Cavitation

The intuition: boiling without heat

We're taught that water boils at 100 °C. That's only true at sea-level pressure. What actually causes boiling is the liquid's vapor pressure rising to meet the pressure pushing down on it. Heating is just one way to make that happen — it raises the vapor pressure until it matches the ambient pressure.

Cavitation takes the other road. Hold the temperature fixed and instead drop the pressure. Once the local pressure falls below the vapor pressure, the liquid can't stay liquid: it flashes into vapor right there, mid-flow, forming bubbles. At 20 °C, water's vapor pressure is only about 2.34 kPa — roughly 2.3% of atmospheric pressure. A fast-spinning propeller blade, a constricted pump inlet, or a slammed shrimp claw can pull the local pressure that low in microseconds. The water "boils" cold.

Then comes the violent half. As the bubble drifts into a region of higher pressure, the surrounding liquid crushes it back in. There's almost no gas inside to cushion the implosion, so the collapse accelerates until the bubble is a fraction of its original size — and all the energy it gathered while expanding releases in a single hammer blow.

How it works: two stages, two physics

Stage 1 — inception (growth). Real water always contains microscopic gas nuclei: tiny pockets of dissolved air trapped in crevices on particles or surfaces. When the local pressure drops below p_v, these nuclei become unstable and grow explosively into vapor-filled bubbles. This is why ultra-pure, degassed water can be pulled to large negative (tension) pressures without cavitating — there are no nuclei to seed bubbles.

Stage 2 — collapse (implosion). When the bubble is swept into higher-pressure fluid, the inward rush of liquid is governed by the Rayleigh–Plesset equation, which tracks the bubble radius R(t). Lord Rayleigh's 1917 result for an empty bubble collapsing under a constant driving pressure p∞ gives a collapse time

t_collapse ≈ 0.915 · R0 · √(ρ / p∞)

For a 1 mm bubble in water (ρ = 1000 kg/m³) at 1 atm driving pressure, that's about 90 microseconds — and the wall velocity diverges as the radius shrinks. Near a solid surface the collapse is asymmetric: the side away from the wall accelerates faster and pierces the bubble with a liquid microjet aimed at the wall, while the rebound radiates a shockwave. Both deliver the damage.

The math: vapor pressure, the cavitation number, and Rayleigh–Plesset

Cavitation begins when the local pressure falls to the vapor pressure:

p_local ≤ p_v(T)        (inception condition)

To predict where a flow will cavitate, engineers use the dimensionless cavitation number:

σ = (p∞ − p_v) / (½ · ρ · v²)

Here p∞ is the reference (ambient/far-field) pressure, p_v the vapor pressure, ρ the liquid density, and v the reference flow speed. The numerator is the pressure margin keeping the liquid from boiling; the denominator is the dynamic pressure ½ρv² that Bernoulli's principle converts into low static pressure wherever the flow speeds up. Cavitation appears when σ falls to the inception number σ_i, which for a given body shape is approximately equal to −C_p,min (the magnitude of the minimum pressure coefficient).

The full collapse dynamics follow the Rayleigh–Plesset equation for the bubble radius R(t):

ρ ( R·R̈ + (3/2)·Ṙ² ) = p_B − p∞ − 4μṘ/R − 2S/R

where p_B is the pressure inside the bubble, μ the dynamic viscosity, and S the surface tension. The R·R̈ + (3/2)Ṙ² term is the inertia of the radially in-rushing liquid — the reason an empty bubble's collapse accelerates without limit until it's stopped by the compression of whatever residual gas remains. That trapped gas is what gets compressed to thousands of kelvin, producing the light flash of sonoluminescence.

Regimes: from a few bubbles to a full supercavity

As you lower the cavitation number (faster flow, lower ambient pressure, warmer liquid), cavitation grows through distinct stages:

Regime	What you see	Roughly when	Consequence
No cavitation	Clear flow	σ well above σ_i	Normal, efficient operation
Inception (traveling bubbles)	Scattered transient bubbles in low-pressure zone	σ ≈ σ_i	First noise; minor erosion begins
Sheet / cloud cavitation	Attached vapor sheet that periodically sheds clouds	σ < σ_i	Most aggressive erosion; strong vibration
Supercavitation	One cavity envelops the whole body	σ ≪ σ_i	Drag collapses; used in fast torpedoes

Cloud cavitation is, counter-intuitively, often more damaging than a steady attached sheet: the periodic shedding and collective collapse of a bubble cloud focuses energy, concentrating erosion in a band downstream of the sheet.

Worked example: would this propeller cavitate?

Consider a propeller blade tip moving through 20 °C seawater. Take the reference (ambient) pressure at the operating depth as p∞ = 150 kPa, the vapor pressure of water at 20 °C as p_v = 2.34 kPa, and seawater density ρ ≈ 1025 kg/m³.

Case A — moderate tip speed, v = 12 m/s.

½ρv² = 0.5 · 1025 · 12²  = 73,800 Pa
σ    = (150,000 − 2,340) / 73,800 ≈ 2.0

A cavitation number of ~2.0 is comfortably above typical inception values (σ_i for many blade sections is well under 1). No cavitation — the blade has plenty of pressure margin.

