## General Mechanical

#### Torsional Vibration

Subscriber

How can I model the torsional stiffness and damping equation (in the fig) in Transient Structural Solver? Also how can I specify the damping values?

• peteroznewman
Subscriber

Please insert images directly into your posts because ANSYS staff are not allowed to open attachments. I am not staff.

You build numerical models in ANSYS, so you need to provide numbers for J, k, etc. There is a spring model that can include damping.

Other than the spring, is the mass rigid or flexible?  If it is rigid, you could build the model in Rigid Dynamics.

Or when you talk about torsion, do you mean a solid shaft that is itself the spring to which the spring constant k is referring?

Please expand on what you are trying to do.

Subscriber

I want to simulate the angular rotation of a tracker about z-axis, hinged at (0,0), due to fluid forces. I have a plate fixed over a torsional shaft as shown. The flat plate has a mass moment of inertia about the rotation axis of J= 260 kgm2, The shaft has a spring stiffness of 6 KNm/rad. I know the values of J,K,damping ratio. Not interested in stress, strains, etc.I am interested in only rigid body rotation. I want to plot the angular displacement wrt time due to fluid forces.

For this I want to couple CFD with APDL. I am ok with CFD but the APDL part is unclear.

How to build the equation of motion model in Rigid Dynamics? Where to insert torsional equation and the values of J,K, etc?

The CS view is shown here.

• peteroznewman
Subscriber

I drew a plate, but as you can see, the value of J is way too small. It would be easy if you have the dimensions of the plate.

I can add a Point Mass and assign the value of J, but that is about the point, not the Z axis.
It would be better to know the coordinates of the plate relative to (0,0) and calculate J around the origin.

A Revolute Joint is defined between the plate and ground. Here the value of k and damping can be defined.

The center of rotation of this joint is (0,0).

Apply a force for 10 ms, then let it ring for 5 seconds.

Subscriber

Thanks for the reply. Actually I want to simulate Fluid Structure interaction in 2-d. But the fluent solver doesn't support 2-d fsi. For that I modeled the geometry with 1 mm thickness in the z-direction. Also in Fluent I have an option for inserting the moment of inertia and the spring stiffness about the z-axis. The concept is to simulate the transient fluid forces which rotates the body about z-axis. It would be helpful if you describe this setting.

• peteroznewman
Subscriber

I'm a beginner with Fluent and FSI, but if you ask a specific and detailed question and insert images to help explain your question, you may get an answer from an expert.

Subscriber

Ansys doesn't support coupling Fluent with Rigid dynamics. So I am using transient structural. Can you model the same in transient structural and share screenshots?

• peteroznewman
Subscriber

The screen shots are exactly the same as I have above, just start with a Transient Structural model.

Subscriber

Hey thanks a lot!

One more help needed though!

I am using Transient structural to model rigid body rotation using FSI. Not interested in stresses or strains.

I have set the material properties of the plate such that Ip3 shows 259 kgm2 (I believe it is the mass moment of inertia about z-axis!). Doing so, I am not using any point mass. Now by customizing mat properties, I also get large values Ip1 and Ip2. I believe these are mass moment of inertia about x and y axes respectively. In such a case, the plate rotates about x and y also and displaces.

Can you specify which constraint or fixity conditions to be used to get only rotation about z-axis and no displacements in x,y,z or rotation in x,y?

• peteroznewman
Subscriber

Add a Revolute Joint to ground. You can edit the Z axis to point in the direction you want and put the origin where you want it.