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This work performs a direct simulation on forced convection heat transfer of flow past a square cylinder in a channel with an upstream promoter. The promoter is small in size that the incoming flow...

In this study the two-dimensional flow over a square cylinder placed in between parallel plates is simulated numerically by using the lattice Boltzmann method (LBM) at low Reynolds numbers. Both the plates are obstructed by solid rectangular blocks of variable length. The fluid was allowed to flow between the parallel plates for Reynolds number (Re) from 75 to 150, and blockage ratio (g*) from 1 to 3. The numerical investigation does not simply yield the predictable primary region of recirculating flow connected to the obstructions; it also shows supplementary regions of the flow downstream of the single cylinder placed in a computational domain. These supplementary separation zones were not already described in the research. The numerical analysis shows that the flow downstream of obstructions and the single cylinder remained two-dimensional for Re varied from 75 to 150. Results available in previous research are reported and compared with both of the available experimental and numerical results for code validation with a single cylinder. Furthermore, the effects of various Re and blockage ratio on the lift forces and drag coefficient is analyzed. Under these circumstances, good agreement between experimental and numerical results are obtained. The hydrodynamic forces of the cylinder are strongly influenced by the spacing ratios.

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.

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

This paper proposes a method to identify low-pressure vortices with swirling motions around a vehicle by considering the two-dimensional pressure minimum. The existing sectional-pressure-minimum-and-swirl method combined with the finite difference method can be used to identify low-pressure vortices with swirling motions in homogeneous isotropic turbulence. To apply this method to the flow field around a vehicle, a method that extends the existing method to the finite volume method on unstructured grids and prevents the fragmentation of the vortex core lines was developed. To verify the proposed method, it was applied to the von Karman vortices of the square cylinder on the unstructured grids. The results indicate that the von Karman vortices, which involve low-pressure vortices with swirling motions, could be effectively captured using the proposed method. Finally, the proposed method was applied to the flow field around a vehicle. Compared with the existing method, the proposed method could better prevent the fragmentation of the vortex core lines. In addition, four known vortex structures around the vehicle could be identified by using the proposed method in combination with the isosurface method. Compared to other generally used methods in the field of vehicle aerodynamics, the proposed method could better identify the vortex core lines within a few minutes. These results demonstrate that the proposed method is effective for identifying the vortices around a vehicle.

The present paper reports numerical simulation of turbulent flow over a square cylinder using a novel scale resolving computational fluid dynamics technique named Partially-Averaged Navier–Stokes (PANS), which bridges Reynolds-Averaged Navier–Stokes (RANS) with Direct Numerical Simulation (DNS) in a seamless manner. All stream-wise and wall normal mean velocity components, turbulent stresses behavior have been computed along the flow (streamwise) as well as in transverse (wall normal) direction. The measurement locations are chosen based on the previous studies so that results could be compared. However, the Reynolds number ( Re) of the flow is maintained at 21,400 and K– ω turbulence model is considered for the present case. All the computations are performed in OpenFOAM framework using a finite volume solver. Additionally, turbulent kinetic energy variations are presented over a wide range of measurement planes in order to explain the energy transfer process in highly unsteady turbulent flow field. The fluctuating root mean square velocities in the streamwise as well as in the wall normal direction have been discussed in the present work. It has been found that Partially-Averaged Navier–Stokes (PANS) model is capable of capturing the properties of highly unsteady turbulent flows and gives better results than Reynolds-Averaged Navier–Stokes (RANS). The results obtained using Partially-Averaged Navier–Stokes (PANS) are quite comparable with Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) data available in literature. The partially-averaged Navier–Stokes results are compared with our simulated Reynolds-Averaged Navier–Stokes (RANS) results, available experimental as well as numerical results in literature and it is found to be good in agreement.

The effect of cylinder alignment on the turbulent bursting events and sediment entrainment around submerged circular and square cylinder within the scour hole are investigated in the present study....

