Waterjet-Hull interaction

Mechanical Maritime and Materials Engineerin

EU projects contribute to continuing development of ship propulsion

Is funding from the European Commission’s Framework Programme an effective investment? And how could you demonstrate the effectiveness? This paper aims to give an answer to those questions from the perspective of MARIN’s involvement in several consecutive projects in the field of ship resistance and propulsion. Previous projects such as SMOOTH, MoVeIT, STREAMLINE, GRIP and SONIC have laid a solid foundation for MARIN’s involvement in current projects such as MOTOR, LeanShips, HOLISHIP and NOVIMAR. An ongoing research line is recognised through these projects. This paper aims to show how research is brought forward and brought to the market through these EU projects. Suggestions are given into the requirements for future FP projects to continue the development in improving the efficiency of ships and shipping

Simulating compressibility in cavitating flows with an incompressible mass transfer flow solver

The effect of finite mass transfer rate in combination with temporal resolution on the dynamics of caviting flows is subject of this study. It will be shown that global flow quantities exhibit convergent behaviour with respect to mass transfer rate and time step size in incompressible pressurebased simulation of cavitating flows. It is concluded that large mass transfer rates are required in combination with sufficiently small time steps to focus the local phase transition process to time intervals which are small with respect to both the time scale of the flow (Sezal 2009) and the characteristic cavity collapse time. Koukouvinis & Gavaises (2015) as well as Bhatt et al (2015) came to similar conclusions. The effect of finite mass transfer is demonstrated by numerical studies of an isolated bubble collapse and a cavitating wedge flow. It is further shown how a conventional finite mass transfer approach must be modified to achieve homogeneous equilibrium states as given by an arbitrary barotropic equation of state in the presence of advective density change.Ship Hydromechanics and Structure

Finite mass transfer effects in cavitation modelling

One of the key aspects classifying the various approaches in numerical simulation of cavitating flows is the equilibrium flow assumption. It states that internal processes in the flow always occur instantaneously compared to the time scale of the flow (s. Sezal (2009)). As a consequence, the density-pressure trajectory in a barotropic flow may follow a unique curve. Contrary to the equilibrium flow assumption, one may assume that the time to achieve a new state is governed by the magnitude of a finite mass transfer source term in a volume fraction transport equation (s. Asnaghi et al. (2015)). In this case, the set of possible density-pressure states is not predefined, but strongly depends on the rate at which pressure changes. Although it has been pointed out by Koukouvinis and Gavaises (2015) that the equilibrium assumption for a barotropic flow would theoretically be mimicked by the mass transfer model if the finite transfer rate tended to infinity, the model parameters triggering the finite transfer rate are generally considered as empirical (s. Frikha et al. (2008)).In this paper, effects of the finite mass transfer rate with special focus on condensation will be studied in detail. First, a cavity collapse will be considered to demonstrate how the finite transfer source term must be modified to satisfy the equilibrium flow assumption. Second, a single bubble collapse is studiednumerically and effects of the finite mass transfer rate will be discussed.Accepted Author ManuscriptShip Hydromechanics and Structure

Numerical prediction of vortex dynamics in inviscid sheet cavitation

Recent studies have indicated that mass transfer models are able to correctly reflect the sheet cavitation dynamics of inertia driven flows, given that the mass transfer model constants governing the source term magnitude are sufficiently large (Koukouvinis and Gavaises 2015) and that enough temporal resolution is provided (Schenke and Van Terwisga 2017). The inertia driven dynamics, characterised by cavity collapse time, shedding frequencies and local pressure impact frequencies, were shown to be insensitive to variations of the mass transfer coefficients in this limit.This study focuses on an inviscid cavitating flow around a NACA0015 hydrofoil. The flow dynamics are driven by the re-entrant jet as the main mechanism of cavity shedding. A threshold of mass transfer magnitude, temporal and spanwise spatial resolution is identified, beyond which the frequency of local pressure impacts is model parameter independent. Although the excact values of peak pressure loads remain time step size, grid size and model parameter dependent, the sheet cavitation dynamics are considered as well resolved in this regime as far as shedding frequency and characteristic cavity collapse time are concerned. The results are compared to experimental results by Van Rijsbergen et al. (2012).Based on this, the study further focuses on the mechanism of vorticity generation and vorticity break-up, causing potentially erosive cavitating structures such as horseshoe cavities (Dular and Petkovˇsek 2015).Accepted Author ManuscriptShip Hydromechanics and Structure

An energy conservative method to predict the erosive aggressiveness of collapsing cavitating structures and cavitating flows from numerical simulations

A new technique is proposed in this study to assess the erosive aggressiveness of cavitating flows from numerical flow simulations. The technique is based on the cavitation intensity approach by Leclercq et al. (2017), predicting the instantaneous surface impact power of collapsing cavities from the potential energy hypothesis (see Hammitt, 1963; Vogel and Lauterborn, 1988). The cavitation intensity approach by Leclercq et al. (2017) is further developed and the amount of accumulated surface energy caused by the near wall collapse of idealized cavity types is verified against analytical predictions. Furthermore, two different impact power functions are introduced to compute a weighted time average of the impact power distribution caused by the cavity collapses in cavitating flows. The extreme events are emphasized to an extent specified by a single model parameter. Thus, the impact power functions provide a physical measure of the cavitating flow aggressiveness. This approach is applied to four idealized cavities, as well as to the cavitating flow around a NACA0015 hydrofoil. Areas subjected to aggressive cavity collapse events are identified and the results are compared against experimental paint test results by Van Rijsbergen et al. (2012) and the numerical erosion risk assessment by Li et al. (2014). The model is implemented as a runtime post-processing tool in the open source CFD environment OpenFOAM (2018), employing the inviscid Euler equations and mass transfer source terms to model the cavitating flow.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Ship Hydromechanics and Structure

Erosive aggressiveness of collapsing cavitating structures

The erosive aggressiveness of idealized cavities collapsing on a flat surface is investigated by numerical simulation, employing the cavitation intensity approach by Leclercq et al (2017) as a measure of the local energy impact rate. We propose a more straight forward formulation of the cavitation intensity model and verify that it satisfies energy conservation requirements for the accumulated surface energy. Based on the cavitation intensity model, statistical aggressiveness indicators are proposed. The indicators account for the rapidness and frequency of the collapse events. The aggressiveness indicators are further applied to a NACA0015 hydrofoil surface.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Ship Hydromechanics and Structure

Cavitation erosion risk assessment for a marine propeller behind a Ro-Ro container vessel

A novel cavitation erosion risk model, developed by Schenke et al. ["On the relevance of kinematics for cavitation implosion loads,"Phys. Fluids 31, 052102 (2019)], is applied to compute the cavitation implosion loads. The instantaneous energy balance during the collapse of cavitating structures is considered, where the initial potential energy is first converted into collapse-induced kinetic energy, before it is radiated to the surrounding surface at the final stage of the collapse. In this study, we focus on assessing the cavitation development and the risk of erosion on the blades of propellers operating behind a Ro-Ro container vessel. The presence of the hull contributes to the non-uniformity of the inflow. The consequent variation in velocities and angles of attack leads to the amplification of the cavitation dynamics, especially when the blade passes through the top position. Two designs are investigated that experience cavitation erosion on the pressure side. A statistical filter is used to attenuate low-amplitude implosion loads and identify the extreme events on the blade. The results show a very good correlation with the position of the actual erosion damage on the real propeller blades. </p