Hello,
I am trying to simulate the wave propagation of a focused (continuous) wave through a layered medium. The media are: water (35 mm), oil (25 mm), polymer (2 mm) and steel (5 mm), respectively. A linear array (fc=2.72MHz) is placed at the beginning of the water and focused on the polymer by using delays. Attenuation is not included.
The system works quite well if I do not include the steel after the polymer layer. By reducing cfl or increasing ppw, the maximum pressure amplitude in the powder is converging. However, when using the steel layer (c0=5000 and rho0=4000) the simulation is not converging for increasing values of ppw (see https://ibb.co/Zdh7gvS for image). It seems that the convergence get worse if the grid size is decreased, which doesn't make sense?
Since the impedance mismatch is quite large between the polymer and steel, it is almost a perfectly reflecting surface. Therefore, I tried replacing the steel with a dirichlet BC with particle velocity is zero. Unfortunately, again no convergence with ppw was found (see https://ibb.co/TqnR6HX for image). Increasing the PML size did also not solve this problem. However, it does have an effect on the maximum pressure in the powder as shown in this image: https://ibb.co/T4LTfJs.
Any ideas why all of this is happening and how to solve this?
Best,
Frank