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[siretb]

I would suggest you read a book or a section about the fundamentals of a Venturi tube. Internet is full of references and pages.

Pressure is related to velocity by Bernouilly's law
P + 1/2 * rho * V^2 = Constant P pressure (Pa) ; rho specific mass (kg/m3) ; V velocity (m/s)

Experimental measurements have shown that the pressure decreases at the throat, where velocity is maximum. Pressure drop is maximum at throat and then pressure is recovered by the long, slowly diverging tube.
In terms of engineering, in order to properly recover pressure (convert back velocity to pressure) it is critical that the half angle of the diverging cone is small.