Fluid dynamics
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In physics and engineeringfluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids - liquids and gases. It has several liquid dynamics def pumps, including aerodynamics the study of air and other gases in motion and liquid dynamics def pumps the study of liquids in motion.
Fluid dynamics has a liquid dynamics def pumps range of applications, including calculating forces and moments on aircraftdetermining the mass flow rate of petroleum through pipelinespredicting weather patternsunderstanding 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 liquid dynamics def pumps. The solution to a fluid dynamics problem typically involves the calculation of various properties of the fluid, such as flow velocitypressuredensityand temperatureas functions of space and time.
Before the twentieth century, hydrodynamics was synonymous with fluid dynamics. This liquid dynamics def pumps still reflected in names of some fluid dynamics topics, like magnetohydrodynamics and hydrodynamic stability liquid dynamics def pumps, both of which can also be applied to gases.
The foundational axioms of fluid dynamics are the conservation lawsspecifically, conservation of massconservation of linear momentum also known as Newton's Second Law of Motionand 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 solutionso 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:. 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.
All fluids are compressible to some extent; that is, changes in pressure or liquid dynamics def pumps 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 liquid dynamics def pumps the flow can be modelled as an incompressible flow. Otherwise the more general compressible flow liquid dynamics def pumps must be used.
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. 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 liquid dynamics def pumps 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 liquid dynamics def pumps resistance to deformation: Isaac Newton showed that for many familiar fluids such as water and airthe 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 liquid dynamics def pumps 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 slurriessome viscoelastic materials such as blood and some polymersand sticky liquid dynamics def pumps such as latexhoney liquid dynamics def pumps lubricants.
The liquid dynamics def pumps of fluid parcels is described with the help of Newton's second law. An accelerating parcel liquid dynamics def pumps 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. In such cases, inertial forces are sometimes neglected; this flow regime is called Stokes or creeping flow.
In high Reynolds number flows, the flow is often modeled as an inviscid flowan 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 liquid dynamics def pumpsbecause liquid dynamics def pumps velocity field may be expressed as the gradient of liquid dynamics def pumps 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 layerin which viscosity effects dominate and which thus generates vorticity. Therefore, to calculate net forces on bodies such as wingsviscous flow equations must be used: A commonly used [ citation needed ] model, especially in computational fluid dynamicsis to use two flow models: The two solutions can then be matched with each other, using the method of matched asymptotic expansions.
A flow that is not a liquid dynamics def pumps 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 liquid dynamics def pumps 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.
The random field U xt 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, eddiesand 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 decompositionin which the flow is broken down into the sum of an average component liquid dynamics def pumps a perturbation component. It is believed that turbulent flows can be described well through the use of liquid dynamics def pumps Navier—Stokes equations.
Direct numerical simulation DNSbased on the Navier—Stokes equations, makes it possible to simulate turbulent flows at moderate Reynolds numbers. Restrictions depend on the power of the computer liquid dynamics def pumps and the efficiency of the solution algorithm. The results of DNS have been found to agree well with experimental data for some flows.
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. Transport aircraft wings such as on an Airbus A or Boeing 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 stressesalthough the turbulence also enhances the heat and mass transfer.
Another promising methodology is large eddy simulation LESespecially in the guise of detached eddy simulation DES liquid dynamics def pumps is a combination of RANS turbulence modelling and large eddy simulation. While many flows e. 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 enginepropulsion devices Rocketsjet engines etc.
Magnetohydrodynamics is the multi-disciplinary study of the flow of electrically conducting fluids in liquid dynamics def pumps fields. Examples of such fluids include plasmasliquid 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. 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 liquid dynamics def pumps below. 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.
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