Upwind Research Articles

Influence of thermal radiation and magnetic field on convective transfer within an inclined square enclosure filled with Cu-water nanofluid

The present work investigates the effects of thermal radiation and magnetic field on the laminar convective transfer process on copper-based nanofluid within an inclined square enclosure. The studied configuration is a rigid-walled cavity subjected to a horizontal temperature gradient where the left wall is maintained at a hot temperature Th and subjected to a magnetic field B0. In contrast, the right wall is kept at a cold temperature Tc. The horizontal walls are adiabatic. The dimensionless governing equations are solved using the finite difference method and the UPWIND scheme to solve the convective terms. Formulated using the stream function, vorticity, and temperature. Effects on mean Nusselt numbers are investigated at different Rayleigh numbers 104,105,106, Hartmann numbers HA=0to100, tilt angles γ=0,π/6,π/4andπ/3, nanofluid volume fractions 0%≤φ≤6% and different radiation parameters ( Rd=0,1,2,3 ). The findings show that the HTR increased 3.6 times by augmenting the Ra from 104 to 106. Also, increasing the Ha number from 0 to 100 causes an 83.16% reduction in the HTR. Considering the radiation mode of heat transfer causes an increase in HTR. The average Nusselt values increase by 59.72% for the concentration of 6% while increasing the radiation parameter from 0 to 3. On the other hand, an increase in volume fraction leads to a deterioration in mean Nusselt numbers (Numean).

Singularly perturbed time-fractional convection–diffusion equations via exponential fitted operator scheme

In this paper, we proposed an accurate ϵ-uniformly convergent numerical method to solve singularly perturbed time-fractional convection–diffusion equations via exponential fitted operator scheme. The time-fractional derivative is defined in the sense of Caputo with order η∈(0,1). The time-fractional derivative is discretized by employing the Crank–Nicolson method on a uniform mesh, and an exponential fitted operator scheme along with the standard upwind method is used to mesh-grid the space domain. The truncation error and uniform stability of the discretized problems are examined in order to prove the parameter uniform convergence of the proposed scheme. It is demonstrated that the scheme is ϵ-uniformly convergent of order O((Δt)2−η+Δx), where Δt and Δx represent the step sizes of the time and space domains, respectively. Two numerical examples are provided in order to assess the accuracy of the suggested scheme and validate the theoretical concepts discussed. To demonstrate the efficiency of the numerical scheme presented, comparisons have been made with the numerical solution obtained by the finite difference method that exists in the literature. Consequently, it is observed that the results obtained by the present scheme are more accurate and have a better convergence rate.

A numerical analysis of wave type and steepness effects on cylindrical buoy dynamic characteristics

In the intricate realm of ocean-atmosphere dynamics, the sustained and stable operation of buoy observation systems under rough seas is crucial for ensuring marine resource security. This study employs a coupled Level-Set and Volume of Fluid (CLSVOF) method alongside the immersed boundary method. It investigates the physical processes of wind-wave interaction, the dynamic characteristics of cylindrical buoys, as well as the underlying mechanisms. The research highlights the significant impact of wave types (wind waves, swells) and wave steepness on the buoy's drift and oscillatory behavior. An increase in wave steepness and a shift from wind waves to swell markedly intensify both the buoy's drift and its oscillatory motions. Additionally, a thorough analysis of the loads on the buoy under varying maritime conditions is performed. By examining the vertical momentum equation on the windward and leeward sides, the secondary load cycle phenomenon is found to originate from collapse of the water column formed by the convergence of water flow behind the buoy. Wind waves are found to be less conducive to secondary load cycles compared to swell waves. Furthermore, the study investigates the relationship between the drag coefficient and the wave crest height on the windward side of the buoy, revealing a robust negative correlation between the two. This research not only illuminates the complex dynamics of wind-wave couplings but also offers theoretical insights for the optimization of buoy design.

