Two-dimensional Numerical Analysis Research Articles

Flow-induced vibration of two tandem square cylinders at low Reynolds number: transitions among vortex-induced vibration, biased oscillation and galloping

The flow-induced vibrations (FIVs) of two identical tandem square cylinders with mass ratio m* = 3.5 at Reynolds number Re = 150 are investigated through two-dimensional direct numerical simulation (DNS) and linear stability analysis over a parameter range of spacing ratio 1.5 ≤ L* ≤ 5 and reduced velocity 3 ≤ Ur ≤ 34. Three kinds of FIV responses, namely vortex-induced vibration (VIV), biased oscillation (BO) and galloping (GA), are identified. The FIVs are then further classified into the branches of initial VIV (IV), resonant VIV (RV and RV′), flutter-induced VIV (FV), desynchronized VIV (DV), VIV developing from GA (GV), transitional state between VIV and GA (TR), BO and GA based on the characteristics of the vibration responses. The transitions among different FIV branches are examined by combining the DNS with linear stability analysis, where the transition boundaries among the VIV, BO and GA branches over the concerned parameters are identified on the branch maps. The transition from IV to RV or RV′ is found to be related to the unstable wake mode, while the FV, transiting from RV or RV′, is induced by the unstable structural factor in the wake-structure mode. The structural instability is considered as the physical origin of GA, whereas the mode competition between unstable wake and structure leads to DV, GV and TR, and thus delays the appearance of GA. The transition from DV to BO with biased equilibrium position, accompanied by the even-order harmonic frequencies, is essentially induced by the symmetry breaking bifurcation.

Two-dimensional Numerical Analysis of Waves at Interface of Laminar Core-annular Flow in Vertical Pipe: Detailed Investigation of Influence of Several Physical Parameters

Two-dimensional Numerical Analysis of Waves at Interface of Laminar Core-annular Flow in Vertical Pipe: Detailed Investigation of Influence of Several Physical Parameters

Numerical Analysis of Block Type Quay Wall with Piles for Restraining Horizontal Deformation

A two-dimensional numerical analysis was performed on the depth of pile embedment, the magnitude of the residual water level, and the condition of the presence or absence of cap concrete to understand the behavior of the block-type quay wall with piles. The results showed the control effect of the lateral displacement of the quay wall depending on the embedment of the pile. When the piles were not embedded, the lateral displacement of the quay wall increased proportionally as the residual water level difference increased. In contrast, when the piles were embedded into the ground, the control of the lateral displacement of the quay wall was greatly exerted even if the residual water level difference increased. There was little difference in the lateral displacement of the block-type quay wall regardless of the presence or absence of cap concrete. Under the condition where the piles were embedded down to the rubble mound layer, the piles exhibited the rotational behavior seen in the short piles. As the embedment depth of the piles increased, the piles showed the same bending behavior as the intermediate piles. Thus, the piles significantly contribute to the control of lateral displacement in the block-type quay wall with piles.

A numerical study on the blade–vortex interaction of a two-dimensional Darrieus–Savonius combined vertical axis wind turbine

To investigate power losses of a Darrieus–Savonius combined vertical axis wind turbine (hybrid VAWT) associated with the interaction between blades and wake, it is crucial to understand the flow phenomena around the turbine. This study presents a two-dimensional numerical analysis of vortex dynamics for a hybrid VAWT. The integration of a Savonius rotor in the hybrid VAWT improves self-starting capability but introduces vortices that cause transient load fluctuations on the Darrieus blades. This study attempts to characterize the flow features around the hybrid VAWT and correlate them with the Darrieus blade force variation in one revolution. Results demonstrate the capability of numerical modeling in handling a wide range of operational conditions: the relevant position of Savonius and Darrieus blades (attachment angle γ=0°−90°) and Savonius' tip speed ratio λS (0.2–0.8, varied Savonius' rotational speed). The torque increase in the Darrieus blade in hybrid VAWT (compared to a single Darrieus rotor) due to the appearance of the vortex shedding from the advanced Savonius blade is independent of the attachment angle and tip speed ratio. Apart from start-up and power performances of the hybrid VAWT, the most rapid force fluctuation is identified when the Darrieus blade interacts with Savonius' wake at γ=0° and λS=0.8, which is considered undesirable. Furthermore, attachment angles of 60° and 90° exhibit better power coefficients compared to those of 0° and 30° for the hybrid VAWT. This study contributes to a comprehensive understanding of flow dynamics in hybrid VAWTs, revealing the correlation between torque variation and vortex development.