Unstable flow patterns around arrangements of bluff bodies in different engineering applications can give rise to pressure oscillations, leading to excitation of strong acoustic resonance that can interrupt operation. In certain conditions, flow fluctuations arising from vortex shedding downstream of a circular cylinder are reported to excite severe acoustic resonance. On the other hand, cylinders of a square cross section are known to be particularly susceptible to mechanisms that involve coupling between the flow and a structural mode. It is not documented, however, if such coupling would occur between an acoustic mode and flow fluctuations downstream of a square cylinder. In this work, the possibility of excitation of acoustic resonance due to coupling between unsteady flow downstream of a single square cylinder with an acoustic cross mode of a rectangular duct is experimentally investigated. During the experiments, acoustic resonance was self-excited. Measurements of the acoustic pressure and the flow velocity are carried out for a single square cylinder of an edge length of 25.4 mm. Results show that aeroacoustic response characteristics for this configuration are not completely analogous to the case of a circular cylinder, with a number of features not reported before. A brief summary of the results is presented in this work.

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 ([Formula: see text]). The increasing slope of the viscosity entropy generation rate at a range of [Formula: see text] is greater than that of [Formula: see text]. It is also found that the viscosity entropy generation rate is kept as a constant when the nondimensional distance [Formula: see text] is greater than 5. At [Formula: see text], the effect of downstream square cylinder becomes negligible on the viscosity entropy generation rate. The fluctuating amplitude increases with increasing the nondimensional distance [Formula: see text]. 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 [Formula: see text]. 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.

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.

This work represents an investigation of the performance of characteristic numerical boundary conditions on the unconfined 2D mixed convective flow of air past a heated square cylinder gove...

Flow past a square cylinder has been studied extensively for over a century, because of its interesting flow features and practical applications. This problem is of fundamental interest as well as important in many engineering applications. The characteristics of flow around a square cylinder placed at symmetric condition are governed by the Reynolds number (Re). In the present study two dimensional simulations of flow past a square cylinder have been carried out for a Reynolds number of 21400. It has been studied numerically using the large-eddy simulation technique. The modeling of the problem is done by ANSYS 17.1 preprocessing software.

Flow passing a heated square cylinder is investigated using a hybrid LES-RANS approach on unstructured grids at a moderate Reynolds number of 22, 050. The effects of inflow turbulence on the flow f...

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.

The “sliding-wall concept” has been introduced for the case of convective heat transfer past a square cylinder with an angle of incidence. Based on this concept, all walls of the cylinder t...

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...

The influence of Prandtl number on laminar, unsteady flow past a heated square cylinder placed in a free-stream has been studied computationally. The flow has been investigated for 0.02 < Pr < 100 at Re = 100 and Ri = 1. Effects of Prandtl number in unsteady mixed convection flow have been reported for the first time in the present study. The finite-volume based open source code OpenFOAM was used for the numerical simulations. An efficient algorithm (PIMPLE) has been used for the pressure-velocity coupling. Streamlines, isotherms, vorticity production and force coefficients have been studied in detail. The role of buoyancy on the baroclinic production has been discussed. Variation of lift coefficient with Pr was found to be quite interesting. The mean lift coefficient was found to be negative at low values of Pr but surprisingly it became positive at very high values of Pr.

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.

This 3D study deals with the effect that a low Reynolds number (Re) pulsating water flow has on both the heat transfer to the walls of a square section channel in which a square section prism was l...

