Line Sizing for Low-Viscosity and Turbulent Flow

Note that our choice of standard pipe diameter becomes more restricted for larger diameter pipe, where the increments of standard pipe diameter are greater than for small diameter pipes. 

I suppose that was easy enough to follow. Let s just backtrack a little now so that we can put the information we have had so far in its proper context. Certainly that equation for AP is simple enough to use, but how do we know if it is really valid for the fluid and the flow we are dealing with How do we know if the fluid is in turbulent flow, and for that matter, what is turbulent flow anyway Why are most fluids piped and pumped in turbulent flow To answer these questions, we have to back track a little, and so we should. Don t panic. Although there are many excellent and far, far more complete texts on this subject (to some of which we will refer), we will now attempt to give you a thumbnail sketch or working guide.  

Low-viscosity materials such as we have been discussing in the previous section have flow behavior that has been classified as Newtonian, because at constant temperature and pressure the fluid has constant viscosity, irrespective of the way in which the fluid is flowing. Other types of fluid that we will look at more closely later do not follow this straightforward flow behavior, but may exhibit varying apparent viscosity depending on the amount of shear used to move the fluid or even exhibit some kind of yield stress which must be overcome before they can begin to flow. Such fluids are known collectively as non-Newtonian.  

Let us return to the discussion on Newtonian fluids. By the way, it is worthwhile to remember that liquids and gases are both thought of as fluids. 

Early pioneering work by Reynolds in 1883 showed evidence of two distinctly different types of flow patterns or flow regimes, in circular cross-section pipes. Reynolds used a glass pipe immersed in a constant-temperature water bath fitted with a quill to introduce a single filament of colored water at the center line of the glass pipe. 

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