616 research outputs found
Synthetic aperture radar images of ocean waves, theories of imaging physics and experimental tests
The physical mechanism for the synthetic Aperture Radar (SAR) imaging of ocean waves is investigated through the use of analytical models. The models are tested by comparison with data sets from the SEASAT mission and airborne SAR's. Dominant ocean wavelengths from SAR estimates are biased towards longer wavelengths. The quasispecular scattering mechanism agrees with experimental data. The Doppler shift for ship wakes is that of the mean sea surface
Spectrograms of ship wakes: identifying linear and nonlinear wave signals
A spectrogram is a useful way of using short-time discrete Fourier transforms
to visualise surface height measurements taken of ship wakes in real world
conditions. For a steadily moving ship that leaves behind small-amplitude
waves, the spectrogram is known to have two clear linear components, a
sliding-frequency mode caused by the divergent waves and a constant-frequency
mode for the transverse waves. However, recent observations of high speed ferry
data have identified additional components of the spectrograms that are not yet
explained. We use computer simulations of linear and nonlinear ship wave
patterns and apply time-frequency analysis to generate spectrograms for an
idealised ship. We clarify the role of the linear dispersion relation and ship
speed on the two linear components. We use a simple weakly nonlinear theory to
identify higher order effects in a spectrogram and, while the high speed ferry
data is very noisy, we propose that certain additional features in the
experimental data are caused by nonlinearity. Finally, we provide a possible
explanation for a further discrepancy between the high speed ferry spectrograms
and linear theory by accounting for ship acceleration.Comment: 21 pages, 10 figures, submitte
SEASAT views oceans and sea ice with synthetic aperture radar
Fifty-one SEASAT synthetic aperture radar (SAR) images of the oceans and sea ice are presented. Surface and internal waves, the Gulf Stream system and its rings and eddies, the eastern North Pacific, coastal phenomena, bathymetric features, atmospheric phenomena, and ship wakes are represented. Images of arctic pack and shore-fast ice are presented. The characteristics of the SEASAT SAR system and its image are described. Maps showing the area covered, and tables of key orbital information, and listing digitally processed images are provided
The Use of Multi-beam Sonars to Image Bubbly Ship Wakes
During the past five years, researchers at Penn State University (PSU) have used upward-looking multi-beam (MB) sonar to image the bubbly wakes of surface ships. In 2000, a 19-beam, 5° beam width, 120° sector, 250 kHz MB sonar integrated into an autonomous vehicle was used to obtain a first-of-a-kind look at the three-dimensional variability of bubbles in a large ship wake. In 2001 we acquired a Reson 8101 MB sonar, which operates at 240 kHz and features 101-1.5º beams spanning a 150º sector. In July 2002, the Reson sonar was deployed looking upward from a 1.4 m diameter buoy moored at 29.5 m depth in 550 m of water using three anchor lines. A fiber optic cable connected the sonar to a support ship 500 m away. Images of the wake of a small research vessel provided new information about the persistence of bubble clouds in the ocean. An important goal is to use the MB sonar to estimate wake bubble distributions, as has been done with single beam sonar. Here we show that multipath interference and strong, specular reflections from the sea surface adversely affect the use of MB sonars to unambiguously estimate wake bubble distributio
Large -eddy simulation of ship wakes
The objective of the present study is to improve our understanding of turbulent wake flows. Large-Eddy Simulation (LES) technique is applied for this purpose. A readily available code was used with necessary modifications. Three dimensional incompressible Navier-Stokes equations are solved in non-orthogonal curvilinear coordinates. Finite-volume approach is implemented on a non-staggered grid. The core of the numerical scheme is a fractional step method. The overall accuracy of the method is second order in both space and time.;The LES approach has been validated for four cases: channel flow, flow past a square body, a shear layer flow, and open channel flow. Different subgrid-scale models and numerical schemes have been tested for these benchmarks. Comparisons between the simulations and experiments show the capability of this LES method.;An efficient and accurate Random Flow Generation (RFG) approach has been improved to provide turbulent initial and inflow conditions for developing wake flow calculations. The RFG method can handle anisotropy and inhomogeneity, and it satisfies the instantaneous continuity equations. This approach has been verified by reproducing a turbulent channel flow based on a Direct Numerical Simulation (DNS). Perfect agreements have been obtained.;A typical one equation sub-grid scale model has been selected and modified to include the backscatter of energy by applying the RFG algorithm. Meanwhile, an attempt was made to implement the one equation sub-grid scale model dynamically. Comparisons have been made between the experiments and the simulation results using different one equation sub-grid scale models.;The RFG approach along with the LES technique has been applied to the wake behind a flat plate. Effects of grid resolution and SGS models on the turbulent flow field have been investigated. Good results are obtained as compared to experiments.;Wake flows behind a Naval ship model (5415) have been studied in details by applying the combined LES-RFG method. The calculation domain starts from a plane behind the ship model. Because of the lack of experimental data, Reynolds Averaged Navier-Stokes (RANS) calculations are used to provide the RFG procedure with the information on the inflow boundary. The ship wake flow with an impose surface wave has been studied. The result shows that the wave surface has a significant influence on the turbulent kinetic energy distribution. Finally, part of the ship hull is included in the simulation of the ship wake with a static wave surface (Fr = 0.28) to investigate the difference between RANS and LES results in the near wake of a ship model. The overall quality of the LES calculations is found to be very good. In particular the large coherent structures with significant vorticity concentration and gradients could be captured in the wake of a ship model which is not possible to accomplished by RANS. These structures have significant importance in the transplant of entrained air bubbles
