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-   -   Outflow results do not tally w/ hand-calculation? (https://www.cfd-online.com/Forums/fluent/30055-outflow-results-do-not-tally-w-hand-calculation.html)

 Julie July 19, 2002 05:08

Outflow results do not tally w/ hand-calculation?

Hi all,

In my model,there are two square outlets at one end of the room each of size 0.6*0.6m And another smaller sqaure outlet at one end of size 0.4*0.4m

ANd a group of four rectangular inlets in the middle of the room each of size 0.2*1m

I am given the values in CMH(cubic meters per hour) of air entering and leaving the room THe four inlets each having 450CMH of air entering, hence a total of 1800CMH entering, The two larger outlets has each 850CMH of air leaving, and the smaller outlet 100CMH air leaving

Hence, the CMH in and out of the room thus balanced.

A quick calculation gives the velocity of air entering at 0.625m/s each and the velocity of air leaving at the two oulets each of 0.655m/s

For the inlets, I specify them as Velocity_inlets and put in value of 0.625m/s in Fluent And for the outlets, I specify them as Outflows, where I put in 0.472 weightage at each of the two larger Outlets and 0.056 at the smaller oulets.

However after iterations, and with the solution converged, I did a display vector of the velocity magnitues. I realised that the velocity at the inlets are only 0.578 m/s each and not the 0.625m/s which I have inputed. And the velocity of at each of the larger outlets is only 0.347m/s and not the 0.655m/s that I have calculated.

Then I did a report/surface integrals/mass flow rate The values computed are in accordance to the my calculations.

I wonder what went wrong, the outlets are supposed to have a higher flow rate 850CMH, and when divided by (60*60) to give CM per sec, and divided by its cross-sectional area should give a value of 0.655m/s as velocity

But the vectors results does not show this. Why?

 Laika July 19, 2002 10:27

Re: Outflow results do not tally w/ hand-calculati

Do you use compressibility? Ask for a report of the average velocity in each inlet and outlet.

Laika, still orbiting

 Julie July 20, 2002 09:48

Re: Outflow results do not tally w/ hand-calculati

I am not sure of the values of pressure. And I guess the closest thing I could ask for is Mass Flow Rate in Fluent. ANd using Velocity=mass flow rate/(density)(cross-section area), I could calculate the velocities

The mass flow rate I got from Fluent after using Report/surface integral or Report/Flux tallies with my calculations

The problem lies in the values of velocities. After getting the value of mass flow rate at each outlet and inlet from Fluent, I applied Velocity=mass flow rate/(density)(cross-section area), but the velocity calculated is different from the values displayed in Display/Plot/vectors/velocity magnitude in Fluent!

I really do not know what went wrong So I decided not to use OUTFLOW but Velocity_Inlet at even the outlets but with the direction of the flow in a negative direction and the magnitude of velocity inputed by me, 0.655m/s

To my surprise, after iteration, I still got 0.347m/s at the outlets! but the mass flow rate calculted by Fluent is still Correct! So now, I'm really stuck...

 Ashu July 22, 2002 01:05

Re: Outflow results do not tally w/ hand-calculati

Let Fluent compute how much air should go through each outlet, not you! See the mass gets balanced after your interation. Mass flow rate you can see by report-> fluxes. See the average velocity at inlet and outlet velocity by report -> surface integral -> facet average value of velocity in x, y or z directions. Hope this might work well.

 Julie July 22, 2002 06:06

Re: Outflow results do not tally,grid-refinement?

HI Ashu,

thanks for replying. Like I said, the mass DID get balanced after iteration. I saw the results under report/fluxes and I was happy with that.

But since v=mass flow rate/(density of air)(cross section area of the inlet/outlet)

with the mass flux obtained at the inlets or outlets, using the above equation, I should get the velocity of air at the inlets/outlets. However this value does not tally with the one shown in display/vectors/velocity magnitude/ nor does it tally with the one report -> surface integral -> facet average value of velocity in x, y or z

even as I have already indicated(keyed in/typed in) under the VELOCITY_INLET panel, that the velocity of air is -0.625m/s at the inlets, after iteration, the velocity vector I get at each inlet is -0.578m/s!

ACtually, recently, it just occurred to me that perhaps, the size of the elements used at the inlets and outlets are perhaps too big thus resulted in the inaccuarcy?

 Ashu July 24, 2002 08:45

Re: Outflow results do not tally,grid-refinement?

Did your result vary after refining the mesh? I think with little tweeking of model you would be able to match the results.

Bye Ashu

 Julie July 27, 2002 00:02

Re: Outflow results do not tally,grid-refinement?

Yes it did, when I refine the mesh from the 0.2 intervel size to 0.1 interval size, I saw there was an improvemnet from 0.55m/s to 0.59m/s but still not enough, my actual input of the magnitude of velocity is 0.625 in the y direction.

I don't think I could refine it further, the file gets too big.

Why is it that Fluent do not read my inputs at the velocity_inlets? How come, when I specify 0.625m/s at the inlets, I got 0.59m/s from the report?

How does Fluent do the computation? It got the mass flow rate right.

Does Fluent work in a way that is : from my velocity inputs, i does a mathematical calculations of the mass flow rate then using mass flow rate, does a computation of what is going on inside my model using mass balance and ended up a different velocity at the inlets (from what I have keyed in)?

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