Fluid dynamics and heat transfer behaviour are studied past side-by-side square cylinders at incidence in vertical flow arrangement numerically considering laminar, Newtonian, steady, incompressible and two-dimensional flow using Finite Volume Method based ANSYS Fluent solver. Using air (Pr = 0.7) as the working fluid, computations are performed at a representative value of Re = 100. The angle of incidence (alpha) is varied from 0 degrees to 45 degrees in the step of 5 degrees, whereas, the lateral distance between the cylinders is kept constant. To consider all possibilities, cylinders are rotated either clockwise or counter-clockwise, simultaneously or individually, which generates thirty-four different orientations. Buoyancy assisting phenomenon is created by changing the Richardson number (Ri) from 0 to 1 in step of 0.25. Due to the variation of the angle of incidence and its orientations, vortex generation is observed in the buoyancy assisting case at a critical value of Ri. To study heat transfer characteristics, time average and the local Nusselt number are analysed. For a fixed lateral distance between the cylinders, maximum heat transfer is found to occur at an incident angle of 45 degrees. Other important parameters, such as drag coefficient (C-D), lift coefficient (C-L), Strouhal number (St) are also studied. (C) 2018 Elsevier Ltd. All rights reserved.

In this paper, the Implicit Large-Eddy Simulation (ILES) is investigated on the flow around a square cylinder incorporating an unstructured Weighted Essential Non-Oscillatory (WENO) reconstruction method for a Reynolds number of 22,000. Simulations are undertaken in the framework of open-source package OpenFOAM and additional implicit 2nd/3rd-order WENO scheme on the convective term of the viscous incompressible Navier-Stokes Equations. A 2nd-order Euler implicit time integration and Pressure-Implicit Splitting-Operator (PISO) algorithm is used to for the pressure-velocity coupling. Conventional LES with Wall Adapting Local Eddy Viscosity (WALE) model is also carried out as a baseline. The results are compared to high fidelity experiment, DNS data and conventional LES with dynamic Smagorinsky model from previous work. Results show favorable performance for ILES with 3rd-order WENO scheme compared with the conventional LES with dynamic Smagorinsky model and similar performance against LES with WALE model. Results also show acceptable predictions over time-averaged statistics with less computational effort for the ILES of 2nd-order WENO scheme. Shear layer flow analysis suggests that both ILES and LES face similar challenges with small quantities, such as shear stress. Finally, simulations are capturing Von Krmn vortex, Kelvin-Helmholtz instability and induced frequency changes.

[Abstract] The phenomenon of turbulence is present in almost every type of flow in practical applications. Depending on its level of intensity and length scale, it can modify both the aerodynamic and aeroelastic performance of a body under flow action. In wind tunnel tests, the desired turbulence level is achieved by placing obstacles, spires, grids and extra roughness generators upwind the tested model. On the other hand, when trying to reproduce turbulence effects by means of a computational fluid dynamics (CFD) approach, two options have usually been considered: synthetic turbulence generation and the reproduction of velocity and pressure fluctuations recorded from previous simulations or wind tunnel tests. Another option, whose feasibility in CFD applications is addressed in this work by means of a 2D URANS (unsteady Reynolds averaged Navier–Stokes) consists of placing a rod upstream of the studied body, near the stagnation line. This approach is based on the generation of small scale turbulence upstream of the studied body, so that the turbulent wake generated by an upwind rod impinges on the body located downwind. In the present study, by means of 2D URANS simulations, the smooth flow over a circular cylinder (the upwind rod) is studied focusing on its wake turbulence characteristics. Furthermore, the aerodynamic performance of a square cylinder, first under smooth flow, and later immersed in the turbulent wake of the upstream rod, are analysed. A substantial effort has been devoted in the verification studies of the numerical models. It has been found that the adopted numerical approach is able to reproduce the turbulent characteristics of the rod wake and assess the impact of the turbulent flow on a square cylinder, providing a promising agreement with experimental data. Xunta de Galicia; ED431C2017/72

