Mixing Efficiency in the Presence of Stratification: When Is It Constant?

The efficiency of the conversion of mechanical to potential energy, often expressed as the flux Richardson number, Rif, is an important determinant of vertical mixing in the ocean. To examine the dependence of Rif on the buoyancy Reynolds number, ReB, we analyze three sets of data: microstructure profiler data for which mixing is inferred from rates of dissipation of turbulent kinetic energy (ε) and temperature variance (χ) measured in the open ocean, time series of spectrally fit values of ε and covariance-derived buoyancy fluxes measured in nearshore internal waves, and time series of spectrally fit values of ε and χ measured in an energetic estuarine flow. While profiler data are well represented by Rif ≈ 0.2 for 1 < ReB < 1,000, the covariance data have much larger values of ReB and, consistent with direct numerical simulation results, show that Rif ~ ReB −0.5. The estuarine data have values of ReB that fall between those of the other two data sets but also shows Rif ≈ 0.2 for ReB < 5000. Overall, these data suggest that Rif is in general not constant and may be substantially less than 0.2 when ReB is large, although the value at which the transition from constant to ReB-dependent mixing may depend on additional parameters that are yet to be determined. Nonetheless, for much of the ocean, ReB < 100 and so Rif is constant there

Extracting Reynolds stresses from acoustic Doppler current profiler measurements in wave-dominated environments

Surface waves introduce velocity correlations that bias and often dominate Reynolds stress estimates made using the traditional variance method for acoustic Doppler current profilers (ADCPs). This analysis shows that the wave bias is the sum of a real wave stress and an error due to instrument tilt, both of which have a large uncertainty. Three alternative extensions to the variance method for calculating Reynolds stress profiles from ADCP measurements in wavy conditions are analyzed. The previously proposed variance fitting method (Variance Fit) is evaluated and two more general methods that use along- and between-beam velocity differencing with adaptive filtering (Vertical AF and Horizontal AF) are derived. The three methods are tested on datasets containing long-period monochromatic swell (Moorea, French Polynesia) and shorter-period mixed swell (Santa Barbara, California). The Variance Fit method leaves a residual wave bias in beam velocity variances, especially for intermediate waves, but gives physically reasonable Reynolds stress estimates because most of the residual wave bias cancels when the variance method is applied. The new Vertical AF method does not produce inherent wave bias in beam velocity variances, but yields comparable Reynolds stresses to the Variance Fit method. The Horizontal AF method performs poorly for all but monochromatic waves. Error remaining after one of the above methods is applied can be attributed to residual wave error, correlation of turbulence between points chosen for differencing, or correlation between waves and turbulence. A simple procedure is provided for determining the minimum bin separation that can be used

A field investigation into the effects of a kelp forest (Macrocystis pyrifera) on coastal hydrodynamics and transport

Macrocystis pyrifera (Giant Kelp) forests form important habitats in temperate coastal regions. Hydrodynamics control the transport of nutrients, food particles, larvae and spores at scales ranging from boundary layers around individual blades to entire kelp forests. Our measurements include vertical profiles of current and temperature, and concurrent wave measurements, at a number of different locations in and around a kelp forest at Santa Cruz, California. We find that flow at the site is dominated by variations at diurnal and semidiurnal frequencies. A vertically sheared across-shore flow, consistent with flow driven by an across-shore density gradient, is thought to be important for exchange between the kelp forest and the surrounding coastal ocean. Within the kelp forest, currents are reduced by a factor that correlates with surface canopy coverage, higher frequency internal waves are damped, and onshore transport due to waves (Stokes drift) is estimated to be similar in magnitude to that due to currents. Richardson numbers within the kelp forest are higher than those outside the kelp forest and indicate that the water column within the kelp forest is usually stable to turbulence generation by mean velocity shear

Large-eddy simulation of the lid-driven cubic cavity flow by the spectral element method

This paper presents the large-eddy simulation of the lid-driven cubic cavity flow by the spectral element method (SEM) using the dynamic model. Two spectral filtering techniques suitable for these simulations have been implemented. Numerical results for Reynolds number $\text{Re}=12'000$ are showing very good agreement with other experimental and DNS results found in the literature

Strong and auxiliary forms of the semi-Lagrangian method for incompressible flows

