Английская Википедия:Bernoulli's principle

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Файл:VenturiFlow.png

A flow of air through a venturi meter. The kinetic energy increases at the expense of the fluid pressure, as shown by the difference in height of the two columns of water.

Файл:Venturi Tube en.webm

Video of a venturi meter used in a lab experiment

Bernoulli's principle is a key concept in fluid dynamics that relates pressure, speed and height. Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or the fluid's potential energy.Шаблон:RpШаблон:Rp The principle is named after the Swiss mathematician and physicist Daniel Bernoulli, who published it in his book Hydrodynamica in 1738. Although Bernoulli deduced that pressure decreases when the flow speed increases, it was Leonhard Euler in 1752 who derived Bernoulli's equation in its usual form.

Bernoulli's principle can be derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of energy in a fluid is the same at all points that are free of viscous forces. This requires that the sum of kinetic energy, potential energy and internal energy remains constant.Шаблон:Rp Thus an increase in the speed of the fluid—implying an increase in its kinetic energy—occurs with a simultaneous decrease in (the sum of) its potential energy (including the static pressure) and internal energy. If the fluid is flowing out of a reservoir, the sum of all forms of energy is the same because in a reservoir the energy per unit volume (the sum of pressure and gravitational potential Шаблон:Math) is the same everywhere.Шаблон:Rp

Bernoulli's principle can also be derived directly from Isaac Newton's second Law of Motion. If a small volume of fluid is flowing horizontally from a region of high pressure to a region of low pressure, then there is more pressure behind than in front. This gives a net force on the volume, accelerating it along the streamline.Шаблон:EfnШаблон:EfnШаблон:Efn

Fluid particles are subject only to pressure and their own weight. If a fluid is flowing horizontally and along a section of a streamline, where the speed increases it can only be because the fluid on that section has moved from a region of higher pressure to a region of lower pressure; and if its speed decreases, it can only be because it has moved from a region of lower pressure to a region of higher pressure. Consequently, within a fluid flowing horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest.

Bernoulli's principle is only applicable for isentropic flows: when the effects of irreversible processes (like turbulence) and non-adiabatic processes (e.g. thermal radiation) are small and can be neglected. However, the principle can be applied to various types of flow within these bounds, resulting in various forms of Bernoulli's equation. The simple form of Bernoulli's equation is valid for incompressible flows (e.g. most liquid flows and gases moving at low Mach number). More advanced forms may be applied to compressible flows at higher Mach numbers.

Incompressible flow equation

In most flows of liquids, and of gases at low Mach number, the density of a fluid parcel can be considered to be constant, regardless of pressure variations in the flow. Therefore, the fluid can be considered to be incompressible, and these flows are called incompressible flows. Bernoulli performed his experiments on liquids, so his equation in its original form is valid only for incompressible flow.

A common form of Bernoulli's equation is: Шаблон:NumBlk

where:

• <math>v</math> is the fluid flow speed at a point,
• <math>g</math> is the acceleration due to gravity,
• <math>z</math> is the elevation of the point above a reference plane, with the positive <math>z</math>-direction pointing upward—so in the direction opposite to the gravitational acceleration,
• <math>p</math> is the pressure at the chosen point, and
• <math>\rho</math> is the density of the fluid at all points in the fluid.

Bernoulli's equation and the Bernoulli constant are applicable throughout any region of flow where the energy per unit mass is uniform. Because the energy per unit mass of liquid in a well-mixed reservoir is uniform throughout, Bernoulli's equation can be used to analyze the fluid flow everywhere in that reservoir (including pipes or flow fields that the reservoir feeds) except where viscous forces dominate and erode the energy per unit mass.Шаблон:Rp

The following assumptions must be met for this Bernoulli equation to apply:Шаблон:Rp

• the flow must be steady, that is, the flow parameters (velocity, density, etc.) at any point cannot change with time,
• the flow must be incompressible—even though pressure varies, the density must remain constant along a streamline;
• friction by viscous forces must be negligible.

