Preliminary instability-analysis of deepwater riser with fairings

Instability of deepwater riser with fairings is investigated in this study. Despite the advantages over other devices for suppressing vortex-induced-vibration (VIV), fairings may be susceptible to flutter type instability. A two-body mathematical model is established for the coupled transverse-torsion motion of a top tensioned riser with fairings. The inner part (riser) can only move transversely while the outer part (fairing) has transverse-torsion motion. The effect of the transverse velocity on the angle of attack is taken into account and damping is considered for both degrees of freedom. An eigenvalue analysis is employed to examine the issue of stability. The emphasis is on identifying the critical current speed for a given riser and fairing configuration. The effects of key parameters are investigated and the results indicate that the section hydrodynamic characteristics of the fairings have a significant impact on the instability

Numerical study of asymmetric keel hydrodynamic performance through advanced CFD

The hydrodynamics of an asymmetric IACC yacht keel at angle of yaw are presented using simulations performed by advanced computational fluid dynamics using state-of-the-art software. The aim of the paper is to continue working on the improvement of numerical viscous flow predictions for high-performance yachts using Large Eddy Simulation and Detached Eddy Simulation on unstructured grids. Quantitative comparisons of global forces acting on the keel and wake survey are carried out. Qualitative comparisons include flow visualisation, unsteady and separated flow and other features. Star-CCM+ and the trimmed cell method give better forces and wake prediction compared to the unstructured mesh of ANSYS Fluent. Both solvers give good flow visualisation near and far field of the keel

Smart materials application on high performance sailing yachts for energy harvesting

Piezoelectric patches are bounded on a keel bulb in order to harvest vibration energy by converting electrical output. Unsteady computational fluid dynamics method is also used to find the structural boundary condition such as the hydrodynamic pressure fluctuation. Finite element analysis (FEM) is used to find structural and electrical responses

On the importance of antifouling coatings regarding ship resistance and powering

This paper aims to introduce one of the latest investigations on development of marine antifouling coatings and also to demonstrate the importance of the type of antifouling coatings on fouling accumulation and ship resistance/powering. First, marine biofouling and fouling prevention methods are reviewed. A recent research study (EU FP7 FOUL-X-SPEL Project) concerning a novel and environmentally friendly antifouling coating is presented and discussed. Next, a case study is carried out to assess the effect of fouling on ship resistance and powering. A vessel is selected and the roughness on the hull surface induced by different level of fouling is considered. The increase in frictional resistance and effective power is evaluated for each particular case by using boundary layer similarity law analysis and experimental data. The results emphasise that the type of antifouling coatings has a great importance on the amount of fouling accumulation, hence on ship performance especially in low speed

Instability analysis of deepwater riser with fairings

The paper investigates the mechanism of instability of deepwater risers fitted with fairings and presents an analytical model to predict the instability onset conditions. The simplified case of a two-dimensional (2D) problem was considered. The governing equations were derived, and the hydrodynamic forces were calculated and the effect of motion in these forces was taken into consideration. The final equations were linearised and an eigenvalue analysis was employed to systematically examine the stability with the emphasis on identifying the critical current speed for a given system. This model was validated against the available test results and showed a good agreement. A parametric study was also carried out. It showed the significant role of the hydrodynamic coefficients as well as mass distribution in the stability of the system

Root Canal Treatment of a Geminated Maxillary Second Molar with C-shaped Canal System: A Case Report

Gemination is a rare phenomenon in the maxillary posterior teeth. Endodontic treatment of these teeth requires special care due to the bizarre anatomy particularly when it is accompanied by a C-shaped canal system. This report illustrates a patient with a rare geminated C-shaped maxillary second molar comprised of two sections in its crown, including a geminated section attached to a normal coronal of a second maxillary molar diagnosed with the pulpal status “necrosis” and “irreversible pulpitis” in geminated section and the molar respectively. Thus, endodontic treatment was performed on both parts of the tooth. Two months follow-up revealed well-functioning teeth with normal status of periapical tissue with no mobility or abnormality. Successful treatment of unusual anatomical teeth requires adherence to biomechanical principles of canal preparation and coronal restoratio

