Hi Brad,
Recently I started to simulate shear wave propagation in tissue-mimicking phantoms by using the "pstdElastic2D" function. I am simulating a uniform phantom (100 mm x 100 mm) with a sinusoidal velocity source on the left side. I thought that in this situation the shear wave only propagates from left to right and therefore PMLs are only needed on the left and right sides but not needed on the top and bottom sides. (a) When I set the PMLSize to be zero on the top and bottom sides, I got uniform vibration amplitudes in the entire domain, which is expected. On the other hand, it is written in the k-Wave manual that "To remove the PML, set the appropraite PMLAlpha to zero rather than forcing the PML to be of zero size. (Page 63)" (b) When I set the PMLSize to be nonzero but PMLAlpha = 0 on the top and bottom sides, I got different results from those in (a). This leads me to the following question: How should the PMLs be set up in k-Wave? For example, should PMLs always be set up on all sides or only on the sides in the wave propagation direction? Why "PMLSize = 0" is not equivalent to "PMLSize=/=0 & PMLAlpha = 0"?
Additionally, I would like to ask whether it is correct to set the compression wave speed (c_p) to be a small (nonphysical) value (e.g., 0.001 m/s) to reduce the compute time, if the focus is on the shear wave (I used c_s = 4.59 m/s). I tried simulations with different c_p values and found that the wave image results with c_p = 1000 m/s and c_p = 0.001 m/s seems nearly the same (for shear wave propagation in a uniform phantom). As far as I know, c_p should be quite different (either much larger or much smaller) from c_s to avoid numerical errors. I also noticed that the PMLs worked fine in attenuating the shear wave when c_p = 1000 m/s but did not work when c_p = 0.001 m/s. Is this behavior of the PMLs expected?
Thank you very much and I look forward to your reply.
Best regards,
Chris