Series Of 3D Numerical Simulations Research Articles

Analysis of the interaction between bolt-reinforced rock and surface support in tunnels based on convergence-confinement method

To investigate the interaction of the bolt-reinforced rock and the surface support, an analytical model of the convergence-confinement type is proposed, considering the sequential installation of the fully grouted rockbolts and the surface support. The rock mass is assumed to be elastic-brittle-plastic material, obeying the linear Mohr-Coulomb criterion or the non-linear Hoek-Brown criterion. According to the strain states of the tunnel wall at bolt and surface support installation and the relative magnitude between the bolt length and the plastic depth during the whole process, six cases are categorized upon solving the problem. Each case is divided into three stages due to the different effects of the active rockbolts and the passive surface support. The fictitious pressure is introduced to quantify the three-dimensional (3D) effect of the tunnel face, and thus, the actual physical location along the tunnel axis of the analytical section can be considered. By using the bolt-rock strain compatibility and the rock-surface support displacement compatibility conditions, the solutions of longitudinal tunnel displacement and the reaction pressure of surface support along the tunnel axis are obtained. The proposed analytical solutions are validated by a series of 3D numerical simulations. Extensive parametric studies are conducted to examine the effect of the typical parameters of rockbolts and surface support on the tunnel displacement and the reaction pressure of the surface support under different rock conditions. The results show that the rockbolts are more effective in controlling the tunnel displacement than the surface support, which should be installed as soon as possible with a suitable length. For tunnels excavated in weak rocks or with restricted displacement control requirements, the surface support should also be installed or closed timely with a certain stiffness. The proposed method provides a convenient alternative approach for the optimization of rockbolts and surface support at the preliminary stage of tunnel design.

Hydrodynamic Loads on a Group of Six Structures of Different Cross-Sections in Uniform and Sheared Flow

The estimates of hydrodynamic forces for a group of structures represent a challenge for the design of offshore systems, as they are subject to changes with a variation in flow profiles. The fluctuating effects may be more pronounced or, on the contrary, suppressed if the cross-sectional shape of structures in an array is altered. The present work performs a series of 2D numerical simulations for the flow past six identical stationary cylinders of three distinct geometrical shapes arranged in a 2 × 3 matrix configuration. The flow profiles considered have an averaged velocity corresponding to the critical flow regime of a Reynolds number of 2.5 × 105. The detached eddy simulation k–ω SST turbulence model is employed to perform a comprehensive investigation of the fluid force coefficients, their frequencies and vortex formation patterns. The effect of the spacing ratio varied simultaneously among the structures from 2 to 7 is considered in conjunction with the change in the flow profile and the cylinders’ cross-section. The results of simulations show a higher mean drag on the upstream cylinders, reduced mean drag on the mid- and downstream cylinders with the second cross-sectional shape, and a higher mean drag on the cylinders with the third cross-sectional shape, compared to the original circular cylinders.

The impact of street level particulate emissions on the energy performance of roof level building ventilation systems

Localised vehicular fine particulate matter (PM2.5) emissions in an urban canyon can influence the energy performance of a building ventilation system at roof level. This paper examines the energy demands of air filtration through an air handling unit (AHU) located in different positions and orientations on a building rooftop. A series of 3D numerical simulations examined the impact of aspiration efficiency (AE) on filter loading rates as the distance from the source increases and AHU orientation relative to ambient wind direction. The ventilation PM2.5 concentration was equal to the ambient when positioned near the windward wall of the target building. A decrease of 33% and 60% in the filter loading rate occurred at a wind speed of 7.5 m/s and 2.5 m/s at the leeward wall. There was no energy savings when the AHU is positioned on the windward side of the target building but a reduction in energy consumption of 9.8% at the centre and 19.4% at the leeward side. Comparing the effect of wind orientation for identical AHU positions on the rooftop centre resulted in a 26–35% reduction in AE when the AHU inlet is not facing into the particle laden wind and incurred energy savings of 25.8%.

