Towed Body Research Articles

Optimal Trajectory Generation Using Model Predictive Control for Aerially Towed Cable Systems

This paper studies trajectory generation for a mothership that tows a drogue using a flexible cable. The contributions of this paper include model validation for the towed cable system described by a lumped mass extensible cable using flight data, and optimal trajectory generation for the towed cable system with tension constraints using model predictive control. The optimization problem is formulated using a combination of the squared error and -norm objective functions. Different desired circular trajectories of the towed body are used to calculate optimal trajectories for the towing vehicle subject to performance limits and wind disturbances. Trajectory generation for transitions from straight and level flight into an orbit is also presented. The computational efficiency is demonstrated, which is essential for potential real-time applications. This paper gives a framework for specifying an arbitrary flight path for the towed body by optimizing the action of the towing vehicle subject to constraints.

Towed body measurements of flow noise from a turbulent boundary layer under sea conditions.

Results from an underwater experiment under sea conditions on flow noise beneath a flat-plate turbulent boundary layer are presented. The measurements were performed with a towed body at towing speeds U=2.3,…,6.1 m/s and depths h=-150,…,-100 m. Flow noise is measured with a linear array of equally spaced hydrophones (Δx=70 mm) that is orientated in streamwise direction and embedded within a laterally attached flat plate. In order to separate flow noise from ocean ambient noise and other acoustical noise sources wavenumber-frequency filtering is applied. The (nondimensionalized) spectral power density of flow noise Φ(ω) ⟨U∞⟩/ (⟨δ(∗)⟩ (1/2ρ ⟨U∞⟩)(2)) is found to scale like (ω⟨δ(∗)⟩/⟨U∞⟩)(-4.3) in a wide frequency range at higher towing speeds. Here, ω, ⟨δ(∗)⟩, and ⟨U∞⟩ denote frequency, boundary layer displacement thickness, and potential flow velocity in the array region, respectively. Potential flow velocity is estimated from numerical simulations around a symmetrical, two-dimensional body with a semi-elliptical nose. Evidence is given that a χ(2)-(Tsallis) superstatistics provides a reasonable representation of the probability distribution function of flow noise at higher towing speeds.

Propagation and dissipation of the internal tide in upper Monterey Canyon

[1] Submarine canyons are sites of intense turbulence and mixing. Monterey Canyon cuts into the continental shelf off California, and is defined by its sinuous nature. Temperature, salinity, and current velocity measurements were made over 21 days in April 2009 with a depth-cycling towed body to understand internal tide propagation and dissipation through the canyon bends. Cross-canyon transects reveal complex flow patterns that follow large-scale bathymetry on scales greater than 5 km. Changes in thalweg direction deflect baroclinic energy flux, but the bends in the measurement region are too sharp for the flux to follow the thalweg. Ridges that form the bends in the canyon act as obstacles to the flow, and turbulent dissipation rates greater than 1 × 10−5 m2 s−3 were observed on their flanks, especially at the largest meander (the Gooseneck). The canyon-integrated baroclinic energy flux increased from 2.7 MW at the most western section to 3.7 MW at the Gooseneck Ridge, which has a nearly critical bottom slope with respect to the semidiurnal baroclinic tide on the western side; baroclinic energy flux was 50% less on the eastern side of the ridge. While measured dissipation near the Gooseneck Meander was sufficient to explain the flux divergence, turbulence near the Gooseneck may have been undersampled. Between the Gooseneck Ridge and the most eastern cross-canyon transect, dissipation may account for the decrease in the energy flux; though a local energy balance does not hold, the energy budget is balanced over the larger scale of the measurement region east of the Gooseneck Ridge.

Couple Model for Predicting Behaviors of Underwater Towed Systems under Effect of Heave and Pitch Motion of Ship

Underwater towed systems have the many applications in exploitation of underwater environments. While in the research exists which is about the numerical models of towed system, the relatively simple model of the towed body is used and the dynamic of the towing ship is neglected. The towing ship as part of towed system, its heave and pitch motion are coupled and highly non-linear. Considering the dynamic of three parts of towed system is important. In the paper, a newly couple model is developed to predict the behaviors of underwater towed systems under effect of the heave and pitch motion of towing ship. We use a finite difference method as the towed cable and is coupled with the non-linear numerical models of a towing ship and a towed body. Illustrative application of the coupling model is presented. The numerical results show the heave and pitch motion of towing ship affect the motion of towed body.