For conservative force fields (not limited to the gravitational field), Bernoulli's equation can be generalized as:Шаблон:Rp <math display="block">\frac{v^2}{2} + \Psi + \frac{p}{\rho} = \text{constant}</math> where Шаблон:Math is the force potential at the point considered. For example, for the Earth's gravity Шаблон:Math.

By multiplying with the fluid density Шаблон:Mvar, equation (Шаблон:EquationNote) can be rewritten as: <math display="block">\tfrac{1}{2} \rho v^2 + \rho g z + p = \text{constant}</math> or: <math display="block">q + \rho g h = p_0 + \rho g z = \text{constant}</math> where

• Шаблон:Math is dynamic pressure,
• Шаблон:Math is the piezometric head or hydraulic head (the sum of the elevation Шаблон:Mvar and the pressure head) and
• Шаблон:Math is the stagnation pressure (the sum of the static pressure Шаблон:Mvar and dynamic pressure Шаблон:Mvar).

The constant in the Bernoulli equation can be normalized. A common approach is in terms of total head or energy head Шаблон:Mvar: <math display="block">H = z + \frac{p}{\rho g} + \frac{v^2}{2g} = h + \frac{v^2}{2g},</math>

The above equations suggest there is a flow speed at which pressure is zero, and at even higher speeds the pressure is negative. Most often, gases and liquids are not capable of negative absolute pressure, or even zero pressure, so clearly Bernoulli's equation ceases to be valid before zero pressure is reached. In liquids—when the pressure becomes too low—cavitation occurs. The above equations use a linear relationship between flow speed squared and pressure. At higher flow speeds in gases, or for sound waves in liquid, the changes in mass density become significant so that the assumption of constant density is invalid.

Simplified form

In many applications of Bernoulli's equation, the change in the Шаблон:Mvar term is so small compared with the other terms that it can be ignored. For example, in the case of aircraft in flight, the change in height Шаблон:Mvar is so small the Шаблон:Mvar term can be omitted. This allows the above equation to be presented in the following simplified form: <math display="block">p + q = p_0</math> where Шаблон:Math is called total pressure, and Шаблон:Mvar is dynamic pressure. Many authors refer to the pressure Шаблон:Mvar as static pressure to distinguish it from total pressure Шаблон:Math and dynamic pressure Шаблон:Mvar. In Aerodynamics, L.J. Clancy writes: "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."Шаблон:Rp

The simplified form of Bernoulli's equation can be summarized in the following memorable word equation:Шаблон:Rp Шаблон:Block indent

Every point in a steadily flowing fluid, regardless of the fluid speed at that point, has its own unique static pressure Шаблон:Mvar and dynamic pressure Шаблон:Mvar. Their sum Шаблон:Math is defined to be the total pressure Шаблон:Math. The significance of Bernoulli's principle can now be summarized as "total pressure is constant in any region free of viscous forces". If the fluid flow is brought to rest at some point, this point is called a stagnation point, and at this point the static pressure is equal to the stagnation pressure.

If the fluid flow is irrotational, the total pressure is uniform and Bernoulli's principle can be summarized as "total pressure is constant everywhere in the fluid flow".Шаблон:Rp It is reasonable to assume that irrotational flow exists in any situation where a large body of fluid is flowing past a solid body. Examples are aircraft in flight and ships moving in open bodies of water. However, Bernoulli's principle importantly does not apply in the boundary layer such as in flow through long pipes.

Unsteady potential flow

The Bernoulli equation for unsteady potential flow is used in the theory of ocean surface waves and acoustics. For an irrotational flow, the flow velocity can be described as the gradient Шаблон:Math of a velocity potential Шаблон:Mvar. In that case, and for a constant density Шаблон:Mvar, the momentum equations of the Euler equations can be integrated to:Шаблон:Rp<math display="block">\frac{\partial \varphi}{\partial t} + \tfrac12 v^2 + \frac{p}{\rho} + gz = f(t),</math>

