Cavitation meaning refers to the formation and collapse of vapor bubbles when local pressure drops below the vapor pressure of a liquid. This phenomenon is particularly critical in industrial applications, especially cavitation in pump systems, where it can cause efficiency loss, noise, and cavitation erosion, leading to significant mechanical damage. However, cavitation also has beneficial applications, such as cavitation ultrasound, which is widely used in medical treatments and cleaning processes. Understanding its causes, impacts, and prevention strategies is essential for engineers working with pumps, turbines, and hydraulic systems. Advancements in Computational Fluid Dynamics (CFD) tools like ANSYS Fluent enable accurate cavitation simulation, prediction, and mitigation. By integrating theoretical insights with numerical modeling, engineers can better understand and control cavitation, ensuring optimal system performance and longevity.
What is the Cavitation Phenomenon?
Cavitation is a process where vapor bubbles form and collapse within a liquid due to low local pressure. While this phenomenon can lead to cavitation erosion (see Fig.1), noise, and efficiency loss in cavitation in pump systems, turbines, and marine propellers, it can also have useful applications. Cavitation ultrasound is widely used in medical treatments, such as shock wave lithotripsy for breaking kidney stones. Understanding both the destructive and advantageous aspects of cavitation is essential for engineers to design efficient systems that mitigate damage while leveraging its beneficial applications.
Figure 1- Pump impellor cavitation
Figure 2 illustrates flow behavior through a valve, showing how pressure changes affect normal flow, cavitation, and flashing. As fluid passes through the vena contracta, pressure drops sharply. If it falls below vapor pressure , cavitation bubbles form and later collapse, causing cavitation erosion. If pressure remains below , flashing occurs, leading to continuous vapor formation. Flashing is a phenomenon where a liquid suddenly turns into vapor due to a drop in pressure below its vapor pressure in a two-phase mixture. Therefore, Proper valve design and pressure control are essential to prevent cavitation in pumps and hydraulic systems, ensuring efficiency and longevity.
Figure 2- Pressure Behavior and Cavitation Phenomenon in a Valve
The process of cavitation
Figure 3 illustrates the cavitation meaning, demonstrating how cavitation bubbles form, collapse, and cause cavitation erosion in pipe walls or valve components:
- Vapor Bubble Formation: When local pressure drops below the cavitation number, vapor bubbles form due to phase change from liquid to gas.
- Bubble Collapse (Implosion): As the bubbles travel into higher-pressure zones, they rapidly collapse under liquid pressure.
- Supersonic Liquid Microjet Formation: The collapsing bubble generates an intense shock wave, forming a high-speed liquid jet directed at nearby surfaces.
- Cavitation Damage (Surface Erosion): The force from collapsing bubbles and microjets leads to cavitation erosion, causing pitting and wear in pipes, valves, and pump components.
Figure 3- The Cavitation Process: From Bubble Formation to Surface Erosion
Cavitation Number
The cavitation number (σ) is a dimensionless parameter used to predict and analyze cavitation in fluid systems. It helps engineers assess whether cavitation in pumps, turbines, or other hydraulic components is likely to occur. Formula for Cavitation Number can be written as follows:
![\[ \sigma = \frac{P_{inlet} - P_{vapor}}{\frac{1}{2}\rho v^2} \]](https://cfdland.com/wp-content/ql-cache/quicklatex.com-14b0c1db4bac6796cc4b154cb66dd2af_l3.png "Rendered by QuickLaTeX.com")
Where:
- = Local (or inlet) pressure (Pa)
- = Vapor pressure of the fluid (Pa)
- ρ = Fluid density (kg/m³)
- 𝑣= Flow velocity (m/s)
Interpretation of Cavitation Number:
High σ (σ > 1) → Low risk of cavitation
Moderate σ (0.1 < σ < 1) → Possible cavitation, depending on system conditions
Low σ (σ < 0.1) → High risk of cavitation formation
How to Predict and Prevent Cavitation?
Predicting cavitation is crucial to prevent damage and inefficiencies in hydraulic systems, pumps, and turbines. Several methods help in identifying potential cavitation risks:
Cavitation Number (𝜎):
A dimensionless parameter that indicates the likelihood of cavitation by comparing pressure differences in a system. A lower cavitation number suggests a higher probability of cavitation.
Cavitation Computational Fluid Dynamics (CFD) Simulation:
Advanced tools like ANSYS Fluent simulate flow conditions, pressure drops, and cavitation-prone areas. Figures 4 and 5 showcase some of CFDL’s website products. These products can help you learn how to solve problems related to the cavitation phenomenon using ANSYS Fluent software.
- Cavitation in Orifice Plate CFD Simulation, ANSYS Fluent Training – Learn how to simulate cavitation in an orifice plate using ANSYS Fluent, covering key parameters and best practices (Fig.4).
Figure 4- Cavitation in Orifice Plate CFD Simulation
Cavitation around Wedge CFD Simulation, Numerical Paper Validation – A detailed CFD simulation of cavitation around a wedge, validated with numerical research for accuracy (Fig.5).
Figure 5- Cavitation around Wedge CFD Simulation
Pump Performance Curves:
Manufacturers provide Net Positive Suction Head (NPSH) values to ensure that the pump operates above the cavitation threshold (Fig. 6). During a transient event, pressure decay can occur, which may lead to cavitation. Cavitation happens when the NPSH available (NPSHa) is less than the NPSH required (NPSHr).”
