In physics and engineering, **fluid dynamics** is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, 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 forces and moments on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns, understanding nebulae in interstellar space and modelling fission weapon detonation,

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 the calculation of various properties of the fluid, such as flow velocity, pressure, density, and temperature, as functions of space and time.

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.^{[1]}

The foundational axioms of fluid dynamics are the conservation laws, specifically, conservation of mass, conservation of linear momentum, 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.

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 assumes that fluids are continuous, rather than discrete. Consequently, it is assumed that properties such as density, pressure, temperature, and flow velocity are well-defined at infinitesimally small points in space and vary continuously from one point to another. The fact that the fluid is made up of discrete molecules is ignored.

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 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 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 the simplifications allow some simple fluid dynamics problems to be solved in closed form.

In addition to the mass, momentum, and energy conservation equations, a thermodynamic equation of state that gives the pressure as a function of other thermodynamic variables is required to completely describe the problem. An example of this would be the perfect gas equation of state:

where *p* is pressure, ρ is density, *T* the absolute temperature, while *R _{u}* is the gas constant and

Three conservation laws are used to solve fluid dynamics problems, and may be written in integral or differential form. The conservation laws may be applied to a region of the flow called a *control volume*. A control volume is a discrete volume in space through which fluid is assumed to flow. The integral formulations of the conservation laws are used to describe the change of 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 infinitesimally small volume (at a point) within the flow.

- 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,
^{[2]}and can be translated into the integral form of the continuity equation:

- Above, is the fluid density,
**u**is the flow velocity vector, and*t*is time. The left-hand side of the above expression is the rate of increase of mass within the volume and contains a triple integral over the control volume, whereas the right-hand side contains an integration over the surface of the control volume of mass convected into the system. Mass flow into the system is accounted as positive, and since the normal vector to the surface is opposite the sense of flow into the system the term is negated. The differential form of the continuity equation is, by the divergence theorem:

- Conservation of momentum: Newton's second law of motion applied to a control volume, is a statement that any change in momentum of the fluid within that control volume will be due to the net flow of momentum into the volume and the action of external forces acting on the fluid within the volume.

- In the above integral formulation of this equation, the term on the left is the net change of momentum within the volume. The first term on the right is the net rate at which momentum is convected into the volume. The second term on the right is the force due to pressure on the volume's surfaces. The first two terms on the right are negated since momentum entering the system is accounted as positive, and the normal is opposite the direction of the velocity and pressure forces. The third term on the right is the net acceleration of the mass within the volume due to any body forces (here represented by
*f*_{body}). Surface forces, such as viscous forces, are represented by**, the net force due to shear forces acting on the volume surface. The momentum balance can also be written for a***moving*control volume.^{[3]}

- The following is the differential form of the momentum conservation equation. Here, the volume is reduced to an infinitesimally small point, and 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 at a point in a flow.

- 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, but it can be expressed as three scalar equations in three coordinate directions. The conservation of momentum equations for the compressible, viscous flow case are called the Navier–Stokes equations.
^{[2]}

- Conservation of energy: Although energy can be converted from one form to another, the total energy in a closed system remains constant.

- Above,
*h*is enthalpy,*k*is the thermal conductivity of the fluid,*T*is temperature, and 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.^{[4]}The expression on the left side is a material derivative.

All fluids are 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.,

where *D*/*Dt* is the material derivative, which is the sum of local and 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). 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.

All fluids are 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 rate; it has dimensions . Isaac Newton showed that for many familiar fluids such as water and air, the 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 that is independent of the strain rate.

Non-Newtonian fluids have a more complicated, non-linear stress-strain behaviour. The sub-discipline of rheology describes the stress-strain behaviours of such fluids, which include emulsions and slurries, some viscoelastic materials such as blood and some polymers, and *sticky liquids* such as latex, honey and lubricants.

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 or creeping flow.

In contrast, 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. Eliminating viscosity allows the Navier–Stokes equations to be simplified into the Euler equations. The integration of the Euler equations along a streamline in an inviscid flow yields Bernoulli's equation. When, in addition to being inviscid, the flow is irrotational everywhere, Bernoulli's equation can completely describe the flow everywhere. Such flows are called potential flows, because the velocity field may be expressed as the gradient of a potential energy expression.

