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Cavitation Risk and Cavitation Index

Cavitation risk and Cavitation index are both triggered simultaneously by checking check box Cavitation risk in the physics section of the TCFD menu, see figure [*]. Both methods work as a post-processing, so triggering them on/off does not affect results. In order for this feature to work properly, user must fill in proper Reference pressure , Reference density and Reference temperature . Reference temperature is used for calculation of the saturated vapor pressure using Antoine equation (see [18]) with coefficients taken from [19]. Reference temperature is expected to be in Kelvins. Most incompressible calculations are done with zero pressure at the outlet. Reference pressure is way to offset the pressure to real physical value. Basically, there are two possible setups:
  • Pressure at outlet is set to zero. Reference pressure must be set to the value the flow will experience in reality, e.g. atmospheric pressure.
  • Pressure at outlet is set to the value the flow will actually experience. Reference pressure should be left zero.
Reference pressure is expected to be in pascals. Reference density is for proper conversion between dynamic and kinematic pressure. Cavitation risk evaluates saturated vapor pressure of water and then marks all cells, where the pressure drops under the saturated vapor pressure level. The result can be visualized by seeing cavitation field. Cells with value $ 0$ are above saturated vapour pressure level, cells below that level have value $ 1$. TCFD also evaluates volume of all cells with cavitation risk and surface of rotating parts with cavitation risk. Both those quantities can be seen in report, see example graph in figure [*].

Figure: Example of Cavitation risk graph from report.

Turbomachinery CFD Cavitation flowRateVsCavitationRisk

Cavitation index is standard measure of cavitation calculated by formula

where $ p$, $ \rho$ and $ U$ are upstream values of pressure, density and velocity, all evaluated at inlet, $ p_V$ is saturated vapor pressure. For more on cavitation index, refer to [6].

Figure: User can switch on and off both "Cavitation Risk" and "Multiphase cavitation" in the physics section of the menu.

cfd processor TCFDSource physical settings incompressible 1