Numerical Analysis on The Aerodynamic Performance of A High-speed Train Operating on Various Embankment Heights Under The Influence of Crosswinds

Given the unavoidable geographical surface, railings must be raised above ground level in some cases, which is known as an embankment. It was discovered that the height of the embankment had a significant influence on the slipstream on the train's leeward side especially during crosswind conditions. The primary objectives of this study were to investigate the impact of varying embankment heights on the aerodynamic characteristics of a high-speed train under different crosswind conditions using computational fluid dynamics (CFD) analysis. The German Aerospace Center DLR's Next-Generation High-speed Train (NG-HST) model has been used for this study. The yaw angles (ψ) are ranging from 10° to 50° in 10° increments. The Reynolds number based on the model's height and freestream velocity at the computational domain is 1.3 x 106. In the results, it shows that embankment height and crosswind ψ has a significant impact on the aerodynamic characteristics. Essential aerodynamic parameters that have a significant impact on train stability, such as the drag, lift, and side force coefficients, as well as the roll, yaw, and pitch moment coefficients, revealed that the higher the ψ, which included 40° and 50°, produced poor results compared to the lower ψ, which included 10°, 20°, and 30°. In terms of visual appearance, rising of crosswind angles have a greater impact on the formation of vortices on the leeward side of the train body and embankment. Thus, it can be concluded that the embankment heights and crosswind angles are crucial in determining train safety operations.

PARAMETER-UNIFORM HYBRID NUMERICAL SCHEME FOR SINGULARLY PERTURBED PARABOLIC CONVECTION-DIFFUSION PROBLEMS WITH DISCONTINUOUS INITIAL CONDITIONS

This article presents a parameter-uniform hybrid numerical technique for singularly perturbed parabolic convection-diffusion problems (SPPCDP) with discontinuous initial conditions (DIC). It utilizes the classical backward-Euler technique for time discretization and a hybrid finite difference scheme ( which is a proper combination of the midpoint upwind scheme in the outer regions and the classical central difference scheme in the interior layer regions(generated by the DIC)) for spatial discretization. The scheme produces parameter-uniform numerical approximations on a piecewise- uniform Shishkin mesh. When the perturbation parameter ε(0 < ε ≪ 1) is small, it becomes difficult to solve these problems using the classical numerical methods (standard central difference or a standard upwind scheme) on uniform meshes because discontinuous initial conditions frequently appear in the solutions of this class of problems. The method is shown to converge uniformly in the discrete supremum with nearly second-order spatial accuracy. The suggested method is subjected to a stability study, and parameter-uniform error estimates are generated. In order to support the theoretical findings, numerical results are presented.

Evolution of riparian vegetation in response to anthropogenic effects: The Taleqan River, Iran

AbstractDuring the last decades, human interventions, such as dam construction and land‐use change, have caused changes in geomorphic patterns and riparian zones in many rivers in Iran. This study aimed to assess the spatio‐temporal evolution of riparian vegetation and the spatial area of biogeomorphic feedback windows (BFWs) located in the riparian Salicaceae vegetation patches and their traits responding to human interventions in the Taleqan River during 1990–2022. The river was divided into five reaches using the geomorphic unit system (GUS) method, and the river sensitivity index (SI), location of BFWs, vegetation typology and main cover typology were investigated in the reaches using aerial photographs and satellite images from 1990 to 2022. For this purpose, the main cover typology was classified into grassland, woody vegetation, bank‐attached bars, islands, vegetated islands, river channels and urban areas. The stem diameter and density, degradation level and longitudinal/latitudinal alluvial bars in each of the BFWs were investigated using the field survey. Based on the findings, the lowest values of the SI (braiding index [BI], channel activity [CA] and channel width [CW]) and the highest expansion of BFWs were observed in the upstream reaches of the Taleqan River. Human disturbances in these reaches were less than in the reaches downstream, allowing the expansion of vegetation habitats. The vegetation patches were unstable and scattered in the midstream and downstream, and the width decrease of the river channel was created by land‐use change and channelization. A significant direct relationship was observed between BFW establishment, stem diameter and density, and vegetation height. The BFWs create an opportunity for engineer vegetation to prevent erosion and expand new habitats by creating biogeomorphic accumulation landforms and trapping alluvial bars on the lee side of engineer woody vegetation. This study highlights the interactions between riparian areas and hydrogeomorphic processes within riparian corridors exposed to human disturbances to help better define a comprehensive plan for river management.

