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Copyright of Karaelmas Science & Engineering Journal / Karaelmas Fen ve Mühendislik Dergisi is the property of Karaelmas Science & Engineering Journal and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Under the action of incoming flow, the square cylinder can generate more intense vibration responses than the circular cylinder, which is beneficial for energy harvesting. Numerical simulations for FIV of the square-cylinder energy conversion system are carried out. URANS equations are used in conjunction with the shear stress transport k-ω turbulence model to predict the flow, and the equations for vibrations are solved by the Newmark-β algorithm. The present numerical method is validated against the published data with good consistency. The Reduced velocity Ur is varied from 1-20, with corresponding Reynolds numbers of 24 000-160 000. The numerical results indicate that the Reynolds number significantly affects the frequency response, amplitude response, vortex shedding mode, and energy conversion efficiency. The highest efficiency point locates at Re=88 000, with a value of 7.156%. When Re>120 000, the system transits from vortex-induced vibration into galloping, and its vibration responses as well as energy harvesting characteristics change sharply. Fully developed galloping motion occurs when Re>144 000.

Sınırlandırılmış bir kanal içerisinde yer alan tek kare silindir (KS) üzerinden iki boyutlu laminer sürekli akış için blokaj (=B/H) oranının ısı transferi ve akış karakteristiklerine olan etkisi incelenmiştir. Çalışmada Reynolds sayısı Re=40 değerinde sabit tutulurken blokaj oranı =0.125-0.8 değerleri arasında değiştirilmiştir. Hesaplamalarda ANSYS CFX 14.0 kullanılmıştır. Blokaj oranı etkisinin KS yüzeyleri üzerindeki sürükleme katsayısı (Cd), sürtünme katsayısı (Cf), boyutsuz yeniden birleşme uzunluğu (Lr/B) ve ortalama Nusselt sayısı (Nu) üzerine olan etkileri incelenmiştir. Blokaj oranı arttıkça sürükleme katsayısı (Cd), sürtünme katsayısı (Cf) ve ortalama Nusselt sayısı (Nu) değerlerinin arttığı ancak boyutsuz yeniden birleşme uzunluğu (Lr/B) değerinin azaldığı bulunmuştur. Sürükleme katsayısı (Cd), sürtünme faktörü (Cf), boyutsuz yeniden birleşme uzunluğu (Lr/B) ve ortalama Nusselt sayısı değerlerinin blokaj oranına göre değişimini veren bağıntılar elde edilmiştir.

An experimental study was carried out to identify the effect of angle of attack on flow-induced vibration (FIV) of square cylinders. The experiment was conducted at the Aeronautical and Wind Engineering Laboratory (AEROLAB), UTM Kuala Lumpur using a wind tunnel that was free from external wind conditions. A supporting structure was designed and fabricated to conduct this experiment. The importance of this support structure was to enable the rigid cylinder to suspend and vibrate freely upon excitation of wind speed. The results were analysed through the response of amplitude and frequency of the rigid cylinder over a velocity range of 0.5m/s to 4.0m/s. The results showed that for a square cylinder of ?=0°, vortex-induced vibration (VIV) occurred at low reduced velocity (UR) in range of 5 ? UR ? 10 and galloping occurred at higher reduced velocity which started at UR=15. A tranquil zone was found between VIV and galloping in the reduced velocity range of 10 ? UR ? 15. As for ?=22.5° and 45°, only VIV response was found at low reduced velocity in range of 4? UR ? 9. ABSTRAK: Satu kajian eksperimentasi telah dilakukan bagi mengenal pasti pengaruh sudut serangan oleh getaran cetusan-aliran (FIV) dalam silinder persegi. Eksperimen ini dijalankan di Makmal Kejuruteraan Aeronautika dan Angin (AEROLAB), UTM Kuala Lumpur dengan menggunakan terowong angin yang bebas dari pengaruh angin luar. Struktur sokongan telah direka dan difabrikasi bagi tujuan eksperimen ini. Ini penting bagi membolehkan silinder pegun tergantung dan bergetar dengan bebas semasa ujian kelajuan angin. Dapatan kajian dianalisis melalui tindak balas amplitud dan frekuensi silinder pegun pada kadar halaju 0.5m/s sehingga 4.0m/s. Hasil kajian menunjukkan bahawa bagi silinder persegi ? = 0 °, getaran pengaruh-vorteks (VIV) berlaku pada halaju rendah (UR) dalam julat 5 ? UR ? 10 dan getaran lebih teruk telah ketara berlaku pada kadar halaju berkurang iaitu bermula pada UR = 15. Zon tenang dijumpai antara VIV dan getaran teruk pada kadar halaju berkurang 10 ? UR ? 15. Adapun pada ? = 22.5° dan 45°, hanya tindak balas VIV dijumpai pada halaju rendah dalam kadar 4? UR ? 9.

New data and review of the spanwise coherence length is provided for flows over cylinders of different cross-sections: circular of diameter d, and rectangular of sectional aspect ratios (breadth (b) to height (d) ratio AR = b/d) of 1, 2 and 3. In the present measurements, the body has both d and spanwise length of 70d fixed, and the Reynolds number (based on d) range 6000–27,000 is covered. Two-point data are obtained from two hot-wire probes, one fixed in the symmetry plane and the other moving on the corresponding spanwise axis. Their position in a cross plane are deduced from preliminary measurement of the mean flow with a single probe, allowing fair comparisons between the different geometries and the introduction of uncertainty bars on coherence length values. At all tested regimes, a very good agreement is noticed between velocity-based and pressure-based coherence experimental data. Coherence length definitions are revisited, and the aeroacoustically consistent, integral length definition is selected, allowing fair synthesis of literature data into a single chart and empirical functions. Definitions for coherence decay models (e.g. Gaussian or Laplacian) are also adapted so that coherence length and coherence integral shall be equivalent. This preliminary work on coherence data and its spanwise integration enables transparent regressions and model selection. Generally, the Gaussian model is relevant for the lift peak, while the coherence exhibits a Laplacian decay at harmonics. On average, at peak Strouhal number, the coherence length for the circular and square cylinders is of 5d while it is of the order of 15d for the rectangular sections. It is concluded that the flow over those latter geometries is still a two-dimensional dynamics at the tone frequency. These values are almost preserved over the tested Reynolds number range. Coherence length value at harmonics is extensively documented. Spanwise coherence length is also discussed as an ingredient of acoustic efficiency.

