Good evening, kind forum users.
I'm trying to understand the microscopic mechanisms that justify the Bernoulli principle. I found an interesting discussion on the site of NASA that tries to describe, in a general way, the mechanism underlying the pressure reduction in favor of the increase in speed. I propose the text in question, and then I explain to you my doubt.
"We can make another interpretation of the Bernoulli equation by considering the motion of the gas molecules. The molecules within a fluid are in constant random motion and collide with each other and with the walls of an object in the fluid. The motion of the molecules gives the molecules a linear momentum and the fluid pressure is a measure of this momentum. If a gas is at rest, all of the motion of the molecules is random and the pressure that we detect is the total pressure of the gas. If the gas is set in motion or flows, some of the random components of velocity are changed in favor of the directed motion. We call the directed motion "ordered," as opposed to the disordered random motion. We can associate a "pressure" with the momentum of the ordered motion of the gas. We call this pressure the dynamic pressure. The remaining random motion of the molecules still produces a pressure called the static pressure. From a conservation of energy and momentum, the static pressure plus the dynamic pressure is equal to the original total pressure in a flow (assuming we do not add or subtract energy in the flow). The form of the dynamic pressure is the density times the square of the velocity divided by two."
[Text from: https://www.grc.nasa.gov/www/k-12/airplane/bern.html ]
From the text we learn that the pressure reduction with speed can be explained by thinking that part of the "chaotic" kinetic energy translates into "ordered" kinetic energy. The remaining chaotic kinetic energy gives rise to static pressure, while the ordered kinetic energy is the so-called "dynamic pressure". If a particle of fluid initially at rest is set in motion, then a force must have acted. This force comes from the pressure difference on the sides of the fluid particle. The text suggests that the motion of the particle is due to the conversion of a part of the chaotic kinetic energy into ordered kinetic energy, thanks to the pressure unbalance on the sides of the fluid particle. Therefore the speed increases and the static pressure decreases."
I'm trying to understand this better. The force acting on the fluid particle, performing work, should add kinetic energy to the system, not converting an energy already present in the particle into another. Why do we have a conversion of potential energy (static pressure) into ordered kinetic energy to the action of a force on our system (the fluid particle)? Should we not maintain the same chaotic kinetic energy and, if anything, have that extra due to the work of force?
Thank you all!