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The flow characteristics around the frontal face of a square cylinder with a frontal jet injection were experimentally studied in a closed-loop wind tunnel using the PIV technique. Four characteristic flow modes were identified based on the injection ratio: swinging jet, deflected oscillating jet, deflection jet, and penetrating jet. At low injection ratio, the flow mode denoted as the swinging jet is characterised by two recirculation regions and a four-way saddle point. At moderate injection ratio, the flow mode termed deflected oscillating jet is characterised by a recirculation region and a counterclockwise rotating vortex within the recirculation region. At moderately high injection ratio, the flow mode is denoted as deflection jet, where a clockwise rotating vortex appears in the flow field. At high injection ratio, the flow mode is termed as penetrating jet, with no vortex formation in the flow field. The strength of the turbulence intensities increases considerably with high injection ratios. The time histories of instantaneous velocity for non-injection case, swinging jet, and deflected oscillating jet modes show periodic oscillations, whereas those of deflection jet and penetrating jet modes do not show periodic oscillations. The study presents and discusses the time-averaged velocity vectors, and streamlines, vorticity contours, velocity properties, velocity time histories, and power spectral density function under different flow modes. [ABSTRACT FROM AUTHOR]

This study aims to investigate the wake of two tandem square cylinders based on the Proper Orthogonal Decomposition (POD) analyses of the PIV and hotwire data. The cylinder centre-to-centre spacing ratio L/w examined is from 1.2 to 4.2, covering the four flow regimes, i.e., extended body, reattachment, transition and co-shedding. The Reynolds number examined was 1.3 × 104. A novel Proper Orthogonal Decomposition (POD) technique (hereafter referred to as PODHW) is developed to analyse data from single point hotwire measurements, offering a new perspective compared to the conventional POD analysis (PODPIV) based on Particle Image Velocimetry (PIV) data. A key finding is the identification of two distinct states, reattachment and co-shedding, within the transition flow regime at L/w = 2.8, which PODPIV fails to capture due to the limited duration of the PIV data obtained. This study confirms, for the first time, the existence of these states as proposed by Zhou et al. (2024), highlighting the advantage of using PODHW for capturing intermittent flow phenomena. Furthermore, the analysis reveals how the predominant coherent structures contribute to the total fluctuating velocity energy in each individual regime. Other aspects of the flow are also discussed, including the Strouhal numbers, the contribution to the total fluctuating energy of the flow from the first four POD modes, and a comparison between different regimes. [ABSTRACT FROM AUTHOR]

This study aims to numerically investigate the optimal conditions for fluid flow control around a single square cylinder with the help of a pair of attached flat plates. It is comparatively a new approach for controlling fluid flows as compared to the traditional solo plate flow control devices. The plates are attached adjacent to the both rear corners of the cylinder and their length (l) is varied from 0.1 to 4 times size of main cylinder while fixing the height (h) at 0.2. By varying the length, the plates manage to control the flow gradually. This study discusses how a steady wake can be achieved through control plates. Results indicate that the flow regime changes from unsteady to transitional at l=2.7 while for l>3.1 the steady flow appears. The streamlines visualizations reveal different flow structures termed as the oval-eye vortex, chain necklace vortex, sphere vortex, hair pin vortex and wooden eyes vortex-like structures. Among these the oval-eye vortex structure is found to have higher flow induced forces and shedding frequencies while the wooden eyes vortex structure is found to have minimal flow induced forces and shedding frequencies. After l=3.2, the plates’ efficacy is proven by a 100% reduction in Strouhal number, root-mean-square values of lift coefficient and amplitudes of lift and drag coefficients. This study reveals that l=3.2 is the best optimal value of plates length for complete wake and fluid forces control. [ABSTRACT FROM AUTHOR]

In the present study, the effect of taper on the lateral sides on fluid flow and heat transfer characteristics on a square cylinder has been analyzed numerically. Two-dimensional simulations have been carried out in the laminar flow regime (Re 100–800) for divergent blockage ratios (ϵ = H/D) and taper angles on the forward and backward sides of the lateral cylinder. Compared to the square cylinder, the forward tapered modification cylinders led to significant total drag reductions, pressure drop, and higher heat dissipation for low Re. The backward taper cylinder illustrates a minor increase in heat dissipation; however, the high drag coefficient and pressure drop penalty cannot be neglected. Moreover, an optimized geometry has been identified as a function of all the parameters. The reduction in blockage ratio has been determined to suppress the vortex shedding for higher Re completely. Results also indicate that the decrease in blockage ratio reduces the critical Re of the domain. The slightly tapered surfaces on either side of the square geometry significantly influence heat transfer and apply to modern industrial electronics and their cooling. [ABSTRACT FROM AUTHOR]

Airflow over a square cross-section at high Reynolds numbers and different angles of incidence is investigated with the aim of providing deeper insight into wind actions on elongated structures and, in particular, tall buildings. The flow around bluff bodies is characterized by separation at sharp corners, as well as possible flow reattachment at side surfaces. The alternate shedding of vortices is also generated in the wake of bluff bodies due to the unsteady nature of flow separation. Two-dimensional (2D) URANS numerical simulations were conducted in order to model transient flow and examine wind actions on a square used as a model of a typical cross-section of a tall building far from its roof and the ground. For validation purposes, the study's numerical results on drag and lift coefficients, Strouhal numbers, as well as pressure coefficient distribution were found to be in good agreement with available experimental and numerical results in the literature for relatively low Reynolds numbers. The numerical study was then extended to higher Reynolds numbers, approaching values that are pertinent for wind flow around buildings, thus addressing the lack of such results in the literature. On the basis of these results, the impact of Reynolds numbers and angles of incidence on drag and lift coefficients, as well as the pressure coefficient distribution along the walls of the cross-section, is highlighted. [ABSTRACT FROM AUTHOR]

