Cavitation is the evolution of vapor and/or gas bubbles
within liquid in the regions of low pressure in the flow, or due to heating
that raises the vapor saturation pressure. The sudden appearance and subsequent
collapse of bubbles may cause large oscillations of pressure within
incompressible fluid that in turn result in severe mechanical damage to the
surrounding structures.
Advantages and disadvantages
Cavitation can cause damage to turbines and pipes, erode
concrete from the dam spillways, etc.
Figure 1 shows the erosion of concrete near the bottom of the spillway in a
dam. Concrete used in dams is typically high strength but cavitation can still
erode it.
Figure 1. Eroded concrete due to cavitation on the spillway of a dam
In high pressure dies casting, die erosion can occur where
fast movement of the molten alloy through constrictions and curves in the die
results in rapid pressure drops and lead to cavitation. The resulting vapor
bubbles can lead to porosity in the final casting, or worse, lead to damage of
the die, contaminating the casting and leading to early die failure.
Cavitation is sometimes intentionally induced for certain
industrial applications like water purification by breaking down the pollutants
and organic molecules, joining hydrophobic chemicals, destroying kidney stones
through shock waves created due to implosion of cavitation bubbles, increasing
turbulence for mixing, etc.
Therefore, it is critical to understand where cavitation is
likely to occur, and how intense it is likely to become. Since initiating and
visualizing cavitation experimentally is difficult and can be potentially
damaging, it is important to be able to simulate the process.
Modeling cavitation
in FLOW-3D v11.1 and FLOW-3D Cast v4.1
The Cavitation model has been successfully used to simulate cavitation in thermal
bubble jets and MEMS devices. In the new version, the model has been upgraded
for even better accuracy. FLOW-3D v11.1 and FLOW-3D
Cast v4.1 provide a better estimate of the location and amount of cavitation
in the computational domain.
Users have the choice to choose between Simplified model and Empirical
model. The former is controlled by a user-defined characteristic time for
nucleation of bubbles, while in the latter the nucleation of bubble is
controlled by the by local turbulence. Opening of the actual cavitation voids
can be controlled by selecting Passive
model (voids are not opened) or Active
model (voids are opened). Passive model is best for simulations where the
brief appearance of small bubbles is expected, while the active model is best
for cases where larger cavitation regions are expected that will significantly affect
the flow field.
A new variable called Cavitation
gas volume fraction has been added to the model and can be used to
visualize cavitation.
Sample simulations
Simulation 1 shows a constricting nozzle. The animation shows
the evolution of cavitation bubbles demonstrating a highly transient,
oscillatory behavior. The cavitation volume fraction is plotted to visualize the
onset of cavitation in the initially continuous liquid.
Simulation 1.
Cavitation in a constricting nozzle
Simulation 2 shows cavitation within a venturi with an entry velocity of 8m/s, a convergent slope of 18°,
and a divergent slope of 8°. Again, the transient behavior of cavitation is
well modeled, with the model predicting a cavitation cycle period of 17.4ms
compared with the experimental result of 22ms.
Simulation 2.
Cavitation in a venturi
Labels: cavitation, constricting nozzle, FLOW-3D Cast v4.1, FLOW-3D v11.1, gas volume fraction, nucleation, software development, vapor saturation pressure, venturi
