KEYWORDS: Wave energy, Fluidic diode, Oscillating water column, Uni-directional turbine, Turbine duo, Optimization

An oscillating water column (OWC) device is one of the most studied wave energy converter devices. It generates bi-directional (oscillatory airflow) from the action of the ocean wave. A pair of turbines can operate in bi-directional flow without mechanical flow rectification (Turbine duo). However, due to inadequate flow blockage, they perform poorly. A fluidic diode (FD) offers a variable resistance to the fluid based on the flow path. Its performance is given by diodicity, a ratio of reverse to forward pressure drop. It can be used with the turbine to enhance flow blockage and improve performance. It is expected that with higher diodicity, the FD can block the airflow better, thereby enhancing the flow rectification of the turbine duo (TD).
FD proposed for the OWC was numerically validated. The six geometrical parameters of the FD were varied to obtain sample points using the sampling technique, which is numerically investigated by solving steady-state Reynolds averaged Navier-Stokes (RANS) equations. These numerical results were used to construct a surrogate model that produced an optimal FD design.

In the first step, the optimum geometry of the FD yielded 36.5% higher diodicity than the base model at a 0.35 m3/s flow rate. Coupled with TD, it improves the system efficiency by 13.3%. With higher diodicity, the FD enhanced the performance of the TD. However, it also increased the losses across the turbine; hence TD performance dropped at a higher flow coefficient.
In the second step, multi-objective optimization is performed to obtain an FD design that can offer a higher diodicity and, at the same time, lesser resistance through it. The popular evolutionary search algorithm (NSGA-II) with an artificial neural network produced non-dominated optimal solutions (Pareto optimal set). Compared to the base model, the optimal design set shows improved diodicity from 17.2 to 21.57% and pressure drop from -2.535% to 78.67%, respectively.
Finally, the transient performance of both the optimized (single and multi-objective) and base FD models was investigated when subjected to the oscillatory flow. The air turbine with diode assembly used in the OWC is subjected to oscillatory (bi-directional) flow. A steady-state flow analysis cannot predict the dynamic flow interaction formed due to the oscillatory flow. Understanding the performance of the devices in oscillatory flow is necessary as this helps better predict the performance and design an efficient model for the actual sea state.
This comprehensive study provides an application of FD for the wave energy application and its effect on the performance of the TD.