Methods in Oceanography最新文献

This paper presents a multi-objective optimization design framework that is integrated with the Computer Aided Design (CAD) for geometric variation and Computational Fluid Dynamics (CFD) software for hydrodynamic computations for the design of Autonomous Underwater Vehicles (AUVs). The optimization model utilizes the ‘Non-dominated Sorting Genetic Algorithm (NSGA-II)’. In the present model hull geometric parameters (i.e. length of nose $\left(L_n\right)$, length of the parallel middle body $\left(L_m\right)$, length of the tail $\left(L_t\right)$, maximum diameter $\left(D_{\max }\right)$, and two shape variation coefficients of nose $\left(n_n\right)$ and tail $\left(n_t\right)$) are considered as the design parameters and minimization of viscous drag, and maximization of nominal wake fraction and total volume are considered as the objective functions for the integrated design approach. CFD software (Shipflow^∗™) is used to evaluate the viscous drag and it is integrated with the CAD definition. The optimization framework NSGA-II is implemented in MATLAB^∗∗™. Finally, we present a design example of an existing AUV Cormoran and show that the integration of NSGA-II with CFD and CAD is effective for AUV hull form design. Our reported results show that for the given bounds on the design parameters, the optimization design framework is able to produce more efficient hull forms than the existing design.

We present an integrated framework for joint estimation and pursuit of dynamic features in the ocean, over large spatial scales and with multiple collaborating vehicles relying on limited communications. Our approach uses ocean model predictions to design closed-loop networked control at short time scales, and the primary innovation is to represent model uncertainty via a projection of ensemble forecasts into local linearized vehicle coordinates. Based on this projection, we identify a stochastic linear time-invariant model for estimation and control design. The methodology accurately decomposes spatial and temporal variations, exploits coupling between sites along the feature, and allows for advanced methods in communication-constrained control. Simulations with three example datasets successfully demonstrate the proof-of-concept.

An experimental deployment of a new type of unmanned vessel is presented. The Christian Michelsen Research SailBuoy, a remotely-controlled surface vehicle, sampled near-surface properties during a two-month mission in the northern Gulf of Mexico in March–May, 2013. Averaged over the entire deployment, the vessel speed over ground was $42\pm 30{\mathrm{cm s}}^{-1}$ ($\pm$ one standard deviation) with a maximum of $180{\mathrm{cm s}}^{-1}$. During the 62 days of the mission, the SailBuoy covered a total range of approximately 400 km in both meridional and zonal directions, with a cumulative total distance of approximately 2400 km. Three parameters were recorded: sea surface temperature, conductivity, and dissolved oxygen. Observed surface temperature and salinity records are compared with remote sensing data and the salinity fields from a regional ocean modeling system, respectively. The absolute difference between remote sensing data to surface temperature is on an average approximately 0.5 °C. The comparison with the full Gulf of Mexico and the nested Northern Gulf of Mexico HYCOM models demonstrates the validity and usefulness of SailBuoy measurements and the instrument’s utility in evaluating fields produced by ocean models having different attributes. The potential of the SailBuoy for mapping a large-scale river plume, which would be challenging or costly with conventional ship surveys and/or remote sensing, is demonstrated.

The hadal trenches and the biology and ecology of the organisms that inhabit them remain one of the least understood marine environments. The study of hadal trenches which is often referred to as hadal science, needs special technical equipment support such as landers, unmanned submersibles and manned submersibles. Sending sampling devices or exploratory vehicles to hadal depths is technically challenging and expensive, consequently, our current understanding of hadal ecological structure is still very much in its infancy. In recognition of the significance that hadal science holds and the unique and challenging requirements that work in the deep ocean presents, Shanghai Ocean University has made a significant commitment to develop operational support for the promotion of hadal science in China. The present authors from the JIAOLONG development team were invited by Shanghai Ocean University to establish a hadal science and technology research center (HAST). The first focus of HAST is to construct a movable laboratory for hadal trenches which includes a mothership, an Human Occupied Vehicle, an Autonomous and Remotely-operated Vehicle and several landers. The purpose of this paper is to introduce the basic philosophy and concepts for the movable laboratory and the preliminary designs for the manned submersible, unmanned submersible and landers. Through these designs all the technical problems to be solved in the development of the full ocean depth surveying and sampling tools are identified and possible solutions to the key technical issues are discussed.

Measurements of the wake behind an experimental 3.5 m wide hydrofoil fabricated to accommodate a large acoustic receiving array on an autonomous underwater vehicle are reported. Results from laboratory experiments using particle image velocimetry of a full scale prototype in a large flume tank are compared to 2D numerical simulations. A parameter space of four flow speeds and five angles of attack are examined. An empirical model to predict the downstream velocity in the wake is developed based on the experimental results and the numerical simulations.

