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Mechanics of Fluids

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The roles <strong>of</strong> experimentation and theory in fluid mechanics 39<br />

flow about basic shapes, such as flat plates, circular cylinders and so on.<br />

Similarly, in the field <strong>of</strong> internal flow, straight pipes and other components<br />

such as flow-metering devices, bends, diffusers etc. have been widely<br />

tested. This information forms an important database for the study <strong>of</strong> fluid<br />

mechanics, and will be referred to extensively throughout this book. But for<br />

more complicated geometries, to gain an understanding <strong>of</strong> the fluid mechanics,<br />

it is <strong>of</strong>ten necessary to carry out tests using purpose-made models or<br />

test rigs.<br />

Experiments can be <strong>of</strong> various kinds. They can be full-scale or can involve<br />

scale models. They can involve the measurement <strong>of</strong> various quantities, such<br />

as velocity or pressure, either at a specified point in a flow or to determine<br />

pr<strong>of</strong>iles in the form <strong>of</strong> velocity or pressure distributions. Alternatively, using<br />

flow visualization techniques, experiments may be designed to show up the<br />

patterns <strong>of</strong> flow. Various kinds <strong>of</strong> special facilities are used. The wind tunnel<br />

is an important tool in the investigation <strong>of</strong> the flow <strong>of</strong> air around aircraft,<br />

vehicles and buildings. Water tanks and water tables are used to study the<br />

hydrodynamics <strong>of</strong> ships and the flow about bridges, respectively.<br />

1.10.2 Mathematical modelling and the equations<br />

<strong>of</strong> fluid dynamics<br />

We have seen that information about the behaviour <strong>of</strong> flowing fluids can<br />

be obtained from experiments, using observation and measurement. Mathematics<br />

also has an essential role to play in furthering our understanding <strong>of</strong><br />

fluid mechanics. It is important for the student new to fluid mechanics to<br />

appreciate a fundamental difference between experimental and theoretical<br />

data. A result from a carefully conducted experiment is (with minor qualification)<br />

unique; in contrast, a theoretical value is not. The outcome from a<br />

theoretical study <strong>of</strong> the flow through or about a specified geometry depends<br />

upon the assumptions on which the mathematical analysis is based. If the<br />

assumptions are sound, the theoretical results will closely match the corresponding<br />

experimental data. If the theory is incomplete, unduly simplified<br />

or unsoundly based, then the theoretical results will differ from the experimental<br />

data. It follows that to refer, without qualification, to the so-called<br />

theoretical value <strong>of</strong> a quantity is meaningless.<br />

In whatever way we choose to represent a flow mathematically, the fundamental<br />

laws <strong>of</strong> fluid mechanics, and in the case <strong>of</strong> compressible flow <strong>of</strong><br />

thermodynamics, must be satisfied. The equations <strong>of</strong> fluid mechanics can be<br />

expressed in two fundamentally different ways. The differential equations <strong>of</strong><br />

motion are a set <strong>of</strong> equations which result when attention is focused on an<br />

infinitesimally small element <strong>of</strong> fluid. Information on the detailed variation<br />

in time and space <strong>of</strong> flow properties, such as velocity and pressure, results<br />

from the solution <strong>of</strong> these equations. When information on the overall<br />

characteristics <strong>of</strong> a flow, such as mass flow rate through a pipe system, is<br />

required then the integral forms <strong>of</strong> the equations <strong>of</strong> motion are relevant.<br />

The representation <strong>of</strong> any physical system by means <strong>of</strong> a set <strong>of</strong> mathematical<br />

equations is referred to as mathematical modelling. We have seen that, to

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