The flow around a square cylinder with a synthetic jet positioned at the rear surface is numerically investigated with unsteady Reynolds-averaged Navier–Stokes (URANS) method. Different exciting signals are utilized to generate the synthetic jets, including the square signal, triangle signal, sinusoidal signal and the varying duty-cycle signal. The wake vortices evolution and the frequency characteristic are analyzed for all the cases. In the case with the square signal, the vortex shedding pattern is not essentially changed due to the deficiency of the synthetic jet strength. The synchronization at half of the exciting frequency is observed for both of the sinusoidal and triangle signal, where the alternative deflection of the jet vortex pairs is detected correspondingly. The complete synchronization is discovered in the case with the duty-cycle k = 2 signal, where the wake is symmetrical and totally controlled by the synthetic jets. The drag coefficient is calculated to contrast the control efficiency of different signals. The most satisfactory control performance is achieved in the case with the duty cycle k = 2 signal as well, where a 33.71 % of drag reduction is realized compared to that without control. [ABSTRACT FROM AUTHOR]

In this work, we present a grid study oriented to capture 3D flow simulations around smooth and wrinkled cylinders that could have practical applications in various engineering areas. The study considers three Reynolds numbers, namely, a benchmark Re = 2.14 × 10 4 and two orders of magnitude above and below it. The main contributions of the paper relate to the optimization of the computational mesh for the spanwise direction of the wind flow that results from the computational-mathematical framework employed, in addition to a novel visualization technique that unfolds features in the recording data that could otherwise be hidden when using traditional plots. We compare our benchmark results with those reported by other authors to conclude that the intermediate resolution grids employed with the widest spanwise provide acceptable results. Furthermore, the new visualization technique offers significant advantages compared to traditional pressure maps, regarding clarity for observing and interpreting local flow disturbances, making variations with Re clearer, and by enabling the detection of asymmetries. [ABSTRACT FROM AUTHOR]

This study aims to investigate the wake of two tandem square cylinders based on the Proper Orthogonal Decomposition (POD) analyses of the PIV and hotwire data. The cylinder centre-to-centre spacing ratio L/w examined is from 1.2 to 4.2, covering the four flow regimes, i.e., extended body, reattachment, transition and co-shedding. The Reynolds number examined was 1.3 × 104. A novel Proper Orthogonal Decomposition (POD) technique (hereafter referred to as PODHW) is developed to analyse data from single point hotwire measurements, offering a new perspective compared to the conventional POD analysis (PODPIV) based on Particle Image Velocimetry (PIV) data. A key finding is the identification of two distinct states, reattachment and co-shedding, within the transition flow regime at L/w = 2.8, which PODPIV fails to capture due to the limited duration of the PIV data obtained. This study confirms, for the first time, the existence of these states as proposed by Zhou et al. (2024), highlighting the advantage of using PODHW for capturing intermittent flow phenomena. Furthermore, the analysis reveals how the predominant coherent structures contribute to the total fluctuating velocity energy in each individual regime. Other aspects of the flow are also discussed, including the Strouhal numbers, the contribution to the total fluctuating energy of the flow from the first four POD modes, and a comparison between different regimes.

The characteristics of forced convection heat transfer across a row of heated square cylinders kept in side‐by‐side arrangement are numerically investigated to examine the combined effects of Reynolds number and cylinder spacing for Ri = 0, 60 ≤ Re ≤ 160, Pr =.71, and s/d = 1.0–8.0, where the space between cylinder surfaces is s and the cylinder size is d. A numerical study was carried out using the thermal lattice Boltzmann method. The goal of this work is to explore the transitions in heat transfer phenomenon that occurs behind the cylinder and to report the corresponding regimes for heat transfer namely synchronous, quasiperiodic, and chaotic. The proposed regime of heat flow is a function of Reynolds number and spacing. The synchronous heat regime is obtained for s/d ≥ 5.0 and quasiperiodic, chaotic regimes are observed for 3.0 ≤ s/d < 5.0, s/d < 3.0, respectively at Re = 100. The instantaneous isotherms, the power spectra of the corresponding Nusselt number signals, and the significance of cylinder Nusselt number frequency are used to examine these heat flow regimes. The heat transfer regimes for a row of heated cylinders and flow regimes for a row of unheated cylinders both have comparable appearances except for the fact that the heat transfer regime is synchronous at s/d ≥ 5.0 and flow is synchronous at s/d ≥ 4.0. The chaotic or quasiperiodic heat transfer regimes occur due to merging and strong interactions between thermal blobs shed from the cylinders. Heat transfer is synchronous at a higher spacing and characterized by independent thermal blobs shedded from the cylinders. It is reported that as spacing reduces and Reynolds number increases, the mean value of the Nusselt number experienced by all cylinders increases. The important outcome of the present numerical work is that for understanding heat transfer from bluff body, the transitions that occur in heat transfer are useful. [ABSTRACT FROM AUTHOR]

The main focus of the current paper is the detection of vortices in fluid flow around a circular cylinder and a square cylinder, with an emphasis on the identification of the parameters used for vortex detection. The authors aim to enhance the practicality of an existing vortex detection method (Lagrangian-averaged vorticity deviation) by providing recommendations for the settings of the vortex detection parameters. The simulations were carried out using ANSYS Workbench 2022 R2, encompassing Reynolds numbers between 12 and 140, and angles of incidence from 0° to 45°. The vortex detection was performed using MATLAB R2020b. The paper provides a comprehensive description of the parameters involved in the detection process and their significance, as well as the implementation of the parameter identification. The study results in the determination of the suggested parameter ranges, and a comparative analysis of different vortex detection methods is also presented for the case of the circular cylinder. [ABSTRACT FROM AUTHOR]

