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Simulation of Stochastic Ice Force Process of Vertical Offshore Structure Based on Spectral Model
Get PDFThe action of drifting ice floes may induce strong vibrations of offshore structures, and further reduce the structural safety and serviceability. The aim of this paper is to develop a method by considering ice crushing as a stochastic process. On the basis of ice force spectrum which considers the time and spatial correlation between the local ice forces, a simulation methodology to generate the stochastic ice forces process of vertical offshore structure is proposed. The crucial segment in the simulation is to accurately calculate the effective ice pressure, and it is accomplished by an empirical formula which can provide a high calculation accuracy. Considering the effect of incidence angle and tangential ice force, global ice force spectrum is established, and the synthesis of the local ice force is realized. The presented simulation methodology is conducted on the Norströmsgr und lighthouse to verify its efficacy. The results show that the simulated maximum global ice forces are close to those measured data, and the frequency contents of the generations coincide well with the target. They directly proves the validity of the proposed simulation method. Compared with the traditional methods, such as field measurement and ice force identification, the present simulation method has advantages in the application range. It can be used to simulate the stochastic global ice force for an arbitrary width offshore structure. In addition, it can also be used for extreme ice force analysis and dynamic response analysis of offshore structures
Neural Network-Based Second Order Reliability Method (NNBSORM) for Laminated Composite Plates in Free Vibration
Get PDFMonte Carlo Simulations (MCS), commonly used for reliability analysis, require a large amount of data points to obtain acceptable accuracy, even if the Subset Simulation with Importance Sampling (SS/IS) methods are used. The Second Order Reliability Method (SORM) has proved to be an excellent rapid tool in the stochastic analysis of laminated composite structures, when compared to the slower MCS techniques. However, SORM requires differentiating the performance function with respect to each of the random variables involved in the simulation. The most suitable approach to do this is to use a symbolic solver, which renders the simulations very slow, although still faster than MCS. Moreover, the inability to obtain the derivative of the performance function with respect to some parameters, such as ply thickness, limits the capabilities of the classical SORM. In this work, a Neural Network-Based Second Order Reliability Method (NNBSORM) is developed to replace the finite element algorithm in the stochastic analysis of laminated composite plates in free vibration. Because of the ability to obtain expressions for the first and second derivatives of the NN system outputs with respect to any of its inputs, such as material properties, ply thicknesses and orientation angles, the need for using a symbolic solver to calculate the derivatives of the performance function no longer exists. The proposed approach is accordingly much faster, and easily allows for the consideration of ply thickness randomness. The present analysis showed that dealing with ply thicknesses as random variables results in 37% increase in the laminate’s probability of failure
Angle of Attack Between Blood Flow and Mitral Valve Leaflets in Hypertrophic Obstructive Cardiomyopathy: An <i>In Vivo</i> Multi-patient CT-based FSI Study
Get PDFThe mechanisms of systolic anterior motion (SAM) of the mitral valve in hypertrophic obstructive cardiomyopathy (HOCM) remain unclear. To investigate the angle of attack between blood flow and mitral valve leaflets at pre-SAM time point, patient-specific CT-based computational models were constructed for 5 patients receiving septal myectomy surgery to obtain pre- and post-operative 2D vector flow mapping. The comparisons between pre- and post-operative angles of attack based on 2D vector flow mapping of 5 patients were performed. It was found that there was no statistically significant difference between pre- and post-operative angles of attack (61.1±t wao vs. 56.2±56.o, p=0.306, n=5). Therefore, we propose that the angle of attack might not play an important role in the initiation of SAM
Computational Fluid Dynamics Analysis of Shroud Design on Hemodynamic Performance and Blood Damage in a Centrifugal Blood Pump
