Cavitation—Introduction
Cavitation is the phenomenon where vapor cavities, which are small and mostly liquid-free zones known as bubbles or voids, are generated in a liquid due to the imbalance of the local dynamic forces. This usually occurs when a liquid is subjected to rapid changes of pressure under isothermal conditions. An example is if the pressure falls below a threshold (saturation vapor pressure), the liquid would rupture and form vaporous cavities, while the voids would implode (bubbles collapse) and generate intense shock waves when the vapor bubbles are subjected to a pressure higher than the threshold pressure.
For a liquid flow, its tendency to cavitate is characterized by the cavitation number, which is given below:
equation 2.157
where p is the absolute value of the flow reference pressure, such as inlet pressure; p
v is saturation vapor pressure, which is a material property depending on the temperature and pressure; the denominator represents the flow dynamic head in which ρ
l is the liquid density, and U
∞ is the free-stream velocity. Therefore,
equation 2.157 indicates that as the cavitation number decreases, a liquid flow more likely tends to be cavitated.
Steady and unsteady cavitating flows can occur in many fluid engineering systems such as fuel injectors, liquid pumps, propellers, impellers, hydrofoils, hydrostatic bearings, and bio heart valves. Cavitation is often an undesirable occurrence. It can cause significant degradation in performance, as manifested by reduced mass flow rates, lower head rise in pumps, load asymmetry, vibration, and noise. Cavitation also causes physical damage to a device due to bubble impact on surfaces, which can ultimately affect structural integrity. To minimize cavitation or account for its presence, you must be aware of the existence and extent of cavitation during the initial design stages. Therefore, it is of importance in CFA to provide an accurate and reliable modeling capability of cavitation. Creo Flow Analysis offers a complete cavitation module along with customized tools (templates), for the simulation of cavitating flows occurring in a wide range of fluids systems.
This topic describes the modeling theory and the cavitation models used in
Creo Flow Analysis. The model parameters and settings, the work flow, and the post-processing quantities are also described. Since cavitation is a thermal phase change process between the liquid and vapor phases, it is modeled as an interface mass transfer under multiphase flows. In
Creo Flow Analysis, however, cavitation is modeled independent of the
Multiphase module. To access this module, follow the steps listed below:
1. Click
Physics Module. The
Physical Model Selection dialog box opens.
2. Select the module from Available Modules. In the Flow Analysis Tree, under Physics, Cavitation is added.
See below for a description of items in the Cavitation module:
• Physics—Definitions, terminology, constants, models and methods used in the cavitation module.
• Conditions—Conditions specified for the entities such as boundaries, interfaces, volumes, and the outputs from the module.
• Numerics and Convergence—Parameters and models controlling the numerical solution.
• Output Variables—Expressions created for specifying conditions and postprocessing associated with cavitation.
