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

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

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

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

Vibration suppression of offshore wind turbine foundations using tuned liquid column dampers and tuned mass dampers

Highly dynamic nature of the applied loads on flexible and lightly damped offshore wind turbine (OWT) foundations affects the lifetime and serviceability of the system. In this study, the excessive vibration responses of OWTs are minimized using tuned mass dampers (TMD) and tuned liquid column dampers (TLCD). Due to high efficiency of TLCDs and TMDs for certain loading conditions, a combined TLCD-TMD is also utilized to improve the overall performance in a wide range of loading conditions. First, a parametric study was performed that highlights the sensitivity of these structural control devices. The effect of two devices on fixed offshore wind turbine foundations for the benchmark 5MW NREL turbine in various loading patterns was investigated. Then, the model was subjected to stochastically generated wind loading in operational, parked, startup, and shutdown conditions. The results suggest that the standard deviation of the dynamic responses can be greatly reduced with all structural control devices. However, TMDs are more efficient in operational conditions, whereas TLCDs show better performances in parked conditions. This highlights the possibility and efficiency of a combined TLCD-TMD system in which the dynamic responses are minimized efficiently in a wider selection of loading conditions

An improved mesh adaption and refinement approach to cavitation simulation (MARCS) of propellers

This paper presents the improvements of cavitation modelling for marine propellers particularly developing tip vortex cavitation. The main purpose of the study is to devise a new approach for modelling tip vortex cavitation using Computational Fluid Dynamics (CFD) methods with commercial software, STAR-CCM+. The INSEAN E779A model propeller was used for this study as a benchmark propeller. Utilizing this propeller, firstly, validation studies were conducted in non-cavitating conditions together with grid and time step uncertainty studies. Then, the cavitation was simulated on the propeller using a numerical cavitation model, which is known as the Schnerr–Sauer model, based on the Rayleigh-Plesset equation. While a Reynolds Averaged Navier Stokes (RANS) model was used for open water simulations, Detached Eddy Simulations (DES) and Large Eddy Simulations (LES) models were preferred for cavitation simulations to capture the cavitation and evaluate its effect on propeller performance accurately. Although the comparison with the benchmark experimental data showed good agreement for the thrust and torque coefficients as well as sheet cavitation pattern, tip vortex cavitation could not be adequately simulated using the existing method. After an evaluation of the interaction between cavitation modelling and generated meshes, two techniques, which involved volumetric control and adaptive mesh refinement, were used in combination on the region where the tip vortex cavitation is likely to occur. The first technique, which is called a ‘volumetric control method’, was developed using spiral geometry around the propeller tip region to generate a finer mesh for capturing tip vortex cavitation. Although this method gave better tip vortex cavitation extension than the method without any mesh refinement or with tube refinement, it still required to be improved to extend the tip vortices further into the propeller slipstream. The second method, which is called ‘adaptive mesh refinement’, was introduced using the pressure distribution data from the results of the ‘volumetric control method’. This improved approach, which is called “Mesh Adaption and Refinement for Cavitation Simulation (MARCS)”, has been successfully applied to simulate the tip vortices trailing from the blades of the INSEAN E779A propeller as demonstrated in the paper. The results of the simulations showed an excellent agreement with the experiments in the open literature by tracking the tip vortex cavitation along this propeller’s slipstream

What to expect from the hydrodynamic energy saving devices

Many retrofitting technologies have been proposed to improve the hydrodynamic performance of existing fleets with the aim of reducing the fuel consumption and consequently CO2 emission. The magnitudes of savings predicted by manufacturers are very promising however ship owners are often still doubtful whether they can achieve what is claimed in operations. This study evaluates the performance of four energy saving devices (ESDs) at ship scale with the aim of assisting ship owners with the decision of selecting suitable devices for their ships. Due to the uncertainties associated with extrapolation of viscous flows from model to full scale it is proposed that investigations must be carried out at full scale; hence a full-scale com putational model was adopted as the only feasible method at the design stage. Two vessels representing di fferent types of ship were selected: a gas carrier and a container ship. Various retrofitting technologies to reduce resistance or to improve the propulsive efficiency were considered. The latter group is subdivided into devices located before, at and aft of the propeller. The resistance induced by large openings on the hull, such as a bow thruster tunnel was quantified and several devices designed to streamline the flow in this region were evaluated. Pre-swirl fins technology was the ESD investigated from preswirl devices. The existing propeller of the gas carrier was replaced with a new type profile propeller which improved the propulsive efficiency. Twisted rudder was the technology investigated from post-swirl ESDs. The level of savings obtained from these technologies was generally less than the values published in the literature. It was concluded that this discrepancy arose for one of three reasons: either the metric used to evaluate the savings was inappropriate, or that the method used to quantify the measure was in accurate, or finally, because the designs examined in the case studies were not suitable optimised. However if some of these devices did not deliver the expected savings because the designs considered in this study were not sufficiently optimised, then the question arises as to whether these devices must be optimised for a specific operational conditions and how well these ESDs behave when the vessel is not operating in the design conditions