Home > GENERAL FLUID DYNAMICS

GOVERNING EQUATIONS OF FLUID DYNAMICS

AND

AN AERODYNAMIC APPLICATION: “COANDA EFFECT”

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1. GOVERNING EQUATIONS

1.1 Introduction

We are all familiar with fluids and we have all been fascinated at some time by fluid behavior: pouring, flowing, splashing and so on.� At some level we have all built an intuitive understanding on how fluids behave.

Understanding fluid dynamics has been one of the major advances of physics, applied mathematics and engineering over the last hundred years. Starting with the explanations of aerofoil theory (i.e., why aircraft wings work), the study of fluids continues today with looking at how internal and surface waves, shock waves, turbulent fluid flow and the occurrence of chaos can be described mathematically. At the same time, it is important to realize how much of engineering depends on a proper understanding of fluids: from flow of water through pipes, to studying effluent discharge into the sea; from motions of the atmosphere, to the flow of lubricants in a car engine.

Fluid dynamics is also the key to our understanding of some of the most important phenomena in our physical world: ocean currents and weather systems, convection currents such as the motions of molten rock inside the Earth and the motions in the outer layers of the Sun, the explosions of supernovae and the swirling of gases in galaxies.

Like many fascinating subjects, understanding is not always easy. In particular, for fluid dynamics there are many terms and mathematical methods which will probably be unfamiliar. Although the basic concepts of velocity, mass, linear momentum, forces, etc., are the building blocks, the slippery nature of fluids means that applying those basic concepts sometimes takes some work.

Fluids can fascinate us exactly because they sometimes do unexpected things, which means we have to work harder at their mathematical explanation.

1.2 The continuity assumption

There are basically two ways of deriving the equations which governs the motion of a fluid. One of these methods approaches the question from the molecular point of view. The alternative method which is used to derive the equations which govern the motion of a fluid uses the continuum concept.

The continuity assumption considers fluids to be continuos. That is, properties such as density, pressure, temperature, and velocity are taken to be well-defined at infinitely small points, and are assumed to vary continuously from one point to another. The discrete, molecular nature of a fluid is ignored.

Those problems for which the continuity assumption does not give answers of desired accuracy are solved using statistical mechanics. In order to determine whether to use conventional fluid dynamics (a subdiscipline of continuum mechanics) or statistical mechanics, the Knudsen number:

where

T, temperature (K)

kB, Boltzmann's constant

P, total pressure (Pa)

is evaluated for the problem. Problems with Knudsen numbers at or above unity must be evaluated using statistical mechanics for reliable solutions.

������������� ������������� The vast majority of� phenomena encoutered in fluid mechanics fall well within the continuum domain and may involve liquids as well as gases. The continuum method is generaly used to describe fluid dynamics.

1.3 Modeling fluids

When it comes to deducing the equations that govern fluid motion, there are two fundamentally different approaches: the Eulerian description and the Lagrangian description. The two viewpoints have been named in honor of the Swiss mathematician Leonhard Euler (1707–1783) and the French mathematician and mathematical physicist Joseph-Louis Lagrange (1736-1813), respectively.

In the Eulerian description a fixed reference frame is employed relative to which a fluid is in motion. Time and spatial position in this reference frame, {t, r} are used as independent variables. The fluid variables such as mass, density, pressure and flow velocity which describe the physical state of the fluid flow in question are dependent variables as they are functions of the independent variables. Thus their derivatives are partial with respect to {t, r}. For example, the flow velocity at a spatial position and time is given by: u(r, t) and the corresponding acceleration at this position and time is then:

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1.3.2 Lagrangian description

In the Lagrangian description the fluid is described in terms of its constituent fluid elements. Different fluid elements have different “labels”, e.g., their spatial positions at a certain fixed time t say q. The independent variables are thus {t, q} and the particle position r{t, q} is a dependent variable. One can then ask about the rate of change in time in a reference frame commoving with the fluid element, and this then depends on time and particle label, which particular fluid element is being followed.

For example, if a fluid element has some velocity u(t, q) then the acceleration

it feels will be:

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The method used to derive the basic equations of motion from the conservation laws is to use the continuum concept and to follow an arbitrarily shaped control volume in a lagrangian frame of reference. The combination of the arbitrary control volume and the lagrangian coordinate system means that material derivative of volume integrals will be encountered. Though it is necessary to transform such terms into equivalent expressions involving volume integrals of eulerian derivatives. The theorem which permits such transformation is called Reynolds’ transport theorem.

Typical Volume |

Consider a specific mass of fluid and follow it for a short period of� time dt as it flows. Let be any property of the fluid such as mass, momentum in some direction or its energy. Since a specific mass of fluid is being considered and since x, y, z and t are the independent variables in the lagrangian framework, the quantity will be a function of t only as the control volume moves with the fluid. That gives the preferred form of Reynolds’ transport theorem:

It is conventional in fluid mechanics to use D instead of d in time derivatives if it is the derivative of a Lagrangian quantity.

