One question we are seeing more often is how to represent a propulsion system with contra-rotating propellers in a self-propelled simulation with Orca3D Marine CFD. As you probably remember, for self-propelled simulations Orca3D Marine CFD uses an approximate representation of the propeller effects called an "actuator disk" or momentum disk. This approximate representation does not require knowledge of the detailed propeller blade geometry, which is often not easily obtained. Instead it inserts momentum sources into the simulation inside an annular disk of the fluid matching the gross propeller geometry (e.g., propeller diameter, hub diameter, hub length) and matching the average propeller performance (producing the open water thrust and absorbing the open water torque). These momentum sources impart both an axial and rotational acceleration to the flow field around the propeller.


With this is background, one might wonder if you should use two actuator disks to represent a contra-rotating propeller system. While you could make the case that using two actuator disks with opposite rotational directions might be a way to try to simulate the net effects on the flow field behind the propeller, using two propeller models in this way can be a little tricky. In this arrangement, Orca3D Marine CFD is treating them as two separate propellers that happen to be placed one in front of the other, but otherwise are providing as much thrust from each propeller as if they were on two separate engines. While you could reduce the thrust of each propeller such that their total thrust matched that of the contra-rotating propeller system, an easier and more straightforward approach is to just use a single propeller model that represents the whole system. In other words choose a single propeller that will provide the same amount of thrust as the contra-rotating propulsion system.

It is worth noting that it is possible to simulate the flow around the real propeller geometry for contra-rotating propellers as shown in the example below. However, doing this type of analysis takes a fair bit of effort to set up (compared to the simple approach of using the actuator disk representation), takes a relatively long time to run, and requires the Premium version of Orca3D Marine CFD. Typically unless you are a propeller designer, the actuator disk model is much more practical.