Self-similar degeneration of the turbulent wake behind a body towed in a passively stratified medium

A mathematical model of the far turbulent wake behind a towed body in a passively stratified medium, based on the known semi-empirical e-ɛ model of turbulence, is considered. A grouptheoretical analysis of the model is performed. With the help of the method of B-determining equations, the model is reduced to a system of ordinary differential equations, which is solved numerically. The resultant solution is compared with a self-similar solution obtained by direct numerical integration of the differential equations at large distances from the body.

On numerical modeling of the dynamics of turbulent wake behind a towed body in linearly stratified medium

Two numerical models of the dynamics of a turbulent wake behind a towed body in a linearly stratified medium are compared, namely, the model based on direct numerical integration of Navier-Stokes equations in the Oberbeck-Boussinesq approximation and the mathematical model with applying a semiempirical turbulence model of the second order. The calculation results of the two models are similar to the known experimental data and are in good agreement.

The improved methods and results of HRBSSS for Mapping the Deep Sea Bottom

High resolution bathymetric sidescan sonar (HRBSSS) is designed by the Institute of Acoustic, Chinese Academic of Sciences (IACAS). HRBSSS have used the technique of direction of arrival (DOA) estimation. It can work near the sea bottom and mapping the deep sea bottom, and get the high resolution bathymetry and sidescan map, concurrently. HRBSSSs have been mounted on the DTA-6000 deep towed body (maximum work depth 6000m) and the Jiaolong HOV (maximum work depth 7000m). In order to upgrading the performance of the system, a new method for improving the depth accuracy on the nadir of the vehicle and an error correct method for minimizing the influence of the inconsistency among the channels have been developed by IACAS. With these method, we have got the good survey results on the area of the seamount (>2000m depth) and on the area of the sea plain (>5000m depth) in the Pacific Ocean in 2011. These methods and results will be introduced in this article.

Improved methods and results of hrbsss for mapping the deep sea bottom

High resolution bathymetric sidescan sonar (HRBSSS) is designed by the Institute of Acoustic, Chinese Academic of Sciences (IACAS). HRBSSS have used the technique of direction of arrival (DOA) estimation. It can work near the sea bottom and mapping the deep sea bottom, and get the high resolution bathymetry and sidescan map, concurrently. HRBSSSs have been mounted on the DTA-6000 deep towed body (maximum work depth 6000m) and the Jiaolong HOV (maximum work depth 7000m). In order to upgrading the performance of the system, a new method for improving the depth accuracy on the nadir of the vehicle and an error correct method for minimizing the influence of the inconsistency among the channels have been developed by IACAS. With these method, we have got the good survey results on the area of the seamount (>2000m depth) and on the area of the sea plain (>5000m depth) in the Pacific Ocean in 2011. These methods and results will be introduced in this article.

密度分层流体中不同长径比拖曳潜体激发内波特性实验

Experiments are conducted for the kinematic and dynamic characteristics of the internal waves excited by horizontally towed bodies with three aspect ratios in a stratified fluid with a pycnocline. Based on the dynamic measurements of the density perturbation generated by towed bodies, it is shown that the body-generated waves are a type of stationary Lee wave structures with multi-modes; moreover the wake-generated waves are a type of non-stationary quasi-Lee wave structures with respect to the towed bodies, where the quasi-Lee waves are shown to be produced by the large-scale coherent structure in turbulent wakes acting as a moving excitation. Results show that the transition between the Lee and quasi-Lee waves occurs at a critical Froude number Fr c which is shown to be linearly dependent of the aspect ratio, and the Froude numbers with respect to the correlation velocities of the quasi-Lee waves remain at relatively constant value of approximately 0.8 regardless of the aspect ratios. Results further show that there exists a Froude number Fr p which is linearly dependent of the aspect ratio. The wave patterns for Fr Fr p are dominated by Lee waves and the dimensionless peak-to-peak amplitudes for such waves increase with Fr . Though the wave patterns for Fr p Fr Fr c are also dominated by Lee waves, the dimensionless peak-to-peak amplitudes for such waves decrease with Fr and the influence of quasi-Lee waves gradually increases; the wave patterns for Fr > Fr c are dominated by quasi-Lee waves and the dimensionless peak-to-peak amplitudes for such waves linearly increase with Fr regardless of the aspect ratios, where Fr is the Froude number for the characteristic diameter of the submerged body.