Nonmodal linear stability analysis of hypersonic flow over an inclined cone

Nonmodal linear stability analysis results are presented for hypersonic flow over a cone at 6° angle of attack complementing earlier modal stability analysis. Based on the parallel flow assumption, singular value decomposition is applied to obtain the optimal linear combination of global crossflow modes. The optimal disturbance exhibits significant transient growth in the initial short distance and progressively follows the path of the most unstable mode downstream. The largest transient energy gain is observed for disturbances at around 40 kHz close to the most amplified modal frequency and tends to increase with the Reynolds number. The optimal disturbance initially exhibits two amplitude peaks in the azimuthal direction, one lying in the leeward region where the unstable crossflow modes reside and the other in the windward region where the adjoint modes exist. As the optimal disturbance travels downstream, the second amplitude peak rapidly shifts toward the leeward side and reaches the optimal energy gain when it eventually merges with the first amplitude peak. The evolution process of the optimal disturbance indicates that the optimal disturbance might have exploited the locally crossflow instability through traveling from the windward side to the leeward side.

CFD study of heat transfer in power‐law fluids over multiple corrugated circular cylinders in a heat exchanger

AbstractThe heat transfer in power‐law fluids across three corrugated circular cylinders placed in a triangular pitch arrangement is studied computationally in a confined channel. Continuity, momentum, and energy balance equations were solved using ANSYS FLUENT (Version 18.0). The flow is assumed to be steady, incompressible, two‐dimensional, and laminar. A square domain of side 300Dh is selected after a detailed domain study. An optimized grid with 98,187 cells is used in the study. The convergence criteria of 10−7 for the continuity, x‐momentum, and y‐momentum balances and 10−12 for the energy equation were used. Constant density and non‐Newtonian power‐law viscosity modules were used. The diffusive term is discretized using a central difference scheme. Convective terms are discretized using the Second‐Order Upwind scheme. Pressure–velocity coupling between continuity and momentum equations was implemented using the semi‐implicit method for pressure‐linked equation scheme. Streamlines show wake development behind the cylinders, which is very dominant at large ReN and n. Isotherm contours are cramped at higher values of ReN and PrN, implying higher heat transfer. Global parameters, like, Cd and Nu, are computed for the wide ranges of controlling dimensionless parameters, such as power‐law index (0.3 ≤ n ≤ 1.5), Reynolds (0.1 ≤ ReN ≤ 40), and Prandtl (0.72 ≤ PrN ≤ 500) numbers. The NuLocal plot attains a pitch near the corrugation of the surface due to abrupt changes in velocity and temperature gradients. Nu increases with ReN and/or PrN and decreases with n under ot herwise identical situations. Nu is correlated with pertinent parameters, namely, ReN, PrN, and n.

A linear second-order maximum bound principle preserving finite difference scheme for the generalized Allen–Cahn equation

In this paper, we consider the numerical method for generalized Allen–Cahn equation with nonlinear mobility and convection term. We propose a linear second-order finite difference scheme which preserves the discrete maximum bound principle (MBP). The scheme is discretized by stabilized Crank–Nicolson in time, upwind scheme for convection term and central-difference scheme for diffusion term. We show that the proposed scheme preserves the discrete MBP under some constraints on temporal step size and stabilizing parameter. Optimal L∞-error estimate is obtained for our scheme. Several numerical experiments are performed to validate our theoretical results.

