Vicinity Of Free Surface Research Articles

An isogeometric FE-BE method to investigate fluid–structure interaction effects for an elastic cylindrical shell vibrating near a free surface

In this study, an isogeometric finite element-boundary element (FE-BE) framework is proposed to investigate frequency-dependent hydrodynamic characteristics of elastic structures vibrating at free surface or in the vicinity of free surface. The overall numerical framework consists of two parts. In the first part, in vacuo dynamic characteristics are obtained by an isogeometric finite element method (IGAFEM) formulation. Then, in the second part, frequency-dependent generalized added mass and generalized hydrodynamic damping coefficients are obtained by an isogeometric boundary element method (IGABEM) formulation, assuming that the surrounding fluid is ideal, i.e., inviscid, incompressible and its motion is irrotational. To include free surface effects, the free surface boundary conditions are satisfied with an appropriate Green’s function in the mathematical formulation. To show the applicability of the presented framework, an elastic cylindrical shell which was studied previously in the literature is investigated considering that the structure is partially or fully submerged into fluid of infinite depth. It is shown that the results obtained in this study compare very well with the results available in literature.

Numerical study of high-lift hydrofoil near free surface at moderate Froude number

Shallow hydrofoils are known to produce lower lift in normal operating conditions in comparison with deep hydrofoils. However, the maximum lift capability of shallow hydrofoils at moderate speeds, which is important for transitional regimes of hydrofoil boats, is studied insufficiently. In this work, two-dimensional flow around a high-lift hydrofoil at a moderate Froude number is numerically simulated in a broad range attack angles up to the stall occurrence in both single-phase fluid and in the vicinity of free surface. It is found that nearly the same maximum lift coefficient can be produced by the shallow foil in the modeled condition as by the deep foil, but much higher attack angle is required near the free surface, which also results in larger drag. Additionally, it is shown that higher Reynolds numbers lead to higher lift coefficients, especially at large attack angles.

A study of Txx-stress on mixed mode I-II semi-elliptical surface crack in plates

A new finite element modeling approach is proposed in this study to analyze Txx (in-plane T-stress) on mixed mode I-II semi-elliptical surface crack in plates. Txx-stress is calculated by varying inclined angle (β), aspect ratio (a/c), relative crack depth (a/t) and the eccentric angle of ellipse (φ). As is found in this study, it is accurate and efficient to simulate mixed mode I–II crack with this new approach. As the inclined angle (β) increases, Txx-stress decreases; as a/t increases, Txx-stress varies widely in accordance with different angle of φ. Based on the stress decomposition of an arbitrary point along the crack front, it can be concluded that Txx-stress is related to sin2β, and the predicted formulas of Txx-stress match the finite element results well, which proves that the deduction of Txx-stress on surface crack in this present work is reasonable. It is also found discrepancy between Txx-stress of through crack and surface crack is great at different points along the crack front. Only in the vicinity of free surface, Txx-stress with different inclined angles is similar to that of through crack.

Simulation on the interaction between multiple bubbles and free surface with viscous effects

Based on the boundary layer theory, a general Bernoulli equation involving normal and tangential stresses has been derived and the weak viscous effects have been considered. Three-dimensional boundary element method and Green's function have been adopted to solve the interaction between bubbles and free surface. Numerical results have been validated by convergence study and comparison with published results. On this basis, two in-phase and out-phase bubbles in the vicinity of free surface are chosen as cases at different Froude number. Influence of fluid viscosity or Reynolds number is mainly investigated. Physical relevance of numerical computation and the range of validity of numerical simulation are further discussed. It is found that viscous effects depress the interaction between bubbles and free surface, which hinders the formation of the downward jet on the upper bubble surface and declines bubble volume and the height of free-surface spike.

A position-aware linear solid constitutive model for peridynamics

A position-aware linear solid (PALS) peridynamic constitutive model is proposed for isotropic elastic solids. The PALS model addresses problems that arise, in ordinary peridynamic material models such as the linear peridynamic solid (LPS), due to incomplete neighborhoods near the surface of a body. We improved model behavior in the vicinity of free surfaces through the application of two influence functions that correspond, respectively, to the volumetric and deviatoric parts of the deformation. Furthermore, the model is position-aware in that the influence functions vary over the body and reflect the proximity of each material point to free surfaces. Demonstration calculations on simple benchmark problems show a sharp reduction in error relative to the LPS model.

