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Craft

International Journal of Solids and Structures

Volume 40, Issue 22, November 2003, Pages 5973-5999

Craft––a plastic-damage-contact model for concrete. I. Model theory and thermodynamic considerations

Author links open overlay panelA.D.Jefferson

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https://doi.org/10.1016/S0020-7683(03)00390-1Get rights and content

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A framework is described for the development of a thermodynamically consistent plastic directional-damage-contact model for concrete. This framework is used as a basis for a new model, named Craft, which uses planes of degradation that can undergo damage and separation but which can regain contact according to a contact state function. The thermodynamic validity of the resulting model is considered in detail, and is proved for certain cases and demonstrated numerically for others. The model has a fully integrated plasticity component that uses a smooth triaxial yield surface and frictional hardening–softening functions. A new type of consistency condition is introduced for simultaneously maintaining both local and global constitutive relationships as well as stress transformation relationships. The introduction of contact theory provides the model with the ability to simulate the type of delayed aggregate interlock behavior exhibited by fully open crack surfaces that subsequently undergo significant shear movement. The model has been implemented in a constitutive driver program as well as a finite element program. The model is assessed against a range of experimental data, which includes data from uniaxial tension tests with and without unloading–reloading cycles, tests in which cracks are formed and then loaded in shear, and uniaxial, biaxial and triaxial compression tests.

Information and Organization

Volume 17, Issue 1, 2007, Pages 2-26

The qualitative interview in IS research: Examining the craft

Author links open overlay panelMichael D.MyersaMichaelNewmanbc

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https://doi.org/10.1016/j.infoandorg.2006.11.001Get rights and content

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The qualitative interview is one of the most important data gathering tools in qualitative research, yet it has remained an unexamined craft in IS research. This paper discusses the potential difficulties, pitfalls and problems of the qualitative interview in IS research. Building on Goffman's seminal work on social life, the paper proposes a dramaturgical model as a useful way of conceptualizing the qualitative interview. Based on this model the authors suggest guidelines for the conduct of qualitative interviews.Slamming pressure assessment is an important topic for shell plating and stiffener design of bow flare. In this study, the pressure distribution on the bottom plating of an high speed planning craft is evaluated through measurements of the impact pressures on scale model running in regular waves.

The planing hull model, monohedral hard chine built with clear bottom and deck, in order to allow the visual inspection of the fluid flow and the exact points of impact has been extensively studied as reported in previous works.

From the time histories of vertical motions (heave and pitch) and bow acceleration of the model measured in "standard" seakeeping tests, preliminary assessment of the slamming impact pressure according to Zhao and Faltinsen method is performed. The experimental campaign presented in this paper is focused on the pressure field assessment in nine points of the hull bottom surface running at four velocities and two regular waves. Results analysis in time and frequency domain is given, identifying the pressure distribution along the bottom panel. Furthermore, comparison of measured, analytical and normative values has been performed. From the cross-correlation of measured responses further comments and indications for future work has been withdrawn. The reported results provide an useful data set for all transportation-engineering systems under dynamic load.2.3.3 Hydrofoil craft

Hydrofoil craft make use of hydrodynamic lift generated by hydrofoils attached to the bottom of the craft. When the craft moves through the water a lift force is generated to counteract the craft's weight, the hull is raised clear of the water and the resistance is reduced. High speeds are possible without using unduly large powers. Once the hull is clear of the water and not, therefore, contributing buoyancy, the lift required of the foils is effectively constant. As speed increases either the submerged area of foil will reduce or their angle of incidence must be reduced. This leads to two types of foil system:

(1)

Completely submerged, incidence controlled. The foils remain completely submerged, reducing the risk of cavitation, and lift is varied by controlling the angle of attack of the foils to the water. This is an 'active' system and can be used to control the way the craft responds to oncoming waves.

(2)

Fixed surface piercing foils. The foils may be arranged as a ladder either side of the hull or as a large curved foil passing under the hull. As speed increases the craft rises so reducing the area of foil creating lift. This is a 'passive' system.

Foils are provided forward and aft, the balance of area being such as to provide the desired ride characteristics. The net lift must be in line with the centre of gravity of the craft. Like the SES, the hydrofoil has been used for service on relatively short-haul journeys. Both types of craft have stability characteristics which are peculiarly their own.

