Can water boil without being heated?
It sounds impossible. Put water in a kettle, turn on the heat, and eventually you get boiling water.
But imagine water flowing through a pipe. There is no heater. No flame. No red-hot surface.
And suddenly...
🫧 A vapor bubble appears.
This strange phenomenon is called cavitation.
And the surprising part is that the water does not necessarily need to become hotter. A sufficiently low local pressure can cause liquid water to form vapor.
🫧 What Is Cavitation?
Cavitation occurs when the local static pressure inside a liquid becomes sufficiently low for vapor cavities to form.
The important word here is local.
The pressure does not have to be extremely low everywhere in the system. A small region of very low pressure can be enough.
If the local pressure falls below the liquid's vapor pressure at that temperature, vapor formation can occur.
So the basic mechanism is:
📉 Pressure drops → 🫧 Vapor forms
Think of the water as having a small disagreement with physics:
“You lowered the pressure THAT much? Fine. I'll become vapor.” 😄
🔥 Is Cavitation the Same as Boiling?
Not exactly.
Both phenomena involve the formation of vapor, but the dominant trigger is different.
| Normal boiling | Cavitation |
|---|---|
| Temperature increases | Local pressure decreases |
| Vapor forms because of heating | Vapor forms because pressure becomes sufficiently low |
| Typical example: kettle | Typical example: pump, valve or Venturi |
So when you see cavitation, don't look for a hidden kettle inside the pipe.
The pressure is doing the interesting work. 🕵️♂️
💨 The Venturi Effect
One of the simplest ways to understand cavitation is with a Venturi tube.
Imagine a pipe that becomes narrower in the middle:
WIDE → NARROW → WIDE
Water flows through the pipe.
As the cross-sectional area decreases, the average flow velocity increases.
For approximately incompressible flow, the continuity equation can be written as:
A₁V₁ = A₂V₂
where:
- A = cross-sectional area
- V = average velocity
Therefore, if the area becomes smaller while the volumetric flow rate remains approximately constant, the velocity must increase.
📉 Why Does Pressure Drop?
For simplified steady, incompressible flow along a streamline, Bernoulli's equation is:
P + ½ρV² + ρgz = constant
where:
- P = static pressure
- ρ = fluid density
- V = velocity
- g = gravitational acceleration
- z = elevation
If elevation changes are small and the simplified assumptions are reasonable, an increase in velocity corresponds to a decrease in static pressure.
Therefore, inside the Venturi throat we can have:
💨 Velocity ↑ → 📉 Static Pressure ↓
And now the conditions for cavitation may appear.
🫧 Step 1 — The Bubble Is Born
As water accelerates toward the Venturi throat, the local static pressure decreases.
If the pressure becomes sufficiently low relative to the vapor pressure of water at the local temperature, vapor formation can begin.
A tiny vapor nucleus appears.
This is the beginning of our cavitation bubble.
Importantly, this vapor cavity should not simply be confused with an ordinary air bubble. Cavitation is fundamentally associated with vapor formation from the liquid.
🫧 Step 2 — The Bubble Grows
If the surrounding pressure remains favorable for vapor formation, the cavity can grow.
Its behavior depends on several factors:
- local pressure
- liquid temperature
- vapor pressure
- flow velocity
- pressure gradients
- nucleation conditions
- time available for bubble growth
Real cavitation is therefore much more complicated than simply saying:
“Low pressure = giant bubble.” 😅
➡️ Step 3 — Pressure Recovers
After the throat, the Venturi expands.
The flow decelerates and, under suitable conditions, the static pressure begins to recover.
Our vapor bubble is now moving from a low-pressure region toward a higher-pressure region.
And this is where the bubble's life gets considerably less comfortable.
💥 Step 4 — The Bubble Collapses
As the surrounding pressure rises, the vapor cavity becomes unstable.
Liquid begins moving inward around the cavity and the bubble rapidly decreases in size.
Eventually, the vapor condenses back into liquid and the cavity disappears.
The collapse can happen extremely quickly.
When collapse occurs close to a solid surface, it can generate strong localized effects, including:
- ⚡ pressure pulses
- 💧 high-speed liquid microjets
- ⚙️ localized material loading
- 🔊 noise
- 📳 vibration
This is why cavitation can become a serious engineering problem.
The bubble looks tiny and innocent.
The component experiencing the collapse may disagree. 😬
⚠️ Why Can Cavitation Damage Metal?
Repeated vapor formation and collapse near a solid surface can contribute to cavitation erosion.
Over time, repeated loading can produce microscopic pits and progressive surface damage.