Three-dimensional structure of a swarm of the salpThalia democraticawithin a cold-core eddy off southeast Australia

[1] Swarms of the salp Thalia democratica periodically occur off southeast Australia following the austral spring bloom of phytoplankton. In October 2008 a filament of upwelled water was advected south by the adjacent East Australian Current and formed a 30 km diameter cold-core eddy (CCE). The three-dimensional structure of a subsurface swarm of T. democratica within the eddy was examined using both oblique and vertical hauls and an optical plankton counter (OPC) deployed on a towed body. The CCE displayed distinct uplift of the nutricline and elevated fluorescence. Net samples show the zooplankton community was dominated by T. democratica, comprising 73%–88% of zooplankton abundance. The size distribution of T. democratica measured from net samples was 0.5–5 mm and was used to interpret the OPC transects, which showed the swarm formed a 15 km diameter disc located 20–40 m deep in the center of the eddy. The maximum salp abundance was in the pycnocline and coincided with the subsurface fluorescence maximum. The mean abundance of T. democratica size particles within the disc was 5003 individuals m−3 (ind. m−3), contrasted with only 604 ind. m−3 at the outer edge of the eddy. The vertically concentrated and horizontally constrained disc-shaped salp swarm occurred at the interface of salp-bearing inner shelf water and nutrient-rich upwelled water in a CCE. The physical processes that formed the CCE on the inshore edge of the western boundary current led to the largest density of salps recorded.

Similarity reduction of a three-dimensional model of the far turbulent wake behind a towed body

Semi-empirical three-dimensional model of turbulence in the approximation of the far turbulent wake behind a towed body in a passively stratified medium is considered. The sought-for quantities of the model are the velocity defect, kinetic turbulent energy, kinetic energy dissipation rate, averaged density defect and density fluctuation variance. The full group of transformations admitted by this model is found. The governing equations are reduced into ordinary differential equations by similarity reduction and method of the B-determining equations (BDE method). System of ordinary differential equations was solved numerically. The obtained solutions agree with experimental data.

Concept Tests for a New Wire Flying Vehicle Designed to Achieve High Horizontal Resolution Profiling in Deep Water

Abstract Efficiently profiling the water column to achieve both high vertical and horizontal resolution from a moving vessel in deep water is difficult. Current solutions, such as CTD tow-yos, moving vessel profilers, and undulating tow bodies, are limited by ship speed or water depth. As a consequence, it is difficult to obtain oceanographic sections with sufficient resolution to identify many relevant scales over the deeper sections of the water column. This paper presents a new concept for a profiling vehicle that slides up and down a towed wire in a controlled manner using the lift created by wing foils. The wings provide a novel low-power method of propulsion along the cable by using the free stream velocity of the wire moving through the water in similar fashion to a sailboat sailing up wind. Scale model tests show a wide range of achievable profiling glide slopes for tow cable angles between vertical and 45°, and effective isolation of cable strum vibration from the towed vehicle body. The concept is not depth limited and will offer two-dimensional resolution that meets or exceeds current undulating tow bodies over the full water column. Additionally, this system could be used simultaneously with many other deep towed instrument packages to produce complementary datasets.

Field applications of broadband acoustic scattering techniques for quantifying zooplankton distribution, abundance, and size.

Discriminating among sources of scattering remains a key problem in ecological applications of active acoustics. This is particularly true in studies of zooplankton, which are often found in communities of heterogeneous species composition, and in the deployment of acoustics on autonomous platforms, where independent sampling with nets to ground-truth acoustic observations is often not feasible. Broadband measurements can provide substantial improvements in species discrimination by characterizing more fully the frequency spectrum of scatterers relative to traditional single- and multi-frequency narrowband techniques. Here, we present findings from a series of three cruises to the margins of Georges Bank examining patchiness in the distribution of krill and other zooplankton, as well as interactions with predators (fish, seabirds, and marine mammals). A heavily modified commercially-available broadband system spanning a frequency band of 30–600 kHz, with some gaps, was deployed in a towed body to depths of 200 m. Concurrent measurements made with a surface-towed multi-frequency system (43, 120, 200, and 420 kHz) along with ground-truthing information from depth-stratified net sampling and a video plankton recorder allow an assessment of the strengths and limitations of broadband methods for remotely discriminating among sources of scattering and for estimating the abundance and size of animals.

