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October 1, 2004, 04:01 
Area weighted or mass weighted average

#1 
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I wonder wat is difference b/w the area weighted and mass weighted average values of any varaible.
and which one is better to use and in what type of situations Regards 

October 1, 2004, 08:33 
Re: Area weighted or mass weighted average

#2 
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Hi,
For pressure use area weighted averaged value. For rest mass averaged is fine. Thanks Apurva 

October 1, 2004, 10:55 
Re: Area weighted or mass weighted average

#3 
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Won't this depend on the profile through your boundary?
If you want ave vel. you want area ave, if you want ave. temp. you will want Mf ave. 

October 2, 2004, 17:44 
Re: Area weighted or mass weighted average

#4 
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consult. "Computational Fluid Dynamics"  T J Chung. This things are better explained in that. hope this helps


October 4, 2004, 02:55 
Re: Area weighted or mass weighted average

#5 
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For incompressible flow it is physical correct to use mass average for the pressure. For velocities you should use area weighted. You can also do a search in the forum, it has been discussed before.


October 4, 2004, 03:21 
Re: Area weighted or mass weighted average

#6 
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thank i will look into it
but any body else can give me more suggestion , coz my experience says that both normally gives same resutls 

October 4, 2004, 03:33 
Re: Area weighted or mass weighted average

#7 
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Take a look at below paper.
Junseok INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS Int. J. Numer. Meth. Fluids 2003; 41:551560 (DOI: 10.1002/d.455) Interfacial transfer in Tryggvason's method Joel H. Ferziger∗; Department of Mechanical Engineering; Stanford University; Stanford CA; U.S.A. SUMMARY Tryggvason's method computes two phase ows by smoothing the regions near the boundaries between the two uids. This requires averaging of the uid properties, which may dier by orders of magnitude and can lead to large errors. Traditional linear averaging produces a rst order error that can be extremely large. In this paper, we show that for a number of simple test cases, averaging of the inverses of the properties can lead to a signicant improvement in the quality of the results. 

October 4, 2004, 04:31 
Re: Area weighted or mass weighted average

#8 
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Regarding the average methode for pressure, you can find that from the energy equation. It should actually be volume flow averaged, but if the density is constant you can use mass flow average. Only if the velocity is near constant, same mass flow at each position, will result in same value for mass flow and area average.


October 4, 2004, 05:29 
Re: Area weighted or mass weighted average

#9 
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If the flow is reasonably spatially constant w.r.t. the variable you are intersted in AND the velocity then both will be similar.
If not, see above. 

October 4, 2004, 09:33 
Re: Area weighted or mass weighted average

#10 
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in fluent how can we take the inverse (as stated in above mentioned paper )
I will grateful to u for ur help Regards 

October 4, 2004, 09:35 
Re: Area weighted or mass weighted average

#11 
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can u send me papers or any refernce in fluent users guide .


October 4, 2004, 11:42 
Re: Area weighted or mass weighted average

#12 
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I don't use fluent.


October 5, 2004, 02:30 
Re: Area weighted or mass weighted average

#13 
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OR DO U HAVE ANY REFERECNE


October 14, 2004, 07:00 
Re: Area weighted or mass weighted average

#14 
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Dear friend, U use area weighted avg. when ever u r using incompressible flow. The reason is that the density of the fuild is constant from inlet to outlet and hence area weigh. avg. will do. But for incompressible flow the density vary with temp and pressure and hence during that time u should used mass weight avg. All the best. with regards deepesh


October 14, 2004, 11:59 
Re: Area weighted or mass weighted average

#15 
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thanks for ur reply
but i have to give reason for this in my thesis , could u please give some references i will be grateful to u regards 

October 16, 2004, 07:53 
Re: Area weighted or mass weighted average

#16 
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Buddy, If u r using fluent software then u can go to the fluent help file where they clearly give the difference and the reason for the same. By chance if u do not have the same, mail me, i will mail the required document.
all the best, deepesh nanda 

October 16, 2004, 07:54 
Re: Area weighted or mass weighted average

#17 
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please mail me


October 18, 2004, 11:16 
Re: Area weighted or mass weighted average

#18 
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Correction,
I replied for the area/mass weight avg, u use mass weight avg. only when u have compressible flow becoz. density varies with pressure and temperature for such flow. i will soon mail u the document. thanks 

January 31, 2011, 01:40 
area weighted or mass weighted average

#19 
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Amit Chauhan
Join Date: May 2010
Location: Chennai, India
Posts: 18
Rep Power: 15 
I have tried for both area weighted and mass weighted average for my problem. And I am giving incompressible flow, still the averages values are not same which contradicts many of u all posts here above.
Can any one please counter check and tell us what is the reason and with correct explanation. 

January 31, 2011, 05:31 

#20 
Administrator

Areaaveraging and massaveraging will give different results and what you should use depends on what variable you are interested in. For conserved variables like totalpressure you should use massaveraging, whereas for other variables, like staticpressure, you should use areaaveraging. Check out some of the refences mentioned.
To make things even more complex another type of averaging often used is fluxaveraging. With fluxaveraging all gradients are mixed out and a fluxaveraged value is the value you would get if you measure at the outlet of an infinently long frictionfree pipe put at the boundary. In turbomachinery applications fluxaveraging is common, and sometimes you do not fluxaverage everything, you do fluxaveraging circumferentially to mixout wakes, but you do not do fluxaveraging radially because you do not want to mixout the endwall boundary layers. 

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