Ross Vennell

Tuning turbines in a tidal channel

As tidal turbine farms grow they interact with the larger scale flow along a channel by increasing the channels drag coefficient. This interaction limits a channels potential to produce power. A 1D model for a tidal channel is combined with a theory for turbines in a channel to show that the tuning of the flow through the turbines and the density of turbines in a channels cross-section also interact with the larger scale flow, via the drag coefficient, to determine the power available for production. To maximise turbine efficiency, i.e. the power available per turbine, farms must occupy the largest fraction of a channels cross-section permitted by navigational and environmental constraints. Maximising of power available with these necessarily densely packed farms requires turbines to be tuned for a particular channel and turbine density. The optimal through-flow tuning fraction varies from near 1/3 for small farms occupying a small fraction of the cross-section, to near 1 for large farms occupying most of the cross-section. Consequently, tunings are higher than the optimal through-flow tuning of 1/3 for an isolated turbine from the classic turbine theory. Large optimally tuned farms can realise most of a channels potential. Optimal tunings are dependent on the number of turbines per row, the number of rows, as well as the channel geometry, the background bottom friction coefficient and the tidal forcing.

Tuning tidal turbines in-concert to maximise farm efficiency

Tuning is essential to maximise the output of turbines extracting power from tidal currents. To realise a large fraction of a narrow channels potential, rows of turbines not only have to be tuned for a particular tidal channel, they must also be tuned in the presence of all the other rows, i.e. ‘tuned in-concert’. The necessity for in-concert tuning to maximise farm efficiency occurs because the tuning of any one row affects a channels total drag coefficient and hence the flow through all other rows. Surprisingly, in several circumstances the optimal in-concert tunings are the same or almost the same for all rows. Firstly, in both constricted and unconstricted channels, rows with the same turbine density have the same optimal tuning. Secondly, turbine rows in channels with a quasi-steady dynamical balance typically have almost the same optimal in-concert tunings, irrespective of their turbine density or any channel constrictions. Channel constrictions, occupying a large fraction of the cross-section or adding more rows of turbines, also make optimal tunings more uniform between rows. Adding turbines to a cross-section increases a farms efficiency. However, in a law of diminishing returns for quasi-steady channels, turbine efficiency (the output per turbine) decreases as turbines are added to a cross-section. In contrast, for inertial channels with only moderate constrictions, turbine efficiency increases as turbines are added to a cross-section.

Acoustic Doppler Current Profiler measurements of tidal phase and amplitude in Cook Strait, New Zealand

Abstract Strong tidal flows are observed in Cook Strait which separates the North and South Islands of New Zealand. The high velocities within the 30 km wide Strait result from a 140° phase difference in the M 2 tide between the ends of the Strait. Extraordinarily 135° of this phase difference occurs over just 40 km in the narrowest section of the Strait. Measurements from a ship mounted Acoustic Doppler Current Profiler (ADCP) over a single tidal cycle are used to determine the horizontal and vertical variation of tidal phase and amplitude in the Strait. Results show that tidal velocity amplitude ranges from 70 cm s −1 on the west of the Strait to 140 cm s −1 on the east. There was little amplitude variation over most of the water column. The eastern side of the Strait led the west by 20°. Near bottom velocity led surface velocity by approximately 10° due to the effect of bottom friction on the oscillating flow. Results from a subsequent 1 month deployment of ADCPs on the same line as the ship track are used to hindcast the semi-diurnal tide on the day of the shipboard measurements. The shipboard measured semi-diurnal tidal amplitude and phase agree extremely well with the hindcast composite of the three largest tidal constituents. Thus shipboard measurements over a single tidal cycle were able to accurately determine the horizontal and vertical variation of phase and amplitude of the semi-diurnal tide in Cook Strait.

Optimization of multiple turbine arrays in a channel with tidally reversing flow by numerical modelling with adaptive mesh

At tidal energy sites, large arrays of hundreds of turbines will be required to generate economically significant amounts of energy. Owing to wake effects within the array, the placement of turbines within will be vital to capturing the maximum energy from the resource. This study presents preliminary results using Gerris, an adaptive mesh flow solver, to investigate the flow through four different arrays of 15 turbines each. The goal is to optimize the position of turbines within an array in an idealized channel. The turbines are represented as areas of increased bottom friction in an adaptive mesh model so that the flow and power capture in tidally reversing flow through large arrays can be studied. The effect of oscillating tides is studied, with interesting dynamics generated as the tidal current reverses direction, forcing turbulent flow through the array. The energy removed from the flow by each of the four arrays is compared over a tidal cycle. A staggered array is found to extract 54 per cent more energy than a non-staggered array. Furthermore, an array positioned to one side of the channel is found to remove a similar amount of energy compared with an array in the centre of the channel.

Observation of a Fast Continental Shelf Wave Generated by a Storm Impacting Newfoundland Using Wavelet and Cross-Wavelet Analyses

Abstract Wavelet and cross-wavelet power spectra of sea level records from tide gauges along the Atlantic coast of Canada showed a low-frequency barotropic response after Hurricane Florence crossed the Newfoundland shelf in September 2006. In comparison with two other storms, the results showed that Florence was the only one that excited a propagating sea level disturbance with a period range similar to the passage time of the storm over the shelf (26–30 h) and phase shifts consistent with a barotropic continental shelf wave (CSW). The high amplitude of the oscillations generated by Florence along the shore diminished from approximately 45 to 12 cm as the CSW propagated from the south coast of Newfoundland to the southern Nova Scotia seaboard. This paper presents the first direct measurement of a remarkably high alongshore group speed (11.4 ± 5.9 m s−1), in the manner of free-barotropic CSW, by examination of sea level wavelet power spectra at different locations. Furthermore, using cross-wavelet analysis...

