Free Surface Effects Research Articles

An efficient method for predicting train-induced vibrations from a tunnel in a poroelastic half-space

This paper presents an efficient method for the prediction of vibrations induced by underground railways in a poroelastic half-space. The proposed method accounts for both the saturated porous characteristic of the soil and the free surface effect. An analytical tunnel model, which is coupled with a train-track system, is firstly developed to calculate the dynamic response of the tunnel–soil interface in a poroelastic full-space. By assuming that the near field response of the tunnel is not affected by the existence of the free surface, vibrations of the poroelastic half-space is then calculated by the two-and-a-half-dimensional (2.5-D) boundary integral equation for saturated porous media along with the Green's function for a poroelastic half-space. Soil vibrations generated by the quasi-static and dynamic train load are presented. It is found that an increase of the soil permeability leads to a decrease of the soil displacement. A saturated soil model may be more suitable for calculating the train-induced vibration in water-rich region. Isolation effectiveness of a float slab is also investigated. The simulation results show that floating the track slab can moderately induces the ground vibration, but also causes more transmission of vibration under certain conditions.

A case study on tandem configured oscillating foils in shallow water

Previous research on the oscillating-foil turbine system has demonstrated its great potential for energy extraction. However, not much is known about the interaction of this device with its working environment. To determine the performance and environmental impact of an oscillating-foil turbine in shallow water, a case study have been conducted which was made of the dual oscillating energy extraction foils system with a tandem configuration which operates at two different water depths: i.e., D = 5c and D = 10c. The performance and the environmental effects of the device were compared between shallow-water and deep-water cases. The results show a 10% efficiency loss in the D = 5c case compared with that of the deep water case, because of the interaction between the oscillating-foils and the seabed. It is also observed that the foil vortices dissipation rate of the D = 5c case is 13% less than that of the deep-water case due to the free-surface effect. The water level also rises 23% around the oscillating-foils location of the D = 5c case because of the blockage effect of the device.

Penetration of Rigid Projectile into Concrete Target with Effect of Attack Angle: Theory and Experiment

The existing prediction methods of penetration depth do not take the influence of attack angle or angular velocity of projectile (i.e. effect of attack angle) into account. The main purpose of this work is to develop an approximate approach to predict the penetration depth for a rigid projectile into concrete target covered with yaw-inducing overlays. A quasi-static, elastic–plastic contact model of projectile impacting on irregular barrier is built based on Hertz’s contact theory, and the expression of effect of attack angle are also presented. The stress on projectile–target interface is derived according to spherical cavity expansion model and mechanical property of target, where reduction coefficient is introduced to consider the effect of free surface, and the penetration resistances are further obtained. Differential equations of motion for projectile arbitrarily oblique into target covered with yaw-inducing overlays are given by employing the initial conditions of projectile striking on irregular barrier. Then a simplified formula of depth-of-penetration (DOP) with effect of attack angle is presented by introducing an influence coefficient of oblique impact and an influence coefficient of yaw-inducing overlays into the depth of normal penetration. Predictions from the simplified formula are validated by comparison with ballistic experimental data for a 57[Formula: see text]mm diameter, 4.44[Formula: see text]kg, and ogive-nose, semi-armor-piercing with striking velocities between 320 and 705[Formula: see text]m/s.

Investigation of thickness effects on the bending singularities of a notched plate

AbstractIn this paper, the bending singularities of a notched plate are investigated through the finite element method. The computed singular parameters, including the singularity orders and associated corner functions, are compared with those of classical plate theory and first‐order shear deformation plate theory (FSDPT); the comparisons demonstrate the inaccuracy of the boundary conditions in classical plate theory. The thickness effects are then investigated through the finite element method. The results show that although the singularity parameters computed by finite element agree with those of the FSDPT in the interior of the plate, the FSDPT is not applicable at the notch tip on the top or bottom surface because of substantial free surface effects. For a very thin plate, the inconsistence in the comparison slightly increases because of the free surface effects.