비대칭 장력 모델을 이용한 예인 물체의 유체-구조 상호작용 모사

The fluid-structure interaction of a towed body is simulated using a developed code, which is based on the flux-difference splitting scheme on the hybrid Cartesian/immersed boundary method. To improve the stability in the coupling between the fluid and structure domains, a scheme is used, in which the effects of structure deformation are treated implicitly. The developed code is validated for the fluid-structure interaction problem through comparisons with other results on the vortex-induced vibration of elastically mounted cylinders. To simulate behavior of a towed body, an asymmetric tension modelling for a towing cable is suggested. In the suggested model, the tension is proportional to the elongation of the cable, but the cable has no effect on the body motion whenever the distance between the endpoints of the cable is smaller than the original length of the cable. The fluid-structure interactions of a towed body are simulated on the basis of different parameters of the towing cables. It is observed that the suggested tension model predicts the snapping for a shorter towing cable, which is in accordance with the reported results.

Resolving estimation of movement in a vertically migrating pelagic fish: Does GPS provide a solution?

Determining geo-positions from light data collected on broadbill swordfish has traditionally been problematic. Diving behaviour in this species is typically diel in nature, with the majority of time during the day spent at depths of approximately 600–800m and the majority of time at night spent in waters typically less than 200m. Descent into deep waters occurs at dawn and ascent into surface waters occurs at dusk. Diving behaviour such as this result in little light data being collected by archival tags deployed on this species and, as a consequence, calculated positions may be few and far between. This reduces the scale at which movement and habitat interaction can be inferred. Swordfish, however, do spend time right at the ocean surface at night and in some regions basking during the day also occurs. Periods of surface behaviour may provide the opportunity to determine position in this species utilising alternative technologies. In an effort to address this problem and assess the suitability of satellite based technologies for determining movement in swordfish, we developed a towed body tag incorporating Fastloc™ GPS technology that functions similarly to a pop-up satellite archival tag. Ten prototype tags were developed with deployments occurring on swordfish off the east coast of Australia during 2008. While tags were deployed on swordfish, GPS locations were recorded from eight of the 10 fish across 1.8–33.3% of days deployed (in comparison to 5.3–54.6% of positions determined from light). Utilisation of GPS technology in a configuration similar to a pop-up tag provides the potential for determining point estimates of position at higher accuracies than light data. By incorporating GPS positions in movement models with light-based positions, it also provides an ability to resolve movement at finer spatial scales than previously achievable. This in turn allows for the determining of habitats of importance, migratory corridors and the responses of individuals to spatial environmental variability at finer scales than previously possible and has broader application for marine wildlife management.

Dynamic modeling of cable towed body using nodal position finite element method

This paper analyses nonlinear dynamics of cable towed body system. The cable has been modeled and analyzed using a new nodal position finite element method, which calculates the position of the cable directly instead of the displacement by the existing finite element method. The newly derived nodal position finite element method eliminates the need of decoupling the rigid body motion from the total motion, where numerical errors arise in the existing nonlinear finite element method, and the limitation of small rotation in each time step in the existing nonlinear finite element method. The towed body is modeled as a rigid body with six degrees of freedom while the tow ship motion is treated as a moving boundary to the system. A special procedure has been developed to couple the cable element with the towed body. The current approach can be used as design tool for achieving improved directional stability, maneuverability, safety and control characteristics with the cable towed body. The analysis results show the elegance and robustness of the proposed approach by comparing with the sea trial data.

