This paper presents the results of a fundamental numerical analysis of the performance of a reversible axial flow air turbine designed to service Oscillating Water Column (OWC) ocean wave energy systems. A new approach to the analysis of such devices is presented wherein an actuator disc/blade-element analysis is formulated purely in terms of non-dimensional variables (ie flow factor, blade lift/drag data, angle of attack, etc) rather than the usual mixture of non-dimensional and dimensional input parameters. The non-dimensional pressure drop, torque, input and efficiency coefficients were then determined using the numerical model for a variable-pitch Denniss-Auld turbine developed by Energetech Australia. Numerical results are presented for simulations using the lift and drag characteristics of both an isolated aerofoil and a cascade of blades. Agreement between numerical and laboratory-scale experimental data was found to be reasonably good, although the quantitative predictions of efficiency were significantly higher than in practice. The implications for optimal design of such air turbines are also discussed.