5 replies to this topic

[Kiran T]

Dear experts, 

I know this is silly question but i want to clear it, here is my question

 

In Bernoulli's theorem as per sketch A₂ < A_{1 ,} V2 > V1 and P2 < P1 but pressure is equal to Force/area then how is it possible ( As per P=F/A, Reduce area should give higher pressure in our case higher P2).

 

 

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[Francisco Angel]

Dear Kiran T:

Your reasoning is based in the assumption that F is constant (so a smaller A2 will correspond to a greater P2, as P*A = constant).

So you can start by analysing if this assumption is valid or not.  In this particular case, remember that the pressures need to balance the force exerted by the walls over the fluid, and also must be consistent with the velocity change experienced by the fluid.

Best regards.

[Ganesh Buddha]

this is a good question; and is taught with the wrong approach through most of the academic and industrial institutions.

 

for this u need to do a simple exercise. take 2 sheets of paper (A4 size will do) and hold them parallely with both your hands with about 2-3 inchs gap. then blow air between the papers. instead of going apart, the papers will come together....

 

i.e. when you induce velocity between the papers by blowing air between them, the perpendicular pressure of static air is partially diverted parallely. i.e. the pressure energy  is partially diverted by flow of air parallel to the face of the sheets of paper.

 

do this and you can revert back for further questions...

 

happy experimenting!!!

[breizh]

hi,

Hope this helps :

 

http://www.wermac.or...iflowmeter.html

 

Breizh

[samayaraj]

Dear Kiran,

 

F = P x A applies to fluid in static. (i.e. hydraulic actuators etc). Small amount of work from small area cylinder is being displaced in large area cylinder. Here your kinetic energy is converted to potential energy.

 

If you consider venturi, its a dynamic system. Here your pressure energy is converted in to kinetic energy. So here the reduction in pressure increases the velocity and vice versa. Ganesh had said a very good example for this. The same examples applies to aeroplane wing and how it flies. To support you more, look at the attached link.

 

Edited by samayaraj, 10 February 2016 - 06:49 AM.

[S.AHMAD]

Hi Kiran T

You should be doing a total energy balance instead of partial energy balance.

You can eliminate other forms of energy except for pressure and kinetic energy:

I do not show the methematical energy equation since it is too easy you can figure-out by yourselves.

Edited by S.AHMAD, 11 February 2016 - 08:47 AM.