Fluid pressure

Fluid pressure is the isotropic normal compressive stress in a fluid at rest.

Cavitation is the formation of gas or vapour cavities when a liquid's pressure becomes sufficiently low. A journal bearing model with cavitation must specify how a partly filled film and its boundaries behave, so periodic pressure alone cannot determine the flow there.

The pressure gradient is the vector $\nabla p$ pointing in the direction of fastest pressure increase. The force density exerted by pressure on a continuum is $-\nabla p$.

Along a boundary-layer streamline directed by increasing $s$, an adverse pressure gradient is a rise in pressure in the direction of flow. The associated pressure gradient force opposes the velocity. In a steady inviscid outer stream the Euler equation gives $UdU/ds=-{\rho }^{-1}dp/ds$, so a positive gradient decelerates a positive outer velocity. Within a viscous boundary layer the slower fluid near the wall has less streamwise momentum and can reverse while the outer stream still moves forward. The onset of flow separation is therefore associated with vanishing and then reversed wall shear stress. A pressure rise does not by itself locate separation: the full boundary-layer solution and upstream history also matter.

A spatially varying pressure exerts net force per unit volume $-\nabla p$ on a small fluid element. Its contribution to acceleration is $-\nabla p/\rho$. Pressure increases in the direction opposite to this force.