# Darcys formula

Darcy's formula for loss of head in pipe

**Contents**

**Henry Philibert Gaspard Darcy**(1803-1858) was a French engineer who made several important contributions to hydraulics.

## Overview

When the water is flowing in a pipe, it experiences some resistance to its motion, whose effect is to reduce the velocity and ultimately the head of water available. An empirical formula for the loss of head due to friction was derived by Henry Darcy. The loss of head due to friction according to Darcy is, where,- = Loss of head due to friction
- = Length of pipe
- = Diameter of the pipe

## Theory

Consider a uniform long pipe through which water is flowing at a uniform rate as shown in figure. Let,- = Velocity of water in the pipe
- = Frictional resistance per unit area at unit velocity

- = Intensity of pressure at section (1-1)
- = Intensity of pressure at section (2-2)

_{1}and p

_{2}would have been equal, if there would have been no frictional resistance. Now considering horizontal forces on water between sections (1-1) and (2-2) and equating the same,

Dividing both sides by -
But
We know that as per Froude's experiment, frictional resistance
Substituting the value of frictional resistance in the above equation,
Let us introduce another coefficient () such that,
We know that the discharge,
Substituting the value of in equation (1)

Example:

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##### Example - Determination of the loss of head

Problem

Find the loss of head due to friction in a pipe of 500mm diameter and 1.5km long. The velocity of water in the pipe is 1.0 m/s. Take coefficient of friction as 0.005.

Workings

Given,

- = 500 mm = 0.5 m
- = 1.5 km = 1500 m
- = 1 m/s
- = 0.005

Solution

Loss of head due to friction = 3.01 m