Flow and Hydraulics near the Sill of Hood Canal, a Strongly Sheared, Continuously Stratified Fjord

Abstract Hood Canal, a long fjord in Washington State, has strong tides but limited deep-water renewal landward of a complex constriction. Tide-resolving hydrographic and velocity observations at the constriction, with a depth-cycling towed body, varied markedly during three consecutive years, partly because of stratification variations. To determine whether hydraulic control is generally important and to interpret observations of lee waves, blocking, and other features, hydraulic criticality is estimated over full tidal cycles for channel wide internal wave modes 1, 2, and 3, at five cross-channel sections, using mode speeds from the extended Taylor–Goldstein equation. These modes were strongly supercritical during most of ebb and flood on the gentle seaward sill face and for part of flood at the base of the steep landward side. Examining local criticality along the thalweg found repeated changes between mode 1 being critical and supercritical approaching the sill crest during flood, unsurprising given local minima and maxima in the cross-sectional area, with the sill crest near a maximum. Density crossing the sill sometimes resembled an overflow with an internal hydraulic control at the sill, followed by a hydraulic jump or lee wave. Long-wave speeds, however, suggest cross waves, particularly along the shallower gentler side, where flow downstream of a large-amplitude wave was uniformly supercritical. Supercritical approaching the sill, peak ebb was critical to mode 1 and supercritical to modes 2 and 3 at the base while forming a sluggish dome of dense water over the sill. Full interpretation exceeds observations and existing theory.

Three-dimensional finestructure and vertical microstructure in thermohaline intrusions.

Thermohaline intrusions have intrigued physical oceanographers since the first XBT section was published by Spilhaus in 1939. Characterized by vertical gradients of temperature and salinity having compensating density effects, intrusions are meters to tens of meters thick and extend laterally for hundreds of meters to many kilometers. Boundaries often have TS staircases produced by salt fingering or diffusive layering. Large diapycnal fluxes have been inferred from sections showing intrusions sloping across isopycnals, but these inferences are questionable, as isopycnal movements can give the illusion of diapycnal motion. To address these issues, in 2007 we used a depth-cycling towed body, a neutrally buoyant Lagrangian float, and microstructure profilers to map and follow intrusions in the subtropical front north of Hawaii. All intrusions were connected to the front, i.e., none was isolated lenses, some as long, thin folds, and others resembled tongues. About 10 m thick, the sheets protruded 10 km into the warm, saline side of the front, were coherent laterally for 10–30 km, and lasted at least 1 week, evolving at both inertial (23 h) and sub-inertial (∼10 day) time scales. They appeared to be formed by advective deformation of the front driven by sub-mesoscale two-dimensional motions.

An investigation of the combustive sound source.

The combustive sound source (CSS) is a versatile impulsive underwater sound source with an adjustable bandwidth and output amplitude. Unlike typical impulsive acoustic sources, CSS can maintain a wide bandwidth at low amplitude; hence, it is a more environmentally friendly impulsive source for ocean acoustics experiments and surveys. The CSS consists of a submersible combustion chamber, open to the water, which is filled with a combustible fuel/oxidizer mixture. The mixture is ignited and a combustion wave propagates through the mixture. During this process the ensuing bubble expands due to an increase in temperature and can collapse to a smaller volume than before ignition. This bubble activity radiates acoustic pulses. The CSS can be used as a source for low-frequency sediment characterization and TL measurements, and when deployed on the bottom can produce seismic interface waves. In addition to stationary deployments in the water column, CSS can be deployed in a tow body and as an array. Discussion will focus on the latest CSS design including functionality, acoustic output, long-term operational stability, and future development plans.

Pairing gap energy correction in Shell model for the neutron-rich tin isotopes

The region in the vicinity of 132Sn, gathering numerous isotopes of important masses, shows lots of interests for explaining the most interesting phenomena. It offers among others the possibility to extract the N-N empirical interactions so as to test theoretical shell model description of nuclear structure in this region. The researches on the neutron-rich tin isotopes present the anomalies in a systematic variation on various spectroscopic data such as the first 2+ excited state and reduced probabilities B(E2;2+? 0+). In order to improve the OXBASH interactions which are based on the shell model calculations, we have modified some neutron-neutron and proton-proton tow body matrix elements of CW5082 interaction of 132Sn region by adding to them the pairing gap energy. The calculations turn out a new interaction so called CWΔ5082, allowing to determine the excited energies and the reduced probabilities B(E2) of Sn and Te isotopes for A = 134 and 136 masses. The obtained results display a remarkable enhancement in the predictive power of experimental values.