Hydrofoil Craft

Hydrofoil craft make use of hydrodynamic lift generated by hydrofoils attached to the hull. As the craft accelerates, the resistance increases due to increasing wave resistance and then drops off as the main hull leaves the water. It is the forward movement through the water that causes a lift force on the foils counteracting the craft's weight. Once the hull is clear of the water, the resistance is reduced to that of the foils. With high lift to drag sections, high speeds are possible at relatively low powers. Once the hull is clear of the water, the lift required of the foils is effectively constant. As speed increases either the submerged area of foil will reduce or their angle of incidence must be reduced. This leads to the following two foil systems:

•

Completely submerged, incidence controlled: these foils remain completely submerged, reducing the risk of cavitation, and lift is varied by controlling the angle of attack of the foils to the water. This is an 'active' system and can be used to control the way the craft responds to oncoming waves. It can be made to contour the waves.

•

Fixed surface-piercing foils: these foils may be arranged as a ladder either side of the hull or as a large curved foil passing under the hull. As speed increases the craft rises thereby reducing the area of foil needed to create the lift. When meeting a wave, the forward foil is submerged more deeply, generates more lift and raises the bow. This is a 'passive' system.

Foils are provided forward and aft, the balance of area being such as to provide the desired ride characteristics. The net lift must be in line with the centre of gravity of the craft. Usually there is a large foil close to the LCG. This provides most of the lift and the other, smaller, foil maintains the trim angle of the craft. The surface-piercing foil will automatically provide a degree of transverse stability due to the greater foil area immersed on the lower side. With fully submerged foils ailerons either side of the foil can be moved to provide a moment opposing heel. Like the SES, the hydrofoil has been used for service on relatively short-haul journeys. Both types of craft have stability characteristics which are peculiarly their own.Hydrofoil Cavitation

The foils of a hydrofoil craft are never far away from the free surface of the water and, consequently, the static head over the foil tends to be low. This coupled with the dynamic head, which is a function of ship speed increases the risk of cavitation. When cavitation occurs, depending upon its type and severity, this may either result in a modification of the lifting capabilities of the foils due to its influence on the pressure distributions generated around the foil or in some cases to the destruction of the foil material by cavitation erosion. Figs. 9.21 and 9.22 outline the general influence that cavitation can have pressure distributions and lifting properties of hydrofoils.

In general, hydrofoils can be designed to operate at speeds of up to around 50–60 knots without incurring the significant effects of cavitation, although some will be present. Beyond these speeds, increasing amounts of cavitation should be expected until a supercavitating state is eventually reached. Because the foils of hydrofoil craft are complex structures when combined with their supporting struts, the cavitation is usually very three dimensional: this applies to both the leading and trailing sets of foils. Systems of three-dimensional cavitation do have a tendency to become unstable from which significant fluctuating loads can be created.

Cavitation buckets, of the type seen in Fig. 9.23, define the likelihood and the type of cavitation that may occur for a given set of design situations. While in many applications some limited amounts of leading edge cavitation are likely to be encountered, and is perhaps unavoidable, this is unlikely to have a marked effect on the hydrodynamic properties of the foils and has the possibility of reducing the foil's response to wave-induced variations in the section angle of attack. More serious, however, is the occurrence of midchord cavitation and this needs to be avoided for both its adverse hydrodynamic and erosion potential.

To control hydrofoil craft, the use of flaps is commonly used. Their modes of operation are not dissimilar to the use of control surfaces fitted to airplanes in that the lifting characteristics of the foil are modified by the chosen angle of flap, Abbott and von Doenhoff (1959). In the case of hydrofoils the effects of cavitation and, if it occurs, ventilation has to be considered. With regard to cavitation, its influence will be dependent upon its type and extent. Up to a certain point in the case of leading edge cavitation, the effect will be marginal; however, as cavitation progresses toward supercavitation, the effect becomes progressively more pronounced as is the case with propeller blade sections. Indeed, the use of flaps will modify the pressure distribution around the foil, so cavitation may transform from one form to another. Fig. 12.44 outlines the effects of flap angles at different foil angles of attack on the lift for cavitating and ventilated aerofoils.The systems of aerofoils such as are found in these craft to support the weight of the vessel foils are placed in forward and aft locations. Clearly the forward foils system will influence the aft located foils since cavitation inception and development is to a very large extent dependent upon the angle of attack of the flow incident on the hydrofoil. The forward foil, because it is of finite dimensions, generates a downwash due to the shed vorticity as discussed in Section 7.9. This variable downwash along its span alters the flow incident upon the after set of hydrofoils and, consequently, the cavitation characteristics of that foil. This interaction effect can be most pronounced if the span of the forward system of hydrofoils is greater than that of the aft foil system.

