Pressure At A Point In A Fluid

Question

I have a doubt regarding pressure in a fluid. Going through a few examples and questions on Bernoulli's equation, one thing which is common is to choose a suitable point where we know the pressure, and maybe the velocity and then calculate the unknowns.

For example in the velocity of efflux we take the pressure on the fluid moving out to be the atmospheric pressure. Similarly, in a question involving fluid flowing vertically downwards through a pipe from the bottom of a tank takes the pressure to be the atmospheric pressure at the opening (lower end, from where the fluid flows out).

Now this is where I am getting confused. What about the pressure of the rest of the fluid above the opening? Why don't we consider that pressure? Why is that the fluid flowing out experiences only the atmospheric pressure and not the pressure of the fluid above it?

Answer 1

Inside the tank itself, the fluid flow streamlines converge toward the exit opening. This takes place within a small spatial region in close proximity to the exit opening, on the order of just a few orifice diameters from the exit. As the fluid flow converges toward the opening, its flow velocity increases rapidly, and, by Bernoulli, its pressure correspondingly decreases rapidly, from the pressure determined by that of the fluid above to the exit (atmospheric) pressure. When you learn about the Bernoulli equation, they don't usually tell you about this part of the flow.