The simplest way to model waterjet propulsion in Orca3D Marine CFD is to use a force vector as your propulsion source. This is very easy to do using the following steps:
- Set up your CFD problem in Rhino/Orca3D as a “Resistance” simulation. For the “Simulation Speed” use a value that is representative of the speed you expect the model to reach. It does not need to be precise, only close enough for the marine template to make estimations for the required time step, similar to the “Nominal Speed” you would use in a powered simulation.
- Once you are in SimericsMP, select the "Marine" module in the Models window. In the Properties window you will see the "Propulsion Option" is set to "Prescribed Profile" as shown below for a 25 knot simulation.
- Change the Propulsion Option to "Propulsion Source" as shown below.
- Set the propulsion source to "Force Vector" by clicking on the Propulsion Source dropdown where it says "No" and clicking "Add Force Vector" as shown below.
- Now set the "Position" of the waterjet force vector by specifying the x,y,z values as well as the "Direction" of the force vector by specifying the x,y,z components. Note that the direction is the direction of the force imparted on the fluid. This will be opposite the direction of the force imparted on the model. Finally enter the value of the waterjet thrust in the "Force Expression" noting the input units. If desired the Force Expression can be something more complicated than a scalar value. For example, it could be a value based on the simulation time such that the thrust is gradually increased to a target value.
One issue to consider when modeling waterjet propulsion is whether or not to model the waterjet tunnels in the simulation. The answer to this depends on the context of the problem being solved. For small to mid-size boats where the volume of the waterjet tunnels are relatively small compared to the vessel displacement, and where the waterjets are being provided as part of an integrated system in which you are not able to obtain the geometry of the tunnels, it is probably ok to neglect the flow in the tunnels for basic studies. However, for larger vessels where the tunnel volume is large and the hydrodynamic impacts of the fluid flow through the tunnels can have a significant effect on vessel dynamics the inclusion of the tunnel geometry is important.
It is also possible to perform a higher fidelity analysis of waterjet propulsion by modeling some of the internal geometry of the waterjet such as the rotor and stator hub. In this scenario, the propulsion source would be a "Propeller Model" placed at the location of the waterjet rotor. A "Uniform Thrust" model would be selected instead of the typical "Hough-Ordway Model" and the propeller and hub diameter would be selected such that the propeller model fits in the annulus between the tunnel wall and rotor hub. The assignment of the thrust value here becomes an iterative computation, because given a specified propeller thrust the net thrust produced by the waterjet will be less due to losses in the tunnel. This higher fidelity approach is generally not needed for most designs, and the earlier approach discussed above is sufficient.