Cavitation

What cavitation actually is

Cavitation is what happens when you drop the pressure of a liquid low enough that it boils — without ever heating it. Every liquid has a vapor pressure: the pressure at which it is in equilibrium with its own vapor at a given temperature. For water at 20 °C that vapor pressure is just 2.34 kPa, roughly one-fortieth of atmospheric. So if any region of flowing water momentarily dips below 2.34 kPa absolute — on the suction side of a pump, across the low-pressure face of a propeller blade, in the core of a shed vortex, or downstream of a half-closed valve — the water tears apart into vapor-filled cavities. That nucleation is the first half of cavitation, and on its own it is harmless.

The destructive half is the collapse. The little vapor cavities are swept downstream into regions where the pressure has recovered above vapor pressure. There they are no longer stable, and they implode. Because the vapor inside has almost no pressure to resist it, the surrounding liquid accelerates inward and slams shut in microseconds. Far from any wall the collapse is spherical and emits a sharp pressure pulse. Close to a solid surface — which is exactly where it matters — the collapse is asymmetric: the wall blocks inflow from one side, so the far side of the bubble caves in faster and forms a reentrant microjet of liquid, 100 to 300 m/s, that pierces the bubble and strikes the surface. The combination of that jet impact and the subsequent rebound shockwave produces local pressures on the order of 1 GPa over a spot a few micrometres across. Repeat that millions of times a second and even hardened steel fatigues, cracks, and erodes away.

That is the entire story of cavitation in one breath: vapor pressure lets the liquid boil cold, and bubble collapse turns the latent energy of that phase change into a microscopic hammer. Everything else — net positive suction head, the cavitation number, supercavitating torpedoes, ultrasonic cleaners — is a consequence of those two facts.

Vapor pressure and the boiling-by-pressure idea

The thing that confuses people first is that cavitation and boiling are the same phase change crossing the same saturation line — just from opposite directions. Boiling raises temperature at fixed pressure until the temperature exceeds the saturation temperature. Cavitation lowers pressure at fixed temperature until the pressure falls below the saturation (vapor) pressure. The two are numerically identical conditions:

Boiling:      heat water at 101 kPa  →  vapor forms at  100 °C
Cavitation:   drop pressure at 20 °C  →  vapor forms at  2.34 kPa

Same saturation curve  p_sat(T):
   T = 20 °C   →  p_v = 2.34 kPa
   T = 50 °C   →  p_v = 12.3 kPa
   T = 80 °C   →  p_v = 47.4 kPa
   T = 100 °C  →  p_v = 101.3 kPa

This is why cavitation gets dramatically worse with hot liquids: the vapor pressure climbs steeply, so a pump that handles 20 °C water happily can cavitate badly on 80 °C boiler feedwater because the margin between system pressure and vapor pressure has collapsed. It is also why a syringe of water at room temperature will fizz and "boil" if you seal the nozzle and pull the plunger hard — you have dropped the pressure below 2.34 kPa, and the water cavitates in your hand.

Where the low pressure comes from — Bernoulli on a blade

The low pressure that triggers cavitation almost always comes from acceleration. Bernoulli's equation for an incompressible flow says that along a streamline the total head is conserved:

p + ½ρV² + ρgz = constant

Where the fluid speeds up, the static pressure p must drop. On the suction face of a propeller blade or the back of a hydrofoil, the flow accelerates to wrap around the curvature, and the local pressure can fall far below ambient. The depth of that dip is captured by the pressure coefficient:

C_p = (p − p_∞) / (½ρV_∞²)

Minimum C_p on a blade section, say  C_p,min = −1.6

Local minimum pressure:
   p_min = p_∞ + C_p,min · ½ρV_∞²

If p_min reaches the vapor pressure, cavitation begins. Worked example for a propeller blade tip running at 30 m/s in seawater (ρ ≈ 1025 kg/m³) at 1 m depth (p_∞ ≈ 111 kPa absolute):

Dynamic pressure:  ½ρV² = 0.5 · 1025 · 30²   = 461 kPa
Pressure dip:      C_p,min · 461 = −1.6 · 461 = −738 kPa
Local pressure:    p_min = 111 − 738          = −627 kPa  (impossible)
                   → so p_min clamps at p_v ≈ 1.7 kPa: the blade cavitates heavily.

The "impossible" negative absolute pressure is the tell-tale: the flow cannot sustain it, so instead of going negative the liquid vaporizes and a cavity forms. That clamping at vapor pressure is cavitation inception.

The cavitation number and inception

Non-dimensionalize the whole problem and you get the single most useful parameter in the field, the cavitation number σ:

σ = (p − p_v) / (½ρV²)

It compares the pressure margin above vapor pressure to the dynamic pressure of the flow. Cavitation begins when σ falls to the inception value σ_i, and to a first approximation σ_i ≈ −C_p,min. The picture is intuitive:

• High σ — slow flow, high ambient pressure, deep submergence: no cavitation. A submarine cruising deep and slow runs quiet.
• σ falling toward σ_i: incipient cavitation — a few bubbles flicker on and off at the lowest-pressure spot.
• σ < σ_i: developed sheet, cloud, or vortex cavitation covering a real fraction of the surface, with audible noise and erosion.
• σ → 0: supercavitation — a single stable cavity envelops the whole body.