The objective of this research is to study the fluid flow control allowing the reduction of aerodynamic drag around a square cylinder using two parallel partitions placed downstream of the cylinder using the lattice Boltzmann method with multiple relaxation times (MRT-LBM). In contrast to several existing investigations in the literature that study either the effect of position or the effect of length of a single horizontal or vertical plate, this work presents a numerical study on the effect of Reynolds number (Re), horizontal position (g), vertical position (a), and length (Lp) of the two control partitions. Therefore, this work will be considered as an assembly of several results presented in a single work. Indeed, the Reynolds numbers are selected from 20 to 300, the gap spacing (0 ≤ g ≤ 13), the vertical positions (0 ≤ a ≤ 0.8d), and the lengths of partitions (1d ≤ Lp ≤ 5d). To identify the different changes appearing in the flow and forces, we have conducted in this study a detailed analysis of velocity contours, lift and drag coefficients, and the root-mean-square value of the lift coefficient. The obtained results revealed three different flow regimes as the gap spacing was varied. Namely, the extended body regime for 0 ≤ g ≤ 3.9, the attachment flow regime for 4 ≤ g ≤ 5.5, and the completely developed flow regime for 6 ≤ g ≤ 13. A maximal percentage reduction in drag coefficient equal to 12.5%, is given at the critical gap spacing (gcr = 3.9). Also, at the length of the critical partition (Lpcr = 3d), a Cd reduction percentage of 12.95% was found in comparison with the case without control. Moreover, the position of the optimal partition was found to be equal to 0.8d i.e. one is placed on the top edge of the square cylinder and the second one is placed on the bottom edge. The maximum value of the lift coefficient is reached for a plate length Lp = 2d when the plates are placed at a distance g = 4. On the other hand, this coefficient has almost the same mean value for all spacings between the two plates. Similarly, the root means the square value of the lift coefficient (Clrms) admits zero values for low Reynolds numbers and then increases slightly until it reaches its maximum for Re = 300.

In this study, a novel flow control scheme characterized by active suction through the structured porous surfaces on the side-walls of a square cylinder is introduced and experimentally investigated to control the shear-layer separation and dynamic wake flows. The Reynolds number (Re) is about 1.05 × 104, given the incoming flow speed and diameter of the square cylinder test model. 55 pores are regularly arranged on the top and bottom side-walls of the square cylinder to form communicating channels for the distributed suction flow. In the wind tunnel experiment, a high-speed particle image velocimetry (PIV) system is used to measure the flow field, and then a proper orthogonal decomposition (POD) method is adopted to obtain dynamic patterns of the wake flow. The experimental results demonstrated that the control effects are closely related with the dimensionless suction momentum coefficient C μ. As C μ increases to 0.0197 and 0.0338, the unsteady wake of the square cylinder is significantly attenuated and the process of flow separation is notably restrained. It is shown that distributed suction through side-walls effectively controls the unsteady shear-layer separation and wake flows of the square cylinder. [ABSTRACT FROM AUTHOR]

The aerodynamic behaviour and wake flow of a cluster of two-dimensional sharp-edged bluff bodies exhibits extremely complex unsteady phenomena in both near and far fields. Due to the high cost of wind-tunnel experiments and numerical simulations, a complete understanding of wake flows and a description of their characteristics are lacking. This paper presents large-eddy simulations (LES) in different flow/wind directions for a cluster of 2 × 2 aligned square cylinders, at a separation distance in streamwise and cross-wind directions equal to cylinder side length, and at Reynolds number Re = 22 , 000 based on the single cylinder side length D. The case at 0 ∘ incidence shows an evident channel-type flow in the along-wind street/gap, and at its exit an irregularly pulsing jet with an intense shedding of large vortices. The wavelet analyses of the side force/lift coefficient and instantaneous velocities in the wake show that the characteristic length and time scales of the large vortical structures in the far-field wake are close to the cluster size 2D; this is the so called 'cluster effect'. The cluster effect increases monotonically as the flow incidence angle increases. At a large incidence angle in the near-field wake, the cylinder-scale flow structures are much weaker compared to the cluster-scale structures. At the incidence angle of 45 ∘ , the overall wake flow and the aerodynamic characteristics are well scaled by the scale approximately equal to 2D. Nevertheless, the interaction between cylinders significantly affects the aerodynamics performance of the individual cylinders. The drag and lift coefficients of the individual cylinders differ substantially from each other in the cluster, and are significantly different from observations on a single isolated cylinder too. [ABSTRACT FROM AUTHOR]

Airflow over a square cross-section at high Reynolds numbers and different angles of incidence is investigated with the aim of providing deeper insight into wind actions on elongated structures and, in particular, tall buildings. The flow around bluff bodies is characterized by separation at sharp corners, as well as possible flow reattachment at side surfaces. The alternate shedding of vortices is also generated in the wake of bluff bodies due to the unsteady nature of flow separation. Two-dimensional (2D) URANS numerical simulations were conducted in order to model transient flow and examine wind actions on a square used as a model of a typical cross-section of a tall building far from its roof and the ground. For validation purposes, the study’s numerical results on drag and lift coefficients, Strouhal numbers, as well as pressure coefficient distribution were found to be in good agreement with available experimental and numerical results in the literature for relatively low Reynolds numbers. The numerical study was then extended to higher Reynolds numbers, approaching values that are pertinent for wind flow around buildings, thus addressing the lack of such results in the literature. On the basis of these results, the impact of Reynolds numbers and angles of incidence on drag and lift coefficients, as well as the pressure coefficient distribution along the walls of the cross-section, is highlighted.