A Posteriori Error Analysis of Defect Correction Method for Singular Perturbation Problems With Discontinuous Coefficient and Point Source

Abstract The paper presents a defect correction method to solve singularly perturbed problems with discontinuous coefficient and point source. The method combines an inexpensive, lower-order stable, upwind difference scheme and a higher-order, less stable central difference scheme over a layer-adapted mesh. The mesh is designed so that most mesh points remain in the regions with rapid transitions. A posteriori error analysis is presented. The proposed numerical method is analyzed for consistency, stability, and convergence. The error estimates of the proposed numerical method satisfy parameter-uniform second-order convergence on the layer-adapted grid. The convergence obtained is optimal because it is free from any logarithmic term. The numerical analysis confirms the theoretical error analysis.

Heat transfer enhancement of air flow through new types of perforated dimple/protrusion fins

Heat transfer enhancement technologies of air flow are critical to energy conservation and emission reduction in industrial applications. This paper explores the numerical simulation analysis of 18 kinds of perforated dimple/protrusion fins. The wind speed varies within the range of 2 ∼ 6 m/s, and the investigation delves into the impact of different structural parameters on strengthening factor Nu/Nu0, resistance factor f/f0 and other comprehensive evaluation factor performance evaluation criteria (PEC) of perforated dimple/protrusion fins. The study specifically explores the influence of structural parameters, including intercepting perforated dimple/protrusion height (S0), hole diameter (D), hole structure offset (a), and the arrangement of perforated dimple/protrusion structure, on heat transfer performance. The comprehensive evaluation factor PEC of the new fin increased by 23.7 % compared with the porous fin. The overall heat transfer performance of the perforated dimple/protrusion fins rises with height under the same operating conditions. It first increases and then declines with the increasing hole diameter, and first drops and then increases with the increasing offset. The perforated protrusion/dimple fins obtain the best enhancement factor when the hole diameter is 3.4 mm. The hole structure offset is better on the inflowing leeward side than on the inflowing side. The fins with both perforated dimple and perforated protrusion structures have advantages over the one with either of two structures, and the alignment arrangements are identified as the optimal configuration. The new structural characteristics of perforated dimple/protrusion fins in heat exchangers can be utilized to create new designs that potentially outperform conventional models.

A parameter uniform numerical method on a Bakhvalov type mesh for singularly perturbed degenerate parabolic convection–diffusion problems

AbstractWe are focused on the numerical treatment of a singularly perturbed degenerate parabolic convection–diffusion problem that exhibits a parabolic boundary layer. The discretization and analysis of the problem are done in two steps. In the first step, we discretize in time and prove its uniform convergence using an auxiliary problem. In the second step, we discretize in space using an upwind scheme on a Bakhvalov-type mesh and prove its uniform convergence using the truncation error and barrier function approach, wherein several bounds derived for the mesh step sizes are used. Numerical results for a couple of examples are presented to support the theoretical bounds derived in the paper.

A discontinuous Galerkin method for the Brinkman–Darcy-transport problem

In this paper, a weighted discontinuous Galerkin finite element method and an upwind format are presented to solve the coupled Brinkman–Darcy flow and transport model. The flow in a highly permeable region of the model is governed by the Brinkman equations, and the percolation in the porous media is controlled by the Darcy equations, which are coupled by three interface conditions. The permeability coefficients in this model are strongly nonhomogeneous, anisotropic, and discontinuous; the transport equation is a convection-dominated problem; and the velocity field in the porous media domain does not satisfy the divergence-free condition. A weighted discontinuous Galerkin finite element method is used to solve the complex permeability coefficient problem and an upwind scheme is used to solve the convection dominated problem. The interface conditions can be naturally incorporated into the discrete formulation without introducing additional variables. Optimal error estimates are obtained for the semi-discretization and the full discretization with the backward Euler scheme in suitable energy norm. A series of numerical experiments are provided to illustrate the proposed method, including testing the convergence and accuracy of various types of meshes; simulating fluid flow in complex porous media such as obstacles, layered media, and curved interfaces; and studying the coupled flow behavior of surface-subsurface flows with contaminant transport.

