Hydraulic Diameter Explained: 4.4 vs 4.5

[foo9008]

Homework Statement

why in 4.4 the author gave D =4Rh? in 4.5, D= 2Rh ?

Homework Equations

The Attempt at a Solution

 

[Chestermiller]

In some developments, the hydraulic radius is defined as the area divided by the wetted perimeter. Get used to the idea.

 

[foo9008]

In some developments, the hydraulic radius is defined as the area divided by the wetted perimeter. Get used to the idea.

why in 4.4 the author gave D =4Rh? in 4.5, D= 2Rh ? can you explain?

 

[Chestermiller]

why in 4.4 the author gave D =4Rh? in 4.5, D= 2Rh ? can you explain?

Yes. He is consistent in defining the hydraulic radius as S/Lp, and the equivalent diameter as 4S/Lp. However, in 4.5, he makes the mistake of calling the radius of the cylinder R_H. The term in brackets should not have a subscript H. It should just have an R. And, for a cylinder, R_H=R/2.