International Shipbuilding Progress - Volume 60, issue 1-4

Authors: Hendrickson, K. | Weymouth, G. | Banerjee, S. | Yue, D.K.-P.

Article Type: Research Article

Abstract: Accurate prediction of the highly-mixed flow in the near field of a surface ship is a challenging and active research topic in Computational Ship Hydrodynamics. The disparity in the time and length scales and the scales of entrainment dictates the use of bubble source and mixed-phase flow models in which the current state of the art models are ad hoc. This paper presents the air entrainment characteristics and multiphase turbulence modeling of the near-field flow of a canonical stern with the inclusion of simple geometry effects. Using state of the art Cartesian-grid numerical methods with the full field equations, high-resolution …two-phase flow data sets of a canonical stern with three different half-beam to draft ratios are simulated down to the scales of bubble entrainment. These data sets are used as the foundation for: (i) characterization of wake structure and near-wake air entrainment of the stern; (ii) analysis of turbulent mass flux in the wake of the stern; and (iii) a priori testing of multiphase turbulence models for turbulent mass flux. We obtain results to show that these techniques enable analysis and physics-based parameterization of near-field air entrainment about surface ships for use in Computational Ship Hydrodynamics. Show more

Keywords: Entrainment, turbulence, wake, iLES, modeling

DOI: 10.3233/ISP-130102

Citation: International Shipbuilding Progress, vol. 60, no. 1-4, pp. 375-401, 2013

Authors: Castro, Alejandro M. | Carrica, Pablo M.

Article Type: Research Article

Abstract: The modeling of polydispersed bubbly flows in practical applications requires the resolution of large systems of equations with multiple unknowns. Since the solution to this problem lives in a four dimensional space the number of unknowns increases quickly with finer discretizations. In addition, the problem is highly non-linear, strongly coupled and stiff. In this work a series of novel ideas to solve these issues in a robust and efficient manner is presented. Further, a simulation of the bubbly flow around a real self-propelled ship where the propellers are fully discretized is performed. This highly demanding computation serves as a test …for the ideas presented in this work. Comparisons with available experimental data are shown. Show more

Keywords: Polydispersed flow, bubbly flow, multigroup discretization, ship flow, high performance computing

DOI: 10.3233/ISP-130094

Citation: International Shipbuilding Progress, vol. 60, no. 1-4, pp. 403-433, 2013

Authors: Hsiao, C.-T. | Wu, X. | Ma, J. | Chahine, G.L.

Article Type: Research Article

Abstract: Free surface disruption, bubble entrainment, and resulting bubbly wake due to a stationary and moving horizontal jets plunging into a quiescent liquid were studied both numerically and experimentally. The moving jet wake showed significantly different flow characteristics than the stationary jet wake. High speed videos revealed that large vortical structures with entrapped air were generated periodically from the horizontal plunging jet. Each vortical air pocket broke up into multiple bubbles due to local shear flows as it returned toward the free surface and moved downstream. The frequency of the air pocket occurrence was analyzed and found to scale with the …plunging jet flow and geometry conditions. The plunging dynamics was simulated with an Eulerian/Lagrangian one-way coupled two-phase flow model, which included a Level-Set method and a sub-grid bubble entrainment model. These captured free surface dynamics and predicted the bubble generation and entrainment. The flow structures, velocity field, and overall bubble spreading region near the plunging region were well captured by the numerical model. Further improvement on downstream wake flow is being sought through two-way coupling between the two phases since the one-way coupling does not account properly for the effective density. Show more

Keywords: Plunging jet, bubble entrainment, two-phase flow model

DOI: 10.3233/ISP-130093

Citation: International Shipbuilding Progress, vol. 60, no. 1-4, pp. 435-469, 2013

Authors: Mäkiharju, S.A. | Perlin, M. | Ceccio, S.L.

Article Type: Research Article

Abstract: Gas cavities, natural and ventilated, can occur on ship propulsors, control surfaces and hulls. These cavities can be significant sources of small bubbles in the ship wake. We present a two-dimensional time resolved X-ray densitometry system developed for investigation of natural, ventilated and mixed cavities. First we consider the limitations and performance of our X-ray system, and compare the measured void fraction to stationary known void fraction, and to data for a ventilated cavity obtained using dual fiber optical probes. Second, we present preliminary data from time-resolved X-ray used to observe the overall dynamics and time dependent void fraction distribution …of a cavitating backward facing step with and without gas injection. Show more

Keywords: Partial cavity, ventilated cavity, X-ray, densitometry, cavitation

DOI: 10.3233/ISP-130086

Citation: International Shipbuilding Progress, vol. 60, no. 1-4, pp. 471-494, 2013

Authors: Fullerton, A.M. | Beale, K.L.C. | Terrill, E. | Dommermuth, D.

