L.F.Moody.

Moody was a professor of engineering and his paper was presented in 1944. It is interesting to note that throughout the long history of the investigation of friction loss in pipes no-one had considered how engineers might actually use the outcome. Engineers in the main, do not need to know all the physics of all the data they use. A reliable graph that can be used easily and successfully is what is required. Moody provided just such a graph and it is interesting to read Hunter Rouse's somewhat tetchy communication to the A.S.M.E. on Moody's paper. There, the different objectives of engineers and physicists is evident for all to read. The chart that Rouse recommended to replace Moody's diagram has been forgotten.

 

Moody observed that the implication of Colebrook's work was that the  against  curve for a given commercial pipe would be a smooth curve starting tangentially from the smooth pipe curve produced by Stanton and Pannell and becoming horizontal at high values of Re. Clearly many such curves could now be plotted for a range of values of , which Moody called the relative roughness, on Stanton and Pannell's graph. Moody says that he used Nikuradse's values of  against  as the basis for his roughness scale. We already have, in Graph 7-3, a plot of Nikuradse's values of  for hydraulically rough flow against the relative roughness for the six values of roughness he used. As the practical values of relative roughness for practical pipes lie in the region of  and below Moody had to extrapolate through this region. Moody does not say how he provided the figures for the lower values of  but he does refer to available data on the friction loss in large steel pipes and to the work of R.J.S.Pigott who in turn refers to a mass of data compiled by Emory Kemler. It may be possible to make a more successful extrapolation by reworking this data in terms of equivalent sand grain roughness but it must be remembered that it is very difficult to provide the flows needed to take a large pipe into the hydraulically rough range. Moody suggests that an accuracy of 10% may reasonably be expected from the chart. We have seen that this new graph shown in Diagram 7-5 is now called the Moody diagram or the Moody chart. It has been widely used since 1944.

 

It is interesting to note that this chart is, in the end, wholly empirical in just the same way as the other coefficients have been empirical. However only the contributions that led to the successful conclusion are recorded here. The many analyses, both quantitative and qualitative, which at once gave stop-gap methods and helped form the successful mode of thought do not now appear. Those analyses are still on record and, with the ever-expanding use of computing, may possibly find favour again.

 

The success of the Moody diagram lies as much in the physics of a pipe system as in the diagram itself and we must now get an insight to this.

 

éNote that, in America the Darcy expression is used in the form  and we have to be prepared to meet two figures for , one four times the other. This is an unnecessary nuisance.û