This is especially important in hydraulic machinery.
⚙️ Pumps
Cavitation can reduce pump performance and contribute to noise, vibration and component damage.
💧 Valves
High local velocities and pressure drops can create conditions favorable for cavitation inside valves.
🚢 Marine Systems
Low-pressure regions around fast-moving underwater components can produce cavitation.
🏭 Hydraulic Machinery
Turbines and other hydraulic machines can also experience cavitation-related erosion and performance losses.
📐 The Cavitation Number
Engineers often use dimensionless parameters to describe cavitation conditions.
One commonly used form of the cavitation number is:
σ = (P∞ − Pv) / (½ρV²)
where:
- P∞ = reference pressure
- Pv = vapor pressure
- ρ = liquid density
- V = characteristic velocity
The exact definition can vary depending on the application and choice of reference conditions.
The concept is nevertheless simple:
How close is the flow to conditions where vapor formation becomes possible?
🌡️ Temperature Still Matters
Cavitation is driven by pressure, but temperature still plays an important role.
Why?
Because the vapor pressure of a liquid changes with temperature.
As water becomes warmer, its vapor pressure increases.
Therefore, warmer water can reach cavitation conditions at a higher absolute pressure than colder water, assuming other conditions are comparable.
So temperature isn't necessarily the direct trigger in our Venturi example, but it changes the threshold at which vapor formation becomes possible.
💻 What Would CFD Show?
Computational Fluid Dynamics can help us visualize the physics that our eyes cannot directly see.
In a simplified Venturi simulation, we would expect:
Converging section
💨 Velocity increases
📉 Static pressure decreases
Throat
💨 Highest velocity
📉 Lowest static pressure
Diverging section
💨 Velocity decreases
📈 Static pressure recovers
If the local pressure becomes sufficiently low, a multiphase CFD model can be used to investigate vapor formation and condensation.
Depending on the application, cavitation simulations can involve multiphase formulations and mass-transfer models describing evaporation and condensation between liquid and vapor.
In other words, a colorful CFD contour is only the beginning.
The physics underneath it is the real story.
🧠 The Entire Phenomenon in One Sequence
↓
📉 STATIC PRESSURE DROPS
↓
🫧 VAPOR FORMS
↓
🫧 CAVITATION BUBBLE GROWS
↓
➡️ BUBBLE MOVES DOWNSTREAM
↓
📈 PRESSURE RECOVERS
↓
💥 BUBBLE COLLAPSES
🤯 The Really Strange Part
You can have water that is nowhere near its normal atmospheric boiling temperature and still create vapor locally.
How?
Change the pressure.
The same liquid can therefore remain perfectly stable in one part of a system and form vapor only a short distance away where the pressure becomes sufficiently low.
This is one reason fluid mechanics can be so fascinating.
You cannot see pressure directly.
But sometimes you can see what pressure does.
🫧 One Tiny Bubble Can Tell You a Lot
Consider that single cavitation bubble.
Where did it form?
Probably a region of very low local pressure.
Where did it grow?
The surrounding pressure and thermodynamic conditions allowed vapor formation to continue.
Where did it collapse?
The surrounding pressure recovered sufficiently to destabilize the vapor cavity.
And if this happens repeatedly near a solid surface...
Now you have an engineering problem. ⚙️
🎬 Making Invisible Physics Visible
The accompanying visualization is AI-generated and is designed to make the physics of cavitation easier to understand.
It shows a conceptual Venturi flow and follows one vapor bubble through its formation, growth and collapse.
The animation is intentionally simplified.
Real cavitation depends on geometry, pressure history, temperature, turbulence, dissolved gases, nucleation sites, surface conditions and multiphase dynamics.
Therefore, the visualization should not be interpreted as quantitatively validated CFD or experimental data.
🎯 The Takeaway
If you remember only one thing about cavitation, remember this:
🫧 CAVITATION IS FUNDAMENTALLY ABOUT PRESSURE.
A simplified chain looks like this:
💨 Velocity ↑
↓
📉 Static Pressure ↓
↓
🫧 Vapor Formation
↓
📈 Pressure Recovery
↓
💥 Bubble Collapse
It may look like boiling.
But there is no kettle hiding inside the Venturi. ☕😄
The pressure field is responsible for the phenomenon.
Welcome to the fascinating world of cavitation. 🫧
Scientific note: This article is an educational simplification of cavitation physics. Actual cavitation onset, growth and collapse depend on the complete thermodynamic and hydrodynamic conditions of the system. The AI-generated visualization is conceptual and is not a substitute for experimentally validated measurements or quantitatively validated CFD simulations.
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