Cavitation Model
What is Cavitation?
Cavitation occurs when the local pressure in a liquid drops below the fluid’s vapor pressure. This can occur due to very low pressure in a flowing fluid or due to a rise in temperature of a liquid that then raises the local vapor pressure. No matter the mechanism this causes the liquid to vaporize and form vapor-filled voids or bubbles. When these bubbles are carried into regions of higher pressure, they collapse violently, generating pressure waves, noise, vibration, and potentially severe surface erosion. Cavitation is a critical concern in hydraulic machinery, valves, pumps, propellers, spillways, and high-pressure casting processes.
The FLOW-3D product family offers two distinct approaches to cavitation simulation: a physics-based Cavitation Model and a simplified Cavitation Potential Model. Each serves a different purpose, and choosing between them depends on the engineering question you are trying to answer.
Cavitation Model
The Cavitation Model, available across the FLOW-3D product family, models the physical process of cavitation directly. When local pressure falls below the vapor pressure threshold, the model drives a phase change from liquid to vapor, forming and tracking vapor regions within the flow field in real time. The reverse process, condensation and bubble collapse, is also captured as pressure recovers.
Two modes are available:
- Active cavitation — the model actively enforces the vapor pressure constraint, preventing fluid pressure from falling below the vapor pressure. Vapor voids form and collapse dynamically as the simulation evolves. This model is appropriate when cavitation materially affects the flow behavior, for example when void formation alters velocity fields, or when the fate of the vapor voids is important.
- Passive cavitation — the model tracks and transports the presence of cavitation bubbles through the fluid as a dispersed phase but does not form vapor voids. This model is useful when cavitation is secondary effect in the flow, like when only a brief appearance of small bubbles is expected.
Example: Butterfly Valve
In this FLOW-3D HYDRO simulation, water flows through a butterfly valve at conditions where cavitation occurs downstream of the valve body. Using the Active Cavitation Model, vapor voids form and are tracked in real time. The visualization shows the void region alongside the pressure distribution and velocity field with streamlines, capturing not just where cavitation occurs, but how it interacts with the surrounding flow.
FLOW-3D HYDRO simulation of a butterfly valve showing cavitation downstream of the valve body (left). Pressure distribution (top right) and velocity magnitude with streamlines (bottom right) show how the vapor void interacts with the surrounding flow.
Applications include:
- Hydraulic valves and gates
- Pump inlets and impellers
- Propellers and underwater vehicles
- Orifices and flow control structures
- Spillways
- Pipe systems with rapid pressure transients
- Boiling
Cavitation Potential Model
The Cavitation Potential Model is a diagnostic tool rather than a full multiphase model. Rather than modeling the physical cavitation, it identifies regions in the flow where the local pressure falls below the vapor pressure, flagging zones where cavitation is possible under the simulated conditions.
Because the model does not simulate the phase change itself, it runs with lower computational cost and produces a straightforward spatial map of cavitation risk. This makes it well suited for early-stage design screening, parametric studies across many geometry variants, or applications where the primary question is location of risk rather than quantification of its effects.
Additionally, cavitation can occur in high pressure die casting. The rapid movement of molten alloy through constrictions and curves in the die can result in rapid pressure drops and lead to subsequent cavitation. The resulting vapor bubbles can cause defects in the final casting and cause damage to the die, leading to reduced die life. For these reasons, it is critical to understand where cavitation is likely to occur and its potential severity. Since initiating and visualizing cavitation through physical experimentation is difficult and potentially damaging, it is preferable to simulate the process.
Example: HPDC Filling
Cavitation bubbles collapsing on the die surface can create pitting, but simulation of the full cavitation bubble can be simplified to improve performance. FLOW-3D CAST‘s cavitation potential model is a powerful tool for identifying areas on the die where cavitation is likely to occur by tracking the conditions which cause cavitation instead of capturing the cavitation bubbles themselves.
Side-by-side views of an HPDC die casting simulation showing cavitation potential. Cyan regions indicate where local pressure drops below the vapor pressure threshold, identifying areas at risk for die erosion and casting defects.
Choosing Between the Two Models
If cavitation is a central phenomenon in your simulation, one that will influence the behavior of the overall flow, use the full Cavitation Model. If you want to quickly determine where cavitation is likely before committing to a full multiphase run, the Cavitation Potential Model is the right starting point.