which is a Bernoulli equation valid also for unsteady—or time dependent—flows. Here Шаблон:Math denotes the partial derivative of the velocity potential Шаблон:Mvar with respect to time Шаблон:Mvar, and Шаблон:Math is the flow speed. The function Шаблон:Math depends only on time and not on position in the fluid. As a result, the Bernoulli equation at some moment Шаблон:Mvar applies in the whole fluid domain. This is also true for the special case of a steady irrotational flow, in which case Шаблон:Mvar and Шаблон:Math are constants so equation (Шаблон:EquationNote) can be applied in every point of the fluid domain.Шаблон:Rp Further Шаблон:Math can be made equal to zero by incorporating it into the velocity potential using the transformation:<math display="block">\Phi = \varphi - \int_{t_0}^t f(\tau)\, \mathrm{d}\tau,</math> resulting in: <math display="block">\frac{\partial \Phi}{\partial t} + \tfrac12 v^2 + \frac{p}{\rho} + gz = 0.</math>

Note that the relation of the potential to the flow velocity is unaffected by this transformation: Шаблон:Math.

The Bernoulli equation for unsteady potential flow also appears to play a central role in Luke's variational principle, a variational description of free-surface flows using the Lagrangian mechanics.

Compressible flow equation

Bernoulli developed his principle from observations on liquids, and Bernoulli's equation is valid for ideal fluids: those that are incompressible, irrotational, inviscid, and subjected to conservative forces. It is sometimes valid for the flow of gases: provided that there is no transfer of kinetic or potential energy from the gas flow to the compression or expansion of the gas. If both the gas pressure and volume change simultaneously, then work will be done on or by the gas. In this case, Bernoulli's equation—in its incompressible flow form—cannot be assumed to be valid. However, if the gas process is entirely isobaric, or isochoric, then no work is done on or by the gas (so the simple energy balance is not upset). According to the gas law, an isobaric or isochoric process is ordinarily the only way to ensure constant density in a gas. Also the gas density will be proportional to the ratio of pressure and absolute temperature; however, this ratio will vary upon compression or expansion, no matter what non-zero quantity of heat is added or removed. The only exception is if the net heat transfer is zero, as in a complete thermodynamic cycle or in an individual isentropic (frictionless adiabatic) process, and even then this reversible process must be reversed, to restore the gas to the original pressure and specific volume, and thus density. Only then is the original, unmodified Bernoulli equation applicable. In this case the equation can be used if the flow speed of the gas is sufficiently below the speed of sound, such that the variation in density of the gas (due to this effect) along each streamline can be ignored. Adiabatic flow at less than Mach 0.3 is generally considered to be slow enough.

It is possible to use the fundamental principles of physics to develop similar equations applicable to compressible fluids. There are numerous equations, each tailored for a particular application, but all are analogous to Bernoulli's equation and all rely on nothing more than the fundamental principles of physics such as Newton's laws of motion or the first law of thermodynamics.

Compressible flow in fluid dynamics

For a compressible fluid, with a barotropic equation of state, and under the action of conservative forces, <math display="block">\frac {v^2}{2}+ \int_{p_1}^p \frac {\mathrm{d}\tilde{p}}{\rho\left(\tilde{p}\right)} + \Psi = \text{constant (along a streamline)}</math> where:

• Шаблон:Mvar is the pressure
• Шаблон:Mvar is the density and Шаблон:Math indicates that it is a function of pressure
• Шаблон:Mvar is the flow speed
• Шаблон:Math is the potential associated with the conservative force field, often the gravitational potential

In engineering situations, elevations are generally small compared to the size of the Earth, and the time scales of fluid flow are small enough to consider the equation of state as adiabatic. In this case, the above equation for an ideal gas becomes:Шаблон:Rp <math display="block">\frac {v^2}{2}+ gz + \left(\frac {\gamma}{\gamma-1}\right) \frac {p}{\rho} = \text{constant (along a streamline)}</math> where, in addition to the terms listed above:

• Шаблон:Mvar is the ratio of the specific heats of the fluid
• Шаблон:Mvar is the acceleration due to gravity
• Шаблон:Mvar is the elevation of the point above a reference plane

In many applications of compressible flow, changes in elevation are negligible compared to the other terms, so the term Шаблон:Mvar can be omitted. A very useful form of the equation is then: <math display="block">\frac {v^2}{2}+\left( \frac {\gamma}{\gamma-1}\right)\frac {p}{\rho} = \left(\frac {\gamma}{\gamma-1}\right)\frac {p_0}{\rho_0}</math>

where:

