[nelos23]

Hi everyone,

I am trying to simulate a ship in self propulsion using DFBI Superposed Rotation. I started the simulation with a big time step(0.05s) to let the resistance converge and then I decreased the time step (0.0005s) to let solve the motions of the propeller. When I changed the time step the simulation huge osculations appeared in the signal. Am I doing something wrong? Also the sign of thrust change so negative to positive.
Im attaching some pictures so you can have a look.

Thanks in advance.

[Stefanos M]

Hi Andreas, I am reading your post as I am dealing with the same situation, did you find a way to overcome this behaviour, thank you very much!

[a.gmira]

Hi everyone,

I am also trying to simulate a ship in self propulsion using DFBI Superposed Rotation. I am using an overset mesh that encloses both, the ship and the propeller, for the ship motions (heave and pitch). However, I am having some trouble a few seconds after the release of the ship, the ship moves up too much (translation in Z), the drag becomes negative and the .log file gives me the following error message:

"Created cell set Overset_Acceptor_with_Zero_Donor_in_Overset.
Created cell set Overset_Inactive_Face_in_Domain 3.
Error 2 of 2: Found 124 Inactive Faces in Region "Domain". This might be due to too fine or too coarse mesh. Please check cells with Overset Error Status: 30.
Found Acceptors in Region "Overset" with zero donors. This might be due to too fine or too coarse mesh. Please check cells with Overset Error Status: 6. Subsequent errors are listed in the output window."

Does anyone have experience with this error?

Thank you very much in advance

[Stefanos M]

Hi A.gmira,

I have not used the overset mesh for SP, however, if the motion on the vertical axis is not normal you should recheck the settings of DFBI, especially the Initial Values.

Furthermore, in the Physics --> VOF --> HRIC, what values do you currently use ?


Stefanos M.

[Stefanos M]

Originally Posted by Stefanos M

Hi Andreas, I am reading your post as I am dealing with the same situation, did you find a way to overcome this behaviour, thank you very much!

UPDATE ON 12/12/2023

After reaching convergence in sinkage and trim, a good strategy is to fix the motions on DFBI and continue with propeller motion.

[a.gmira]

Hi Stefanos M,

Thank you for your response. The error I had was caused by some mistakes in the DFBI parameters of my Body.

However, do you know how can I monitor the rotation rate of my propeller? I only see examples of this in simulations with virtual disk, but nothing for a rigid body propeller

Thank you in advance.

[Stefanos M]

Originally Posted by a.gmira

Hi Stefanos M,

Thank you for your response. The error I had was caused by some mistakes in the DFBI parameters of my Body.

However, do you know how can I monitor the rotation rate of my propeller? I only see examples of this in simulations with virtual disk, but nothing for a rigid body propeller

Thank you in advance.

Do you use MRF or Sliding Mesh?

[a.gmira]

Hi Stefanos M,

I think I am using Sliding mesh, but I'm not sure. I have configurate my simulation from the one in Tutorials > Solid Stress > FSI and 6-DOF Motion: Stress Analysis on Boat Propeller (but I am not doing the Stress analysis)

Thank you for your response.

[Stefanos M]

Originally Posted by a.gmira

Hi Stefanos M,

I think I am using Sliding mesh, but I'm not sure. I have configurate my simulation from the one in Tutorials > Solid Stress > FSI and 6-DOF Motion: Stress Analysis on Boat Propeller (but I am not doing the Stress analysis)

Thank you for your response.

You can set & check the rotation rate:
1)MOTION->ROTATION for Sliding Mesh
2)REFERENCE FRAME -> ROTATING for MRF

Then in the relevant Region you should specify the above motion to use it.

[a.gmira]

Hi Stefanos M,

I am using sliding mesh definitely. I already have the rotation motion defined, what I need now is to get a report in order to monitor the rotation rate of my propeller.

Since I have the boat and the propeller all together in a 6-dof Body, if I go for a 6-dof report > Angular velocity, it gives me zero rad/s.

