The unmixed isobaric two-fluid test at times t = 0, t = 0.4, t = 1 and t = 2 (columns from left to right) calculated with different volume elements. The initial model is that of two nested squares with density contrast of four (the one with higher density being the inner yellow square). First and second rows use particles with identical mass but unevenly spaced grid, while the opposite applies for the two last rows. The first row depicts the evolution with the volume estimator (p = 0.9). The second row is the same, but using either X_a = 1, m_a, and (k = 0.5) (all three giving similar results). The evolution shown in the third row was calculated using either , and , while the outcome using X_a = m_a is shown in the fourth row.