• Шаблон:Math is the total pressure
• Шаблон:Math is the total density

Compressible flow in thermodynamics

The most general form of the equation, suitable for use in thermodynamics in case of (quasi) steady flow, is:Шаблон:RpШаблон:RpШаблон:Rp

<math display="block">\frac{v^2}{2} + \Psi + w = \text{constant}.</math>

Here Шаблон:Mvar is the enthalpy per unit mass (also known as specific enthalpy), which is also often written as Шаблон:Mvar (not to be confused with "head" or "height").

Note that <math display="block">w =e + \frac{p}{\rho} EducationBot (обсуждение)\left(= \frac{\gamma}{\gamma-1} \frac{p}{\rho}\right)</math> where Шаблон:Mvar is the thermodynamic energy per unit mass, also known as the specific internal energy. So, for constant internal energy <math>e</math> the equation reduces to the incompressible-flow form.

The constant on the right-hand side is often called the Bernoulli constant and denoted Шаблон:Mvar. For steady inviscid adiabatic flow with no additional sources or sinks of energy, Шаблон:Mvar is constant along any given streamline. More generally, when Шаблон:Mvar may vary along streamlines, it still proves a useful parameter, related to the "head" of the fluid (see below).

When the change in Шаблон:Math can be ignored, a very useful form of this equation is: <math display="block">\frac{v^2}{2} + w = w_0</math> where Шаблон:Math is total enthalpy. For a calorically perfect gas such as an ideal gas, the enthalpy is directly proportional to the temperature, and this leads to the concept of the total (or stagnation) temperature.

When shock waves are present, in a reference frame in which the shock is stationary and the flow is steady, many of the parameters in the Bernoulli equation suffer abrupt changes in passing through the shock. The Bernoulli parameter remains unaffected. An exception to this rule is radiative shocks, which violate the assumptions leading to the Bernoulli equation, namely the lack of additional sinks or sources of energy.

Unsteady potential flow

For a compressible fluid, with a barotropic equation of state, the unsteady momentum conservation equation <math display="block">\frac{\partial \vec{v}}{\partial t} + \left(\vec{v}\cdot \nabla\right)\vec{v} = -\vec{g} - \frac{\nabla p}{\rho}</math>

With the irrotational assumption, namely, the flow velocity can be described as the gradient Шаблон:Math of a velocity potential Шаблон:Math. The unsteady momentum conservation equation becomes <math display="block">\frac{\partial \nabla \phi}{\partial t} + \nabla \left(\frac{\nabla \phi \cdot \nabla \phi}{2}\right) = -\nabla \Psi - \nabla \int_{p_1}^{p}\frac{d \tilde{p}}{\rho(\tilde{p})}</math> which leads to <math display="block">\frac{\partial \phi}{\partial t} + \frac{\nabla \phi \cdot \nabla \phi}{2} + \Psi + \int_{p_1}^{p}\frac{d \tilde{p}}{\rho(\tilde{p})} = \text{constant}</math>

In this case, the above equation for isentropic flow becomes: <math display="block">\frac{\partial \phi}{\partial t} + \frac{\nabla \phi \cdot \nabla \phi}{2} + \Psi + \frac{\gamma}{\gamma-1}\frac{p}{\rho} = \text{constant}</math>

Derivations

Шаблон:Math proof{\mathrm{d}x} \left( \rho \frac{v^2}{2} + p \right) =0</math> by integrating with respect to Шаблон:Mvar <math display="block"> \frac{v^2}{2} + \frac{p}{\rho}= C</math> where Шаблон:Mvar is a constant, sometimes referred to as the Bernoulli constant. It is not a universal constant, but rather a constant of a particular fluid system. The deduction is: where the speed is large, pressure is low and vice versa.

In the above derivation, no external work–energy principle is invoked. Rather, Bernoulli's principle was derived by a simple manipulation of Newton's second law.