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 forces, a limitation known as the d'Alembert's paradox.

A commonly used 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.

A flow that is not a function of time is called **steady flow**. Steady-state flow refers to the condition where the fluid properties at a point in the system do not change over time. Time dependent flow is known as unsteady (also called transient^{[6]}). 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.

Turbulent flows are unsteady by definition. A turbulent flow can, however, be statistically stationary. According to Pope:^{[7]}

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.

Turbulence is flow characterized by recirculation, eddies, and apparent randomness. Flow in which turbulence is not exhibited is called laminar. 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.^{[8]}

Most flows of interest have Reynolds numbers much too high for DNS to be a viable option,^{[9]} 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 20 m/s (72 km/h) 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 dimension). 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 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.

While many flows (e.g. flow of water through a pipe) occur at low Mach numbers, many flows of practical interest in aerodynamics or in turbomachines occur at high fractions of M=1 (transonic flows) or in excess of it (supersonic or even hypersonic flows). New phenomena occur at these regimes such as instabilities in transonic flow, shock waves for supersonic flow, or non-equilibrium chemical behaviour due to ionization in hypersonic flows. In practice, each of those flow regimes is treated separately.

Reactive flows are flows that are chemically reactive, which finds its applications in many areas such as combustion(IC engine), propulsion devices (Rockets, jet engines etc.), detonations, fire and safety hazards, astrophysics etc. In addition to conservation of mass, momentum and energy, conservation of individual species (for example, mass fraction of methane in methane combustion) need to be derived, where the production/depletion rate of any species are obtained by simultaneously solving the equations of chemical kinetics.

Magnetohydrodynamics is the multi-disciplinary study of the flow of electrically conducting fluids in electromagnetic fields. Examples of such fluids include plasmas, liquid metals, and salt water. The fluid flow equations are solved simultaneously with Maxwell's equations of electromagnetism.

Relativistic fluid dynamics studies the macroscopic and microscopic fluid motion at large velocities comparable to the velocity of light.^{[10]} This branch of fluid dynamics accounts the relativistic effects both from the special theory of relativity and the general theory of relativity. The governing equations are derived in Riemannian geometry for Minkowski spacetime.

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*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 gradients are small. *Darcy's law*is used for flow in porous media, and works with variables averaged over several pore-widths.- In rotating systems, the
*quasi-geostrophic equations*assume an almost perfect balance between pressure gradients and the Coriolis force. It is useful in the study of atmospheric 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 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.

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.

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.

In a compressible fluid, it is convenient to define the total conditions (also called stagnation conditions) for all thermodynamic state properties (e.g. total temperature, total enthalpy, total speed of sound). These total flow conditions are a function of the fluid velocity and have different values in frames of reference with different motion.

To avoid potential ambiguity when referring to the properties of the fluid associated with the state of the fluid rather than its motion, the prefix "static" is commonly used (e.g. static temperature, static enthalpy). Where there is no prefix, the fluid property is the static condition (i.e. "density" and "static density" mean the same thing). The static conditions are independent of the frame of reference.

Because the total flow conditions are defined by isentropically bringing the fluid to rest, there is no need to distinguish between total entropy and static entropy as they are always equal by definition. As such, entropy is most commonly referred to as simply "entropy".

- Acoustic theory
- Aerodynamics
- Aeroelasticity
- Aeronautics
- Computational fluid dynamics
- Flow measurement
- Geophysical fluid dynamics
- Hemodynamics
- Hydraulics
- Hydrology
- Hydrostatics
- Electrohydrodynamics
- Magnetohydrodynamics
- Metafluid dynamics
- Quantum hydrodynamics