Two-dimensional numerical analysis on the rock/bolt interaction considering shear and normal relative displacements

Fully-grouted passive bolts are widely used in underground or surface rock excavations and in particular in stabilizing potentially unstable blocks of rock due to sliding on natural discontinuities. Their operating mechanism is complex, but it is possible to consider two stabilizing forces that each bolt applies to the block of rock. These forces depend on the mechanical parameters governing the bolt-rock interaction, which are difficult to evaluate. In this work, specific numerical analyses have been developed, able of evaluating the bolt-rock interaction (in shear and perpendicular to the interface that separates them) for numerous cases that were obtained by varying the main geometric parameters of the bolt, the mechanical properties of the binder material and rock. Thanks to this complex study, it was possible to describe the variability of the interaction parameters and to define, through graphs, the trend of the stabilization forces as the main geometric and mechanical parameters that can be encountered in practice change. The graphs obtained are a useful tool for the correct design of fully-grouted passive bolts and the stabilization of potentially unstable rock blocks on the walls of underground cavities or on the faces of surface excavations.

Investigation of Numerical Irregular Wave Generation Using Transient Discrete Data as Boundary Conditions of Prescribed Velocity

This work presents a two-dimensional numerical analysis of a wave channel. The goal is to investigate a methodology that uses transient velocity data as means to impose velocity boundary condition for generating numerical waves. To do so, a numerical wave channel was simulated using irregular waves. These waves were obtained using the WaveMIMO methodology, which converts sea data from a spectral model into time series of free surface elevation, and then converts this elevation into transient discrete wave velocity data. For the numerical analysis, computational fluid dynamics ANSYS Fluent software was employed, which is based on the finite volume method. The computational domain and mesh were generated using GMSH software, and the nonlinear multiphase model volume of fluid (VOF) was applied to tackle water-air interaction. In general, the results obtained through the use of discrete data as velocity boundary condition presented a good agreement with free surface elevation converted from the spectral model, and the tests performed provided further insight into the parameters which affect this methodology. Since many studies use regular waves, the proposed investigation stands out for its capacity to improve the use of realistic sea state data in numerical simulations of wave energy converters.

Undrained uplift resistance of under-reamed open caisson shafts

Deep, large-diameter caisson shafts are a popular means of constructing underground storage and attenuation tanks and pumping stations for the water and wastewater industry. One of the key design concerns for these structures is resistance to flotation during periods when the tanks are only partially filled or empty. In this paper, two-dimensional numerical analysis is used to explore the undrained uplift resistance provided by under-reaming the walls of the caisson shaft to create an enlarged base. The primary aim of the study is to assess the influence of the taper angle of the anchor (i.e. the protruded base) on the resulting uplift resistance. The effects of the anchor–soil interface roughness factor, soil weight, surcharge pressure and caisson radius are also investigated. The results indicate that the effect of the taper angle on both the uplift bearing capacity and the developed horizontal reaction can be very significant. The numerical output informs the development of a closed-form approach for application in routine design. The new design method is shown to provide an excellent agreement with both finite-element and additional finite-element limit analysis calculations. By way of example, the proposed design method is applied to a hypothetical design scenario.

