Sverre Steen

EVALUATION OF ADDED RESISTANCE OF KVLCC2 IN SHORT WAVES

The added resistance of KVLCC2 in short and regular head waves has been studied theoretically and experimentally. Model tests are performed to determine how well the asymptotic formula (Faltinsen et al. 1980) predicts the typical level of added resistance in short waves. Because the asymptotic formula neglects the effects of ship motions, it is combined with theoretical methods to calculate the added resistance in long waves using an R function to predict the added resistance in the intermediate wavelength region where both ship motions and wave reflection are important. A unique feature of this experiment is that the ship model is divided into three segments to explore the added resistance distribution with respect to hull segment. This paper discusses the sensitivity of experimental results to the quality of the incident regular head waves. Moreover, a novel procedure for analyzing added resistance is described. Finally, the experimentally determined added resistance of KVLCC2 is compared with theoretical results. It is shown that the added resistance from the combined theoretical methods agrees well with experimental results in both the intermediate and short wave regions. The use of hull segments shows that added resistance is concentrated primarily at the bow.

Experiments on the stability and drag of a flexible sheet under in-plane tension in uniform flow

Abstract A flexible sheet in uniform parallel flow is studied in order to quantify its fluid dynamic drag and fluid–elastic stability characteristics. An experimental campaign is undertaken that involves a cantilevered sheet in air flow characterised by Reynolds numbers of order R = 10 4 – 10 6 . The properties of the sheet include: constant mass per unit area; small but finite flexural rigidity; varying aspect ratios from within the range 0.43 l / L 1 , where L and l denote the length and width, respectively; and tension applied at the trailing edge. The unique aspect of the present work is an investigation into the influence of in-plane tension on both the fluid drag and fluid–elastic stability of the sheet. In the absence of tension, the configuration resembles a flag and the drag coefficient is observed to decrease with increasing aspect ratio and Reynolds number. In the presence of tension, the fluid drag is significantly reduced in the region below the critical flow velocity at which convected wave instabilities appear. This critical flow velocity can be increased through the moderate application of in-plane tension. Under lateral tension, the drag of the sheet is given to good approximation by the turbulent boundary layer drag law for a flat plate. Once stability is lost, however, the drag coefficient increases rapidly with Reynolds number due to convected waves travelling over the sheets surface.

Added Resistance of a VLCC in Short Waves

In quantifying speed loss due to waves, added resistance in short waves plays an important role, because most of the time a ship travels in small sea states. It is known that added resistance in small sea states is difficult to predict accurately. In order to study improved prediction methods, numerical calculations and experiments on added resistance of a VLCC in head waves have been carried out. This paper explores a radiated energy method, an asymptotic method (Faltinsen 1980) and their combination. In addition, experimental determination of added resistance in short waves is challenging. The paper discusses the sensitivity of the results to the quality of the waves, and it is also discussed how different approaches for measuring and analyzing waves might influence the results. A novel procedure for carrying out and analyzing added resistance test results is described, and the table for the precision error of the experiment is given. Finally the results from experiments and numerical calculations are compared. The results show that the added resistances predicted by th e combined numerical calculation are a bit smaller than the experimental results at lower Froude number, while they are in good agreement at higher Froude number.Copyright

Verification and validation of numerical calculation of ship resistance and flow field of a large tanker

Ship resistance and flow field of KVLCC2 (KRISO Very Large Crude Carrier 2) are studied experimentally and numerically. A systematic analysis of both experimental and numerical uncertainty is performed. The experiment focuses on resistance distribution along the hull, which is divided into three segments. A detailed experimental uncertainty analysis with bias and precision limits is investigated. The validation of numerical calculation is performed for four grid densities and two turbulence models: ‘Shear-Stress Transport k-ω’ (SST) and ‘Explicit Algebraic Stress k-ω’ model (EASM). The validation and verification show that the numerical results are reasonable and reliable. The numerical study shows that both turbulence models can give good prediction of resistance, ships sinkage and trim, as well as flow field around the hull. Comparatively, the ‘EASM’ model has higher accuracy. The prediction of resistance on the aft segment confirms the superiority of anisotropic ‘EASM’ model, and shows that the present measurement data are a valuable test case for both numerical verification and turbulence model validation.

