within the volume. In the integral formulation of this equation, [[Body force|body forces]] here are represented by ''f''<sub>body</sub>, the body force per unit mass. [[Surface force|Surface forces]], such as viscous forces, are represented by '''<math>\mathbf{F}_\text{surf}</math>''', the net force due to [[Stress (mechanics)|stresses]] on the control volume surface.
:: <math> \frac{\partial}{\partial t} \iiint_{\scriptstyle V} \rho\mathbf{u} \, dV = -\, {} </math> {{oiint|integrand|preintegral=|intsubscpt=<math>_{\scriptstyle S}</math>}} <math> (\rho\mathbf{u}\cdot d\mathbf{S}) \mathbf{u} -{}</math> {{oiint|intsubscpt=<math>{\scriptstyle S}</math>|integrand=<math> {}\, p \, d\mathbf{S}</math>}} <math>\displaystyle{}+ \iiint_{\scriptstyle V} \rho \mathbf{f}_\text{body} \, dV + \mathbf{F}_\text{surf}</math>
 
: The differential form of the momentum conservation equation is as follows. Here, both surface and body forces are accounted for in one total force, ''F''. For example, ''F'' may be expanded into an expression for the frictional and gravitational forces acting on an internal flow.
*Phương pháp [[Leonhard Euler|Euler]]: Nghiên cứu vận tốc của phần tử chất lỏng tại nhiều điểm trong dòng chảy ở những thời điểm khác nhau. (Hình vẽ trên) Ví dụ: Xét một điểm <math>M</math> có toạ độ <math>x,y,z</math> cố định trong không gian chứa chất lỏng chuyển động và một phần tử chất lỏng chuyển động với vận tốc <math>u</math> qua điểm <math>M</math> tại thời gian <math>t</math>. Lúc đó, <math>u</math> phụ thuộc vào cả toạ độ điểm <math>M</math> và thời gian <math>t</math>, ta có: <math>u = f(x,y,z,t)</math> và <math>x,y,z,t</math> gọi là biến số Euler.
:: <math>\ {D \mathbf{u} \over D t} = \mathbf{F} - {\nabla p \over \rho} </math>
 
: In aerodynamics, air is assumed to be a [[Newtonian fluid]], which posits a linear relationship between the shear stress (due to internal friction forces) and the rate of strain of the fluid. The equation above is a vector equation: in a three-dimensional flow, it can be expressed as three scalar equations. The conservation of momentum equations for the compressible, viscous flow case are called the Navier–Stokes equations.{{citation needed|date=May 2014}}
Phương pháp Euler thường dùng hơn vì: - Đối với một số dòng chảy thường nghiên cứu trong thủy lực học, gọi là dòng chảy ổn định với tốc độ dòng chảy <math>u</math> không phụ thuộc vào thời gian: <math>u =f(x,y,z)</math>. - Toàn bộ các véc tơ vận tốc của các phần tử chất lỏng chuyển động qua các điểm cố định tạo thành một trường vận tốc, có những tính chất của những [[trường vector|trường véctơ]], do đó việc tính toán được thuận tiện hơn. Có thể xác định được đường dòng khi biết được biến Euler. (xem mục 3 sau)<!-- 3 mục nào? --> - Phương pháp này tiện lợi cho việc nghiên cứu thực nghiệm. Hoàn toàn có thể suy từ biến Euler sang biến Lagrange và ngược lại.
* [[Conservation of energy]]: Although [[energy]] can be converted from one form to another, the total [[energy]] in a given closed system remains constant.
{{thể loại Commons|Fluid dynamics}}
:: <math>\ \rho {Dh \over Dt} = {D p \over D t} + \nabla \cdot \left( k \nabla T\right) + \Phi </math>
 
: Above, ''h'' is [[enthalpy]], ''k'' is the [[thermal conductivity]] of the fluid, ''T'' is temperature, and <math>\Phi</math> is the viscous dissipation function. The viscous dissipation function governs the rate at which mechanical energy of the flow is converted to heat. The [[second law of thermodynamics]] requires that the dissipation term is always positive: viscosity cannot create energy within the control volume.<ref>White, F.M., ''Viscous Fluid Flow'', McGraw–Hill, 1974.</ref> The expression on the left side is a [[material derivative]].
==Tham khảo==
{{tham khảo}}
 
=== Compressible vs incompressible flow ===
All fluids are [[Compressibility|compressible]] to some extent, that is, changes in pressure or temperature cause changes in density. However, in many situations the changes in pressure and temperature are sufficiently small that the changes in [[density]] are negligible. In this case the flow can be modelled as an [[incompressible flow]]. Otherwise the more general [[compressible flow]] equations must be used.
 