- Airy wave theory
- Benjamin–Bona–Mahony equation
- Boussinesq approximation (water waves)
- Different types of boundary conditions in fluid dynamics
- Helmholtz's theorems
- Kirchhoff equations
- Knudsen equation
- Manning equation
- Mild-slope equation
- Morison equation
- Navier–Stokes equations
- Oseen flow
- Poiseuille's law
- Pressure head
- Relativistic Euler equations
- Stokes stream function
- Stream function
- Streamlines, streaklines and pathlines
- Torricelli's Law

- Aerodynamic force
- Cavitation
- Compressible flow
- Couette flow
- Free molecular flow
- Incompressible flow
- Inviscid flow
- Isothermal flow
- Open channel flow
- Pipe flow
- Secondary flow
- Stream thrust averaging
- Superfluidity
- Transient flow
- Two-phase flow

- List of hydrodynamic instabilities
- Newtonian fluid
- Non-Newtonian fluid
- Surface tension
- Vapour pressure

- Balanced flow
- Boundary layer
- Coanda effect
- Convection cell
- Convergence/Bifurcation
- Darwin drift
- Drag (force)
- Hydrodynamic stability
- Kaye effect
- Lift (force)
- Magnus effect
- Ocean current
- Ocean surface waves
- Rossby wave
- Shock wave
- Soliton
- Stokes drift
- Thread breakup
- Turbulent jet breakup
- Upstream contamination
- Venturi effect
- Vortex
- Water hammer
- Wave drag
- Wind

- Acoustics
- Aerodynamics
- Cryosphere science
- Fluid power
- Geodynamics
- Hydraulic machinery
- Meteorology
- Naval architecture
- Oceanography
- Plasma physics
- Pneumatics
- 3D computer graphics

*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*

- 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

- Aileron
- Airplane
- Angle of attack
- Banked turn
- Bernoulli's principle
- Bilgeboard
- Boomerang
- Centerboard
- Chord (aircraft)
- Circulation control wing
- Currentology
- Diving plane
- Downforce
- Drag coefficient
- Fin
- Flipper (anatomy)
- Flow separation
- Foil (fluid mechanics)
- Fluid coupling
- Gas kinetics
- 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 third law
- Propeller
- Pump
- Rudder
- Sail (aerodynamics)
- Skeg
- Spoiler (automotive)
- Stall (flight)
- Surfboard fin
- Surface science
- Torque converter
- Trim tab
- Wing
- Wingtip vortices

**^**Eckert, Michael (2006).*The Dawn of Fluid Dynamics: A Discipline Between Science and Technology*. Wiley. p. ix. ISBN 3-527-40513-5.- ^
^{a}^{b}Anderson, J. D. (2007).*Fundamentals of Aerodynamics*(4th ed.). London: McGraw–Hill. ISBN 0-07-125408-0. **^**Nangia, Nishant; Johansen, Hans; Patankar, Neelesh A.; Bhalla, Amneet Pal S. (2017). "A moving control volume approach to computing hydrodynamic forces and torques on immersed bodies".*Journal of Computational Physics*.**347**: 437–462. arXiv:1704.00239. Bibcode:2017JCoPh.347..437N. doi:10.1016/j.jcp.2017.06.047.**^**White, F. M. (1974).*Viscous Fluid Flow*. New York: McGraw–Hill. ISBN 0-07-069710-8.**^**Shengtai Li, Hui Li "Parallel AMR Code for Compressible MHD or HD Equations" (Los Alamos National Laboratory) [1]**^**"Transient state or unsteady state? -- CFD Online Discussion Forums".*www.cfd-online.com*.**^**See Pope (2000), p. 75.**^**See, for example, Schlatter et al, Phys. Fluids 21, 051702 (2009); doi:10.1063/1.3139294**^**See Pope (2000), p. 344.**^**Landau, Lev Davidovich; Lifshitz, Evgenii Mikhailovich (1987).*Fluid Mechanics*. London: Pergamon. ISBN 0-08-033933-6.

