Upwind Research Articles

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.

Application Of Onboard PIV To Front Wheelhouse Wake Behavior Under Stationary Crosswind Conditions

When driving a car, the driver and passengers may feel larger drag from a headwind, the steering wheel would be taken away by a crosswind or pushed by a tailwind. Anytime, anywhere, we will encounter various scenes with natural wind under real world conditions such as urban, suburb and highway. We will develop the appropriate vehicle by simulating these conditions to satisfy with various customer requirements. For vehicle aerodynamic research and development, we are simply running simulations in physical- and cyber-space, that is, wind tunnel facility and computational fluid dynamics to reproduce these real worlds. However, it is not easy to simulate these wind inlet conditions, such as the temporal change in crosswind, especially, in wind tunnel facility. The stationary yaw angle by rotating turntable would reproduce a crosswind, if there is no turbulent generator system like in Pininfarina and Swing in FKFS. Therefore, we focus on the unsteady and asymmetrical flow from the front wheelhouse of the vehicle with tire rotation. There is three-dimensional complicated flow that are interacted by the flow from various directions, such as the flow along the front bumper, the flow along the rotating tire and wheel, the blowing flow from the rotating wheel opening area and the flow from the tire house. These complicated flow produces the front wheelhouse wake along the body side that affects the vehicle aerodynamic performance. Furthermore, there are asymmetrical flows around the vehicle by different front wheelhouse wake in windward and leeward sides. The larger side wake increases the aerodynamic drag, and the fluctuating side wake may induce the instability of the vehicle handling stability. Here, the proposed onboard 2D-3C PIV measurement system in the previous study enables us to measure same measurement area along the vehicle body side without change in optical configuration effectively, that indicates to reduce the time by resetting the optical configuration. We will investigate what kind of flow field should be focused on and controlled to prevent the vehicle instability by investigating these asymmetrical flow field around the vehicle under crosswinds.

Indian ocean warming, extreme positive Indian Ocean Dipole events, and their impact on monthly Indian Monsoon rainfall from June to November in NMME models

Indian Ocean (IO) warming, the frequency of extreme positive Indian Ocean Dipole (EPIOD) events, and its impact on Indian monsoon rainfall (IMR) are more prominent in the recent era. A proper understanding of the relationship between the EPIOD and IMR is very important because two thirds of the people in India are dependent on agriculture and related economy. The primary focus of this study is on three main aspects: (i) analysing the monthly trend of sea surface temperature (SST) in the IO; (ii) the spatial variability of SST anomalies (SSTA) linked to the EPIOD and its impact on the IMR; and (iii) the factor responsible for the intensification of the EPIOD. These aspects are thoroughly assessed using 12 NMME models, comparing them with observations and reanalysis datasets from 1990 to 2020. The analysis reveals a warming trend in the central IO during June and July, with significant increases observed from August to November. Over the past 31 years, the central IO has experienced an approximate rise of 0.9 to 1 °C from June to November. Among the 12 models scrutinised, GFDL_FLOR_A, FLOR_B, and RSMAS_CCSM4 closely align with observed spatial trends. In contrast, models such as GFDL_SPEAR and NASA_GEOSS tend to overestimate these spatial trends. An increase in easterlies over the equatorial eastern IO was identified as a possible driver for the rising SST in the central IO during October and November. In response to EPIOD on the monthly spatial rainfall distribution over India, all NMME models often underestimate rainfall over the west coast and exhibit wet biases over the leeward side of the Western Ghats. Grid-wise skill scores for accumulated monthly rainfall demonstrated variations among NMME models, with GFDL_SPEAR, RSMAS_CCSM4, and other IC3 models showing higher skill. The interhemispheric pressure gradient (IHPG) has been identified as a precursor to EPIOD events. A positive IHPG from March to June is correlated with EPIOD occurrences from October to December. It is observed that half of the NMME models are capable of simulating a positive relationship between IOD and IHPG, consistent with the observed data.

