Understanding Static Pressure: Causes of Pressure Changes in Air Flow

[leonpalios]

When air flows through a variable cross - section tube, as the cross - section area increases, the average flow speed of air decreases (due to continuity equation) and according to Bernoulli’s theorem the static pressure increases. Regardless of the mathematical proof of Bernoulli’s theorem, what physical process causes the static pressure increase?
What physical process causes the static presssure decrease when the air speeds up passing through a narrow part of the tube?

 

[enigma]

See this thread.