# Flow in partially filled gravity pipe/tunnel

1. Dec 8, 2015

### kcljk009

Hi everybody, I am working on a project to design an inspection system in a gravity tunnel. Can anyone advise me if the flow velocity in a partially filled pipe is the same as the flow velocity in a fully filled pipe?

2. Dec 9, 2015

### Mech_Engineer

Can you define what you mean by gravity tunnel? Are you referring to a hypothetical design for an evacuated tunnel through the earth's mantle to travel between two cities using gravity alone?

https://en.wikipedia.org/wiki/Gravity_train

3. Dec 10, 2015

### gjonesy

Are you referring to the "gradient" on an ordinary pipe? It will vary depending on the fall of the pipe from start to finish. For example you can generally maintain a downwardly gradient over some distance but depending on the topographical features of where you are going to locate the structure you may have 5 degrees fall in one area in others 3 or less which will affect the volume of fluid the system can handle.

4. Dec 10, 2015

### 256bits

What you are looking at is open channel flow, which does have its own pecularities, one of which is the free surface at the top. Friction to flow is along the wetted area, ie the sides and bottom, which as you can now see is the major difference from pipe flow.
Surface waves, hydraulic jumps or drops from changes in gradient are two occurances that one can visually see in particular occurances of channel flow.

The Manning equation is the one equation mostly used for analysis.