Mathematically, incompressibility is expressed by saying that the density ρ of a [[fluid parcel]] does not change as it moves in the flow field, i.e.,
: <math>\frac{\mathrm{D} \rho}{\mathrm{D}t} = 0 \, ,</math>
where ''D''/''Dt'' is the [[substantial derivative]], which is the sum of [[Time derivative|local]] and [[Convective derivative|convective derivatives]]. This additional constraint simplifies the governing equations, especially in the case when the fluid has a uniform density.
 
For flow of gases, to determine whether to use compressible or incompressible fluid dynamics, the [[Mach number]] of the flow is evaluated. As a rough guide, compressible effects can be ignored at Mach numbers below approximately 0.3. For liquids, whether the incompressible assumption is valid depends on the fluid properties (specifically the critical pressure and temperature of the fluid) and the flow conditions (how close to the critical pressure the actual flow pressure becomes). [[Acoustics|Acoustic]] problems always require allowing compressibility, since [[sound waves]] are compression waves involving changes in pressure and density of the medium through which they propagate.
 
=== Inviscid vs Newtonian and non-Newtonian fluids ===
[[Tập_tin:Potential_flow_around_a_wing.gif|nhỏ|Potential flow around a wing]]
All fluids are [[Viscosity|viscous]], meaning that they exert some resistance to deformation: neighbouring parcels of fluid moving at different velocities exert viscous forces on each other. The velocity gradient is referred to as a [[Strain (materials science)|strain rate]]; it has dimensions <math>T^{-1}</math>. [[Isaac Newton]] showed that for many familiar fluids such as [[water]] and [[Earth's atmosphere|air]], the [[Stress (physics)|stress]] due to these viscous forces is linearly related to the strain rate. Such fluids are called [[Newtonian fluids]]. The coefficient of proportionality is called the fluid's [[viscosity]]; for Newtonian fluids, it is a fluid property independent of the strain rate.
 
[[Non-Newtonian fluid|Non-Newtonian fluids]] have a more complicated, non-linear stress-strain behaviour. The sub-discipline of [[rheology]] studies the stress-strain behaviours of these fluids, which include [[Emulsion|emulsions]] and [[slurries]], some [[Viscoelasticity|viscoelastic]] materials such as [[blood]] and some [[Polymer|polymers]], and ''sticky liquids'' such as [[latex]], [[honey]] and [[lubricants]].{{citation needed|date=June 2015}}
 
The dynamic of fluid parcels is described with the help of [[Newton's second law]]. An accelerating parcel of fluid is subject to inertial effects.
 
The [[Reynolds number]] is a [[dimensionless quantity]] which characterises the magnitude of inertial effects compared to the magnitude of viscous effects. A low Reynolds number (''Re''<<1) indicates that viscous forces are very strong compared to inertial forces. In such cases, inertial forces are sometimes neglected; this flow regime is called [[Stokes flow|Stokes or creeping flow]].
 
On the contrary, high Reynolds numbers (''Re''>>1) indicate that the inertial effects have more effect on the velocity field than the viscous (friction) effects. In high Reynolds number flows, the flow is often modeled as an [[inviscid flow]], an approximation in which [[viscosity]] is completely neglected. The [[Navier–Stokes equations|Navier-Stokes equations]] then simplify into the [[Euler equations (fluid dynamics)|Euler equations]]. Integrating these along a streamline in an inviscid flow yields [[Bernoulli's equation]]. When in addition to being inviscid, the flow is everywhere [[Lamellar field|irrotational]], Bernoulli's equation can be used throughout the flow field. Such flows are called [[Potential flow|potential flows]], because the velocity field may be expressed as the [[gradient]] of a potential.
 