In this paper, effects of nondimensional distance between two square cylinders on the dissipation characteristics of the complex flow are investigated. The viscosity entropy generation rates around two serial square cylinders and the lift coefficient are analyzed to fully reveal the statistical features of the flow dissipation. Numerical results mainly show that the major viscosity entropy generation rate appears in the shear intersection region of the main flow and local stationary vortex. The viscosity entropy generation rate increases with increasing nondimensional distance ( S ∗ ). The increasing slope of the viscosity entropy generation rate at a range of S ∗ ≤ 2 is greater than that of 2 ≤ S ∗ ≤ 5. It is also found that the viscosity entropy generation rate is kept as a constant when the nondimensional distance S ∗ is greater than 5. At S ∗ ≥ 5 , the effect of downstream square cylinder becomes negligible on the viscosity entropy generation rate. The fluctuating amplitude increases with increasing the nondimensional distance S ∗ . The high-frequency peak is ascribed to the strong vortex shedding around the downstream square cylinder, and the low-frequency peak is ascribed to the weak vortex shedding around the up square cylinder at S ∗ = 1. Although our focus is mainly on the complex flow around two square cylinders, this work demonstrates the viscosity entropy generation rate with increasing nondimensional distance, which provides nice physical insight into understanding the local flow dissipation characteristics around the two serial square cylinders. [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.

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.

This study investigated wind-induced vibrations in square cylinders, commonly encountered in practical engineering. Passive-jet flow control was achieved by arranging a hollow pipe with openings on a square cylinder surface, thereby suppressing the wake and wind-induced vibrations. Large eddy simulation was used to determine a reasonable grid and time-step scheme and an optimal chamfered scheme. The effects of various parameters such as the hollow pipe height, radial thickness, opening number, and opening height on the wake vortex state and aerodynamic forces of the cylinder were investigated at R e D = 50 , 000 based on the side length (D) of the cylinder. The results indicate that the passive-jet flow control method could effectively suppress the wake and reduce the aerodynamic forces acting on the cylinder. The optimal control effect was achieved when the hollow pipe height, radial thickness, opening number, and opening height were 0.6 D , 0.05 D , six per side, and 0.4 D , respectively. Compared to the uncontrolled chamfered square cylinder, the lift coefficient fluctuation and mean drag coefficient decreased by 89.2 % and 19.2 %, respectively. • Passive-jet flow control method suppresses wake and reduces aerodynamic forces. • Passive-jet hollow pipe control can blow the main vortices farther downstream. • The optimal opening parameters of the hollow pipe is determined. • Flow state in the near wake region is more stable under the hollow pipe control. [ABSTRACT FROM AUTHOR]

The paper reports on the prediction of the turbulent flow field around a three-dimensional, surface mounted, square-sectioned cylinder at Reynolds numbers in the range 104-105. The effects of turbulence are accounted for in two different ways: by performing large-eddy simulations (LES) with a Smagorinsky model for the subgrid-scale motions and by solving the unsteady form of the Reynolds-averaged Navier-Stokes equations (URANS) together with a turbulence model to determine the resulting Reynolds stresses. The turbulence model used is a two-equation, eddy-viscosity closure that incorporates a term designed to account for the interactions between the organized mean-flow periodicity and the random turbulent motions. Comparisons with experimental data show that the two approaches yield results that are generally comparable and in good accord with the experimental data. The main conclusion of this work is that the URANS approach, which is considerably less demanding in terms of computer resources than LES, can reliably be used for the prediction of unsteady separated flows provided that the effects of organized mean-flow unsteadiness on the turbulence are properly accounted for in the turbulence model. Copyright © 2014 by ASME.

Abstract Unsteady RANS (URANS), hybrid LES/RANS and IDDES simulations were conducted to numerically investigate the velocity field around, and pressures distribution and forces over a square cylinder immersed in a uniform, steady oncoming flow with Reynolds number Re = 21,400. The vortex shedding responses in terms of Strouhal number, the pressure distribution, the velocity profile and the velocity fluctuations obtained by numerical simulations are compared with experimental data. Compared with 2D URANS simulation, 3D simulations using hybrid LES/RANS and IDDES models provide more accurate prediction on the responses in the wake, including mean streamwise velocity profile and rms velocity fluctuations. This also results in more accurate prediction of time-averaged surface pressure coefficient on the rear surface obtained by 3D hybrid LES/RANS and IDDES simulations than by URANS simulation. When a hybrid LES/RANS model or IDDES model is used, a more accurate prediction for either pressure coefficient or velocity profile (especially in the far wake region) is not guaranteed by increasing the mesh resolution along the spanwise direction of the square cylinder.

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this paper, we propose a model for the transverse oscillation of a square-section cylinder under flow. The fluctuating transverse force due to vortex shedding is represented using a coupled nonlinear wake oscillator, while the unsteady force for galloping caused by the varying incidence angle effects is modelled using the quasi-steady approach. First, we analytically investigate the lift behavior and phase angle variation of the square cylinder under forced vibrations. Comparison with experimental data is used to determine the form of the coupling terms and its values. The present model shows advantages in predicting the phase angle, and it successfully captures the change in sign of the phase. Second, the proposed model is directly applied in predicting free oscillation cases without any tuning. The dynamical behaviors predicted by this model are compared with published experiments under different Scruton numbers, and reasonable agreement can be found. The results indicate that the model can not only be applied in simulating the "pure galloping" and "pure VIV," but also is able to capture the interactions of VIV and galloping, including combined and separate VIV-galloping motions. [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.

