Bernoulli’s principle from Equation in Fluid Mechanics

In fluid dynamics, Bernoulli’s principle state that an increase in fluid speed occurs with pressure decrease or decrease in fluids potential energy. This principle can be applied to many types of fluid flow as result in various forms of Bernoulli’s equation. So far, we have discussed fundamental aspects of Bernoulli equation. This article is about Bernoulli’s principle and equation in fluid dynamics with examples.

What is Bernoulli’s Principle?

Bernoulli’s Principle is important principle involving in movement of fluid through pressure difference. Suppose fluid is moving horizontal direction and meet pressure difference. This pressure difference will result in net force will cause an increase of fluid. Also read Pressure Scales in Fluid Mechanics 

What is Bernoul li’s Equation?

Bernoulli’s equation is one of most important equations in fluid mechanics. Let’s take a look at Bernoulli’s equation and what it says and how one would go about using it. Bernoulli’s equation is based on conservation of energy for flowing fluids. If no energy is added to system as work or heat then total energy of fluid is sealed.

Also, it puts into relation velocity and pressure in an inviscid incompressible flow. There are different forms of Bernoulli’s equation for different types of flow. Bernoulli’s equation is used anytime which relate pressures and velocities in place where flow conditions are close enough to derive Bernoulli equation. Since Bernoulli’s equation can be used to describe how air moves across surface. Also, this equation is applicable in pipes of changing diameters and height.

Limitations on used of Bernoulli’s Equation

Bernoulli’s Equation is regularly used in fluid mechanics. Also, this equation is very valuable tool to use in analysis. Therefore, it is important to understand Bernoulli’s equation limits in its use, as following points.

  • stable flow system
  • Constant density (this means fluid is incompressible)
  • No work is done by fluid
  • From fluid no heat is transferred
  • There is no change occurs in internal energy
  • This equation is applicable along streamline

Under these conditions, Bernoulli equation can be between any two points on same streamlines as follows.

Bernoulli’s equation

Bernoulli’s equation relates pressure, speed and height of any two points in steady flowing fluid of density P. It states that neglecting friction for perfect incompressible fluid, flowing in continuous stream and total energy of particle remains constant while particle move from one point to another. Bernoulli’s equation is written as follows,

Bernoulli’s equation is relationship between pressure and velocity in fluids is described by Bernoulli’s equation named by Daniel Bernoulli. Bernoulli’s equation states that for frictionless fluid, an in-compressible sum is constant-

P_{{1}}+\frac{1}{2}\rho v^{2}+ \rho gh= constant

And finally, this is Bernoulli’s equation that P is pressure plus kinetic energy density  \frac{1}{2}\rho v^{2}plus gravitational potential energy density is pgh at any points they will be equal.

Where P is absolute pressure, ρ is fluid density, v is velocity of fluid, h is height, and g is acceleration due to gravity. So if we follow small volume of fluid along its path, various amounts may change in sum but total remains constant. Let 1 and 2 refer to any two points along path that bit of fluid follows then Bernoulli’s equation becomes

How to derive Bernoulli’s equation

Energy per unit volume before = Energy per unit volume after

P_{1}+\frac{1}{2}\rho v \frac{2}{1}+\rho gh_{1}=P_{2}+\frac{1}{2}\rho v \frac{2}{2}+ \rho gh_ {2}

These variables P1, v1, h1 refer to pressure, speed and height of fluid at point 1, whereas variables P2, v2 and h2 refer to pressure, speed and height of fluid at point 2. Where P is pressure in the, ρ is density of fluid, g is acceleration due to gravity (9.80 m/s^2), h is height of fluid off ground, and v is velocity of fluid.

From above equation, you can understand that if velocity increases, it will be decrease in pressure and vice-versa. This Bernoulli Effect is reduce in pressure which occurs when fluid speed increase. For Bernoulli’s equation, point 1 and 2 must be present on same line, not different lines. But people place these points on different streamline. Moreover, Bernoulli’s equation or principle becomes invalid when flow becomes compressible. This equation is just small part of explaining lift on wing.

Explain Bernoulli’s Equation

This equation is well-known equation in fluid dynamics. Bernoulli’s equation tells about behavior of flowing fluid that is labeled with term Bernoulli’s effect. On other hand, this effect causes lowering of fluid pressure in regions where flow velocity is raise.

This low of pressure flow is limit, so when you think pressure to energy density. In high velocity flow through limit but kinetic energy must rise at pressure energy. In terms of dimensions this equation is kinetic energy per unit volume. Check What is a Dynamometer? 

This equation tells us that, in static fluids, pressure increases with depth. As we go from point 1 to point 2 in fluid, depth increases by h1, and so, P2 is greater than P1 by an amount ρgh1. In simple case, P1 is zero at top of fluid, and we get familiar relationship P = ρgh.

Bernoulli’s Principle from Bernoulli’s Equation

Another important is one in which fluid moves but its depth is constant that is.h_{1}=h_ {2} Under that condition, Bernoulli’s equation becomes

P_{1}+\frac{1}{2}\rho v \frac{2}{1}=P_{2}+\frac{1}{2}\rho v \frac{2}{2}

In this equation, fluid flows at constant depth are important so that this equation is called Bernoulli’s principle. It is Bernoulli’s equation for fluids at constant depth. As pressure drops as speed increase in moving fluid and we can see this from Bernoulli’s principle. For example, if is greater than  in equation, then  is less than  to hold equally.

Application of Bernoulli’s Principle

There are number of devices in which fluid flows at constant height with Bernoulli’s principle.

Air flight

For example, airplane wing is example of Bernoulli’s principle in action.  In an airplane wing, top of wing is curved while bottom of wing is flat. In the sky, air travels across both top and bottom because both top part and bottom part of plane are designed differently. This allows for air on bottom to move slow which creates more pressure on bottom and allows top air to move faster which gives less pressure.

In simple words, this will creates lift which allows planes to fly. Wings can also gain lift by pushing air downward by conservation of momentum principle. On other hand, an airplane is acted by pull of gravity that enables airplane to move forward.


In engines carburetor is used in many engines contains venture to create low pressure to draw fuel into carburetor and mix it well with incoming air.  This low pressure in throat of venture can be explained by Bernoulli’s principle. Thus in narrow throat, air is moving fast and therefore it is at low pressure.

Assumption of Bernoulli’s Therom

  • Fluid is ideal
  • Flow is steady and continuous
  • Flow is in-compressible
  • Flow is one diamentional
  • Irrotational flow
  • Velocity of fluid is uniform over cross section
  • No other force energy exist except gravity and pressure force