Three-Dimensional Spatial Distribution of the Sedimentation Rate of Chloride Ions on a Tropical Island

Chlorine ions in the air are a key factor in the corrosion of offshore buildings. Mastering the distribution law of the chloride ion settlement rate (RCl−) in three-dimensional (3D) spatiality is helpful in protecting offshore buildings. The self-designed “kite-hanging wet candle method” was used to collect chloride ions in the air, using ion chromatography to analyze the chloride ion concentration of the sample solution, and obtained the annual RCl− in the offshore atmosphere at different vertical heights, using the Pearson correlation coefficient method to analyze the influence of environmental factors on the RCl−. The results show that the RCl− has a significant linear relationship with temperature, relative humidity and wind speed. Among them, the RCl− is positively correlated with temperature and negatively correlated with wind speed and relative humidity. In the vertical height range of 10–100 m, the RCl− presents a “⊂”-shaped distribution. In the range of 10–30 m, the RCl− is mainly controlled by the impact of ocean spray; in the range of 30–80 m, the RCl− is mainly controlled by marine aerosols; and in the range of 80 m–100 m, the RCl− is mainly controlled by marine aerosols and wind speed. Under the influence of wind direction and wind speed, the RCl− of the windward side is higher than that of the leeward side at different monitoring points, which are close to the coastline and at a low vertical height. Studying the distribution of the RCl− in 3D spatiality can effectively prevent and reduce its impact on offshore buildings.

A Study on the Impact of Window Partition Walls on the Spread of Fire on Building Facades

This paper investigates the impact of window partition walls on the spread of fire on building facades under the impact of environmental wind through Fire Dynamics Simulator simulation experiments. A four-story building model was constructed using a Fire Dynamics Simulator incorporating six different wind speed conditions and six different partition wall widths. The fire-blocking performance of window partition walls of varying widths was systematically compared and analyzed, and the data indicated: (1) Under calm wind conditions, the installation of window partition walls is observed to facilitate the vertical spread of facade fires. Moreover, as the width of these partition walls increases, this facilitative effect becomes increasingly prominent; (2) Under wind speeds of 0 to 5 m/s, the temperature on the leeward side is lower when window partition walls are present than when they are absent. This indicates that window partition walls inhibit the horizontal spread of building facade fires, and wider window partition walls have better horizontal fire resistance performance.

2DH Numerical Study of Solitary Wave Processes around an Idealized Reef-Fringed Island

In order to better understand the role of coral reefs around an isolated island in mitigating tsunami hazards, this study performed a horizontally two-dimensional (2DH) numerical study of tsunami-like solitary wave propagation and run-up around an idealized reef-fringed island. The shock-capturing Boussinesq wave model, the FUNWAVE-TVD is used in the present study and well-validated with existing experimental data for its robustness in predicting 2DH solitary wave processes around an island. Based on the validated model, the typical solitary propagation process around the reef-fringed island and the effects of morphological and hydrodynamic parameters on the maximum run-up heights were systematically investigated. It is found that coral reefs can effectively reduce maximum run-up heights around an isolated island. The reef flat’s water depth, reef flat width, and reef surface roughness are the main factors affecting maximum run-up heights around an island, while the fore-reef slope has little impact. For the idealized reef-fringed island in this study, sea-level rise will cause coral reefs to lose their protective capability on the lee side, and the presence of coral reefs may even enhance tsunami hazards around an island when the reef flat width is very narrow or coral bleaching happens.

Characteristics of the jet shear layer of a round elevated jet in crossflow

The present research aims at an experimental study to investigate the effect of velocity ratio (R) on the evolution of elevated round jet issued from a stack having an aspect ratio (AR) of 9.0 in a uniform crossflow at a Reynolds number of 2000. The experiments are conducted in a low-speed, recirculating water tunnel. Laser Doppler Velocimetry (LDV) is employed to characterize the water tunnel. Particle Image Velocimetry (PIV) is used to measure the flow field, and the dye visualization technique is utilized to capture the accompanying instantaneous flow structures. Depending on the velocity ratio (R = 0.16 - 1.5), distinct jet shear layer (JSL) structures are observed in the symmetric plane (XY), confirmed by both the flow visualization and PIV data. These structures include clockwise vortices, anticlockwise vortices, swing-induced mushroom vortices, and jet-like vortices. For R<0.5, entrainment of a substantial amount of jet fluid into the stack-wake region (downwash) has been observed. At R=0.5, the streak image captured in the wall parallel plane reveals the ring-like vortices that appear to wrap around the jet core while upright vortices are detected downstream on the lee side of the jet. Additionally, variations in average velocity and turbulent fluctuations in the near field, contingent on different velocity ratios, are discussed.

