Predictions of the flux of (turbulent) kinetic energy in three different 3D RHD simulations (top panel: solar case, middle panel: DA white dwarf; bottom panel: solar case with solid plate at the bottom). The same colour and linestyle coding is used as for Fig. 1. For the models and for the computation from only the vertical component of the kinetic energy flux an identical scaling factor of 2/3 was applied (see text in Appendix C). The evaluation of the exact expression for this quantity from the 3D RHD simulations is shown in bright red with data points denoted by filled squares.