A combined boundary-finite element procedure for dynamic analysis of plates with fluid and foundation interaction considering free surface effect

This study combines a mixed finite element formulation and a boundary element procedure for the free vibration analysis of Mindlin plates resting on Pasternak foundation and interacting with a quiescent fluid with a free surface on the other side. The plate-foundation system is represented by a two field mixed finite element formulation, based on the Hellinger-Reissner variational principle, and the fluid-structure interaction is incorporated into the analysis through a boundary element solution. The proposed formulation provides the added mass matrix in terms of the plate deflection, which is appended into the plate equation of motion. The method is applied to the free vibration problem of circular and elliptical bottom plates of rigid fluid storage tanks supported by elastic foundation. Effects of system parameters, such as thickness to width and fluid depth ratios, foundation parameters and plate ellipticity, are extensively studied.

Study on the free edge effect on finite line contact elastohydrodynamic lubrication

Abstract This paper incorporated a recently developed algorithm, which directly gives the elastic deformation of loaded quarter-space (QS) bodies, into a finite line contact elastohydrodynamic lubrication (EHL) analysis. Half-space (HS) and QS models were adopted in the analysis and the results were compared with the data obtained from conventional optical EHL tests of steel rollers on a glass disc. This study found that contact deformation was dominated by glass disc deformation. Thus, the use of a steel roller on a glass disc in conventional optical EHL technique is not appropriate for studying the free edge surface effect. Free edge surfaces reduce the magnitude of pressure spikes near the ends of contact. Rollers with different end profiles were also evaluated.

Cavity nucleation and growth in dual beam irradiated 316L industrial austenitic stainless steel

Thin foils of 316L were simultaneously ion irradiated and He implanted in situ in a Transmission Electron Microscope at elevated temperatures. The resulting microstructure is carefully investigated in comparison with previous single ion irradiation experiments with a focus on the nucleation and growth of cavities. Helium is found to strongly enhance the nucleation of cavities in dual beam experiments. On the contrary, it does not induce more nucleation when implanted consecutively to an in situ ion irradiation but rather the growth of cavities by absorption at existing cavities, which shows the importance of synergistic effects and He injection mode on the microstructural changes. In both dual beam and single beam experiments, the characteristics of the populations of cavities, either stabilized by He or O atoms, are in qualitative agreement with the predictions of rate theory models for cavity growth. The evolutions of cavity population as a function of irradiation conditions can be reasonably well explained by the concept of relative sink strength of cavities and dislocations and the resulting partitioning of defects at sinks, or conversely recombination when either of the sinks dominates. The dislocations whose presence is a prerequisite to cavity growth in rate theory models are not observed in all studied conditions. In this case, the net influx of vacancies to cavities necessary to their growth and conversion to voids is believed to result from free surface effects, and possibly also segregation of elements close to the cavity surface. In any studied condition, the measured swelling is low, which is ascribed to the dilution of gaseous atoms among a high density of cavities as well as a high rate of point defect recombination and loss at traps. This high rate of recombination enhanced when dislocations are absent appears to result in the formation of overpressurized He bubbles.

Austenite reversion kinetics and stability during tempering of a Ti-stabilized supermartensitic stainless steel: Correlative in situ synchrotron x-ray diffraction and dilatometry

Correlative physical simulation, synchrotron x-ray diffraction and laser dilatometry were used to characterize the surface and volumetric austenite reversion kinetics and stability in a Ti-stabilized supermartensitic stainless steel. A fast heating rate of 500 °C s−1 was used to minimize any martensite to austenite reversion related to the heating stage. This allowed the characterization of the austenite reversion kinetics and its corresponding thermal stability on cooling for tempering temperatures between 600 and 700 °C. In all cases, a soaking time of 9000 s and a cooling rate of 5 °C s−1 were used. The isothermal transformation was divided in two regimes: At and above 625 °C, the kinetics of the transformation was faster and the austenite equilibrium volume fraction was reached. Below 625 °C, the transformation was slower and incomplete. The reverted austenite was stable during cooling after tempering at and below 610 °C, partially stable for temperatures between 625 and 650 °C, and unstable for temperatures between 670 and 700 °C. The austenite Ni content should be higher than 8 wt % in order to effectively stabilize austenite at room temperature. Correlated bulk (dilatometry) and surface (diffraction) analyses showed very good agreement during the isothermal stage. However, martensitic transformation at the sample surface was evidenced at higher temperatures related to the bulk due to the free surface effect. A reversion TTT diagram and the austenite stability curve were constructed from the in situ x-ray diffraction data, providing tools for microstructural and performance optimization of this material.

