## General Mechanical

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#### Harmonic Acoustics – Free surface sloshing water body

• Victor Gallardo Torres
Subscriber

Hello,

I have been trying to model the free surface response of a water body that is harmonically excited by a solid particle moving in the water domain using Harmonic Acoustics.

I created a rectangular water body (70x17m) with the free surface and rigid wall BC. Inside the fluid body a solid steel particle (1x1x1m) is located with FSI and displacement BC. The solid particle has a displacement along the length (z-axis) of the basin and maintains a constant depth.

Apperently the elevation of the free surface (spatial patterns of the eigenmodes of the water body) can be displayed with directional deformation of the free surface and acoustic pressure of the water body. Analysing this and the frequency response graph it can be concluded that the spatial sloshing patterns and eigenfrequencies match up with the theory (Raichlen 1966). However, I am not sure how to interpret the amplitudes.

Firstly, I am interested if it makes sense to setup the model like this. Can the linear sloshing response of free surface under this type of harmonic forcing be modelled with Harmonic Acoustics? Or is the model schematized to bluntly?

Secondly, I would like to know how to interpret the aplitudes of the free surface deformation.

I would like to hear your opinion.

Kind regards.

• peteroznewman
Subscriber

Hello Victor,

You may find it helpful to plot each plot with a Phase of 180 degrees. Once you do that, it should be clear that the 3.6e-2 Hz mode is sloshing where the levels rise and fall at each end out-of-phase with each other (one rises while the other falls) while the center does not change.  The 7.1e-2 Hz mode is where the two ends rise and fall in-phase (both rise and fall together) while the center moves in the opposite direction.

I suggest you remesh with global elements half the size, look at the results, then make the elements half the size again and check the results again.

Regards

• Erik Kostson
Ansys Employee

Hi

The issue is that the acceleration needed here to activate the sloshing formulation is also applied (structural load) to all the structural bodies (so this will always be a problem so you can not do what you are trying to do).

If you just need the sloshing frequencies and modes, just use a modal acoustic analysis with acceleration and free surface.

All the best
Erik

• Victor Gallardo Torres
Subscriber

Dear Peter and Erik,

I performed a modal acoustics analysis on the basin with acceleration and free surface. I attached the results of Mode 1, 2 and 6. The the eigen frequencies and the spatial patterns of the modes agree with the theory (Raichlen 1966). I am still not completely sure how to interpret the value of the amplitudes (relative to one other). Is this the maximum amplitude for the solution to be a linear wave?

For the next step of my research I would like to model the free surface response of the fluid body to some kind of periodic excitation. For example an earthquake type of excitation that works on the basin in the xz-plane. Eventually I would like to create a frequency response function to evaluate amplification of certain nodes. I want to avoid setting up a computationally expensive transient fluent model.

Can such a model be made with Harmonic Acoustics? Do you have any recommendations on the setup of such a model?

Kind regards,

Victor

• Erik Kostson
Ansys Employee

Hi

THe values of the amplitudes are just scaled eigenvectors in modal acoustics so they do not mean anything (the actual value). Only the overall mode shape means something, so the eigenvector.

They (values) might have some meaning when you do a harmonic excitation (say base excitation of a fluid filled tank), unfortunately this is not supported in WB mechanical.
All the best
Erik