We present a review of the semi-Lagrangian method for advection-diusion and incompressible Navier-Stokes equations discretized with high-order methods. In particular, we compare the strong form where the departure points are computed directly via backwards integration with the auxiliary form where an auxiliary advection equation is solved instead; the latter is also referred to as Operator Integration Factor Splitting (OIFS) scheme. For intermediate size of time steps the auxiliary form is preferrable but for large time steps only the strong form is stable

Fluid dynamics of double diffusive systems. Final report

Over the past seven years the authors have conducted an experimental, numerical, and theoretical study of the stability of doubly diffusive systems, and of mixing processes in stratified turbulence. For the study of the stability of doubly diffusive systems continuous gradients of two diffusing components (heat and salinity in this case) were used as the initial condition, and forcing was introduced by lateral heating and surface shear. The goals of this work included (1) quantification of the effects of finite amplitude disturbances on stable, double diffusive systems, particularly with respect to lateral heating, (2) development of an improved understanding of the physical phenomena present in sheardriven flows in doubly diffusive stratified environments, (3) increasing their knowledge-base on turbulent flow in stratified environments and how to represent it, and (4) formulation of a numerical code for such flows. In particular, the overall goals of this aspect of the research were as follows: (1) develop more general stability and scaling criteria for the destabilization of doubly-stratified systems, (2) study the variation of flow structure and scales with Rayleigh ratio and lateral heating ratio, (3) delineate the mechanisms governing convective layer formation and merging, (4) study the mixing processes within the convective layers and across interfaces, and estimate the heat and mass fluxes in such a system, (5) quantify the effects of turbulence and coherent structures (due to a wind-driven surface shear) on a doubly stratified system, and (6) study the interaction between surface shear and side-wall heating destabilization mechanisms. Goals 1 through 4 have been successfully completed and the results are described in this report

The effect of bivalve excurrent jet dynamics on mass transfer in a benthic boundary layer

International audienc

The coral proto free ocean carbon enrichment system (CP-FOCE): Engineering and development

Ocean acidification is driven by increasing atmospheric CO and represents a key threat to the Great Barrier Reef (GBR) and other coral reefs globally. Previous investigations have depended on studies in aquaria that are compromised by reduced ecological complexity and buffering capacity, and problems associated with containment. These aquaria studies also include artifacts such as artificial flow, light, temperature, and water quality conditions. In order to avoid these issues a new technology was needed for in situ science. This need was the driver behind development of the Free Ocean Carbon Enrichment (FOCE) approach. FOCE is similar in approach to the Free Air Carbon Enrichment (FACE) experiments pursued on land for almost two decades. FOCE as a systems concept was developed at the Monterey Bay Aquarium Research Institute (MBARI) to perform controlled in situ studies on the effects of increased carbon dioxide on ocean environments. FOCE systems inject carbon dioxide enriched water into the desired control volume to lower the environmental pH to a specified value. The challenge of maintaining reef conditions while manipulating the carbonate chemistry further advanced the FOCE concept. A shallow water reef version of FOCE was needed to perform this research at the University of Queensland. Working with MBARI the University of Queensland developed the Coral Proto - Free Ocean Carbon Dioxide Enrichment (CP - FOCE) system. Although the CP-FOCE does not differ conceptually from the original FOCE it is different in a couple of respects. First, it requires that a region of the coral flat be semi-enclosed in the chamber section of CP-FOCE. This allows the required amount of CO to be optimised. Second, by closing the enclosure off fully for a short time, the oxygen levels and carbonate chemistry can be accurately measured to determine net production/respiration as well as the calcification/dissolution rates of the organisms living within the chamber. In this paper we present the engineering details of the CP-FOCE system design. This paper details the unique engineering design and challenges of the CP-FOCE system The paper briefly outlines the chemical and biological requirements that provided the technical specifications for CP-FOCE to successfully study the impacts of the changing water chemistry on the physiology of calcareous reef organisms including corals and calcareous algae. We have also a brief outline of the methods used to perform measurements of calcification and dissolution rates. Additionally, we include discussion on production and respiration rates in CP-FOCE systems when maintained at ambient and two different increased pCO scenarios. We present technical results of this first deployment and address future plans for modifications and deployments of CP-FOCE. Forthcoming peer reviewed papers will describe the biological, chemical, and geochemical responses