
[Sponsors] 
August 13, 2012, 00:01 
mass flow and (rho*A*v) are not matching!!!

#1 
Member
Join Date: Aug 2011
Posts: 53
Rep Power: 6 
Hi all,
I am simulating a flow through an internal passage (quite large). It is a incompressible flow case I have one velocity inlet and 2 pressure outlets The converged results show mass conservation i.e; Massflow@in = massflow@out1+massflow@out2 but when i try the rho*A@in *V@in, it is not the sum of rho*A@out1 *V@out1 and rho*A@out2 *V@out2 pl. do help asap Regards 

August 13, 2012, 03:30 

#2 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,242
Rep Power: 96 
The flow is the integral of the velocity over the area. You are multiplying the area by the average velocity. This is not equivalent to the intergal.


August 13, 2012, 09:42 

#3 
Member
Join Date: Aug 2011
Posts: 53
Rep Power: 6 
Thanks Glenn.
Can u pl. explain little more? (i have checked it with area averaged velocity and mass flow averaged velocity also!!) Regards 

August 13, 2012, 09:51 

#4 
Member
Felipe Gobbi
Join Date: Apr 2012
Location: Brazil
Posts: 76
Rep Power: 6 
Hello,
What Glenn is trying to tell you is that you're doing it wrong by multiplying these variables intead of integrating it. You should do int(rho*V)dA if I'm not wrong. Good luck 

August 13, 2012, 11:20 

#5 
Senior Member
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,212
Rep Power: 22 
I may be wrong here, but isn't Studentraks approach correct for the incompressible (rho=const; can be written before the integral) case?


August 13, 2012, 13:38 

#6 
Member
Felipe Gobbi
Join Date: Apr 2012
Location: Brazil
Posts: 76
Rep Power: 6 
Yes you're right, but maybe the problem is with (v*dA)!
rho doesn't vary with the area, but v does. Not sure if integrating would give the same results as just multiplying the average value... You should be alert if there's no backflow in your domain inlets and outlets, this could mess your results too. 

August 14, 2012, 04:49 

#7 
Member
Max
Join Date: May 2011
Location: old europe
Posts: 84
Rep Power: 7 
Studentrak, what expressions did you use?
If V@in is the areaaveraged velocity (areaAve(Velocity)@IN), than multypling it with the area should yield the same result as the integration of Velocity. The easiest way to calculate the masss flow is probably "massFlow()@IN". If you want to do it by hand, a correct way is "areaAve(Density)@IN*areaInt(Velocity)@IN". 

August 20, 2012, 03:06 

#8 
Member
Join Date: Aug 2011
Posts: 53
Rep Power: 6 
Thanks all
Will try "areaAve(Density)@IN*areaInt(Velocity)@IN" n update in the forum 

August 21, 2012, 21:26 

#9 
Member
Join Date: Aug 2011
Posts: 53
Rep Power: 6 
Hi all
I have mass flow rate(kg/s): @in = 2.4302, @out1 = 1.4818, @out2 = 0.9485 mass flow ave velocity(m/s): @ in~64.9m/s,@out1 = 41.7,@out2 = 36.157 AreaInt Velocity (m^3/s) :@in = 2.0487, @out1 = 1.8601, @out2 = 0.9258 Density is a const= 1.185 (kg/m^3) mass flow rate computed from the velocity or Areaint Velocity does not match with the mass flow rate mentioned Is there a problem? 

August 21, 2012, 22:22 
Hi

#10 
Member
Dynampally Pavitran
Join Date: Mar 2010
Location: India
Posts: 74
Rep Power: 8 
I usually set mass flow rate as my inlet boundary condition for the internal flow simulations (Pipe flow). After going through your discussions, I have cross checked my mass flow rate at the inlet using function calculator. Below are the values which I got:
Boundary condition at Inlet (mass flow rate) : 393.55 kg s^1 Density (Water) : 997 kg m^3 inlet area : 0.122457 m^2 After convergence Conservative value is areaAve(Velocity)@inflow : 3.2229 m s^1 therefore Rho*U*A = 393.48 kg s^1 Hybrid value is areaAve(Velocity)@inflow : 3.22345 m s^1 therefore Rho*U*A = 393.55 kg s^1 ( matches) I believe that Hybrid values should be used when cross checking your inlet conditions. Last edited by pavitran; August 21, 2012 at 23:06. 

August 21, 2012, 22:35 

#11 
Member
Join Date: Aug 2011
Posts: 53
Rep Power: 6 
Hi
i have checked it with hybrid also not matching pl do help 

August 22, 2012, 06:38 

#12 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,242
Rep Power: 96 
Pavitran,
Conservative variables are called conservative as they are the ones which should achieve conservation. So the fact that you have got "conservation" with hybrid variables but not conservative variables shows something is wrong with your analysis. 

August 22, 2012, 07:46 
Hi

#13 
Member
Dynampally Pavitran
Join Date: Mar 2010
Location: India
Posts: 74
Rep Power: 8 
Yes, obviously for checking global conservation we need to use conservative variables. But in my above post I just cross checked my inlet condition with the predicted value of velocity. I usually go through the out file for checking the imbalances.
Last edited by pavitran; August 22, 2012 at 23:53. 

August 23, 2012, 10:23 

#14 
Member
Join Date: Aug 2011
Posts: 53
Rep Power: 6 
Can anyone explain me the problem behind this massflow rate being conserved but rho*A*V or Areaave(Density)*AreaInt(Velocity) not being conserved (looks so)
pl do reply asap Regards Studentdrak 

September 21, 2015, 15:26 

#15 
New Member
Imtiaz
Join Date: Sep 2015
Posts: 5
Rep Power: 2 
I am facing this kind of problems. I need all of your suggestion. Please follow the link.
Mass conservation problem, Finite Element 

Thread Tools  
Display Modes  