Simulation of mold filling and prediction of gas entrapment on practical high pressure die castings

Element parameters including volume filled ratio, surface dimensionless distance, and surface filled ratio for DFDM (direct finite difference method) were proposed to describe shape and location of free surfaces in casting mold filling processes. A mathematical model of the filling process was proposed specially considering the mass, momentum and heat transfer in the vicinity of free surfaces. Furthermore, a method for gas entrapment was established by tracking flow of entrapped gas. The model and method were applied to practical ADC1 high pressure die castings. The gas entrapment prediction was compared with the fraction and maximum size of porosities in the different casting parts. The comparison shows validity of the proposed model and method. The study indicates that final porosities in high pressure die castings are dependent on both gas entrapment during mold filling process and pressure transfer within solidification period.

Complete flow field computation around an ACV (air-cushion vehicle) using 3D VOF with Lagrangian propagation in computational domain

In this study an algorithm and a 3D solver is developed to solve the flow field around air-cushion vehicles (ACV) in vicinity of free surface. A single set of dimensionless equations is derived to handle both liquid and air phases in viscous 3D incompressible free surface flows in general curvilinear coordinates. The momentum equations are solved using the SIMPLE method in a staggered grid. The Lagrangian approach in the computational domain is also applied in the context of the VOF method to resolve the free surface. To demonstrate the robustness and versatility of the code, an application of this method on the impact problem of a circular cylinder is presented that is compared to experimental, theoretical and other computational results. In a 3D problem, the gravity-fill test is solved in both Cartesian and randomly generated curvilinear grid system. Comparison of results with the available experimental data and other numerical results is presented as well. At last, the code is applied to solve the two and three dimensional air and water flow field around an ACV and the effects of several parameters including the under skirt pressure distribution, initial air gap and effect of Fr. number on the wave elevation and wave making drag are investigated. It is shown that the presented method and the code are, robust and accurate enough to produce complicated 3D flow fields around ACV’s.

Modeling of mold filling of Al gravity casting and validation with X-ray in-situ observation

In the mold filling simulation, element parameters including volume filled ratio, surface dimensionless distance, and surface filled ratio, were proposed to describe the shape and location of free surfaces in DFDM (Direct Finite Difference Method) elements. A model of the filling process was established, specially taking into account the mass, momentum and heat transfer in the vicinity of free surfaces. It was applied to an experimental AC4C (Al–7Si–0.4Mg) gravity casting. With a special X-ray apparatus, in-situ observation and record of actual mold filling process of the casting were carried out. The simulation results were validated and analyzed by comparing with the observation. The liquid flowing in the casting runner and ingate as well as the evolution of free surfaces were analyzed and discussed.

Study on Fabrication of a Large Al-Cu-Mn Wheel with Indirect Squeeze Casting Process

The fabrication of a large Al-4.5wt%Cu-0.5wt%Mn wheel with 670mm diameter by indirect squeeze casting process was studied. The casting system including modified hydraulic press and die structure was introduced, and the casting procedures was designed and described. The filling behavior of the casting process was simulated with software Flow3D. It was found that while flowing from spoke to rim, turbulence flow of liquid melt in the vicinity of free surface was found, and that at the end of the filling, the unwanted solidification might occur. To reduce the turbulent flow and the unwanted solidification, a modified injection condition was proposed. The simulation results with the modified injection indicated that the turbulence was prevented and the unwanted solidification during the filling was reduced as well. The squeeze wheel castings were fabricated with the modified injection condition. Cross sections of the castings were checked. A number of specimens were obtained from different parts of the castings to evaluate microstructure of the wheel, and both high density and fine grain microstructures were found in the specimens. Mechanical properties of the tensile samples from the wheels with T5 heat treatment were measured. The average tensile strength and elongation were 390 MPa and 10%, respectively.

