6DOF wind turbine not rotating

[Daniel.dejan.gunde]

Hi,

I ran previously 2D cases for a wind turbine using 6DOF solver - 1DOF rotation without problems. For the 3D case the turbine doesn't want to rotate with a reasonable angular velocity. Normally the rotation is defined by UDF but even if I define it internally via UI the problem remains identical - the turbine rotates extremely slowly (in the order of 1e-04rad/s) despite monitoring a good amount of torque induced by aerodynamic forces. I isolated many possible problems to no success. I tested defining a similar torque via spring preload in the 6DOF Properties menu and the problem remained identical. If I define very large preload (100x larger) the angular velocity increases to about 1e-02 and stays there. Moment center and rotation axis vector seem to be correct because, if I define the movement outside 6DOF Dynamic mesh using mesh motion of the turbine cell zone, the rotation is calculated correctly.

Some more things I tested:

-Moving turbine center to the global center coordinate

-testing low and high values of moment of inertia

-imlicit update

-recreated a fresh cas. file while I imported only the complete mesh via Fluent meshing

-normaly the axis of rotation is tilted but I tilted the whole domain so that I could use a simple rotation vector (0, 1, 0)

To conclude, the turbine is moving extremely slowly despite correct fluid flow and torque calculation.

I hope you can help me this time.

Best regards

Daniel

 

 

[mpowens]

It should work. You can compute the torque about the turbine axis in the Fluent post processing making sure to get the axis specification correct. The turbine should spin up to a velocity such that the torque goes to zero. To see the initial torque, turn off the mesh motion and compute the torque on the blades. The torque determines the angualar acceleration not the velocity. By definition, the angular acceleration will be zero under steady state conditions though it may take a large number of time steps to reach steate state if such a steady state even exists.

[Daniel.dejan.gunde]

I think I'm seeing the inital torque the whole time as the turbine is near to stationary. The axis seems to be correct as if I use the same axis to "manually" rotate the turbine via mesh/zone rotation it rotates correctly.

[mpowens]

Could the constraints be limiting the motion such that the torquce about what the 6dof solver thinks is the allowed motion is greatly reduced. Are you setting them in DEFINE_SDOF_PROPERTIES? You can specify whether they are set in the global or local coordinates. You can change the local coordinates by setting the initial orientation of the rigid body in the dynamic mesh settings. The default is that the rigid body x,y,z directions point in the same direction as the global mesh x,y,z directions. I guess it doesn't apply for the remodelled domain with the rotation about (0,1,0), which should set

        prop[SDOF_ZERO_TRANS_X] = TRUE;

        prop[SDOF_ZERO_TRANS_Y] = TRUE;

        prop[SDOF_ZERO_TRANS_Z] = TRUE;

 

        prop[SDOF_ZERO_ROT_X] = TRUE;

        prop[SDOF_ZERO_ROT_Y] = FALSE;

        prop[SDOF_ZERO_ROT_Z] = TRUE;

[Daniel.dejan.gunde]

I checked those settings as well. Interestly, if I avoid UDF and set the motion inside UI it is the same. If I turn all zones to passive and apply a similar external torque via spring preload the result is also the same. Like there is some very strong brake resisting the rotation, because it is rotating, just very slowly.

[mpowens]

What are your dynamic mesh settings? Do you have a sliding/rotating mesh inside the background mesh. Typically there should be 2 dynamic mesh obejcts. The 2D blades of the turbine are set to be rigid body with 6dof turned on and pasive turned off. Then the 3D cell zone of the whole turbine region is set to be rigid body with 6dof turned on and passive turned on. The motion is computed from the forces and torques only on the dynamic mesh 6dof objects in which passive is turned off. Any other obejct can be made to move the same way by turning on passive.  

[Daniel.dejan.gunde]

They seem to be correct and they are also identical to my 2D cases. I think the only thing I haven't tested yet is to start from scratch, from a completely empty .cas file in case that an error involving 6DOF settings managed to be imported through Fluent meshing. The domain and the mesh is quite complex therefore I started over by exporting the .cas file to Fluent Meshing, saving, and importing back to Fluent. My hope was that all data except the mesh was lost in the process.

[mpowens]

Fluent meshing cas files can contain solver data as well. If you turn on beta features you can revert from the solver back to Fluent meshing with a TUI command and then switch back to the solver and it will preserve most settings. You tend to lose the mesh interfaces which need to be re-created but most other settings are preserved. It may be worth trying to create a fresh case from scartch incase some settings got imported incorrectly.  Sometimes the order in which dynamic mesh objects are created can be important especially if relative motion is turned on.

[Daniel.dejan.gunde]

I created a completely fresh case and the identical problem persist. Now I ran out of ideas what I can test... Are there any known 6DOF solver problems?

[mpowens]

It should work. Have you set the momment of inertia tensor correctly? Try setting a non-zero value for at least all three primary axis values Ixx, Iyy, Izz even if it is constrained to rotate about one axis.  Not only should  the inertia tensor be symmetric positive but also its eignevalues must satisfy the triangle inequality: the sum of any two must be greater than the third.  We are not aware of any issures in respect of a rotation not resulting from a torque. We have run similar cases many times. Which version of Fluent are you running? There is a limit to how much help can be provided via the forum pages.