# how to get the value of the flow rate in FLOW-3D

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 November 11, 2013, 06:16 how to obtain the Value of the discharge Q in FLOW-3D #1 Member   ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 3 DEAR experts. i'm simulating a rectangular channel with a weir which is placed in the middle of channel. I want to obtain the Value of the discharge Q.knowing that I made ​​the BC of the output channel as outflow . THANKS mirmahna likes this. Last edited by ounifiras; November 11, 2013 at 14:40.

 November 13, 2013, 13:06 #2 Member   Join Date: Mar 2012 Posts: 71 Rep Power: 5 You'll want to use a baffle defined as a flux surface. This measures quantities that pass through the baffle and reports the results in the Analyze->Probe->General History data

 November 13, 2013, 15:35 #3 Member   ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 3 thank you But I have defined the BAFFLE as a flux surface at the end of the channel and I found negative flow rate values, and more when I open the file fluid volume I was surprised by the presence of the volume value for a very short time, before the flow reaches the BAFFLE, I did not find an explanation. thank you for the explanation

 November 14, 2013, 18:32 #4 Senior Member   Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 8 The negative value refers to the baffle orientation relative to the flow. The baffle has a positive and a negative side. For flow magnitude, you can omit the sign when reporting your results. I do not know why you have volumetric flow across the baffle before the front reaches it. Check your boundary conditions, and check to see if a small depth of flow is advancing before the main wave front. ounifiras likes this.

 November 14, 2013, 19:24 #5 Member   ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 3 thank you greatly for explaining; for the boundary conditions I set Xmin = Pressure; Xmax = Outflow ; Ymin and Ymax = Balanced, for Zmin = pressure with bcz baffle (1) = 1, and Zmax = Balanced, I don 'I did not understand how to change the sign? or change the direction of BAFFLE, is that I can work with absolut values ​​of the value found?

 November 15, 2013, 12:21 #6 New Member   Flowdy Join Date: Nov 2013 Posts: 5 Rep Power: 3 mmmmm I´m not sure if this is what you want but... try Analyse->Probe->Mesh dependent history and in the options of "variable names" you would have Volume flow rate of the fluid at a boundary... you can find that in F1 with the name "Probing Mesh Dependent History to Determine the Flow Rate" cheers ounifiras likes this.

 November 17, 2013, 08:03 #7 Member   ouni firas Join Date: Oct 2013 Posts: 33 Rep Power: 3 thank you , but Is there a difference between flow rate and volume flow rate ? i want to obtain Q(m3/s). in the endd off channel. thank youu mirmahna likes this.

 November 17, 2013, 09:29 #8 New Member   Flowdy Join Date: Nov 2013 Posts: 5 Rep Power: 3 Well I was wondering that also. But following the example of the f1 (How Do I Model...? -> Volume Flow Rate (VFR) Boundary Condition) we can say that yes, this is what you need, it is Q and the units are in [m³/s] If one reads the meanings also, I would say: Flow rate is the amount of something which crosses a surface in a unit of time. But this something could be measure in mass units (kg/s mass flow rate) or in volume units (m³/s volume flow rate). I think that everyone got used to say "flow rate" to "volume flow rate" and this is why we were confused but I think is the same. I would like that someone else confirm what I´m saying (I´m still not sure) ounifiras and mirmahna like this.

 February 11, 2014, 17:42 #9 New Member   Join Date: Sep 2013 Posts: 4 Rep Power: 3 Flowdy You are correct. Volume flow rate and flow rate are the same things.

December 15, 2014, 01:25
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Yi-An Chen
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Quote:
 Originally Posted by dmilano You'll want to use a baffle defined as a flux surface. This measures quantities that pass through the baffle and reports the results in the Analyze->Probe->General History data
DEAR experts:

I set a baffle,and defined it as a flux surface "yes".
But when I save it ,"defined as a flux surface" will change to "NO"

Ican't see the Value of the discharge Q (Volume flow rate) in the Analyze->Probe->General History data

 December 18, 2014, 13:42 #11 New Member   RoyDaniel Join Date: Oct 2014 Location: Taiwan Posts: 9 Rep Power: 2 Dear Ounifiras.. From you previous discussion, If I am not mistake..hopefully.. , you are trying to get the Outflow discharge, aren't you. I think we have the same case, but for me in the upstream, the BC is a inflow hydrograph ( volume rate). and what I have done is just simple one , in the analyze-> probe-> I check the Xmin boundary fluid 1 volume rate & Xmax boundary fluid 1 volume flow rate, and then after I click render, it showed two hydrograph which are for me they make sense, where the Xmax hydrograph is bit lower that Xmin because the effect of attenuation. I hope you also can solve as I do, but please correct me If I am wrong.. My Best Regards, Roy Daniel

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