������������� ������������� Having established the method to be used to derive the basic conservation equations it remains actually to invoke the various conservation principles.

The mathematical modeling of flows is described by the theory of continuum mechanics. The governing equations consist of conservation equations and constitutive equations. The conservation equations are derived from the principle of conservation of mass, the principle of balance of linear momentum, and the principle of balance of energy.

Constitutive equations are relating the stresses in the fluid to the deformation history.

Whereas conservation equations apply whatever the material studied, the constitutive equations depend from the material. The conservation and constitutive equations are used to calculate flows in complex geometries. For such problems, it is generally not possible to obtain an analytical solution of the governing equations.

1.4.1 Conservation of mass

Consider a specific mass of� fluid whose volume V is arbitrarily chosen. If this given mass is followed as it flows, its size and shape will be observed to change but the mass will remain unchanged. This is the principle of mass conservation which applies to fluids in which no nuclear reactions are taking place.

The equation which express conservation of mass is:

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The above equation express more than the fact that mass is conserved. Since it is a partial differential equation, the implication is that the velocity is continuous. For this reason is usually called the continuity equation.

������������� In many practical cases of fluid flow the variation of density of the fluid may be ignored, for example in most cases of the flow of liquids. In such cases the fluid is said to be incompressible, which means that as a given mass of fluid is followed, not only will its mass be observed to remain constant but it’s volume and hence its density.

Mathematically this special simplification of the continuity equation can be written as:

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Equation of continuity, either in the general form or the incompressible form is the first condition which has to be satisfied by the velocity and the density of the fluid.

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1.4.2 Conservation of momentum

The principle of conservation of momentum is in fact an application of Newton’s second law of motion to an element of fluid. That is, when considering a given mass of fluid in a lagrangian frame of reference, it is stated that the rate at which the momentum of the fluid mass is changing is equal to the net external force acting on the mass. Thus the mathematical equation which results from imposing the physical law of conservation of momentum is:

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where the left-hand side represents the rate of change of momentum of a unit the volume of fluid. The first term is the familiar temporal acceleration term, while the second term is a convective acceleration and allows for local accelerations (around obstacles) even when the flow is steady. This second term is also nonlinear since the velocity appears quadratically. On the right hand side are the forces which are causing the acceleration.

There are two types:

- surface forces (proportional to area)
- body forces (proportional to volume)

Surface forces are usually expressed in terms of stress (= force per unit area):

force=stressarea

The major surface forces are:

- pressure p: always acts normal to a surface
- viscous stresses ij : frictional forces arising from relative motion of fluid layers.

Body forces

The main body forces are:

- gravity: the force per unit volume is

������������� g= (0,0,-g)

(For constant-density fluids the effects of pressure and weight can be combined

in the governing equations as a piezometric pressure p* = p +gz)

- Coriolis forces (in rotating reference frames).

Separating surface forces (determined by a stress tensor ) and body forces (f per unit

volume), the integral equation for the i component of momentum may be written:

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The principle of conservation energy amounts to an application of the first law of thermodynamics to a fluid element as it flows. When applying the first law of thermodynamics to a flowing fluid the instantaneous energy of the fluid is considered to be the sum of the internal energy per unit mass and the kinetic energy per unit mass. In this way the modified form of the first law of thermodynamics which will be applied to an element of fluid states that the rate of change in the total energy (intrinsic plus kinetic) of the fluid as it flows is equal to the sum of the rate at which work is being done on the fluid by external forces and the rate on which heat is being added by conduction. In this way the mathematical expression becomes:

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������������� ������������� The basic conservation laws discussed in the previous section represent five scalar equations which the fluid properties must satisfy as the fluid flows. The continuity and the energy equation are scalar equations while the momentum equation is a vector which represents three scalar equations. But our basic conservation laws have introduced seventeen unknowns: the scalars and e, the density and internal energy respectively, the vectors uj and qj, the velocity and heat flux respectively, each vector having three components and the stress tensor ij, which has in general nine independent components.

In order to obtain a complete set of equations the stress tensor ij and the heat-flux vector qj must be further specified. This leads to so-called “constitutive equations” in which the stress tensor is related to the deformation tensor and the heat-flux vector is related to the temperature gradients.

In order to achieve this end the postulates for a newtonian fluid are used. It should pointed out that some fluid do not behave in a newtonian manner and their special characteristic are among the topics of current research. One example is the class of fluids called viscoelastic fluids whose properties may be used to reduce the drag of a body.

The stress tensor is supposed to satisfy the following condition: when the fluid is at rest the stress is hydrostatic and the pressure exerted by the fluid is the thermodynamic pressure.

The above figure represents stress acting on a small cube, where ij are the components of the shear-stress tensor which depends upon the motion of the fluid only.

