This functionality requires the Premium version of Orca3D Marine CFD (Simerics-MP).

To perform a roll decay analysis you need to start with an existing Orca3D Marine CFD analysis created from Rhino/Orca3D. This can be either a towed resistance or self-propelled simulation. Remember to select the option to perform an "asymmetric" analysis since a rolling simulation does not have port/starboard symmetry in the flow.

If you are simulating roll damping with forward speed, you will need to first run the simulation without roll until the simulation reaches a steady state. If you are simulating roll damping at zero speed, you can skip this step and go right to the next step.

To convert the simulation to a roll decay simulation, follow these steps:

1. Add the roll degree of freedom to the simulation. This is done by selecting the "Marine" module in the Model panel and then changing the "Analysis Type" to "User Analysis." Then modify the Dynamic Option to enable Heave, Roll, and Pitch as shown below.

2. You will also need to change the "Numerical Option" to one of Transient settings like "Medium (Transient)" or "Conservative (Transient)" to accurately capture the dynamic behavior. In the image above we used Medium (Transient).

3. Next you must remove all of the numerical damping so that it does not affect the dynamic behavior. Set all of the numerical damping to zero by selecting each of the dynamics modules and setting the corresponding damping coefficient to zero as shown below for marinePitch (do same for marineHeave and marineRoll). Then save this simulation with a new name.

4. If you are simulating a zero speed roll damping analysis, you can simply give the simulation an initial roll velocity like shown below where we set the inital velocity to 15 deg/s. Then click Start to start the simulation and the rolling should begin using the initial velocity.

5. If you are doing a roll damping with forward speed, you can take either of two approaches:

(a) In the first approach, you would load the results from the simulation without roll as the starting point. Then set the initial roll velocity to the desired value such as 15 deg/sec as shown earlier. To make this approach work, you must choose the simulation option "Start from Solution" before clicking the Start button. This sets the simulation time back to zero so that the entered "Initial Angular Velocity" is applied. If you just did a Continuation Run here there would not be any roll velocity applied. After choosing this option click Start to run the roll decay simulation.

(b) In the second approach, you load the results from the no-roll simulation, but you choose "Continuation Run" as the starting condition. In this approach instead of an initial roll velocity you will apply an external roll torque to the model for a specified amount of time. To do this add an expression for the External Torque option in the marine module as shown below. This would be an "X" torque and it would only be applied for a few timesteps before turning it back to zero. In this example the X torque expression might be defined in the Expression Editor as something like:

xTorque = time <= startTime + deltaTime ? rollTorque : 0

where startTime is the beginning time of the continuation run, deltaTime is the amount of time in seconds you want to apply the external torque, and rollTorque is the magnitude of the roll torque in N-m in this case. Click Start to run the roll decay simulation.