1,721,052 research outputs found
A probabilistic approach for the quantification of prediction error in deterministic phase-resolved wave forecasting
No full textThis paper presents a semi-analytical methodology for the determination of prediction error statistics in deterministic sea wave predictions (DSWP), based on linear wave models. The underlying wave elevation is modelled as a Gaussian stochastic process and the coefficients of the wave propagation model are assumed to be determined by linear fitting on available measurements in time and/or space. The possible data contamination due to measurement error is also explicitly considered. The resulting approach eventually provides a Linear Estimator of Prediction Error (LEPrE) in time and space, in terms of prediction error standard deviation, given the fitting procedure and the sea spectrum. The presented approach allows supplementing deterministic predictions based on phase-resolved linear wave models with a sound prediction error measure, and allows defining the concept of “Predictability Region” in a consistent probabilistic framework. Example applications are reported, both for long-crested and short-crested waves, with verification through Monte Carlo simulations. Single point wave gauge/wave buoy measurements as well as wave radar measurements have been considered as simulated examples. The developed methodology is also compared with existing approaches highlighting and discussing both the differences and the interesting qualitative commonalities
Role of Morphology and Flexibility in Maneuvering for Fish and Bio-Inspired Marine Robots
No full textIn this paper we present a semi-analytical approach based on slender-body and Euler-Bernoulli beam theory to investigate the role of body morphology and flexibility in fish maneuvering via stability as a first research step towards gaining knowledge for use in future bio-inspired robotic design. Non-dimensional parameters representing the body morphology and fin lift showed that both the stable and unstable fin configuration, in addition to body shape, should be considered simultaneously during design of the robot to ensure targeted maneuverability characteristics. The role of flexibility and body morphology for tuna, sailfish and barracuda body shapes were examined. From these investigations it was found that for a flexible robot, its stiffness distribution and body morphology can be used to change stability
Quantifying error in deterministic predictions based on phase-resolved linear wave models
No full textShip operational safety is most frequently addressed from a statistical point of view, making reference to probabilistic measures of ship motions and/or wave characteristics. However, in the recent years, thanks to the technological advances in wave sensing, complementary approaches, based on the deterministic forecasting of wave elevation and ship motions, have been developed. Nevertheless, deterministic wave forecasting approaches are still lacking an associated sound measure of the prediction uncertainty. In trying to contribute filling this identified gap, a theoretically consistent measure of prediction error is developed in this work, starting only from fundamental assumptions about the water wave elevation field and the employed phase-resolved wave prediction model. Specifically, assuming linear wave theory and gaussianity of the wave elevation field, a Linear Estimator of Prediction Error (LEPrE) is derived. Some example applications are re-ported where the developed approach is applied and verified
The relevance of recoil and free swimming in aquatic locomotion
Get PDFThe study of the free swimming of undulating bodies in an otherwise quiescent fluid has always encountered serious difficulties for several reasons. When considering the full system, given by the body and the unbounded surrounding fluid, the absence of external forces leads to a subtle interaction problem dominated, at least at steady state conditions, by the equilibrium of strictly related internal forces, e.g. thrust and drag, under the forcing of a prescribed deformation. A major complication has been dictated by the recoil motion induced by the non linear interactions, which may find a quite natural solution when considering as unknowns the velocity components of the body center of mass. A simplified two-dimensional model in terms of impulse equations has been used and a fruitful separation of the main contributions due to added mass and to vorticity release is easily obtained. As main results we obtain either the mean locomotion speed and the oscillating recoil velocity components which have a large effect on the overall performance of free swimming. Several constrained gaits are considered to highlight the relevance of recoil for realizing graceful and efficient trajectories and to analyze its potential means for active control
The fish ability to accelerate and suddenly turn in fast maneuvers
Get PDFVelocity burst and quick turning are performed by fish during fast maneuvers which might be essential to their survival along pray-predator encounters. The parameters to evaluate these truly unsteady motions are totally different from the ones for cruising gaits since a very large acceleration, up to several times the gravity, and an extreme turning capability, in less than one body length, are now the primary requests. Such impressive performances, still poorly understood, are not common to other living beings and are clearly related to the interaction with the aquatic environment. Hence, we focus our attention on the water set in motion by the body, giving rise to the relevant added mass and the associated phenomena in transient conditions, which may unveil the secret of the great maneuverability observed in nature. Many previous studies were almost exclusively concentrated on the vortical wake, whose account, certainly dominant at steady state, is not sufficient to explain the entangled transient phenomena. A simple two-dimensional impulse model with concentrated vorticity is used for the self-propulsion of a deformable body in an unbounded fluid domain, to single out the potential and the vortical impulses and to highlight their interplay induced by recoil motions
Coupled dynamic simulations of offshore wind turbines: influence of wave modeling on the fatigue load assessment
No full textNonlinear waves influence to a large extent the coupled hydro-aero-elastic response of offshore wind turbines. Higher-order contributions in the hydrodynamic forcing are responsible for resonant springing-like vibrations of the tower causing an increase of stress cycles and amplitudes. The present study investigates the effects of these amplifications in terms of fatigue load. Equivalent fatigue loads are estimated by means of both time and frequency domain methods. A comparison between linear, second-order and fully nonlinear wave models is proposed and it is shown that the weakly nonlinear model, widely used in the state-of-the-art simulations, may significantly underestimate the actual fatigue load. Hydrodynamic loads associated with the different wave models are coupled with aerodynamic loads acting on the rotor of a 5-MW wind turbine (fixed-bottom)
Wave dispersion in moderate channel turbulence
Get PDFWe study channel turbulence by interpreting its vorticity as a random sea of ocean wave packet analogues. In particular, we investigate the ocean-like properties of vortical packets applying stochastic methods developed for oceanic fields. Taylor's hypothesis of frozen eddies does not hold when turbulence is not weak, and vortical packets change shape as they are advected by the mean flow, altering their own speed. This is the physical manifestation of a hidden wave dispersion of turbulence. Our analysis at the bulk Reynolds number Reb = 5600 suggests that turbulent fluctuations behave dispersively as gravity-capillary waves, with capillarity being dominant near the wall region
Prediction error statistics in deterministic linear ship motion forecasting
No full textDeterministic ship motions predictions methodologies represent a promising emerging approach, which could be embedded in decision support systems for certain types of operation. The typically envisioned prediction chain starts from the remote sensing of the wave elevation through wave radar technology. An estimated wave field is then fitted to the data, it is propagated in space and time, and it is finally fed to a ship motion prediction model. Prediction time horizons, typically, are practically limited to the order of minutes. Deterministic predictions are, however, inevitably associated with prediction uncertainty which is seldom quantified. This paper, therefore, presents a semi-analytical methodology for the estimation of ship motion prediction error statistics in ensemble domain as function of the forecasting time, assuming linear Gaussian irregular waves and stationary linear ship motions. This information can be used, for instance, to supplement deterministic forecasting with corresponding confidence intervals. The paper describes the theoretical background of the developed methodology and reports some numerical application examples
Sloshing in a rotating liquid inside a closed sea cage for fish farming
Get PDFSloshing in a sea cage with a slowly rotating liquid is investigated. The cage is axisymmetric, and the liquid is subjected to a nearly uniform angular velocity about the vertical axis of the cage. Both experimental and theoretical investigations are presented. It is shown that rotation modifies the sloshing regimes of a non-rotating liquid by splitting the natural frequencies. Therefore, resonant sloshing regimes can be manipulated by varying the rotation rate of the liquid
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