Craig L. Stevens

The initial response of a stratified lake to a surface shear stress

Laboratory experiments are used to study the initial response of a stratified fluid to the action of a wind stress. The experiments are described in the context of a parameterization scheme that quantifies the strength of the applied stress relative to the bulk stability of the fluid and also the duration of the wind stress relative to the periods of the waves generated by the stress. This study concentrates on the first fundamental internal wave period in experiments where the fluid is considered to have upwelled, i.e. the stratified region of the fluid reaches the surface at the upwind endwall. The majority of the experiments use three-layer initial density profiles as an approximation to a continuously stratified water column. A linear model using normal modes proved successful prior to the commencement of upwelling and this enabled an estimate to be made of the time at which upwelling occurred. At this point the wave development ceased and the flows developed via entrainment mechanisms. Consideration of the energy budget showed that little of the input energy was stored in the system. The initial mixing efficiency, defined as the ratio of the mean potential energy gained to the energy imparted by the belt, never exceeded 30 %. Peak efficiency occurred when the surface stress was just sufficient to bring the interfacial region to the surface.

Hydrodynamic implications of large offshore mussel farms

Around 3000 Ha of New Zealands coastal waters are presently allocated to mussel farming of the Greenshell mussel Perna canaliculus mainly in small inshore farms of 30 000 to 80 000 m/sup 2/ each. The farms are constructed from blocks of parallel moored long-lines, with loops of spat-impregnated line hanging vertically in the upper 10-15 m of the water column (provided sufficient water depth). When fully laden, the mussel loops can present a rough diameter of 0.20 m or more to the flow. Industry expansion is likely to be in the form of larger offshore farms. Some of the larger proposed farms will have more than 1000 km of mussel line. This presents a potentially significant drag to currents and waves. In this paper, observations are used from one of the first large offshore farms constructed in New Zealand, in conjunction with scaling estimates of energy loss, to investigate the effect of the farm on waves, currents, and stratification. The farm, consisting of 230 long-lines arranged into 20 blocks, measured 650 m /spl times/ 2450 m. The hanging mussel loops at this site reached an average depth of 8 m, and water depth at midtide varied across the site from 10-12 m. Transects were taken through the farm using both conductivity, temperature, depth (CTD), and Acoustic Doppler Profiler (ADP) instruments. A second ADP was moored inside the farm providing velocity time series. The average current was reduced within the farm by 36%-63%. The moored ADP detected an undercurrent beneath the farm with velocities nearly twice that within the farm. Sharp changes in stratification coincide with the start of fully stocked mussel long-lines. Furthermore, other than at the very upstream end of the farm, stratification appears to have prevented significant vertical mixing between the fluid within the farm and that flowing under the farm. Wave energy was low during the 7-d deployment, with significant wave heights peaking at 0.25 m. The attenuation of transitional waves through the farm is examined by comparing simultaneous pressure sensor measurements from offshore and inshore of the farm. The observations show that wave energy attenuation was frequency dependent. Observed wave energy attenuation across the 650 m wide farm was approximately 5%, 10%, and 17% at wave frequencies of 0.1, 0.2, and 0.25 Hz, respectively.

Boundary-layers around bladed aquatic macrophytes

This paper describes a model of the diffusive boundary-layer around anaquatic macrophyte blade. Nutrient uptake at the base of the modelledboundary-layer was examined using previously published laboratory data. Theanalysis showed that, over the outer velocity range 0.01–0.16ms−1, nutrient uptake varied between that predicted byflat-plate boundary-layer theory and that expected with no mean advection. Aratio of theoretical and actual nutrient uptake rates was defined as a meansof categorising the transition between the two transport models. Themodelling was extended to show how fluctuating boundary conditions mayenhance nutrient uptake by the macrophyte; the scenario examined heresuggested that at otherwise low outer velocities, periodic stripping of theboundary-layer by passing waves increased nutrient uptake by a factor of 10.

Experimental and Numerical Study of the Turbulence Characteristics of Airflow around a Research Vessel

Airflow distortion by research vessels has been shown to significantly affect micrometeorological measurements. This study uses an efficient time-dependent large-eddy simulation numerical technique to investigate the effect of the R/V Tangaroaon the characteristics of the mean airflow and the turbulent wake. Detailed comparison is given between the numerical results and an extensive experimental dataset. The study is performed for the whole range of relative wind directions and for instruments located in regions of high and low flow distortion. The experimental data show that both the normalized wind speed and normalized standard deviation are only weakly dependent on wind speed, ship speed, ship motion, and sea state, but strongly dependent on relative wind direction. Very good agreement is obtained between the experimental and numerical data for the mean flow, standard deviation, and turbulence spectra in the wake, even in areas of strong turbulence.