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 paper investigates the dynamic response of two freely rotatable rigid square cylinders to two-dimensional laminar flow in a microchannel. The square cylinders are laterally pinned side-by-side in the microchannel with a single freedom of rotation. Finite volume method coupled with a dynamic mesh technique is developed and validated to reveal the detailed motion characteristics of the cylinders and nearby flow structures. Under small Reynolds number (Re = 50), both cylinders oscillate periodically. The oscillate curves (rotating angle v.s. time) are symmetrical with each other but with a certain phase difference. At Re = 150, both cylinders oscillate randomly. Under high Reynolds number (Re = 300), the two cylinders both keep rotating in the opposite direction with the velocity magnitude fluctuating drastically around 1.75. Important motion details are presented to understand the Fluid-Structure interaction mechanism under different Reynolds number, including the time history of rotating angles and rotating velocities, lift and drag coefficients on the cylinders, distribution of pressure around the cylinder sides. Both pressure-induced torque and the shear induced one are obtained and their contributions to both cylinders’ rotation characteristics are quantitatively evaluated. Vortex structures and streamlines around the cylinders at specific moments are also revealed in this paper to help understanding the fluid-structure interaction phenomenon.

The present project consists on the computational study and resolution of the Navier-Stokes equations and the physical phenomena involved. The main objective is the development of C++ programming codes to solve flow’s governing equations using numerical methods. The project comprises the fluid dynamic and thermal study and analysis of both laminar and turbulent regimes. In addition to that, in case of turbulent flow, there has been selected to implement a RANS turbulence model called Spalart-Allmaras. The computational codes developed will be used to simulate the study cases LID Driven Cavity, LID Differentially Heated and Square Cylinder for the laminar regime and a supersonic pipe in case of the turbulent part. Additionally, the results obtained will be extensively analysed and verified using scientific publications.

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.

WOS:000453247900025 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.

WOS:000453247900020 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

The natural ocean/river currents energy can be harvested using Flow Induced Motion (FIM) phenomena. The effect of damping-ratio and mass-ratio on Flow Induced Motion energy harnessing of a square cylinder are numerically investigated for Reynolds number 15500

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.

A numerical study has been carried out to elucidate the effects of rounding the sharp edges on flow-induced forces on a square cylinder immersed in a laminar cross flow. Rounding reduces both the upstream stagnation pressure and the downstream base pressure. Consequently, competition between these two pressure reductions yields the minimum drag on the cylinder when its edges are partially rounded. It was also found that the leading-edge rounding is mainly responsible for the topological change thus the drag reduction, while the trailing- edge rounding alone just enhances lift fluctuation. However, trailing-edge rounding plays a role of stabilizing the flow when all of the four edges are rounded.

The unsteady fluid stream and warmth transmission nearby a square cylinder with sharp and rounded cornered edges are numerically examined, and then the roundness of the corner is predicted and optimized for the minimum fluid forces and maximum heat transmission rate. The roundness of the cylinder corner is changing 0.5D (circle) to 0.71D (square); D is the depth of the cylinder. The fluid flow and the heat transmission features around the sharp and curved cornered square cylinder are evaluated with the streamline, isotherm patterns, pressure coefficient, drag and lift coefficients, local Nusselt number (Nulocal) and average Nusselt number (Nuavg) at different Re and for several roundness values. These characteristics are predicted by the gene expression programming, and then the multi-objective genetic algorithm is utilized for the optimization. A number of combinations of values of corners have been found in the form of Pareto-optimal solution to compromise the minimum fluid forces with maximum heat transfer rate.

The prospect of harvesting energy from flow-induced vibration using an elastic square cylinder with a detached flat plate is experimentally investigated. The feasibility of flow-induced vibration to supply an adequate base excitation for micro-scale electrical power generation is assessed through a series of wind tunnel tests. The current test model of a single square cylinder is verified through a comparable pattern of vibration amplitude response with previous experimental study and two-dimensional numerical simulations based on the unsteady Reynolds averaged Navier–Stokes (URANS). In addition, a downstream flat plate is included in the wake of the square cylinder to study the effects of wake interference upon flow-induced vibration. A downstream flat plate is introduced as the passive vibration control to enhance the magnitude of flow-induced vibration and simultaneously increases the prospect of harvesting energy from the airflow. The study is conducted by varying the gap separation between the square cylinder and flat plate for 0.1≤ G/ D ≤3. The highest peak amplitude is observed for the gap G/ D = 1.2 with yrms/ D = 0.46 at UR = 17, which is expected to harvest ten times more energy than the single square cylinder. The high amplitude vibration response is sustained within a relatively broader range of lock-in synchronization. Meanwhile, for G/ D = 2 the vibration is suppressed, which leads to a lower magnitude of harvested energy. Contrarily, the amplitude response pattern for G/ D = 3 is in agreement with the single square cylinder. Hence, the flat plate has no significance to the wake interference of the square cylinder when the gap separation is beyond 3 D.