This article shows the results of a study of the structure of turbulent flow behind a square profile ALUTEC 45 × 45 mm with T-slots. The angle of rotation of the profile relative to its axis varied α = 0 ° , 15 ° , 30 ° and 45 °. During the experiment, the flow velocity was 5 m⋅s − 1 , Reynolds number was 7. 7 ⋅ 1 0 4 . The Constant Temperature Anemometry technique was used for experimental studies. To avoid backflow, the measuring plane was positioned at the rear of the profile at a distance of x ⋅ d − 1 ≈ 2. 2. As a result of the studies, it was found that the highest Taylor microscale Reynolds number and standard deviation for turbulent flow was observed in the area behind the cylinder. The width of this area is 3.5 times the width of the cylinder. With the distance from the center of the cylinder in the spanwise direction to flow the Taylor microscale Reynolds number and standard deviation sharply decreases. The maximum values of the Taylor microscale Reynolds number are observed at α = 45 ° is 426 and 398. It has also been found that behind the cylinder there is some area in which the some parameters of the turbulent flow vary greatly with the change angle α. The lowest energy dissipation rate in this range is observed for α = 15 ° − 68 m 2 ⋅ s − 3 and the largest for α = 0 ° − 138 m 2 ⋅ s − 3 . We also found that the minimum value of the Kolmogorov scale and the Kolmogorov time is observed at α = 0 °. The minimum values of the Kolmogorov scale η = 71 μ m and the Kolmogorov time τ η = 0. 33 ms. The maximum values for previous parameters observed at α = 15 ° is η = 84 μ m and τ η = 0. 47 ms. We also found that the flow rate and standard deviation distributions between the ALUTEC profile and the ordinary square cylinder different. This can be observed when the upper part of the profile is tightly closed and the lower part is open. In this case, along with the profile inside of the T-slot, there is a generation of internal flow. This reduces the total backflow area behind the ALUTEC profile by 20% compared to an ordinary square profile. • Measurement by using Constant Temperature Anemometry (CTA). • Investigation of the structure of turbulence on a square cylinder. • The average flow velocity is 5 m s − 1 . • The maximum values R λ is observed at α = 45 ° is 426 and 398. • Influence of T-slot of the ALUTEC profile on the flow structure behind it. [ABSTRACT FROM AUTHOR]

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 2-dimensional 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 [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.

Turbulent uniform flows past a two-dimensional square cylinder are investigated numerically. By varying the turbulence intensity and turbulence length scale of the approaching flow, the flow effect of the cylinder are compared to that in a laminar approaching-flow case. In addition, the variations of drag and lift coefficients with respect to the changes of turbulence intensity and turbulence length scale are analyzed on a systematic basis. In the large eddy simulations, the approaching-flow turbulence is generated by a spectral method according to Kármán spectrum. Two levels of turbulence intensities (5% and 10%) and three turbulence length scales (0.25, 0.50 and 1.0 times of the cylinder width) are selected in the study to examine the effect on the cylinder. Results show that the Strouhal number remains almost unchanged when the uniform approaching-flow changes from a laminar state to a turbulent one. The approaching-flow turbulence has noticeable effect in promoting the resulting drag and lift fluctuations. However, its effect on the mean drag appears negligible. In contrast, an increase of the approaching-flow turbulence length scale leads to mild increases of the mean and root-mean-square values of drag. On the other hand, the resulting lift fluctuation is insensitive to the change of the turbulence length scale.

Copyright of Journal of Heilongjiang University of Science & Technology is the property of Journal of Heilongjiang University of Science & Technology Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Two-dimensional numerical simulations of flow-induced vibration (FIV) of a square cylinder in laminar flow with a Reynolds number Re = 170 are carried out, and the influence of the mass ratio on the FIV characteristics is studied. The results show that the mass ratio affects the vibration responses distinctly in different regimes. In the vortex-induced vibration (VIV) regime, a larger mass ratio results in a smaller amplitude. On the contrary, in the galloping regime, a larger mass ratio leads to a greater amplitude, causing the galloping starts at a lower reduced velocity. Furthermore, the VIV-galloping coupling effect, as observed in the galloping regime, becomes stronger with increasing mass ratio. To further elucidate the vibration mechanisms, the instantaneous phase lag between displacement and lift force, the work done by the vortex lift and the potential lift, and their influences on vibration responses are carefully investigated. In the VIV regime, the vortex lift contributes primarily to the vibration while the potential lift becomes significant in the galloping regime. More specifically, both forces do positive work in the initial and upper branches of VIV, leading to an increase in the vibration amplitude. However, in the lower branch, the potential lift continues to contribute positively, while the vortex lift does negative work, extracting energy from the vibration system. The counteracting vortex and potential lift cause a decrease in the vibration amplitude. With further increasing the reduced velocity, the vortex lift alternately promotes or inhibits the vibration in the galloping regime, while the potential lift consistently contributes positively to the work done, playing a leading role. As a result, the vibration amplitude increases in the galloping regime. • In VIV, a higher mass ratio corresponds to a smaller amplitude. • In galloping, a higher mass ratio leads to more pronounced vibration. • Different to the vortex-force dominated VIV, galloping is primarily affected by the potential force. [ABSTRACT FROM AUTHOR]

A square cylinder may gallop if subjected to fluid flow, experiencing a self-excited vibration mode that can harvest energy for low-power applications. The harvested power is typically low and depends on the upstream flow velocity and system dynamic parameters. In this study, the influence of nonlinear stiffness induced by two repulsive magnetic poles on the galloping response of square rods is investigated at a mass ratio of 10 and a Reynolds number of 200. The vibration response of two identical coaxial square rods with magnetic poles attached to their opposite ends is analyzed. Two simplified configurations are selected to model the magnetic forces: a pair of identical monopoles and a pair of identical dipoles. Results show that the magnet configuration and strength significantly affect the vibration amplitude and the onset flow velocity. A weak magnetic force can enhance vibration and reduce onset flow velocity, while a stronger pair of dipoles may push the bistable system to a state of low-amplitude vibration with a mean displacement. The influence of magnets in this configuration has potential as a control technique for energy harvesting applications. [Display omitted] • Nonlinear added stiffness can be achieved through magnetic repulsion to enhance or suppress galloping. • Magnetic monopoles enhance vibration and reduce the required onset velocity. • Magnetic dipoles' strength ratio to stiffness affects bistable response stability. • Enhanced vibration due to negative stiffness from magnet repulsion. [ABSTRACT FROM AUTHOR]