A combination of scientific, economic, technological and policy drivers is behind a recent upsurge in the use of marine autonomous systems (and accompanying miniaturized sensors) for environmental mapping and monitoring. Increased spatial–temporal resolution and coverage of data, at reduced cost, is particularly vital for effective spatial management of highly dynamic and heterogeneous shelf environments. This proof-of-concept study involves integration of a novel combination of sensors onto buoyancy-driven submarine gliders, in order to assess their suitability for ecosystem monitoring in shelf waters at a variety of trophic levels. Two shallow-water Slocum gliders were equipped with CTD and fluorometer to measure physical properties and chlorophyll, respectively. One glider was also equipped with a single-frequency echosounder to collect information on zooplankton and fish distribution. The other glider carried a Passive Acoustic Monitoring system to detect and record cetacean vocalizations, and a passive sampler to detect chemical contaminants in the water column. The two gliders were deployed together off southwest UK in autumn 2013, and targeted a known tidal-mixing front west of the Isles of Scilly. The gliders’ mission took about 40 days, with each glider travelling distances of >1000 km and undertaking >2500 dives to depths of up to 100 m. Controlling glider flight and alignment of the two glider trajectories proved to be particularly challenging due to strong tidal flows. However, the gliders continued to collect data in poor weather when an accompanying research vessel was unable to operate. In addition, all glider sensors generated useful data, with particularly interesting initial results relating to subsurface chlorophyll maxima and numerous fish/cetacean detections within the water column. The broader implications of this study for marine ecosystem monitoring with submarine gliders are discussed.

My professional career has largely revolved around developing and operating Autonomous Underwater Vehicles (AUVs) for ocean science. It has proven immensely gratifying; there are very few other enterprises that offer a similar combination of interesting people, tough intellectual problems, opportunity to work in the most remote and beautiful parts of the planet, and the satisfaction of contributing to an important endeavor. This article tracks my research and development activity, starting with early technology exploration when research funding was thin, to my first field programs, to leadership of larger enterprises where AUVs became elements of integrated observation-modeling systems. Not all of the activity was in the laboratory; as the platforms matured, and applications become better defined, commercialization activity became the dominant vector of AUV capability to the larger community. Most recently, my AUV work has focused on a new generation of long-range AUVs and the biological investigations they are designed to enable. Today AUVs are accepted oceanographic tools, and science users are increasingly sophisticated. However, in the late 80s, when I started, it was not at all clear how oceanographers would employ AUVs, or what operational AUVs would look like.

A key challenge for ecosystem science in the 21st century is to characterize emerging trends in ecosystem productivity due to climate change and to better predict cycles in ecosystem variability. A first step toward this goal is to be able to characterize phytoplankton variability across a wide range of spatial and temporal scales. In this paper, 15 months of Wave Glider (WG) fluorometer measurements made across the Pacific Ocean were used to understand how WGs complement existing chlorophyll-a-based measurements of phytoplankton biomass from satellite platforms. Extensive analysis of the WG transects demonstrated that WG fluorometer readings reliably characterized similar large-scale variability in satellite Chl-a measurements in four distinct ecosystem types including coastal upwelling, transition zone, oligotrophic and equatorial upwelling regions. Complementary information provided by WG measurements included better resolution of coastal Chl-a patches and prominent diel cycles in the open ocean. The decorrelation scales computed from WG fluorometer measurements in this study provide necessary information for designing observing systems, process experiments, and data assimilation studies. We conclude this paper by suggesting how WGs can be used to anchor satellite measurements and to develop better predictive models.

The last decade has seen the eagerly anticipated introduction of marine autonomous systems as a pragmatic tool for ocean observation. However, outstanding reliability problems means that these vehicles are not yet fulfilling their true potential. Of the classes of problems, loss of communication with a marine autonomous system is both fundamental and difficult to diagnose. In our view, this is due to two reasons: first in many cases users are not technologists and secondly if a vehicle is lost the task of diagnosing the root cause is subject to epistemic uncertainty that users are often reluctant to quantify in a formal manner. As a result users may accept the first hypothesis considered as the main root cause for loss of communication. We show that this approach can result in an increased unreliability of marine autonomous systems through failure to ascertain and then address the true root causes. Consequently, we propose a probability tree approach to help diagnose root cause(s) for loss of communication with a marine autonomous system (MAS). The model was developed based on the results of two detailed investigations and a body of failure data collected from 205 undersea glider operations.

Microstructure measurements of temperature and current shear are made using an autonomous underwater glider. The glider is equipped with fast-response thermistors and airfoil shear probes, providing measurements of dissipation rate of temperature variance, $\chi$, and of turbulent kinetic energy, $\epsilon$, respectively. Furthermore, by fitting the temperature gradient variance spectra to a theoretical model, an independent measurement of $\epsilon$ is obtained. Both Batchelor (${\epsilon }_B$) and Kraichnan (${\epsilon }_K$) theoretical forms are used. Shear probe measurements are reported elsewhere; here, the thermistor-derived ${\epsilon }_B$ and ${\epsilon }_K$ are compared to the shear probe results, demonstrating the possibility of dissipation measurements using gliders equipped with thermistors only. A total of 152 dive and climb profiles are used, collected during a one-week mission in the Faroe Bank Channel, sampling the turbulent dense overflow plume and the ambient water above. Measurement of $\epsilon$ with thermistors using a glider requires careful consideration of data quality. Data are screened for glider flight properties, measurement noise, and the quality of fits to the theoretical models. Resulting dissipation rates from the two independent methods compare well for dissipation rates below 2×10⁻⁷  W kg⁻¹. For more energetic turbulence, thermistors underestimate dissipation rates significantly, caused primarily by increased uncertainty in the time response correction. Batchelor and Kraichnan spectral models give very similar results. Concurrent measurements of $\epsilon$ and $\chi$ are used to compute the dissipation flux coefficient $\Gamma$ (or so-called apparent mixing efficiency). A wide range of values is found, with a mode value of $\Gamma \approx 0.14$, in agreement with previous studies. Gliders prove to be suitable platforms for ocean microstructure measurements, complementary to existing methods.