Study on the non-linear deformation and failure characteristics of EPS concrete based on CT-scanned structure modelling and cloud computing

To understand the complex deformation features and failure mechanisms of expanded polystyrene (EPS) concrete and reveal the composite effect of expanded polystyrene beads and polypropylene fibers, a series of experiments were conducted on the poured EPS concrete specimens. Meanwhile, a cloud computing system for 3D realistic failure process analysis (RFPA3D) was established to model the fine failure process of a real concrete structure. The micromorphology of the EPS concrete specimens was obtained via CT scanning and further processed using digital image processing technology. The Otsu algorithm was applied to automatically recognize the segmentation thresholds of each partition image and a procedure for CT image processing was designed to automatically realize digital image segmentation and merging. Then, the numerical models reflecting the microstructures of the EPS concrete specimens were built using the processed digital images and a series of 3D numerical simulations were performed using cloud-computing-based RFPA3D. The results show that for concrete with low EPS volume fracture, the non-smooth convex-step-shaped failure morphology, which is a typical brittle fracture characteristic, appears. In contrast, ductile fracture occurs for concrete with a high EPS volume fracture. Simultaneously, the addition of polypropylene fibers of a certain length can effectively prevent the formation and expansion of new cracks in the cement matrix. In addition, the peak strength of concrete increases with an increase in homogeneity while the residual strength generally decreases with an increase in homogeneity. Moreover, a more heterogeneous material presented more acoustic emission precursors before macro fracture. All these achievements greatly improve our knowledge of the design, construction, and maintenance of EPS concrete in civil engineering.

Amplification and generation of turbulence during self-gravitating collapse

Context. The formation of astrophysical structures, such as stars, compact objects, and also galaxies, entail an enhancement of densities by many orders of magnitude, which occurs through gravitational collapse. Aims. The role played by turbulence during this process is important. Turbulence generates density fluctuations, exerts a support against gravity, and possibly delivers angular momentum. How exactly turbulence behaves and is amplified during the collapse remains a matter of investigation and is the aim of the present paper. Methods. We carried out spherical averaging of the fluid equations, leading to 1D fluid equations that describe the evolution of mean quantities in particular the mean radial velocity as well as the mean radial and transverse turbulent velocities. These equations differ from the ones usually employed in the literature. We then performed a series of 3D numerical simulations of collapsing clouds for a wide range of thermal and turbulent supports with two polytropic equations of state, P ∝ ρΓ, with Γ = 1 and 1.25. For each 3D simulation, we performed a series of 1D simulations using the spherically averaged equations and the same initial conditions. Results. By performing a detailed comparison between 3D and 1D simulations, we can analyse the observed behaviours in great detail. Altogether, we find that the two approaches agree remarkably well, demonstrating the validity of the inferred equations; although, when turbulence is initially strong, major deviations from spherical geometry certainly preclude quantitative comparisons. The detailed comparisons lead us to an estimate of the turbulent dissipation parameter, which, when the turbulence is initially low, is found to be in good agreement with previous estimates of non self-gravitating supersonic turbulence. When turbulence is initially dynamically significant, larger values of the dissipation appear necessary for the 1D simulations to match the 3D ones. We find that the behaviour of turbulence depends on the cloud thermal support. If it is high, initial turbulence is amplified, as proposed earlier in the literature. However, if thermal support is low, turbulence is also generated by the development of local non-axisymmetric gravitational instabilities reaching values several times larger and in equipartition with gravitational energy. Conclusions. The inferred 1D equations offer an easy way to estimate the level reached by turbulence during gravitational collapse. Depending on the cloud thermal support, turbulence is either amplified or locally generated.

Multiscale evaluation method of the drag effect on shallow water flow through coastal forests based on 3D numerical simulations

AbstractThis study presents a method for determining the drag parameter in the 2D shallow water (SW) equation for flows through a coastal forest by conducting a series of 3D numerical simulations (3D NSs). Following the theory of multiscale modeling, an evaluation method procedure is proposed. We first prepare a local test domain that contains a sufficient number of trees to constitute part of a coastal forest. Then, 3D NSs are conducted in this test domain with various inflow conditions. Based on the corresponding results, the momentum losses over the test domain are converted into the drag parameter of the global SW equation. A response surface of the drag parameter is constructed as a function of the flow conditions. The stabilized finite element method is employed for both the local and the global NSs, and the phase‐field method is utilized to represent 3D free surfaces. Comparisons between the 2D SW calculation results and the 3D NS results are also performed to verify the validity of the proposed method.

