×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

plug flow versus center of mass

plug flow versus center of mass

plug flow versus center of mass

(OP)
In my state, the regulators typically look at the COM detention time to determine if their criteria are being met.  In the model I'm working on now, the plug flow time is nearly double the COM time.  Reviewing HydroCAD's website entry on the difference between the two, I believe that using COM results in an overly conservative design.

In my model, it says "66% of inflow" next to the plug flow time.  I've read the help file, and understand that this is the volume included in the calculation.  Why wasn't the whole volume included?  What is the practical effect on the accuracy of the plug flow number?  Does this make the calculation more conservative, or is it not counting part of my flow?

RE: plug flow versus center of mass

"66% of inflow" can be caused by several things:

1) Some of the inflow is permanently retained in the pond
2) The time span isn't long enough to include the entire discharge
3) You have a permanent pool which must be displaced before the plugs can exit.

Having a permanent pool will increase the plug-flow time, but has no effect on the COM method.  Otherwise the two method will usually produce similar results.

For details see www.hydrocad.net/detentiontime.htm

If you have further questions please post your file or send it to HydroCAD support.

Peter Smart
HydroCAD Software
www.hydrocad.net
 

RE: plug flow versus center of mass

(OP)
This is for a wet pond, so there will be some inflow held in the basin.  I take it that means that 34% of my flow becomes the replaced volume of my permanent pool?

RE: plug flow versus center of mass

Correct.  Since the plug flow calculation is based on a first-in first-out scenario, the existing volume must be displaced before any of the new inflow can exit the pond.  This means that the existing volume will increase the plug flow detention time, but it will not alter the center-of-mass detention time which doesn't take the constant pool into account.
 

Peter Smart
HydroCAD Software
www.hydrocad.net
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources