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Strange result in transient acoustic-structure interaction

Posted Dec 9, 2011, 6:57 a.m. EST Acoustics & Vibrations, Modeling Tools & Definitions, Parameters, Variables, & Functions, Results & Visualization 4 Replies

Robert Malkin
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Hi everyone!

I have what may turn out to be a rather simple question, but I am rather stumped!

I have a simple 2D model with a Gaussian pulse, looking at transient acoustic-structure interaction. The model simply has a point sound source in air a distance away from a metal rod.

I am plotting the pressure in the air and the von Mises stresses in the metal rod. However the stress in the rod evolves before the sound pulse has reached the rod.

Please see the attached GIF of the first 0.01 milliseconds of the model.

Given the speed of sound in air the pulse should reach the bar at 0.001 seconds. However stresses can be seen in the bar much before this time (at all time increments).

Does anyone have an idea whats going on?

Appreciate your help on this :-)

Rob


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4 Replies Last Post Dec 9, 2011, 9:38 a.m. EST
Robert Malkin
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Posted: 1 decade ago Dec 9, 2011, 7:22 a.m. EST
In addition to the above I found the following in the documentation.

"Avoiding Strong Transients

If you start solving a time dependent problem with initial conditions that are inconsistent, or if you use boundary conditions or sources that switch instantaneously at a certain time, you induce strong transient signals in a system. The time-stepping algorithm then takes very small steps to resolve the transient, and the solution time might be very long, or the solution process might even stop. Stationary problems can run into mesh-resolution issues such as overshooting and undershooting of the solution due to infinite flux problems.

Unless you want to know the details of these transients, start with initial conditions that lead to a consistent solution to a stationary problem. Only then turn on the boundary values, sources, or driving fluxes over a time interval that is realistic for your model.

In most cases you should turn on your sources using a smoothed step over a finite time. What you might think of as a step function is, in real-life physics, often a little bit smoothed because of inertia. The step or switch does not happen instantaneously. Electrical switches take milliseconds, and solid-state switches take microseconds."
In addition to the above I found the following in the documentation. "Avoiding Strong Transients If you start solving a time dependent problem with initial conditions that are inconsistent, or if you use boundary conditions or sources that switch instantaneously at a certain time, you induce strong transient signals in a system. The time-stepping algorithm then takes very small steps to resolve the transient, and the solution time might be very long, or the solution process might even stop. Stationary problems can run into mesh-resolution issues such as overshooting and undershooting of the solution due to infinite flux problems. Unless you want to know the details of these transients, start with initial conditions that lead to a consistent solution to a stationary problem. Only then turn on the boundary values, sources, or driving fluxes over a time interval that is realistic for your model. In most cases you should turn on your sources using a smoothed step over a finite time. What you might think of as a step function is, in real-life physics, often a little bit smoothed because of inertia. The step or switch does not happen instantaneously. Electrical switches take milliseconds, and solid-state switches take microseconds."
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Dec 9, 2011, 8:52 a.m. EST
The Mises stress in your animation is of the order of 1e-30. What you’re seeing is just numerical round-off errors.

Nagi Elabbasi
Veryst Engineering
The Mises stress in your animation is of the order of 1e-30. What you’re seeing is just numerical round-off errors. Nagi Elabbasi Veryst Engineering
Robert Malkin
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Posted: 1 decade ago Dec 9, 2011, 9:17 a.m. EST
Thank you for your quick reply Nagi.

Do you know if it is possible to avoid these artefacts? I shall run the simulation again and let it run longer to see what the internal stresses are like after the pulse has reached the boundary of the rod.

Thanks again,

Rob
Thank you for your quick reply Nagi. Do you know if it is possible to avoid these artefacts? I shall run the simulation again and let it run longer to see what the internal stresses are like after the pulse has reached the boundary of the rod. Thanks again, Rob
Robert Malkin
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Posted: 1 decade ago Dec 9, 2011, 9:38 a.m. EST
Hello again,

Just to add the the reply from Nagi.

If you plot the average von Mises stress within the metal bar you see it increase from insignificantly small magnitude to realistic stresses ONLY when the pressure wave reaches the bar.

Perhaps this will help others.

Rob
Hello again, Just to add the the reply from Nagi. If you plot the average von Mises stress within the metal bar you see it increase from insignificantly small magnitude to realistic stresses ONLY when the pressure wave reaches the bar. Perhaps this will help others. Rob

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