Evolution of the transverse kinetic energy of the gas in six different simulations with the same dimensionless wavenumber κ = 0.7968 (the most unstable wavenumber in the gas-only case). The vertical axis has been set to logarithmic scale to clearly show the exponential growth in the initial linear growth phase. Three simulations start with a lower initial transverse kinetic energy. This is because they start with small random perturbations in the middle layer, instead of being excited with the perturbation described in Eq. (8). These three simulations are discussed in the Sect. 4.3. The behavior of the simulation with δ = 1 is different; this is explained in Sect. 4.1.1.