- Acheson, D. J. (1990).
*Elementary Fluid Dynamics*. Clarendon Press. ISBN 0-19-859679-0. - Batchelor, G. K. (1967).
*An Introduction to Fluid Dynamics*. Cambridge University Press. ISBN 0-521-66396-2. - Chanson, H. (2009).
*Applied Hydrodynamics: An Introduction to Ideal and Real Fluid Flows*. CRC Press, Taylor & Francis Group, Leiden, The Netherlands, 478 pages. ISBN 978-0-415-49271-3. - Clancy, L. J. (1975).
*Aerodynamics*. London: Pitman Publishing Limited. ISBN 0-273-01120-0. - Lamb, Horace (1994).
*Hydrodynamics*(6th ed.). Cambridge University Press. ISBN 0-521-45868-4. Originally published in 1879, the 6th extended edition appeared first in 1932. - Landau, L. D.; Lifshitz, E. M. (1987).
*Fluid Mechanics*. Course of Theoretical Physics (2nd ed.). Pergamon Press. ISBN 0-7506-2767-0. - Milne-Thompson, L. M. (1968).
*Theoretical Hydrodynamics*(5th ed.). Macmillan. Originally published in 1938. - Pope, Stephen B. (2000).
*Turbulent Flows*. Cambridge University Press. ISBN 0-521-59886-9. - Shinbrot, M. (1973).
*Lectures on Fluid Mechanics*. Gordon and Breach. ISBN 0-677-01710-3. - Nazarenko, Sergey (2014),
*Fluid Dynamics via Examples and Solutions*, CRC Press (Taylor & Francis group), ISBN 978-1-43-988882-7 - Encyclopedia: Fluid dynamics Scholarpedia

- National Committee for Fluid Mechanics Films (NCFMF), containing films on several subjects in fluid dynamics (in RealMedia format)
- List of Fluid Dynamics books

Ansys Inc. is an American public company based in Canonsburg, Pennsylvania. It develops and markets engineering simulation software. Ansys software is used to design products and semiconductors, as well as to create simulations that test a product's durability, temperature distribution, fluid movements, and electromagnetic properties.

Ansys was founded in 1970 by John Swanson. Swanson sold his interest in the company to venture capitalists in 1993. Ansys went public on NASDAQ in 1996. In the 2000s, Ansys made numerous acquisitions of other engineering design companies, acquiring additional technology for fluid dynamics, electronics design, and other physics analysis.

Circulation (fluid dynamics)In fluid dynamics, circulation is the line integral around a closed curve of the velocity field. Circulation is normally denoted Γ (Greek uppercase gamma). Circulation was first used independently by Frederick Lanchester, Martin Kutta and Nikolai Zhukovsky.

Computational fluid dynamicsComputational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flows. Computers are used to perform the calculations required to simulate the free-stream flow of the fluid, and the interaction of the fluid (liquids and gases) with surfaces defined by boundary conditions. With high-speed supercomputers, better solutions can be achieved, and are often required to solve the largest and most complex problems. Ongoing research yields software that improves the accuracy and speed of complex simulation scenarios such as transonic or turbulent flows. Initial validation of such software is typically performed using experimental apparatus such as wind tunnels. In addition, previously performed analytical or empirical analysis of a particular problem can be used for comparison. A final validation is often performed using full-scale testing, such as flight tests.

CFD is applied to a wide range of research and engineering problems in many fields of study and industries, including aerodynamics and aerospace analysis, weather simulation, natural science and environmental engineering, industrial system design and analysis, biological engineering and fluid flows, and engine and combustion analysis.

Craik–Leibovich vortex forceIn fluid dynamics, the Craik–Leibovich (CL) vortex force describes a forcing of the mean flow through wave–current interaction, specifically between the Stokes drift velocity and the mean-flow vorticity. The CL vortex force is used to explain the generation of Langmuir circulations by an instability mechanism. The CL vortex-force mechanism was derived and studied by Sidney Leibovich and Alex D.D. Craik in the 1970s and 80s, in their studies of Langmuir circulations (discovered by Irving Langmuir in the 1930s).

Eddy (fluid dynamics)In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid behind the obstacle flows into the void creating a swirl of fluid on each edge of the obstacle, followed by a short reverse flow of fluid behind the obstacle flowing upstream, toward the back of the obstacle. This phenomenon is naturally observed behind large emergent rocks in swift-flowing rivers.