Article Type: Research Article

Abstract: Wind over the ocean can be divided into two parts: the mean wind and the unsteady, time-varying wind. The unsteady component of the wind profile is not often included when modeling wind and predicting its impact on ship motions, but may have an effect, particularly in extreme cases. The mean velocity profiles for the wind have been thoroughly investigated and documented, however the time varying portion is not as well understood, mostly due to insufficient data. A portion of the time-varying wind is due to wind gusts, which are typically modeled as a Gaussian stochastic process and can be …fully described by a wind gust spectrum [Ocean Engineering 34 (2007), 354–358]. Wind gust spectra measured over land typically have less energy at lower frequencies than measured spectra over the ocean, and this low frequency energy can be important in design of ships and offshore structures. Several wind gust spectra have been proposed, with various dependencies on frequency, height above sea level, and mean wind speed, including Ochi [Proceedings of 20th Offshore Technology Conference, Vol. 2, 1988, pp. 461–467], Wills [Marine Structures 19 (2006), 173–192] and Froya [Marine Structures 19 (2006), 173–192]. These spectra are compared with data collected from the High Resolution Air Sea Interaction (HIRES) research program, sponsored by the Office of Naval Research (ONR), from surface buoy data at 1 m as well as from the R/P FLIP at 30 m above sea level. The various spectra from the literature bound the data at the 1 m level, but all tend to over predict the spectra at the 30 m level. In addition to gusts, the mean wind profile may be modified by the time-varying wave profile, another aspect of wind behavior that can affect the design of ships and offshore structures. Air flow separation over water waves has been observed by many researchers over the years, though a global criterion for separation and a model for the separated flow have not yet been conclusively determined. Evidence in the literature shows that both the wave age and wave slope play a role in separation (for example, in: Proceedings of the ASME 2010 29th International Conference on Ocean, Offshore, and Arctic Engineering, 2010). A separation criterion is proposed, based on data from the literature, as well as a model for the adjusted wind profile based on a separation profile developed for land. The adjusted profile compares well with data from the literature. The criterion and proposed model are also compared with preliminary results from the ONR supported computational fluid dynamics code, Numerical Flow Analysis (NFA). The objective of this numerical effort is to research and develop the capability to couple advanced free-surface hydrodynamic predictions with numerical wind distributions in a marine atmospheric boundary layer. By developing a wind/wave boundary interaction model for the Numerical Flow Analysis (NFA) code, an important step towards carrying out this objective is achieved. It is the final goal of this development to provide a wind/wave interaction prediction capability that includes the possibility of correctly predicting the impact of breaking ambient waves. Show more

Keywords: Wind, waves, NFA

DOI: 10.3233/ISP-130095

Citation: International Shipbuilding Progress, vol. 60, no. 1-4, pp. 495-521, 2013

Authors: Degtyarev, Alexander B. | Reed, Arthur M.

Article Type: Research Article

Abstract: The problem of wave climate description and modeling are considered on both a short-term and synoptic basis. The wave climate is considered as an ensemble of conditions of spatio-temporal wave fields characterized by frequency-directional spectra. Using an expanded set of wave and wind characteristics makes it possible to correctly introduce the concept of a wave weather “scenario” and to use it to evaluate vessel safety. The mathematical basis of the short-term wave description is by the representation of the stochastic processes by a generalized autoregressive model (ARM) and related models of periodically correlated random process. An ARM is used to …generate space-time realizations of the wave surface of any short-term duration. Application of the aforesaid model takes into account the true stochastic process (lack of repetition in quasi-steady implementation of the process inherent in other models of wind waves). Ways of treating synoptic and seasonal variability of wave forecasts for a specific geographic region are also described. Here, for the first time using the general positions of stationary wave processes and non-stationary synoptic processes of average wave height variation are combined. In this case we have nested mathematical models of the same type. Show more