This idea can work fairly well when the Reynolds number is high. However, problems such as those involving solid boundaries may require that the viscosity be included. Viscosity cannot be neglected near solid boundaries because the [[no-slip condition]] generates a thin region of large strain rate, the [[boundary layer]], in which [[viscosity]] effects dominate and which thus generates [[vorticity]]. Therefore, to calculate net forces on bodies (such as wings), viscous flow equations must be used: inviscid flow theory fails to predict [[Drag (physics)|drag forces]], a limitation known as the [[d'Alembert's paradox]].
 
A commonly used{{citation needed|date=June 2015}} model, especially in [[computational fluid dynamics]], is to use two flow models: the Euler equations away from the body, and [[boundary layer]] equations in a region close to the body. The two solutions can then be matched with each other, using the [[method of matched asymptotic expansions]].
 
=== Steady vs unsteady flow ===<!-- [[Steady flow]] redirects here -->
[[Tập_tin:HD-Rayleigh-Taylor.gif|nhỏ|320x320px|Hydrodynamics simulation of the [[Rayleigh–Taylor instability]] <ref>Shengtai Li, Hui Li "Parallel AMR Code for Compressible MHD or HD Equations" (Los Alamos National Laboratory) [http://math.lanl.gov/Research/Highlights/amrmhd.shtml]</ref>]]
When all the time derivatives of a flow field vanish, the flow is considered '''steady flow'''. Steady-state flow refers to the condition where the fluid properties at a point in the system do not change over time. Otherwise, flow is called unsteady (also called transient<ref>[http://www.cfd-online.com/Forums/main/118306-transient-state-unsteady-state.html Transient state or unsteady state?]</ref>). Whether a particular flow is steady or unsteady, can depend on the chosen frame of reference. For instance, laminar flow over a [[sphere]] is steady in the frame of reference that is stationary with respect to the sphere. In a frame of reference that is stationary with respect to a background flow, the flow is unsteady.
 
[[Turbulence|Turbulent]] flows are unsteady by definition. A turbulent flow can, however, be [[Stationary process|statistically stationary]]. According to Pope:<ref>See Pope (2000), page 75.</ref>{{quote|
The random field ''U''(''x'',''t'') is statistically stationary if all statistics are invariant under a shift in time.
}}This roughly means that all statistical properties are constant in time. Often, the mean field is the object of interest, and this is constant too in a statistically stationary flow.
 
Steady flows are often more tractable than otherwise similar unsteady flows. The governing equations of a steady problem have one dimension fewer (time) than the governing equations of the same problem without taking advantage of the steadiness of the flow field.
 
=== Laminar vs turbulent flow ===
[[Turbulence]] is flow characterized by recirculation, [[Eddy (fluid dynamics)|eddies]], and apparent [[Random|randomness]]. Flow in which turbulence is not exhibited is called [[Laminar flow|laminar]]. It should be noted, however, that the presence of eddies or recirculation alone does not necessarily indicate turbulent flow—these phenomena may be present in laminar flow as well. Mathematically, turbulent flow is often represented via a [[Reynolds decomposition]], in which the flow is broken down into the sum of an [[average]] component and a perturbation component.
 
It is believed that turbulent flows can be described well through the use of the [[Navier–Stokes equations]]. [[Direct numerical simulation]] (DNS), based on the Navier–Stokes equations, makes it possible to simulate turbulent flows at moderate Reynolds numbers. Restrictions depend on the power of the computer used and the efficiency of the solution algorithm. The results of DNS have been found to agree well with experimental data for some flows.<ref>See, for example, Schlatter et al, Phys. Fluids 21, 051702 (2009); {{doi|10.1063/1.3139294}}</ref>
 