The Flow Past square cylinder with tandem arrangement is numerically analyzed using Commercial Finite volume code. The fixed Reynolds number (Re.) 100 is selected for the present study. However, corner radius to diameter ratio, R/D = 0 to 0.5 and L/D = 1.5 to 7.5 spacing between two cylinders is used as a varying parameter. The flow visualization parameters, the drag and lift coefficients are comprehensively presented and compared for different cases in order to reveal the effect of corner radius and gap spacing on the behavior of the flow. The obtained results have shown that flow aerodynamic characteristics are strongly influenced by cylinders rounded corners and spacing. It was also found that the total drag force can be reduced for the downstream cylinder when cylinders are placed within L/D ≤ 4.5.

Direct numerical simulation (DNS) is performed to investigate the modes of shedding of the wake of a wall-mounted finite-length square cylinder with an aspect ratio (AR) of 7 for six different boundary layer thicknesses (0.0–0.30) at a Reynolds number of 250. For all the cases of wall boundary layer considered in this study, two modes of shedding, namely, anti-symmetric and symmetric modes of shedding, were found to coexist in the cylinder wake with symmetric one occurring intermittently for smaller time duration. The phase-averaged flow field revealed that the symmetric modes of shedding occur only during instances when the near wake experiences the maximum strength of upwash/downwash flow. The boundary layer thickness seems to have a significant effect on the area of dominance of both downwash and upwash flow in instantaneous and time-averaged flow field. It is observed that the near-wake topology and the total drag force acting on the cylinder are significantly affected by the bottom-wall boundary layer thickness. The overall drag coefficient is found to decrease with thickening of the wall boundary layer thickness.

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.

A numerical investigation was conducted to study the forced and mixed convection of nanofluid in a horizontal channel with a built-in-heated square cylinder. The nanofluid considered in this study is composed of metal nanoparticles(Cu) suspended in water (base fluid). The governing equations are solved using the finite volume method based SIMPLER algorithm. Different Reynolds numbers and volume fractions of nanoparticles ranging respectively from Re= 85 to 200 and from φ= 0%to12%, have been considered. The effect of the nanoparticles volume fraction on the critical Reynolds number value defining the transition between two flow regimes (stationary and periodic)as well as on the overall flow coefficients is firstly studied. In the thermal study, we have established correlations to evaluate the heat flux transferred from the obstacle to the flow for different nanoparticles volume fractions. Results show a marked improvement in heat transfer compared to the base fluid. This improvement is more pronounced for higher Richardson numbers and higher nanoparticles volume fractions.

In the present study, a method of Partial-Averaged Navier-Stokes (PANS) equations, purported to perform variable resolution modeling, is used to predict the heat transfer over a square cylinder in a cross-flow. The PANS closure is based on the RANS SST k-ω model paradigm. The simulations were carried out using an open source software, namely, Open FOAM, at Reynolds number = 22000. The open source code and the PANS model are validated against the experimental work reported in the literature and it was observed that both the mean flow properties and turbulent statistics were in good agreement with the experimental results. Further the capability of the PANS approach in predicting heat transfer in turbulent flow is also studied. An algebraic wall function is used for the near wall treatment of the energy equation. The computed, average and local Nusselt numbers are compared with the experimental and LES results reported in the literature. The phase-averaged analysis of the shedding phenomenon is studied to understand the heat transfer phenomenon at different faces of the cylinder and turbulent heat fluxes are also considered to understand the effect of turbulence on convection.