Figure 6- The relation between Cavitation and the NPSH of a pump
Acoustic Monitoring:
Cavitation generates high-frequency noise and vibrations, which can be detected using specialized sensors (Fig.7).
Figure 7- A typical application using a 4-20mA sensor.
Visual Inspection and Bubble Detection:
Transparent test sections and high-speed cameras can help observe cavitation bubble formation in experimental setups (Fig.8).
Figure 8- A schematic of the experimental setup for observing the behaviors of a spark-induced cavitation bubble near a soft membrane
Cavitation in ANSYS Fluent
In ANSYS Fluent, three cavitation models are available:
- Singhal et al. Model: Also known as the Full Cavitation Model, this model allows cavitation effects to be incorporated into two-phase flows when using the mixture model.
- Zwart-Gerber-Belamri Model: This model is compatible with both the mixture and Eulerian multiphase models.
- Schnerr-Sauer Model: As the default cavitation model in ANSYS Fluent, it can be used with both the mixture and Eulerian multiphase models.
Cavitation models can be used to achieve the following:
- The Singhal et al. model can account for the effects of non-condensable gases in the flow, whereas the other two models do not consider this effect.
- The Schnerr-Sauer and Zwart-Gerber-Belamri models are compatible with all turbulence models available in ANSYS Fluent.
- Both segregated (Pressure-Based) and coupled solvers can be used with cavitation models.
- Cavitation models are fully compatible with dynamic mesh and non-conformal interfaces.
- Both the liquid and vapor phases can be compressible or incompressible. For compressible liquids, density is defined using a User-Defined Function (UDF).
Limitations of Cavitation Models
- The Singhal et al. model is limited to a single cavitation process, meaning only one liquid phase can undergo cavitation.
- To use the Singhal et al. model, the primary phase must be liquid, and the secondary phase must be vapor.
- The Singhal et al. model is not compatible with the Eulerian multiphase model.
- The Singhal et al. model does not support the LES turbulence model.
- As mentioned earlier, the Schnerr-Sauer and Zwart-Gerber-Belamri models do not account for non-condensable gases by default.
- It is recommended to avoid using the explicit VOF formulation in cavitation simulations.
- When using the implicit VOF multiphase model, if a sharp phase interface needs to be captured, turbulent effects can be disabled.
Important Considerations
- Numerical solution stability in cavitation simulations is highly sensitive. Factors such as high pressure differences between inlet and outlet, large liquid-to-vapor density ratios, and high phase change rates can negatively impact convergence. Additionally, inappropriate initial conditions often lead to unrealistic pressure fields, which may cause unwanted and non-physical cavitation regions in the simulation.
- The Singhal et al. model is not enabled by default. To use this cavitation model, the user must first activate the Mixture multiphase model, then enter the text command solve/set/expert and respond “yes” when prompted with “use Singhal-et-al cavitation model?”
Figure 9 shows the panel to setup the Singhal et al. model.
Figure 9- Singhal et al. model setting panel.
If the TUI command is not used, the setup panel turn out to be as shown in Figure 10. Then select the Cavitation at the Mechanism of Phase Interaction panel.
Figure 10- : Phase Interaction panel
Clicking ‘Edit’ allows the selection of the cavitation model (Figures 12).
Figure 12- Zwart-Gerber-Belamri model selection
Conclusion
Understanding cavitation meaning is crucial for industries relying on fluid dynamics, as it directly affects the efficiency and lifespan of hydraulic systems. Cavitation in pump systems is one of the most common challenges, leading to cavitation erosion, noise, and performance loss. Engineers and researchers continuously explore ways to predict and prevent this phenomenon using cavitation number calculations and cavitation devices to optimize fluid flow and minimize damage. By leveraging computational tools like ANSYS Fluent and improving predictive models, industries can mitigate the adverse effects of cavitation bubbles while harnessing its potential benefits. A deeper understanding of cavitation will lead to better system efficiency, reduced maintenance costs, and new technological advancements in fluid mechanics.
FAQs
- What is the meaning of cavitation?
Cavitation refers to the formation and collapse of vapor bubbles in a liquid when the local pressure drops below the liquid’s vapor pressure. It often occurs in pumps, turbines, and hydraulic systems, causing efficiency loss and potential damage.
- How does cavitation occur in a pump?
Cavitation in pump systems happens when the pressure at the pump inlet falls below the liquid’s vapor pressure, forming cavitation bubbles that collapse and cause cavitation erosion on impellers and other components.
- What is the cavitation number, and why is it important?
The cavitation number is a dimensionless parameter used to predict cavitation risk in fluid systems. A lower cavitation number indicates a higher likelihood of cavitation, helping engineers design systems to minimize its impact.
- How Can I Predict Cavitation?
- Cavitation Number (σ): A dimensionless parameter used to assess cavitation risk.
- Computational Fluid Dynamics (CFD) Simulations: Tools like ANSYS Fluent predict cavitation-prone areas.
- Acoustic Monitoring: High-frequency noise detection can indicate cavitation.
- How can computational tools help in cavitation analysis?
Software like ANSYS Fluent allows engineers to simulate cavitation in pumps and other systems, helping them predict cavitation-prone areas, optimize designs, and implement effective prevention strategies.
- What Cavitation Models Are Available in ANSYS Fluent?
- Singhal et al. Model (Full Cavitation Model)
- Zwart-Gerber-Belamri Model
- Schnerr-Sauer Model