This article is intended to clarify the pressure definitions used by Orca3D Marine CFD.
As shown in the image below, when plotting pressure distributions in the 3D graphic views, there are 3 pressure variables available. "Pressure" from the Flow module under Variables is the pressure you would measure with a pressure tap, sometimes called gauge pressure. In OMCFD atmospheric pressure is defined as zero so the Flow module Pressure is the pure fluid pressure (dynamic plus hydrostatic).
"Dynamic Pressure" from the Marine module under Derived Variables is defined as the (gauge) Pressure minus the hydrostatic pressure. The Marine module definition of Dynamic Pressure can be both positive and negative (suction). The hydrostatic pressure is referenced to the initial static waterplane. In other words the dynamic pressure is the non-hydrostatic fluid pressure.
"Total Pressure" from the Flow module under Derived Variables is defined as the (gauge) Pressure + 0.5 * rho * V^2. Although perhaps of limited applicability in marine CFD, it can be relevant in other flow problems like internal pump flows. In the case of OMCFD, the reference for velocity here is the inertial coordinate system so the vessel is assumed to be moving at the forward velocity that you can plot.
"Dynamic Pressure" from the Marine module under Derived Variables is defined as the (gauge) Pressure minus the hydrostatic pressure. The Marine module definition of Dynamic Pressure can be both positive and negative (suction). The hydrostatic pressure is referenced to the initial static waterplane. In other words the dynamic pressure is the non-hydrostatic fluid pressure.
"Total Pressure" from the Flow module under Derived Variables is defined as the (gauge) Pressure + 0.5 * rho * V^2. Although perhaps of limited applicability in marine CFD, it can be relevant in other flow problems like internal pump flows. In the case of OMCFD, the reference for velocity here is the inertial coordinate system so the vessel is assumed to be moving at the forward velocity that you can plot.
If you are interested in the absolute pressure, add 1 atmosphere to the Pressure from the Flow module (under Variables). This will then include atmospheric pressure, hydrostatic pressure, and dynamic pressure.
To look for pressure that is low enough for water to flash to vapor, you would plot the Pressure from the Flow module under Variables. If the vapor pressure at the temperature of interest was 2290 Pa and the atmospheric pressure is 101325 Pa, you would look for Pressure that was below (2290-101325)= -99035 Pa.
Note that if you are specifically interested in the potential for cavitation, experience has shown that there are really two significant parts to cavitation inception. The first is when the pressure on the model drops below the local vapor pressure of water, and the water flashes to vapor. The second, which we have often found happens before the vapor flash and can generally be responsible for the majority of the cavitation, is related to dissolved air in the fluid coming out of solution.
Orca3D Marine CFD provides two approaches to evaluating cavitation inception, both require the Premium version.
1. In the first approach, there are some settings within the Marine template, that will set a reference pressure and volume fraction and apply the Ideal gas law to evaluate cavitation. This approach only simulates the cavitation due to dissolved air in the fluid coming out of solution, but our experience has been that this is often the vast majority of the cavitation that occurs.
2. The second approach involves using the dedicated "Cavitation Module" within Simerics. This dedicated module accounts for both the dissolved air and the vaporization of the fluid. However, it cannot be used simultaneously with the Marine template, and must be obtained as an additional module. This means you need to create your simulation without the Orca interface.