2D Numerical investigation of surface wettability induced liquid water flow on the surface of the NACA0012 airfoil

The accumulation of ice on wind turbine blades is a serious threat to the safety of wind power generation. Anti-icing techniques with low energy consumption, especially when combined with superhydrophobic surfaces, have recently attracted increasing levels of interest. In this work, the anti-icing mechanism of superhydrophobic surfaces was investigated by conducting the two-dimensional numerical analysis of the flow patterns of liquid water on surfaces with different wettability, which was based on the phase field method (PFM) and a dynamic contact angle (DCA) model. It is found that the wettability of surfaces significantly influences the flow, break, and shedding behaviors of liquid water on the surfaces. An increase in contact angle (CA) leads to more frequent break and shedding behaviors, and the flow pattern of liquid water gradually transformed from continuous water film to fragmented droplets because of the vortical flow field. In addition, it is also investigated that when the CA of surfaces exceeded 145°, the broken liquid water completely shed away in 0–0.1 chord length. Moreover, the average chordwise location (x‾/c) and maximum chordwise location (xmax/c) of the liquid water that had traveled the farthest on the surface in 30 ms gradually decreased with increasing contact angle, which is beneficial to superhydrophobic-dry anti-icing. What's important, the wettability characteristics should be responsible for the unique water flow patterns coupled with shear stress, which has been validated again, and previously proposed by our work. The results of this study provide theoretical support for the incorporation of superhydrophobic surfaces in anti-icing technology applied to wind turbine blades, which is promising for solving the icing issue of wind turbine blades with low energy consumption.

Numerical modelling of enhancement of the capacity of buried metal culverts by using geogrids

This paper presents a two-dimensional numerical modelling analysis of a flexible buried corrugated metal arch culvert in Enkoping, Sweden. The numerical results of this study are validated against field measurements of the culvert crown deformation, thrust, and bending moments recorded during backfilling. To model the backfill soil, the hardening soil with small strains (HSs) material model is used because of its efficiency in simulating the soil-structure interaction. Furthermore, in a numerical investigation of the stress distribution at the culvert invert, it is found that weakness of the foundation soil has an insignificant impact, due to stress dissipation resulting from arching actions. The numerical modelling analysis also investigates the use of geogrid layers with dead end bolts in the soil cover above the culvert crown during the application of static surface loads, as an innovative technique to improve the load capacity of the soil-culvert system. The results show a reduction in culvert crown deformation and internal forces when geogrid layers are used. This indicates the efficiency of geogrid layers in improving the load capacity of existing buried culverts or overcoming deficiencies in culvert serviceability by reducing the impact of applied loads.

Evolving a Bio-Inspired Blade Shape of the Drag-based Vertical-axis Wind Rotor Derived from Orange Sea-pen (Ptilosarcus Gurneyi)

Abstract Inspired by the polyp leaf of the Orange sea-pen (Ptilosarcus gurneyi), a novel blade shape of the Savonius vertical-axis wind rotor is developed. The similarities between the aerodynamic and the hydrodynamic aspects of the Savonius rotor blade profile and the sea-pen leaf are reviewed, and an appropriate analogy is thereby established. The shape of the sea-pen leaf is then extracted to fabricate the rotor blades. The performance of this sea-pen bladed rotor is evaluated in a low-speed subsonic wind tunnel at different wind velocities. The two-dimensional (2D) numerical analysis is also performed to support the experimental findings and to study the influence of blade shape on the pressure and the torque distributions of the rotor. The novel sea-pen bladed rotor, having lesser material requirements, is seen to demonstrate higher performance than that of the conventional semicircular bladed rotor in the tested range of low tip-speed ratio.

