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My problem is practical - I have a boat that has a cabinet in it which has a persistent epoxy smell from recent work, causing all clothing stored in the cabinet to also smell of epoxy. So it needs to be ventilated. Inspired by carburetors, I was thinking of running a tube from inside this cabinet out the window of the boat, where there is typically wind - in the hopes that this wind would suck air out of the tube the same way fuel gets sucked up through the jet of a carburetor, thereby ventilating the cabinet [Editor's note, the cabinet is NOT airtight].

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The situation, with arrows indicating (desired) speed and direction of airflow.

Then thinking about it more, I realized this requires that in some cases (if wind is blowing from left-to-right, as in diagram) the wind would actually have to be travelling in a loop - which seems somehow unnatural, so I'm not sure if it would actually work. It might actually cause air to flow from the cabinet into the cabin and make the whole cabin smell of epoxy.

Can anyone who understands something about fluid dynamics comment on whether this scheme makes any sense? Does it only work when air flows right-to-left? In general, in a passive system like this with no moving parts, is it possible to drive fluid in a loop?

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  • $\begingroup$ How does air leak from the cabin to the cabinet? Are there gaps? $\endgroup$ – Alex Trounev Mar 26 at 17:07
  • $\begingroup$ The air outside will generally like to be pulled out by the faster moving, lower pressure air outside, like to how chimney's work and how things get sucked out of your car when you open the window. The problem is that the air in the cabin has to get replaced from somewhere, which will happen from some complicated combination of either or both of these openings and any other openings in the boat. In short, you will most likely get sufficient airflow depending mostly on the diameter of tube and size of vents in the cabinet, but the air may not flow in the right direction. $\endgroup$ – Sigma Mar 26 at 17:44
  • $\begingroup$ @Alex. Yes, the cabinet is not airtight. $\endgroup$ – Peter Mar 26 at 18:54
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It is quite possible to drive fluid in a loop (in fact a loop or circulation component is essential to the lifting airfoil).

However your proposed solution depends on the relative pressures at the venturi and the cabin door. If the pressure at the door is lower then the circulation will go the wrong way. It will depend very much on wind speed and direction.

My suggestion would be to put up two or three ventilator tubes and turn their tops to point sideways in different directions, and down a bit (but not vertically) as well. This would have two benefits:

  • Differential pressure in most any direction will drive circulation without impacting the cabin.
  • Rain and spray would not soak your clothing so much.

If you might head for green water, do include caps to close the vents off!

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  • $\begingroup$ Thinking about it more - I suspect it is impossible without some kind of dynamics in the flow of the wind (ie a vector field that changes with time) - regardless of which way the tubes face. Air follows a pressure gradient, and you can't follow a gradient in a loop. For the airfoil, I looked it up and see what you mean, but i think that the loop around an airfoil is only possible in 3d - the 2D-projected components of the vector field are forming a loop, but the depth component is moving towards the end of the swept-back wing - so no air is actually moving in a loop. Not sure though. $\endgroup$ – Peter Mar 27 at 4:20
  • $\begingroup$ Seems to me that whenever the wind is flowing left-to-right, then pressure must be lower at the cabin door, no? If it weren't, the wind wouldn't be blowing that way. $\endgroup$ – Peter Mar 27 at 5:17
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    $\begingroup$ It is the relation of the vector field to the mouth of the tube which matters, you can change either. The circulation theory of lift is a 2D theory. By your pressure gradient argument there would never be local eddies in fluid flows and airfoils would not work. The Navier-Stokes equations are not that simple! Bear in mind that wind directions can change but your clothes will still be in that locker. $\endgroup$ – Guy Inchbald Mar 27 at 9:21
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    $\begingroup$ Yes there is a circulatory component to the airflow over a wing; lift is impossible without it. If I may say so, you seem not to know enough aerodynamics to pass sensible comment. Pressure-gradient theory is one thing, your argument purporting to invoke it is quite another. But can you explain why you say that an eddy is not a loop? $\endgroup$ – Guy Inchbald Mar 27 at 14:27
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    $\begingroup$ You said that pressure-gradient theory does not hold with eddies but is necessary for loops. This implies that an eddy is not a loop. But now you deny that is what you meant. Whatever you do mean must remain a mystery to me. $\endgroup$ – Guy Inchbald Mar 27 at 15:12
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Having thought about it, I'm pretty sure the answer is:

No, this scheme is impossible (when the wind is flowing left-to-right) if the airflow does not change with time. We can imagine associating an air-pressure to every pixel in the diagram, creating a "pressure-field". The flow (arrows) point in the direction of the gradient of this pressure-field. Gradients point from high-to-low pressure. It is impossible to follow gradients in a loop, and thus there can be no loops in your flow field.

However, it is possible when the flow oscillates with time. In this case, air could be pushed along a high pressure wave that takes it from an on-average-low to an on-average-high pressure, like how you could push a ball up a slightly sloped mattress by always pushing down on the bit of the mattress slightly uphill from it. One way to have such an oscillating airflow would be to put one of these on the top of the tube:

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  • $\begingroup$ Two things that you're ignoring in this answer are gravity and compressibility. If you were to light a candle in the cabinet, the hot air would rise out of the tube, cool outside, and sink back down. The airflow could be completely static and still go in a loop. This is what keeps hot air flowing up a chimney, being replaced by cool air getting coming in through the edges of doors/windows. So I guess the above conclusion is only true for incompressible fluids? $\endgroup$ – Peter Mar 27 at 5:10

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