Using that condition the constitutive relation for stress in a newtonian fluid becomes:

The nine elements of the stress tensor have now been expressed in terms of the pressure and the velocity gradients and two coefficients and . These coefficients cannot be determined analytically and must be determined empirically. They are the viscosity coefficients of the fluid.

The second constitutive relation is Fourier’s Law for heat conduction:

where qj is the heat-flux vector, k is the thermal conductivity of the fluid and T is the temperature.

The equation of momentum conservation together with the constitutive relation for a Newtonian fluid yield the famous Navier-Stokes equations, which are the principal conditions to be satisfied by a fluid as it flows:

The Navier-Stokes equations for an incompressible fluid of constant density is:

In the special case of negligible viscous effects, Navier-Stokes equations become:

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known as Euler equations.

1.4.6 In conclusion

the equations which govern the motion of a Newtonian fluid are:

The continuity equation:

The Navier-Stokes equations:

The energy equation

The thermal equation of state:

The caloric equation of state:

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The above set of equations represents seven equation which are to be satisfied by seven unknowns. Each of the continuity, energy and state equations supplies one scalar equation, while the Navier-Stokes equation supplies three scalar equations. The seven unknowns are: the pressure (p), density (), internal energy (e), temperature (T) and velocity components (uj). The parameters , and k are assumed to be known from experimental data, and they may be constants or specified functions of the temperature and pressure.

1.4.6 Special form of the governing equation: Bernoulli equation

For an inviscid fluid in which any body forces are conservative and either the flow is steady or it is irrotational the equations of momentum conservation may be integrated to yield a single scalar equation which is called the Bernoulli equation:

This result is referred to as the Bernoulli integral or the Bernoulli equation. It should be recalled that is valid only for the steady flow of a fluid in which viscous effects are negligible and in which any body forces are conservative.

"That we have written an equation does not remove from the flow of fluids its charm or mystery or its surprise." --Richard Feynman [1964]

2. Aerodynamics application: The Physics of flight-Coanda Effect

If a stream of water is flowing along a solid surface which is curved slightly away from the stream, the water will tend to follow the surface. This is an example of the Coanda effect and is easily demonstrated by holding the back of a spoon vertically under a thin stream of water from a faucet. If you hold the spoon so that it can swing, you will feel it being pulled toward the stream of water. The effect has limits: if you use a sphere instead of a spoon, you will find that the water will only follow a part of the way around. Further, if the surface is too sharply curved, the water will not follow but will just bend a bit and break away from the surface.The Coanda effect works with any of our usual fluids, such as air at usual temperatures, pressures, and speeds.

A word often used to describe the Coanda effect is to say that the air stream is "entrained" by the surface.

������������� Coanda effect was used by Jeff Raskin to explain how a wing generate lift and drag.

Henri Coanda was a Romanian scientist born in Bucharest on June 7, 1886. As he later stated he has been attracted by the 'miracle of wind' since he was a boy.

Henri Coanda attended high-school in Bucharest and in Iasi. After this he joined the Bucharest Military School where he graduated as an artillery officer. Fond of technical problems, especially of flight technics, in 1905 he built a 'missile-airplane' in Bucharest for the Army. Then he went up to Berlin to attend studies at Technische Hochschule in Charlottenburg, after which he followed with studies at the Science University in Liege, part of the Electrical Institute in Montefiore. He registered at the Superior Aeronautical School in Paris where he graduated in 1909.

The most known, studied, and applied discovery of Henri Coanda is the 'Coanda Effect'. Henri stated that the first time he realized something about what would become known as the Coanda Effect was while he was testing what he termed was his reactive airplane, Coanda-1910. After the plane took off, Coanda observed that the flames and burned gases exhausted from the engine tended to remain very close to the fuselage. For a long time this phenomenon of the burned gases and flames hugging the fuselage remained a great mystery which he explored by exchanging opinions with specialists in aerodynamics around the world. After studies which lasted more than 20 years, (carried out by Coanda and other scientists) it was recognized as a new aeronautical effect. Prof. Albert Metral named the phenomenon for Coanda.

In 1970, Coanda returned to Romania and settled for the last years of his life in Bucharest. In 1971, he and Prof. Elie Carafoli reorganized the Aeronaurical Engineering discipline at Bucharest Polytechnic Institute, splitting the Mechanical and Aeronautical Engineering Department into two departments of study -- Mechanical Engineering and Aircraft Engineering.

H. Coanda died on November 25, 1972.

References:

2. Fluid mechanics, Landau, L. D., London, Pergamon Press; Reading, Mass., Addison-Wesley Pub. Co., 1959

3. http://www.allstar.fiu.edu/aero/Flow2.htm

4. http://www.maths.qmul.ac.uk/~hve/MAS209/

5.http://personalpages.umist.ac.uk/staff/david.d.apsley/lectures/comphydr/goveqns.

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