Orifice vs Pipe Flow

[MattZ]

I am modeling several detention ponds and am curious to know which approach is better for modeling the outlets for these ponds.

I am using the EXTRAN block and have used orifices for the outlets and actual conduits for the outlets. I get results that are similar to each other, but not close enough.

I would like guidance on which method is the best to use and what the computational differences are between the two methods.

Thanks in advance.

Matt

Matt,

If you call for an orifice, EXTRAN actually substitutes an equivalen "pipe" with a Manning's n determined by a formula to convert from the orifice loss equation to the Manning.s loss equation. I recommend you use a conduit which allows you to set the "minor" loss coefficients. You will want to set the pipe length to take care of the short pipe problem (see p 13-14 of the EPA SWMM Manual).

Ron Kilmartin
ronkilmartin@attbi.com

Matt,

You may use an orifice connected with the actual conduits. This way it may better simulate the real discharge, and seems better to explain. Because there are really two controls here, one is the orifice equation, one is the real conduit. Extran will convert the orifice to an equivalent pipe. With the orifice you can adjust the coefficient according to some manual or experience. Just an idea, you may find it good or bad with your test.

Lianwu
liul@clarkson.edu