ship motion control with cfx

ehsanspp63 · November 22, 2018, 15:10

hi everyone,
I am going to simulate a towing tank test on a ship (in regular
waves) provided with an active controller to reduce heave and pitch motions using a T-foil installed on the aft section of the ship. I have carried out lots of ship simulations using cfx but in this case, I don't know how to include controller operations in simulation to change T-foil's angle of attack during the simulation to improve heave or pitch motions.

urosgrivc · November 23, 2018, 01:00

if you can prescribe motion of your controller to be the same as the motion of the ship than you can add or subtract some prescribed relative movement to the controller maybe.
didn't try it yet but should work I think.

ghorrocks · November 23, 2018, 03:21

Yes, theoretically you can implement a controller in CFX, but they are not simple to implement and are even harder to make stable. A better approach is often to model a range of positions (heave, pitch etc) to get a buoyancy and drag force map and then use a simple time integration scheme using that force map outside of CFX to model the motion. In this case your CFX models are simple fixed position models which I think you will find are complex enough in themselves.

ehsanspp63 · November 23, 2018, 06:03

Dear urosgrivc and ghorrocks,
thanks a lot for your quick responses and valuable information. please correct me if I am mistaken.
I think it can be done if i inter mathematical functions of the controlers to the cfx as expressions (ccl) and then in each time step provide mathematical ccls with controler inputs (heave or pitch velocities), which had been drived at that time step, and extract controler output (fin angles) from mathematical functions and finally prescribe these angles for fins at the next time step using moving mesh or remeshing. in this strategy simulation should be adequately converged in each time step.

urosgrivc · November 23, 2018, 06:18

Yes I was thinking something like you have described now. expressions plus mesh deformation (displacement).
Some care vould have to be taken so the mesh dosent colapse near the boat
I probably do not understand your problem completly so this is more of a sugestion

November 22, 2018, 15:10	ship motion control with cfx	#1
ehsanspp63 New Member ehsan Join Date: Nov 2018 Posts: 4 Rep Power: 7	hi everyone, I am going to simulate a towing tank test on a ship (in regular waves) provided with an active controller to reduce heave and pitch motions using a T-foil installed on the aft section of the ship. I have carried out lots of ship simulations using cfx but in this case, I don't know how to include controller operations in simulation to change T-foil's angle of attack during the simulation to improve heave or pitch motions.

November 23, 2018, 03:21		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,719 Rep Power: 143	Yes, theoretically you can implement a controller in CFX, but they are not simple to implement and are even harder to make stable. A better approach is often to model a range of positions (heave, pitch etc) to get a buoyancy and drag force map and then use a simple time integration scheme using that force map outside of CFX to model the motion. In this case your CFX models are simple fixed position models which I think you will find are complex enough in themselves. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

November 23, 2018, 06:18		#5
urosgrivc Senior Member urosgrivc Join Date: Dec 2015 Location: Slovenija Posts: 365 Rep Power: 11	Yes I was thinking something like you have described now. expressions plus mesh deformation (displacement). Some care vould have to be taken so the mesh dosent colapse near the boat I probably do not understand your problem completly so this is more of a sugestion ehsanspp63 likes this.

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November 23, 2018, 01:00		#2
urosgrivc Senior Member urosgrivc Join Date: Dec 2015 Location: Slovenija Posts: 365 Rep Power: 11	if you can prescribe motion of your controller to be the same as the motion of the ship than you can add or subtract some prescribed relative movement to the controller maybe. didn't try it yet but should work I think.

November 23, 2018, 06:03		#4
ehsanspp63 New Member ehsan Join Date: Nov 2018 Posts: 4 Rep Power: 7	Dear urosgrivc and ghorrocks, thanks a lot for your quick responses and valuable information. please correct me if I am mistaken. I think it can be done if i inter mathematical functions of the controlers to the cfx as expressions (ccl) and then in each time step provide mathematical ccls with controler inputs (heave or pitch velocities), which had been drived at that time step, and extract controler output (fin angles) from mathematical functions and finally prescribe these angles for fins at the next time step using moving mesh or remeshing. in this strategy simulation should be adequately converged in each time step.