The paper presents an experimental investigation of the flow around two bluff bodies in a tandem arrangement. Two different configurations were investigated. The first one concerns two square cylinders (SS case) whereas in the second setup, a triangular cylinder was used as an upstream object (TS case). Particular attention was devoted to examining the effect of the gap space G between cylinders (from 1 to 6) on the flow structure and dynamics of vortices. The study also includes measurements conducted for single square and single triangular cylinders to show how the second object modifies the flow field. It was found that the wake behind each object is always wider for the SS case regardless of G. When G is less than or equal to 3 for the TS case, the slender body regime is seen in the flow field which for the SS case is normally present for G < 1. 5. Consequently, distinctive vortex shedding frequencies cannot be distinguished in the gap for the TS case when G ≤ 3. When G ≥ 4 , twice as high values of the Strouhal number S t are reported in the gap for the TS case compared to the SS case. Downstream the second object, S t gradually decreases from ∼ 0.245 to ∼ 1.14 with increasing G from 1 to 4 for the TS case, whereas for 5 ≤ G ≤ 6 distinctive vortex shedding frequencies are no longer observed. • Flow around tandem configuration of triangular and square cylinders at Re = 104. • Effect of gap spacing between objects and upstream cylinder shape are examined. • Strouhal numbers downstream the first and second object are examined. • Slender body regime is identified for 4 < G for the studied configuration. • Effect of upstream cylinder shape is seen in the gap and downstream the 2nd object. [ABSTRACT FROM AUTHOR]

A comparative analysis is presented to illustrate the benefits of finite impulse response (FIR)-based spectral proper orthogonal decomposition (SPOD) over traditional proper orthogonal decomposition (POD) in separating coherent contributions within narrow spectral bandwidth. The wake of a cantilevered square cylinder of height-to-width ratio h / d = 4 protruding a thin laminar boundary layer (δ / d = 0. 21) is investigated at a Reynolds number (R e) of 10600 using time-resolved stereoscopic particle image velocimetry. In the vicinity of the obstacle-plate junction region (z < 0. 5 d), spectral analysis of the temporal modes obtained with both POD and FIR-based SPOD show fluctuation energy concentration centered on the vortex shedding frequency (f s), as well as weaker contributions at 0.1, 0.4, 0.9, 0.95, 1.05, 1.1, and 2 f s. Broad-band spectral energy concentrations around 0.4 f s are mainly observed close to the ground plane, while the energy contents at (1 ± 0. 05) f s and (1 ± 0. 1) f s increase at higher planes. With POD, the frequencies are not separated in the modes and it is very difficult to associate each to specific phenomena within the wake region. However, with tuning of the filter bandwidth, FIR-based SPOD represents each of the identified frequencies in a separate mode pair, which permits a better interpretation of the related dynamics. Notably, the improved modal separation assists in associating the 0.4 f s spectral contribution with the interactions between the extensions of horseshoe vortex system and Kármán vortices in the wall-body junction region and the (1 ± 0. 05) f s and (1 ± 0. 1) f s frequencies to perturbation of the shedding cycle resulting from interactions with the lower frequency dynamics. • Interactions of coherent motions give rise to inter-harmonic frequencies. • Spectral Proper Orthogonal Decomposition separates motions at different time scales. • Separation of inter-harmonics facilitates interpretation in time domain. • Improved frequency separation results in less scatter in modal phase relationships. • Refined spatial functions can be more easily related to interactions of flow dynamics. [ABSTRACT FROM AUTHOR]

A finite-volume method simulation is used to investigate two-dimensional unsteady flow of nanofluids and heat transfer characteristics past a square cylinder inclined with respect to the main flow in the laminar regime. The computations are carried out of nanoparticle volume fractions varying from 0 ≤ ≤ 5% for an inclination angle in the range 00≤ δ ≤ 450 at a Reynolds number of 100. The variation of stream line and isotherm patterns are presented for the above range of conditions. Also, it is noticed that the addition of nanoparticles enhances the heat transfer. Hence, the local Nusselt number is found to increase with increasing value of the concentration of nanoparticles for the fixed value of the inclination angle. [ABSTRACT FROM AUTHOR]

A numerical investigation of flow past a square cylinder with a circular bar upstream and a splitter plate downstream is carried out in this paper by lattice Boltzmann method (LBM). The combination of the three obstacles and LBM is the main novelty of the present study. The flow patterns are analyzed by a uniform flow of Reynolds number 100 based on the side length of the square cylinder, D. Numerical simulations are performed in the range of 1 ≤ Ds/D ≤ 5, 0 ≤ G/D ≤ 7 and 1 ≤ L/D ≤ 6, where Ds, G and L are the center-to center distance, surface-to-surface distance and the splitter plate length, respectively. Six flow patterns are observed in the present study. The maximum percentage reduction in mean drag coefficient is 68.76% at (ds, g, l) = (2.5, 0, 3) which is in pattern VI. The vortex shedding from the square cylinder and the circular bar can be completely suppressed in pattern VI. The small distance between the square cylinder and the splitter plate plays a more vital role in suppression of vortex shedding as compared with large distance and length. [ABSTRACT FROM AUTHOR]

• Review on numerical and experimental studies pertaining to natural convection in enclosures. • Effect of internal bodies such as square, circular and elliptical cylinder on flow mechanism. • Summarization of different methodologies used to improve the thermal behavior of buoyancy driven flows within an enclosure. • Effect of wall boundary conditions, position and number of internal bodies. • Categorization of flow regimes and the factors affecting the transition from steady to unsteady state. Natural convection has been extensively studied due to its presence in many engineering applications. It is one of the most important modes of heat transfer and arises due to buoyancy-induced flows resulting from temperature differences. This review presents a detailed summary of numerical and experimental studies related to laminar natural convection in enclosures with and without internal bodies. Square, circular, and elliptical cylinders are mostly considered as internal bodies. The presence of internal bodies makes significant changes in the flow characteristics within the enclosure. The effects on the flow regime and thermal fields of various parameters have been discussed in detail, including the Rayleigh number, aspect ratio, position of internal bodies, number of internal bodies, and inclination angle. The different flow regimes depending on the input parameters are categorized based on observations made from flow and thermal patterns. This review discusses various methodologies used by a large group of researchers to improve the hydrodynamic and thermal behavior for buoyancy-driven flows within an enclosure. [ABSTRACT FROM AUTHOR]

