# Fluid Inertia

Pressure differential across tube or channel due to change in fluid velocity

## Library

Hydraulic Elements

## Description

The Fluid Inertia block models pressure differential, due to change in fluid velocity, across a fluid passage of constant cross-sectional area. The pressure differential is determined according to the following equation:

`$p=\rho \frac{L}{A}\frac{dq}{dt}$`

where

 p Pressure differential ρ Fluid density L Passage length A Passage area q Flow rate t Time

Use this block in various pipe or channel models that require fluid inertia to be accounted for.

The block positive direction is from port A to port B. This means that the flow rate is positive if it flows from A to B.

### Variables

Use the Variables tab in the block dialog box (or the Variables section in the block Property Inspector) to set the priority and initial target values for the block variables prior to simulation. For more information, see Set Priority and Initial Target for Block Variables.

## Assumptions and Limitations

Fluid density remains constant.

## Parameters

Passage area

Fluid passage cross-sectional area. The default value is `8e-5` m^2.

Passage length

Length of the fluid passage. The default value is `1` m.

Initial flow rate

Initial flow rate through the passage. This parameter specifies the initial condition for use in computing the block's initial state at the beginning of a simulation run. For more information, see Initial Conditions Computation. The default value is `0`.

## Global Parameters

Parameter determined by the type of working fluid:

• Fluid density

Use the Hydraulic Fluid block or the Custom Hydraulic Fluid block to specify the fluid properties.

## Ports

The block has the following ports:

`A`

Hydraulic conserving port associated with the passage inlet.

`B`

Hydraulic conserving port associated with the passage outlet.