Increasingly high-order hybrid multi-resolution WENO schemes in multi-dimensions

In this paper, a new type of increasingly high-order hybrid multi-resolution weighted essentially non-oscillatory (HMR-WENO) schemes is presented in the finite difference framework for solving hyperbolic conservation laws in one, two, and three dimensions. Based on the reconstruction polynomials defined on the one-point, three-point, five-point, seven-point, and nine-point spatial stencils, we reconstruct one zeroth degree reconstruction polynomial, one quadratic reconstruction polynomial, one quartic reconstruction polynomial, one sextic reconstruction polynomial, one octave reconstruction polynomial, and their derivative polynomials together with a new hierarchical bisection method to design a series of new troubled cell indicators which can precisely find all extreme points of associated unequal degree reconstruction polynomials located inside the smallest interval in one dimension. The new troubled cell indicators do not introduce any manual parameters related to different problems. The new hybrid methodology is divided into two parts: if all extreme points of the reconstruction polynomials are nonexistent or outside the smallest interval, the target cell is not a troubled cell and the simple linear upwind schemes are utilized to obtain high-order approximations. Otherwise, the target cell is a troubled cell and the MR-WENO spatial reconstruction procedures with excellent shock-capture ability are adopted. Then a series of HMR-WENO schemes are proposed by using these new troubled cell indicators, which can be easily expanded to arbitrarily high-order accuracies in multi-dimensions. The main benefits of these HMR-WENO schemes are their efficiency, since they could save about 22%-75% CPU time than that of the same order MR-WENO schemes when simulating some benchmark examples in multi-dimensions.

Upwind Scheme Using Preconditioned Artificial Dissipation for Unsteady Gas-liquid Two-phase Flow and Its Application to Shock Tube Flow

Upwind Scheme Using Preconditioned Artificial Dissipation for Unsteady Gas-liquid Two-phase Flow and Its Application to Shock Tube Flow

Mitigation of crosswind effects on high-speed trains using vortex generators

Vortex generators can enhance the operational safety of high-speed trains and offer effective anti-rolling performance. This paper investigates the influence of vortex generator installation angles on the aerodynamic characteristics of trains. The Improved Delayed Detached Eddy Simulation method is used to analyze the leeward side vortex structure. It is found that when the angle between the vortex generators and the relative wind is 30°, the rolling moment of the train is minimized, as it significantly reduces side forces while preventing excessive growth of lift force inducing rolling moment. The reduction in rolling moment of the train by vortex generators is attributed to the suppression of leeward side trailing vortices of the train, which delays flow separation at the roof of the train, inducing a downward trend in the separated flow. Dynamic Mode Decomposition reveals that vortex generators do not alter the stability of near-body trailing vortices but enhance the pulsatile characteristics of far-body trailing vortices, which do not affect the pressure distribution on the leeward side of the train.

Angle-of-attack characteristics of opposing jet for improving drag and heat reduction

While the opposing jet technique has the potential to achieve efficient drag and heat reduction, it can be severely affected by the incoming angle of attack. To analyze the angle-of-attack characteristics of opposing jet for improving drag and heat reduction, a three-dimensional blunt model was studied under various jet stagnation pressure ratios and angles of attack using the verified numerical method. The results showed that the enhanced reattachment shock on the windward side resulted in a higher pressure and temperature rise, which led to the deterioration of drag and heat reduction. Under the influence of the incoming angle of attack, the recirculation vortex transformed into a longitudinal vortex, resulting in a slanted U-shaped distribution of the surface pressure coefficient and Stanton number. Increasing the jet stagnation pressure ratio widened the coverage of the recirculation vortex on both the windward and leeward sides, which brought an improvement in drag and heat reduction. The interaction between the incoming angle of attack and the opposing jet caused a double-peak distribution of Stanton number due to the recirculation vortex reattachment and the compression of the incoming flow. The inclined opposing jet could reduce the peak values of pressure coefficient and Stanton number when subjected to the incoming flow with an angle of attack by spreading the recirculation vortex along the windward side. There should exist an optimal inclination angle that can effectively reduce the peak caused by the compression of the incoming flow without generating an excessive peak due to the recirculation vortex reattachment.

Experimental study on a combined sand barrier: Integration of porous mesh plate and novel porous sand-fixing bricks

Wind and sand disasters frequently impact the newly constructed railroad along the Hongyi line in Qinghai Province's Qaidam Basin. The existing wind and sand prevention measures, primarily comprising polyethylene (PE) nets, have shown limited effectiveness. To this end, this study introduces an innovative combined sand barrier: A new type of porous sand fixing bricks (PSBs) are made from local aeolian sand, arranged in a square grid like low vertical sand barrier, and combined with a high vertical porous mesh plate (PMP) to form a combined sand barrier. Wind tunnel experiments were conducted to optimize the combined sand barrier's structure. The results indicate that incorporating porous sand-fixing brick sand barrier (PSBSB) on the leeward side of the PMP substantially impacts the near-surface flow field and plays a crucial role in decreasing wind velocity while enhancing sand control. The PSBSB with five rows of PSBs, positioned 18H from the PMP's leeward side, maximizes windproof and sand control efficiencies. Notably, the PSBs exhibit excellent self-stability under wind load, eliminating the need for additional fixing measures. This combined sand barrier presents a replicable, cost-effective solution for wind-blown sand disaster prevention in aeolian sand regions like the Hongyi line, promising advancements in sand barrier technology.