• Experimental study of the flow around an array of four square cylinders arranged in square configuration at Re = 8000. • Effects of the cylinders' center-to-center pitch (P / D = 2–5) and the array orientation (α = 0° – 45°) are examined. • Flow field around cylinders measured using PIV, in conjunction with measurements of hydrodynamic forces on each cylinder. • Flow broadly classified into three regimes (shielding, reattachment and impinging) depending on the value of P / D and α. • A new criterion is proposed to quantitatively classify the different flow regimes based on the average force coefficients. Experiments were conducted to study the flow around an array of four square cylinders arranged in square configuration under a subcritical Reynolds number Re = 8000. The goal is to systematically examine the effects of the cylinders' center-to-center pitch (with 6 pitch ratios P / D = 2–5) and the array orientation with reference to oncoming flow (with 7 incidence angles α = 0°–45°), i.e., 42 cases in total. The flow field was measured using digital Particle Image Velocimetry (PIV), in conjunction with direct measurement of the hydrodynamic forces (lift and drag) on each cylinder using a piezoelectric load cell. The flow interference among the cylinders in the array is complex, which could be non-synchronous, quasi-periodic or synchronized. The flow patterns, Reynolds shear stress distributions, temporal variation and power spectra of drag and lift coefficients on each cylinder, force statistics and vortex shedding frequencies are discussed in detail. Similar to that of four-circular-cylinder array, the flow could be broadly classified into three regimes (shielding, reattachment and impinging) governed by the combined value of P / D and α. However, it differs noticeably from that of the four-circular-cylinder array, including: (i) a lack of reattachment regime for the inline square configuration (α = 0°); (ii) the unique feature of attractive lift for the pair of cylinders arranged side-by-side; and (iii) higher sensitivity to the variation in incidence angle. A new criterion is proposed to quantitatively classify the different flow regimes based on the averaged RMS drag/lift coefficients of the four cylinders in the array. [ABSTRACT FROM AUTHOR]

Laminar forced convection heat transfer from 2-D flow over a square cylinder placed in a confined adiabatic channel is studied numerically. The governing equations are solved using unsteady stream function-vorticity method. The effect of volume fraction of the nanoparticles are tested for different Reynolds number in laminar range. Fluid dynamics and heat transfer results were reported for steady-state condition. Local Nusselt number and average Nusselt number are reported in connection with volume fraction and Reynolds number for blockage ratio of 0.25. Square cylinder's front wall results maximum Nusselt number whereas rear wall shows smaller Nusselt number. Wall attached pair of vortices appeared rear side of cylinder for Reynolds number varying from 10 to 40 and volume fraction varies from 0.0 to 0.1. The size of symmetry vortices linearly increases when Reynolds number or volume fraction is increased. The drag coefficient is more pronounced to the variation in Reynolds number and volume fraction rather lift coefficient. [ABSTRACT FROM AUTHOR]

The closeness or matching of the self-sustained vortex shedding excitation with the structural frequency of the body may cause resonance resulting in structural failures. The present study aims at the suppression of vortex shedding using a passive method, by placing a small control cylinder in the near wake of the main cylinder. Square control cylinders of sides either 20%, 25% or 30% of the main cylinder are employed to change the dynamics of the flow. Direct numerical simulation (DNS) of two-dimensional, unsteady, incompressible flow past a square cylinder at Reynolds numbers of 75, 100 and 125 has been carried out. The Navier–Stokes equations are solved using Marker and Cell (MAC) method with the second-order spatial and temporal discretizations. Computed results indicate that there exists an explicit spatial domain present on the edges of the separating shear layers in the near wake within which the placement of the control cylinder guarantees complete suppression. The mechanism of the suppression may be attributed to an effective increase in vorticity diffusion of the separating shear layer. Also, changes in the control cylinder size and Reynolds number have a considerable effect on the vortex shedding suppression. A remarkable drag reduction of the main cylinder is also achieved for partial as well as complete suppression. The study also highlights that the partial suppression of vortex shedding results in significant reductions in Strouhal number, and fluctuating lift and drag forces. The control behavior of the flow past a square cylinder surprisingly resembles that of the flow past a circular cylinder. • Control of vortex shedding in flow around a square cylinder has been undertaken. • Both partial and complete suppression has been achieved using a control cylinder. • Similarity of control behavior of wake of square and circular cylinders is observed. • Both control cylinder size & Re decide the extent of vortex shedding suppression. • Vorticity diffusion of separating shear layer is the driving factor for suppression. [ABSTRACT FROM AUTHOR]

In the present paper, physical mechanism responsible for origin of streamwise vortices in mode A appeared in the three-dimensional (3-D) wake transition of a square-section cylinder is investigated. Direct numerical simulations at a Reynolds number of 180 firstly show that such streamwise vorticity is not originated from lateral surfaces. Then through the analysis of local flow field in the immediate neighborhood of rear surface, based on the theory of vortex-induced vortex, a new physical mechanism is identified. At first, the vertical vorticity on rear surface is generated by the intrinsic three-dimensional instability with the same instability wavelength of mode A. Then the streamwise vorticity at a specific sign is induced by such vertical vorticity, convected and concentrated in the shear layers. Finally, streamwise vortices are formed and shed with alternatively shedding spanwise vortices in the near wake. Moreover, the effect of induced spanwise vorticity on original two-dimensional (2-D) spanwise vorticity is also presented in detail. [ABSTRACT FROM AUTHOR]

This study reports on an experimental investigation of the effect of mass ratio on the transverse flow-induced vibration (FIV) response of an elastically mounted square cylinder placed at three different incidence angles: two symmetric with respect to the centreplane, α = 0 ° and 45 ° ; and one asymmetric at 20 °. These three angles display different dominant FIV phenomena: transverse galloping and combined vortex-induced vibration (VIV) and galloping for α = 0 ° , VIV for α = 45 ° , and higher branch subharmonic (period-doubled) VIV for α = 20 ° (Nemes et al., 2012). The mass ratio is defined as the ratio of the total oscillating mass (m) to the displaced fluid mass (m d), m ∗ = m ∕ m d. The present results show that the mass ratio (m ∗ = 2. 64 – 15.00) has a significant influence on the structural vibration response for all FIV phenomena, and can dictate whether two of these modes exist at all. Three primary observations are presented: for the α = 0 ° case, the combined VIV–galloping response is diminished as the mass ratio is increased, and it ceases to exist for m ∗ ⩾ 11. 31 ; for the α = 45 ° case, the peak values of the normalised oscillation amplitude during VIV are only reduced marginally with increasing m ∗ , however the body oscillation amplitude in the desynchronised regions is significantly attenuated; for the α = 20 ° case, there exists a critical mass ratio (m crit ∗ ≃ 3. 50) above which the higher branch subharmonic VIV does not persist. [ABSTRACT FROM AUTHOR]

