Comparison between different methods to access velocity of gas entering in the discontinuity in its co-moving frame (for the OT simulation at P_m = 1 at time t = t_turnover/3). [Wave] is the gas velocity derived from the stationary wave frame (see Sect. 3.3.2). [Most conservative frame] is determined in the frame that minimises the violation of all fluxes conservation relations, mass flux excepted. [Mass conservation] is the mass flux conserving frame.