A further cause for concern is the design of the intersections between the supporting struts and the hydrofoil. In much the same way as with propeller blade roots, trailing vortices are generated because both the foil and strut are lifting surfaces. Consequently, without due care over the detail of these component parts drag in its various forms and cavitation development can arise. Nevertheless, apart from the hydrodynamic consideration, strength considerations must prevail.13.3 Conformal Mapping

Let us consider a plane in which we define points z=x+iy, and a second plane in which we define points w=u+iv. If there exists a function f such that to each point z corresponds one point w=f(z), we say that the function f is a mapping or transformation of the plane z into the plane w. If the function f(z) is complex differentiable at every point in a region R of the complex plane we say that the function is analytic in the region R. Another term for this notion is holomorphic function. Mappings by an analytic function preserve angles; therefore, such mapping are qualified as conformal. In other words, if in the plane z the angle between two curves is α, in the plane w the angle between the conformal mappings of the given curves is also α.

There is another invariant of conformal mappings and the application discussed in this chapter is based on it. If we study a two-dimensional flow and assume that it is irrotational and inviscid, there is a potential function, ϕ, such that the components of the fluid velocity are the partial derivatives of this function with respect to coordinates. The potential must be a solution of the Laplace equation

(13.1)∂2ϕ1∂x2+∂2ϕ1∂y2=0

and fulfill adequate boundary conditions. If in the plane z we are given a potential Φ1, and apply to it a conformal transformation, in the plane w we obtain a potential Φ2 that is a solution of the Laplace equation

(13.2)∂2ϕ2∂u2+∂2ϕ2∂v2=0

A proof of this theorem is given in Wylie (1966). Now, let us assume that we want to study the flow around a given ship section. We have a solution for the flow around a circle. We must find a function that maps the circle into a shape that approximates well the given section and then we use that function to map the potential flow.

Example 13.1 The Joukovski transformation

Zhukovski (Nikolai Egorovich, Russian, 1847–1921) published in 1910 a paper that describes what is known as the famous Joukovski transformation. This conformal mapping transforms a circle into an airfoil and its main use is in aeronautical engineering. Airfoil shapes, however, appear also in naval architecture. Ruders have often the form of a symmetrical airfoil, while cambered shapes are used for hydrofoil craft or roll stabilizers. To exemplify the Joukovski transformation we represent the circle in the z-plane by the polar equations

(13.3)x=x0+r0cos⁡θy=y0+r0sin⁡θ

and define the mapping function

(13.4)w=(z+b2z)/2

A MATLAB function that carries on the mapping is

function Joukovski(x0, y0, b)

%JOUKOVSKI Joukowski transformation of circle with centre

% at x0, y0, and parameter b.

% define and plot circle in z plane

theta = 0: pi/60: 2*pi; % circle parameter

r0 = 1.5; % circle radius

x = x0 + r0*cos(theta); y = y0 + r0*sin(theta);

z = x + i*y;

subplot(1, 2, 1) % z-plane

plot(x, y, 'r-'), grid, axis equal

tt = [ 'x_0 = ', num2str(x0), ', y_0 = ', num2str(y0) ];

tt = [ tt, ', b = ', num2str(b) ];

title({'z-plane'; tt})

xlabel('\Re'), ylabel('\Im')

hold on

point([ x0; y0 ], x0/8)

hold off

% Joukowski transformation

w = (z + b^2./z)/2;

subplot(1, 2, 2) % w plane

plot(real(w), imag(w), 'r-'), grid

axis equal

title({'w-plane'; 'w = (z + b^2/z)/2'})

xlabel('\Re'), ylabel('\Im')

Three examples appear in Figs 13.2 to 13.4; they show how the centre of the circle in the z-plane determines the shape of the foil.

Figure 13.2. The Joukovski transformation, circle to ellipse

Figure 13.3. The Joukovski transformation, symmetrical airfoil

Figure 13.4. The Joukovski transformation, cambered airfoil

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Waterjet Propulsion

J.S. Carlton FREng, in Marine Propellers and Propulsion (Fourth Edition), 2019

16.4.1 Tunnel, Inlet, and Supporting Structures

The inlet to the tunnel, in order to protect the various internal waterjet components, is frequently fitted with an inlet guard to prevent the ingress of large objects. Clearly the smaller the mesh of the guard, the better it is at its job of protection. However, the design of the guard must strike a balance between undue efficiency loss because of the flow restriction and viscous losses, the size of the object allowed to pass, and the guard's susceptibility to clog with weed and other flow-restricting matter. For small tunnels, a guard may be unnecessary or indeed undesirable since a compromise between these constraints may prove unviable, but for the large tunnels this is not the case. In this latter event, the strength of guard needs careful attention since the flow velocities can be high.

The profile of the tunnel needs to be designed so that it will provide a smooth uptake of water over the range of vessel operating trims and, therefore, avoid any significant separation of the flow or cavitation at the tunnel intake.