A low-cost solution for the collection of fine particles in square cyclone: A numerical analysis

Generally, the square cyclone separator suffers from low separation efficiency. Therefore, a deeper investigation for developing an effective method to enhance this type of cyclone's overall performance is of interest. Although different methods are available, optimizing the vortex finder's shape for enhancing separation efficiency is a low-cost solution. The present study assesses the effect of vortex finder on the performance of a square cyclone, thus guiding the design for improving the square cyclone separation efficiency. Three different vortex finders namely, cylindrical, convergent, and divergent, are studied. A series of 3D numerical simulations are carried out by solving the Reynolds-Averaged Navier-Stokes equations with the Reynolds Stress Model (RSM) and performing the Eulerian-Lagrangian particle tracking analysis. The results demonstrate that using a convergent vortex finder improves the square cyclone's separation efficiency while producing an increase in pressure drop. Using a convergent vortex finder is beneficial for reducing the 50% cut-size. Among all vortex finders, the convergent one with vortex finder diameter ratio (D2/D1 = 0.75) allows the square cyclone to collect finer particles. As a result, the 50% cut size decreases by approximately 33% at an inlet velocity of 28 m/s.

An asymmetric flow-focusing droplet generator promotes rapid mixing of reagents

Nowadays droplet microfluidics is widely used to perform high throughput assays and for the synthesis of micro- and nanoparticles. These applications usually require packaging several reagents into droplets and their mixing to start a biochemical reaction. For rapid mixing microfluidic devices usually require additional functional elements that make their designs more complex. Here we perform a series of 2D numerical simulations, followed by experimental studies, and introduce a novel asymmetric flow-focusing droplet generator, which enhances mixing during droplet formation due to a 2D or 3D asymmetric vortex, located in the droplet formation area of the microfluidic device. Our results suggest that 2D numerical simulations can be used for qualitative analysis of two-phase flows and droplet generation process in quasi-two-dimensional devices, while the relative simplicity of such simulations allows them to be easily applied to fairly complicated microfluidic geometries. Mixing inside droplets formed in the asymmetric generator occurs up to six times faster than in a conventional symmetric one. The best mixing efficiency is achieved in a specific range of droplet volumes, which can be changed by scaling the geometry of the device. Thus, the droplet generator suggested here can significantly simplify designs of microfluidic devices because it enables both the droplet formation and fast mixing of the reagents within droplets. Moreover, it can be used to precisely estimate reaction kinetics.

A note on the cavitation phenomenon in metallic plates perforated by sharp-nosed rigid projectiles

We explore the perforation process of metallic plates impacted by rigid sharp-nosed projectiles at high velocities. In particular, we are looking at the diameters of the penetration hole in the plates through a series of 2D numerical simulations, in order to check for the occurrence of cavitation in finite-thickness plates. This phenomenon has not been observed by previous workers and we were looking for its effect on the perforation process. Our simulations show that for every projectile/plate pair there is a certain impact velocity which marks the onset of cavitation. These threshold velocities depend on the normalized thickness of the plates, as well as on their effective strength. Our simulations are supported by the results from perforation tests on plates made of a low strength lead-antimony alloy. The main conclusion from our work is that analytical models for plate perforation should take into account the cavitation phenomenon, especially for high velocity impacts.

Jet propagation in expanding medium for gamma-ray bursts

ABSTRACT The binary neutron star (BNS) merger event GW170817 clearly shows that a BNS merger launches a short gamma-ray burst (sGRB) jet. Unlike collapsars, where the ambient medium is static, in BNS mergers the jet propagates through the merger ejecta that is expanding outward at substantial velocities (∼0.2c). Here, we present semi-analytical and analytical models to solve the propagation of GRB jets through their surrounding media. These models improve our previous model by including the jet collimation by the cocoon self-consistently. We also perform a series of 2D numerical simulations of jet propagation in BNS mergers and in collapsars to test our models. Our models are consistent with numerical simulations in every aspect (the jet head radius, the cocoon’s lateral width, the jet opening angle including collimation, the cocoon pressure, and the jet–cocoon morphology). The energy composition of the cocoon is found to be different depending on whether the ambient medium is expanding or not; in the case of BNS merger jets, the cocoon energy is dominated by kinetic energy, while it is dominated by internal energy in collapsars. Our model will be useful for estimating electromagnetic counterparts to gravitational waves.

