Learning Forum

[Pegah Mehrabian]

Hi all,

I am trying to simulate fluid-structure interaction in 2D using ANSYS transient Structural and CFX. One can see the tube which should have one head attached to a spring and the other one free (simple mass-spring problem but in 2D). The tube is allowed to move just in the transverse direction. I consider "Elastic support" for the attached head. The problem is the unit of spring stiffness that I need to apply is in N/m but "Elastic support" has Foundation Stiffness and the unit is in N/m3. How should I use Foundation stiffness? considering that in reality, the tube has a length.

Thanks for your thoughts in advance.  

[mjmiddle]

If you don't understand the elastic support you can assign a body-to-ground spring which is similar. The elastic support only has spring stiffness in the normal direction for each element. A spring has direction from the ground location to the remote point on the other side of the spring. Both of these methods can have changing direction as the model deforms.
Your model is 3D. Do you mean the motion is 2D? The Ansys documentation states the elastic support is "spring behavior" and the foundation stiffness "is defined as the pressure required to produce a unit normal deflection of the foundation":

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v231/en/wb_sim/ds_Elastic_Support.html

For a spring, f=kx, but it is dealing with pressure so p=kx. For units: [N/m^2] = k * [m]. This means the foundation stiffness, k, must be units of [N/m^3] to produce a pressure. In 3D the foundation stiffness can ony be applied to a face (edge in 2D). If you know a force per unit length deflection, you will have to account for area of the face selected to apply the foundation stiffness: p=f/a. You can select the face, and Mechanical will show the area at the bottom of the GUI. k=p/x, so just set k=p with the knowledge it is units [N/m^3] since x=1 for one unit deflection.
For 2D plane stress, this is thin assumption, and you enter a body thickness. Applied pressure, which must be applied to an edge, uses the assigned thickness and the edge length to compute on an area.
For 2D plane stress, this in infinite length assumption, but analysis is actually run on a thickness of one unit length. Applied pressure uses the edge length and one unit thickness to compute on an area.

[Pegah Mehrabian]

Thank you for your helpful reply. My model is 3D with one element thickness just for the sake of CFX. As I don't have a force per unit length of deflection for this case and I just have a spring stiffness, I am not sure how much it is appropriate to consider elastic support in this case considering your explanation.

If I consider a body-to-ground spring, I just need to add the spring stiffness [k/m], right? but the problem arises when I need to apply the damping ratio. So in analysis settings >>damping control you need to add two numbers in order to apply the damping ratio: Frequency and damping ratio. If I add natural frequency in here with a damping ratio and apply a body-to-ground spring for connections, It seems I consider the spring stiffness twice. I am getting confused now. Please help me to find out.

[mjmiddle]

The spring stiffness is force per unit length.

[Pegah Mehrabian]

It was a typo, sorry, Do you have any idea regarding my problem?

[mjmiddle]

So you can use the elastic support or spring. I was just telling you how to compute or specify the stiffness on the same basis -- knowing a force per unit length stiffness. I am not so good at answering the damping question. Maybe you can make a new forum post since it's a different question. Damping can be applied many ways. There are material damping properties, global damping, and local damping like the damping coefficient on the spring. If you apply damping through the Analysis Settings, this is global damping. With the spring damping also, it should be applying additional damping upon the global damping, but I haven't tested it. Refer to the Mechanical help page first for the damping in the analysis settings:

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v231/en/wb_sim/ds_damping_controls.html

When specifying by critical damping ratio and frequency, this uses DMPRAT command, so you can then go to the APDL help:

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v231/en/ans_cmd/Hlp_C_DMPRAT.html