Orca3D Marine CFD automatically adds external forces and moments to your simulation to account for things like gravitational force and propulsion forces from a user-defined propeller. However, sometimes you might want to include additional forces to your model to account for specific situations, such as when your vessel is towing another vessel. Note that in this context we are not talking about a "towed simulation" which refers to a resistance simulation in which the speed of the vessel is specified. We are instead considering a simplified approach to consider the effects of the vessel being analyzed towing another vessel without specifically including the towed vessel in the analysis.
SimericsMP provides two methods to add external forces/moments to your simulation. The first is through the use of the "External Force" and "External Torque" options in the Marine template. As described elsewhere, the user can specify an External Force and/or External Torque directly in the Marine module settings when the "Extended Mode" Setup Option is selected as shown below. Both of these inputs are vectors, having x, y, and z components. These vectors are specified in the inertial (i.e., world) coordinate system and are especially useful when the force and moment are known relative to that system. The main limitation of this method is that you can only specify a single force and single moment, so if there are multiple external forces/moments they must be summed to get the net force/moment.
The second method is through the use of Propulsion force vectors. These can be added through the Marine module under the Propulsion Option where the user can choose to add one or more Propulsion Sources as Force Vectors as shown below. Here the user can add multiple vectors and since the force vectors include a magnitude, location, and direction any moment is implicitly included in the simulation. Furthermore, these force vectors remain rigidly connected to the model as dynamics are experienced. Clearly this method is more suitable to represent a propulsor which moves with the model during pitch and heave dynamics.
In the case of a simulation of a vessel towing another vessel, where the tow force has been estimated, the second approach seems better suited since the towing bit is rigidly connected to the vessel, moving with the vessel as it experiences any model dynamics. The location of the towing force will be at the location of the towing bit, and the magnitude will be the estimated towing force. However, contrary to the case of a propulsor on the vessel, the direction of the towing force relative to the body will vary as the boat heaves and pitches (both in body and world coordinates). To account for this effect, we need to create expressions in the Expression Editor representing the direction of the tow force. The image below shows the expressions used to compute the direction of the tow line as the vessel heaves and pitches as well as for the towing force. Note how those expressions are used to specify the direction of the force vector, and keep in mind that a towing drag force like this will have it's x-component as a positive number (as opposed to a propulsion force which has its x-component direction as a negative value).
It is important to note that the ability to specify the direction of a Propulsion Source force vector as an expression that varies with time requires the Premium version of Orca3D Marine CFD.
