# Bernoullis Theorem

The background to Bernoulli's Theorem.

## Overview

**Daniel Bernoulli**(1700 - 1782) investigated not only mathematics but also such fields as medicine, biology, physiology, mechanics, physics, astronomy, and oceanography.

**Key Facts:**

**Definition**

*v*is velocity [m/s]*P*is pressure [N/m^{2}]*g*is accretion due to gracity [m/s^{2}]*z*is vertical elevation- ρ is the density of the liquid

**Bernoulli's**principle states,

" For a perfect incompressible liquid, flowing in a continuous stream, the total energy of a particle remains the same, while the particle moves from one point to another."

This statement is based on the assumption that there are no losses due to friction in the pipe. Mathematically,
where,
*z*= Potential Energy- = Kinetic Energy
- = Pressure Energy

**Figure 1**for example, the fluid speeds up in constricted areas so that the pressure the fluid exerts is least where the cross section is smallest. This phenomenon is also called the

**Venturi effect**. The Venturi effect has several applications including that associated with generating lift atop an aerofoil, making all forms of flight on aeroplanes, gyroplanes, and helicopters possible.

**Note:**The Bernoulli's theorem is also the law of conservation of energy, i.e. the sum of all energy in a steady, streamlined, incompressible flow of fluid is always a constant.

## Theorem Proof

Consider a perfect incompressible liquid, flowing through a non-uniform pipe as shown in fig.
Let us consider two sections
Total work done by the pressure
Loss of potential energy =
and again in Kinetic Energy = =
We know that, Loss of potential energy + Work done by pressure = Gain in kinetic energy
which proves Bernoulli's Equation.

**AA**and**BB**of the pipe and assume that the pipe is running full and there is a continuity of flow between the two sections. Let,- = Height of
**AA**above the datum - = Pressure at
**AA** - = Velocity of liquid at
**AA** - = Cross sectional area of the pipe at
**AA**, and - = Corresponding values at
**BB**

**AA**and**BB**move to**A'A'**and**B'B'**through very small lengths and as shown in fig. This movement of liquid between**AA**and**BB**is equivalent to the movement of the liquid between**AA**and**A'A'**to**BB**and**B'B'**, the remaining liquid between**A'A'**and**BB**being unaffected. Let,*W*be the weight of the liquid between**AA**and**A'A'**. Since the flow is continuous, therefore or Similarly, We know that the work done by pressure at**AA**, in moving the liquid to**A'A'**Similarly, the work done by pressure at**BB**, in moving the liquid to**B'B'****Note:**Minus (-) sign is taken as the direction of is opposite to that of .

Example:

[metric]

##### Example - Bernoulli's Theorem

Problem

The diameter of a pipe changes from 200mm at a section 5m above datum to 50mm at a section 3m above datum. The pressure of water at first section is 500kPa. If the velocity of the flow at the first section is 1m/s, determine the intensity of pressure at the second section.

Workings

Given,

- ,
- ,
- ,
- ,
- and

- = Velocity of flow at section
**(2**), and - = Pressure at section
**(2**).

**(1**), and area of pipe at section**(2**), Since the discharge through the pipe is continuous, therefore Applying Bernoulli's equation for both the ends of the pipe,Solution

The intensity of pressure at the second section = 392.4 kPa

Last Modified: 23 Nov 11 @ 12:28 Page Rendered: 2022-03-14 17:28:48