[butterflymuzzy]

I am continuing my studies on ship maneuvering with Ansys CFX. The short question I will ask you will be the following. While the ship is making the sinusoidal sway and yaw movement, I want to see the force x,y and torque Z generated on the ship instantaneously from the bow direction of the ship. So I want the axis to move along with the ship. How does the added axis move together with the ship at the same time ? Thank you very much.

[Gert-Jan]

In Pre, you can generate a coordinate frame from a surface that is part of your boat. You can determine the forces and torques along one the main axes of this coordinate frame (instead of the global coordinate frame).

[butterflymuzzy]

How can i move the added axis with the ship at the same time? Because I always want to see the force from the bow side of the boat, other wise if the axis does knot turn with the hull, the forces that I take from the hull won’t yaw or sway force. They will be drift forces. But I want to see the yaw forces each time from the bow side of the hull.

[Gert-Jan]

If I'm correct, the coordinate frame will move with your surface as long as the surface is part of your boat.

[butterflymuzzy]

I think no. If I add new a new coordinate system. And if I monitor my hull surface’s x-y forces and z moments with this coordinate system, I got all the results from “fixed coordinate system”. Hull is mowing, no problem. But the axis couldn’t make yaw or sway movement with the hull. For instance in the yaw movement, the ship can yaw very well. But the coordinate system which I add custom to this hull surface could not move at the same time with hull. May be Glenn or Opaque can answer this 🙏🏽

[ghorrocks]

If I could answer it I would have posted something. My silence represents my ignorance.

If the coord system does not move with your boat then try some work-arounds:
* If the motion is pre-defined then you know the position of the boat, so you can take the forces in the global coord system and do the transform yourself. OR
* If the motion is not pre-defined (eg rigid body motion) so you do not know the position in advance then you can access the variables which have the Euler angles of the body. The you can do the transform yourself from the forces in the global coord system.

[butterflymuzzy]

Glenn you are right. In the rigid body hull motion Side There is no problem. Because I put the fixed coordinate frame to the longitudinal center of buoyancy of the hull, then I can easily see the sinkage and trim motions of the hull from the rigid body monitors and force x or y and torque Z.

But in the maneuvering side of the ships, literature uses adaptive grid refinement technic for this type of problems. In the adaptive grid refinement hull goes with coordinate frame whether yaw or sway motion. Cfx does not have adaptive grid refinement ability. So what I want to do is I can yaw or sway my hull with “specified location” mesh motion technique. But the coordinate frame could not follow the hull at the same time.

May be my mistake is I m choosing axis points when I put the coordinate frame. If I choose “point and Normal” Option and choose the face of the cylinder moving domain, and click the “frame motion-stationary” option, coordinate frame can move with hull ??? 🤯

[Opaque]

I do not think this can be done out of the box.

Perhaps just evaluate the forces in a fixed frame, and then convert them into "your local frame" via expressions

ForceMyLocalX = a * force_x().. + b * force_y().. + c * force_z()..
ForceMyLocalY = d * force_x().. + e * force_y().. + f * force_z()..
ForceMyLocalZ = g * force_x().. + h * force_y().. + l * force_z()..

Where the coefficients are the entries of the rotation matrix from fixed to local, i.e.

{ForceMyLocal} = [Fixed_2_MyLocal} {ForceFixed}