Highlights • Mixed convection heat transfer and flow characteristics of a square cylinder are investigated using a 3D numerical model • Two working fluids, Pr = 0.7 and 7 are considered for 55 ≤ Re ≤ 250 and 0 ≤ Ri ≤ 2. • For pure forced, pure free and mixed convection empirical correlations for the mean Nusselt number were developed. • The effects of buoyancy (Richardson number) on Strouhal number, rms/mean drag and rms/mean lift coefficients are discussed. Abstract Unsteady flow and heat transfer from a horizontal isothermal square cylinder is studied numerically using a three-dimensional computational model to investigate the influence of buoyancy on the forced flow and heat transfer characteristics. The numerical model is based on a horizontal square cylinder subjected to laminar fluid flow in an unconfined channel. The governing equations in 3D form are solved using a fractional step method based on the finite difference discretization in addition to a Crank–Nicholson scheme employed to the convective and the viscous terms. Two working fluids–air (Pr = 0.7) and water (Pr = 7)–are considered, and the flow and heat transfer simulations were carried out for the Reynolds and Richardson numbers in the intervals 55 ≤ Re ≤ 250 and 0 ≤ Ri ≤ 2, respectively. The flow characteristics such as time-averaged drag/lift, rms drag/rms lift coefficients as well as Strouhal number were computed. The heat transfer from the cylinder is assessed by mean Nusselt number (and rms Nusselt number) over the total heated cylinder walls. As the buoyancy increases, the mass and the velocity of the fluid flowing underneath the cylinder increases. The fluid is injected into the near wake region with an upward motion which significantly alters the flow field in the downstream as well as upstream regions. The effects of Reynolds, Richardson and Prandtl numbers on the flow field and temperature distributions are discussed in detail. It is shown that the flow and heat transfer characteristics are influenced more for air than water. To fill the void in the literature, useful empirical correlations of practical importance are derived for pure forced and pure natural as well as mixed convection. The mixed convection correlations, in terms of the ratio of pure forced convection, are also developed, and their implications are discussed. [ABSTRACT FROM AUTHOR]

Abstract In the present study, flow behind a square cylinder is investigated using control rods. The control rods are placed in the shear layer and studied using particle image velocimetry, hotwire anemometry and flow visualization techniques. Experiments are conducted in a low-speed wind tunnel for three different Reynolds numbers 485, 805 and 1290. However, detailed analysis of flow is only presented at a Reynolds number of 485. The aim is to modify the wake behind a square cylinder for possible suppression of vortex Street and drag reduction. The control rod diameter is kept constant as 0.2 times of the cylinder diameter. Depending upon the gap between control rods and the cylinder, the flow structure, vortex dynamics, the distance of the shear layer roll up behind the cylinder is modified. An attempt is made to correlate the different flow structures with corresponding drag forces. Several parameters such as time-averaged drag coefficient, Strouhal number, recirculation length, fluctuating velocity, turbulence intensity, and Reynolds stress are measured. To understand the flow phenomena and its intricate structure, flow visualization images are also captured in the same setup with smoke visualization. Maximum up to 22% drag reduction is observed at a particular control rod position. Highlights • Wake modification behind a square cylinder using control rods. • Particle image velocimetry (PIV), hotwire anemometer and flow visualization techniques are used to characterize the flow. • Effect of control rod position on the flow structure, vortex shedding and the drag coefficient. • The maximum reduction in drag coefficient (up to 22%) is observed for particular control rod position in the shear layer. • The flow visualization images give a clear picture of flow structure modification and suppression of vortex shedding. [ABSTRACT FROM AUTHOR]

This study focuses on the computational implementation of structured non-uniform finite volume method for the 2-D laminar flow of viscoelastic fluid past a square section of cylinder in a confined channel with a blockage ratio 1/4 for Re = 10-4, 5, 10 and 20. Oldroyd-B model (constant viscosity with elasticity) and the PTT model (shear-thinning with elasticity) are the constitutive models considered. In this study effects of the elasticity and inertia on the drag coefficients and stress fields around the square cylinder are obtained and discussed in detail. With an increase elasticity, drag coefficients get smaller due to stronger shear thinning effects for PTT fluid, however, the drag coefficients show slightly enhancement for the Oldroyd-B fluid. [ABSTRACT FROM AUTHOR]

The confined flow of a Newtonian fluid around a square cylinder mounted in a rectangular channel was investigated both numerically and experimentally. Ratio between the pipe and channel height, the blockage ratio, is kept constant at 1/4. The flow variables including streamlines, vorticity and drag coefficients were calculated at 0 = Re = 50 using finite volume method. The velocity terms in the momentum equations are approximated by a higher-order and bounded scheme of Convergent and Universally Bounded Interpolation Scheme for the Treatment of Advection (CUBISTA). Particle Image Velocimetry (PIV) was also used to obtain the twodimensional velocity field. The flow measurements were conducted for 1 = Re = 50. Streamline and vorticity results obtained by PIV are compared with those of the numerical simulation. Based on this comparison, good agreement is found between the numerical and experimental results in a qualitative manner. [ABSTRACT FROM AUTHOR]

