Learning Forum

[pohigor]

I am currently simulating a ship's turning circle using Aqwa Hydrodynamic Response. In my setup, I apply propulsion forces, drag resistance, maneuvering forces, and rudder forces as functions of the velocity vector using user-defined forces. My code receives the velocity vector at the center of gravity (CG) in the FRA reference frame. I then convert this vector to the LSA frame, calculate the forces at the CG relative to the LSA, and finally convert the forces back to the FRA frame before applying them to the model.

 

The Hydrodynamic Response module uses data from the Hydrodynamic Diffraction analysis, which is run for wave directions between -180 to 180 degrees and accounts for forward speed. During the simulation, the initial surge velocity and drag are zero, while the propulsion force is at maximum. After some time, the propulsion force balances with the drag, and the ship reaches its service speed. At this point, the rudder angle begins to increase until it reaches its maximum position. From here, the ship should start turning.

 

However, after running some cases in calm water (no waves), I noticed that the ship only develops a proper turning circle under one of these two conditions:

1. When using the NCNV option, or

2. When using the CONV option but running the Hydrodynamic Diffraction analysis without forward speed.

 

Otherwise, the ship starts to turn but eventually straightens out with a new heading, exiting the circle tangentially. This makes me wonder whether the Hydrodynamic Response reads the Diffraction data in the FRA frame, which does not rotate with the yaw angle. In this case, the direction of the radiation and damping forces might not follow the turning motion of the ship.

 

Has anyone done this type of simulation before? I would appreciate any insights or suggestions.

[Shuangxing Du]

For analyzing a turning circle maneuvering in wave, please do not define the forward speed in Hydrodynamic Diffraction.  The forward speed is for a ship travelling with a constant speed in the global x-direction. It is not the case for a turning circle maneuvering.

Please use Hydrodynamic Response - Time Response Analysis with Regular/Irregular Wave Response type, and switch on Include Maneuvering Force (See 13.10. Low Frequency Maneuvering Loads (ansys.com)).

[pohigor]

When the maneuvering hydrodynamic derivatives are known (added mass and potential damping linear terms related to yaw and sway motions: Yv_dot, Yr_dot, Nv_dot, Nr_dot, Yv, Yr, Nv, Nr), and the goal is to model the maneuvering forces using these coefficients, is it possible, during a hydrodynamic response analysis, to substitute these terms calculated in hydrodynamic diffraction with these known coefficients?

[Shuangxing Du]

Please see Aqwa theory manual Section 13.10, the maneuvering forces are calculated and included in the time domain response in irregular/regular wave.