Get PDFPatients with extracorporeal membrane oxygenation still suffer from high rates of complication that linked to the flow field within the blood pump. So it is essential to optimise the geometry of the pump. The specification of shroud design is arguably the necessary design parameter in the centrifugal pump. However, the hemodynamic performances of the different shroud designs have not been studied extensively. In this study, ten different shroud designs were made and divided into two groups as the different covering locations (A: Covering the blade leading edge, B: Covering the blade trailing edge). In every group, six shroud designs with the covering proportions of 0,1/5,2/5,3/5,4/5,1 were made. Detailed computational fluid dynamics (CFD) analyses were performed to investigate their effects on hemodynamics and hydraulic performance at the constant flow condition (4000 rpm, 5 L/min). The percentage volumes of the scalar shear stress in specific threshold (τ9 Pa: the destruction of von Willebrand factor, τ>50 Pa: Platelet activation, τ>150 Pa: Break of red blood) were used to compare the blood damage of the different shroud designs. Also, the modified index of hemolysis (MIH) were calculated based on a Eulerian approach for different pumps. CFD simulations predicted an increase in the pump head, hydraulic efficiency, a fraction of fluid volume with scalar shear stress values above a threshold (9 Pa, 50 Pa, 150 Pa) and MIH with increasing shroud covering proportions from 0 to 1 in the same covering location. Also, these above results were higher in group B than group A. This means that the risks of the hemolysis, thrombosis and bleeding increased as the rise of the covering proportion and they were higher in the pump whose shroud covers the blade trailing edge
Eddy Current Analyses by Domain Decomposition Method Using Double-Double Precision
Get PDFA matrix equation solved in an eddy current analysis, A-ϕ method based on a domain decomposition method becomes a complex symmetric system. In general, iterative method is used as the solver. Convergence of iterative method in an interface problem is improved by increasing an accuracy of a solution of an iterative method of a subdomain problem. However, it is difficult to improve the convergence by using a small convergence criterion in the subdomain problem. Therefore, authors propose a method to introduce double-double precision into the interface problem and the subdomain problem. This proposed method improves the convergence of the interface problem. In this paper, first, we describe proposed method. Second, we confirm validity of the method by using Team Workshop Problem 7, standard model for eddy current analysis. Finally, we show effectiveness of the method from two numerical results
Spline Fictitious Boundary Element Alternating Method for Edge Crack Problems with Mixed Boundary Conditions
Get PDFThe alternating method based on the fundamental solutions of the infinite domain containing a crack, namely Muskhelishvili’s solutions, divides the complex structure with a crack into a simple model without crack which can be solved by traditional numerical methods and an infinite domain with a crack which can be solved by Muskhelishvili’s solutions. However, this alternating method cannot be directly applied to the edge crack problems since partial crack surface of Muskhelishvili’s solutions is located outside the computational domain. In this paper, an improved alternating method, the spline fictitious boundary element alternating method (SFBEAM), based on infinite domain with the combination of spline fictitious boundary element method (SFBEM) and Muskhelishvili’s solutions is proposed to solve the edge crack problems. Since the SFBEM and Muskhelishvili’s solutions are obtained in the framework of infinite domain, no special treatment is needed for solving the problem of edge cracks. Different mixed boundary conditions edge crack problems with varies of computational parameters are given to certify the high precision, efficiency and applicability of the proposed method compared with other alternating methods and extend finite element method
Pivot-Point Beam Steerer
Get PDFA simplified beam-steering mount for a mirror or beam splitter can be adjusted quickly with three degrees of freedom, pivoting about the single point in space where the chief ray of the light beam strikes the reflecting surface, before being locked in place by tightening a single screw. The design goal is demonstrated by multiple superimposed images taken with different positions of the mount
Combining Smaller Patch, RV Remodeling and Tissue Regeneration in Pulmonary Valve Replacement Surgery Design May Lead to Better Post-Surgery RV Cardiac Function for Patients with Tetralogy of Fallot