Resonance and trapping of topographic transient ocean waves generated by a moving atmospheric disturbance

Proudman resonance amplifies the oceanic forced wave beneath moving atmospheric pressure disturbances. The amplification varies with water depth; consequently, the forced wave beneath a disturbance crossing topography radiates transient free waves. Transients are shown to magnify the effects of Proudman resonance for disturbances crossing the coast or shelf at particular angles. A Snell like reflection law gives rise to a type of resonance for relatively slow moving disturbances crossing a coast in an otherwise flat-bottomed ocean. This occurs for translation speeds less than the shallow water wave speed for disturbances approaching the coast at a critical angle given by the inverse sine of the Froude number of the disturbance. A disturbance crossing the shelf at particular angles can also excite seiche modes of the shelf via generation of a transient at the continental slope. Beyond a typically small angle of incidence, transients generated by a disturbance crossing the continental slope and coast will be trapped on the shelf by internal reflection. The refraction law for a fast-moving forced wave crossing an ocean ridge at greater than a small angle of incidence also results in trapped free-wave transients with tsunami-like periods propagating along the ridge. The subcritical resonance, excitation of shelf modes and trapping of the transients may have implications for storm surges and the generation of destructive meteotsunami.

Measurements of an Oceanic Front Using a Front-Following Algorithm for AVHRR SST Imagery

The Subtropical Front (STF) is a global front which extends around the Southern Ocean and is the boundary where the Subantarctic Surface Water mass (SAW) converges with the Subtropical Surface Water mass (STW). The Southland Front (SF) is part of the STF, which lies off the east coast of the South Island, New Zealand. The SF is narrow, approximately 8 km, with a temperature difference of approximately 1.8°C which can be detected using remote sensing Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data. The work presented here is an application of remote sensing for the first detailed study of the surface spatial and temporal variability of the SF. The variability of the SF was quantified using an algorithm developed to follow the Front using AVHRR SST imagery. The algorithm used a new approach to determine the position, mean SST, SST difference, width, and gradient across the Front. Three time scales of variability were examined: long-term (3 years), annual, and seasonal. The algorithm efficiently followed the SF and consistently showed the 3-year mean position was stable and constrained by the bathymetry of the continental slope. Seasonally, the front moved inshore during summer. The temperature gradient across the front was strongest and the front narrowest in winter. The decrease in SST gradient during the 3-year data set coincided with the decrease in Southern Oscillation Index (SOI).

Evolution of supercooling under coastal Antarctic sea ice during winter

Abstract Here we describe the evolution through winter of a layer of in situ supercooled water beneath the sea ice at a site close to the McMurdo Ice Shelf. From early winter (May), the temperature of the upper water column was below its surface freezing point, implying contact with an ice shelf at depth. By late winter the supercooled layer was c. 40 m deep with a maximum supercooling of c. 25 mK located 1–2 m below the sea ice-water interface. Transitory in situ supercooling events were also observed, one lasting c. 17 hours and reaching a depth of 70 m. In spite of these very low temperatures the isotopic composition of the water was relatively heavy, suggesting little glacial melt. Further, the waters temperature-salinity signature indicates contributions to water mass properties from High Salinity Shelf Water produced in areas of high sea ice production to the north of McMurdo Sound. Our measurements imply the existence of a heat sink beneath the supercooled layer that extracts heat from the ocean to thicken and cool this layer and contributes to the thickness of the sea ice cover. This sink is linked to the circulation pattern of the McMurdo Sound.

Variability of water masses through the Mernoo Saddle, South Island, New Zealand

Abstract The Mernoo Saddle is situated c. 100 km east of Banks Peninsula, Canterbury, New Zealand at 44°S 174°E. The Saddle separates the South Island of New Zealand from an underwater ridge known as the Chatham Rise. The Rise acts as a partial barrier to the flow of the subantarctic surface water mass (SAW) and the subtropical surface water mass (STW), which are part of the global Subtropical Front extending around the Southern Ocean. This study examined the variability of the water masses flowing through the Mernoo Saddle using a 3‐year data set of Advanced Very High Resolution Radiometer (AVHRR) Sea Surface Temperature (SST) imagery. This investigation revealed that SAW extended north through the western edge of the Mernoo Saddle for most of the year, however in winter and early spring a southward extension of STW was observed.

Prediction of fluid forces acting on a hand model in unsteady flow conditions

The aim of this study was to develop a method to predict fluid forces acting on the human hand in unsteady flow swimming conditions. A mechanical system consisting of a pulley and chain mechanism and load cell was constructed to rotate a hand model in fluid flows. To measure the angular displacement of the hand model a potentiometer was attached to the axis of the rotation. The hand model was then fixed at various angles about the longitudinal axis of the hand model and rotated at different flow velocities in a swimming flume for 258 different trials to approximate a swimmers stroke in unsteady flow conditions. Pressures were taken from 12 transducers embedded in the hand model at a sampling frequency of 200Hz. The resultant fluid force acting on the hand model was then determined on the basis of the kinetic and kinematic data taken from the mechanical system at the frequency of 200Hz. A stepwise regression analysis was applied to acquire higher order polynomial equations that predict the fluid force acting on the accelerating hand model from the 12 pressure values. The root mean square (RMS) difference between the resultant fluid force measured and that predicted from the single best-fit polynomial equation across all trials was 5N. The method developed in the present study accurately predicted the fluid forces acting on the hand model.