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andrew700andrew

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Hi, I'm trying to understand vortex shedding and how the Karman vortex street occurs when air flows around a cylindrical object, so far it's going OK but then I came across this part of the explanation which leaves me confused:

"Looking at the figure above, the formation of the separation occurs as the fluid accelerates from the centre to get round the cylinder

[Taken from here http://www.efm.leeds.ac.uk/CIVE/CIVE1400/Section4/boundary_layer.htm]

What I'm unsure about is why the air must accelerate to travel around the cylinder, which in turn creates a low pressure on top of the cylinder. I've tried to figure it out using the Continuity Equation p1*A1*v1 = p2*A2*v2 (p=density or pressure, A=area, v=velocity) and Bernoulli's equation but I come to a problem because assuming that flow Area decreases as you approach the middle of the cylinder, either p (i.e. pressure) or v could increase to maintain the equal relationship. If it's true that the pressure could increase (given this is air) then that would contradict the Bernoulli equation which suggests that the pressure should decrease as velocity increases. Bearing in mind that I haven't started University yet is there some way you could explain this to me?

Also, how can the Bernoulli's equation apply to this when air is compressible?

Thanks allot.

"Looking at the figure above, the formation of the separation occurs as the fluid accelerates from the centre to get round the cylinder

**. It reaches a maximum at Y, where it also has also dropped in pressure. The adverse pressure gradient between here and the downstream side of the cylinder will cause the boundary layer separation if the flow is fast enough, (Re > 2.)"**__(it must accelerate as it has further to go than the surrounding fluid)__[Taken from here http://www.efm.leeds.ac.uk/CIVE/CIVE1400/Section4/boundary_layer.htm]

What I'm unsure about is why the air must accelerate to travel around the cylinder, which in turn creates a low pressure on top of the cylinder. I've tried to figure it out using the Continuity Equation p1*A1*v1 = p2*A2*v2 (p=density or pressure, A=area, v=velocity) and Bernoulli's equation but I come to a problem because assuming that flow Area decreases as you approach the middle of the cylinder, either p (i.e. pressure) or v could increase to maintain the equal relationship. If it's true that the pressure could increase (given this is air) then that would contradict the Bernoulli equation which suggests that the pressure should decrease as velocity increases. Bearing in mind that I haven't started University yet is there some way you could explain this to me?

Also, how can the Bernoulli's equation apply to this when air is compressible?

Thanks allot.

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