The effect of yaw on the penetration of rigid rods

We explored the effect of yaw angles on the penetration depths of rigid long rods through a series of 3D numerical simulations. In particular, we looked for normalized relations between the penetration depths and the yaw angles, in order to account for our simulation results through non-dimensional parameters. These numerically-based relations highlight the physics behind the interaction of rigid yawed rods and metallic targets, and they can be used in future attempts to account for the yawed impact by analytical modeling. We also explored the behavior of rods with real constitutive properties, impacting aluminum targets at high velocities and non-negligible yaw angles. Our simulations reproduced unique aspects of this interaction which were found experimentally two decades ago.

Fracture evolution mechanism of hollow sandstone under conventional triaxial compression by X-ray micro-CT observations and three-dimensional numerical simulations

The knowledge of the fracture evolution mechanism of hollow rock under conventional triaxial compression is very important to evaluate the stability and safety in deep tunnel engineering. In this research, a series of X-ray micro-CT observations and three-dimensional (3D) numerical simulations were conducted on hollow sandstone specimens with various borehole diameters (d = 0, 15 and 26 mm) under different confining pressures (0, 8, 16, 24 and 35 MPa). A 3D realistic failure process analysis method (RFPA3D) was established for the simulation of the deformation failure behavior of hollow sandstone under conventional triaxial compression. First, a group of micro-parameters used in RFPA3D, which reflect the macro mechanical behavior of the tested sandstone material, was calibrated using the experimental triaxial compression results obtained for intact specimens. Subsequently, a series of 3D numerical simulations was conducted on hollow sandstone specimens with various borehole diameters under different confining pressures. The quantitative comparison of the axial deviatoric stress-strain curve and peak strength of hollow sandstone obtained from the experiments and simulations indicates that the 3D numerical results agrees very well with the experimental results. The nonlinear Hoek-Brown criterion better reflects the peak strengths of intact and hollow sandstone materials than the linear Mohr-Coulomb criterion. The effect of the borehole diameter on the peak strength of hollow sandstone depends on the confining pressure, which is interpreted by radial and tangent stress distribution around the borehole. In addition, the surface and internal fracture characteristics of the hollow sandstone obtained by X-ray micro CT observations and simulations were compared. Generally, the internal cracks of the intact and hollow sandstone specimens determined using numerical simulations are very similar to those observed using X-ray micro CT. The influence of confining pressure on the fracture evolution mechanism of hollow sandstone with various borehole diameters was analyzed in detail based on the results of numerical simulations. The conclusions drawn in this study are significant for the prediction of the instability around deep well-bores in petroleum engineering and to ensure the safety of deep excavation damage zones in tunnel engineering.

Influence of Pressure Difference Between Reservoir and Production Well on Steam-Chamber Propagation and Reservoir-Production Performance

Summary Steam-assisted gravity drainage (SAGD) is the most-effective thermal recovery method to exploit oil sand. The driving force of gravity is generally acknowledged as the most-significant driving mechanism in the SAGD process. However, an increasing number of field cases have shown that pressure difference might play an important role in the process. The objective of this paper is to simulate the effects of injector/producer-pressure difference on steam-chamber evolution and SAGD production performance. A series of 2D numerical simulations was conducted using the MacKay River and Dover reservoirs in western Canada to investigate the influence of pressure difference on SAGD recovery. Meanwhile, the effects of pressure difference on oil-production rate, stable production time, and steam-chamber development were studied in detail. Moreover, by combining Darcy's law and heat conduction along with a mass balance in the reservoir, a modified mathematical model considering the effects of pressure difference is established to predict the SAGD production performance. Finally, the proposed model is validated by comparing calculated cumulative oil production and oil-production rate with the results from numerical and experimental simulations. The results indicate that the oil production first increases rapidly and then slows down when a certain pressure difference is reached. The pressure difference has strong effects on steam-chamber-rising/expansion stages. However, at the expansion stage, lower pressure difference can achieve the same effect as high pressure difference. In addition, it is shown that the steam-chamber-expansion angle is a function of pressure difference. Using this finding, a new mathematical model is established considering the modification of the expansion angle, which (Butler 1991) treated as a constant. With the proposed model, production performance such as cumulative oil production and oil-production rate can be predicted. The steam-chamber shape is redefined at the rising stage, changing from a fan-like shape to a hexagonal shape, but not the single fan-like shape defined by (Butler 1991). This shape redefinition can clearly explain why the greatest oil-production rate does not occur when the steam chamber reaches the caprock. Literature surveys show few studies on how pressure difference influences steam-chamber development and SAGD recovery. The current paper provides a modified SAGD production model and an entirely new scope for SAGD enhanced oil recovery (EOR) that makes the pressure difference a new optimizable factor in the field.