Get PDFPatients with repaired Tetralogy of Fallot (ToF), a congenital heart defect which includes a ventricular septal defect and severe right ventricular outflow obstruction, account for the majority of cases with late onset right ventricle (RV) failure. The current surgical approach, which includes pulmonary valve replacement/insertion (PVR), has yielded mixed results. A computational parametric study using 7 patient-specific RV/LV models based on cardiac magnetic resonance (CMR) data as "virtual surgery" was performed to investigate the impact of patch size, RV remodeling and tissue regeneration in PVR surgery design on RV cardiac functions. Two patch sizes, three degrees of scar trimming (RV volume shrinkages: 9%, 17%, 25%) and hypothetical use of regenerated myocardium as replacement of patch and scar were considered in these models. Our preliminary results indicate that each of the three techniques (smaller patch, RV remodeling, and myocardium regeneration) had modest improvement on post-PVR RV ejection fraction (from 1.76%-4% over the conventional PVR procedure) and combination of all three techniques had the best performance (a 4.74% improvement in ejection fraction over the conventional PVR, for the patient studied). Changes in RV stress, strain and curvatures were also observed. However, their linkages to RV ejection fraction were less clear. Further investigations are required to confirm our findings
Microtubular Protofilament Analysis Based on Molecular Level Tubulin Interaction
Get PDFNonlinear microstructure of the microtubules (MTs) plays an important role in their mechanical properties. Despite the extensive efforts into the development of continuum models for microtubules, a mesoscale finite element model that can link the molecular level information to the overall performance of microtubules is still missing. The aim of this study is to develop a molecular dynamics model (MDM), finite element model (FEM) and structural mechanics beam model (SMBM) for tubulins of protofilament (PF). In MDM, the backbone atoms of α-tubulin were fixed while the backbone atoms of β-tubulin were attached to a molecular dynamics (MD) atom through a virtual spring. In FEM, both α and β tubulins are modeled as spherical shells and adjacent tubulins are connected by linear springs. The spherical shells were framed as beams in SMBM. Corresponding parameters such as the elasticity of tubulin-tubulin interaction (TTI) and the stiffness of springs and beam are derived from MD simulation. Marginal differences in the force-deflection curve among the FEM, the MDM and SMBM indicate the good accuracy in describing the mechanical properties of microtubules. Simulation results show that the protofilament behaves non-linearly under tension and torsion but linearly under bending. Deformation pattern of a PF from the SMBM frame bending can be well captured by the classical Euler-Bernouli beam theory and the flexural rigidity derived from FEM is in good agreement with SMBM. These findings lend compelling credence in our developed models of PF to deepen our understanding of the underlying mechanism of statics and dynamics of MTs. In perspective our approach provides a tool for the analysis of MTs mechanical behavior under different conditions
Optimization of Casing Design Parameters to Mitigate Casing Failure Caused by Formation Slippage
Get PDFThere has been lack of work efforts on how to optimize cementing and completing parameters in order to prevent casing failure induced by formation slippage in pertroleum industry scope. Once the weak plane fails, the formation will become easily undertaken slippage across a large area along its interface. The plenty of horizontal planes of weakness in reservoir formations, as reported for a number of oilfields, can easily undertaken slippage once it fails. To address the problem, three-dimensional finite element models were established by taking into considerations the elastoplastic mechanical characteristics of both the casing and the near-wellbore rock. Two types of casing impairment scenarios were considered: Casing collapse (that causes tubing stuck in the well) and complete casing shear-off. In this study, the critical slip displacement of casing shear damage under both cemented and un-cemented conditions was calculated, and the critical displacement of casing with various wall thicknesses and steel grades was compared. A new cementing practice for the Daqing oilfield was then proposed by optimizing casing parameters according to API standards, and a new research method was also put forward by proposing new casing materials to effectively mitigate casing failure caused by formation slippage for the future. Modeling results indicate that the stress and deformation associated with casing in the un-cemented condition is more diffused and the critical slippage displacement is larger than that in the cemented condition. Therefore, the un-cemented condition is more effective in preventing casing shear failure and easier for casing repair, for the case of casing damage caused by formation shear slippage. Casing elongation is the key parameter of casing shear failure in the un-cemented condition. Lower grade casing exhibits a larger critical slippage displacement because of its higher elongation capacity under stress. Casing with lower grade and smaller thickness provides more advantages in preventing casing damage in formations abundant with horizontal weak layers. If the elongation of casing can be largely improved, the critical displacement value can be increased by 21.40%. Higher grade and thicker casing is adapted for mitigate casing failure caused by formation slippage