  Active fluids exhibit complex turbulent-like flows at low Reynolds number.
Recent work predicted that 2d active nematic turbulence follows universal
scaling laws. However, experimentally testing these predictions is conditioned
by the coupling to the 3d environment. Here, we measure the spectrum of the
kinetic energy, $E(q)$, in an active nematic film in contact with a passive oil
layer. At small and intermediate scales, we find the scaling regimes $E(q)\sim
q^{-4}$ and $E(q)\sim q^{-1}$, respectively, in agreement with the theoretical
prediction for 2d active nematics. At large scales, however, we find a new
scaling $E(q)\sim q$, which emerges when the dissipation is dominated by the 3d
oil layer. In addition, we derive an explicit expression for the spectrum that
spans all length scales, thus explaining and connecting the different scaling
regimes. This allows us to fit the data and extract the length scale that
controls the crossover to the new large-scale regime, which we tune by varying
the oil viscosity. Overall, our work experimentally demonstrates the emergence
of universal scaling laws in active turbulence, and it establishes how the
spectrum is affected by external dissipation.