Cloud Cavitating Flow That Surrounds a Vertical Hydrofoil Near the Free Surface

Unstable cavitation presents an important speed barrier for underwater vehicles such as hydrofoil craft. In this paper, the authors concern about the physical problem about the cloud cavitating flow that surrounds an underwater-launched hydrofoil near the free surface at relatively high-Froude number, which has not been discussed in the previous research. A water tank experiment and computational fluid dynamics (CFD) simulation are conducted in this paper. The results agree well with each other. The cavity evolution process in the experiment involves three stages, namely, cavity growth, shedding, and collapse. Numerical methods adopt large eddy simulation (LES) with Cartesian cut-cell mesh. Given that the speed of the model changes during the experiment, this paper examines cases with varying constant speeds. The free surface effects on the cavity, re-entry jet location, and vortex structures are analyzed based on the numerical results.

Hydrodynamic simulation of an oscillating hydrofoil near free surface in critical unsteady parameter

In this research study, unsteady, viscous and turbulent fluid flow around a plunging hydrofoil is simulated near the water free surface for different submergence depths and oscillation frequencies. The Navier-Stokes (N-S) equations are discretized by employing the finite volume method (FVM) and are solved by using the Pimple algorithm. The volume of fluid (VOF) technique is applied to capture the free surface. To validate the present simulations, a part of the results is compared with experimental results. The aim of the current study is to clarify the physics of flow in the critical unsteady parameter range. Furthermore, the effects of free surface and surface waves on hydrofoil drag are analyzed. Moreover, instantaneous drag behavior, the shape of the surface waves and the effect of the free surface on trailing edge vortices (TEV) are investigated at subcritical, supercritical and especially near critical Strouhal number. The results demonstrate that the main reason behind the sudden drag increment in the critical unsteady parameter is the generated powerful waves in this region which transfer more momentum and finally lead to drag increment. In addition, the free surface affects TEV and causes drag increment at all frequencies at the submergence depth of 0.5c and critical frequency of other depths. However, the free surface does not have any effects on the vortices in other cases.

Influence of flooding compartments on the stability safety of the training warship

Abstract The paper presents computational stability issues with a flooded ship compartment situated on boat deck. As the main criteria for stability of the warship, calculation algorithm of metacentric height and righting levers have been defined, with a free surface effect taken into account. Based on the results of the calculations presented in tabular and graphical forms, suitable conclusions have been drawn up.

Prediction of Far-Field Sound Pressure of a Semisubmerged Cylindrical Shell With Low-Frequency Excitation

A sound–structure interaction model is established to study the vibroacoustic characteristics of a semisubmerged cylindrical shell using the wave propagation approach (WPA). The fluid free surface effect is taken into account by satisfying the sound pressure release condition. Then, the far-field sound pressure is predicted with shell's vibration response using the stationary phase method. Modal coupling effect arises due to the presence of the fluid free surface. New approaches are proposed to handle this problem, i.e., diagonal coupling acoustic radiation model (DCARM) and column coupling acoustic radiation model (CCARM). New approaches are proved to be able to deal with the modal coupling problem efficiently with a good accuracy at a significantly reduced computational cost. Numerical results also indicate that the sound radiation characteristics of a semisubmerged cylindrical shell are quite different from those from the shell fully submerged in fluid. But the far-field sound pressure of a semisubmerged shell fluctuates around that from the shell ideally submerged in fluid. These new approaches can also be used to study the vibroacoustic problems of cylindrical shells partially coupled with fluid.

Compressed Bi-crystal micropillars showing a sigmoidal deformation state – A computational study

It is the aim of this paper to show the mechanisms behind the experimental observations of rather smooth sigmoidal deformations in bi-crystal micropillar tests (in contrast to single crystal micro-compression tests) and to point out that the appearance of such deformation modes are a further reason for being careful when interpreting the force-axial displacement behavior in terms of stress-strain curves.Instabilities, i.e., buckling and subsequent post-buckling deformations, inhomogeneous strain fields and substantial deformations of the base as well as pronounced free surface effects are considered. The influences of imperfections and of friction as well as a possible clearance in the guidance of the loading device are taken into account, too. From these studies, the experimenter may get information how and with which limitations material parameters can be obtained from such compression tests in combination with simulations.

Direct measurement of critical resolved shear stress of prismatic and basal slip in polycrystalline Ti using high energy X-ray diffraction microscopy

Knowledge of the critical resolved shear stress (CRSS) values of different slip modes is important for accurately modeling plastic deformation of hexagonal materials. Here, we demonstrate that CRSS can be directly measured with an in-situ high energy X-ray diffraction microscopy (HEDM) experiment. A commercially pure Ti tensile specimen was deformed up to 2.6% strain. In-situ far-field HEDM experiments were carried out to track the evolution of crystallographic orientations, centers of masses, and stress states of 1153 grains in a material volume of 1.1 mm × 1 mm × 1 mm. Predominant prismatic slip was identified in 18 grains, where the orientation change occurred primarily by rotation around the c-axis during specimen deformation. By analyzing the resolved shear stress on individual slip systems, the estimated CRSS for prismatic slip is 96 ± 18 MPa. Predominant basal slip was identified in 22 other grains, where the orientation change occurred primarily by tilting the c-axis about an axis in the basal plane. The estimated CRSS for basal slip is 127 ± 33 MPa. The ratio of CRSSbasal/CRSSprismatic is in the range of 1.7–2.1. From indirect assessment, the CRSS for pyramidal 〈c+a〉 slip is likely greater than 240 MPa. Grain size and free surface effects on the CRSS value in different grains are also examined.

Fast algorithm for simulation of normal and oblique penetration into limestone targets

A fast algorithm is proposed to predict penetration trajectory in simulation of normal and oblique penetration of a rigid steel projectile into a limestone target. The algorithm is designed based on the idea of isolation between the projectile and the target. Corresponding factors of influence are considered, including analytical load model, cratering effect, free surface effect, and separation-reattachment phenomenon. Besides, a method of cavity ring is used to study the process of cavity expansion. Further, description of the projectile’s three-dimensional gesture is presented. As a result, the algorithm is coded for fast calculation, named PENE3D. A series of cases with selected normal and oblique penetrations are simulated by the algorithm. The predictions agree with the results of tests, showing that the proposed algorithm is fast and effective in simulation of the penetration process and prediction of the penetration trajectory.

Role of free surface on gas‐induced liquid mixing in a shallow vessel

The present work is carried out to understand the effect of free surface on liquid velocity distribution, dynamics and liquid phase mixing in a shallow basic oxygen furnace (BOF). Three‐dimensional/transient Euler–Lagrange (EL) without/with volume‐of‐fluid (VOF) simulations of dispersed gas–liquid flow in a scaled‐down model of the BOF were performed. For lower H/D ratios, EL simulations performed with no‐slip and free‐slip boundary conditions led to oscillatory plume behavior and higher liquid velocity regions which in turn led to lower mixing time. In contrast, EL + VOF simulations led to reduced meandering motion of bubble plumes and lower liquid velocities resulting in higher mixing times. Interestingly, the mixing time predicted using EL + VOF approach was found to be in a good agreement with the measurements. The results presented in this work show that free surface has a significant effect on dynamics of gas–liquid flow and liquid phase mixing for shallow vessels with H/D ≤ 0.5. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3582–3598, 2017

