For example, when air is blown through a spoon in the atmosphere, the air flow bends toward the spoon. People say it's the Coandă effect. So, outside the atmosphere, when we let air blow through the spoon, does the air flow also bend towards the spoon? I don't think it will, because atmospheric pressure bends the air toward the spoon.

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You should not expect a Coandă effect without an atmosphere.

The Coandă effect comes from entrainment, which requires a pressurized fluid surrounding the jet to be entrained. This creates a low pressure zone around the jet, which then generates a net force towards any nearby surfaces (if there are any, if not the jet just generates entrainment without the Coandă effect). This means that there must be some sort of pressurized environment for this effect, or else you could not generate the low pressure zone (because you can't go lower than "none").

That said, I'm not sure if this tells the full story. Although without an atmosphere of some sort, you will not get the Coandă effect, you may or may not still get curvature of the path due to viscous effects.

In fluid mechanics, the no-slip condition is a common assumption, which should cause a slight redirection of the flow to follow the curved surface, as the layers closest to the surface would drag the other layers along with it, curving the path somewhat. The thing to consider here is that in low pressure environments, the no slip condition does not hold (see exceptions here).

In this case, I think it would depend significantly on the jet speed, the pressure of the jet, and the distance from the curved surface. If the jet were close enough to the surface, and high enough pressure, it should slightly follow the curvature of the surface as the jet travels forward for some distance. At some distance away from the jet, the jet will have dispersed significantly into the vacuum, and pressure will be too low for the no-slip condition to hold, and at that point it will skip from the surface more, and thus not follow the curvature as well.

So basically, a high pressure jet close to a curved surface should still experience some curvature outside of an atmosphere, but it will not be nearly as significant as when it is in an atmosphere, as the Coandă effect needs an atmosphere, and it induces much more curvature than viscosity alone.


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