Fluids

Fluids are very familiar to us. Our body itself is mostly water while what surrounds us is largely air which again is a fluid. In fact, Greeks and Indians in the past worshiped earth, fire, water and air; three of these being fluids. We encounter motion of fluids almost everywhere- inside our bodies, in our daily activity such as taking a shower, cleaning, swimming etc. We also fly and sail which are nothing but motion of an object through a fluid. Fluid Mechanics is a science that studies the behaviour of fluids and its effect on other bodies. It comprises of fluid statics, which is a study of fluids at rest and fluid dynamics, which concerns fluids in motion. Then we also classify them further as aerodynamics, which specialises in flow of air. If we concentrate on water, we have hydrostatics and hydrodynamics.

Figure 1.1: Three Approaches to fluid Mechanics

There are three approaches to Fluid Mechanics – Experimental, Theoretical and Computational. Experimental approach is the oldest approach, perhaps also employed by Archimedes when he was to investigate a fraud. It is a very popular approach where you will make measurements using a wind tunnel or a similar equipment. But this is a costly venture and is becoming costlier day by day. Then we have the theoretical approach where we employ the mathematical equations that govern the flow and try to capture the fluid behaviour within a closed form solution i.e., formulas that can be readily used. This is perhaps the simplest of the approaches, but its scope is somewhat limited. Not every fluid flow renders itself to such an approach. The resulting equations may be too complicated to solve easily. Then comes the third approach- Computational. Here we try to solve the complicated governing equations by computing them using a computer. This has the advantage that a wide variety of fluid flows may be computed and that the cost of computing seems to be going down day by day. With the result the emerging discipline Computational Fluid Dynamics, CFD, has become a very powerful approach today in industry and research. It is worth noting that any theoretical calculation or a numerical computation has to be validated. For this it is usual to rely on experiments. After all seeing is believing. Theory guides and experiment decides.

In this course, we concentrate mainly on the theoretical approach as it gives us a good insight into fluid mechanics. Of course, we will be referring to the information provided by the experimental studies. Computational approach becomes too involved for a first course in fluid mechanics.

(c) Aerospace, Mechanical & Mechatronic Engg. 2005
University of Sydney