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So we have a moving fluid and we know from Bernouli's equation that there is a term called dynamic pressure(not to be confused with the hydrostatic pressure of the fluid). So,what exactly is it and how can it be explained microscopically? Note:do not involve relativity please!

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Dynamic pressure of fluids is the kinetic energy per unit volume of the fluid. Its unit are the same as pressure, and Bernoulli's equation,

$0.5\rho v^2 + \rho gy + p = constant$

can be written as

$P_{dynamic} + \psi_{gravity} + P = constant$, where $P_{dynamic} = 0.5\rho v^2$ and $\psi_{gravity}$ is the force potential of gravity.

As for microscopic explanation, it is the total kinetic energy per unit volume of a fluid (average sum of KE of particles per unit volume).

Update: Microscopic explanation in more detail

Consider all the particles in a (tiny) region with constant flow speed $v$. Consider one particle. This particle has an arbitrary velocity, $v_i$, and mass $m_i$. Hence, the kinetic energy of this particle is $0.5mv_i^2$. The total kinetic energy in the region is

$\Sigma 0.5m_iv_i^2 = 0.5\Sigma m_iv_i^2 = 0.5 \Sigma m_iv_i \cdot v_i = 0.5\Sigma m_iv_i \cdot \Sigma v_i = 0.5m_{total}v_{cm} \cdot v_{flow} = 0.5m_{total}v_{cm}^2$, using the fact that the flow velocity is equal to the center of mass velocity in a region of constant flow velocity (so, any tiny region in a fluid).

Hence, the total kinetic energy per unit volume of a fluid is equal to the dynamic pressure term, explaining the motivation for the dynamic pressure term. Note: This term comes from an energy analysis, not a force analysis.

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Wiki/google quote:

Dynamic pressure is the kinetic energy per unit volume of a fluid particle. Dynamic pressure is in fact one of the terms of Bernoulli's equation, which can be derived from the conservation of energy for a fluid in motion.

So Dynamic Pressure is just local impulse/energy of movement that is being passed from particles to other particles in solution or in your measuring instrument

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  • $\begingroup$ But,microscopically,how can we explain it using forces of the particles? $\endgroup$ – TheQuantumMan Mar 22 '15 at 22:38
  • $\begingroup$ by realizing that single particles collide with each other and pass impulse/energy like that, and in flow they will have average speed pointed somewhere. Hence, average force will have direction of the flow $\endgroup$ – aaaaa says reinstate Monica Mar 22 '15 at 22:45
  • $\begingroup$ yes,you are right! $\endgroup$ – TheQuantumMan Mar 22 '15 at 22:52

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