Gregor Macfarlane

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.

A Novel Method for Generating Continuously Surfable Waves: Comparison of Predictions With Experimental Results

In this paper, a novel idea to produce continuous breaking waves is discussed, whereby a pressure source is rotated within an annular wave pool. The concept is that the inner ring of the annulus has a sloping bathymetry to induce wave breaking from the wake of the pressure source. In order to refine the technique, work is being conducted to better understand the mechanics of surfable waves generated by moving pressure sources in restricted water. This paper reports on the first stage of an experimental investigation of a novel method for generating continuously surfable waves utilising a moving pressure source. The aim was to measure and assess the waves generated by two parabolic pressure sources and a wavedozer [I] for their suitability for future development of continuous breaking surfable waves. The tests were conducted at the Australian Maritime College (AMC), University of Tasmania (UTas) 100 metre long towing tank. The experimental results as variations in wave height ( H ) divided by water depth ( h ) as functions of depth Froude number ( Fr h ) and h , together with predictions from both methods, are presented in this paper. Finally, measures of the wave making energy efficiency of each pressure source, and the surfable quality of the waves generated by it, were developed and are presented.

Investigation into float-over installations of minimal platforms by hydrodynamic model testing

The idea of using float-over installations for minimal facilities platforms was shown to offer significant advantages, especially when coupled with a substructure installed by a jackup rig. Recently, float-over installations of minimal facilities have been conducted by the cantilevered method by ICON Engineering PlY Ltd (ICON). The operation involves the platform topsides being loaded and transported to site on a barge, skidded over the barge bow, and lowered onto the jacket. The paper presents results of a research project undertaken by the Australian Maritime College (AMC) in conjunction with ICON, with the objective to investigate motions of a barge and loads exerted on the jacket when the two are docked together for a smooth load transfer operation. The model of an installation barge has been tested in the AMC wave basin and response amplitude operators of the barge motions have been determined for both the free floating and docked conditions. A range of wave periods and heights has beeninvestigated. Model test results have been used to verify numerical predictions used in the design, and to get insight into uncertainties, which may otherwise be difficult to assess using standard software.

A Novel Method for Generating Continuously Surfable Waves—Comparison of Predictions With Experimental Results

In this paper, a novel idea to produce continuous breaking waves is discussed, whereby a pressure source is rotated within an annular wave pool, with the inner ring of the annulus having a sloping bathymetry to induce wave breaking. In order to refine the technique, work is being conducted to better understand the mechanics of surfable waves generated by moving pressure sources in restricted water. The pool aims to be capable of creating waves suitable for surfers from beginner to expert level, with an added benefit being by providing a safe learning environment, the overall surfing ability of the participants should be improved. The method of approach reported in this paper is the first stage of an experimental investigation of a novel method for generating continuously surfable waves utilizing a moving pressure source. The aim was to measure and assess the waves generated by two parabolic pressure sources and a wedge-shaped wavedozer (Driscoll, A., and Renilson, M. R., 1980, The Wavedozer. A System of Generating Stationary Waves in a Circulating Water Channel, University of Glasgow, Naval Architecture and Ocean Engineering, Glasgow, UK) for their suitability for future development of continuous breaking surfable waves. The tests were conducted at the University of Tasmania (UTas) Australian Maritime College (AMC) 100 m long towing tank. The predictions and experimental results for the wave height (H) at different values of depth Froude number (Frh) are presented in this paper. Finally, the preferred pressure source is determined based on the wave making energy efficiency and the quality of the waves for surfing.

Investigation Into the Sensitivity of the Dynamic Hook Load During Subsea Deployment of a Suction Can

Suction cans are commonly used as foundations of fixed offshore structures, subsea equipment, and anchors of mooring lines. During the offshore installation phase, when a suction can is submerged, it attracts large heave added mass, which may be an order of magnitude higher than the mass of the can in air. Due to motions of an installation vessel the dynamic hook load may significantly exceed the submerged weight of the can. The dynamic hook load must be accurately predicted, as it governs selection of the vessel, lifting gear and rigging, and defines the allowable installation sea state. The objective of this paper is to examine the sensitivity of the dynamic hook load to hydrodynamic properties of the suction can, in particular its heave added mass and damping. This research is motivated by the lack of data on such properties, which are usually estimated by simplified methods with some engineering judgement and assumptions. A single degree of freedom system is considered and the frequency domain spectral analysis is used, which employs the stochastic linearization of the nonlinear damping component. The added mass and damping of a 6-meter diameter suction can of dimensions typical for Australian North West Shelf developments have been determined by testing a 1:10 model in the 4.1 m deep basin of the Australian Maritime College. Free decay tests were conducted at several frequencies and the added mass, linear and nonlinear damping components determined. The effect of open hatches on the hydrodynamic properties was examined by fitting the model with hatches of various diameters, with up to 4.8% of the relative area open. Results of the tests demonstrate that the added mass and damping are higher, when compared with estimates based on empirical data for non-oscillatory flow. Within the Keulegan-Carpenter number range of 0.1–1.0, open hatches impact significantly on the added mass and produce additional damping, which is found to be linear with the heave velocity. Results of the tests and their interpretation are discussed. Sensitivity analysis shows that if the model test results are used in the dynamic lift analysis for an installation vessel and sea states considered, the predicted hook load is generally less than its values obtained by using simplified estimates. In particular, the increase in linear damping due to open hatches is responsible for up to 20% reduction in the dynamic hook load, with 2.4% of the relative top area open.© 2007 ASME

Time-frequency analysis of ship wave patterns in shallow water: modelling and experiments

A spectrogram of a ship wake is a heat map that visualises the time-dependent frequency spectrum of surface height measurements taken at a single point as the ship travels by. Spectrograms are easy to compute and, if properly interpreted, have the potential to provide crucial information about various properties of the ship in question. Here we use geometrical arguments and analysis of an idealised mathematical model to identify features of spectrograms, concentrating on the effects of a finite-depth channel. Our results depend heavily on whether the flow regime is subcritical or supercritical. To support our theoretical predictions, we compare with data taken from experiments we conducted in a model test basin using a variety of realistic ship hulls. Finally, we note that vessels with a high aspect ratio appear to produce spectrogram data that contains periodic patterns. We can reproduce this behaviour in our mathematical model by using a so-called two-point wavemaker. These results highlight the role of wave interference effects in spectrograms of ship wakes.

Wave Wake: Focus on Vessel Operations within Sheltered Waterways

This article describes the development of an empirical tool that can rapidly and accurately predict the characteristics of the wave wake generated by vessels that typically operate within sheltered waterways, including small commercial craft and recreational vessels. A wave wake regulatory criterion is also proposed and incorporated within the prediction tool.

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.