THE 456th FIGHTER INTERCEPTOR SQUADRON

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Bernoulli's Principle

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A graphic showing Bernoulli's equations which relates the
 velocity and static pressure of a flow.

 

Bernoulli's Principle states that for an ideal fluid (low speed air is a good approximation), with no work being performed on the fluid, an increase in velocity occurs simultaneously with decrease in pressure or a change in the fluid's gravitational potential energy.

This principle is a simplification of Bernoulli's equation, which states that the sum of all forms of energy in a fluid flowing along an enclosed path (a streamline) is the same at any two points in that path. It is named after the Dutch/Swiss mathematician/scientist Daniel Bernoulli, though it was previously understood by Leonhard Euler and others. In fluid flow with no viscosity, and therefore, one in which a pressure difference is the only accelerating force, the principle is equivalent to Newton's laws of motion.

 

Incompressible flow

The original form, for incompressible flow in a uniform gravitational field, is:

 {v^2 \over 2}+gh+{p \over \rho}=\mathrm{constant}

where:

v = fluid velocity along the streamline
g = acceleration due to gravity
h = height of the fluid
p = pressure along the streamline
ρ = density of the fluid

These assumptions must be met for the equation to apply:

An increase in velocity and the corresponding decrease in pressure, as shown by the equation, is often called Bernoulli's principle. The equation is named for Daniel Bernoulli although it was first presented in the above form by Leonhard Euler.

A common example used to illustrate the effect of Bernoulli's principle is when air flows around an airplane wing; the velocity of the air is higher and the pressure is lower on the top surface of the wing when compared to the bottom surface. This pressure differential creates an upwards lift force on the wings making flight possible.

This can be rewritten as:

 {v^2 \rho \over 2}+\rho g h+p=\mathrm{constant}

or:

q + ρgh + p = constant

where:

q = dynamic pressure

 

Compressible flow

A second, more general form of Bernoulli's equation may be written for compressible fluids, in which case, following a streamline:

 {v^2 \over 2}+ \phi + w =\mathrm{constant}
\phi \, = gravitational potential energy per unit mass,  \phi = gh \, in the case of a uniform gravitational field
 w \, = fluid enthalpy per unit mass, which is also often written as  h \, (which conflicts with the use of  h \, in this article for "height"). Note that  w = \epsilon + \frac{p}{\rho} where  \epsilon \, is the fluid thermodynamic energy per unit mass, also known as the specific internal energy or "sie".

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

When shock waves are present, in a reference frame moving with a shock, many of the parameters in the Bernoulli equation suffer abrupt changes in passing through the shock. The Bernoulli parameter itself, however, 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.

 

 Derivations of Bernoulli equation

 

 Incompressible fluids

The Bernoulli equation for incompressible fluids can be derived by integrating the Euler equations, or applying the law of conservation of energy in two sections along a streamline, ignoring viscosity, compressibility, and thermal effects.

The simplest derivation is to first ignore gravity and consider constrictions and expansions in pipes that are otherwise straight, as seen in Venturi effect. Let the x axis be directed down the axis of the pipe.

The equation of motion for a parcel of fluid on the axis of the pipe is

m \frac{dv}{dt}= -F
\rho  A  dx \frac{dv}{dt}= -A dp
\rho \frac{dv}{dt}= -\frac{dp}{dx}

In steady flow, v = v(x) so

\frac{dv}{dt}= \frac{dv}{dx}\frac{dx}{dt} = \frac{dv}{dx}v=\frac{d}{dx} \frac{v^2}{2}

With ρ constant, the equation of motion can be written as

\frac{d}{dx} \left(  \rho \frac{v^2}{2} + p \right) =0

or

 \frac{v^2}{2} + \frac{p}{\rho}= C

where C is a constant, sometimes referred to as the Bernoulli constant. We deduce that where the speed is large, pressure is low. In the above derivation, no external work-energy principle is invoked. Rather, the work-energy principle was inherently derived by a simple manipulation of the momentum equation. The derivation that follows includes gravity and applies to a curved trajectory, but a work-energy principle must be assumed.

Click on Picture to enlarge

A stream-tube of fluid moving to the right. Indicated are pressure, height, velocity, distance (s), and cross-sectional area.

Applying conservation of energy in form of the work-kinetic energy theorem we find that:

the change in KE of the system equals the net work done on the system;
W=\Delta KE. \;

Therefore,

the work done by the forces in the fluid + decrease in potential energy = increase in kinetic energy.

The work done by the forces is

F_{1} s_{1}-F_{2} s_{2}=p_{1} A_{1} v_
{1}\Delta t-p_{2} A_{2} v_{2}\Delta t. \;

The decrease of potential energy is

m g h_{1}-m g h_{2}=\rho g A
_{1} v_{1}\Delta t h_{1}-\rho g A_{2} v_{2} \Delta
t h_{2} \;

The increase in kinetic energy is

\frac{1}{2} m v_{2}^{2}-\frac{1}{2} m v_{1}^{2}=\frac{1}{2}\rho A_{2} v_{2}\Delta t v_{2}
^{2}-\frac{1}{2}\rho A_{1} v_{1}\Delta t v_{1}^{2}.

