Approximate neutrino oscillations in the vacuum

It is well known that neutrino oscillations may damp due to decoherence caused by the separation of mass eigenstate wave packets or by a baseline uncertainty of order the oscillation wave length. In this note we show that if the particles created together with the neutrino are not measured and do not interact with the environment, then the first source of decoherence is not present. This demonstration uses the saddle point approximation and also assumes that the experiment lasts longer than a certain threshold. We independently derive this result using the external wave packet model and also using a model in which the fields responsible for neutrino production and detection are treated dynamically. Intuitively this result is a consequence of the fact that the neutrino emission time does not affect the final state and so amplitudes corresponding to distinct emission times must be added coherently. This fact also implies that oscillations resulting from mass eigenstates which are detected simultaneously arise from neutrinos which were not created simultaneously but are nonetheless coherent, realizing the neutrino oscillation paradigm of Kobach, Manohar and McGreevy.

Heavy neutrino-antineutrino oscillations in quantum field theory

It has been proposed that the coherent propagation of long-lived heavy neutrino mass eigenstates can lead to an oscillating rate of lepton number conserving (LNC) and violating (LNV) events, as a function of the distance between the production and displaced decay vertices. We discuss this phenomenon, which we refer to as heavy neutrino-antineutrino oscillations, in the framework of quantum field theory (QFT), using the formalism of external wave packets. General formulae for the oscillation probabilities and the number of expected events are derived and the coherence and localisation conditions that have to be satisfied in order for neutrino-antineutrino oscillations to be observable are discussed. The formulae are then applied to a low scale seesaw scenario, which features two nearly mass degenerate heavy neutrinos that can be sufficiently long lived to produce a displaced vertex when their masses are below the W boson mass. The leading and next-to-leading order oscillation formulae for this scenario are derived. For an example parameter point used in previous studies, the kinematics of the considered LNC/LNV processes are simulated, to check that the coherence and localisation conditions are satisfied. Our results show that the phenomenon of heavy neutrino-antineutrino oscillations can indeed occur in low scale seesaw scenarios and that the previously used leading order formulae, derived with a plane wave approach, provide a good approximation for the considered example parameter point.

Research on Resonant Fluid Behaviors and Induced Violent Slamming Effects in a Stepped Moonpool Under Wave Excitations With Ship Forward Speed

Moonpools on drilling vessels are structures located at the mid-ship position to facilitate drilling and other marine operations. The existence of the moonpools is prone to result in intense resonant fluid behaviors even other unexpected violent nonlinear impact on the flow system under external excitations, especially for those types with one or more recesses. The present study focuses on the resonant fluid behaviors and the induced violent nonlinear effects so-called slamming in a moonpool with a recess within the frameworks of both theoretical and numerical investigations. The natural frequencies of the piston mode and the first-three modes of the studied moonpool are approximated based on the theoretical formulas extracted from the linear potential flow theory. Furthermore, the calculated frequency of piston mode are carried out as the external wave excitations combined with varying current velocity to investigate the slamming effects at the typical locations such as the wall of the moonpool and the bottom of the recess. Finally, the characteristics of the slamming pressure both in space and time are discussed in detail based on the RANS-based numerical simulations. The result shows that the resonant fluid behaviors under the wave-current interactions are greatly different from the wave-only excitations. Moreover, intense slamming occur in a moon-pool with a recess under some particular wave-current interactions due to the shallow water effects in the recess region and the phenomenon of energy transfer. The phenomenon of slamming must be treated with special cautions in practical engineering because of the fact that some induced adverse effects could weaken the hydrodynamic performance of the drilling vessel, as well as the structural strength of the moonpool.

