Seminar 5B

MOMENTUM ANALYSIS OF FLOW SYSTEMS

PREPARATION

Read Chap 13 – Fundamentals of Thermal-Fluid Sciences 4th

Edition

Read Chap 6 – Thermofluids II 3rd Edition

EAS109 Thermo – Fluid Mechanics

January 2018

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Objectives

• Identify the various kinds of forces and

moments acting on a control volume

• Use control volume analysis to determine the

forces associated with fluid flow

• Use control volume analysis to determine the

moments caused by fluid flow and the torque

transmitted

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Examples for MOMENTUM ANALYSIS

OF FLOW SYSTEMS

Example 13-2

Example 13-3

Example 13-4

Linear Momentum Equation

Problem 13-11C & 13-16C

Propulsion Systems

Worked Examples Exercises

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NEWTON’S LAWS 1

Newton’s laws: Relations between motions of bodies and the

forces acting on them.

Newton’s first law: A body at rest remains at rest, and a body in

motion remains in motion at the same velocity in a straight path

when the net force acting on it is zero.

Therefore, a body tends to preserve its state of inertia.

Newton’s second law: The acceleration of a body is

proportional to the net force acting on it and is inversely

proportional to its mass.

Newton’s third law: When a body exerts a force on a second

body, the second body exerts an equal and opposite force on the

first.

Therefore, the direction of an exposed reaction force depends

on the body taken as the system.

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Linear momentum is the

product of mass and velocity,

and its direction is the

direction of velocity.

Newton’s second law is also expressed as

the rate of change of the momentum of a

body is equal to the net force acting on it.

Linear momentum or just the momentum of the body: The product of

the mass and the velocity of a body.

Newton’s second law is usually referred to as the linear momentum

equation. Conservation of momentum principle: The

momentum of a system remains constant

only when the net force acting on it is zero.

NEWTON’S LAWS 2

CHOOSING A CONTROL VOLUME

A control volume can be selected as any arbitrary

region in space through which fluid flows, and its

bounding control surface can be fixed, moving, and

even deforming during flow.

Many flow systems involve stationary hardware firmly

fixed to a stationary surface, and such systems are

best analyzed using fixed control volumes.

When analyzing flow systems that are moving or

deforming, it is usually more convenient to allow the

control volume to move or deform.

In deforming control volume, part of the control

surface moves relative to other parts.