In some waterjet applications, typically hydrofoil craft applications where the water flow has to pass up the foil legs, it is necessary to introduce guide vanes into the tunnel in order to assist the water flow around bends in the tunnel. The strength of these guide vanes needs careful attention, both from steady and fluctuating loadings, and if they form an integral part of the bend by being, for example a cast component, then adequate root fillets need to be provided. The guide vanes also need to be carefully aligned to the flow, and the leading and trailing edges of the vanes should be faired so as not to cause undue separation or cavitation. Guide vanes, where fitted, need to be inspected periodically for fracture or impending failure during service. Therefore, some suitable means for inspection needs to be provided for either direct visual inspection or indirectly using a boroscope.

Within the tunnel, the dimensions are sometimes such that the drive shaft for the pump needs support from the tunnel walls. In such cases the supports, which should normally number three arranged at 120 degrees spacing if there is danger of shaft lateral vibration, must be aligned to the flow and have an aerofoil section to minimize the flow disturbance of the incident flow into the pump and, additionally, the probability of cavitation erosion on the strut. The form and character of the wake field immediately ahead of the impeller is generally unknown. Some model tests have been undertaken in the past by, for example, Holden et al. (1981) and Sayers (1990). An example of these measurements is shown in Fig. 16.9. However, the aim should be to provide the pump with as small a variation in the flow field as possible to minimize the fluctuating blade loading. This can be done only by scrupulous attention to detail in the upstream tunnel design and with the aid of RANS computational codes.

Fig. 16.9. Typical wake survey of a waterjet inlet just upstream of the pump impeller.

(Reproduced with permission from Haglund, K., et al., 1982. Design and testing of a high performance waterjet propulsion unit. In: Proc. 2nd Symp. on Small Fast Warships and Security Vessels. Trans RINA, London , May (Paper No. 17).)

The integrity of the tunnel wall both in intact and failure modes of operation is essential. If the wall fails this can lead to extensive flooding of the compartment in which the waterjet is contained and hence, by implication, have ship safety implications. There is, therefore, a need for attention to the detail of the tunnel design; for example, in adequately radiusing any penetrations or flanged connections and in terms of producing an adequate stress analysis of the tunnel both in the global and detailed senses. In addition to considering the waterjet as an integral component, the tunnel must be adequately supported, framed, and fully integrated into the hull structure, taking due account of the different nature, response, and interactions of the various materials used; for example, GRP, steel, and aluminum.An approach is investigated in this study for structural dynamic analysis of a high-speed planing hull, in which the pointwise acceleration data collected from sea trials are enforced as base excitation. The paper first performed the full boat analysis of an 11-meter high speed craft for a period of one wave impact selected from each of nine seakeeping runs. The sea trial acceleration data collected from 11 accelerometers placed close to the centerline and the keel are enforced as input, while those from 3 accelerometers placed around the pilot cabin are selected for validation. The substructure dynamic analysis of the isolated pilot cabin was then conducted and validated, in which 7 pointwise enforced accelerations are selected from the simulation output of the full boat dynamic analysis. The substructure dynamic analysis enables detailed investigation of local stress concentrations where critical equipment and personnel are located. The proposed approach can be extended to rigid-flexible body coupling analysis of a high-speed craft when it is running with large pitching and yawing motion.

High Speed Craft Fire Resisting Divisions

For use in high speed craft constructions, the fire resistance of the polymer composite-based fire resisting divisions is determined in accordance with Resolution MSC.45(65)19 which, in general, uses IMO A.754(18)11 test procedures. The test should continue for a minimum of 30 min for fire resisting division 30, or 60 min for fire resisting division 60. In addition, the Resolution MSC.45(65)19 requires that the load bearing divisions of polymer composites should be tested with the prescribed static load and they should maintain their load-bearing ability within the classification period. It also provides the static loading level for bulkhead and decks (overheads), and also provides the performance criteria for load-bearing ability.

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Control Engineering Practice

Volume 8, Issue 2, February 2000, Pages 185-190

Autopilot and track-keeping algorithms for high-speed craft

Author links open overlay panelClaes G.Källström

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https://doi.org/10.1016/S0967-0661(99)00167-7Get rights and content

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A track autopilot predictor (TAP) system for high-speed craft with waterjet propulsion systems has been developed and installed on-board four catamarans. The TAP system includes automatic control modes for steady-state course-keeping, turning and track-keeping. A special harbour mode and a hydrodynamic predictor based on a non-linear manoeuvring model of the ship are also included. The paper describes the design of the control algorithms used in the autopilot and track-keeping systems. The design is based on a combination of velocity scheduling, pole placement and a feed-forward model for turning. The performance of the algorithms is illustrated in full-scale experiments.

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