In this work, we present a grid study oriented to capture 3D flow simulations around smooth and wrinkled cylinders that could have practical applications in various engineering areas. The study considers three Reynolds numbers, namely, a benchmark Re =2.14×104 and two orders of magnitude above and below it. The main contributions of the paper relate to the optimization of the computational mesh for the spanwise direction of the wind flow that results from the computational-mathematical framework employed, in addition to a novel visualization technique that unfolds features in the recording data that could otherwise be hidden when using traditional plots. We compare our benchmark results with those reported by other authors to conclude that the intermediate resolution grids employed with the widest spanwise provide acceptable results. Furthermore, the new visualization technique offers significant advantages compared to traditional pressure maps, regarding clarity for observing and interpreting local flow disturbances, making variations with Re clearer, and by enabling the detection of asymmetries.

The effects of the synthetic jet momentum coefficient (C μ) and excitation frequency ( f e *) on the flow characteristics around a square cylinder at an incident angle of 0° are systematically studied. The synthetic jet is deployed at the front stagnation and separation points (θ = 0° and 45°, respectively). Numerical studies based on the large eddy simulation (LES) method are carried out at a Reynolds number of 2.2 × 104. Subsequently, the suppression effects on the time-averaged drag and fluctuating lift forces at two jet exit angles are compared. For θ = 0°, the time-averaged drag coefficient appears to be more sensitive to C μ than to f e * , and the maximum reduction (98.3%) is achieved at C μ = 0.5. At θ = 45°, even a lower C μ has a significant suppressive effect on the fluctuating lift coefficient, and the maximum reduction (95.2%) occurs at C μ = 0.1 and f e * = 2. Furthermore, the frequency characteristics, stress distribution, time-averaged flow, and flow structure evolution are discussed to understand the mechanism of the optimal reduction at θ = 45°. The results reveal that symmetrical shedding of the exterior away from the centreline and asymmetrical shedding of the interior near the centreline can effectively mitigate the fluctuating lift force. The synthetic jet momentum coefficient determines whether the synthetic jet vortices can penetrate the shear layers and completely suppress wake vortex shedding, whereas the excitation frequency ensures that the synthetic jet vortices maintain their structures downstream of the convection. • The effects of synthetic jet on the flow around a square cylinder at subcritical Reynolds number were numerically studied. • The fluctuating lift coefficient achieves maximum reduction (95.2%) when the jet exit angle is 45°. • Symmetrical outside shedding and asymmetrical inside shedding mode may result in the reduction in fluctuating lift force. • The momentum coefficient determines that the jet vortices penetrate shear layers and completely suppress wake vortex shedding. • The excitation frequency ensures that the vortices maintain their structures downstream of the convection. [ABSTRACT FROM AUTHOR]

Large-Eddy Simulations were conducted to investigate accelerated flows around a square cylinder. The study explores flow characteristics for Gaussian-type accelerations of the inflow within the range of Reynolds numbers Re = 1. 720 × 1 0 4 to Re = 6. 536 × 1 0 4 . Three different inflow acceleration intensities within the same range of Re have been considered. For all analyzed acceleration values, time cells with a constant frequency in vortex shedding were identified in agreement with the experimental findings of Brusco et al. (2022a). The temporal behavior of the vorticity field indicates that vortex packets are shed at a constant frequency within these cells. Moving from one cell to another, there is a discontinuity in the vortex-shedding mechanism, leading to an unsteady but more symmetrical wake and a reduction in crossflow-force fluctuations. Subsequently, vortex shedding restarts at a higher frequency. As acceleration intensity increases, the time length of the constant-frequency time cells decreases. A similar behavior of the Strouhal number with the Reynolds number is obtained for all acceleration intensities. • Large-Eddy Simulations to investigate accelerated flows around a square cylinder. • Time cells with a constant frequency in vortex shedding. • Discontinuities in the vortex-shedding mechanism occur when moving from one cell to another. • Remarkable variation of the vortex-shedding Strouhal number during flow acceleration. • The Strouhal number has a similar behavior with Reynolds for all acceleration intensities. [ABSTRACT FROM AUTHOR]

This study investigates the unsteady incompressible flow around a square cylinder with different chamfer ratios (CRs) using a commercial finite volume code, ANSYS Fluent. CR ranges from 0.0 (sharp square cylinder) to 0.5 (diamond cylinder) with variable increments. Detailed analysis of flow characteristics is conducted at Reynolds number (Re) = 2100. Additionally, simulation is extended to cover Re, i.e., Re = 100, 500, and 10000. The simulation results show that cylinder with CR = 0.1 outperforms all other cases by enabling a drag reduction of about 60 % at Re = 104. Drag has an inverse relationship with the wake closure length. Time-averaged coefficient of pressure, streamlines, and vorticity contours are also discussed to better understand near-wake features and the physics of drag reduction. [ABSTRACT FROM AUTHOR]

Corner modifications can reduce wind loads acting on supertall buildings and modify the corresponding flow structures. The present study investigated the aerodynamic shape optimization of the corner recession square cylinders with multiple geometric parameters in a large design space via the GA-GRNN surrogate model updating-based multi-objective optimization framework. Six typical optimal aerodynamic shape sections M1~M6 were selected from the Pareto optimal front, and the effects of multiple geometric parameters of these sections on the aerodynamic performance and flow field were analyzed. The results showed that the present multi-objective optimization framework can significantly reduce the computational load and time cost, and significantly improve the optimization efficiency in solving complex engineering problems. The optimal corner recession sections can obviously reduce the mean drag coefficient CD and root mean square lift coefficient CσL while significantly increasing the Strouhal number St of the square cylinder, and it is concluded that the aerodynamic shape optimization can significantly improve the aerodynamic performance of square-sectional supertall buildings. When compared with the benchmark section, the CD and CσL of the optimal section M1 can be reduced up to 45.7% and 84.5%, respectively. Based on the analysis of the flow structures around the optimal sections, the flow mechanism can be attributed to the fact that the corner recession modifications postpone the flow separation, and deflect the separated shear layer towards the side surfaces and suppress the development of vortex shedding in the wake, which leads to significant elongation of the wake length and reduction of the width of the recirculation region. The proposed multi-objective optimization framework in this study can provide an important reference for the aerodynamic shape optimization of building structures and relevant studies. [ABSTRACT FROM AUTHOR]