The same propeller that is silent on a deep submarine becomes a loud, eroding mess on a surface ship at full power, purely because surfacing lowers p and full power raises V, and both drive σ down. Naval architects plot a cavitation bucket — the band of σ versus angle of attack within which a blade section stays cavitation-free — and design propellers to live inside it across the operating envelope.

Bubble dynamics — the Rayleigh–Plesset equation

The collapse itself is governed by the Rayleigh–Plesset equation, which tracks the radius R(t) of a single spherical bubble responding to the pressure difference between its interior and the far field:

ρ ( R·R̈ + 1.5·Ṙ² ) = p_B(t) − p_∞(t) − 4μṘ/R − 2S/R

where p_B is the pressure inside the bubble, μ the viscosity, and S the surface tension. Lord Rayleigh's 1917 simplification — an empty bubble collapsing under constant pressure with no viscosity or surface tension — gives the collapse time directly:

τ_collapse = 0.915 · R₀ · √(ρ / Δp)

For R₀ = 1 mm in water, Δp = 100 kPa:
   τ = 0.915 · 1e-3 · √(1000 / 1e5) ≈ 9.2e-5 s ≈ 92 µs

A millimetre bubble collapses in under a hundred microseconds; the final stage is faster still, and the inward wall velocity diverges toward the centre. In that last instant the trapped non-condensable gas is compressed adiabatically to thousands of kelvin and emits a flash of light — sonoluminescence, a literal spark born from collapsing water. Near a wall the spherical symmetry of Rayleigh's solution breaks and the reentrant microjet forms, which is why the damage is concentrated and directional rather than diffuse.

Pumps and NPSH — the practical engineering number

For pumps the controlling quantity is Net Positive Suction Head (NPSH), the absolute-pressure margin above vapor pressure available at the pump inlet, expressed as a height of the liquid being pumped. It splits into what the installation supplies and what the pump demands:

NPSH_available = (p_atm + p_tank − p_v)/(ρg)  −  z_static  −  h_friction

NPSH_required  = measured on the pump test rig
                 (suction head at which head drops 3 %)

Design rule:   NPSHa  ≥  NPSHr + margin   (margin ≈ 0.5–1 m, or ×1.1–1.3)

Worked example: a centrifugal pump lifting 20 °C water from a sump 3 m below the pump, with 0.8 m of suction-line friction loss, open to atmosphere (101.3 kPa):

p_atm/(ρg)  = 101300 / (998·9.81)    = 10.34 m
p_v/(ρg)    = 2340  / (998·9.81)     =  0.24 m
NPSHa = 10.34 − 0.24 − 3.0 − 0.8     =  6.30 m

If the pump's NPSHr at this flow = 5.0 m  →  margin 1.3 m  ✓  (no cavitation)
Now pump 80 °C water:  p_v/(ρg) = 47400/(972·9.81) = 4.97 m
NPSHa = 10.6 − 4.97 − 3.0 − 0.8      =  1.83 m   <  NPSHr 5.0 m  ✗  (cavitates)

That single change — pumping hot water instead of cold — flips a healthy installation into a cavitating one, because vapor pressure ate up the margin. The classic field causes of pump cavitation all reduce NPSHa or raise NPSHr the same way: hot or volatile liquids, a clogged suction strainer, too high a static lift, an undersized suction pipe, or running the pump far past its best-efficiency point where NPSHr climbs steeply. The symptoms are unmistakable — a sound "like pumping gravel," a vibration spike, a falling and fluctuating discharge head, and pitting found on the impeller leading edges and the shroud during overhaul.

The four forms cavitation takes

• Inception (travelling-bubble) cavitation. Isolated bubbles nucleate at the low-pressure point, grow, sweep downstream, and collapse. The mildest form; the onset that σ_i predicts.
• Sheet (attached) cavitation. A continuous vapor film attaches to the leading edge of a blade or foil. Stable sheets are relatively benign, but their trailing edge sheds clouds.
• Cloud cavitation. The sheet periodically breaks off into a collapsing cloud of bubbles. The collective collapse focuses energy and is the most aggressively erosive form — the main eroder of marine propellers and pump impellers.
• Vortex cavitation. The low pressure in the core of a tip vortex or a draft-tube vortex drops below vapor pressure, producing the trailing helical "rope" you see streaming off a propeller tip or a Francis-turbine runner. Often the first form to appear and a major noise source for submarines.

Cavitation versus flashing — two ways a valve can boil

Engineers constantly confuse cavitation with flashing, the related phenomenon in throttling valves. Both start with a pressure drop below vapor pressure; the difference is whether the pressure recovers. The distinction dictates entirely different mitigation.

