Alan Fleming

Phase Averaged Flow Analysis in an Oscillating Water Column Wave Energy Converter

The paper presents the application of phase averaging to experimental data obtained during scale model testing of a forward facing bent duct oscillating water column (OWC). Phase averaging is applied to both wave probe data and a two-dimensional velocity field at the centerline plane of the OWC model obtained using particle imaging velocimetry (PIV). Results are presented for one monochromatic wave condition. The influence of varied wave frequency is briefly discussed.

Phase averaging of the velocity fields in an oscillating water column using splines

The principal objective of this paper is to present a phase averaging method by curve fitting using B-splines. The method was designed specifically to process phase-clumped and discontinuous periodic data. Phase averaging by ensemble averaging of data clumped in phase will cause phase bias error if the mean phase of the clumped data does not equal the desired phase. The curve-fitting method presented here avoids phase bias error. The performance of the curve-fitting method was compared favourably with the ensemble averaging method when processing phase-clumped experimental data. Generally, the curve-fitting method exceeded the performance of the ensemble averaging method when the data were clumped in phase and matched the performance of the ensemble averaging method when the data were randomly distributed in phase. Experimentally obtained phase-clumped two-dimensional velocity fields at the centre-line of a forward-facing bent-duct oscillating water column were processed using the curve-fitting method. The phase-averaged velocity fields were combined for visualization purposes in the form of animated graphics interchange format (GIF) images to show the velocity fields over a wave cycle. The GIF images correspond to the four separate monochromatic wave frequencies tested and accompany the online version of this paper. The GIF images reveal the two-dimensional phase-averaged flow characteristics at the centre-line of the model with phenomena including oscillating flow, water column heave and slosh, front-wall swash and down wash, an outflow jet and vortices. It was concluded that phase averaging using splines finds a niche for the phase averaging of data which are dispersed (or clumped) in phase.

Effect of RANS-based turbulence models on nonlinear wave generation in a two-phase numerical wave tank

Ocean waves are the most important exciting source acting on marine structures such as ships, offshore platforms, wave energy converters and wavebreakers. In order to efficiently design the aforementioned structures, accurate modelling of these waves is of importance. In this paper a two dimensional Numerical Wave Tank (NWT) has been established based on the Reynoldsaveraged NavierStokes (RANS) equations for viscous, incompressible fluid and Volume of Fluid (VOF) method and a commercial software code ANSYS FLUENT (Release 15.0) has been used to numerically investigate ocean wave generation. Impact of different turbulence models such as standard k-ɛ, realizable k-ɛ, Shear Stress Transport (SST) and Reynolds Stress Models (RSM) on the generated ocean surface waves were investigated. With all uncertainties associated with various numerical setting aspects, experimental wave measurements over a wide range of wave conditions covering intermediate and deep water regimes have been conducted in a physical wave basin to validate the numerical results. The excessive generation of eddy viscosity resulted from using eddy viscosity turbulence models especially at the free surface interface, leads to a significant unphysical damping on the generated waves. Good numerical agreement with both experimental measurements and analytical wave theory was successfully achieved either with the RSM or implementing artificial turbulence damping at the airwater interface with the SST model.

Application of photogrammetry for spatial free surface elevation and velocity measurement in wave flumes

This article presents a method for obtaining the spatial free surface elevation and velocity field for the water surface in a wave flume over a relatively large measurement area for this type of application (approximately 1.5 m × 1.5 m). The technique employs proprietary videogrammetry software to post-process stereo images captured by multiple synchronised machine vision cameras. Dimensional resolution and other limitations are similar to that experienced for particle imaging velocimetry systems (x, y resolution of 2 mm). Imaging of the free surface was enabled by the use of millions of bespoke slightly positively buoyant fluorescent flakes. Ultraviolet light was used as the primary light source to excite the fluorescent flakes. Reflected ultraviolet light was attenuated by a high-pass filter fitted to the cameras so that only the emitted light from the fluorescent flakes was visible. The software was validated using a simple linear translation experiment. An application is demonstrated for the radiated wave field generated from a submerged sinusoidal heaving sphere for two cases: one single and five consecutive oscillations. Results agree with linear wave theory which indicates that the floating flakes had minimal impact on the water surface particle motion at the scale tested. It is, therefore, concluded that spatial measurement of the free surface elevation and velocity using the method presented has good resolution over a large measurement field. The flakes were found to follow the free surface well, but the measurement area is constrained to where the pattern of flakes exists in the image. Hence, application of floating markers is not suitable for experiments with significant outflow/upwelling which would wash away the floating markers from the intended measurement area.

Tracking the vortex core from a surface-piercing flat plate by particle image velocimetry and numerical simulation

The wake flow around the tip of a surface piercing flat plate at an angle of incidence was studied using two-dimensional particle image velocimetry as part of benchmarking the particle image velocimetry technique on the moving carriage in the Australian Maritime College towing tank. Particle image velocimetry results were found to be in close agreement with those of the benchmarking work presented by the Hydro Testing Alliance, and a method of tracking the tip-vortex core near a free surface throughout numerical simulation has been demonstrated. Issues affecting signal to noise ratio, such as specula reflections from the free surface and model geometry were overcome through the use of fluorescing particles and a high-pass optical filter. Numerical simulations using the ANSYS CFX Solver with the volume of fluid method were validated against the experimental results, and a methodology was developed for tracking the location of the wandering vortex core experimentally and through simulation. The ability of the scale-adaptive simulation shear stress transport turbulence model and the shear stress transport model to simulate three-dimensional flow with high streamline curvature was compared. The scale-adaptive simulation shear stress transport turbulence model was found to provide a computationally less resource-intensive method of simulating a complex flow topology with large eddies, providing an insight into a possible cause of tip-vortex aperiodic wandering motion. At high angles of attack, vortex shedding from the leading edge separation of the test geometry is identified as a possible cause of the wandering phenomena. In this study, the vortex centre and point of extreme core velocity were found not to be co-located. The point of extreme stream wise velocity within the vortex core was found to be located within half the vortex radius of the vortex centre.

Model predictive Control-based power take-off control of an oscillating water column wave energy conversion system

Australias extended coastline asserts abundance of wave and tidal power. The predictability of these energy sources and their proximity to cities and towns make them more desirable. Several tidal current turbine and ocean wave energy conversion projects have already been planned in the coastline of southern Australia. Some of these projects use air turbine technology with air driven turbines to harvest the energy from an oscillating water column. This study focuses on the power take-off control of a single stage unidirectional oscillating water column air turbine generator system, and proposes a model predictive control-based speed controller for the generator-turbine assembly. The proposed method is verified with simulation results that show the efficacy of the controller in extracting power from the turbine while maintaining the speed at the desired level.