Most flows of interest have Reynolds numbers much too high for DNS to be a viable option,<ref>See Pope (2000), page 344.</ref> given the state of computational power for the next few decades. Any flight vehicle large enough to carry a human (L > 3 m), moving faster than 72&nbsp;km/h (20&nbsp;m/s) is well beyond the limit of DNS simulation (Re = 4 million). Transport aircraft wings (such as on an [[Airbus A300]] or [[Boeing 747]]) have Reynolds numbers of 40 million (based on the wing chord). Solving these real-life flow problems requires turbulence models for the foreseeable future. [[Reynolds-averaged Navier–Stokes equations]] (RANS) combined with [[turbulence modelling]] provides a model of the effects of the turbulent flow. Such a modelling mainly provides the additional momentum transfer by the [[Reynolds stresses]], although the turbulence also enhances the [[Heat transfer|heat]] and [[mass transfer]]. Another promising methodology is [[large eddy simulation]] (LES), especially in the guise of [[detached eddy simulation]] (DES)—which is a combination of RANS turbulence modelling and large eddy simulation.
 
=== Subsonic vs transonic, supersonic and hypersonic flows ===
While many terrestrial flows (e.g. flow of water through a pipe) occur at low mach numbers, many flows of practical interest (e.g. in aerodynamics) occur at high fractions of the Mach Number M=1 or in excess of it (supersonic flows). New phenomena occur at these Mach number regimes (e.g. shock waves for supersonic flow, transonic instability in a regime of flows with M nearly equal to 1, non-equilibrium chemical behaviour due to ionization in hypersonic flows) and it is necessary to treat each of these flow regimes separately.
 
=== Magnetohydrodynamics ===
{{main|Magnetohydrodynamics}}
[[Magnetohydrodynamics]] is the multi-disciplinary study of the flow of [[Electrical conduction|electrically conducting]] fluids in [[Electromagnetism|electromagnetic]] fields. Examples of such fluids include [[Plasma (physics)|plasmas]], liquid metals, and [[Saline water|salt water]]. The fluid flow equations are solved simultaneously with [[Maxwell's equations]] of electromagnetism.
 
=== Other approximations ===
There are a large number of other possible approximations to fluid dynamic problems. Some of the more commonly used are listed below.
* The ''[[Boussinesq approximation (buoyancy)|Boussinesq approximation]]'' neglects variations in density except to calculate [[buoyancy]] forces. It is often used in free [[convection]] problems where density changes are small.
* ''[[Lubrication theory]]'' and ''[[Hele–Shaw flow]]'' exploits the large [[aspect ratio]] of the domain to show that certain terms in the equations are small and so can be neglected.
* ''[[Slender-body theory]]'' is a methodology used in [[Stokes flow]] problems to estimate the force on, or flow field around, a long slender object in a viscous fluid.
* The ''[[shallow-water equations]]'' can be used to describe a layer of relatively inviscid fluid with a [[free surface]], in which surface [[Slope|gradients]] are small.
* The ''[[Boussinesq equations (water waves)|Boussinesq equations]]'' are applicable to [[surface waves]] on thicker layers of fluid and with steeper surface [[Slope|slopes]].
* ''[[Darcy's law]]'' is used for flow in [[Porous medium|porous media]], and works with variables averaged over several pore-widths.
* In rotating systems, the ''[[quasi-geostrophic equations]]'' assume an almost [[Balanced flow#Geostrophic flow|perfect balance]] between [[Pressure gradient|pressure gradients]] and the [[Coriolis force]]. It is useful in the study of [[atmospheric dynamics]].
 
== Terminology in fluid dynamics ==
The concept of [[pressure]] is central to the study of both fluid statics and fluid dynamics. A pressure can be identified for every point in a body of fluid, regardless of whether the fluid is in motion or not. Pressure can be [[Pressure measurement|measured]] using an aneroid, Bourdon tube, mercury column, or various other methods.
 
Some of the terminology that is necessary in the study of fluid dynamics is not found in other similar areas of study. In particular, some of the terminology used in fluid dynamics is not used in [[fluid statics]].
 