Bearing capacity of ring footings on cohesionless soil under eccentric load

This paper presents a study on the bearing capacity of eccentrically-loaded rough ring footings resting over cohesionless soil. To this aim, a series of 3D numerical simulations were performed using the finite difference method. In order to consider the effect of load eccentricity, reduction factor method is applied. In this method, the ratio of an eccentrically-loaded bearing capacity to the bearing capacity of the same footing under vertical load is defined. Comparison between the results of the numerical simulations with those of analytical solutions and experimental data indicates good agreement. A mathematical expression is also introduced for eccentrically-loaded ring footings.

Edge Fracture Prediction Using Uncoupled Ductile Fracture Models for DP780 Sheet

This paper is a contribution to the prediction of edge fracture behavior using uncoupled ductile fracture models. A fully integrated simulation framework for the edge fracture prediction is proposed with the shear-induced pre-damage considered. User-defined material subroutines are coded with uncoupled ductile fracture models (Lou-Huh, Oh, Brozzo) incorporated, which are calibrated using the fracture strains of various loading paths. A series of 3D numerical simulations are performed and compared with the results of hole-expansion tests. The effects of pre-damage field and fracture models are analyzed and discussed.

A Novel Model to Investigate the Effects of Injector-Producer Pressure Difference on SAGD for Bitumen Recovery

Steam-assisted gravity drainage (SAGD) provides many advantages compared to alternate thermal recovery methods for bitumen recovery. Nowadays, most of researchers believe that the gravity mechanism is the main drive in SAGD recovery, ignoring the injector-producer pressure difference, which makes the field prediction deviate from reality. To tackle this problem, this paper makes further investigation on the injector-producer pressure difference. A series of 2D numerical simulations are conducted on the basis of Mackay River reservoir in Canada to investigateon influence of injector-producer pressure difference. Meanwhile, a new mathematical model considering injector-producer pressure difference is established. The results indicate that when the injector-producer pressure difference exists, SAGD usually has better recovery. Pressure difference can effectively improve SAGD operating performance to achieve a high economic efficiency. More pressure difference does not necessarily lead to better recovery, for when the pressure difference increases to some certain degrees, it will cause steam breakthrough. [Received: May 2, 2016; Accepted: November 11, 2016]

Numerical modeling and validation for 3D coupled-nonlinear thermo-hydro-mechanical problems in masonry dams

We introduce in this article a fully coupled thermo-hydro-mechanical (THM) model describing the physical phenomenon present in masonry materials. A series of 3D numerical simulations are carried out for the THM analysis of a dam during its impounding process. The results of the coupled THM analyzes are validated with measurements recorded during that process in terms of transient displacement, pore-water pressure and temperature values. An agreement between numerical simulations and measured data proves that the coupled THM model can well reproduce the multi-physical behavior of masonry dams. Furthermore, we introduce herein a solution for a large system of balance equations by combining parallel computation, storage in Compressed Sparse Row format, and iterative pre-conditioned Conjugate Gradient Squared method in order to improve the computational cost.