The present study experimentally investigates the aerodynamic noise from the flow around cylinders of square and equilateral triangle cross-sections at different angles of incidence (α). The cylinder models have a side dimension of 10 mm and a span of 300 mm. The free stream velocity (U 0 ) is in the range of 12–36 m/s, and the corresponding Reynolds numbers are 7.8 × 103 to 2.3 × 104, which is in the subcritical flow regime. The characteristic acoustic tones are generated at α = 30° and 45° for square and triangular cylinders. The frequency of acoustic tones linearly increases with the free stream velocity, and the corresponding Strouhal numbers are found to be in the range of 0.13–0.16. Depending on the angle of incidence, the overall sound pressure level is higher than the background noise by 4–24 dB for the square cylinder and 3–15 dB for the triangular cylinder at U 0 = 36 m/s. The highest noise level of the square cylinder is 90 dB at α = 45° and 79 dB at α = 30° for the triangular cylinder. The spectral scaling with the sixth power of the free stream velocity indicates the dipole behaviour of the acoustic tones. The mean and root-mean-square velocity profiles in the wake region characterise the noise emissions at different angles of incidence. The comparative acoustic study of the non-circular cylinders with a circular counterpart showed that the highest noise level is from the square cylinder at α = 45°. The directivity study shows that the noise level of the square cylinder at α = 45° at 90° angular location (θ) is higher by 6.5 dB than that at θ = 30°. [ABSTRACT FROM AUTHOR]

The objective of this research is to study the fluid flow control allowing the reduction of aerodynamic drag around a square cylinder using two parallel partitions placed downstream of the cylinder using the lattice Boltzmann method with multiple relaxation times (MRT-LBM). In contrast to several existing investigations in the literature that study either the effect of position or the effect of length of a single horizontal or vertical plate, this work presents a numerical study on the effect of Reynolds number (Re), horizontal position (g), vertical position (a), and length (Lp) of the two control partitions. Therefore, this work will be considered as an assembly of several results presented in a single work. Indeed, the Reynolds numbers are selected from 20 to 300, the gap spacing (0 ≤ g ≤ 13), the vertical positions (0 ≤ a ≤ 0.8d), and the lengths of partitions (1d ≤ Lp ≤ 5d). To identify the different changes appearing in the flow and forces, we have conducted in this study a detailed analysis of velocity contours, lift and drag coefficients, and the root-mean-square value of the lift coefficient. The obtained results revealed three different flow regimes as the gap spacing was varied. Namely, the extended body regime for 0 ≤ g ≤ 3.9, the attachment flow regime for 4 ≤ g ≤ 5.5, and the completely developed flow regime for 6 ≤ g ≤ 13. A maximal percentage reduction in drag coefficient equal to 12.5%, is given at the critical gap spacing (gcr = 3.9). Also, at the length of the critical partition (Lpcr = 3d), a Cd reduction percentage of 12.95% was found in comparison with the case without control. Moreover, the position of the optimal partition was found to be equal to 0.8d i.e. one is placed on the top edge of the square cylinder and the second one is placed on the bottom edge. The maximum value of the lift coefficient is reached for a plate length Lp = 2d when the plates are placed at a distance g = 4. On the other hand, this coefficient has almost the same mean value for all spacings between the two plates. Similarly, the root means the square value of the lift coefficient (Clrms) admits zero values for low Reynolds numbers and then increases slightly until it reaches its maximum for Re = 300. [ABSTRACT FROM AUTHOR]

The flow past a square cylinder under the influence of a one dimensional gust was investigated using computational fluid dynamics (CFD). The effect of upstream wind gusts of the same amplitude but different duration was investigated with respect to their effect on the flow, the vortex-shedding, and the pressure distribution around the square cylinder. For the computations, a very large eddy simulation (VLES) model was implemented in an in-house code and validated against numerical and experimental results from the literature. The gusts of different duration were found to have a distinctly different effect. The short-duration gust causes a lock-on behavior with cessation of the alternating vortex shedding, and a symmetric pair-vortex was created above and below the square cylinder. It was observed that the pressure distribution on the lateral sides of the cylinder has the same magnitude and phase, which resulted in a zero total lift coefficient. In terms of a free-standing structures, such as a building, this would lead to zero instantaneous forces and pressure difference in the lateral direction with obvious implications for dynamic response and cross ventilation. [ABSTRACT FROM AUTHOR]

Rectangular cylinders are recurrent in engineering, a particular case is a square. Structures with square shapes are of great interest and their real applications can be seen in columns, towers, counterwind structures of tall buildings, etc. The present work analyses, in bidimensional domain, the external incompressible flow over a cylinder by turbulence models SST-kω and RSM through ANSYS Fluent. In the studied cases the objective is to analyze comparatively relevant parameters of the flow: drag coefficient and Strouhal number. The average pressure coefficient over the cylinder is also evaluated. In all cases analyzed the flow parameters converged to the experimental data. The average pressure coefficient over the cylinder did not present good correlation with the experiments for both turbulence models. The main differences are in the cylinder faces parallel to the flow. The analysis shows that RSM model was able to reproduce accurately the flow parameters. However, this model has a greater computational cost since it adds more equations to be solved and has performed worse than SST-kω model. [ABSTRACT FROM AUTHOR]

