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02-05-2011
01:30 AM

02-05-2011
01:30 AM

I constructed the attached worksheet for partially full open channel flow in a pipe using the Manning's and Colebrook-White equation, the problem is that the calculation proceeds as would be expected if the pipe diameter is 2 m but if one changes the diameter to be small or bigger than 2 m (change the highlighted yellow value) then the whole calculation gives very unexpected results and in some cases division by zero errors. Can some of the more learned users examine this worksheet and explain to me why this is the case?

Thanking you in advance.

Kind regards, Mark Buckton

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02-05-2011
08:05 AM

02-05-2011
08:05 AM

In your definition of wetted area function P_w(r,d) in the d>r sectionyou had

P_full as 2 pi m - s_arc

should have been

2 pi r - s_arc

Mathcad was taking m as meter, so when r = 1 the equation worked

5 REPLIES 5

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02-05-2011
01:58 AM

02-05-2011
01:58 AM

Re: Partially filled pipe flow

I hope this WebSheets will be useful for your pipe calculation:

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02-05-2011
02:10 AM

02-05-2011
02:10 AM

Re: Partially filled pipe flow

See please (the red formula) how we can work with empirical formulas!

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02-05-2011
04:46 AM

02-05-2011
04:46 AM

Re: Partially filled pipe flow

Valery

Thank you for this insight, I was not aware of this work around for empirical formulas hense why I have given the normally unitless Manning's n units of m^-1/3/sec^-1. I will file your suggestion for later use. However this workaround does not answer the specific problem i.e. why does the answer "blow up" (as seen in the graphs) for pipe diameter values less than or bigger that 2m.

Mark

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02-05-2011
08:05 AM

02-05-2011
08:05 AM

In your definition of wetted area function P_w(r,d) in the d>r sectionyou had

P_full as 2 pi m - s_arc

should have been

2 pi r - s_arc

Mathcad was taking m as meter, so when r = 1 the equation worked

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02-05-2011
04:35 PM

02-05-2011
04:35 PM

Re: Partially filled pipe flow

Thank you f.kohlhepp this change has corrected the problem.

kind regards, Mark