=== Terminology in incompressible fluid dynamics ===
The concepts of total pressure and [[dynamic pressure]] arise from [[Bernoulli's equation]] and are significant in the study of all fluid flows. (These two pressures are not pressures in the usual sense—they cannot be measured using an aneroid, Bourdon tube or mercury column.) To avoid potential ambiguity when referring to [[pressure]] in fluid dynamics, many authors use the term [[static pressure]] to distinguish it from total pressure and dynamic pressure. [[Static pressure]] is identical to [[pressure]] and can be identified for every point in a fluid flow field.
 
In ''Aerodynamics'', L.J. Clancy writes:<ref>Clancy, L.J. ''Aerodynamics'', page 21</ref> ''To distinguish it from the total and dynamic pressures, the actual pressure of the fluid, which is associated not with its motion but with its state, is often referred to as the static pressure, but where the term pressure alone is used it refers to this static pressure.''
 
A point in a fluid flow where the flow has come to rest (i.e. speed is equal to zero adjacent to some solid body immersed in the fluid flow) is of special significance. It is of such importance that it is given a special name—a [[stagnation point]]. The static pressure at the stagnation point is of special significance and is given its own name—[[stagnation pressure]]. In incompressible flows, the stagnation pressure at a stagnation point is equal to the total pressure throughout the flow field.
 
=== Terminology in compressible fluid dynamics ===
In a compressible fluid, such as air, the temperature and density are essential when determining the state of the fluid. In addition to the concept of total pressure (also known as [[stagnation pressure]]), the concepts of total (or stagnation) temperature and total (or stagnation) density are also essential in any study of compressible fluid flows. To avoid potential ambiguity when referring to temperature and density, many authors use the terms static temperature and static density. Static temperature is identical to temperature; and static density is identical to density; and both can be identified for every point in a fluid flow field.
 
The temperature and density at a [[stagnation point]] are called stagnation temperature and stagnation density.
 
A similar approach is also taken with the thermodynamic properties of compressible fluids. Many authors use the terms total (or stagnation) [[enthalpy]] and total (or stagnation) [[entropy]]. The terms ''static enthalpy'' and ''static entropy'' appear less common, but where they are used they mean enthalpy and entropy respectively, using the prefix "static" to avoid ambiguity with their 'total' or 'stagnation' counterparts. Because the 'total' flow conditions are defined by [[Isentropic|isentropically]] bringing the fluid to rest, the total (or stagnation) entropy is by definition always equal to the "static" entropy.
 
== See also ==
 
=== Fields of study ===
{{columns-list|3|
*[[Acoustic theory]]
*[[Aerodynamics]]
*[[Aeroelasticity]]
*[[Aeronautics]]
*[[Computational fluid dynamics]]
*[[Flow measurement]]
*[[Geophysical fluid dynamics]]
*[[haemodynamics|Hemodynamics]]
*[[Hydraulics]]
*[[Hydrology]]
*[[Hydrostatics]]
*[[Electrohydrodynamics]]
*[[Magnetohydrodynamics]]
*[[Metafluid dynamics]]
*[[Quantum hydrodynamics]]
*[[Rheology]]
}}
 
=== Mathematical equations and concepts ===
{{columns-list|3|
*[[Airy wave theory]]
*[[Bernoulli's equation]]
*[[Reynolds transport theorem]]
*[[Benjamin–Bona–Mahony equation]]
*[[Boussinesq approximation (buoyancy)]]
*[[Boussinesq approximation (water waves)]]
*[[Conservation laws]]
*[[Euler equations (fluid dynamics)]]
*[[Different types of boundary conditions in fluid dynamics]]
*[[Darcy's law]]
*[[Dynamic pressure]]
*[[Fluid statics]]
*[[Hagen–Poiseuille equation]]
*[[Helmholtz's theorems]]
*[[Kirchhoff equations]]
*[[Knudsen equation]]
*[[Manning equation]]
*[[Mild-slope equation]]
*[[Morison equation]]
*[[Navier–Stokes equations]]
*[[Oseen flow]]
*[[Pascal's law]]
*[[Poiseuille's law]]
*[[Potential flow]]
*[[Pressure]]
*[[Pressure head]]
*[[Relativistic Euler equations]]
*[[Reynolds decomposition]]
*[[Static pressure]]
*[[Stokes flow]]
*[[Stokes stream function]]
*[[Stream function]]
*[[Streamlines, streaklines and pathlines]]
*[[Torricelli's Law]]
}}
 