The current manifestation is utilized to explicate the inspiration of combined convection flows in a cavity with a square cylinder placed at the center of the cavity having coordinates (0.5, 0.5). To be more specific, right- and left-sided vertical walls are kept cold, and the lower wall is to be heated in uniform and non-uniform manners, while the upper horizontal wall is moved with a constant velocity ULid and is thermally adiabatic. The obstacle is treated as cold as well as thermally adiabatic, and the no-slip velocity boundary conditions are specified at its surface. For the purpose of computing the velocity profile and temperature, a space including the quadratic polynomials (P2) is chosen; however, the pressure has been approximated using a linear ( P1 ) finite element space of functions. The Newton technique is used to perform the computations needed to solve the discrete systems of non-linear algebraic equations. The non-linear iterations are terminated at residual below 10–6, whereas the inner core linear solver is based on Gaussian elimination with special reordering of unknowns. To show the consistency of the implemented numerical technique, the parametric study is designed based on the most relevant non-dimensional parameters, namely, the Grashof number Gr ranging from 102 to 105, the Hartmann number Ha changing from 0 to 50, and the Reynolds number Re varying from 10 to 200. Computations in the forms of velocity streamlines and isotherm contour profiles are adorned. In addition, the production of temperature disparities is associated with an increase in Nuavg as Re increases. In contrast, a diminishing aptitude in kinetic energy is observed due to the creation of Lorentz forces.

Square cylinders are widely used in various fields. For example, they are common structures in fishways. The flow around square cylinders has been a common problem in various fields. However, reducing the flow drag of the square cylinder is a problem that remains unexplored. Many previous studies have reported the drag reduction of 2D square cylinders, which failed to reflect the drag of real structures. Also, some studies focus on the drag force of the inner wall of the square cylinder modified by the microstructure. Achieving drag reduction by microstructuring the surface of the 3D square cylinder is a challenging problem. This study applied a 3D numerical simulation and deep neural network to study the drag reduction performance of the square cylinder under different patch sizes. We studied the drag reduction performance of protrusion and pit-patched square cylinders and tried to find the rule between drag reduction performance and patch configuration. The results show that the square cylinder has better drag reduction performance in some cases. However, its drag reduction performance is greatly affected by the protrusion structure. Also, too large protrusions will increase the drag force of the structure. When the surface protrusion accounts for 10% of the total area of the square cylinder, the drag reduction performance is the best (22.1%). The pit patch structure demonstrated an insignificant drag reduction performance and even increased the drag in most cases. The DNN prediction model demonstrated the robustness of the numerical simulation data.

The transonic flow around a square cylinder at Ma= 0.71 and Re= 4×105 has been studied by using the scale-adaptive simulation (SAS) method, and the characteristics of separated shear layer and wake have been analyzed in depth. To validate the SAS approach, the SAS results are compared with the existing numerical and experimental results. In the present transonic flow, the convective Mach number inside the shear layer is about 0.6. This indicates that the initial evolution of the separated shear layer is dominated by Kelvin-Helmholtz instability, and the roller spanwise eddies in the initial stage of the shear layer can be observed. In the regions near the shear layer and the wake, the doubling frequencies can be obtained indicative of the harmonic phenomenon inside the separated shear layer, which is closely related to the obvious merging of the vortices in the shear layer. Proper orthogonal decomposition of the pressure field shows that the transonic flow field of square cylinder is dominated by the antisymmetric mode, which is associated with the vortex shedding in the wake and the propagation of compression waves induced by the shear layer.

• Peak frequency of the Kármán vortex shedding decreases with the square cylinder height, similar to an empirical equation for the Strouhal number. • The peak PSD value behind the shortest square cylinder was found to be twice as high as those behind the other cylinders. • The spatial peak coherence distribution at the tallest cylinder suggests vertical movement of the Kármán vortex shedding. The pressure fluctuation distribution on the floor surface behind a supported square cylinder in a turbulent boundary layer was measured by using pressure-sensitive paint (PSP). Specifically, the accuracy and frequency response of PSP at a Mach number around M = 0.3 were examined. Four square cylinders with different dimensions were investigated in the same turbulent boundary layer, and the detailed relationship between these conditions and Kármán vortex shedding structures was discussed. The values measured with PSP had a similar trend to those measured with a pressure transducer at up to 5 kHz. The peak power spectral density (PSD) of the pressure fluctuations due to Kármán vortex shedding was observed within an error of approximately 30 % at up to frequencies greater than 3 kHz. Moreover, the peak frequency of Kármán vortex shedding was found to decrease with the cylinder height in a manner similar to a previous empirical equation for the Strouhal number. The peak PSD value for the shortest square cylinder (h / w = 3.5, h / δ = 1.1) was twice as high as that for the other cylinders; also, the high-pressure fluctuation area in this case did not spread downstream, which suggests that the flow from the top of the cylinder affected the Kármán vortex shedding generated from the cylinder's sides. One contour of the two main modes extracted for the highest PSD via the right singular vector v i at the Kármán vortex frequency had an asymmetric distribution behind the supported taller square cylinders (h / w ≥ 7.0, h / δ ≥ 2.3). This result is considered to be derived from the Kármán vortex shedding being mainly generated from the mainstream rather than the boundary-layer flow. [ABSTRACT FROM AUTHOR]

It is of significant importance to reconstruct the high-dimensional global flow field using limited sensor data, considering that sensors are often local and limited in full-scale measurements. In the literature, the reconstruction of laminar flows at low Reynolds number (Re) was mostly investigated. This study focuses on high-Re turbulent flows around a body, and aims to systematically investigate the reconstruction performance of deep learning models, including multilayer perceptron (MLP), convolutional neural network (CNN), and generative adversarial neural network (GAN). The extensive assessment encompasses the prediction accuracy of the instantaneous, phase-averaged, and mean flow fields, and the training efficiency. Results shows that GAN demonstrates a obvious advantage in reconstructing small-scale vortex structures. MLP is slightly better at reconstructing time-averaged flow fields (3%) and has a significant advantage in reconstruction efficiency. We also examine the generalization ability by applying transfer learning at different Re. Additionally, sensitivity of sensor number, training data size, and optimization of sensor layout are studied to further augment the accuracy of flow field reconstruction. • The turbulent flow at high Reynolds number is reconstructed using limit sensors. • The pros and cons of deep learning methods are clarified. • GAN model has advantage in reconstructing small-scale vortex structures. [ABSTRACT FROM AUTHOR]

