Pressure wave reflection compressibleInterIsoFoam
 

Hi all,

I want to model the breakup of a liquid column under the influence of a step change in pressure & velocity (shock wave), using the compressibleInterIsoFoam (VOF) solver. The goal of the case is to determine a minimum grid cell size to properly resolve the breakup behavior, by comparison to literature data (https://doi.org/10.2514/6.2017-1892).

The simulations are two-dimensional. I am not yet taking into account the presence of the liquid (i.e. alpha.liquid = 0 in the whole domain), just considering the propagation of the pressure wave through the domain. The domain is initialised with p=p_rgh=8.9e4 Pa (gravity is neglected), U = (0 0 0).
On the left side of the domain, I create an area with higher pressure, p=p_rgh=1.09e5 Pa, U = (80 0 0), as can be seen in attached picture, for t=0. Left domain boundary is an inlet with these values specified with fixedValue BC. Top and bottom boundaries are zeroGradient for all parameters.
After the pressure wave passes through the domain, I would like it to exit at the right outlet boundary. However, as shown in the picture, the pressure wave reflects back into the domain.

Initially, I used zeroGradient conditions for p, p_rgh, and U at the outlet. Since this resulted in wave reflection, I have also tried the advective and waveTransmissive for p, p_rgh and U. I have also applied pressureInletOutletVelocity or zeroGradient for U in combination with advective or waveTransmissive for p and p_rgh. None of these different conditions made any difference in the wave reflection.

I know that density-based solvers (such as rhoCentralFoam), are better suited for pressure wave modeling as compared to pressure-based solvers such as I am using. Indeed, when using rhoCentralFoam in combination with zeroGradient outlet condition for p and U for my case-setup, there is no wave reflection at the outlet. However, since I want to use this case to determine a proper VOF resolution for droplet breakup, I do not have the option of switching to a density-based solver.

Is it possible at all to avoid these pressure wave reflections when using a pressure-based solver? If so, which combination of boundary conditions is recommendable?

I hope I have provided enough information on my case, if not, please let me know what is missing. Thanks in advance!

Try using something different for U. waveTransmissive can be paired with zeroGrad in U, for instance. I'm not sure you can use waveTransmissive for p_rgh: I'm facing this very problem right now.