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# Mesh and Increment Size-Lamb Wave Propagation Simulation

Sat, 2013-04-27 19:28 - mhdiab

Hey all,

I am simulating Lamb wave propagation in a plate and was wondering if anyone knows where I can find the criteria to chose the mesh size and the time increment size. Thank you.

Regards,

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## Comments

## CFL and Blake's criteria

Hi,

CFL and Blake's criteria are used for time step and mesh size, respectively.

Regards,

- Ramadas

## Hey, I have looked at

Hey,

I have looked at some similar criteria , however, I am not sure how to calculate the following parameters:

-Wavelength : is that simply the speed of the wave divided by the frequency of the excitation wave ?

-Maximum frequency: is that just the excitation wave frequency ?

Thank you for taking the time to reply !

Regards,

## Cell and time step size

Since Lamb mode is dispersive, calculate wavelength taking ratio between phase velocity and excitation frequency. Then cell size (delta x) may be taken as 1/12 th of the wavelength.

To estimate critical time step size, find out the ratio between the delta x and the velocity. Generally, time step size is taken 0.75 times of the critical time step.

Hope this helps.

Regards,

- Ramadas

## Thank you, I am now making

Thank you, I am now making sure I follow the criteria you described. The experiment is however taking a very long time to complete! I ran an experiment with 3000 time increments and a relatively large mesh element and the simulation took more than 8 hours. I will need a very small element and about 16000 time increments for the simulation I need (using a higher simulation frequency).I am only outputting U3 at a certain node from the history output.Do you have any suggestions on how to reduce the computation time?

## Frequency and velocities

Could you please mention the following details.

1. Frequency of excitation and duration of excitation

2. Phase and group velocities of Lamb mode, which you wish to excite

Regards,

- Ramadas

## Frequency

The frequency of excitation is 100KHz, and the duration is 0.01 seconds

The longitudinal wave velocity is about 4000m/s (determined experimentally).

Thank you

## Wavelength and time step

Hi,

From the above data, wavelength is 40 mm. Take 3 mm ( approx 40/12 mm) as mesh size (delta x). The critical time step size is delta x / 4000 = 750 nano sec. Take time step size as 0.75 * critical time step size => 500 nano sec (approx).

Earlier it was mentioned that it was a Lamb wave, but, in the above post it was mentioned that longitudinal wave. Do you mean it is So mode?

Best regards,

- Ramadas

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