Dynamic responses of four rigidly-connected square cylinders in a square con- figuration subjected to two-dimensional steady ow of a constant property Newtonian uid were investigated numerically. The focus of the present study is to investigate the effects of the angle of attack α on the dynamic responses by varying α from 0° to 15° in intervals of 2:5° at a fixed L = 4 (L is the non-dimensional center-to-center distance between two adjacent square cylinders, normalized by the side length of the square cylinder B). For each α, the reduced velocity (Vr) ranges from 1 to 40. The Reynolds number Re, mass ratio m* and structural damping ratio maintain constants of 180, 10 and 0, respectively. Numerical results show that the angle of attack α has a significant inuence on the dynamic response. When α ≤ 5°, galloping occurs in addition to vortex-induced vibration (VIV), while it weak- ens for α = 7:5° and 10°, and finally disappears as α = 12:5° and 15°, leaving only VIV response. The effects of L on the responses of the four-square-cylinder oscillating system were also examined for Re = 180, Vr = 40, and α = 2:5°. Numerical results show that L affects not only the response displacement but also the vortex shedding mode. Galloping with large response amplitude can happen at either large L = 4 or small L = 1:5 and 2. The response amplitude is relatively small as 2:5 ≤ L ≤ 3:5 due to the inuence of the ow in the gap between the square cylinders. For the particular case of L = 3:5, a combined vortex shedding mode is identified, where the vortex shedding from the top row square cylinders behaves as that from an elongated single body while the vortex shedding from the bottom row cylinders presents a co-shedding mode. [ABSTRACT FROM AUTHOR]

Traveling waves generated on the side surfaces of a square cylinder are employed to suppress the oscillating wake for improving the flow behavior around a square cylinder; this method is termed the traveling wave wall (TWW) method. This study aimed to evaluate the influence of the key parameters of TWW on the control of aerodynamic forces and the oscillating wake of the flow around a square cylinder. Unsteady numerical analyses at a low Reynolds number (Re) of 100 were performed using a two-dimensional CFD simulation. First, the grid independence and time step independence tests of the simulation were conducted to verify the rationality of the solving parameter settings, and the validation of flow around the fixed square cylinder at Re =100 was carried out. Subsequently, the lift and drag coefficients and the vortex shedding modes under different combinations of three TWW control parameters, including wave velocity, wave amplitude, and wavenumber, were analyzed in detail. The results show that TWW can remarkably reduce the mean value of drag coefficient and the RMS value of the lift coefficient by more than 12% compared to the method involving a standard square cylinder. Two peaks occur in the lift coefficient spectrum, with the low frequency corresponding to the vortex shedding frequency in the wake of the flow around the square cylinder and the high frequency corresponding to the traveling wave frequency. The vorticity contours show that the alternating vortices in the wake of the square cylinder are not completely suppressed under the selected control parameters. [ABSTRACT FROM AUTHOR]

Simulation of uniform flow past a square cylinder undergoing rotational oscillations at various frequencies is performed using the ANSYS Fluent CFD package. The frequency ratios FR chosen for the study are 0.4, 0.8 and 1.0. At each frequency ratio, a number of angular amplitudes are selected, and the simulations are performed using a dynamically deforming mesh. The results are obtained from the simulations in terms of the nondimensional drag and lift forces, vorticity contours, power spectral density plots of lift coefficients and Strouhal number. A general increase in lift and drag coefficients with increasing frequency ratios as well as with increasing amplitudes at a particular frequency ratio, except a few anomalies, is seen. The flow features in the wake and the vicinity of the cylinder are compared for the various parameters. The use of convective boundary condition for the problem is demonstrated. [ABSTRACT FROM AUTHOR]

Research utilizing piezoelectric fiber composites to power network nodes or micro-sensors by capturing energy from the surrounding environment is becoming a hot spot, and the structural design to efficiently improve the performance of energy harvesting is very important. The performance of a vertical cantilever piezoelectric water energy harvester (PWEH) with three different cross-sections including circular, square and equilateral triangle, each with a mass ratio of 4.8, was investigated at Reynolds number within 770–8800. Emphasis is placed on the effects of the bluff body cross-section, load resistance, and flow velocity on the flow-induced vibration (FIV) and energy response of the vertical cantilever beam of the PWEH. Except for the decreasing amplitude in the lower branch of the circular cylinder water energy harvester (CWEH), there is no distinction in the vibration response branches compared to the classical elastically supported circular cylinder. The resistance has little effect on the vibration response, while it has a significant effect on the voltage and power response. The maximum amplitude and pendulum angle of the square cylinder water energy harvester (SWEH) are slightly larger than those of the CWEH. The maximum voltage and power are smaller for the SWEH than for the CWEH. The equilateral triangular cylinder water energy harvester (TWEH) demonstrates excellent performance. It has a maximum amplitude of 3.8 D, 3.4 times that of the CWEH. The peak pendulum angle of TWCH is 13.9°, 3.2 times that of CWEH. It has a voltage of 0.057 V at a resistance of ∞ , which is 3.4 times that of CEWH. In addition, the TWCH has an output power of 0.38 nW at a resistance of 10 6 Ω , which is 9.5 times that of the CWEH. This study suggests that the best energy efficiency of TWEH is achieved by coupling vortex-induced vibration (VIV) and galloping excitation. • Piezoelectric water energy harvester with three different cross-sections is tested. • Effects of cross-sectional shape, load resistance, and reduced velocity are investigated. • Triangular cylinder water energy harvester exhibited superior performance in energy harvesting. • Optimal efficiency is achieved through vortex-induced vibration and galloping coupling excitation. [ABSTRACT FROM AUTHOR]

This study designs an electromagnetic energy harvester that uses a magnet in translation within a coil to harness energy from flow-induced vibrations when wall confinement affects a change in the wake formation behind a bluff body. An attached permanent magnet surrounded by a coil is attached to a flow channel with a flexible diaphragm, and a square cylinder acts as the bluff body that generates a vortex-induced street, thereby oscillating the diaphragm and generating electrical energy. The main novelty presented in the current investigation is developing a quick and efficient way of capturing fluid energy for practical applications, at the cost of augmented mechatronical complexity. Fluid flow and electromagnetic induction convert flow energy into electrical energy, and the influences of the Reynolds number (Re = 3000, 4000, and 5000) and the position of square cylinders (X*=X/D) on the dynamic characteristics are considered. As a result of different incidence angles (α) and spacing between two identical square cylinders (L*=L/D), Reynolds number and incidence angles have significant effects on the harvesting of energy. Additionally, based on experimental data, an output voltage of 0.04 V can be generated when the excitation pressure oscillates at about 20 Hz. Whether a single square or two identical squares are arranged, the 45-degree incidence angle is the most suitable arrangement for power conversion, and L*=4 can produce maximum energy for all configurations. Finally, the experimental results provide deeper insight into the physics behind the relationship between square spacing, rotation angle, and energy harvesting. Regarding hydroelastic energy extraction, tandem diamond configurations have substantial advantages over square ones. With high amplitudes and high frequencies, tandem diamond cylinders can generate mechanical power even beyond typical circular cross-sections. Furthermore, complementary numerical simulations are presented to provide a solid foundation for the reasoning behind the variation in extracted energy as well as complement the experiment results. By conducting numerical simulations, it is found that changing the incidence angle can significantly alter the flow patterns, which is attributable to the sequential formation of a primary vortex from the lee of the bluff body. [ABSTRACT FROM AUTHOR]

