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The extraction of wave energy through self-rectifying air turbine is one of the emerging technologies for oscillating water column (OWC) based wave energy devices. In the present effort, a bi-directional impulse (BDI) turbine is designed and the performance parameters were found numerically and compared with an existing unidirectional impulse (UDI) turbine. A brief analytical formulation through similarity laws, to find a dynamically similar BDI turbine using pressure drop vs flow characteristics, gives the approximate diameter range equivalent to the reference UDI turbine. The results are used to reduce the range of diameters and it is found that the characteristics are matching with the reference UDI turbine. The maximum and minimum diameters among the selected range are considered for detailed computational fluid dynamics (CFD) analysis. These two BDI turbines are modeled and meshed in ICEM CFD 14.5. The commercial CFD code CFX 14.5 is used for the numerical simulations. The Reynolds-averaged Navier-Stokes (RANS) equations with the standard k-ϵ scalable wall function model are solved to obtain the performance parameters. A detailed flow physics of the BDI turbines has also been included.

The oscillating water column (OWC) based wave energy devices require a turbine for the conversion of wave energy to electrical energy. Customarily, Wells and impulse turbines have been widely accepted for this energy conversion. However, the damping (i.e. the pressure drop developed based on volume flow rate) induced by the turbine significantly influences the performance of the OWC. The present study involves two different impulse turbines that is, reference and optimized. Initially, the reference turbine was numerically validated with two different turbulence models ( k- ε and SST). It was found that the SST predicted better than k- ε in terms of flow separation and efficiency. This numerical methodology was extended to the optimized turbine and detailed flow physics has been presented in this article. Furthermore, both the turbines were compared in terms of pressure drop, turbine pressure coefficient, and turbine damping coefficient. It was observed that the induced damping by the optimized turbine is less than the reference turbine. Finally, different diameters were arrived from the optimized turbine by the principle of dynamic similarity to match with an available optimum range of damping.

Summary A bidirectional axial‐flow impulse turbine installed in an oscillating water column raises the question of damping at different wave conditions. This article reports unsteady performances of such a turbine in an airflow test rig. The rig has a piston‐cylinder arrangement, and the piston, when reciprocating, generates the oscillatory airflow. At a very low wave height (piston stroke length, <0.4 m), the turbine speed is very low. Time series analysis of pressure drop and speed are reported for higher stroke lengths, which shows the rate of increase and decrease in velocity of the turbine. Furthermore, the turbine pressure coefficient and the damping characteristics of the test rig chamber were estimated. Finally, the chamber damping verifies that the turbine‐chamber coupling effect is reciprocal.