Performance study of savonius style wind turbines in a solar chimney power plant under guided flow environment

With the uprise in global population and rapid industrialisation in developing nations, increasing energy demand to compensate for societal and industrial needs is inevitable. However, popular energy resources such as gas, petroleum, and coal, on which the global energy mix is heavily reliant, induce environmental pollution and global warming. Thus, clean energy sources, such as wind and solar, are increasingly sought to mitigate these adverse environmental effects and identify alternative sustainable and renewable energy technologies. The existing literature suggests minimal research has been conducted on the turbine component of the solar chimney power plant (SCPP). The present research proposes the study of a vertical axis wind turbine (VAWT), namely the Savonius-style wind turbine (SSWT), for power augmentation in the SCPP under the influence of guide vanes and shields that are installed in the solar collector area. A two-dimensional numerical analysis is performed using the ANSYS Fluent software. The SSWT and SCPP models used in the simulations were validated using numerical and experimental results from previous literature data. The angle of the guide vanes was varied while the dimensions of the SCPP model and the energy extractor considered were kept constant. The optimum turbine performance was identified based on the flow analysis and the power output of the rotary component from different guide vane configurations. The present study shows an overall improvement in the turbine’s performance coefficient and power output under the influence of the guide vane and shield in the SCPP. The turbine with the guide vane and shield at an angle of 45° enhanced the performance by 22.22% and power output by 22.25% compared to the turbine without the guide vane and shield.

Analyzing the fire performance of concrete columns and slabs under loading and using options, preventing explosive spalling to ensure the pre-set fire resistance

Introduction. The authors focus on preventing the explosive spalling of concrete and the fireproofing of reinforced concrete structures. The relevance of this issue is explained by the insufficient number of fire tests of such structures under loading and thermal engineering calculations, needed for an objective analysis of testing results.Goal and objectives. The authors analyze the results of a series of fire tests, involving concrete columns and slabs with and without polypropylene microfiber, if no fireproofing is applied, as well as the results of the same tests involving the same items fireproofed by plates or plaster.Methods. The fire resistance of full-scale specimens of concrete was evaluated according to a standardized testing in a fire furnace under loading. It encompasses additional thermocouple measurements used to make a thermal engineering analysis. The analysis entailed both one- and two-dimensional problem formulations, methods and programmes for the numerical computation of non-stationary temperature fields in fireproof structures.Results. New data, obtained in the course of the fire experiments, show the efficiency of the polypropylene microfiber used to prevent the explosive spalling of concrete. The fire resistance limit is R 120 and R 150 under constant static loading. The fire resistance limit of similar structures, fireproofed by PROSASK Firepanel plates or IGNIS LIGHT plaster, was demonstrated. The specimens show the efficiency of methods and programmes for the one- and two-dimensional numerical analysis of non-stationary temperature fields in fireproof structures. The calculation results are presented for various fireproofing options.Conclusions. The testing results and their thermal analysis represent important items of information necessary to ensure the fire safety and the pre-set fire resistance of concrete structures under loading. They can also be used to outline the development pattern of this experimental and theoretical research project. The efficiency of thermal engineering calculations as a tool for evaluating fire protection parameters and the fire resistance of concrete structures is demonstrated, also as an option to reduce the number of expensive fire tests.

Numerical Study on the Thermal Enhancement of Phase Change Material with the Addition of Nanoparticles and Changing the Orientation of the Enclosure

The global demand of the heating and cooling applications gives a larger potential to study the thermal energy storage system. Phase change materials (PCM) that are used to charge, store, and discharge the heat energy are inferior in heat transfer characteristics. The properties of PCM can be improved by adding nanoparticles, changing the orientation of the enclosure or both. Two-dimensional transient numerical analysis on the effect of Grashoff number (5000, 13000, and 20000), nanoparticle type (Al2O3, CuO, and MWCNT), and volume concentration (0%, 1%, 3%, and 5%) added in RT 42 PCM and orientation of square enclosure (30, 45, and 60°) to enhance the heat transfer rate is carried out. The thermophysical properties of the nano-PCM are evaluated and presented. From the results, it is affirmed that the melt fraction of the PCM rises with the increase in Gr and volume concentration of the nanoparticles up to an optimum level. The MWCNT-based nano-PCM attained a larger portion of melt fraction followed by Al2O3, CuO, and pure PCM. It is noted that an orientation of 60° and 45° will convert more quantities of pure PCM and nano-PCM into liquid fraction, respectively. The (3% MWCNT/RT-42 PCM) filled in 45° oriented container attained the highest melt fraction by 3.4%, 2.04%, and 2.94% than (3% Al2O3/RT-42 PCM), (1%CuO/RT-42 PCM), and pure PCM. The variation in the maximum melt fraction of the nanomaterial is because of the change in thermophysical characteristics of the nano-PCM.

