PondPack - oriface question 3

[sketchhwyman]

Working in the PondPack outlet structure editor, I want to create a rectangular oriface, for which you select "Area" for your oriface type. However, there seems to be no way to specify the height and width of the oriface, only the area. Two rectangular orifaces with the same area but different H:W ratios could have very different outflow characteristics. Is there a way to specify the dimensions that I am not aware of? Is there a default H:W ratio that PondPack uses in its analysis? Any help would be appreciated.

 

[sam74]

What you are saying about different outflow characteristics does sound logically correct but the orifice equation only for a submerged rectangular orifice has input for only for the area. See link

http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/chap09_05.html

 

[sketchhwyman]

Understood. But the opening will act as a weir up to the point where it is submerged. I would like to think that PondPack would could somehow account for that.

 

[bltseattle]

Can you set up a spreadsheet to compute the flow at each stage, then use a stage-discharge table in PondPack? That way you can be sure that you are accounting for both weir and submerged orifice conditions; check the flow at each stage increment using both flow equations, then use the lowest computed flow as the expected discharge. Hopefully Pondpack, will allow you to enter your own discharge table for atypical control structures.

 

[sketchhwyman]

While I could easily set up a spreadsheet to compute the flow at each stage, that would somewhat defeat the purpose of using PondPack. I'm wondering if there is a way, within PondPack, to address this and get accurate results. I feel like there must be a way to do it and I'm just not familiar enough with the software.

 

[psmart]

Even for a fully-submerged orifice, the head is somewhat different at the top and bottom of the opening, so using the textbook weir equation with a given area is only an approximation, especially when the head is small in relation to the height of the opening. And when the opening is partially submerged, it must be treated as weir flow.

I'm not sure about Pond Pack, but the software really should handle these cases automatically, including the ability to specify the actual dimmensions of the opening. Some programs (such as HydroCAD) will even let you model a custom orifice/weir opening of any shape.

 

[sketchhwyman]

I tried tech support and their response was "no, there is no way to specify the dimensions," but I find that hard to believe. Then the guy told me that I could use the oriface coefficient to accomplish this, i.e. different coefficients correspond to different H:W ratios. That's news to me.

 

[sam74]

If you specify a riser structure you can input both orifice area and weir length. The program computes both orifice and weir flow and defaults to the higher of the two in its calculations.

But this is for a riser type structure with the opening in the horizontal plane. I imagine you are looking to put the rectangular orifice in the vertical plane. This might be the best way to trick the software by just inputing the bottom width of the slot for weir length (but it not designed to work that way and you would want to pay close attention to the orifice/wier calculations output and probably do a hand check to see if it was behaving properly).

Unless you have a lot of time on your hands you are probably better off forming your own rating curve as mentioned above.

 

[KatiLynSki]

I just took the PondPack seminar offered by Haested and this precise question was raised. Unfortunately a colleague borrowed my manual and the so I do not have the precise answer for you but if I remember correctly...

You are going to enter an orifice and a weir structure into the outlet structure system. For each one you are going to click the "Options" button at the bottom of the screen..

For the weir: you are going to have it on as a weir from the bottom elevation of the weir to the bottom elevation of the weir plus 1.2 times the height of the orifice. (I really am unsure about the 1.2 times, I would think it would just be at the top of the orifice but for some reason this is sticking in my head and I don’t have the manual to confirm it but perhaps someone else knows).

For the orifice: you are going to have it on as an orifice from the bottom elevation of the weir plus 1.2 times the height of the orifice (the same number you stopped the weir equation at) and then to the top of the structure.

Hope this all made sense. I hesitated responding because I am unsure but maybe this will point you in the right direction.


-KD in CT

 

[sns777]

I have an older version of pondpack. This is what I do. I input two separate structures (a weir and orifice) and use the "start" and "stop" elevations to specify where the calcuations start and stop for each individual structure. First I input a weir with the the length specified. Then I input the orifice. In other words, at the water rises, the software calculates the outflow for the weir up to a certain head which you specify(I use the midpoint of the orifice), after which the software stops calculating outflow for that structure. Then the software calculates the outflow for the orifice for the higher heads (above the midpoint of the orifice). Then I add the two outflows together to get the total outflow from that structure.

I think this method is somewhat similar to what KD is describing above.