Fluid relaxation doesn't perform well for opening bcs

[WeiyiVirtonomy]

Describe the bug
The inflow region has velocity and pressure singularities if the fluid particles are relaxed.
E.g. T-shaped pipe flow:

This is within the inflow region, where the velocity is prescribed. Weirdly, the velocity within this region is not exactly parabolic.
The simulation can still go on with the parameters used in test_2d_T_shaped_pipe, but in some cases, it can fail after several steps.

To Reproduce
Add relaxation of fluid in test_2d_T_shaped_pipe.

[WeiyiVirtonomy]

@DongWuTUM I reported this issue