Keywords: Autoregressive model (ARM), ocean waves modeling, wave weather “scenario”, synoptic weather model

DOI: 10.3233/ISP-130091

Citation: International Shipbuilding Progress, vol. 60, no. 1-4, pp. 523-553, 2013

Authors: Sclavounos, Paul D. | Lee, Sungho

Article Type: Research Article

Abstract: A new nonlinear seakeeping theory is presented for the prediction of the large amplitude motions and wave induced loads on ships in severe seastates. The hydrodynamic forces on the ship are evaluated using a new fluid impulse theory which expresses the nonlinear Froude–Krylov, radiation and diffraction forces as time derivatives of integrals of velocity potentials over the instantaneous ship wetted surface circumventing the time consuming evaluation of the pressure from Bernoulli's equation. Analogous expressions are derived for the structural loads. The free surface problems for the ship wave disturbance are solved on vertical planes fixed relative to an earth fixed …frame using a 2D+t slender body theory. The linear free surface condition is enforced on the ambient wave profile assumed locally horizontal, a nonlinear boundary condition is imposed on the ship hull and the resulting boundary value problem is solved using the two-dimensional time-domain wave source potential. Computations are presented illustrating the performance of the new theory. The application of the theory to the evaluation of the extreme statistics of the ship responses and structural loads in a stochastic seastate is also addressed. Show more

Keywords: Fluid impulse, slender body theory, seakeeping

DOI: 10.3233/ISP-130085

Citation: International Shipbuilding Progress, vol. 60, no. 1-4, pp. 555-577, 2013

Authors: Belenky, Vadim | Spyrou, Kostas | Weems, Kenneth M. | Lin, Woei-Min

Article Type: Research Article

Abstract: This paper describes recent progress and issues in the development of the split-time method for predicting the probability of a roll motion stability failure – capsize or extreme roll motion event – for a ship operating in irregular ocean waves. The split-time method attempts to address the challenge presented by the rarity and physical complexity of such events by separating the problem into a sequence of non-rare and rare problems that can be practically characterized with advanced numerical methods. In the broadest terms, the non-rare problem can be described as determining the rate of occurrence of an intermediate event such …as the upcrossing of a threshold roll angle, while the rare problem can be described as determining the probability of capsizing when this intermediate event is realized. The current development of the method focuses on two areas related to the application of this method for nonlinear ship motion in quartering seas. The first area of development is the variation of the ship's roll restoring curve in following or quartering seas, which is a key element in a pure loss of stability event. To incorporate this phenomenon, the split-time method has been reformulated with the intermediate roll threshold and the critical roll rate leading to capsizing after upcrossing described by stochastic processes. The implementation of this method with the results of numerical simulation data has led to several important results, including the understanding that roll and roll rates may be dependent processes in quartering seas and the development of a novel procedure for characterizing distribution of the dependent process at the instant of upcrossing. The second area of development is the probabilistic model of surf-riding, which is a necessary step toward evaluating the probability of capsizing or large roll motion due to broaching-to following surf-riding. Recent results include the evaluation of a suitable wave celerity in irregular seas and the use of the existence or non-existence of surf-riding equilibria to describe a ship's transition into and out of surf-riding. Show more

Keywords: Probability of capsizing, pure loss of stability, surf-riding, split-time method

DOI: 10.3233/ISP-130084

Citation: International Shipbuilding Progress, vol. 60, no. 1-4, pp. 579-612, 2013

Authors: Greeley, David S. | Willemann, Simmy D.

Article Type: Research Article

Abstract: The use of lifting potential flow techniques for the calculation of surface ship maneuvering forces in both calm water and waves is presented and demonstrated. By properly treating the trajectory of vorticity shed from salient hull features of slender ships, non-linear forces and moments can be captured using potential flow theory. This approach offers an accurate and rapid turn-around calculation, relative to RANS (Reynolds-Averaged Navier–Stokes) approaches, of maneuvering forces in both calm water and waves.

Keywords: Lifting potential flow, maneuvering forces, weak scatterer

DOI: 10.3233/ISP-130089

Citation: International Shipbuilding Progress, vol. 60, no. 1-4, pp. 613-631, 2013

Article Type: Other

Citation: International Shipbuilding Progress, vol. 60, no. 1-4, pp. 633-635, 2013