=== Types of fluid flow ===
{{columns-list|3|
*[[Aerodynamic force]]
*[[Cavitation]]
*[[Compressible flow]]
*[[Couette flow]]
*[[Free molecular flow]]
*[[Incompressible flow]]
*[[Inviscid flow]]
*[[Isothermal flow]]
*[[Laminar flow]]
*[[Open channel flow]]
*[[Secondary flow]]
*[[Stream thrust averaging]]
*[[Superfluidity]]
*[[Supersonic]]
*[[Transient flow]]
*[[Transonic]]
*[[Turbulence|Turbulent flow]]
*[[Two-phase flow]]
}}
 
=== Fluid properties ===
{{columns-list|3|
*[[Density]]
*[[List of hydrodynamic instabilities]]
*[[Newtonian fluid]]
*[[Non-Newtonian fluid]]
*[[Surface tension]]
*[[Viscosity]]
*[[Vapour pressure]]
*[[Compressibility]]
}}
 
=== Fluid phenomena ===
{{columns-list|3|
*[[Boundary layer]]
*[[Coanda effect]]
*[[Convection cell]]
*[[squeeze mapping#Corner flow|Convergence/Bifurcation]]
*[[Darwin drift]]
*[[Drag (force)]]
*[[Hydrodynamic stability]]
*[[Kaye effect]]
*[[Lift (force)]]
*[[Magnus effect]]
*[[Ocean surface waves]]
*[[Rossby wave]]
*[[Shock wave]]
*[[Soliton]]
*[[Stokes drift]]
*[[Turbulence]]
*[[Fluid thread breakup|Thread breakup]]
*[[Upstream contamination]]
*[[Venturi effect]]
*[[Vortex]]
*[[Vorticity]]
*[[Water hammer]]
*[[Wave drag]]
}}
 
=== Applications ===
{{columns-list|3|
*[[Acoustics]]
*[[Aerodynamics]]
*[[Cryosphere science]]
*[[Fluid power]]
*[[Geodynamics]]
*[[Hydraulic machinery]]
*[[Meteorology]]
*[[Naval architecture]]
*[[Oceanography]]
*[[Plasma physics]]
*[[Pneumatics]]
*[[3D computer graphics]]
}}
 
=== Fluid dynamics journals ===
{{columns-list|3|
* ''[[Annual Review of Fluid Mechanics]]''
* ''[[Journal of Fluid Mechanics]]''
* ''[[Physics of Fluids]]''
* ''[[Experiments in Fluids]]''
* ''European Journal of Mechanics B: Fluids''
* ''Theoretical and Computational Fluid Dynamics''
* ''Computers and Fluids''
* ''[[International Journal for Numerical Methods in Fluids]]''
* ''[[Flow, Turbulence and Combustion]]''
}}
 
=== Miscellaneous ===
{{columns-list|3|
*[[List of publications in physics#Fluid dynamics|Important publications in fluid dynamics]]
*[[Isosurface]]
*[[Keulegan–Carpenter number]]
*[[Rotating tank]]
*[[Sound barrier]]
*[[Beta plane]]
*[[Immersed boundary method]]
*[[Bridge scour]]
* [[Finite volume method for unsteady flow]]
}}
 
