Square Cylinder In Cross Flow Research Articles

Flow and surface heat transfer analysis of a square cylinder in turbulent cross-flow

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 field as well as surface convective heat transfer are studied by adopting a grid-based random-number method (GRM). Validation of the GRM method is carried out by generating and simulating a decaying homogeneous isotropic turbulent flow. Analysis of the turbulent quantities and comparisons with the Synthetic Coherent Eddy Method (SCEM) suggest that the cheaper GRM is able to generate good quality inflow turbulence, despite a longer transition region is required. Studies of the heated square cylinder in crossflow show that the inflow turbulence results in an early breakup of the shear layer, which leads to further effects on the vortex shedding and surface heat transfer. The surface convective heat transfer is increased with inflow turbulence, especially on the front, top and bottom surfaces, while limited influence is found on the rear surface. It is shown by the spectra that the effects of the inflow turbulence mainly focus on the turbulent shear layers, as well as heat transfer of the corresponding surfaces.

Numerical investigation of flow and combined natural-forced convection from an isothermal square cylinder in cross flow

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.

Fluid flow and mixed convective heat transfer around a semi-circular cylinder at incidence with a tandem downstream square cylinder in cross flow

The present paper presents fluid flow and mixed convective heat transfer characteristics past a semi-circular cylinder at incidence with a tandem square cylinder in cross flow. Using air (Pr=0.71) as an operating medium, numerical simulations are performed for the range of Reynolds numbers, 10≤Re≤45 and incidence angles, 00≤α≤1800. The effect of thermal buoyancy is brought about by varying the Richardson number in the range 0≤Ri≤2. The mathematical model is firstly validated with the experimental and numerical results from the literature and found to be in good agreement. The steady separated flow is observed to become unsteady periodic under the superimposed thermal buoyancy. Furthermore, apart from thermal buoyancy effect, angle of incidence is found to play a pivotal role in bringing hydrodynamic instabilities and thereby vortex shedding for such steady mixed convective flows. Functional dependence of drag (CD), lift (CL), and moment (CM) coefficients on the combined influence of α and Ri, is explored and analysed in detail. Additionally, other global quantities, such as local and average Nusselt number distribution, Strouhal number (St) are determined with respect to the various ranges of parameters considered in the present investigation.

Modulating flow and aerodynamic characteristics of a square cylinder in crossflow using a rear jet injection

The temporally evolved flow behaviors around a square cylinder subject to modulation of a planar jet issued from the cylinder’s downstream surface into the wake were studied using the laser-assisted smoke flow visualization method and synchronized hot-wire anemometers. The drag force asserted on the square cylinder was obtained by measuring the surface pressures. Four characteristic flow modes (wake-dominated, transitional, critical, and jet-dominated) were observed in different regimes of freestream Reynolds number and jet injection ratio. In the wake-dominated mode, the jet swung periodically back and forth on the downstream surface due to the wake vortex shedding. In the transitional mode, the vortex shedding in the wake vanished so that the flow around the cylinder presented no periodic oscillations. In the critical mode, the wake width became smaller and therefore made the vortex shedding frequency larger than that observed in the wake dominated mode. In the jet-dominated mode, the jet had a large momentum that entrained wake fluids and therefore stabilized the instabilities of the wake, separated boundary layers on lateral surfaces, and stagnation point on the upstream surface. Two standing vortices appeared in the near wake beside the high-momentum jet. The width of the wake was decreased substantially by jet entrainment. The drag coefficient decreased with an increase in the jet injection ratio. The downstream surface jet injection caused the pressure coefficients to decrease at the upstream surface and to increase at the downstream surface. Therefore, the drag coefficients were decreased significantly by 26%, 33%, and 38% at the injection ratios of 0.5, 1.5, and 2.5, respectively.

Study of Heat Transfer over a Square Cylinder in Cross Flow using Variable Resolution Modeling

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.

Aerodynamic sound from a square cylinder with a downstream wedge

The effect of placing a wedge in the wake of a square cylinder (side length D) at a Reynolds number of 22,000 is numerically investigated. In particular, the effect of the wedge on aerodynamic noise is observed along with its effect on the flow field. Wedge base height (h) and its gap distance (G) downstream of the cylinder are systematically varied. Flow simulations are carried out using an unsteady RANS model employing the k–ω SST turbulence model, whereas the calculation of aerodynamic noise radiated from the flow is solved using Curle's equation. A special correction technique is applied to consider spanwise effects on noise production and validation is provided using new aeroacoustic data for a square cylinder in cross-flow. It is found that the flow behavior can be divided into two main regimes (regime I and regime II), with a linking transition regime. For regime I, the generated sound is lower than that of the isolated square cylinder case. The thinnest wedge produced the best sound reduction (11.79 dB) when the wedge is placed at G=2D. For regime II, the calculated sound level is higher than the case of an isolated square cylinder. This is because the sound emitted from both bodies have about the same magnitude and are in phase. For this case, the maximum increase of sound pressure is 6.24 dB, when the medium wedge is at G=2.5D.

