B Why do the balloons approach each other?

  • Thread starter jaumzaum
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I was solving a question from an entrance exam that said the following:
Consider 2 balloons very close to each other. A person blows air in the place at the middle of the 2 balloons. What happens with them?

The answer is that they approach each other, but why? For me, the blow will increase the pressure in the middle of the balloons, that will be greater than the one in the other side, creating a net force that will distance the balloons. What is wrong with this thinking? I tried to use Bernoulli on this one but as I was told, Bernoulli only can be used in the same current line, and I got no response.

Thank you very much!
Joao
 

rcgldr

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The air tends to follow a convex surface if the speed of the flow and radius of curvature are not excessive. This means the flow is separated on the downwind side of the two balloons, which reduces the pressure. This reduction in pressure affects the flow upwind of the separation. This effect, combined with Bernoulli effect, draws the balloons together.
 

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