Файл:BernoullisLawDerivationDiagram.svg

A streamtube of fluid moving to the right. Indicated are pressure, elevation, flow speed, distance (Шаблон:Mvar), and cross-sectional area. Note that in this figure elevation is denoted as Шаблон:Mvar, contrary to the text where it is given by Шаблон:Mvar.

Derivation by using conservation of energy

Another way to derive Bernoulli's principle for an incompressible flow is by applying conservation of energy. In the form of the work-energy theorem, stating that Шаблон:Block indent Therefore, Шаблон:Block indent The system consists of the volume of fluid, initially between the cross-sections Шаблон:Math and Шаблон:Math. In the time interval Шаблон:Math fluid elements initially at the inflow cross-section Шаблон:Math move over a distance Шаблон:Math, while at the outflow cross-section the fluid moves away from cross-section Шаблон:Math over a distance Шаблон:Math. The displaced fluid volumes at the inflow and outflow are respectively Шаблон:Math and Шаблон:Math. The associated displaced fluid masses are – when Шаблон:Mvar is the fluid's mass density – equal to density times volume, so Шаблон:Math and Шаблон:Math. By mass conservation, these two masses displaced in the time interval Шаблон:Math have to be equal, and this displaced mass is denoted by Шаблон:Math: <math display="block">\begin{align} \rho A_1 s_1 &= \rho A_1 v_1 \Delta t = \Delta m, \\ \rho A_2 s_2 &= \rho A_2 v_2 \Delta t = \Delta m. \end{align}</math>

The work done by the forces consists of two parts:

• The work done by the pressure acting on the areas Шаблон:Math and Шаблон:Math <math display="block">W_\text{pressure}=F_{1,\text{pressure}} s_{1} - F_{2,\text{pressure}} s_2 =p_1 A_1 s_1 - p_2 A_2 s_2 = \Delta m \frac{p_1}{\rho} - \Delta m \frac{p_2}{\rho}.</math>
• The work done by gravity: the gravitational potential energy in the volume Шаблон:Math is lost, and at the outflow in the volume Шаблон:Math is gained. So, the change in gravitational potential energy Шаблон:Math in the time interval Шаблон:Math is

<math display="block">\Delta E_\text{pot,gravity} = \Delta m\, g z_2 - \Delta m\, g z_1. </math> Now, the work by the force of gravity is opposite to the change in potential energy, Шаблон:Math: while the force of gravity is in the negative Шаблон:Mvar-direction, the work—gravity force times change in elevation—will be negative for a positive elevation change Шаблон:Math, while the corresponding potential energy change is positive.Шаблон:Rp So: <math display="block">W_\text{gravity} = -\Delta E_\text{pot,gravity} = \Delta m\, g z_1 - \Delta m\, g z_2.</math> And therefore the total work done in this time interval Шаблон:Math is <math display="block">W = W_\text{pressure} + W_\text{gravity}.</math> The increase in kinetic energy is <math display="block">\Delta E_\text{kin} = \tfrac12 \Delta m\, v_2^2-\tfrac12 \Delta m\, v_1^2.</math> Putting these together, the work-kinetic energy theorem Шаблон:Math gives: <math display="block">\Delta m \frac{p_1}{\rho} - \Delta m \frac{p_2}{\rho} + \Delta m\, g z_1 - \Delta m\, g z_2 = \tfrac12 \Delta m\, v_2^2 - \tfrac12 \Delta m\, v_1^2</math> or <math display="block">\tfrac12 \Delta m\, v_1^2 + \Delta m\, g z_1 + \Delta m \frac{p_1}{\rho} = \tfrac12 \Delta m\, v_2^2 + \Delta m\, g z_2 + \Delta m \frac{p_2}{\rho}.</math> After dividing by the mass Шаблон:Math the result is: <math display="block">\tfrac12 v_1^2 +g z_1 + \frac{p_1}{\rho}=\tfrac12 v_2^2 +g z_2 + \frac{p_2}{\rho}</math> or, as stated in the first paragraph: Шаблон:NumBlk Further division by Шаблон:Mvar produces the following equation. Note that each term can be described in the length dimension (such as meters). This is the head equation derived from Bernoulli's principle: Шаблон:NumBlk The middle term, Шаблон:Mvar, represents the potential energy of the fluid due to its elevation with respect to a reference plane. Now, Шаблон:Math is called the elevation head and given the designation Шаблон:Math.