Flow velocityIn continuum mechanics the macroscopic velocity, also flow velocity in fluid dynamics or drift velocity in electromagnetism, is a vector field used to mathematically describe the motion of a continuum. The length of the flow velocity vector is the flow speed and is a scalar.

It is also called velocity field; when evaluated along a line, it is called a velocity profile (as in, e.g., law of the wall).

FluidIn physics, a fluid is a substance that continually deforms (flows) under an applied shear stress, or external force. Fluids are a phase of matter and include liquids, gases and plasmas. They are substances with zero shear modulus, or, in simpler terms, substances which cannot resist any shear force applied to them.

Although the term "fluid" includes both the liquid and gas phases, in common usage, "fluid" is often used as a synonym for "liquid", with no implication that gas could also be present. This colloquial usage of the term is also common in medicine and in nutrition ("take plenty of fluids").

Liquids form a free surface (that is, a surface not created by the container) while gases do not. Viscoelastic fluids like Silly Putty appear to behave similar to a solid when a sudden force is applied. Also substances with a very high viscosity such as pitch appear to behave like a solid (see pitch drop experiment).

Fluid mechanicsFluid mechanics is the branch of physics concerned with the mechanics of fluids (liquids, gases, and plasmas) and the forces on them. It has applications in a wide range of disciplines, including mechanical,

civil, chemical and biomedical engineering, geophysics, astrophysics, and biology.

Fluid Mechanics can also be defined as the science which deals with the study of behaviour of fluids either at rest or in motion.

It can be divided into fluid statics, the study of fluids at rest; and fluid dynamics, the study of the effect of forces on fluid motion. It is a branch of continuum mechanics, a subject which models matter without using the information that it is made out of atoms; that is, it models matter from a macroscopic viewpoint rather than from microscopic. Fluid mechanics, especially fluid dynamics, is an active field of research, typically mathematically complex. Many problems are partly or wholly unsolved, and are best addressed by numerical methods, typically using computers. A modern discipline, called computational fluid dynamics (CFD), is devoted to this approach. Particle image velocimetry, an experimental method for visualizing and analyzing fluid flow, also takes advantage of the highly visual nature of fluid flow.

Geophysical Fluid Dynamics Laboratory Coupled ModelGeophysical Fluid Dynamics Laboratory Coupled Model (GFDL CM2.5) is a coupled atmosphere–ocean general circulation model (AOGCM) developed at the NOAA Geophysical Fluid Dynamics Laboratory in the United States. It is one of the leading climate models used in the Fourth Assessment Report of the IPCC, along with models developed at the Max Planck Institute for Climate Research, the Hadley Centre and the National Center for Atmospheric Research.

Geophysical fluid dynamicsGeophysical fluid dynamics, in its broadest meaning, refers to the fluid dynamics of naturally occurring flows, such as lava flows, oceans, and planetary atmospheres, on Earth and other planets.Two physical features that are common to many of the phenomena studied in geophysical fluid dynamics are rotation of the fluid due to the planetary rotation and stratification (layering). The applications of geophysical fluid dynamics do not generally include the circulation of the mantle, which is the subject of geodynamics, or fluid phenomena in the magnetosphere.

Hydraulic headHydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum.It is usually measured as a liquid surface elevation, expressed in units of length, at the entrance (or bottom) of a piezometer. In an aquifer, it can be calculated from the depth to water in a piezometric well (a specialized water well), and given information of the piezometer's elevation and screen depth. Hydraulic head can similarly be measured in a column of water using a standpipe piezometer by measuring the height of the water surface in the tube relative to a common datum. The hydraulic head can be used to determine a hydraulic gradient between two or more points.

Ocean gyreIn oceanography, a gyre () is any large system of circulating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity along with horizontal and vertical friction, determine the circulation patterns from the wind stress curl (torque).The term gyre can be used to refer to any type of vortex in the air or the sea, even one that is man-made, but it is most commonly used in oceanography to refer to the major ocean systems.

