## General Mechanical

Topics relate to Mechanical Enterprise, Motion, Additive Print and more

#### Modal Analysis using point mass

• Rohan Pawar
Subscriber

Hi community,

I have done a modal analysis of a container in which a fluid is considered as a point mass. This point mass is defined using free standing remote point. Free-free modal case is considered. After calculation for 20 modes, I find out that 6 modes are zero and additional 7th, 8th and 9th mode frequencies are almost zero. Why the 7th, 8th and 9th modes are also zero when first six modes capture rigid body motion? Following are the snapshots of the analysis.

1) Geometry

2) Remote point

3) Point mass

4) Mode and frequency

Your suggestions for any modifications in analysis are most welcome.

Regards,

Rohan.

• peteroznewman
Subscriber

The first six modes are for the solid body assembly, the next three modes are for the free standing remote point. The free standing remote point has only 3 modes because it has only mass and no moment of inertia. If you add moments of inertia, there will be a total of 12 zero modes.

A free standing remote point is not adding a fluid mass to the structure.

Is the fluid inside the container or is the container submerged in fluid?

I suggest you read this discussion for further information.

• Rohan Pawar
Subscriber

Hello peteroznewman,

Thank you for your prompt reply. The liquid is enclosed within the container and not submerged in liquid. So, can you suggest any solution to model fluid as point mass? Also had a doubt regarding point mass. Point mass with no mass moment of inertia and only mass has 3 modes zero, while point mass with MMI has 6 modes zero. Can you elaborate on this point?

• peteroznewman
Subscriber

Hello Rohan,

Add a solid body inside the container to represent the fluid. Use shared topology so the fluid is connected to the container. Assign a fluid material to the body. Mesh the fluid body. What element shape was created?  If Quadratic Tet elements, assign FLUID221 elements to that body. If Linear Hex elements, assign FLUID30 elements to that body. Solve the model using Modal Acoustics. Does the fluid completely fill the container or is there a free surface?

A point mass with no MMI causes the node to have only 3 translational DOF while a point mass with MMI causes the node to have an additional 3 rotational DOF.

• Rohan Pawar
Subscriber

Hi peteroznewman,

The container is not fully filled, but has a free surface at a distance 10mm from inner top surface of the container. Also, I have considered quadratic order elements.

Regards,

Rohan

• peteroznewman
Subscriber

Hi Rohan,

The free surface of the solid body that is meshed with FLUID221 elements needs to have a Free Surface boundary condition applied. See section 7.1.4 of the Acoustic Analysis Guide in Ansys Help.

`https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v231/en/ans_acous/acous_bc_apply.html`

Did the fluid body get meshed with tet or hex shaped elements?

• Rohan Pawar
Subscriber

Hi,

How to generate the fluid 221 elements? Can we run modal analysis for structure+fluid case with fluid modeled as point mass? I have also modeled a 3d fluid part. Can we define contact between fluid and container walls interface? I read in mechanical help guide where it is mentioned that only bonded with MPC formulation can be used for fluid-structure contact. Will the contact affect reults or shared topology is to be considered between fluid structure interface?

• peteroznewman
Subscriber

Shared topology creates a fluid-structure interface, there is no need for contact.

For the current version of Ansys 2023 R1, in Modal Acoustics, define an Acoustic Region (the fluid body) and a Structural Region, (the container).  That causes elements to have the correct element types assigned. When you solve, look at the text in the Solution Information folder Solution Output to see what elements types were created. You should see FLUID221 elements in the list.

In Ansys version 18.1, look in the Toolbox in Workbench, you will not find Modal Acoustics there. Use a Modal analysis. In Mechanical, use a command object under the fluid body in Geometry to assign the element type.

ET,matid,FLUID221

I don’t know if a fluid Free Surface is supported in Ansys 18.1 for acoustics.

Do you have a second computer where you can install the free ANSYS Student 2023 R1 software?  Then you will have an easier time getting help. There is a risk of messing up the licensing for Ansys 18.1 if you install the free Ansys Student 2023 R1 on the same computer.