A free falling mass from an elevation Шаблон:Math (in a vacuum) will reach a speed <math display="block">v = \sqrt{ {2 g}{z} },</math> when arriving at elevation Шаблон:Math. Or when rearranged as head: <math display="block">h_v =\frac{v^2}{2 g}</math> The term Шаблон:Math is called the velocity head, expressed as a length measurement. It represents the internal energy of the fluid due to its motion.

The hydrostatic pressure p is defined as <math display="block">p = p_0 - \rho g z ,</math> with Шаблон:Math some reference pressure, or when rearranged as head: <math display="block">\psi = \frac{p}{\rho g}.</math> The term Шаблон:Math is also called the pressure head, expressed as a length measurement. It represents the internal energy of the fluid due to the pressure exerted on the container. The head due to the flow speed and the head due to static pressure combined with the elevation above a reference plane, a simple relationship useful for incompressible fluids using the velocity head, elevation head, and pressure head is obtained. Шаблон:NumBlk If Eqn. 1 is multiplied by the density of the fluid, an equation with three pressure terms is obtained: Шаблон:NumBlk Note that the pressure of the system is constant in this form of the Bernoulli equation. If the static pressure of the system (the third term) increases, and if the pressure due to elevation (the middle term) is constant, then the dynamic pressure (the first term) must have decreased. In other words, if the speed of a fluid decreases and it is not due to an elevation difference, it must be due to an increase in the static pressure that is resisting the flow.

All three equations are merely simplified versions of an energy balance on a system. }}

Applications

Файл:Cloud over A340 wing.JPG

Condensation visible over the upper surface of an Airbus A340 wing caused by the fall in temperature accompanying the fall in pressure.

In modern everyday life there are many observations that can be successfully explained by application of Bernoulli's principle, even though no real fluid is entirely inviscid, and a small viscosity often has a large effect on the flow.

• Bernoulli's principle can be used to calculate the lift force on an airfoil, if the behaviour of the fluid flow in the vicinity of the foil is known. For example, if the air flowing past the top surface of an aircraft wing is moving faster than the air flowing past the bottom surface, then Bernoulli's principle implies that the pressure on the surfaces of the wing will be lower above than below. This pressure difference results in an upwards lifting force.Шаблон:Efn Whenever the distribution of speed past the top and bottom surfaces of a wing is known, the lift forces can be calculated (to a good approximation) using Bernoulli's equations, which were established by Bernoulli over a century before the first man-made wings were used for the purpose of flight.

• The carburetor used in many reciprocating engines contains a venturi to create a region of low pressure to draw fuel into the carburetor and mix it thoroughly with the incoming air. The low pressure in the throat of a venturi can be explained by Bernoulli's principle; in the narrow throat, the air is moving at its fastest speed and therefore it is at its lowest pressure.
• An injector on a steam locomotive or a static boiler.
• The pitot tube and static port on an aircraft are used to determine the airspeed of the aircraft. These two devices are connected to the airspeed indicator, which determines the dynamic pressure of the airflow past the aircraft. Bernoulli's principle is used to calibrate the airspeed indicator so that it displays the indicated airspeed appropriate to the dynamic pressure.Шаблон:Rp
• A De Laval nozzle utilizes Bernoulli's principle to create a force by turning pressure energy generated by the combustion of propellants into velocity. This then generates thrust by way of Newton's third law of motion.
• The flow speed of a fluid can be measured using a device such as a Venturi meter or an orifice plate, which can be placed into a pipeline to reduce the diameter of the flow. For a horizontal device, the continuity equation shows that for an incompressible fluid, the reduction in diameter will cause an increase in the fluid flow speed. Subsequently, Bernoulli's principle then shows that there must be a decrease in the pressure in the reduced diameter region. This phenomenon is known as the Venturi effect.
• The maximum possible drain rate for a tank with a hole or tap at the base can be calculated directly from Bernoulli's equation and is found to be proportional to the square root of the height of the fluid in the tank. This is Torricelli's law, which is compatible with Bernoulli's principle. Increased viscosity lowers this drain rate; this is reflected in the discharge coefficient, which is a function of the Reynolds number and the shape of the orifice.