A quasi two-dimensional numerical study is performed to analyze the thermo-magneto-convective transport of liquid metal around a square cylinder in a square duct subjected to a strong externally imposed axial magnetic field. The channel bottom wall is considered heated while the top wall is maintained at the free stream temperature keeping the cylinder adiabatic. The Reynolds and Hartmann numbers are kept in the range and . The flow dynamics in the aforementioned range of parameters reveals the existence of four different regimes out of which the first three ones are similar to the classical non-MHD 2-D cylinder wakes while the fourth one is characterized by the vortices evolved from the duct side walls due to the boundary layer separation which strongly disturbs the Kármán vortex street. The flow dynamics and heat transfer rate from the heated channel wall are observed to depend on the imposed magnetic field strength. With increasing magnetic field, the flow becomes stabilized resulting in a degradation in the forced convection heat transfer. A special case at a very high Reynolds number with Ha = 2160 is also considered to show the development of a Kelvin–Helmholtz-type instability that substantially affects the heat transfer rate.

An unsteady two-dimensional forced convection over a square cylinder with sharp and rounded corner edge is numerically analyzed for the low Reynolds number laminar flow regime. In this study, the analysis is carried out for Reynolds number (Re) in the range of 80 to 180 with Prandtl number (Pr) variation from 0.01 to 1000 for various corner radius (r=0.50, 0.51, 0.54, 0.59, 0.64 and 0.71). The lateral sides of the computational domain are kept constant to maintain the blockage as 5%. Heat transfer due to unsteady forced convection has been predicted by Artificial Neural network (ANN). The present ANN is trained by the input and output data which has been acquired from the numerical simulation, performed in finite volume based Computational Fluid Dynamics (CFD) commercial software FLUENT. The heat transfer characteristics over the sharp and rounded corner square cylinder are evaluated by analyzing the local Nusselt number (Nulocal), average Nusselt number (Nuavg) at various Reynolds number, Prandtl numbers and for various corner radii. It is found that the heat transfer rate of a circular cylinder can be enhanced by 12% when Re is varying and 14% when Prandtl number is varying by introducing a new cylinder geometry of corner radius r=0.51. It is found that the unsteady forced convection heat transfer over a cylinder can be predicted appropriately by ANN. It is also observed that the back propagation ANN can predict the heat transfer characteristics of forced convection very quickly compared to a standard CFD method.

Flow structures and downstream wake behind two identical tandem flat plates and a square cylinder has been studied at high Reynolds number and similarities and dissimilarities in coherent and incoherent structures have been identified. It was found that the square cylinder produces higher coherent turbulence kinetic energy (TKE) with peaks up to seven times higher than the flat plates in tandem. Also, turbulence kinetic energy production contours revealed that while the turbulent peak occurs mainly at the edges of the normal flat plates in tandem, it develops at centreline in the wake of the square cylinder. Interestingly, it was observed that the square cylinder exhibits two alternative shedding frequencies along centreline in the wake region.

This study investigates the effectiveness of a new technique to optimize aerodynamic responses of square cylinder by corner modification. The corner of the square cylinder was modified by introducing a small inclined opening which was measured in terms of corner point dislocation. Four specific opening widths (normalized with the depth of the cylinder) viz. 0.02, 0.08, 0.12 and 0.18 were considered. The normalized length of the inclined opening was 0.05 for all cases. The aerodynamic responses of these four modified square cylinders are compared with the unmodified square cylinder. Direct Numerical simulation was utilized to predict the aerodynamic responses and the flow filed. Second order accuracy was maintained both in space and time. The Reynolds number was kept constant at 100. The mean and RMS values of the force coefficients are calculated and compared. The flow filed is analyzed in terms of vorticity filed, mean flow streamlines and after-body wake characteristics. Due to corner modification, a drag reduction of approximately 5% is achieved for square cylinder with corner opening. Along with the separated flow, the corner opening affected the wake of the cylinder as well. Square cylinder with corner opening had different wake characteristics as compared to the unmodified cylinder.