In this paper, the effect of the dual splitter plates on the fluid flow and heat transfer characteristics around a regular square cylinder for a low Reynolds number flow (Re = 100) is presented. The placement of the dual splitter plates is novel of its kind as these plates are located at the top and bottom surfaces of the cylinder rather than the conventional locations, that is, at the upstream and downstream of the cylinder. Here, two splitter plates of the same width (w) with varying lengths and location are considered. A numerical investigation is performed using the open-source code, OpenFOAM. A base solver, icoFOAM is used after modifying the code by incorporating the energy equation in it. The primary wake bubble is found closer to the cylinder rear surface when the dual plates are introduced. It is also noticed that the separation angle and the recirculation length are smaller in the dual plates cases than that are in the cases without the dual plates. A mixed effect of the dual plates on the fluid forces is observed in the present study. A maximum reduction on the mean drag coefficient and root mean square of the lift coefficient is found as 3% and 24%, and maximum increment as 75% and 87%, respectively. However, a substantial enhancement on the overall heat transfer is noticed with the dual plates compared to that of the bare cylinder. A maximum enhancement of 40% is observed in the heat transfer around the square cylinder. In addition, thermal-hydraulic performance is calculated for finding the trade-off between the fluid forces and the heat transfer. The maximum value of thermal-hydraulic performance is found as 1.35 in the present study depending on the mean drag coefficient and 3.65 depending on the root mean square of the lift coefficient. Further, a novel combined thermo-fluid regime is defined for the square cylinder with dual splitter plates from which the location of the plates can be determined according to the demand on the heat transfer and fluid forces. [ABSTRACT FROM AUTHOR]

A preliminary study is carried out to investigate the aerodynamic characteristics of a square cylinder with Savonius wind turbines and to explain the reason why this kind of structure can suppress wind-induced vibrations. A series of computational fluid dynamics simulations are performed for the square cylinders with stationary and rotating wind turbines at the cylinder corners. The turbine orientation and the turbine rotation speed are two key factors that affect aerodynamic characteristics of the cylinder for the stationary and rotating turbine cases, respectively. The numerical simulation results show that the presence of either the stationary or rotating wind turbines has a significant effect on wind forces acting on the square cylinder. For the stationary wind turbine cases, the mean drag and fluctuating lift coefficients decrease by 37.7% and 90.7%, respectively, when the turbine orientation angle is 45°. For the rotating wind turbine cases, the mean drag and fluctuating lift coefficients decrease by 34.2% and 86.0%, respectively, when the rotation speed is 0.2 times of vortex shedding frequency. Wind turbines installed at the corners of the square cylinder not only enhance structural safety but also exploit wind energy simultaneously. [ABSTRACT FROM AUTHOR]

A channel confined flow around a bluff object is significantly different from its unconfined counterpart. A confined flow is presumably more stabilized due to the presence of the confinement. This causes a delay in the onset of flow separation and vortex shedding in comparison to the unconfined flow. While the above facts have been reported for the confined flow around a circular object in a channel, the same for a square object is infrequent. The present study focuses on the steady flow past a stationary square cylinder placed in a channel at low Reynolds numbers 1 ≤ Re ≤ 30 . The behavior of the flow is observed through numerical simulation for various channel confinements ( 0.01 ≤ B ≤ 0.9 , blockage ratio). The steady state laminar two-dimensional incompressible flow equations are solved using a finite volume based technique. An effort is made to find the separation critical Reynolds number at all blockages. It is observed that the separation critical Reynolds number increases with an increase in the blockage ratio. Unsteady flow characteristics at various blockages are also studied at Re = 100. In the unsteady regime, the frequency of vortex shedding increases with an increase in the blockage. [ABSTRACT FROM AUTHOR]

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This work numerically investigates the effects of combined rotational and transverse oscillations of a square cylinder on the flow field and force coefficients. The primary non-dimensional parameters that were varied are frequency ratio fR (0.5, 0.8), Re (50-200), phase difference (ϕ) between the motions and rotational amplitude (θ0) with the influence of the last three parameters being discussed in detail. The amplitude of transverse oscillations is fixed at 0.2D, where D is the cylinder width. The study has been validated using the mean drag coefficient for stationary and transversely oscillating square cylinders from literature. Output data was obtained in the form of force coefficient (Cd), vorticity and pressure contours. The governing equations for the 2dimensional model were solved from an inertial frame of reference (overset meshing) using finite volume method. The interplay between the convective field and prescribed motion has been used to explain many of the results obtained. The relative dominance of cylinder motion over the flow stream was determined using Discrete Fast Fourier Transform. The influence of Re on Cd disappears when the motions are completely out of phase (ϕ = π). In general, the Cd for low Re flows exhibited low sensitivity to change in other parameters. Direct correlation has been observed between frontal area, vortex patterns and drag coefficient

