This paper theoretically and experimentally investigates the inertial effects of the flap body on the performance of a bottom-hinged oscillating wave surge converter (BH-OWSC). A two-dimensional (2D) hydrodynamic theory for a BH-OWSC based on the assumption of potential flow is developed to show that one simple but critical parameter, i.e., the square of sum of three mechanical-impedance terms associated with the inertial effects, can precisely characterize the performance trend of a BH-OWSC. Model testing in a small-scaled wave basin follows to validate the theoretical formulations with a flap body consisting of multiple hollow cylinders into which water can be filled individually to alter the values of flap's inertial parameters. The performance of each inertial specification of the flap model is evaluated based on the measurement of the mean water discharge from the hydraulic pump (or the power take-off). Finally, the “near resonant condition” has been validated experimentally by altering the inertial parameters of the flap. Thus, the aforementioned parameter is shown to be capable of characterizing the inertial effects on the performance of a BH-OWSC, and the minimization of it will maximize the power capturing performance of a BH-OWSC. Consequently, the parameter can be used for design guidelines of the flap body in its inertial aspect, such as locating the center of mass and determining the geometric dimensions of a flap body.

The distinct engineering advantages of Oscillating Water Column (OWC) systems have driven substantial academic interest lately. This work examines the onshore U-shaped OWC (U-OWC), selected for its cost-effective installation integrated with existing coastal infrastructure and its superior broadband response to diverse wave climates. Time-domain CFD simulations, incorporating the scaling-rematched approach, were conducted to quantify key hydrodynamic and air-compressibility coefficients, including the amplitude of the wave exciting force, fluid damping coefficient, added mass, absorption factor, and the effective PTO (power take-off) damping and air-compressibility coefficients. These parameters collectively elucidate the underlying hydrodynamics and how they are interwoven with the compressibility of the air in the plenum chamber, thereby impacting the U-OWC’s energy-capture performance under incident waves. A principal finding is the identification of a C+ interval wherein air compressibility enhances capture performance in the lower wave-period range examined (<8.0 s). The added mass of the present U-OWC exhibits a remarkably pronounced decrease around the wave period of 8.0 s, which can be verified by a simple resonance formula of heave buoys to underline its strong near-resonance behavior.

The distinct engineering advantages of Oscillating-Water-Column (OWC) systems have driven substantial academic interest lately. This work examines the on-shore U-shaped OWC (U-OWC), selected for its cost-effective installation integrated with existing coastal infrastructure and its superior broadband response to diverse wave climates. Time-domain CFD simulations, incorporating the scaling-rematched approach, were conducted to quantify key hydrodynamic and air-compressibility coefficients, including the amplitude of the wave exciting force, fluid-damping coefficient, added mass, absorption factor, and the effective PTO (Power Take-Off)-damping and air-compressibility coefficients. These parameters collectively elucidate the underlying hydrodynamics and how they are interwoven with the compressibility of the air in the plenum chamber, thereby impacting the U-OWC's energy-capture performance under incident waves. A principal finding is the identification of C+ interval wherein air compressibility enhances capture performance in the lower wave-period range examined (< 8.0s). The added mass of the present U-OWC exhibits a remarkably pronounced decrease around the wave period of 8.0s, which can be verified by a simple resonance formula of heave buoys to underline its strong near-resonance behavior.

Juncture flows associated with surface-mounted obstacles can be characterized by a U-shaped tubular vortical flow known as a horseshoe vortex (HSV). Horseshoe vortices are usually detrimental to engineering applications. Therefore, numerous studies have investigated methods for reducing HSVs’ strength. To design a suitable method for reducing HSVs’ strength, the characteristics of HSVs influenced by external boundary geometries must first be understood. Thus, we experimentally investigated a juncture flow field associated with a fundamental geometry—a circular cylinder mounted perpendicular to a plane surface—with a trailing backward-facing step representing downstream effects on upstream-formed flow structures. This setup is one of the simplest nonplanar geometries that generates flow features such as an unsteady separated shear layer that may considerably affect an HSV. We used the particle image velocimetry (PIV) and PIV-based flow visualization techniques combined with a vortex-fitting algorithm to measure the juncture flow and identify the HSV and its kinematic modes at a low Reynolds number of 1100. We compared the results obtained with and without the backward-facing step and observed that the backward-facing step increased the number and complexity of kinematic modes, i.e. the unsteadiness, of the HSV, and reduced HSV’s stretching, which resulted in an increased vortex diameter and increased circulation; that is, the strength of the HSV was enhanced by the backward-facing step.

Juncture flows associated with surface-mounted obstacles can be characterized by a U-shaped tubular vortical flow known as a horseshoe vortex (HSV). Horseshoe vortices are usually detrimental to engineering applications. In order to identify a boundary geometry that may effectively reduce a HSV’s strength for further design of HSV-reduction devices, or otherwise enhance it for further disaster prevention by avoiding such a geometry in design, the characteristics of HSVs influenced by external boundary geometries must first be understood. Thus, we experimentally investigated a juncture flow field associated with a fundamental geometry—a circular cylinder mounted perpendicular to a plane surface—with a trailing backward-facing step (BFS) representing a novel idea of downstream effects on upstream-formed flow structures. This setup is not only one of the simplest nonplanar geometries that generates flow features such as an unsteady separated shear layer that may considerably affect an HSV, but it is also a new attempt without prior knowledge. We used the particle image velocimetry (PIV) and PIV-based flow visualization techniques combined with a vortex-fitting algorithm to measure the juncture flow and identify the HSV and its kinematic modes at the low Reynolds number of 1166. We observed from the flow-visualization results regarding HSV’s kinematic modes and their duration percentages as follows: (1) without the BFS: “oscillation with a small displacement” (98.6%) and “breakaway to roll-up” (1.4%); (2) with the BFS (step height/cylinder diameter = 1.5): “oscillation with a small displacement” (83.3%), “merging” (4.5%), and “mixed” (12.2%). It is clearly evident that the BFS increased the number and complexity of kinematic modes, i.e., the unsteadiness of the HSV. Moreover, the PIV results show that the BFS reduced HSV stretching, which resulted in increased vortex diameter by 8.24% and increased circulation by 6.37%, i.e., the strength of the HSV was enhanced by the BFS.