Spanwise correlation of unsteady lift due to vortex shedding is of particular interest for slender structures. In this paper, Hilbert transform and Wavelet transform were applied as time–frequency analysis tools to the sectional lift of a square cylinder. For Wavelet transform, the complex Morlet wavelet and the generalized Morse wavelet were considered as the mother wavelets, and the synchrosqueezing technique was used for post-processing. At high Reynolds number and in smooth flow, Hilbert transform and Wavelet transform mutually confirm that vortex shedding is characterized by an instantaneous shedding frequency fluctuating in real-time. As an example, at Reynolds number 3. 9 × 1 0 4 , the results of Wavelet transform suggest a standard deviation of the instantaneous shedding frequency of at least 4% about the conventional Strouhal frequency. The extracted instantaneous frequency is found stochastic, showing reasonably good Gaussian properties for its probability density function. The instantaneous frequencies simultaneously extracted at two cross sections of a spanwise separation are correlated, being the correlation coefficient lower than that calculated with fluctuating lifts. The shape of the coherence of the instantaneous frequency is found quite similar to that of turbulence, and the magnitude overall decreases with the increase of spanwise separation. The fluctuation of the instantaneous vortex shedding frequency, especially the low-frequency part (lower than about one-half of the Strouhal frequency), is believed critical in determining the spanwise correlation of unsteady lift, which was confirmed reconstructable via the extracted instantaneous shedding frequency. Finally, attention in this paper has also been paid to a reliable extraction and quantification of the concerning time–frequency characteristics. For instance, tests of the used time–frequency tools first on the purposely prepared artificial signal suggest, that the high-frequency fluctuation in the instantaneous vortex shedding frequency (higher than the Strouhal frequency) can actually not be reliably extracted. • Time–frequency analysis was applied to the sectional lift of a square cylinder at high Reynolds Number and in smooth flow. • The extracted instantaneous vortex shedding frequency was found to fluctuate appreciably and randomly. • Spanwise correlation of unsteady lift can be reconstructed with the instantaneous vortex shedding frequency. [ABSTRACT FROM AUTHOR]

Based on the fact that the drag coefficient is relatively insensitive to the Reynolds number in many experiments on the flow past a sharp-edged bluff body (SEBB) at high Reynolds numbers, it is speculated that the effect of the wall viscosity on the aerodynamics of the SEBB can be neglected to some extent. To validate this hypothesis for the large-eddy simulation (LES), the slip-wall boundary condition was applied to the SEBB, and this LES was termed slip-wall LES (SWLES). The flow past a square cylinder was chosen to validate the SWLES. The aerodynamic parameters, such as the drag coefficient and Strouhal number, agreed well with the experimental data and those of the traditional wall-resolved LES (WRLES). The surface pressure distributions and energy spectra were close to those of the WRLES. The Kelvin–Helmholtz instability structures, flow separation, and von Kármán vortex shedding were captured accurately by the SWLES. The computational cost was compared with that of the WRLES. An interpretation of the SWLES was discussed based on flow visualizations. After the validation, the SWLES was applied to the flow past a photovoltaic system. The wind loads of the system were compared with the wind tunnel experimental data at different incident angles. The surface pressure distributions, flow structures, and wake were analyzed. The present study demonstrated that the SWLES has potential applications for the flow around an SEBB with a much lower cost than that of the WRLES. • Slip-wall large-eddy simulation (SWLES) is proposed for the flow past a sharp-edged bluff body at high Reynolds numbers. • The SWLES was validated via the flow past a square cylinder. • The SWLES was applied on the flow past a photovoltaic system. • SWLES obtained an accuracy similar to that of the wall-resolved large-eddy simulation with much lower computational cost. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the distributed blowing control of the flow around a square cylinder at a low Reynolds number of R e = 200. The air holes are placed at the windward or leeward stagnation points. The control effect of the unsteady aerodynamic forces is notably different with different spanwise distributions of air holes, which is represented by the dimensionless air hole area ratio G. The simulation results indicate that the windward blowing control (WBC) shows better control effect than the leeward blowing control (LBC) in reducing the time-averaged drag coefficient C d ¯ of the square cylinder while the fluctuating lift coefficient C l ′ can be significantly suppressed by the LBC. Besides, the surface pressure distribution, the turbulence kinetic energy (TKE) and Reynolds shear stress (RSS) distributions and the shedding vortices in the wake are analysed and compared in detail under different control conditions. Finally, through the instantaneous 3D vortex structures and the spanwise fluctuating velocity w ′ distribution in the flow field, the suppression of the unsteady aerodynamic forces is mainly related to the 3D vortex pairs induced by the blowing jet. • Distributed blowing was employed to control the flow around a square cylinder. • The spanwise distribution of air holes notably affected the steady blowing flow control. • A 'Six-Vortices Type' vorticity distribution appeared in the flow field under windward blowing control. [ABSTRACT FROM AUTHOR]

The aerodynamic behavior of a square cylinder with rounded corner edges in steady flow regime in the range of Reynolds number (Re) 5–45; is predicted by Artificial Neural Network (ANN) using MATLAB. The ANN has trained by back propagation algorithm. The ANN requires input and output data to train the network, which is obtained from the commercial Computational Fluid Dynamics (CFD) software FLUENT in the present study. In FLUENT, all the governing equations are discretized by the finite volume method. Results from numerical simulation and back propagation based ANN have been compared. It has been discovered that the ANN predicts the aerodynamic behavior correctly within the given range of the training data. It is additionally observed that back propagation based ANN is an effective tool to forecast the aerodynamic behavior than simulation, that has very much longer computational time.