k-Wave

1. elliotP
   

   Member
   Joined: Mar '19
   Posts: 10

   Hi,

   I have a question about the relationship between elastic simulation time and coefficients.
   For instance, when I use below constants:
   -----------------------------------------
   % water
   cp1 = 1540; % compressional wave speed [m/s]
   cs1 = 0; % shear wave speed [m/s]
   rho1 = 1000; % density [kg/m^3]
   alpha0_p1 = 0; % compressional wave absorption [dB/(MHz^2 cm)]
   alpha0_s1 = 0; % shear wave absorption [dB/(MHz^2 cm)]
   % skull
   cp2 = 3000; % compressional wave speed [m/s]
   cs2 = 1500; % shear wave speed [m/s]
   rho2 = 1850; % density [kg/m^3]
   alpha0_p2 = 3; % compressional wave absorption [dB/(MHz^2 cm)]
   alpha0_s2 = 6; % shear wave absorption [dB/(MHz^2 cm)]
   % create the time array
   cfl = 0.13;
   t_end = 40e-6;
   -----------------------------------------
   estimated simulation time 1hours 15min 41.4135s...
   -----------------------------------------

   When I change the skull's attenuation values to alpha0_p2 = 8.83, alpha0_s2 = 19.5 (belongs to your paper(Modeling power law absorption and dispersion in viscoelastic solids using a split-field
   and the fractional Laplacian)). Unfortunately, simulation gives a black screen. I think I have to change the cfl number. But, when I decrease the cfl coefficient, simulation time takes much longer.
   How can I minimize the simulation time under these circumstances?

   Best,
   Michael

   Posted 5 years ago #

2. genny_p
   

   Member
   Joined: May '19
   Posts: 3

   Hi Michael,

   I had a similar problem with a black screen forming (mine was due to some instability) when I increased the attenuation coefficients. I wonder if we are facing the same problem. In my case, I found the whether the simulation goes black is highly sensitive to the grid spacing. Increasing dx seemed to make the simulation more stable (as with increasing the cfl number), but then I couldn't confirm convergenge. I tried putting the parameters you supplied in the Shear Waves And Critical Angle Reflection Example and everything seems to run fine, even with the higher attenuation coefficients, so perhaps something else in your code is different (grid spacing/size probably considering the longer simulation time). Can you post your complete code (or a big enough subset of it to reproduce the problem) so I can see if your problem seems similar to mine?

   I posted my code at http://www.k-wave.org/forum/topic/instability-in-shear-wave-absorption-simulation a few months ago, and am still searching for a solution. Perhaps you can try increasing the grid spacing in your simulation and see if a coarser grid helps your simulation like mine (though this would only confirm we are facing the same problem, and still need a solution to confirm for convergence).

   Best,
   Genny

   Posted 5 years ago #

3. Bradley Treeby
   

   Developer
   Joined: Mar '10
   Posts: 1,643

   Hi Michael,

   If the screen goes black, it's likely an instability. Unfortunately, the stability criterion for the elastic model means that with high absorption values, you need to use a small CFL. You can try minimising the simulation time by switching to single precision, reducing the size of your domain if possible, using different hardware, etc.

   Hope that helps,

   Brad

   Posted 5 years ago #

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