256 research outputs found
An analysis method to evaluate the added resistance in short waves considering bow wave breaking
No full textA bow wave breaking is one of the most prominent factors to be considered regarding the nonlinearity of added resistance for a ship. Considering the stability of the bow wave breaking, which is mostly influenced by the ship speed and the waterline entrance angle, can enhance understanding of the nonlinearity. Understanding of the nonlinearity can be improved by considering the stability of the bow wave breaking, which is mostly influenced by the ship speed and the waterline entrance angle. New transfer function containing the ship speed is proposed to make a better representative of the nonlinearity. This method is evaluated with the model test data of the Fast Displacement Ship (FDS) under the short waves condition (λ/L= 0.4). This study has shown that new transfer function can be an efficient analysis method of the ship performance prediction offering intuitive consistency with proposed residual resistance concept. The findings lead to better understanding the nonlinearity considering bow wave breaking.Ship Hydromechanics and Structure
The application of particle image velocimetry for the analysis of high-speed craft hydrodynamics
No full textThe particle image velocimetry (PIV) technique has become a reliable method for capturing the velocity field and its derivatives, even in complex flows and is now also widely used for validation of numerical codes. As the imaging system is sensitive to vibrations, the application in environments such as towing tanks makes it a challenging task. Especially when operating the towing tank carriage at higher speeds structural vibrations increase significantly. However, to apply this technique for the analysis of planing and semi-planing vessels, this is absolutely necessary. To assess the ability of a PIV system to capture the flow features close to a ship model while being towed at high speeds, a stereo PIV system was installed in the TU Delft towing tank. Measurements are conducted in the transom region of a generic high-speed vessel hull with a flat bottomed aft section, which is equipped with an interceptor at the transom. It is shown that with increasing carriage speed, vibrations of the imaging system increase, which ultimately can affect the quality of PIV recordings. The effect of vibrations is quantified by analysis of successive recordings and based on this, a shift correction is applied. A comparison with numerical results shows that the flow around the interceptor is well captured by the PIV systemShip Hydromechanics and Structure
Generating and absorbing boundary conditions for combined wave-current simulations
No full textThe CFD simulation tool ComFLOW is extended to investigate the characteristics of wave motions in the presence of steady uniform currents. Initially, the inflow boundary is the superposition of waves and current. Effect of the latter on the former is resolved by solving Navier-Stokes equations within the domain as a next step. A Generating and Absorbing Boundary Condition (GABC) with currents is introduced that allows the simulation of a combined wave-current environment in truncated domain. This GABC is characterized by a rational function approximation of dispersion relation, based on Sommerfeld condition and irrotational wave model. The artificial boundaries where GABC with current is applied are transparent to incoming and outgoing waves and currents simultaneously. The absorption properties of the GABC for various waves and currents are analysed. The temporal and spatial differences of free surface elevation between the small domain and large domain turn out to be small, i.e. the GABC prevents the reflection from the boundaries well. The large domain here is arranged in such a way that the reflected waves and currents will not reach the outflow boundary of the small domain within the simulation time. The behaviour of GABC in 3D domain is also investigated, where waves and currents are traveling under an angle of incidence colinearly.Ship Hydromechanics and Structure
Validation of a simple aerodynamic model capable to predict the interaction effects occuring between two generic wind propulsion systems
No full textIn recent years wind-assisted propulsion for commercial ships has gained an increasing interest as valuable alternative to reduce fuel pollutant emissions. However, the development of feasible and commercially viable wind propulsion systems to partially (or fully) propel a ship is nowadays hindered by the difficulties of modelling the complicated aerodynamic and hydrodynamic aspects involved. From an aerodynamic point of view, it appears that one of the main challenges of predicting the performance of a wind-assisted ship, is to properly evaluate the interaction effects that occur between the various wind propulsion systems mounted on the deck of the ship. This research deals with the validation of a simple and quick-to-use aerodynamic model that is capable of evaluating such effects, i.e. upwash, downwash and wake, occurring between two generic propulsion systems placed at any given relative position on the ship's deck. The wind propulsion systems might assume any given angle of attack; the flow can be attached as well as separated. Such aerodynamic model, that was first presented in [1], consists of the horseshoe vortex method modified with semi-empirical formulas to take into account the effects of viscosity. First, the results provided by the aerodynamic model were compared with results obtained by using more sophisticated tools, i.e. a CFD body force model and RANS CFD. Then, experimental validation was carried out by means of dedicated wind-tunnel tests. It can be concluded that, despite the simplicity of the aerodynamic model employed, it proved to give reasonable results when compared to more sophisticated tools and to experimental data. REFERENCES [1] K. Roncin and J.M. Kobus, “Dynamic simulation of two sailing boats in match racing”, Sports Engineering , Vol. 7, pp. 139-152, (2004)