Effects of windbreak types on aerodynamics of high-speed trains traversing from flat ground to semi-cutting and semi-embankment under crosswinds

Under the operation of strong crosswinds, the aerodynamic performance of high-speed trains (HSTs) will be seriously deteriorated when the transition section of flat ground and semi-cutting and semi-embankment (FGSCSE) is traversed, and the setting of windbreaks will help to slow down the impact of strong crosswinds on the trains. In this study, a three-dimensional coupled computational fluid dynamics numerical model to assess the aerodynamic performance of train–windbreak–FGSCSE–air system is developed. A comparative assessment is carried out to identify the variations in aerodynamic performance on the train carriage: no windbreak (NW), 50% ventilation windbreak (VW), and solid windbreak (SW), and the reasons for these variations are elucidated by examining the flow field structure's evolution. Furthermore, the operational safety of the train is discussed based on the indicator of wheel unloading ratio (fΔQ). Across the three distinct conditions, significant abrupt changes in aerodynamic load coefficients (ALCs) and the shedding of vortex structures are experienced by HSTs traversing the FGSCSE transition sections. Compared to the VW condition, the NW and SW conditions result in a greater number of shedding vortices on the leeward side and the tail of the train, and the VW condition results in the smallest magnitude of ALCs fluctuation. The power spectral density peak values of the aerodynamic loads follow the order: SW &amp;gt; NW &amp;gt; VW. Upon the train fully enters the subsequent operational environment, the VW condition has the smallest standard deviation of these coefficients. The standard deviations of CFy, CFz, CMx, CMy, and CMz for the head train in the VW condition are only 57.17% (46.81%), 55.85% (54.15%), 72.74% (34.62%), 57.99% (51.92%), and 44.60% (43.82%) of the corresponding values in the NW (SW) condition, respectively. In the NW, VW, and SW conditions, the fΔQ exceeds 0.9 when the wind speeds reach 30, 40, and 35 m/s, respectively. The windbreak with a ventilation rate of 30% performs the best, providing the most effective safety and stability for train operation.

Impact of Islands on Tidally Dominated River Plumes: A High‐Resolution Modeling Study

AbstractWhen flow passes over topographic features such as headlands and islands, island wakes can arise at the lee side of the flow. Island wakes are associated with enhanced biological productivity, increased mixing, and water mass transformation. While previous studies have mainly focused on the dynamical and biological effects of island wakes in the open ocean, here we focus on a large tidally‐dominated estuary with numerous islands, aiming to investigate the impact of such wakes on the offshore transport of river plumes. To this end, we use numerical simulations with unprecedented grid resolution in the plume region and around the islands. Our study area is the Pearl River Estuary, a region where satellite images indicate that oscillating wakes occur in the lee and far downstream of the islands. We show that submesoscale island wakes are ubiquitous in the plume‐influenced region and can affect a large area around the islands as the tidal flow reverses. These strong vorticity tails correspond well with the horizontal patterns of salinity gradients and salinity mixing. Sensitivity experiments show that these flow disturbances will largely decrease after the hypothetical “removal” of the islands. Analysis based on an isohaline coordinate framework shows that the isohaline surface area is limited by the presence of islands. It is proven that this “limiting” effect of islands on the plume extension is related to the salinity mixing and the associated diahaline water exchange.

Crosswind effects on thermal performance improvement of mechanical draft cooling towers with deflector plates

In practical operations, crosswinds significantly impact the thermal performance of mechanical draft cooling towers (MDCTs). This paper investigated the beneficial impact of deflector plates to mitigate the adverse crosswind effects on the thermal performance of MDCTs. The mechanism by which deflector plates reduce the crosswind effects was elucidated from both the windward and leeward sides. Additionally, the performance optimization with varying crosswind angles and speeds was comprehensively analyzed. Results revealed that at lower crosswind speeds, deflector plates effectively streamline airflow on both windward and leeward sides. However, when wind speeds exceed 4 m/s, the vortex region on the windward side expands, reducing the effectiveness of airflow optimization by deflector plates. Nonetheless, deflector plates consistently mitigate crosswind effects on the leeward side. Within the investigated parameters, at a 0° crosswind angle and 1 m/s crosswind speed, deflector plates significantly enhance thermal performance, manifesting in a 0.362°C reduction in outlet water temperature, a 10.77% increase in Merkel number, and a 2.48% improvement in cooling efficiency. This research establishes a theoretical framework for optimizing thermal performance in MDCTs with crosswind effects and provides valuable insights for engineering applications.