Case B — high tip speed, v = 25 m/s.

½ρv² = 0.5 · 1025 · 25²  = 320,300 Pa
σ    = (150,000 − 2,340) / 320,300 ≈ 0.46

Now σ ≈ 0.46 — low enough that the suction side of the blade, where local pressure dips well below p∞, will dip under p_v and cavitate. The fix matches the physics: slow the tips, run deeper (raise p∞), or redesign the blade section to raise the minimum pressure (less negative C_p,min, larger σ_i margin).

The real-world bill: erosion, efficiency, and noise

• Propeller and impeller erosion. Microjets at 100–300 m/s and ~1 GPa shockwaves repeated millions of times per second fatigue metal into a spongy, pitted surface. Severe ship-propeller cavitation can remove millimeters of bronze and force dry-dock repairs costing tens to hundreds of thousands of dollars per incident.
• Pump failure. A pump run below its required NPSH (Net Positive Suction Head) cavitates at the impeller eye. Symptoms are a sound "like pumping gravel," a drop in head and flow, and impellers eaten through in weeks. NPSH margin is one of the first checks in pump selection precisely to avoid this.
• Lost efficiency. Vapor pockets disrupt the pressure field a blade or impeller relies on, cutting thrust, head, and efficiency and adding vibration that loosens bearings and seals.
• Acoustic signature. Cavitation is loud — the dominant noise source for ship and submarine propellers. Navies define a "cavitation inception speed" below which a sub runs quiet; exceeding it broadcasts the vessel's position.
• Dam and valve damage. High-velocity spillway flow and throttling valves cavitate against concrete and steel. The 1983 cavitation damage to the Glen Canyon Dam spillway tunnels gouged a crater roughly 10 m deep and tens of meters long through the concrete lining and into the sandstone during high releases.

Cavitation in nature: the shrimp that snaps

The snapping shrimp (Alpheus) turns cavitation into a weapon. It cocks an oversized claw and releases it to slam shut in under a millisecond, ejecting a water jet at roughly 25–30 m/s. By Bernoulli's principle that jet drops the local pressure below the vapor pressure, spawning a cavitation bubble. The bubble's collapse emits a crack measured near 210 dB (in water) — loud enough to stun or kill small fish and rivaling the loudest sounds any animal makes. The collapse is violent enough to flash light: the snap produces a tiny burst of sonoluminescence from a hot spot estimated near 5,000 K, briefly approaching the temperature of the Sun's surface inside a bubble smaller than a grain of sand.

The mantis shrimp does the same with a hammer-like club, striking so fast that the impact and a follow-up cavitation collapse both hit the shell of its prey — two blows from one strike.

Where cavitation is put to work

• Ultrasonic cleaning. A transducer cavitates solvent into a haze of microbubbles whose collapse scrubs grime from jewelry, eyeglasses, surgical instruments, and PCB assemblies — reaching crevices a brush can't.
• Medicine. Phacoemulsification emulsifies cataracts with a cavitating ultrasonic tip; lithotripsy fragments kidney stones with focused cavitation; histotripsy ablates tumors mechanically, with no incision.
• Sonochemistry. The ~5,000 K hot spots inside collapsing bubbles drive reactions and generate radicals, used for advanced water treatment and nanoparticle synthesis.
• Supercavitating vehicles. The Russian VA-111 Shkval torpedo rides inside a single vapor cavity to exceed 100 m/s by slashing skin friction — a regime engineers deliberately push past inception.
• Hydrodynamic propulsion design. Naval architects spend enormous effort preventing cavitation: skewed blades, supercavitating foils, and careful NPSH budgeting all exist to manage σ.

Common misconceptions and edge cases

• "Cavitation bubbles are full of air." Mostly they're vapor — the liquid itself flashed to gas. There's also dissolved gas (gaseous cavitation), but the violent collapse comes from vapor bubbles with little gas to cushion them. A bubble full of air would just compress and bounce gently.
• "It's just boiling, so it's hot." The bulk liquid stays cold. The heat is hyper-local — confined to the picosecond, micron-scale collapse interior. You could hold cavitating room-temperature water and feel nothing unusual.
• "Lowering temperature stops it." Colder liquid has lower vapor pressure, which helps — but warm water cavitates far more easily because p_v rises steeply with temperature. Hot-water pumps need much more NPSH margin than cold ones.
• "The bubble damages the wall by hitting it as a balloon." No — the lethal mechanism is the asymmetric collapse firing a focused liquid microjet plus a radiated shockwave, not the gentle bubble surface touching the wall.
• "Pure water can't be pulled below vapor pressure." It can. Without nuclei, ultra-clean degassed water sustains large tensile (negative) pressures — tens of MPa in lab experiments — before cavitating. Real engineering water always has nuclei, so it cavitates near p_v.
• "Cavitation and flashing are the same." Related but distinct: cavitation involves growth and collapse as the fluid re-pressurizes; flashing is a sustained phase change (e.g. across a throttling valve) where the vapor doesn't recondense downstream.