Fracture spacing behavior in layered rocks subjected to different driving forces: a numerical study based on fracture infilling process

Natural layered rocks subjected to layer-parallel extension typically develop an array of opening-mode fractures with a remarkably regular spacing. This spacing often scales with layer thickness, and it decreases as extension increases until fracture saturation is reached. To increase the understanding of how these opening-mode fractures form in layered rocks, a series of 2D numerical simulations are performed to investigate the infilling process of fractures subjected to different driving forces. Numerical results illustrate that any one of the following could be considered as a driving force behind the propagation of infilling fractures: thermal effect, internal fluid pressure, direct extension loading, or pure compressive loading. Fracture spacing initially decreases with loading process, and at a certain ratio of fracture spacing to layer thickness, no new fractures nucleate (saturated). Both an increase in the opening of the infilled fractures and interface delamination are observed as mechanisms that accommodate additional strain. Interface debonding stops the transition of stress from the neighboring layers to the embedded central layer, which may preclude further infilling of new fractures. Whatever the driving force is, a large overburden stress and a large elastic contrast between the stiff and soft layers (referred to as a central or fractured layer and the top and bottom layers) are key factors favoring the development of tensile stress around the infilled fractures in the models. Fracture spacing is expected to decrease with increasing overburden stress. Numerical results highlight the fracturing process developed in heterogeneous and layered sedimentary rocks which provides supplementary information on the stress distribution and failure-induced stress redistribution., It also shows, in detail, the propagation of the fracture zone and the interaction of the fractures, which are impossible to observe in field and are difficult to consider with static stress analysis approaches.

A numerical investigation of the hydraulic fracturing behaviour of conglomerate in Glutenite formation

Rock formations in Glutenite reservoirs typically display highly variable lithology and permeability, low and complex porosity, and significant heterogeneity. It is difficult to predict the pathway of hydraulic fractures in such rock formations. To capture the complex hydraulic fractures in rock masses, a numerical code called Rock Failure Process Analysis (RFPA2D) is introduced. Based on the characteristics of a typical Glutenite reservoir in China, a series of 2D numerical simulations on the hydraulic fractures in a small-scale model are conducted. The initiation, propagation and associated stress evolution of the hydraulic fracture during the failure process, which cannot be observed in experimental tests, are numerically simulated. Based on the numerical results, the hydraulic fracturing path and features are illustrated and discussed in detail. The influence of the confining stress ratio, gravel sizes (indicated by the diameter variation), and gravel volume content (VC) on the hydraulic fracturing pattern in a conglomerate specimen are numerically investigated, and the breakdown pressure is quantified as a function of these variables. Five hydraulic fracturing modes are identified: termination, deflection, branching (bifurcation), penetration, and attraction. The propagation trajectory of the primary hydraulic fractures is determined by the maximum and minimum stress ratios, although the fracturing path on local scales is clearly influenced by the presence of gravels in the conglomerate, particularly when the gravels are relatively large. As the stress ratio increases, the fractures typically penetrate through the gravels completely rather than propagating around the gravels, and the breakdown pressure decreases with increasing stress ratio. Furthermore, the breakdown pressure is affected by the size and volume content of the gravel in the conglomerate: as the gravel size and volume content increase, the breakdown pressure increases.

Revisiting the dynamic compressive behavior of concrete-like materials

This paper deals with two questions that are related to the dynamic compressive behavior of concrete-like materials: (1) whether the compressive strength enhancement of concrete-like materials with strain-rate is a material property; (2) what is the physical reason for the observed two-stage behavior of the compressive strength enhancement with increasing strain-rate in concrete-like materials. A series of 3D numerical simulations were performed. Analysis of the results indicates that the compressive strength enhancement with increased strain-rate is an indirect result of a lateral confining effect. Furthermore, comparisons between the results obtained with pressure-dependent material model and related tests, show that the lateral confinement is caused by both the lateral inertia and the interface friction between the loading apparatus and the loaded specimen. Under these two joint actions the trend of the strain-rate effect is characterized by two stages. The current study points out the role of these mechanisms in controlling the two stages. Finally, similarities between the stress–strain behaviors obtained in the current uniaxial dynamic compressive loading and in triaxial compressive static loading with a moderate level of lateral confining pressure, further prove that the lateral confinement generated during the dynamic loading process contributes to the strain-rate effect.