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

Hi,

For pressure use area weighted averaged value. For rest mass averaged is fine.

Thanks

Apurva

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.

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

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.

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

Take a look at below paper.

Junseok

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS Int. J. Numer. Meth. Fluids 2003; 41:551–560 (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.

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.

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.

in fluent how can we take the inverse (as stated in above mentioned paper )

I will grateful to u for ur help

Regards

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

I don't use fluent.

OR DO U HAVE ANY REFERECNE

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

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

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

please mail me

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

[chauhan]

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.

[jola]

Area-averaging and mass-averaging will give different results and what you should use depends on what variable you are interested in. For conserved variables like total-pressure you should use mass-averaging, whereas for other variables, like static-pressure, you should use area-averaging. Check out some of the refences mentioned.

To make things even more complex another type of averaging often used is flux-averaging. With flux-averaging all gradients are mixed out and a flux-averaged value is the value you would get if you measure at the outlet of an infinently long friction-free pipe put at the boundary. In turbomachinery applications flux-averaging is common, and sometimes you do not flux-average everything, you do flux-averaging circumferentially to mix-out wakes, but you do not do flux-averaging radially because you do not want to mix-out the end-wall boundary layers.