Theoretical and experimental investigation of propeller shaft loads in transient conditions

This paper investigates experimentally and numerically the effect on the propeller shaft loads of dynamically changing propeller speed and azimuth angle of thruster propellers. Model tests with a six-component shaft dynamometer were performed in the large towing tank at the Marine Technology Centre. A blade element momentum theory (BEMT) model is applied for numerical analysis. It is found that the side forces and bending moments on the propeller in strongly oblique inflow are quite sensitive to dynamically changing azimuth angle; for instance the maximum values of vertical side force and horizontal bending moments effectively double when the azimuth angle is changed dynamically. On the other hand, dynamically changing the azimuth angle is less important for the thrust and torque of the propeller. For ships, the effect of dynamically changing the propeller speed can be ignored and the quasi-static values used.

Real-Time Hybrid Testing for Marine Structures: Challenges and Strategies

Within the field of hydrodynamics, it is fairly easy to find examples of model tests whose performance is impaired by only a subpart of the whole system, which may not be the one of interest.Real-time hybrid testing (RTHT) overcomes this issue by performing scale model testing only on a subpart of the whole structure, the remainder being simulated numerically. The loads acting on the virtual substructure are calculated from online-measured motions of the physical substructure and actuated back on the latter in real-time. RTHT involves data measurement, filtering, force estimation, motion observing and force actuation. The main challenge is to fit all of those items into one time step.A simple case study is suggested. It consists in a linearized one degree of freedom floating wind turbine, whose floating substructure is physically tested while wind loads are numerically simulated and actuated. Design rules to build the corresponding RTHT set up are then presented.© 2013 ASME

Comparison of Numerical Methods for Wave Generation by VOF-Based Numerical Wave Tank

Based on the Reyonlds Averaged Navier-Stokes (RANS) equation for incompressible, viscous fluid, a VOF-based numerical wave tank which could accurately generate and absorb waves is investigated. Three different wave-making functions, namely ‘Inlet-velocity boundary condition (IBC)’, ‘Momentum source (MOS)’, ‘Mass source (MAS)’ are investigated, and the advantage of each method is analyzed to guide an optimal selection of these functions. Moreover, the effects of viscous model, grid size, time step, and discretization method on the accuracy of the wave simulation are discussed. The interaction between the wave and current is also studied. In order to verify the applicability of these methods, the numerical results in both 2D and 3D tanks are compared with analytical solutions.Copyright

Dynamics of a Marine Propulsion System With a Diesel Engine and a Propeller Subject To Waves

When travelling in waves, the efficiency of the propulsion system is in most cases reduced, due to a variety of different effects. Traditionally, the performance of propeller and engine is analyzed separately. However, there might be important interaction effects, which this paper attempts to explore.Engine and propeller models have been coupled to obtain realistic response of the propulsion machinery and to observe the effect of interaction between engine and propeller. Experimental wake data have been used in the simulations. The effects of different factors affecting ship propulsion in waves have been noted and their effects have been calculated separately to analyze the influence of each of the factors on ship operation.A ship travelling in three different wavelengths of three different waveheights has been simulated. Influence of these waves on engine power and fuel efficiency has been studied. This paper is a step towards optimization of installed power through better prediction of sea margin with improved insights into the propulsion performance in waves.Copyright

Identification and Analysis of Full Scale Ventilation Events

The present paper deals with propeller ventilation in full scale. The paper is based on full scale monitoring data from an offshore supply ship during normal operation. The data was collected by the on-line monitoring system HeMoS, developed by Rolls Royce Marine. The data covering one year and a half of ship operations were made available within the framework of the Era-Net Martec project PropSeas. The ventilation events are identified by means of an analysis procedure based on fuzzy logic. The paper contains both a basic introduction to fuzzy logic and a detailed description of the analysis procedure. The analysis procedure is then adopted to process the available data, find ventilation events, and form a set which is further analyzed including weather observations.

The Response of a Flexible Sheet Under Axial Tension Immersed in Parallel Flow

This paper explores the fluid-elastic response of a cantilevered flexible sheet in the presence of uniform airflow. The leading edge of the sheet is clamped, while at the trailing edge, tension is applied to provide additional rigidity to the sheets small but finite bending stiffness. We outline a series of experiments performed in a wind tunnel with the purpose of examining fluid-elastic instabilities. In particular, we examine the role of tension induced rigidity and how it influences static divergence and convected wave instabilities. The flow is characterised by Reynolds numbers of order 105 –106 and we specifically examine a sheet with an aspect ratio of L/l = 1.33 . A unique aspect of this present work, is the direct measurement of sheet displacements through an optical tracking method with a grid of passive markers placed on the sheet surface. We show the evolution of the sheet surface from stability, through to divergence, and then finally into flutter. The frequency composition of the flutter event shows higher harmonic components that suggest significant nonlinearities. Tension induced rigidity plays a crucial role in the response of the sheet to the fluid in both postponing and suppressing instabilities.Copyright