Multiscale modeling of free-surface effect on crack formation in unidirectional off-axis laminates

A multiscale approach based on the mesh superposition method is applied to unidirectional CFRP laminates to evaluate the influence of the deformation field near the free-surface region on crack formation. Our approach employs two different scale analyses: local analysis utilizing a model composed of carbon fibers with micron-scale diameter and matrix resin, and global analysis employing a homogenized model assumed to be an anisotropic elasto-plastic body. Global analysis is conducted to evaluate the macroscopic deformation behavior of laminates. The local model is superimposed on the global model, maintaining the continuity of the displacement field between global and local domains. Local analysis is then performed to predict crack initiation and crack propagation, using the displacement field obtained from the global analysis. Our simulated results indicate that the initial crack occurring on the free-surface region does not affect the final failure strain.

Irreversible Adsorption Erases the Free Surface Effect on the Tg of Supported Films of Poly(4-tert-butylstyrene).

When cooled at constant rate, a 25 nm thin film of poly(4-tert-butylstyrene) vitrifies 50 K lower than in bulk. This record sets the largest depression in thermal glass transition temperature (Tg) ever observed upon confinement at the nanoscale level. Same as for other supported polymer layers, this reduction in Tg has been attributed to the presence of a free surface, the ensemble of molecules at the interface with air remaining in the liquid state also at temperatures well below bulk Tg. Here, we verify that such tremendous shifts can be erased upon prolonged annealing in the liquid state, hinting at a metastable nature of confinement effects. We demonstrate that the recovery of bulk behavior and the manifestation of the free surface are enslaved to the kinetics of irreversible adsorption of chains on the supporting substrate.

Hydroelastic analysis of a semi-submerged propeller using simultaneous solution of Reynolds-averaged Navier–Stokes equations and linear elasticity equations

Numerical analysis of fluid–structure interaction in semi-submerged propellers is considered as one of the complex problems in multiphase flow simulation. This study utilizes a coupled numerical simulation for Reynolds-averaged Navier–Stokes equations and elasticity equations in order to analyze the hydroelastics of a semi-submerged propeller. Also, the free surface effects and existence of cavitation by which the problem may be simulated in a more realistic situation are considered. The governing equations of the fluid and structural parts are discretized based on finite volume and finite element methods, respectively, and solved using the ANSYS software. Validity of the numerical simulation is assessed by comparing its results against available results of semi-empirical equations. This comparison displays good agreement. Hydrodynamic and hydroelastic analyses are performed for a wide range of advanced and submerged ratios. It is clarified that frequency, which is five times greater than that of the shaft speed, is a dominant factor in the oscillations. It is also observed that the existence of cavity patterns and ventilation have a major role in variations in fluctuation amplitudes. Moreover, it is concluded that larger advanced ratio leads to an increase in the displayed oscillations. Based on the current simulation, it is detected that the determined failure point is located at an expected place. It is also observed that distribution of von Mises stress significantly depends on water surface profile. Finally, it is concluded that the Cavitation number has a direct relation with the stress.

A rigid projectile perforation model for metallic targets with the free-surface and fracture effects

The metallic targets are regarded as the compressible power-law strain-hardening materials, and the resistance function coefficients for three kinds of steel and 11 kinds of aluminum targets are obtained based on the spherical cavity-expansion theory. Then, an explicit expression for projectile impact resistance is proposed by curve-fitting, which is simple to use without performing the complex numerical solution. For the projectile perforating on the finite thickness plate, the free-surface effect is realized by multiplying a decay function with the above explicit resistance. Furthermore, a new phenomenological perforation model considering the projectile entry and exit phases, compressibility, rear free-surface and fracture effects of target is established, which is sufficiently verified by comparing with the existing 19 sets of projectile perforation tests and two typical theoretical models. Finally, the influence of the impact velocity on the rear free-surface effect is discussed.