International Shipbuilding Progress - Volume 66, issue 4

Authors: Wellens, Peter

Article Type: Editorial

DOI: 10.3233/ISP-209001

Citation: International Shipbuilding Progress, vol. 66, no. 4, pp. 271-271, 2019

Authors: Rotte, Gem | Kerkvliet, Maarten | van Terwisga, Tom

Article Type: Research Article

Abstract: Background: Air lubrication techniques have the potential to significantly reduce frictional drag, benefiting sustainable employability of ships. However, these techniques are not yet widely applied in the shipping industry, since a complete understanding of the relevant two-phase flow physics is still lacking. Objective: This article aims to explore the limitations and capabilities of RANS-VoF modelling to numerically model air cavity flows. Methods: Simulations were performed including numerical uncertainty verification and compared to experimental data for an external cavity. To study the effect of reduced eddy-viscosity at the cavity interface, two types of eddy-viscosity correction functions were …applied next to a base case, i.e., a power and a Gaussian function. Results: The cavity length and thickness as well as the velocity profiles in the boundary layer just upstream, in the middle and downstream of the external cavity compare well to experimental data. However, in contrast to what was found experimentally, a too strong coupling was found between the computed cavity profile and the air pressure at the nozzle and too much air leaks out of the cavity. For the same nozzle air pressure as in the experiments, similar cavity dimensions were found, but the air flow rate is overestimated by a factor of five. Conclusions: The used methodology is capable of predicting the cavity profile and velocity profiles at different stream-wise locations in the boundary layer around the cavity with respect to experimental findings. However, a mismatch was found in the determination of the required air flow rate for the cavity, which is hypotesized to be mainly caused by the incorrect turbulence modelling around the interface and the advection of a smeared air-water interface in the reattachment zone. This is a direct consequence of the used VoF method. The exact mechanism for air discharge at the cavity closure is still not clear. Show more

Keywords: Ship drag reduction, air lubrication, RANS-VoF modelling, re-entrant jet, eddy-viscosity correction

DOI: 10.3233/ISP-190270

Citation: International Shipbuilding Progress, vol. 66, no. 4, pp. 273-293, 2019

Authors: Bos, R.W. | den Besten, J.H. | Kaminski, M.L.

Article Type: Research Article

Abstract: Highly varying sloshing loads are a superposition of load components resulting from a sequence of different physical phenomena. However, not all features of spatial and temporal variations of sloshing loads and associated phenomena are equally important when failure of structure is considered. Therefore, the prediction of sloshing loads should be focused on those load components which lead to failure. These components can be found by employing a structural model, which should be fast computationally considering the huge number of possible sloshing loads. This paper presents a reduced order model based on the beam-foundation model which is derived for the Mark-III …cargo containment system. The model is validated against a detailed finite element model and it conservatively predicts the stresses at failure locations. The calculation time using the model is approximately two orders smaller in comparison to a finite element model computation, which allows the model to be applied for finding governing load components and associated physical phenomena. Show more

Keywords: LNG, cargo containment system, dynamic response, beam-foundation model, reduced order model

DOI: 10.3233/ISP-190272

Citation: International Shipbuilding Progress, vol. 66, no. 4, pp. 295-313, 2019

Authors: van der Eijk, Martin | Wellens, Peter R.

Article Type: Research Article

Abstract: A significant part of all structural damage to conventional ships is caused by complex free-surface events like slamming, breaking waves, and green water. During these events air can be entrapped by water. The focus of this article is on the resulting air pockets affecting the evolution of the hydrodynamic impact pressure that loads the ship’s structure. ComFLOW is a computationally efficient method based on the Navier–Stokes equations with a Volume-of-Fluid approach for the free surface, designed to perform multiphase simulations of extreme free surface wave interaction with maritime structures. We have extended ComFLOW with a Continuum Surface …Force (CSF) model for surface tension, thereby completing our method for representing gas-water interaction after free surface wave impacts. The implementation was verified with benchmark cases addressing all relevant aspects of the dynamics of entrapped air pockets. The implementation was validated by means of a dam-break experiment, a characteristic model for green water impact events. The method – having been verified and validated – was applied to a dam-break simulation for a different setting in which the impact on a wall leads to an entrapped air pocket. Surface tension was found not to have an influence on entrapped air pocket dynamics of air pockets with a radius larger than 0.08 [m]. For wave impacts it was found that the effect of compression waves in the air pocket dominates the dynamics and leads to pressure oscillations that are of the same order of magnitude as the pressure caused by the initial impact on the base of the wall. The code is available at: https://github.com/martin-eijk/2phase.git . Show more

Keywords: CFD, compressible two-phase flow, Volume-of-Fluid, surface tension, wave impact, dam-break

DOI: 10.3233/ISP-200278

Citation: International Shipbuilding Progress, vol. 66, no. 4, pp. 315-343, 2019

Authors: de Jong, R.G. | Vos, T.G. | Beindorff, R. | Wellens, P.R.

Article Type: Research Article

Abstract: Dynamic positioning (DP) systems are used for station keeping during offshore operations. The safety and operability of several offshore operations can be increased when the roll motion is actively controlled, especially in beam seas. We propose a novel control strategy for combined roll motion control and station keeping, using no additional hardware than the installed DP thrusters. The control strategy is applied to an offshore construction vessel and the performance is demonstrated by time domain simulations. The DP footprint is compared to a conventional dynamic positioning control model. The proposed control model enables active roll reduction while the station …keeping performance remains unaffected. The code has been made open source and is available on https://github.com/pwellens/3dp.git . Show more

Keywords: Active roll reduction, Dynamic Positioning

DOI: 10.3233/ISP-200280

Citation: International Shipbuilding Progress, vol. 66, no. 4, pp. 345-372, 2019