The FIV (Flow-Induced Vibration) responses of an elastically mounted square cylinder near a free surface are numerically investigated using 2-dimensional RANS equations in conjunction with SST k- ω turbulence model. In this work, the effect of submergence depth on the FIV response and energy conversion is studied. The flow field behavior for flow velocities between 0.2 m/s and 2.5 m/s (1.61 × 104

The vortical structures around a yawed square cylinder oscillating in quiescent water are investigated using the particle image velocimetry technique. Following a previous study on the hydrodynamics (Lou et al., 2017), the present experiments are performed at different yaw angles ( α ) and Keulegan-Carpenter (KC) numbers to correlate the independence principle (IP) to vortical flow structures. At KC = 6, the vortex pair shows no shedding. Similar vortex patterns at different yaw angles result in similar hydrodynamic behaviors, which validates the IP at small KC numbers. The single and double pairs of vortex shedding regimes are observed for α = 0° at KC = 11 and 19, respectively, and the vortex shedding process is determined by the movement of the cylinder as well as the interaction between vortices and shear layers. As α increases to 45°, the shear layers are stretched and show attachment to the upper and lower sides of the cylinder for most of the time within one oscillating cycle. The shedding is only observed at the end of each half cycle and the vortices are found to reattach to the cylinder body. The subsequent drag force behavior of the yawed cylinder displays significant differences from that at α = 0° and hence the IP is no longer applicable at KC = 11 and 19. When KC increases to 25, a three pairs of vortex shedding regime can be observed at both α = 0° and 45°. A similar flow feature, characterized by the shear layer attachment when the cylinder is at the neutral position and the vortex shedding at the end of each half cycle, has been found for both α = 0° and 45°. This result indicates that the IP becomes valid for the yawed square cylinder when the KC is sufficiently large until it is analogous to the steady flow.

In this paper, flow structures around a corner modified square cylinder (side dimension, Bo) are presented and discussed. Cylinders with various corner arcs (circular) were considered (arc radius ‘r’). For various Corner Ratios (CR = r/Bo), values ranging from 0 to 0.5, flow visualization experiments were conducted in a water channel and the results are reported at Re = 2100 (based on Bo). Results presented are for two cases (a) stationary cylinders reporting the values of CD (coefficient of drag), St (Strouhal no.), and D (vortex size) and (b) oscillating cylinders at fe/fs = 1 (fe is the cylinder excitation frequency and fs is the vortex shedding frequency) and a/Bo = 0.8 (a is the cylinder oscillation amplitude). The work is aimed to explore the most effective configuration for drag reduction. Cylinder with corner ratio of 0.2 is proved to be the most effective one among the cases considered in this study with 19.3% drag reduction. As a major highlight, in contrast to the results of the previous studies, current study do not reveal a monotonous decrease of drag with increasing corner modification. Instead, it is shown here that, there is a specific value of CR ratio where the drag is the minimum most. A peculiar type of vortex structure was observed in the cases of stationary cylinders with CR > 0.2, contributing to the increase in drag. In the case of oscillating cylinders, description of one complete cycle for all CR ratios at various time instances are presented. The near-wake structures were observed to be dependent on the CR ratio. Counter intuitively, cylinder oscillation does not bring major difference in vortex size compared to the stationary case.

In the present paper, three-dimensional Fluid structures and wake characteristics are evaluated for a square cylinder with an upstream splitter plate. The splitter plate divides the incoming flow in two streams, the upper and lower one, existing between them a velocity ratio. Two different Reynolds numbers of 56 and 200 and three different velocity ratios are here considered. The effect of the mixing enhancement using a square cylinder located downstream of the splitter plate is evaluated. A Floquet analysis to compute the spanwise wavelengths of three dimensional disturbances appearing in the square cylinder wake with and without a splitter plate is performed. It is observed that the use of the detached splitter plate has a stabilizing effect at low velocity ratios. However, when the velocity ratio increases, the vortex shedding suffers a linear increase and the wake resembles that of a mixing layer. Vortex dislocations appear at ratios larger than 2, which points out the onset of a bifurcation to a more chaotic wake. The wavelength of this secondary instability has been measured by means of Floquet analysis and two-point correlations being in the order of 3.5D.

