In the example "example_ewp_plane_wave_absorption.m", only the velocity data at each sensor was recorded finally.
(1) Now, I edited "sensor.record = {'u'};" to "sensor.record = {'u','p'};" in the script to record pressure data simultaneously.
(2) When the distribution of "medium.sound_speed_compression", "medium.sound_speed_shear" and "medium.density" are all uniform, the pressure recorded was zero or noise in the whole simulation time.
(3) When the distribution of "medium.sound_speed_shear" was nonuniform, the pressure recorded was "right".
How to explain this phenomenon? Thanks.
Regards,
Yu
k-Wave
A MATLAB toolbox for the time-domain
simulation of acoustic wave fields
problem facing in shear wave propagation simulation
(7 posts) (2 voices)-
Posted 9 years ago #
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Hi Yu,
Within the elastic code, pressure is calculated by
p = -(sxx + syy)/2. In other words, it is calculated only from the normal components of the stress tensor. If you have a pure shear wave in a homogeneous medium,sxxandshywill be zero, hence the the output pressure will be zero as expected.In the example you mention, two simulations are performed, first with a compressional wave and then with a shear wave. If you run the simulation and type:
plot(sensor_data_comp.p.'); plot(sensor_data_shear.p.');You will see a signal from the first simulation but not the second. If you make the medium heterogeneous in the second simulation, mode conversion will occur at the medium boundaries, and thus the pressure will no longer be zero as you have observed.
For an elastic medium, pressure as defined in k-Wave is not necessarily the most useful quantity to consider!
Hope that helps,
Brad.
Posted 9 years ago # -
Dear Brad,
In the same example, "example_ewp_plane_wave_absorption.m" (uniform media) (impulse excitation) (no absorption). When I analyze the velocity curve recorded by sensor using FFT (Matlab), I can obtain a frequency spectrum (magnitude), it looks like a low pass filter. I mean in the magnitude spectrum, the relation between frequency and magnitude is a horizontal line. Is this right ??????
In my expectation, the relation may be a curve.
Thanks so much!
YuPosted 9 years ago # -
Dear Brad,
In the same example, "example_ewp_plane_wave_absorption.m" (uniform media) (impulse excitation) (no absorption). When I change the shear wave velocity to 2m/s by using FFT. the frequency spectrum is a low pass filter. and the band width is (0-10000) Hz. I think it is wrong, but my FFT is right. How to explain this ?? (when the shear wave velocity is 2m/s, this material is too soft to bear high frequency)
Thanks,
YuPosted 9 years ago # -
Dear Brad,
I found the answer from the kwave manual. But now I have another problem. For the same medium, I can get different frequency spectrum by selecting different dx based on "f_max = min(c_0)/(2*dx)". How to obtain a accurate spectrum of a medium?
Best,
YuPosted 9 years ago # -
I think the frequency spectrum I got is spatial frequency, how to get temperal frequency?
Posted 9 years ago # -
Hi Yu,
In the example you mention, the source is set to a smoothed initial velocity distribution that is applied at the first time step. The smoothing applies a Blackman window in the spatial frequency domain using the k-Wave function
smooth. Because the window is normalised to span between DC and the maximum spatial frequency (which depends on dx), the spectrum which change as you change the grid parameters. The reasons for smoothing the initial distribution are explained in the Source Smoothing Example and in this paper.If you are analysing the sensor data over time, the spectrum is the temporal frequency. However, this will be affected by the spatial frequency content in the initial velocity distribution.
Hope that helps,
Brad.
Posted 9 years ago #
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