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-   -   mass flow and (rho*A*v) are not matching!!! (https://www.cfd-online.com/Forums/cfx/105906-mass-flow-rho-v-not-matching.html)

 Studentdrak August 13, 2012 00:01

mass flow and (rho*A*v) are not matching!!!

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

 ghorrocks August 13, 2012 03:30

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.

 Studentdrak August 13, 2012 09:42

Thanks Glenn.

Can u pl. explain little more?

(i have checked it with area averaged velocity and mass flow averaged velocity also!!)

Regards

 Felggv August 13, 2012 09:51

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

 flotus1 August 13, 2012 11:20

I may be wrong here, but isn't Studentraks approach correct for the incompressible (rho=const; can be written before the integral) case?

 Felggv August 13, 2012 13:38

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.

 murx August 14, 2012 04:49

Studentrak, what expressions did you use?

If V@in is the area-averaged 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".

 Studentdrak August 20, 2012 03:06

Thanks all

Will try "areaAve(Density)@IN*areaInt(Velocity)@IN" n update in the forum

 Studentdrak August 21, 2012 21:26

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?

 pavitran August 21, 2012 22:22

Hi

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.

 Studentdrak August 21, 2012 22:35

Hi
i have checked it with hybrid also
not matching :(
pl do help

 ghorrocks August 22, 2012 06:38

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.

 pavitran August 22, 2012 07:46

Hi

Quote:
 Originally Posted by ghorrocks (Post 378140) 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.
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.

 Studentdrak August 23, 2012 10:23

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)