Hi.

Does anyone know why there are a factor 4 in difference between the hydraulic diameter (d_H) and the hydraulic radius (r_H) ?

d_H will reduce to d for a round tube and that is logical. r_H will reduce to r/2 for a round tube. But where is the logic in that?

Does anyone know why ? What is the story behind this difference?

I noticed this difference and realized that hydraulic diameter is mainly used in close conduits (pipes, annulus, etc.) whereas hydraulic radius is mainly used in open channel flows. In case of pipe considered as an open channel only half of the pipe perimeter enters into the definition of hydraulic radius. This is only my guess.

Angen

Hi.

Well yes, you are correct. But when a open channel is considered as in your example, also only half the areal is entered into the definition.

Yes you are right. It still would be r/2. So, why it is so? Anyone else guess?

Angen

Refer to Fluid Mechanics, by F.M. White, pp. 341-342.

I think its a matter of definition. There is nothing to embaded physical. The hydraulic dia is defined as 4A/P, whereas the other one as A/P. The factor of 4 comes from the definition A=pi (r^2)/4, area of circle.

Adam

sir how to calculate hydraullic dia of the inclined plates in which water is flowing between them Also how to calculate hydraullic diameter in journal bearing in its rotating conditions in which it have some eccentricity. both journal bearing and plates have zero length in direction normal to the paper.

If you need for Re number determination, I`d put there the distance between the plates. As of the bearings, there`re many papers dealing with the modelling of the oil film behaviour; a time ago I bump into some when helping to a friend of mine.

[Jenny]

Any idea about hydraulic diameter and hydraulic radius relationship? is Dh=2Rh?