A mixed soft elastohydrodynamic lubrication model of textures in a rotary lip seal

A two-dimensional elastohydrodynamic numerical analysis is conducted for a rotary lip seal with a textured shaft by coupling flow, deformation and contact equations. In this analysis, an elastic deformation equation is used to obtain the displacement of the discrete lip surface and a correction process is proposed to calculate the contact pressure in operation. Considering the effects of circular and elliptical dimples respectively, the contact pressure distribution, lip deformation, oil film load support and leakage of the lip seal are obtained, and a parametric study for textures is performed. The results show that the effects of textures reshape the lip surface topography and generate secondary fluid pressure. Micro-dimples with appropriate size can significantly reduce the contact pressure.

A phase-field model of quasi-brittle fracture for pressurized cracks: Application to UO[formula omitted] high-burnup microstructure fragmentation

In this paper, we present a phase-field model of quasi-brittle fracture with pressurized cracks, with dedicated applications for polycrystalline materials. The model is formulated as a minimization problem within the variational framework. The external work done by pressure on the crack surfaces is included in the objective function. Several careful modeling choices lead to a regularization-length-independent critical strength. The model is constructed to give a softening response with an underlying linear traction-separation law. The pressure-dependent softening response and the regularization of the prescribed pressure are demonstrated with a (quasi) one-dimensional numerical analysis. In a two-dimensional numerical analysis under plane strain assumptions, the critical stress corresponding to crack propagation (as predicted by our quasi-brittle fracture model) is compared with linear elastic fracture mechanics (LEFM) analytical solutions. Our model is further utilized to simulate fission-gas-induced fragmentation of the UO2 high-burnup structure (HBS). Simulation results show that pressurized bubbles can cause crack nucleation and propagation, and that the bubble size and the surrounding external pressure affect the critical pressure corresponding to crack nucleation. Simulations of a partial HBS at different recrystallization stages show that different grain structures (due to recrystallization) also influence crack paths and fragmentation morphology.

Coupled Fluid-Solid Heat Transfer of A Gas and Liquid Cooling PMSM Including Rotor Rotation

In order to study the problem of the high temperature reached by the end windings, caused by the high current in the armature windings of permanent magnet synchronous motors (PMSM), a 560 kW gas and liquid cooling PMSM used for a compressor is analyzed. The heat source is obtained by a two-dimensional numerical analysis of the electromagnetic field. A three-dimensional fluid-solid coupling heat transfer model is established. Under the action of the cooling medium, one-quarter of the PMSM structure is numerically simulated. The temperature distribution of the 560 kW PMSM is studied, including rotor rotation. The temperature distribution of the PMSM armature winding and the distribution of fluid around the winding under different inlet air flow speeds are analyzed. The calculated values of the end winding temperature at different currents are compared with the experimentally measured values. The rationality of the three-dimensional fluid-solid coupling heat transfer model and the correctness of the calculation method are verified. This paper provides both the analysis and the computation scheme of the fluid-solid heat transfer of the PMSM.

Excavation-induced arching effect below base level and responses of long-collinear underlying existing tunnel

Deep excavation overlying existing tunnel becomes a common recurring problem in congested urban areas. The mechanism behind the tunnel responses to overlying excavation is not yet fully understood. This study investigates the performances of a long and deep excavation and underlying collinear existing tunnel through well-documented field observation and elaborate two-dimensional numerical analysis. Focus was placed on the ground stress transfer and displacement profile, and corresponding tunnel responses. It indicates that two types of concave arches could occur, including “frictional arch” and “end-bearing arch”. This pattern of stress transfer phenomenon was termed as “negative arching effect”, differing from the arching effect above trapdoor and tunnel in light of formation mechanism. In comparison with the end-bearing arch, the frictional arch was not stable and could exhibit appreciable overall heave as the excavation depth increased. The area below the excavation base could be split into three different zones in terms of mechanical responses: loosened zone, stress transfer zone, and stable zone, respectively. Moreover, variations of the tunnel responses with relative tunnel positions with respect to the loosened zone were investigated. It exhibits that the tunnel heave and vertical elongation in tunnel diameter experienced significant increases when the tunnel crown entered into the loosened zone.