Plume (fluid dynamics)In hydrodynamics, a plume is a column of one fluid moving through another. Several effects control the motion of the fluid, including momentum (inertia), diffusion and buoyancy (density differences). Pure jets and pure plumes define flows that are driven entirely by momentum and buoyancy effects, respectively. Flows between these two limits are usually described as forced plumes or buoyant jets. "Buoyancy is defined as being positive" when, in the absence of other forces or initial motion, the entering fluid would tend to rise. Situations where the density of the plume fluid is greater than its surroundings (i.e. in still conditions, its natural tendency would be to sink), but the flow has sufficient initial momentum to carry it some distance vertically, are described as being negatively buoyant.

Power number- For
**Newton number**, see also Kissing number in the sphere packing problem.

The **power number** *N*_{p} (also known as **Newton number**) is a commonly used dimensionless number relating the resistance force to the inertia force.

The power-number has different specifications according to the field of application. E.g., for stirrers the power number is defined as:

with

River mouthA river mouth is the part of a river where the river debouches into another river, a lake, a reservoir, a sea, or an ocean.

Ursell numberIn fluid dynamics, the **Ursell number** indicates the nonlinearity of long surface gravity waves on a fluid layer. This dimensionless parameter is named after Fritz Ursell, who discussed its significance in 1953.

The Ursell number is derived from the Stokes wave expansion, a perturbation series for nonlinear periodic waves, in the long-wave limit of shallow water – when the wavelength is much larger than the water depth. Then the Ursell number *U* is defined as:

which is, apart from a constant 3 / (32 π^{2}), the ratio of the amplitudes of the second-order to the first-order term in the free surface elevation.
The used parameters are:

So the Ursell parameter *U* is the relative wave height *H* / *h* times the relative wavelength *λ* / *h* squared.

For long waves (*λ* ≫ *h*) with small Ursell number, *U* ≪ 32 π^{2} / 3 ≈ 100, linear wave theory is applicable. Otherwise (and most often) a non-linear theory for fairly long waves (*λ* > 7 *h*) – like the Korteweg–de Vries equation or Boussinesq equations – has to be used.
The parameter, with different normalisation, was already introduced by George Gabriel Stokes in his historical paper on surface gravity waves of 1847.

In fluid dynamics, wave setup is the increase in mean water level due to the presence of breaking waves. Similarly, wave setdown is a wave-induced decrease of the mean water level before the waves break (during the shoaling process). For short, the whole phenomenon is often denoted as wave setup, including both increase and decrease of mean elevation. This setup is primarily present in and near the coastal surf zone. Besides a spatial variation in the (mean) wave setup, also a variation in time may be present – known as surf beat – causing infragravity wave radiation.

Wave setup can be mathematically modeled by considering the variation in radiation stress (Longuet-Higgins & Stewart 1962). Radiation stress is the tensor of excess horizontal-momentum fluxes due to the presence of the waves.

Wave–current interactionIn fluid dynamics, wave–current interaction is the interaction between surface gravity waves and a mean flow. The interaction implies an exchange of energy, so after the start of the interaction both the waves and the mean flow are affected.

For depth-integrated and phase-averaged flows, the quantity of primary importance for the dynamics of the interaction is the wave radiation stress tensor.

Wave–current interaction is also one of the possible mechanisms for the occurrence of rogue waves, such as in the Agulhas Current. When a wave group encounters an opposing current, the waves in the group may pile up on top of each other which will propagate into a rogue wave.

Wirth ResearchWirth Research is a group of engineering companies, founded by Nicholas Wirth in 2003, specialising in research, development, design and manufacture for the motor racing industry and other high technology sectors.

The companies use virtual engineering technologies to enable a completely simulated vehicle design, development and testing process. The group is known for deviating from traditional physical development models; most notably neglecting to use a wind tunnel and instead relying solely upon computational fluid dynamics to design the 2010 Virgin Racing VR-01 Formula 1 car.The Wirth Research group have a long-standing partnership with Honda Performance Development Inc (HPD) which is responsible for the design, development and manufacture of the ARX sports cars. Wirth Research also provides client IndyCar teams with full technical support.

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