• The Bernoulli grip relies on this principle to create a non-contact adhesive force between a surface and the gripper.
• During a cricket match, bowlers continually polish one side of the ball. After some time, one side is quite rough and the other is still smooth. Hence, when the ball is bowled and passes through air, the speed on one side of the ball is faster than on the other, and this results in a pressure difference between the sides; this leads to the ball rotating ("swinging") while travelling through the air, giving advantage to the bowlers.

Misconceptions

Airfoil lift

Файл:Equal transit-time NASA wrong1 en.svg

An illustration of the incorrect equal transit-time explanation of airfoil lift.

One of the most common erroneous explanations of aerodynamic lift asserts that the air must traverse the upper and lower surfaces of a wing in the same amount of time, implying that since the upper surface presents a longer path the air must be moving over the top of the wing faster than over the bottom. Bernoulli's principle is then cited to conclude that the pressure on top of the wing must be lower than on the bottom.

However, there is no physical principle that requires the air to traverse the upper and lower surfaces in the same amount of time. In fact, theory predicts and experiments confirm that the air traverses the top surface in a shorter time than it traverses the bottom surface, and this explanation based on equal transit time is false. While this explanation is false, it is not the Bernoulli principle that is false, because this principle is well established; Bernoulli's equation is used correctly in common mathematical treatments of aerodynamic lift.

Common classroom demonstrations

There are several common classroom demonstrations that are sometimes incorrectly explained using Bernoulli's principle. One involves holding a piece of paper horizontally so that it droops downward and then blowing over the top of it. As the demonstrator blows over the paper, the paper rises. It is then asserted that this is because "faster moving air has lower pressure".

One problem with this explanation can be seen by blowing along the bottom of the paper: if the deflection was caused by faster moving air, then the paper should deflect downward; but the paper deflects upward regardless of whether the faster moving air is on the top or the bottom. Another problem is that when the air leaves the demonstrator's mouth it has the same pressure as the surrounding air; the air does not have lower pressure just because it is moving; in the demonstration, the static pressure of the air leaving the demonstrator's mouth is equal to the pressure of the surrounding air. A third problem is that it is false to make a connection between the flow on the two sides of the paper using Bernoulli's equation since the air above and below are different flow fields and Bernoulli's principle only applies within a flow field.

As the wording of the principle can change its implications, stating the principle correctly is important. What Bernoulli's principle actually says is that within a flow of constant energy, when fluid flows through a region of lower pressure it speeds up and vice versa. Thus, Bernoulli's principle concerns itself with changes in speed and changes in pressure within a flow field. It cannot be used to compare different flow fields.

A correct explanation of why the paper rises would observe that the plume follows the curve of the paper and that a curved streamline will develop a pressure gradient perpendicular to the direction of flow, with the lower pressure on the inside of the curve. Bernoulli's principle predicts that the decrease in pressure is associated with an increase in speed; in other words, as the air passes over the paper, it speeds up and moves faster than it was moving when it left the demonstrator's mouth. But this is not apparent from the demonstration.

Other common classroom demonstrations, such as blowing between two suspended spheres, inflating a large bag, or suspending a ball in an airstream are sometimes explained in a similarly misleading manner by saying "faster moving air has lower pressure".

See also

• Torricelli's law
• Coandă effect
• Euler equations – for the flow of an inviscid fluid
• Hydraulics – applied fluid mechanics for liquids
• Navier–Stokes equations – for the flow of a viscous fluid
• Teapot effect
• Terminology in fluid dynamics

External links

• The Flow of Dry Water - The Feynman Lectures on Physics
• Science 101 Q: Is It Really Caused by the Bernoulli Effect?
• Bernoulli equation calculator
• Millersville University – Applications of Euler's equation
• NASA – Beginner's guide to aerodynamics Шаблон:Webarchive
• Misinterpretations of Bernoulli's equation – Weltner and Ingelman-Sundberg Шаблон:Webarchive