224 research outputs found
Effects of wave directionality on the in-line loading of a vertical cylinder
No full textThis paper presents results from an experimental investigation into the loading on a rigid vertical circular cylinder in irregular unidirectional and multidirectional waves in water of uniform depth. The ambient flow at the location of the cylinder was measured directly using perforated-ball velocity meters (PVMs), in order to avoid uncertainties associated with the use of wave theories. Reynolds numbers were in the range of 104 to 7 X 104, and Keulegan-Carpenter numbers ranged from o to 16. In-line Morison coefficients and root-mean-square force coefficients are estimated from the loading measured on force sleeves at three elevations along the length of the cylinder. Results are presented for the probability distributions of forces and peak forces, and for Morison and rms force coefficients. Effects of wave directionality are compared with Dean's analytical predictions
Modelling surface roughening during plastic deformation of metal crystals under contact shear loading
No full textDuring plastic deformation, metal surfaces roughen and this has a deleterious impact on their tribological performance. It is therefore desirable to be able to predict and control the amount of roughening caused by subsurface plasticity. As a first step, we focus on modelling plastic deformation during contact shearing of an FCC metallic single crystal, employing a finite strain Discrete Dislocation Plasticity (DDP) formulation. This formulation allows us to capture the finite lattice rotations induced in the material by shearing and the corresponding local rotation of the crystallographic slip planes. The simulations predict a pronounced material pile-up in front of the contact and a sink-in at its rear, which are strongly crystal-orientation dependent. By comparing finite and small strain DDP, we can assess the effect of slip plane rotation on surface roughening and on metal plasticity in general. Results of the simulations are also compared with crystal plasticity, which is also capable of predicting a pile-up and sink-in, but not the crystal-orientation dependency of roughening.Accepted Author Manuscript(OLD) MSE-
Local forces on a vertical cylinder in regular and irregular waves
No full textThis paper presents results from an experimental investigation on the loading on a rigid slender vertical circular cylinder in undirectional regular and irregular waves. The ambient flow was measured directly, so that the derived results would not be subject to additional uncertainties associated with the use of wave theories. Morison drag, inertia and lift coefficients are computed from forces measured at one force sleeve and compared with data obtained at the same Reynolds and Keulegan Carpenter numbers, but under uniform flow conditions in a U-tube. In regular waves, the results show the importance of the non-uniformity of wave-induced flow, and in irregular waves it is clear that the loading is much influenced by the history of the flow.NRC publication: Ye
OMAE 2004-51315 MODEL TESTING FOR VORTEX INDUCED MOTIONS OF SPAR PLATFORMS
No full textABSTRACT The state-of-the art in model testing for Vortex Induced Vibrations (VIV) of Spars is presented. Important issues related to Spar VIV model testing are highlighted. The parameters that need to be modeled including hull geometry, strake configuration, mass and mooring properties and, considerations of test set-up and instrumentation are discussed. Results are presented from model tests of an as-built Spar deployed in the Gulf of Mexico. It is shown that the model test results compare well with the VIV responses measured in the field
Plastic contact of self-affine surfaces: Persson's theory versus discrete dislocation plasticity
No full textPersson's theory allows for a fast and effective estimate of contact area and contact stress distributions when a flat and a self-affine rough surface are pressed into contact. For elastic bodies, the results of the theory have been shown to be in very good agreement with rather costly simulations. The theory has also been extended to plastic bodies. In this work, the results of Persson's theory for plastic bodies are compared with those of discrete dislocation plasticity. The area–load curves obtained by theory and simulations are found to be in good agreement when the rough surface has a very small root-mean-square (rms) height. For larger rms heights, which are more realistic for metal surfaces, the agreement is no longer good unless in the theory, instead of a size-independent material strength, one uses a rms height- and resolution-dependent yield strength. A modification of this type, i.e., the use of a yield strength dependent on size, does however not lead to agreement between the probability distributions of the contact stress, which is much broader in the simulations than in the theory. The most likely reason for this discrepancy is that the theory, apart from neglecting plasticity size dependence, only applies to elastic-perfectly plastic bodies and therefore, neglects strain hardening.</p
Modeling adhesive contacts under mixed-mode loading
No full textExperiments show that when an adhesive contact is subjected to a tangential load the contact area reduces, symmetrically or asymmetrically, depending on whether the contact is under tension or compression. What happens after the onset of sliding is more difficult to be assessed because conducting experiments is rather complicated, especially under tensile loading. Here, we provide through numerical simulations, a complete picture of how the contact area and tractions of an adhesive circular smooth punch evolve under mixed-mode loading, before and after sliding. First, the Green's function molecular dynamics method is extended to include the description of the interfacial interactions between contacting bodies by means of traction–separation constitutive laws that enforce coupling between tension (or compression) and shear. Next, simulations are performed to model sliding of a circular smooth punch against a flat rigid substrate, under tension and compression. In line with the experimental observations, the reduction in the contact area during shear loading is found to be symmetric under tension and asymmetric under compression. In addition, under tensile loading, full detachment is observed at the onset of sliding with a non-zero value of the tangential force. After the onset of sliding and the occurrence of slip instability, the contact area abruptly increases (reattachment), under both tension and compression. For interfaces with high friction, the reattachment occurs only partially. However, a full reattachment is attainable when friction is low.(OLD) MSE-
Some aspects of marine riser analysis
No full textThe problem of the static and dynamic analysis of marine risers is considered in this thesis.
