SWMM-USERS Digest - 28 Dec 2009 to 2 Jan 2010 (#2010-1)
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Topics of the day:
1. Flow Instability in pipes.
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Date: Sat, 2 Jan 2010 20:13:30 +0100
From: Maurizio Cingi <mrz...@GMAIL.COM>
Subject: Re: Flow Instability in pipes.
Sheila,
if you post your INP file (or a reduced version if too big) I can try to
understand ...
Your points (1-4) are correct (and 0.005 % seems a quite small continuity
error, even if you mean 0.005 => 0.5% ).
About the 5 ft (less than 2 m correct?) diameter storage it is too small to
have any effect (but it depends on pipes diameter).
You must also look at very steep conduits.
Last but not least, are you shure that it is numerical instability and not
hydraulic transient (due to sudden valve closure or opening, inflow
variation etc.)
I can't say more without seeing the model.
Ciao
Maurizio Cingi
2009/12/22 Sheila Reeves <sre...@rkk.com>
> From: Sheila Reeves [mailto:sre...@rkk.com]
> Sent: Tuesday, December 22, 2009 9:21 AM
> To: 'SWMM-USERS'
> Subject: Flow Instability in pipes.
>
>
>
> I am having some issues with flow instability in some pipes of particular
> interest to my study. How can I go about resolving flow instabilities
> within pipes? The overall model continuity error is -0.005. The routing
> timestep is set to 1 sec. I was reading in the Users Guide to PCSWMM on pg
> 190 and it was saying that numerical instabilities under Dynamic Wave flow
> routing can be reduced by
>
> 1) Reducing the routing time step - this has already been done in my
> model as it is down to 1 sec
>
> 2) Utilizing the variable time step option with a smaller time step
> factor - I am using this tool but not sure how sensitive this is. I have
> tried it at 100% and 75% and get similar results
>
> 3) Selecting to ignore the inertial terms of the momentum equation -
> have not done this as I don't understand the ramifications of doing this.
> I
> currently have them dampened. I tried this but it did not help the
> instability.
>
> 4) Selecting the option to lengthen short conduits -I have conduit
> lengthening timestep of 2 sec but have also tried it at 4, 6, and 10 but it
> did not help the instability.I am playing with the conduit lengthening tool
> to help make my model more stable. The manual says to use 0 for no
> lengthening, but it does not specify a typical or max value. I would like
> to know what values are considered low, moderate, or high to get a sense
> for
> how sensitive the tool is.
>
>
>
> OI have also tried making the nodes in the area of this instability a
> storage node that is 5 ft in diameter to simulate a manhole. but this
> didn't
> help either.
>
>
>
> AAny suggestions would be very much welcomed!
>
> Sheila
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End of SWMM-USERS Digest - 28 Dec 2009 to 2 Jan 2010 (#2010-1)
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