５つの山頂部をもつ平板コルゲート翼の翼表面圧力特性

A two-dimensional unsteady numerical analysis of a corrugated airfoil was performed to clarify the variation of flow around the airfoil as a function of leading-edge angle. A corrugated airfoil with five crests was made to follow the outline of NACA4412, and three types of airfoils with leading edge angles of 25° and 35° were selected as the target airfoils. The Reynolds number was set to 1.0 × 104. Comparison of NACA4412 and the three types of corrugated blades showed that the lift coefficient was highest for NACA4412 at angles of attack of 4° or less and was significant for the corrugated blades at angles of attack higher than that. On the other hand, the drag coefficient was the lowest for NACA4412 at all angles of attack. Comparison of corrugated blades with different leading-edge angles revealed that at low angles of attack of 6° or less, the circulating flow in the first valley has a greater effect on aerodynamic characteristics than that in the second valley. On the other hand, at high angles of attack of 10° or more, flow cancellation was observed from the leading edge to the first crest of the upper surface of the wing, which is one of the reasons for the improved wing characteristics.

Experiments and numerical analysis of rebounding behaviors of a laser-induced cloud cavitation

It is well known that a high-speed submerged water jet generates a cloud of cavitation bubbles experiencing a collective unsteady behavior that repeats growth and collapse. In particular, it is considered that a high-pressure shock wave may be emitted associated with the collapse of the cloud. However, its mechanism has not been enough clarified and hence it is required to be explored both from the experimental and numerical analyses. In this study, we fist make an observation of such a unsteady behavior of the cloud cavitation that is generated by a pulsed Ho:YAG laser-induced liquid jet using a high-speed video camera. Then, a two-dimensional numerical analysis based on the SPH method is examined to simulate the unsteady behavior of the cloud, in which the mixture model of liquids and gas bubbles is employed. Finally, the validity of our numerical analysis is illustrated by showing that the unsteady behavior of the cloud from its inception, growth, shrink, collapse to rebound is realized in qualitative agreement with the experimental data.

Electrical сharacteristics study of heterojunction solar сells CdS/CIGS

In this work, we carried out the study of electrical characteristics with two-dimensional numerical analysis by using the Aided Design (TCAD Silvaco) software for CdS/CuInGaSe2 (CIGS) thin solar cells. Their structure is composed of a thin CIGS solar cell in the configuration: ZnO(200 nm)/CdS(50 nm)/CIGS (350 nm)/Mo. Then ZnO is used for conductive oxide contacted transparent front of the cell. For rear contact, the molybdenum (Mo) is used. The layer of the CdS window and the shape of the CIGS absorber is the n-p semiconductor heterojunction. The performance of the cell was evaluated by applying the defects created in the grain joints of polycrystalline CdS and CIGS material and CIGS/CdS interface in the model, and the physical parameters used in the TCAD simulations have been calibrated to reproduce experimental data. The J–V characteristics are simulated under AM1.5 illumination conditions. The conversion efficiency (η) 20.10% has been reached, and the other characteristic parameters have been simulated: the open-circuit voltage (Voc) is 0.68 V, the circuit-current density (Jsc) is equal to 36.91 mA/cm2, and the form factor (FF) is 0.80. The simulation results showed that the molar fraction x of the CIGS layer has an optimal value around 0.31 corresponding to a gap energy of 1.16 eV, this result is in very good agreement with that found experimentally.