The equations of motion for a pipe conveying fluid are derived using a variational approach. The nonlinear expressions for the bending and torsional curvatures are obtained for a beam undergoing finite rotations. The elastic strain energy is expressed in terms of these curvatures and Hamilton's principle used to derive the equations of motion valid for large deformations. The effect of internal flow is included in the variational formulation in a consistent manner.
A finite element formulation is developed for the static analysis of compliant risers. Large deformations are handled by using a convected coordinate system fixed to the elements. The nonlinear problem is solved using a combined incremental-iterative approach based on an instantaneous linearization of the Taylor series expansion of the forces about the displaced configuration. The numerical results presented here show that the large displacement formulation can predict the static deflected shape of different types of compliant riser systems.
The dynamic response of a typical marine riser system to excitations of the ocean waves, current and surface vessel motion is also studied. The hydrodynamic loading is represented by a general form of the Morison equation. The nonlinear drag force in the Morison equation is linearized using the method of equivalent linearization and a solution procedure in the frequency domain developed. Alternatively, the complete nonlinearity of the hydrodynamic loading is retained and the equations of motion are integrated in the time domain. The effect of this linearization on riser response predictions is evaluated. The method of equivalent linearization involves less computational effort and it approximates the response reasonably well. The effect of internal flow on the response of a typical production riser configuration is also studied. It is concluded that internal flow has a negligible effect on the dynamic response.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat
Finite element analysis of viscous flow and rigid body interaction
No full textA finite element method is developed to analyse the interaction of a two-dimensional viscous fluid and an elastically supported rigid body. The stream function form of the Navier-Stokes equations is discretized using 18 degree of freedom C¹ triangular elements. For each fluid element in contact with the body consistent surface traction vectors are derived in terms of the nodal variables to represent the force coupling terms in the equations of motion of the body. Thus a system of nonlinear differential equations is obtained for the coupled fluid-mass system. For the first order solution of the nonlinear problem by the perturbation method, the equations are linearized and the linear problem is solved. In the present study results for the linear problem only are presented, which are valid for "zero" Reynolds number or very slow flow. The problem of a square shaped mass which is excited by a harmonic force in otherwise still fluid and supported by an elastic spring in the direction of its motion is studied. A simple dimensional analysis shows that the response is a function of three basic non-dimensional parameters, namely, (a) the effective viscosity, (b) the ratio of the forcing frequency to the natural frequency of the spring-mass system in vacua, and, (c) the mass to fluid density ratio. A parametric study of the response in terms of these basic non-dimensional parameters is carried out. The influence of the fluid on the response of the mass, represented as added mass and effective damping, is also studied as a function of the frequency Reynolds number. The effect of the fluid, in the form of added mass and damping, is clearly evident in the shift of the resonance peaks of the amplitude response curves of the mass. In the absence of known results no quantitative estimate of the accuracy of the method could be made. But the values of the added mass obtained at high frequencies came to within ten per cent of the predictions by inviscid flow theory. This indicates good accuracy at least in this limiting case.Applied Science, Faculty ofCivil Engineering, Department ofGraduat
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