Property	Cavitation	Flashing
Pressure history	Drops below p_v, then recovers above p_v	Drops below p_v and stays below (downstream p < p_v)
Bubble fate	Bubbles collapse violently downstream	Bubbles persist as a two-phase vapor-liquid mix
Damage mechanism	Implosion microjets and shockwaves — pitting	High-velocity vapor scour — erosion of soft metal
Damage location	Concentrated downstream of the vena contracta	Spread along the outlet passage and pipe wall
Damage appearance	Sharp-edged craters, "cinder"-like pitting	Smooth, polished, directional grooves
Sound	Pumping-gravel rattle, broadband to 1 MHz+	Lower, hissing roar
Primary fix	Stage the pressure drop, raise back-pressure, harden trim	Material choice and geometry — cannot stage it away

In a control valve the fix for cavitation is to break the total pressure drop into several smaller drops in series — multi-stage trim, drilled cages, or labyrinth paths — so the local pressure never reaches p_v. Flashing cannot be staged away because the downstream pressure is below vapor pressure no matter what; there the only defence is hardfacing (Stellite) and generous expansion geometry.

Erosion — how the metal actually goes

Cavitation erosion is a fatigue process, not a single-blow fracture. Each microjet impact is below the yield strength of a tough metal, but the surface is loaded cyclically at megahertz rates. The sequence is: an incubation period where the surface work-hardens with no measurable mass loss; then an acceleration phase as microcracks initiate at slip bands and inclusions; then a steady-state phase where craters coalesce and material spalls away at a roughly constant rate. Resistance correlates with a combination of hardness, toughness, and strain-hardening capacity rather than hardness alone — which is why cobalt-based Stellite and certain austenitic stainless steels beat harder but more brittle alloys.

• Marine propellers. Nickel-aluminium bronze (NAB) is the standard because it combines corrosion resistance with good cavitation-erosion resistance; severely cavitating naval propellers still need periodic weld repair of the trailing-edge and tip craters.
• Hydraulic turbines. Francis and Kaplan runners cavitate near the draft tube and blade outlets; the eroded zones are repaired by welding and ground smooth, and modern runners use 13Cr-4Ni martensitic stainless or Stellite overlays at the worst spots.
• Centrifugal pumps. Impeller-eye and vane leading edges pit; duplex stainless impellers and hardfaced wear rings extend life in cavitating service.
• Diesel cylinder liners. Engine vibration drives a less obvious form — the coolant cavitates against the outer liner wall, drilling pinholes clean through; the fix is a controlled coolant additive package and stiffer liner support.

When cavitation is the product, not the failure

• Ultrasonic cleaning. A 20–40 kHz transducer floods a tank with cavitation clouds whose collapse scrubs grease, oxide, and particulate off jewelry, optics, surgical instruments, and PCBs — reaching into blind holes a brush never could.
• Lithotripsy and histotripsy. Focused ultrasound nucleates and collapses bubbles at a target inside the body, fracturing kidney stones (lithotripsy) or mechanically ablating tissue and tumors (histotripsy) with no incision.
• Supercavitation. Deliberately envelop a whole moving body in one stable gas cavity so only the nose touches water — drag drops by an order of magnitude. The Russian VA-111 Shkval torpedo exceeds 200 knots (≈100 m/s) inside its own vapor bubble, roughly four times a conventional torpedo, by venting gas from a cavitator disc at the nose.
• Sonochemistry. The thousands-of-kelvin hot spot and GPa pressures inside a collapsing bubble drive radical chemistry, emulsification, nanoparticle synthesis, and water-treatment oxidation that would otherwise need extreme bulk conditions.
• The pistol shrimp. Nature got there first — the snapping shrimp closes its claw fast enough to shoot a cavitating jet, collapsing a bubble that emits a ~210 dB crack (one of the loudest sounds in the ocean) and a flash of sonoluminescence, stunning its prey.

Common pitfalls when designing against cavitation

• Sizing NPSH for cold water only. Vapor pressure rises steeply with temperature; a pump fine on 20 °C water can cavitate on hot process fluid. Always evaluate NPSHa at the highest operating temperature.
• Forgetting the 3 % drop is already cavitation. NPSHr is defined at a 3 % head drop, meaning the pump is already cavitating at NPSHr. Erosion-free operation needs a real margin above it, not equality.
• Running far from best-efficiency point. Off-design flow produces recirculation cavitation at part-load and steeply rising NPSHr at high flow. The cavitation-free window is narrow; size the pump to live near its BEP.
• Confusing cavitation with flashing in valves. Staging the pressure drop cures cavitation but does nothing for flashing. Diagnose which one you have from the downstream pressure before choosing trim.
• Trusting hardness alone for erosion resistance. Cavitation erosion is a fatigue process; toughness and strain-hardening matter as much as hardness. The hardest alloy is not always the most cavitation-resistant.
• Ignoring scale effects in model tests. Cavitation inception depends on the dissolved-gas content and nucleus population of the water, which do not scale geometrically. A clean model tunnel can read a higher σ_i than the gas-rich full-scale sea, so model results need careful correction.