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Old   September 16, 2009, 08:23
Default Outflow drag
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Lukasz
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Hi all,
I have a simple question that is maybe more related to physics not CFD itself. The simulated arrangement is very simple: Outflow of the water to air through the pipe (it is a kind of valve), the outlet of the pipe is approximately 5 mm. The velocity of the water is approximately 100m/s. Physical testing shows, that the outflow with this velocity produces spray or mist rather than water flow.
My question is: what is a most appropiate outlet boundary condition (simlating only a pipe, not a pipe outlet and some air volume)?. Setting "Opening" with relative pressure 0 overpredicts mass flow. I guess that there is some kind of aerodynamic drag of such a spray but looking through literature I cannot find anything seriously usefull. Can anyone tell anything about the "drag" of the waterflow into a gas?

Thanks a lot,
Luk
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Old   September 16, 2009, 18:59
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Glenn Horrocks
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It might also be because the 0 pressure at the outlet assumption is not correct. Try moving the domain boundary beyond the outlet a bit and try again - it means you may have to model it as two phase flow also.
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Old   September 17, 2009, 04:04
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Hi,
Yes - it is my opinion that 0 pressure is wrong. However I wanted to omit simulation of 2 phases because spray started by the outflow takes a few cubic meters of the air volume. Because of that I am studing papers of "spray formation", "nozzle exit loss" etc. but so far I didnt find anything usable.

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Old   September 17, 2009, 07:33
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Glenn Horrocks
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I suspect the pressure profile in the outlet pipe is not simple so the only way of being accurate is to include it in the model. You probably won't need to model the entire spray formation, just get far enough away from the outlet so the pressure profile in the outlet is correct. You may well be able to have most of the flow still in a jet as it leaves the domain.
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