Analytical Study on the Effect of a Horizontal Perforated Plate on Sloshing Motion in a Rectangular Tank

Tank sloshing in a liquid cargo ship will cause instability or even overturning of its carrier if the external wave frequency is close to the natural frequency of the tank. The inherent damping of a tank without inner structures has been found to be insufficient for suppressing violent sloshing motion. A variety of damping plates have been designed to increase the inherent damping of the tank. Of them, a horizontal perforated plate (HPP) has been proved to be effective for dissipating energy in a swaying tank through experiments. In this study, the sloshing problem in a tank with an HPP under swaying and rolling excitation is analytically studied based on the potential theory. The quadratic pressure loss boundary at the perforated plate is adopted, and the matched eigenfunction expansion method (MEEM) with iterative calculations is used to develop the analytical model. Based on the different porosities and submerged depths of the plate, both the free surface elevations and the hydrodynamic coefficients are carefully examined. The results give a better understanding in the effect of the inner HPP on the sloshing motion in the ship tank.

RESONANT OSCILLATIONS IN SMALL CRAFT HARBOURS: OBSERVATIONS AND MITIGATION MODELING EXAMPLES FROM ATLANTIC CANADA

Agitation from swells and long waves can pose serious challenges for harbours, in terms of both infrastructure design and operations. Wave gauge observations from Atlantic Canadian harbours of varying sizes were used to assess how combinations of basin dimensions and external wave forcing may lead to swell agitation, resonance, and scour problems. This paper summarizes how basin resonances were investigated with field observations, analytical methods, and then phase-resolving numerical models. The case studies illustrate how resonance mitigation may require substantial (and sometimes impractical) changes in harbour layout. Swell and scour mitigation may be more readily achieved by modifications or additions to existing structures.

Infinite Element Static-Dynamic Unified Artificial Boundary

The method, which obtains a static-dynamic comprehensive effect from superposing static and dynamic effects, is inapplicable to large deformation and nonlinear elastic problems under strong earthquake action. The static and dynamic effects must be analyzed in a unified way. These effects involve a static-dynamic boundary transformation problem or a static-dynamic boundary unified problem. The static-dynamic boundary conversion method is tedious. If the node restraint reaction force caused by a static boundary condition is not applied, then the model is not balanced at zero moment, and the calculation result is distorted. The static numerical solution error is large when the structure possesses tangential static force in a viscoelastic static-dynamic unified boundary. This paper proposed a new static-dynamic unified artificial boundary based on an infinite element in ABAQUS to solve static-dynamic synthesis effects conveniently and accurately. The static and dynamic mapping theories of infinite elements were introduced. The characteristic of the infinite element, which has zero displacement at faraway infinity, was discussed in theory. The equivalent nodal force calculation formula of infinite element unified boundary was deduced from an external wave input. A calculation and application program of equivalent nodal forces was developed using the Python language to complete external wave inputting. This new method does not require a static and dynamic boundary transformation and import of stress field and constraint counterforce of boundary nodes. The static calculation precision of the infinite element unified boundary is more improved than the viscoelastic static-dynamic unified boundary, especially when the static load is in the tangential direction. In addition, the foundation simulation range of finite field can be significantly reduced given the utilization of the infinite element static dynamic unified boundary. The preciseness of static calculation and dynamic calculation and static-dynamic comprehensive analysis are unaffected.

Fully Coupled Analysis of Ship Motion and Sloshing Tanks in Regular and Irregular Waves

Fully coupled analysis of ship motion and sloshing tank in waves is essential for floating structures which store and transports natural gas. For partially filled tanks would generate violent sloshing due to external wave excitation, and the sloshing flow can consequently affect ship motion. Therefore, how to evaluate ship motion and sloshing phenomenon in tank is of great importance, especially under real sea state, when wave induced sloshing would be more complex than that under linear wave condition. In the present work, a CFD-based method is applied to simulate both external wave field and inner sloshing tank field in regular waves and irregular waves. The ship is a simplified FPSO, with two LNG tanks. All the numerical simulations are carried out by the in-house CFD code naoe-FOAM-SJTU, which is developed on the open source platform OpenFOAM. The regular and irregular wave condition is simulated based on open source toolbox waves2Foam. The main parameters of coupling effect of ship motion and sloshing tank, such as the time history of ship motion, sloshing phenomenon in tanks are obtained by our computations. The predicted results for the coupling effects of ship motion and sloshing tank in regular waves are compared with the corresponding experimental data. The comparison is satisfactory and shows that the CFD method has the ability to simulate coupling effects of ship motion and sloshing tank in waves.