Putting these together,

p_{1} A_{1} v_{1}\Delta t-p_{2} A_{2} v_{2}\Delta t+\rho g A_{1} v_{1}\Delta t h_{1}-\rho g A_{2} v_{2}\Delta t h_{2}=\frac{1}{2}\rho A_{2} v_{2}\Delta t v_{2}^{2}-\frac{1}{2}\rho A_{1} v_{1}\Delta t v_{1}^{2}

or

\frac{\rho A_{1} v_{1}\Delta t v_{1}^{
2}}{2}+\rho g A_{1} v_{1}\Delta t h_{1}+p_{1} A_{1
} v_{1}\Delta t=\frac{\rho A_{2} v_{2}\Delta t v_{
2}^{2}}{2}+\rho g A_{2} v_{2}\Delta t h_{2}+p_{2}
A_{2} v_{2}\Delta t.

After dividing by Δt, ρ and A1v1 (= rate of fluid flow = A2v2 as the fluid is incompressible):

\frac{v_{1}^{2}}{2}+g h_{1}+\frac{p_{1}}{\rho}=\frac{v_{2}^{2}}{2}+g h_{2}+\frac{p_{2}}{\rho}

or, as stated in the first paragraph:

\frac{v^{2}}{2}+g h+\frac{p}{\rho}=C


Further division by g implies

\frac{v^{2}}{2 g}+h+\frac{p}{\rho g}=C

A free falling mass from a height h (in vacuum), will reach a velocity

v=\sqrt{{2 g}{h}}, or h=\frac{v^{2}}{2 g}.

The term \frac{v^2}{2 g} is called the velocity head.

The hydrostatic pressure or static head is defined as

p=\rho  g  h \,, or h=\frac{p}{\rho  g}.

The term \frac{p}{\rho  g} is also called the pressure head.


A way to see how this relates to conservation of energy directly is to multiply by density and by unit volume (which is allowed since both are constant) yielding:

v^2 \rho + P = constant \, and
mV^2 + P \cdot volume = constant \,

 

 Compressible fluids

The derivation for compressible fluids is similar. Again, the derivation depends upon (1) conservation of mass, and (2) conservation of energy. Conservation of mass implies that in the above figure, in the interval of time Δt, the amount of mass passing through the boundary defined by the area A1 is equal to the amount of mass passing outwards through the boundary defined by the area A2:

 0 = \Delta M_1 - \Delta M_2 = \rho_1 A_1 v_1 \, \Delta t - \rho_2 A_2 v_2 \, \Delta t .

Conservation of energy is applied in a similar manner: It is assumed that the change in energy of the volume of the stream-tube bounded by A1 and A2 is due entirely to energy entering or leaving through one or the other of these two boundaries. Clearly, in a more complicated situation such as a fluid flow coupled with radiation, such conditions are not met. Nevertheless, assuming this to be the case and assuming the flow is steady so that the net change in the energy is zero,

 0 = \Delta E_1 - \Delta E_2 \,

where ΔE1 and ΔE2 are the energy entering through A1 and leaving through A2, respectively.

The energy entering through A1 is the sum of the kinetic energy entering, the energy entering in the form of potential gravitational energy of the fluid, the fluid thermodynamic energy entering, and the energy entering in the form of mechanical  p\,dV work:

 \Delta E_1 = \left[  \frac{1}{2} \rho_1 v_1^2 + \phi_1 \rho_1 + \epsilon_1 \rho_1  + p_1 \right] A_1 v_1 \, \Delta t

A similar expression for ΔE2 may easily be constructed. So now setting 0 = ΔE1 − ΔE2:

 0 = \left[  \frac{1}{2} \rho_1 v_1^2+ \phi_1 \rho_1 + \epsilon_1 \rho_1  + p_1 \right] A_1 v_1 \, \Delta t  - \left[ \frac{1}{2} \rho_2 v_2^2 + \phi_2\rho_2 + \epsilon_2 \rho_2  + p_2 \right] A_2 v_2 \, \Delta t

which can be rewritten as:

 0 = \left[ \frac{1}{2} v_1^2 + \phi_1 + \epsilon_1  + \frac{p_1}{\rho_1} \right] \rho_1 A_1 v_1 \, \Delta t  - \left[  \frac{1}{2} v_2^2  + \phi_2 + \epsilon_2  + \frac{p_2}{\rho_2} \right] \rho_2 A_2 v_2 \, \Delta t

Now, using the previously-obtained result from conservation of mass, this may be simplified to obtain

 \frac{1}{2}v^2 + \phi + \epsilon + \frac{p}{\rho} = {\rm constant} \equiv b

which is the Bernoulli equation for compressible flow.

 

Wikipedia

 

 

Last Updated

02/10/2014

 

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