Do you know how can I monitor the rotation rate of the propeller ?

Thank you very much for your responses.

[a.gmira]

Hi Stefanos M,

I think I've explained my self badly. I only want to know how can I get a report of the rotation rate of my propeller.

This is because I want to define a function for the rotation rate of my propeller (in MOTIONS > Body 1-Motion > Superposing Motions > Rotation) that increments linearly until the thrust of the propeller reaches de drag of the hull, and from there it turns constant. However to create this function I need report or something to refer to the actual rotation rate of my propeller so that I can define de turn to constant rad/s when they are the appropiate ones.

Please let me know your opinion on this.

Thank you very much.

[Stefanos M]

Originally Posted by a.gmira

Hi Stefanos M,

I think I've explained my self badly. I only want to know how can I get a report of the rotation rate of my propeller.

This is because I want to define a function for the rotation rate of my propeller (in MOTIONS > Body 1-Motion > Superposing Motions > Rotation) that increments linearly until the thrust of the propeller reaches de drag of the hull, and from there it turns constant. However to create this function I need report or something to refer to the actual rotation rate of my propeller so that I can define de turn to constant rad/s when they are the appropiate ones.

Please let me know your opinion on this.

Thank you very much.

Hello a.gmira,

a) you must set up a new scalar parameter and rename this to RotationRate with Value in rps or rpm or radian/s
and Dimensions --> Angular Velocity

this parameter should be used as input in MRF or Sliding mesh --> ${RotationRate}

b) make a report --> Expression --> Definition ${RotationRate} and Dimensions --> Angular Velocity

[a.gmira]

Thank you, Stefanos M.

However, the scalar parameter doesn't allow me to put an expressión as an input. Dou you know if it is possible to do what I am trying to do? I want to define the angular velocity of my propeller as a function of the drag, in order to reach the equilibrium between drag and thrust in one simulation. By doing that, I will know the rotation rate of my propeller for a specific speed.

I know this is possible with the Virtual Disk (body force method) because it lets you define the operation point of the disk with the thrust. But how can I do this with the propeller as a rigis body?

Thank you again!

[Stefanos M]

Originally Posted by a.gmira

Thank you, Stefanos M.

However, the scalar parameter doesn't allow me to put an expressión as an input. Dou you know if it is possible to do what I am trying to do? I want to define the angular velocity of my propeller as a function of the drag, in order to reach the equilibrium between drag and thrust in one simulation. By doing that, I will know the rotation rate of my propeller for a specific speed.

I know this is possible with the Virtual Disk (body force method) because it lets you define the operation point of the disk with the thrust. But how can I do this with the propeller as a rigis body?

Thank you again!

Do a Google search for the paper below:

"Automated self-propulsion point search algorithm for ship performance
CFD simulations"

by Mika Nuutinen

[a.gmira]

Hi Stefanos M,

Thank you very much for help. I am trying to implement the algorithm of the paper above for Fixed Pitch Propellers (equation 10). I have generated one fild function for n_i and another one for n_i+1 (also for the rest of variables in equations 9 and 10).

n_i --> ${Iteration} == 1 ? 12 : ${n_i+1}

n_i+1 --> ${n_i} + (${Mean_R_iReport} - ${Mean_T_iReport}) / (2*${Beta_i}*${Mean_n_iReport}*${relax_T}) * ${deltaT}

When I run the first iteration every variable has its value and everything seems ok, but when I try to run the 2nd iteration the simulation stops and the following error appears:

Cycled detected in ...Field functions > n_i

I understand that I can't do a loop only defining variables because the the way it solves this is not structured, so I need to code a loop somehow. Do you know how can I generate a loop so that the value of n_i+1 becomes n_i in the next iteration to calculate de new n_i+1 ?

Thank you again!

[a.gmira]

Hi StefanosM,

Finally I solved the problem above. However could you tell me if it is necessary to use the morpher motion for the fluid region around the propeller (in the sliding mesh approach)?

Thank you in advance.