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Flow recirculation around a ducted fan

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Old   March 29, 2004, 16:43
Default Flow recirculation around a ducted fan
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Jane
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I have a short duct with a small fan, the flow is sucted from atmosphere in the inlet by the fan, the pressure in the upstream of the fan is lower than atmospheric pressure by Bernoulli. Then the fluids experience a static pressure increase because of the work added by the fan, then eject out to atmospheric. Since the duct is very short, the flow will from a recirculation: Flow ejecting to exit recirculates to inlet. Does anybody have experience on this kind of flow? How is the static pressure distribution in the recirculation area? Suppose if I wanna increase the recirculation area, what should I do? Many thanks.

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Old   March 29, 2004, 20:40
Default Re: Flow recirculation around a ducted fan
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Ananda Himansu
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The static pressure in the vicinity of the duct exit will be higher than atmospheric, while that in the vicinity of the inlet will be lower than atmospheric. The static pressure in the recirculation route outside the duct will vary continuously from the high pressure near duct exit to the low pressure near duct inlet. It is in fact this pressure gradient that causes the air to recirculate. You can almost visualize the whole thing as a closed duct loop, with a pump in it (the fan).

If you want to increase the recirculation area, you can (a) increase the fan and duct diameter, (b) increase the fan speed, (c) improve the duct efficiency (by smoothly flaring the ends of the duct) and thus the air flow rate, (d) increase the duct length, or (e) any combination of the previous four possibilities. Strategy (a) will increase the cross-sectional area of recirculation without changing the speed of the recirculating air. Strategies (b) and (c) will increase both the speed and cross-sectional area of recirculation (increasing the speed should increase the entrainment). Strategy (d) will increase the cross-sectional area of recirculation but at the same time decrease the speed of the air in the recirculation path.

Separating acoustics from general nonlinear velocity variations is a controversial subject. Sound consists of waves that propagate at sonic velocity relative to the medium. However, nonlinear waves propagating faster or slower than sound can eventually decay to sound or give rise to sound, through complex ill-understood mechanisms. The generation mechanisms of self-noise of a fluid are as yet imperfectly understood. It is only in the farfield where purely linear acoustic waves exist possibly superimposed on a uniform flow, that you can compute the sound pressure level unambiguously. Of course linearity is an idealized limit.
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Old   January 24, 2012, 06:17
Default UDF for pressure variation
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Nirav
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Hii friends
My problem is flow through river channel, I have taken a orbitary region so at exit of my geometry flow exits into river itself. so i need to know how to apply UDf at exit, as pressure must vary with P=row*g*h. my exit cross section is not uniform so what should i do please help. is it possible to apply custom field function ???? in fluent please help ASAP
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