http://www.k-wave.org/forum/topic/about-time-step-warning Support for the k-Wave MATLAB toolbox en-US Tue, 12 May 2026 23:36:53 +0000 http://bbpress.org/?v=1.0.2 q http://www.k-wave.org/forum/search.php http://www.k-wave.org/forum/topic/about-time-step-warning#post-372 Thu, 08 Mar 2012 22:28:54 +0000 Bradley Treeby 372@http://www.k-wave.org/forum/ <p>Hi Chao,</p> <p>The check is really just to make sure that if people have manually defined <code>kgrid.t_array</code> (rather than using <code>makeTime</code>), that the time step is within a sensible range. You are right that if the grid spacing happens to be anisotropic, taking the minimum will give a more stringent warning. However, simply replacing <code>max</code> with <code>min</code> won't work in this case because <code>kgrid.dz</code> is zero in 2D and <code>kgrid.dy</code> and <code>kgrid.dz</code> are zero in 1D.</p> <p>A good discussion on the CFL condition required for stability (at least in the case of a heterogeneous sound speed) is given in the paper by Mast et al. (2001) "A k-space method for large-scale models of wave propagation in tissue". It depends on both the maximum sound speed in the medium, and the scalar sound speed used inside the k-space operator.</p> <p>I hope that helps,</p> <p>Brad. </p> http://www.k-wave.org/forum/topic/about-time-step-warning#post-356 Tue, 06 Mar 2012 01:23:10 +0000 huangchao 356@http://www.k-wave.org/forum/ <p>Hi, Dr. Cox and Dr. Treeby,</p> <p>I found that the condition for prompting the warning "time step may be too large for a stable simulation" is:</p> <p>dt &gt; DT_WARNING_CFL*max([kgrid.dz, kgrid.dx, kgrid.dy])/max(medium.sound_speed(:))</p> <p>But in my opinion, "min([kgrid.dz, kgrid.dx, kgrid.dy])" should be used instead of "max([kgrid.dz, kgrid.dx, kgrid.dy])", because for a fixed "DT_WARNING_CFL", "dt" should be less than "DT_WARNING_CFL*min([kgrid.dz, kgrid.dx, kgrid.dy])/max(medium.sound_speed(:))". I don't know if this is right?</p> <p>Another thing I want to make sure is that if the simulation is stable as long as "DT_WARNING_CFL&lt;1".</p> <p>Thank you very much!</p> <p>Best,<br /> Chao </p>