Hydrodynamics of wind-assisted ship propulsion verification and validation of RANS methodology
No full textWind energy as an auxiliary form of propulsion for commercial ships has again become of great interest as a possible response to volatile fuel prices and increasingly stringent environmental regulations. A well-founded performance prediction tool is a key prerequisite for the further development of this promising technology, and with the support of the European Commission and others, a group of researchers at Delft University of Technology is developing a performance prediction program for these hybrid ships. Reynolds-Averaged Navier Stokes (RANS) packages will be one of the primary tools used during the study. The advent of the numerical towing tank brings possibilities but also new challenges. The predominance of large, separated flow structures in the wake of the sailing ship, and the particular interest in the transverse force component points to a conscientious grid verification and validation study. Here, it is sufficient to achieve parity for absolute uncertainty within the larger context of the project. The ‘drift sweep’ procedure is presented with validation levels alongside computational time to demonstrate the utility of this approach in support of the derivation of empirical formulations using systemic hull form variations. Finally, a moving mesh for the rudder may be implemented in an extended drift/rudder sweep. However for this case there is no validation data presently available.Ship Hydromechanics and Structure
Nitric Oxide-releasing Poly(vinyl Alcohol) Film For Increasing Dermal Vasodilation
No full textPathological conditions associated with the impairment of nitric oxide (NO) production in the vasculature, such as Raynaud's syndrome and diabetic angiopathy, have stimulated the development of new biomaterials capable of delivering NO topically. With this purpose, we modified poly(vinyl-alcohol) (PVA) by chemically crosslinking it via esterification with mercaptosuccinic acid. This reaction allowed the casting of sulfhydrylated PVA (PVA-SH) films. Differential scanning calorimetry and X-ray diffractometry showed that the crosslinking reaction completely suppressed the crystallization of PVA, leading to a non-porous film with a homogeneous distribution of -SH groups. The remaining free hydroxyl groups in the PVA-SH network conferred partial hydrophylicity to the material, which was responsible for a swelling degree of ca. 110%. The PVA-SH films were subjected to an S-nitrosation reaction of the -SH groups, yielding a PVA containing S-nitrosothiol groups (PVA-SNO). Amperometric and chemiluminescence measurements showed that the PVA-SNO films were capable of releasing NO spontaneously after immersion in physiological medium. Laser Doppler-flowmetry, used to assess the blood flow in the dermal microcirculation, showed that the topical application of hydrated PVA-SNO films on the health skin led to a dose- and time-dependent increase of more than 5-fold in the dermal baseline blood flow in less than 10. min, with a prolonged action of more than 4. h during continuous application. These results show that PVA-SNO films might emerge as a new material with potential for the topical treatment of microvascular skin disorders. © 2013 Elsevier B.V.116643651Holowatz, L.A., Thompson-Torgerson, C.S., Kenney, W.L., The human cutaneous circulation as a model of generalized microvascular function (2008) J. Appl. 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Limitations On The Creation of Continuously Surfable Waves Generated By A Pressure Source Moving In A Circular Path
No full textThe aim of the research presented in this work was to investigate the novel idea to produce continuous breaking waves, whereby a pressure source was rotated within an annular wave pool. The concept was that the pressure source generates non-breaking waves that propagate inward to the inner ring of the annulus, where a sloping bathymetry (beach) triggers wave breaking. In order to refine the technique, research was conducted to better understand the mechanics of waves generated by moving pressure sources in a constrained waterway, the transformation of these waves as they travel across the channel and the effect of the sloping beach on the wave quality for surfing. The quality of the waves was defined in terms of wave height, speed and shape, with the aim to create plunging waves, known as “barrels”, that are highly desired by surfers. Surfers also require a long steep crestline or “wall”, to allow a full range of manoeuvres to be performed. Finally, the pool was required to create waves suitable for surfers from beginner to expert level. The major finding was that the design parameters were generally in competition, and to determine a balance of limiting values, the parameters could not be considered in isolation. Therefore, a set of empirical relationships between the design parameters were developed to allow the pool to be designed for a combination of desired wave height and shape in a given pool radius. In the early stages of the study, a pressure source operating in a very constrained waterway with high levels of blockage, travelling in a circular track at high depth Froude numbers was found to exhibit highly nonlinear behaviour. This behaviour appeared to invalidate the use of linear and simpler non-linear potential flow numerical modeling tools to analysis the wave pool design. Even considering simplified configurations (such as a linear track, no blockage, and no beach), the predicted wave heights and shape generally did not correlate well with initial experimental results. Thus, a predominantly experimental approach was undertaken. A method of qualitative scoring wave shape from a surfer’s perspective was developed and proved valuable for focusing the research effort. At the end of the test series, high quality continuous breaking waves with the desired plunging shape were able to be generated, with these waves being desirable for surfing. However, it was determined that only a very small range of design parameter values produced the desired high, plunging waves in the very constrained waterway under consideration, and the wave quality was shown to be extremely sensitive to changes in the design parameters.Ship HydrodynamicsMechanical, Maritime and Materials Engineerin