Flow structures around a wavy square cylinder with a perturbation wavelength of 5.6D are investigated using large eddy simulation at the Reynolds number of 23 500. The detailed force characteristics and wake flow structures of the wavy square cylinder are captured and compared with a square cylinder. Under the effect of wavy leading edge up to 27% and 98% reduction in the mean drag and the lift fluctuations are achieved, respectively. The 3D mean flow field implies a high shear flow between node and saddle positions, which is associated with the generation of additional streamwise and vertical vortex pairs. These vortex structures are responsible for the three-dimensionality of the wake flow behind the wavy square cylinder. The instantaneous flow patterns suggest that the staggered pattern of von Kármán vortex shedding is suppressed and replaced by symmetric vortex shedding into the near wake region of the wavy square cylinder. This symmetric vortex structure in the near wake plays a role in preventing the upper and lower shear layers from interacting with each other. The time–frequency analysis exhibits a reduction in the dominating vortex shedding frequency for the case of wavy square cylinder, indicating a relatively steady wake flow. This can be attributed to the vortex dislocation behind the wavy square cylinder. In the range of high frequencies, small streaks associated with small-scale fluctuations are enhanced by the wavy leading edge. The present study suggests that the perturbation wavelength obtained from the wavelength of Mode A instability at laminar flow regime can be served as a basis for flow control in turbulent flow regime. [ABSTRACT FROM AUTHOR]

In this paper, the entire process of the flow around a fixed square cylinder and the moving surface boundary-layer control (MSBC) at a low Reynolds number was numerically simulated. Two small rotating circular cylinders were located in each of the two rear corners of the square cylinder, respectively, to transfer momentum into the near wake behind the square cylinder. The rotations of the two circular cylinders were realized via dynamic mesh technology, when the two-dimensional incompressible Navier–Stokes equations for the flow around the square cylinder were solved. We analyzed the effects of different rotation directions, wind angles θ , and velocity ratios k (the ratio of the tangential velocity of the rotating cylinder to the incoming flow velocity) on the wake of flow around a square cylinder to evaluate the control effectiveness of the MSBC method. In the present work, the aerodynamic forces, the pressure distributions, and the wake patterns of the square cylinder are discussed in detail. The results show that the high suction areas near the surfaces of the rotating cylinders can delay or prevent the separation of the shear layer, reduce the wake width, achieve drag reduction, and eliminate the alternating vortex shedding. For a wind angle of 0 ° , the inward rotation of the small circular cylinders is the optimal arrangement to manipulate the wake vortex street behind the square cylinder, and k = 2 is the optimal velocity ratio between the control effectiveness and external energy consumption. [ABSTRACT FROM AUTHOR]

To attain the goal of sustainable development of resources, a study was conducted around a heated square cylinder placed inside a relatively cooler square enclosure. The temperature difference between the enclosure and cylinder is governed by different Rayleigh numbers (104, 105 and 106). For each case, analysis was done to know how the change in orientation of cylinder about its central axis affects the heat flow and flow characteristics. The problem is analyzed using ANSYS fluent and the plots are built-in Tecplot. The study also hopes to provide necessary analysis on flow characteristics by swotting through the Nusselt number, Skin friction coefficient, and pressure coefficient of each case. To further enrich the study, analysis on its vorticity, velocity magnitude, and drag is done. By comparing a square at the different rotation of 0°, 15°, 30°, 45°, 60°, and 75°, we inferred that 45° angle of rotation provides the most optimum and efficient means of heat transfer higher Rayleigh number. This is caused due to effective formation of the recirculation system around, which is formed due to an increase in velocity and skin friction drag around the cylinder and inside the enclosure. [ABSTRACT FROM AUTHOR]

A two-dimensional numerical investigation on flow past a heated square cylinder at different confinements has been carried out in the presence of two control cylinders placed on the rear side of the cylinder near the wake to control the shedding phenomena. The confined flow has been simulated with a blockage ratio from 4 to 8 (i.e., ratio of the computational domain height to the diameter of the square). The effects of Reynolds number (Re) range 100 ≤ Re ≤ 150 on various flow and heat transfer characteristics are studied and analyzed in the form of streamline plots, variations of pressure and viscous drag coefficients, Strouhal number (St), total pressure drop and Local and average Nusselt number. Different varieties of flow pattern were observed under laminar conditions with different combinations of Re, Blockage ratio (BR), size of the control cylinder and placement of the control cylinder. The St for geometry with control cylinder of size d—30% decreases by 6.5% when shifted from L = 0.6 to L = 1.6 for least BR 4 and maximum Reynolds number 150. The pressure difference decreases by 10.2% for further control cylinder position from 0.6 to 1.6, size from 20 to 30% at BR 4 and Re 100. The control cylinder closer to the bluff body maintains the pressure difference in increasing BR. The convective heat transfer is higher for L = 1.6 for lower BR. An increase of 8.6% observed for Re 150 at d—30%. The increase in BR reduces the effect of the positioning of the control cylinder and its size becomes the decisive factor. [ABSTRACT FROM AUTHOR]

The present study analyses the effect of triangular and circular extensions on lateral sides of square cylinder in a confined domain. Two-dimensional simulation has been carried out for laminar flow across a square cylinder with triangular and circular extensions for Reynolds number (Re) range 75–200. The momentum and energy equations are solved numerically by applying suitable boundary conditions using Ansys Fluent 18. The fluid flow and heat transfer characteristics of the square cylinder with extensions (at constant characteristics length) has been analysed by estimating and comparing the pressure drag, viscous drag, total drag, total pressure drop, Strouhal number, local and average Nusselt numbers. Different models with circular and triangular modifications are considered by modifying the lateral sides of a square cylinder by increasing the transversal height h without changing the characteristic length. Maximum decrease of 45% is observed in total drag coefficient when an ideal square cylinder is geometrically modified to form a cylinder with triangular extensions at h = 0.2 on the lateral side without affecting the characteristic length D at Re 200. Maximum decrease in pressure drop of around 42.8% is observed for square cylinder with triangular modifications with h = 0.2D. It is found that modified square cylinders have higher Strouhal number than an ideal square cylinder. An increase of 23.66% in Nu is observed for circular modification; h = 0.3D with respect to square cylinder at Re 200. Circular modification shows better heat transfer capabilities than triangular modification. Thus, the studies show enhancement in flow and heat transfer properties with lateral modifications. [ABSTRACT FROM AUTHOR]