Compressibility Fluid:

Compressible fluid flow refers to the air flow through a gas turbine engine and, because the air is compressible, flow at subsonic speeds causes a change in the density of the air as it progresses through the engine.

The air entering the duct at section A, consists of air at pressure (P1) and velocity (V1); then as the air enters the increased area of the duct at B it will spread out to fill the increased area and this will cause the air flow to slow down (continuity equation) and give a change in velocity to V2. The static pressure of the air will increase (Bernoulli's theorem) to become P2 in the wider section of the duct and, because air is compressible, the air density will increase as it is compresses by the rise in pressure in section B of the duct.