Aircushion Supported Mega-Floaters
No full textThe increase of the global population and expanding coastal mega-cities will necessitate an innovative pursuit of the utilization of the ocean space in which mega-floaters will play an important role in the future. These types of structures are very large floating artificial islands that can be used for various facilities and purposes similar to those on land. Compared to landfill methods mega-floaters generally have a smaller environmental impact than traditional land reclamation projects. They are indifferent to earthquakes and can be constructed at relatively low cost in a short period of time, independent of ocean depth and seabed conditions. Furthermore, the existing facilities can be easily expanded while they are functional and the space available inside the structure offers prospects for various activities and different use. This thesis describes a method to predict the dynamic behavior of aircushion supported mega-floaters in waves. These types of structures are supported by a large volume of air which is entrapped underneath the structure by vertical walls that extend sufficiently far underneath the water surface in a way that no air will escape when waves pass by. The method is based on a linear three-dimensional potential theory using modal expansions and a linear adiabatic law to describe the air pressure within the aircushion. It is the first method that is able to accurately predict the three-dimensional dynamic behavior and stresses of flexible aircushion supported structures of arbitrary shape in waves. The structure around the aircushion is modeled in the usual way by means of panels representing pulsating sources which are distributed over the mean wetted surface of the body. The free water surface underneath the structure is modeled by panels laying in the mean free surface of each aircushion. All panels associated with an aircushion represent a body without material mass, but having added mass, damping, hydrostatic restoring and aerostatic restoring characteristics. The results of this study indicate that the behavior of aircushion supported structures can be well predicted by means of a three-dimensional linear potential method. In case of rigid bodies, the numerical results were validated with model tests. Model tests with a conventional flexible barge served to validate the hydroelastic method. Unfortunately no experimental results are available for flexible aircushion supported structures. Therefore the numerical results of these structures are verified with analytical and FEM computations. Both the model tests and computations have shown that the application of aircushions can significantly influence the behavior of floating structures. The effect on the structural loads is significant and is particularly pronounced in the wave induced bending moments which are considerably reduced by the aircushions. A conventional mega-float structure has to be protected by breakwaters if it is located in open seas. These breakwaters will reduce the wave loads on the structure, but add to the total costs of the mega-float project. Another option is to support the structure by aircushions to reduce the wave induced bending moments and consequently the stresses. In general, the results of this study have shown that an aircushion supported structure will have significant advantages compared to conventional mega-floaters. In addition, the computational method as developed and proposed proved to be a suitable tool to optimize the cushion configuration for a particular application.Offshore Engineering & Ship HydromechanicsMechanical, Maritime and Materials Engineerin
Proactive Control of Fast Ships: Improving the seakeeping behaviour in head waves
No full textShip Hydromechanics and StructuresMechanical, Maritime and Materials Engineerin
Practical application of viscous-flow calculations for the simulation of manoeuvring ships
No full textThe present work was initiated in order to improve traditional manoeuvring simulations based on empirical equations to model the forces and moments on the ship. With the evolution of the capability of viscous-flow solvers to predict forces and moments on ships, it was decided to develop a practical method to simulate the manoeuvrability of ships in which viscous-flow solvers are utilised and to investigate whether this improves the accuracy of manoeuvring predictions. In this thesis, it is demonstrated that good predictions of the loads on the hull in manoeuvring motion can be obtained for a wide range of ship types. The trends in the forces and moments as a function of the drift angle or yaw rate are simulated well. By using hydrodynamic manoeuvring coefficients derived from the CFD calculations, it is shown that it is possible to improve the prediction of ship manoeuvres compared to predictions using coefficients based on empirical equations. A considerable improvement in the turning circle predictions was obtained. The prediction of the yaw checking and course keeping and initial turning abilities based on zig-zag simulations improved as well, but further improvements are required for more reliable assessment of the manoeuvring performance.Marine and Transport TechnologyMechanical, Maritime and Materials Engineerin
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