This is the knowledge on which this book depends and it is not just a few simple principles. Fluids are particularly troublesome to deal with because they have no shape other than that imposed on them from outside and when they move they do not move like a solid. They move with internal motion at three levels, the motion at molecular level, the motion of small scale turbulence and the larger scale motion of eddying. Just looking at water flowing in a small river leads to the feeling that this is just the same as the last time you looked at it yet the detail of the flow is simply not the same. It is the same old flow with different detail. In a sense this gives us a clue to the possibility of success because it suggests that if we try to avoid the detail we might find an overall order to things that is adequate for engineering purposes. This last proviso is dominant. Once science and experiment can provide us with numbers that are adequate for our purposes there is no incentive for further refinement and it pays to remember that those numbers might be changed for political, financial or commercial expediency.
In this book I want to attempt to assess precisely what we are doing and to spell out how it fits into the overall order of things.
I wrote it originally in 1990 and since then computer programmes like Mathcad have made the exploration of the many equations of fluids very easy to do. This exploration gives insights that were formerly not available because it was too laborious to make the exploration. I hope to re-draft this text to include some of these explanations.