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Chapter 1 Introduction and Basic Concepts
Solutions Manual for
Fluid Mechanics: Fundamentals and Applications
by Çengel & Cimbala



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Chapter 1 Introduction and Basic Concepts

Introduction, Classification, and System

We are to define internal, external, and open-channel flows.

External flow is the flowof an unbounded fluid over a surface such as a plate, a wire, or a pipe. The flow
in a pipe or duct is internal flow if the fluid is completely bounded by solid surfaces. The flow of liquids in a pipe is
called open-channel flow if the pipe is partially filled with the liquid and there is a free surface, such as the flow of
water in rivers and irrigation ditches.

Discussion As weshall see in later chapters, there different approximations are used in the analysis of fluid flows based
on their classification.

We are to define incompressible and compressible flow, and discuss fluid compressibility.

A fluid flow during which the density of the fluid remains nearly constant is called incompressible flow.
A flow in whichdensity varies significantly is called compressible flow. A fluid whose density is practically independent
of pressure (such as a liquid) is commonly referred to as an “incompressible fluid,” although it is more proper to refer to
incompressible flow. The flow of compressible fluid (such as air) does not necessarily need to be treated as compressible
since the density of a compressible fluid maystill remain nearly constant during flow – especially flow at low speeds.

Discussion It turns out that the Mach number is the critical parameter to determine whether the flow of a gas can be
approximated as an incompressible flow. If Ma is less than about 0.3, the incompressible approximation yields results that
are in error by less than a couple percent.

SolutionWe are to define the no-slip condition and its cause.

A fluid in direct contact with a solid surface sticks to the surface and there is no slip. This is known as
the no-slip condition, and it is due to the viscosity of the fluid.

Discussion There is no such thing as an inviscid fluid, since all fluids have viscosity.

We are to defineforced flow and discuss the difference between forced and natural flow. We are also to
discuss whether wind-driven flows are forced or natural.

In forced flow, the fluid is forced to flow over a surface or in a tube by external means such as a pump or a
fan. In natural flow, any fluid motion is caused by natural means such as the buoyancy effect that manifests itself as the...
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