=== See also ===
{{div col|4}}
* [[Aileron]]
* [[Airfoil]]
* [[Airplane]]
* [[Angle of attack]]
* [[Banked turn]]
* [[Bernoulli's principle]]
* [[Bilgeboard]]
* [[Boomerang]]
* [[Centerboard]]
* [[Chord (aircraft)]]
* [[Circulation control wing]]
* [[Diving plane]]
* [[Downforce]]
* [[Drag coefficient]]
* [[Drag (physics)]]
* [[Fin]]
* [[Flipper (anatomy)]]
* [[Flow separation]]
* [[Foil (fluid mechanics)]]
* [[Formula One car]]
* [[Glider (aircraft)|Glider]]
* [[Hydrofoil]]
* [[Keel]] (hydrodynamic)
* [[Küssner effect]]
* [[Kutta condition]]
* [[Kutta–Joukowski theorem]]
* [[Lift coefficient]]
* [[Lift-induced drag]]
* [[Lift-to-drag ratio]]
* [[Lifting-line theory]]
* [[NACA airfoil]]
* [[Newton's laws of motion#Newton's third law|Newton's third law]]
* [[Propeller]]
* [[Pump]]
* [[Rudder]]
* [[Sail]] (aerodynamics)
* [[Skeg]]
* [[Spoiler (automotive)]]
* [[Stall (flight)]]
* [[Surfboard fin]]
* [[Surface science]]
* [[Trim tab]]
* [[Wing]]
* [[Wingtip vortices]]
{{div col end}}
 
== References ==
{{reflist|2}}
 
== Further reading ==
* {{cite book|last=Acheson|first=D. J.|title=Elementary Fluid Dynamics|publisher=Clarendon Press|year=1990|isbn=0-19-859679-0}}
* {{cite book|last=Batchelor|first=G. K.|authorlink=George Batchelor|title=An Introduction to Fluid Dynamics|publisher=Cambridge University Press|year=1967|isbn=0-521-66396-2}}
* {{cite book|last=Chanson|first=H.|authorlink=Hubert Chanson|title=Applied Hydrodynamics: An Introduction to Ideal and Real Fluid Flows|publisher=CRC Press, Taylor & Francis Group, Leiden, The Netherlands, 478 pages|year=2009|isbn=978-0-415-49271-3}}
* {{cite book|last=Clancy|first=L. J.|title=Aerodynamics|publisher=Pitman Publishing Limited|location=London|year=1975|isbn=0-273-01120-0}}
* {{cite book|last=Lamb|first=Horace|authorlink=Horace Lamb|title=Hydrodynamics|edition=6th|publisher=Cambridge University Press|year=1994|isbn=0-521-45868-4}} Originally published in 1879, the 6th extended edition appeared first in 1932.
* {{cite book|last1=Landau|first1=L. D.|author1-link=Lev Landau|last2=Lifshitz|first2=E. M.|author2-link=Evgeny Lifshitz|title=Fluid Mechanics|edition=2nd|series=[[Course of Theoretical Physics]]|publisher=Pergamon Press|year=1987|isbn=0-7506-2767-0}}
* {{cite book|last=Milne-Thompson|first=L. M.|title=Theoretical Hydrodynamics|edition=5th|publisher=Macmillan|year=1968}} Originally published in 1938.
* {{cite book|last=Pope|first=Stephen B.|title=Turbulent Flows|publisher=Cambridge University Press|year=2000|isbn=0-521-59886-9}}
* {{cite book|last=Shinbrot|first=M.|title=Lectures on Fluid Mechanics|publisher=Gordon and Breach|year=1973|isbn=0-677-01710-3}}
* {{citation|last1=Nazarenko|first1=Sergey|year=2014|title=Fluid Dynamics via Examples and Solutions|publisher=CRC Press (Taylor & Francis group)|isbn=978-1-43-988882-7}}
* [http://www.scholarpedia.org/article/Encyclopedia:Fluid_dynamics Encyclopedia: Fluid dynamics] [[Scholarpedia]]
 
== External links ==
{{Commons category|Fluid dynamics}}{{Commons category|Fluid mechanics}}
* [http://www.efluids.com/ eFluids], containing several galleries of fluid motion
* [http://web.mit.edu/hml/ncfmf.html National Committee for Fluid Mechanics Films (NCFMF)], containing films on several subjects in fluid dynamics (in [[RealMedia]] format)
* [http://www.salihnet.freeservers.com/engineering/fm/fm_books.html List of Fluid Dynamics books]
{{NonDimFluMech}} {{physics-footer|continuum='''[[Continuum mechanics]]'''}}
[[Thể loại:Động lực học chất lưu]]
[[Thể loại:Cơ học chất lưu]]