Ship Wake Detection in SAR Images via Sparse Regularization
In order to analyse synthetic aperture radar (SAR) images of the sea surface,
ship wake detection is essential for extracting information on the wake
generating vessels. One possibility is to assume a linear model for wakes, in
which case detection approaches are based on transforms such as Radon and
Hough. These express the bright (dark) lines as peak (trough) points in the
transform domain. In this paper, ship wake detection is posed as an inverse
problem, which the associated cost function including a sparsity enforcing
penalty, i.e. the generalized minimax concave (GMC) function. Despite being a
non-convex regularizer, the GMC penalty enforces the overall cost function to
be convex. The proposed solution is based on a Bayesian formulation, whereby
the point estimates are recovered using maximum a posteriori (MAP) estimation.
To quantify the performance of the proposed method, various types of SAR images
are used, corresponding to TerraSAR-X, COSMO-SkyMed, Sentinel-1, and ALOS2. The
performance of various priors in solving the proposed inverse problem is first
studied by investigating the GMC along with the L1, Lp, nuclear and total
variation (TV) norms. We show that the GMC achieves the best results and we
subsequently study the merits of the corresponding method in comparison to two
state-of-the-art approaches for ship wake detection. The results show that our
proposed technique offers the best performance by achieving 80% success rate.Comment: 18 page
Turbulent ship wakes and their spatiotemporal extent
Shipping activities occur in almost every part of the global oceans and in intensely trafficked shipping lanes there can be up to one ship passage every ten minutes. All these ships impact the marine environment in different ways through pollution or physical disturbance. This thesis is focused on the turbulent ship wake, a physical disturbance from ships and previously overlooked as an environmental impact. When a ship moves through water, the turbulence induced by the propeller and hull, will create a turbulent wake that remains and expands after the ship passage. The turbulence in the wake will govern the spread of contaminants and affect gas exchange in the wake water, physically perturb plankton, and potentially impact local biogeochemistry through increased vertical mixing.To be able to assess the environmental impact of ship-induced turbulence in areas with intense ship traffic, knowledge of the spatiotemporal extent and development of the turbulent wake is necessary. The aim of this thesis is to increase that knowledge, by conducting in situ observations of turbulent ship wakes, which can be used to estimate the spatiotemporal extent of the turbulent wake. By using a collection of methods, the thesis work has resulted in a first estimate of the spatiotemporal extent of the turbulent ship wake, based on more than 200 field observations of different real-size ships in natural conditions. The observed turbulent wakes showed large variation in their spatiotemporal extent, and further studies are needed to fully disentangle how environmental conditions and vessel specifications affect the intensity and extent of the turbulent wake. The results and experiences gained from the in situ observations, give an indication of the complexity entailed in characterising the development of the turbulent wake, and provide valuable input regarding the relevant parameters and spatiotemporal scales to include in future studies. The work of this thesis constitutes the first step in addressing the knowledge gap regarding the environmental impact of ship-induced turbulence and can be used as a road map for further studies within the field
Time-frequency analysis of ship wave patterns in shallow water: modelling and experiments
A spectrogram of a ship wake is a heat map that visualises the time-dependent
frequency spectrum of surface height measurements taken at a single point as
the ship travels by. Spectrograms are easy to compute and, if properly
interpreted, have the potential to provide crucial information about various
properties of the ship in question. Here we use geometrical arguments and
analysis of an idealised mathematical model to identify features of
spectrograms, concentrating on the effects of a finite-depth channel. Our
results depend heavily on whether the flow regime is subcritical or
supercritical. To support our theoretical predictions, we compare with data
taken from experiments we conducted in a model test basin using a variety of
realistic ship hulls. Finally, we note that vessels with a high aspect ratio
appear to produce spectrogram data that contains periodic patterns. We can
reproduce this behaviour in our mathematical model by using a so-called
two-point wavemaker. These results highlight the role of wave interference
effects in spectrograms of ship wakes.Comment: 14 pages, 7 figure
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