Effect of bridge height on airflow and aeolian sand flux near surface along the Qinghai-Tibet Railway, China

In this work, we studied the near-surface flow field structure of railway bridges with different heights through field investigation and wind tunnel simulation experiments. Meanwhile, we simulated the distribution of sand accumulation around a bridge via CFD software based on the sand accumulation around the Basuoqu bridge in the Cuona Lake section of the Qinghai–Tibet Railway. Results show that the sand around this railway bridge is mainly from the lake sediment on the west side of the railway and the weathered detritus on the east side. The height of the railway bridge in the sandy area affects the distribution of the near-surface flow field and the variation in speed on both sides of the bridge. The wind speed trough on both sides of the 6 m high bridge is higher, and the horizontal distance between the wind speed trough and the bridge section is 1.5 times that of the 3 m high bridge. Wind speed attenuates in a certain range on the windward and leeward sides of the bridge, forming an aeolian area; under the beam body, it is affected by the narrow tube effect, forming a wind erosion area. The height of the bridge determines its sand transport capacity. Under certain wind conditions, the overhead area at the bottom of the 3 m high bridge and its two sides do not have the sand transport capacity, so sand accumulates easily. Nevertheless, the sand accumulation phenomenon gradually disappears with the increase in bridge clearance height. The objectives of this study are to reveal the formation mechanism of sand damage for railway bridges, provide theoretical support for the scientific design of railway bridges in sandy areas, and formulate reasonable railway sand prevention measures to ensure the safety of railway running, which have certain theoretical significance and practical value.

Temperature evolution and flame behavior inside a ceiling-vented compartment with different fire source locations under the effect of wind

This work presents an experimental investigation on ceiling-vented compartment fire behavior and thermal characteristics with different fire source locations under the effect of wind associated by CFD simulations. Experiments were carried out under the ambient wind condition, and the fire source was respectively set at windward side, center and leeward side. Results show that: (1) In no-wind cases, a critical heat release rate is found at which the inside temperature decreases with the increasing of fire heat release rate indicating flame flows outside. The critical HRR for fire source attached to sidewall is slightly larger than that when the fire source is located at compartment center, and it increases as vent dimension increases. (2) In ambient wind condition, there is a travelling process that the flame transfers from windward side to leeward side with the increasing of fire HRR. The critical HRR for the transition increases as vent size increases, and decreases as wind speed increases, and is basically independent of fire source location, which is proved to be related to air inflow rate. A Froude number is proposed to characterize the wind effect based on air inflow analysis, and the critical HRR for transition is represented well by the Froude number.

A uniformly convergent method for two-parameter singularly perturbed parabolic partial differential equations with a large shift

A class of singularly perturbed two-parameter parabolic partial differential equations with a large shift is studied. These equations include time-dependent variables and exhibit sensitivity to small perturbations in the system parameters. Moreover, the shift captures the influence of neighbouring states or events on the current evolution of the system. It adds memory-like behaviour to the problem, making their analysis and numerical solution more intricate. The solution of these equations exhibits a multiscale character. The interior and boundary layers exist across which the solution has a steep gradient. A higher-order accurate numerical method is proposed on a non-uniform mesh to solve the problem. The method combines an upwind difference scheme in space and a Crank-Nicolson scheme in time. The method is unconditionally stable, and parameters uniformly convergent with second-order accuracy in time and first-order accuracy in the space variable. Besides, the paper presents numerical results and illustrations to support theoretical estimates.