Introduction.

At one time the study of the flow of liquids in channels was a regular topic in degree courses for students of mechanical engineering. Then it was omitted because of the need to make time for the studies in other fields of fluid flow. I think that there was a case for omitting the flow in long channels because that is properly the job of civil engineers. However water flows through and over all sorts of fittings that are employed in connection with flow in long channels and the very fact that the behaviour of the water in these fittings can be observed directly is of significant value to mechanical engineers. The direct value lies in the similarity between flow in these fittings and the flow of gas in convergent-divergent nozzles and in supersonic flow in general. We cannot see gas but we can see water and this can give a very useful mental picture.

 

I spent a very long time on this chapter concentrating on the mechanical aspects and gradually concluded that this topic is a valuable exercise in the application of science to a very practical problem and, as this text is going on the internet, there is no reason to omit anything on the grounds of space as might be the case for a book. There is a more general value to the student in that the “theory” and “practice” or, as I prefer the physics and its application can be separated.

 

For many years I could not see any line of demarcation between physics as taught in a physics department in a college or university and mechanical engineering. Indeed the student projects in physics were often engineering projects. Then a tele-professor, adjudicating on a design competition, said that the group sponsoring a machine for capturing energy from the wind had clearly understood the physics of the aerofoils that had been used and had also solved the practical problems of their application in the machine. He had no doubt where the line between physics and its application lay. The study of flow in channels is tailor made for a clear division. So I plan to introduce the topic in general, present the physics in the form that it is used and then apply it to some practical applications.