Gravity flow 2

[Durk]

Hello,

I have a question regarding a gravity flow pipe scenario. Water flows through a wide-ish channel at its end it is collected by a 798mm (ID) pipe. I have used the attached Cole-brook White formula to calculate the velocity through the pipe at full flow. I am getting a velocity of about 3.5m/s and a flowrate of 1500l/s with 1% slope(seems high). I want to calculate the level in the pipe for my actual flow of 842l/s. I think i have to swap the diameter terms to the new wetted area e.g. 90% area, 80% area etc to find my 842l/s but i am unsure that its is as simple as this...

Does anyone know how to manipulate the formula to give me what i am looking for?

http://files.engineering.com/getfil...b-43441a787a86&file=Colebrook-brook_White.doc

 

[katmar]

Download

http://www.repwars.com/cispi_ch8.pdf

for the theory and worked tables for water flow.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[bimr]

Here is a free calculator that you can modify to your problem for demonstration purposes:

http://www.wq.uiuc.edu/dg/Equations/Mannings.exe

 

[Durk]

Thank you both for your posts, while not the cole-brook eq, i find this very helpful as a reference.

 

[BRIS]

To use the Colbrook White equation fopr part full flow replace D by 4R. where R is the hydraulic radius = Area / Wetted Perimeter.

For a full pipe 4 x R = 4 x PI X D^2/ (4 x PI X D) = D

It is a lot easier to use Mannings equation !!

 

[BRIS]

http://www.ascinfo.co.uk/9/resourcefile/24/06/54/4of4.pdf

Figure 3.4.1 of the above gives a typical chart for partial full flow.

 

[Durk]

Thanks Bris,

I have previously viewed the above datasheets.Manning's is indeed alot easier especially with Bimr's calculator! But for the purposes of my piece of mind, i would like to continue to look at a method of solving this by colebrook white formula. I have attached my method used, from this you can see that i have incoporated a capacity value into the equation, it just isn't working as i had hoped, i always get a value of around 100% flow even if i sub in the Rh value for the D value in the equation. I was hoping somebody could have a look and maybe point out some obvious mistake in methology.

 

[BRIS]

As noted in my reply above you need to replace D by (4 * R) for all terms in the Colbrook White Equation

 

[BRIS]

V = -2 (2g 4R S) ^0.5 * Log [ks/(3.71. 4R) + 2.51 v/ (4R (2g 4R S)^0.5))]

 

[stanier]

Google Flowpro2 and download this free software for free surface flow.