Vortex flow

[Surcharged]

I am examining an existing storm conveyance system that is flooding at a combination curb inlet/grate. My calculations show that the pipes are undersized. However, the owner thinks that the inlet may be undersized since he saw a swirling, vortex there during a storm. My question: is it possible to have a vortex with the pipes flowing full? And is it possible to have a vortex if the pipe is flowing so full that the hyrdrualic grade line is above the inlet. I think that there is a general feeling that a vortex can only occur on structures that are not full of water, therefore the inlet is undersized. Thanks in advance.

 

[beej67]

Could be they're both undersized.

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[dik]

They could... a vortex can form with either condition. It can also form with proper drainage.

Interesting feature... if you tie a couple of small sticks together to form an 'X' and float it on the vortex it will not rotate... it keeps the same orientatin... there is no curl component to the flow.

Dik

 

[beej67]

Technically that depends on whether it's approximating "solid body rotation" or an "irrotational vortex," Dik. It's been a while, but if I recall correctly, vorticity is constant across a region of solid body rotation, and it's infinite at the center of an irrotational vortex, but zero everywhere else in the flow field. A hurricane, for example, has a region that approximates solid body rotation, surrounded by a region that approximates an irrotational vortex. I've got no idea what a curb inlet approximates.

None of that helps Surcharged with his problem though.

My general impression is that vortex action can form in any case, but will tend to be more noticeable without a high HGL in the pipe system. Producing the science to support my impression would be difficult. What I suspect, is that Surcharged's client witnessed a rain event that was larger than the inlet could handle, yet smaller than the event Surcharged modeled in his analysis.

Did you run a HEC-22 analysis on the inlet yet?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com