An investigation into computational modelling of cavitation in a propeller's slipstream

This paper reports on the ongoing developments of cavitation modelling so far which include preliminary validation studies for simulating the performances of two benchmark model propellers: i.e. PPTC propeller with inclined shaft; and E779A propeller, in non-cavitating and cavitating conditions. The main purpose of this study is to estimate the propeller’s performance in cavitating conditions particularly developing tip vortex cavitation. The simulations in open water and cavitating conditions were carried out in uniform flow using a commercial CFD package. Firstly, the validation studies were conducted for non-cavitating condition. The comparison with the benchmark experimental data showed good agreement for the thrust and torque coefficients as well as for the open water efficiency. Next, the cavitation developed on the propeller was simulated using a numerical model based on the Rayleigh-Plesset equation. Propulsion coefficients (KT, KQ) and the cavity patterns on the benchmark propellers’ blades showed very good agreement with the experimental data. However, the tip vortices off the blades could only be traced for E779A propeller by using a new mesh refinement approach

Unsteady RANSE and detached-eddy simulations of cavitating flow

The Twisted Delft Hydrofoil and the Potsdam Propeller Test Case (PPTC) were used to analyse and compare the capabilities of Reynolds-Averaged Navier Stokes Equations simulations (RANSE simulations) and detached-eddy simulations (DES) to predict three-dimensional cavitating flow. Although the RANSE simulations were able to predict the lift and drag forces in reasonable agreement with the experiments, it has been shown that the accurate numerical simulation of cavitational flow requires the use of an advanced model such as the SST k-omega detached-eddy model

Applications of holistic ship theory for the simulation driven optimization of the design and operation of large bulk carriers

The change of scenery in shipping has been evident over the past 20 years. The changing fuel costs, tough and volatile market conditions, the constant societal pressure for a «green» environmental footprint combined with ever demanding international safety regulations create the new framework in which commercial ship designs are subject to. As a result of this current status of shipping commercial a change of attitude in the philosophy and process of ship design is required in order to shift towards new approaches where holistic approaches are deemed necessary. Apart from considering all the interrelationships between the subsystems that consist the vessel lifecycle and supply chain considerations are the key in successful and «operator oriented» designs. The methodology herein presented is built within the computer aided engineering (CAE) software CAESES that integrates in the design process CFD codes. It can be successfully used for the optimization of either of the basic design of a vessel or the operation of an existing vessel with regards to the maximization of the efficiency, safety and competitiveness of the final design. The model is created based on the design of a large bulk carrier and a simulation model consisting of modules that cover most aspects of ship design. Stability, strength, powering and propulsion, safety, economics, operational and maintenance and in service management considerations are tightly integrated within a fully parametric model. This tight integration enables the user to simulate the response of the model in variations of the geometrical, design variables of the vessel (including its propeller) under conditions of simulation and uncertainty. The uncertainty modelling is extensive and in several levels including but not limited to Economic, Environmental, and Operational uncertainty as well an accuracy modelling of the methodology itself

Optimisation of Offshore Wind Farms Using a Genetic Algorithm

A modular framework for the optimisation of an offshore wind farm using a discrete genetic algorithm is presented. This approach uses a bespoke grid generation algorithm to define the discrete positions that turbines may occupy thereby implicitly satisfying navigational and search and rescue constraints through the wind farm. The presented methodology takes a holistic approach optimising both the turbine placement and inter-array cable network, while minimising the levelised cost of energy and satisfying real world constraints. This tool therefore integrates models for the assessment of the energy production including wake losses; the optimisation of the inter-array cables; and the estimation of costs of the project over the lifetime. This framework will allow alternate approaches to wake and cost modelling as well as optimisation to be benchmarked in the future