## Ansys Products

Discuss installation & licensing of our Ansys Teaching and Research products

#### Modal analysis of simply supported beam with circular cross section

• Abhishek Chauhan
Subscriber

As I add the remote displacement on both ends  with x roations free and all are fixed or constant and the natural frequency is not accurate at mode 2.

• Dave Looman
Ansys Employee

So you are modeling a simply supported beam bending in the yz plane?  What is your theoretical mode 2 frequency?  What does the mode 2 shape look like?

• Abhishek Chauhan
Subscriber

This is the Ansys Value

Calculated value analytical

Ms excel is calculated from formula of different mode and i want this frequency

As the model is extruded z-plane and uses remote displacement with y rotation free and the rest are fixed.

Kindly give way to find the correct values of natural frequency and reply asap

• Dave Looman
Ansys Employee

The problem is you have a simply supported beam in the XZ plane, but a fixed-fixed beam in the YZ plane.  If you display mode 2 you will see that it is in the YZ plane and is the 1st fixed-fixed mode.  Mode 3 is the 2nd simply supported mode.  The mode 3 frequency may be slightly smaller than an analytical calculation because the analytical formula might not include the rotary inertia of the cross-section (may assume the mass is concentrated at the centerline) and may not consider shear deformation.  To get rid of the fixed-fixed modes you could create the cylinder in 4 quadrants so that you end up with a line down the centerline and then constrain that line in the Y direction.

• Abhishek Chauhan
Subscriber

Sir, then I have to make a cylinder on which plane and which rotation i have to free in remote displacement .

• Dave Looman
Ansys Employee

Every thing else would be the same except you would scope the remote displacements to the 4 quarter faces now.  Another ideas is to just model half the cylinder cross-section and constrain UY at the two faces at y=0 to zero.

• Abhishek Chauhan
Subscriber

I do not understand can you give the steps on how to rectify this?

I tried many times but did not get any method can you the way with steps and examples please.

• Dave Looman
Ansys Employee

What have you tried?  Modeling a half the cross-section?  Why didn't that allow you to constrain UY?  If the fixed-fixed frequency is bothering you, don't constrain ROTX at the ends.  Then you will get duplicate simply supported modes.

• Abhishek Chauhan
Subscriber

As i tried half cross-sections by using the named selection mesh node but it is also not working.

As I used avoid rotation x also it is not working.

can you tell me the other method or give any reference?

• Abhishek Chauhan
Subscriber

Is the any way to find the required natural frequecy.

• Dave Looman
Ansys Employee

If you created a half model there should be 2 faces at y = 0 that you could apply a displacement to.  I don't understand why you would need a named selection mesh node.  A simpler way to model a beam is with beam elements.