k-Wave

1. Elseliv
   

   Member
   Joined: Mar '20
   Posts: 2

   Hello Dr.Treeby,

   Thank you for the great toolbox!

   I`m doing shear wave simulation in a thin tissue layer using pstdElastic2D. As a source.ux, I use a line where each element imposes one cycle of a sine wave with a frequency of 50 Hz and max amplitude of 0.03 m/s. Then I record u_split_field with a line of sensors perpendicular to the source line and get values significantly higher (sensor_data.ux_split_s ~ 0.1 m/s sensor_data.uy_split_s ~ 0.5 m/s), than those imposed by source. Even for the sensors placed tight to the source.

   What could become a reason for differences in the velocities imposed and observed? Is there a way to validate the magnitude of output data? May there be a problem with output units or something else?

   Best regards,
   Ekaterina

   Posted 3 years ago #

2. Bradley Treeby
   

   Developer
   Joined: Mar '10
   Posts: 1,643

   Hi Ekaterina,

   By default, a velocity source is a force source. It will inject or "add" to the field that is already there, and thus the actual velocity will be higher than the velocity you specify. If you want the source to act like a boundary condition (exactly impose the values you specify), then you can set source.u_mode = 'dirichlet'. Keep in mind that in this case, the imposed boundary condition will also reflect any ways coming back to the source.

   Brad.

   Posted 3 years ago #

3. Elseliv
   

   Member
   Joined: Mar '20
   Posts: 2

   I actually tried using the Dirichlet condition but this brought instability and the simulation got totally crashed. Here is a link with 2 plots for Dirichlet and NoDirichlet cases https://we.tl/t-Dft5lKj5vd and it depicts in the top row - source velocities as I imposed them and on the 2-nd and 3-d rows - velocities measure by the sensor exactly next to the source. Could you then assume what might be a reason for such a crash?
   Concerning reflections, the source I use is located at the boundary between tissue and water so reflections are expected to happen anyway.

   Posted 3 years ago #

4. Bradley Treeby
   

   Developer
   Joined: Mar '10
   Posts: 1,643

   Hi Ekaterina,

   I couldn't view the image, however, if you experience an instability you could try reducing the time step size.

   On the topic of reflections, I'm not sure these will match any physically realistic scenario, as you are imposing a time-varying boundary condition, thus the reflections will depend on the values for the velocity that you are imposing at each time step in the simulation.

   Brad.

   Posted 3 years ago #

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