Numerical analysis of two dimensional steady flows over a triangular extended solid (thorn) attached to square cylinder positioned at front stagnation point and at rear stagnation point separately is reported at low Reynolds number ( Re = 40). The variation of thorn length ( l ′ = 0.2, 0.4 & 0.6) & inclination angle ( Θ = 5°, 10°, 15° & 20°) and its effect on the drag, pressure, shear stress, boundary layer as well as on the inertia force and viscous force are of interest. There is comparatively large variation on drag when the thorn is placed at the front side instead of placing at rear. The recirculation length is remained constant by varying the length and inclination of the thorn irrespective of its position. The variation of drag is comparatively less by changing thorn inclination. It is found that the drag is minimized by 2–3% compared to square model.

A detailed numerical investigation of the flow behind a square cylinder at a Reynolds number of 21,400 is conducted to assess the ability of the delayed detached-eddy simulation (DDES) modeling approach to accurately predict the velocity recovery in the wake of a bluff body. Three-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) and DDES simulations making use of the Spalart–Allmaras turbulence model are carried out using the open-source computational fluid dynamics (CFD) toolbox OpenFOAM-2.1.x, and are compared with available experimental velocity measurements. It is found that the DDES simulation tends to overestimate the averaged streamwise velocity component, especially in the near wake, but a better agreement with the experimental data is observed further downstream of the body. The velocity fluctuations also match reasonably well with the experimental data. Moreover, it is found that the spanwise domain length has a significant impact on the flow, especially regarding the fluctuations of the drag coefficient. Nonetheless, for both the averaged and fluctuating velocity components, the DDES approach is shown to be superior to the URANS approach. Therefore, for engineering purposes, it is found that the DDES approach is a suitable choice to simulate and characterize the velocity recovery in a wake.

An approach to designing passive devices for control of unsteady flows is presented. The method requires only snapshots of the flow to be controlled as inputs. A temporal correlation based on proper orthogonal decomposition of both fluctuating velocity and nonlinear forcing serves to identify the spatial locations in which the forcing drives the different unsteady flow features. The installation of a passive device in these spatial locations inhibits the fluctuating motion. The potential of the methodology is demonstrated via the suppression of vortex shedding in flow past a square cylinder, paving the way to the control of more complex flows using passive devices. Connections in agreement with previous studies targeting the same flow using different passive flow control strategies are provided.

Large-scale organized motions (commonly referred to coherent structures) and flow topology of a transitional separated-reattached flow have been visualised and investigated using flow visualisation techniques. Two geometrical shapes including two-dimensional flat plate with rectangular leading edge and three-dimensional square cylinder are chosen to shed a light on the flow topology and present coherent structures of the flow over these shapes. For both geometries and in the early stage of the transition, two-dimensional Kelvin-Helmholtz rolls are formed downstream of the leading edge. They are observed to be twisting around the square cylinder while they stay flat in the case of the two-dimensional flat plate. For both geometrical shapes, the two-dimensional Kelvin-Helmholtz rolls move downstream of the leading edge and they are subjected to distortion to form three-dimensional hairpin structures. The flow topology in the flat plate is different from that in the square cylinder. For the flat plate, there is a merging process by a pairing of the Kelvin-Helmholtz rolls to form a large structure that breaks down directly into many hairpin structures. For the squire cylinder case, the Kelvin-Helmholtz roll evolves topologically to form a hairpin structure. In the squire cylinder case, the reattachment length is much shorter and a forming of the three-dimensional structures is closer to the leading edge than that in the flat plate case.