Active gels or active liquid crystals are soft materials which are driven out of equilibrium by their continuum use of chemical energy. Physical realisations of active gels are solutions of cytoskeletal filaments with molecular motors, and concentrated suspensions of bacterial swimmers. Here we focus on the active nematic phase which is exhibited by these materials, and study their hydrodynamics when confined in a slab of a finite height. We solve the hydrodynamic equations of motion by means of a hybrid lattice Boltzmann algorithm. The homogeneous flow is known to be unstable for sufficiently large activity, leading to a state with spontaneous flow in steady state. We characterise the form of this flow in I and 2 dimensions with extensile and contractile flow aligning materials.