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-   -   how to get the value of the flow rate in FLOW-3D (http://www.cfd-online.com/Forums/flow-3d/126214-how-get-value-flow-rate-flow-3d.html)

ounifiras November 11, 2013 06:16

how to obtain the Value of the discharge Q in FLOW-3D
 
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

dmilano November 13, 2013 13:06

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

ounifiras November 13, 2013 15:35

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

JBurnham November 14, 2013 18:32

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 November 14, 2013 19:24

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?

Flowdy November 15, 2013 12:21

mmmmm Im 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 November 17, 2013 08:03

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

Flowdy November 17, 2013 09:29

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 [m/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 (m/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 Im saying (Im still not sure)

spaudel February 11, 2014 17:42

Flowdy You are correct. Volume flow rate and flow rate are the same things.

Yi-An Chen December 15, 2014 01:25

Quote:

Originally Posted by dmilano (Post 461847)
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" :eek:

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

Help me, please!!

hydr_eng1805 December 18, 2014 13:42

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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