The process in a flowing liquid in which local reductions of pressure to or below the liquid's vapour pressure cause the nucleation, growth and subsequent violent collapse of vapour cavities (bubbles), producing intense local shear, shock waves and transient hot spots that alter mass transfer, mixing, chemical reaction rates and can cause material erosion.

Principle

Where flow-driven local static pressure falls below a fluid's vapour pressure, phase change to vapour occurs; subsequent re-pressurization causes bubble collapse that concentrates mechanical and thermal energy at the collapse site.

Demonstration

Illustrative scenario: Liquid flows through a constriction (Venturi) in a pipe. At the throat the static pressure drops below the vapour pressure and vapour cavities form. Downstream, pressure recovers and cavities collapse near a metal wall, producing pitting on the wall surface and locally enhanced mixing and oxidant formation within milliseconds.

Misapplication

Treating any visible bubbles or gas release as cavitation. The error is failing to distinguish vapour cavities produced by local pressure < vapour pressure from gas exsolution of dissolved gases or entrained bubbles, which have different dynamics and collapse effects.

Consequence

Operationally, cavitation can both damage hydraulic components (erosion, vibration, noise, reduced pump efficiency) and be harnessed for process intensification (enhanced mass transfer, emulsification, accelerated chemical reactions) depending on location, intensity and control.

Reversal

If the fluid is near its critical point, compressible or contains intentionally injected microbubbles (e.g., acoustic cavitation), the collapse dynamics, energy concentration and effects differ; in highly viscous flows or at very small scales cavitation may be suppressed.

Boundary

Clearly within: vapour cavities generated by flow-induced local pressure below vapour pressure collapsing in a liquid. Boundary case: bubbles nucleated on a rough surface where pressure transient and surface tension compete—outcome depends on nucleation site energy. Clearly outside: boiling driven by bulk heating, or degassing of dissolved gas without phase-change from liquid to vapour due to pressure drop.

Semantic Tension

Control versus exploitation: operators must choose between minimizing cavitation to protect equipment and intentionally inducing cavitation to obtain process benefits; these objectives constrain design and operating point selection.

Synthesis

Hydrodynamic cavitation is a localized, flow-driven phase-change phenomenon whose practical significance depends on where and how bubble collapse energy is delivered: close to solid surfaces it tends to cause damage, while distributed collapse in the bulk can be exploited for intensified mixing and chemical effects.