Modeling of Mold Filling Process of Al Casting and Validation with X-Ray In-Situ Observation

To describe the shape and location of free surfaces in the mold filling simulation, parameters including volume filled ratio, surface dimensionless distance, and surface filled ratio for the DFDM (Direct Finite Difference Method) elements were proposed. A model of mold filling process was established, considering the mass, momentum and heat transfer in the vicinity of free surface. It had been applied to an experimental Al casting. In-situ observation and record of actual mold filling process of the casting with special X-ray apparatus were carried out, and were used to validate the simulation results. The validation showed that the model could predict the mold filling process well. It was found that back pressure of gas in the mould cavity had minor effect on change of liquid flow when using pressurized feeding system and sand mould with good air permeability.

내화용 고강도 습식 스프레이 폴리머 모르타르의 화재 저항성 평가

초록 · 키워드 목차 오류제보하기 등록된 정보가 없습니다. ABSTRACT1. 서론2. 실험계획 및 방법3. 실험결과4. 결론참고문헌요약

Modelling Surface Properties of Linear Amorphous Polymers

ABSTRACTMolecular dynamics simulations have been carried out on fully atomistic models of amorphous polymers. Both bulk structures and thin films were simulated for a wide range of thermoplastics such as PS, PMMA, poly (phenylene-ether) (PPE), poly (etherimide) (PEI). The influence of molecular weight and functional end-groups was investigated. The comparison between “bulk” and “surface” models clearly shows the increase of molecular mobility and a strong decrease in density at the vicinity of free surfaces. Simulated surface tensions, computed by different methods, are generally overestimated as compared to the experimental ones.PVT (Pressure-Volume-Temperature) measurements have been conducted, as another method to compute physical properties of amorphous polymers. From such measurements, cohesive energy densities, interaction parameters, and then interfacial properties, were obtained for various thermoplastics and the corresponding incompatible melts. Such a tool could be used for designing polymer surfaces and interfaces in polymer blends.

A lattice resolution study of the martensitic transformation of small iron particles in a copper matrix

The martensitic transformation of small, fully coherent, gamma iron precipitates in an epsilon copper matrix, brought about by the bombardment of thin foils with argon ions, has been studied using lattice fringe imaging. It is found, in general, that only partial transformation of individual particles occurs, this corresponding to a simple homogeneous shear (a Bain Strain). In addition, the transformation is limited to particles located within about two particle diameters of the foil surface. It is established that the mere presence of a Frank (or Shockley) loop in a particle, due to the bombardment, is insufficient to trigger the transformation. The implications of this result are discussed with reference to bulk martensite. In the present case, it is concluded that transformation may simply initiate at sites of local decohesion or microslip at the matrix/particle boundaries brought about by vacancy condensation following ion bombardment, these sites being most effective in the vicinity of free surfaces.

Diffusion size effect

On the basis of the fact that the mean square displacement of atoms in the vicinity of the surface is higher than that in the bulk, a thin film diffusion size effect is introduced. In films of the order of 100 Å thick it leads to a marked increase in the diffusivity as the film thickness decreases. The size effect introduced is used to explain experimental data on the high diffusivity in thin films. The approach developed should also be taken into account when consideration is given to other thermally activated processes in thin films as well as in the vicinity of free surfaces, interfaces and grain boundaries (segregation, vacancy distribution and migration to the surface, phase formation etc.).

Critical behavior of semi-infinite magnets (invited)

The local critical behavior of magnets is modified in the vicinity of free surfaces. These surface effects can be studied by using semi-infinite model systems. Recent theoretical work on the semi-infinite n-vector model is reviewed. When possible the theoretical predictions are compared with experimental results. The general new features which show up in the field-theoretical renormalization-group approach are explained. Systems with defect planes and n-vector films are also briefly discussed.

EXPERIMENTAL INVESTIGATION ON NONLINEAR REFLECTION OF SHORT IMPULSIVE UNDERWATER SOUND AT WATER SURFACE

The nonlinear phenomena in the vicinity of free surface of water, induced by the incidence of high intensity underwater compressional pulse, are experimentally investigated. The behaviors of cavitation bubbles and water surface are observed by photographical visualization. It is concluded that the transmission and the reflection at the free surface are linear, and the limitation of negative pressure observed in the reflected pulse must be due to the onset of cavitation. Utilizing the phenomena, the cavitation threshold of water for negative pressure pulse of about 10 µs duration is measured. Obtained threshold value of air-saturated tap water is about 10 atm., and after water is degassified to 20% of saturation it becomes about 20atm., at 20°C. The dependence of the threshold on temperature is also investigated.