Applications of a maximised cross-correlation technique for resolving velocity fields in laboratory experiments

SummaryIn this study a maximised cross-correlation technique is applied to two very different experiments. The first is a laminar, slowly developing experiment where the main scales of the motion are comparable to the image size. Here extensive use of multiple passes with the technique and various filters are applied to replace erroneous measurements. The second experiment has both laminar and turbulent regions of flow, and in addition, many of the scales of motion are smaller than the correlation window size. Mean velocities are resolved in this application, as well as a qualitative impression of the turbulent fluctuations.The results from the experiments are discussed, along with the parameters available and how they affect the efficiency of the process. In addition pre- and post-processing options for the technique are examined. The technique proves a viable method that provides good results while using commonly available hardware.

Estimation of wind-forced internal seiche amplitudes in lakes and reservoirs, with data from British Columbia, Canada

Analyses of observations from four lakes in British Columbia, Canada, compare estimates of the amplitude of thermocline deflections to predictions of wind-driven internal seiche amplitudes made using the Wedderburn number,W. The study sites range from the 750 m diameter Brenda Mines pit-lake to the 107 km long Kootenay Lake. Causal filtering of the wind data with a frequency cut-off based on the fundamental baroclinic time-scale is critical for correct calculation ofW. With the filtering incorporated, good comparison betweenW, its integral equivalent the Lake numberLN and the observations can be made. In all but the mine pit-lake, upwelling or near-upwelling conditions (W≈1) were encountered.

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.

FLOW VISUALIZATION AROUND SINGLE- AND MULTIPLE-BLADED SEAWEEDS WITH VARIOUS MORPHOLOGIES1

Water flow was visualized around 10 seaweeds with various morphologies to determine the onset of turbulence and to estimate the scales of motion generated by the seaweeds themselves. For single‐bladed specimens of the kelps Laminaria setchellii, Costaria costatum, Macrocystis integrifolia, and Alaria marginata, the transition from a laminar to turbulent velocity boundary layer occurred at mainstream velocities of 1.5 cm.s−1. Transition to turbulence fm multiple‐bladed specimens of M. integrifolia, Nereocystis luetkeana, Egregia menziesii, and Fucus gardneri occurred at 2.5–3 cm‐s−1 and at 5 cm.s−1 for the coarsely branched red seaweed, Gelidium coulteri. Flow features such as separation, recirculating eddies, and Von Kánncán vortex streets were observed around various morphologtcal features. We suggest that in the field, flow around larger macroalgae such as kelp is mostly turbulent and that many seaweeds will lie within the wakes of neighboring macroalgae. For small, branched seaweeds such as G. coulteri, however, the meshlike structure may damp turbulence within the thallus, thereby increasing the mainstream velocity at which the transition to turbulence occurs.

Modelling of diffusion boundary-layers in subtidal macroalgal canopies: the response to waves and currents

Abstract. A model is described for nutrient uptake by the giant kelp Macrocystis integrifolia Bory that incorporates blade boundary-layer dynamics. The model input is derived from field measurements of water motion near wave-exposed and wave-sheltered M. integrifolia beds. Factors considered in the model include (i) the distinction between uni-directional and oscillatory flow in wave-exposed and -sheltered locations, (ii) the reduction in boundary-layer thickness due to wave-driven flow and (iii) the tight packing of M. integrifolia beds at low tide which can reduce the in-canopy flow rates. The velocities are incorporated into the model which indicates that uptake rates can vary between 5 to 300% of the steady-state equilibrium value depending on the flow regime.

Field measurement of the dynamics of the bull kelp Durvillaea antarctica (Chamisso) Heriot

Seaweed habitats and morphological development are strongly affected by wave forces. Novel measurements were made of the force dynamics of the large intertidal macroalga Durvillaea antarctica under the influence of wave action. Synchronized video, a pressure sensor and a resistance wave gauge provided data describing the wave field. The response of seaweeds to waves was gauged using instrumentation mounted directly on the seaweed, including accelerometers and displacement and force transducers. These field measurements were used to estimate forces and bending moments acting at the holdfast, where failure is most likely to occur. For waves of the order of 0.5 m high, we measured maximum forces on the stipe of around 300 N and blade accelerations that exceeded 30 m s−2. During large wave events, inferred bending moments at the base of the stipe reached average values of around 140 N m. There was a decoupling between the blade response and the force experienced at the stipe base. Furthermore, changes in water depth throughout the tidal cycle had a systematic effect on blade accelerations and moments at the holdfast.