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How to solve this pipe flow problem

SOLVED

Re: How to solve this pipe flow problem

I haven't checked all dependencies but I see that at least h.in is dependent on V1 and I guess other variables used in the calculation ov V1 at the and, too. So its no surprise you don't get P1=0 by simply changing the value of V1 at the top of the sheet.

Only way out I can see is turning all calculations which depend on V1 and Pc into functions of those variables and use those functions in both solve blocks instead of the variables.

Re: How to solve this pipe flow problem

Thank you for your suggestion. I tried it (I hope in a correct way) but at no avail. Would you be so kind to have another look?

Re: How to solve this pipe flow problem

FDS wrote:

Thank you for your suggestion. I tried it (I hope in a correct way) but at no avail. Would you be so kind to have another look?

Why do you think it doesn't work?

The last solve block yield Vel=304.718 cm/s and if I set V.1 and P.c to their appropriate values before the first solve block, I get the solution with P1=0. Could it be that you forgot to set P.c to -70 mmHg and so the solveblock used P.c=0?

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Re: How to solve this pipe flow problem

Well, when I use Pc = -70mmHg and Q = 5.99 L/min (put in purple background now) then I obtain a P1 of -0.17 mmHg. When I use P1 = 0 and Pc = -70 mmHg in the last solve block I obtain 5.903 L/min. However putting this value in for the flow in the first part with Pc -70 mmHg will generate a P1 of -3.06 mmHg. Also when I put in different values of Pc in the last solve block, for example -40 mmHg I get a V1 of 85 cm/s higher than with a Pc of -70 mmHg so there must be something wrong I presume.

Re: How to solve this pipe flow problem

Not sure but I think you have setup a different situation here because you change Q and not just V1 as before!

All velocities are dependend on Q so all have to be functions in Q, otherwise it can't work of course. Depending on your need it may also be usefull to express V2 to V4 not as functions in Q but as functions in V.1

Some of the discrepancy might also be because of rounding errors. You use just three decimals of the result. It may be a better idea to put the last solve block which yields V1 before the first one which calculates P1 to P4, so you can use the exact result without the need for manual copy and paste.

Re: How to solve this pipe flow problem

Upon rethinkig the whole thing I guess its best to express all as functions in Q.

See attached.

Re: How to solve this pipe flow problem

Werner, I admire the way you support this forum. This is really helpful. I will look at it more closely tomorrow but at first sight this seems to solve all previous problems. Thanks a lot.

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