Flow characteristics and drag force of a square cylinder in crossflow modulated by a slot jet injected from upstream surface

The flow characteristics and drag force of a square cylinder with a front jet injection were experimentally investigated in a wind tunnel. The evolution process of the characteristic flow patterns was recorded by the laser-assisted smoke flow visualization method. The time-averaged velocity fields measured by particle image velocimetry (PIV) were applied to analyze the velocity vectors, streamline patterns, vorticity contours, velocity distributions, and time histories of instantaneous velocities around the upstream region of the square cylinder. The drag force experienced by the square cylinder was obtained by measuring the surface pressures on the front and rear faces. The results show that the jet emitted from the upstream surface of the square cylinder periodically swings left-and-right in the experimental range of injection ratio ⩽0.9. In the time-averaged velocity field, the jet flow impinges the freestream at a four-way saddle and subsequently bifurcates into two streams; one stream goes toward the left edge of the upstream surface, while the other stream directs toward the right edge of the upstream surface. Two recirculation regions formed above the upstream surface of the square cylinder are enclosed by those two streams. The vorticity contours around the upstream surface of the square cylinder are characterized by two adjacent vorticity-concentrated areas of opposite signs. The time histories of the instantaneous velocities around the four-way saddle, jet exit, and shear layer of the swinging jet represent periodic oscillation. This characteristic oscillation frequency is dominated by the wake instability. The recirculation regions formed above the cylinder’s upstream surface prevent impingement from the freestream. Consequently, the surface pressure coefficients on the upstream surface of the square cylinder are reduced. This reduction in the surface pressure coefficient decreases the drag force acting on the square cylinder.

Effect of angle of incidence on mixed convective wake dynamics and heat transfer past a square cylinder in cross flow at Re = 100

In this paper, a numerical investigation is performed to study the mixed convective flow and heat transfer characteristics past a square cylinder in cross flow at incidence. Utilizing air (Pr=0.71) as an operating fluid, computations are carried out at a representative Reynolds number (Re) of 100. Angles of incidences are varied as, 0°⩽α⩽45°. Effect of superimposed positive and negative cross-flow buoyancy is brought about by varying the Richardson number (Ri) in the range −1.0⩽Ri⩽1.0. The detail features of flow topology and heat transport are analyzed critically for different angles of incidences. The thermo fluidic forces acting on the cylinder during mixed convection are captured in terms of the drag (CD), lift (CL), and moment (CM) coefficients. The results show that the lateral width of the cylinder wake reduces with increasing α and the isotherms spread out far wide. In the range 0°<α<45°, CD reduces with increasing Ri. The functional dependence of CM with Ri reveals a linear relationship. Thermal boundary layer thickness reduces with increasing angle of incidences. The global rate of heat transfer from the cylinder increases with increasing α.

Unsteady mixed convection heat transfer from tandem square cylinders in cross flow at low Reynolds numbers

A two-dimensional numerical study is carried out to understand the influence of cross buoyancy on the vortex shedding processes behind two equal isothermal square cylinders placed in a tandem arrangement at low Reynolds numbers. The spacing between the cylinders is fixed with five widths of the cylinder dimension. The flow is considered in an unbounded medium, however, fictitious confining boundaries are chosen to make the problem computationally feasible. Numerical calculations are performed by using a finite volume method based on the PISO algorithm in a collocated grid system. The range of Reynolds number is chosen to be 50–150. The flow is unsteady laminar and two-dimensional in this Reynolds number range. The mixed convection effect is studied for Richardson number range of 0–2 and the Prandtl number is chosen constant as 0.71. The effect of superimposed thermal buoyancy on flow and isotherm patterns are presented and discussed. The global flow and heat transfer quantities such as overall drag and lift coefficients, local and surface average Nusselt numbers and Strouhal number are calculated and discussed for various Reynolds and Richardson numbers.

Mixed Convection Heat Transfer from an In-Line Row of Square Cylinders in Cross-Flow at Low Reynolds Number

This article presents a two-dimensional numerical study on the unsteady laminar mixed convection heat transfer from a row of five in-line isothermal square cylinders placed in an unconfined medium and subjected to cross-flow of a Newtonian fluid at low Reynolds number (Re = 125). The hydrodynamic and thermal transport phenomena are captured for the separation ratios (spacing to cylinder size ratio, s/d) of 0.5, 1, 1.5, 2, 3, and 4. The mixed convection heat transfer is studied for Richardson numbers (Ri) ranging from 0 to 3 with a fixed Prandtl number (Pr = 0.71). Numerical calculations are performed by using a PISO algorithm-based finite volume solver in a collocated grid system. The instantaneous vorticity fields along with the isotherm patterns are systematically presented and discussed for different separation ratios and Richardson numbers. Depending on the engineering application, the temperature difference between the surface and the free stream could vary to make buoyancy of primary importance, entailing major modification of the flow field. Additionally, the instantaneous and mean drag and lift coefficients, Strouhal numbers, and mean Nusselt numbers are determined and discussed.

Effect of thermal buoyancy on vortex shedding behind a square cylinder in cross flow at low Reynolds numbers

Two-dimensional numerical simulation is performed to understand the influence of cross buoyancy on the vortex shedding process behind a stationary heated square cylinder at low Reynolds numbers. The flow is considered in an unbounded medium, however, fictitious confining boundaries are chosen to make the problem computationally feasible. Numerical calculations are carried out by using a finite volume method based on the PISO algorithm in a collocated grid system. The range of Reynolds number is chosen to be 5–40. In this range the flow and thermal fields are found to be steady and separated without the superimposed thermal buoyancy (i.e. for pure forced convection). However, as the buoyancy parameter (Richardson number) increases flow becomes unstable and subsequently, at some critical value of Richardson number, periodic vortex shedding is observed to characterize the flow and thermal fields. The global flow and heat transfer quantities such as the recirculation length, overall drag coefficient and surface average Nusselt number are computed for the steady separated flow (without buoyancy effect) and are found in good agreement with the available results in the literature. The effect of superimposed thermal buoyancy for the range of Reynolds number is studied for various Richardson numbers. The frequency of vortex shedding (Strouhal number) is calculated for the unsteady periodic flow. The critical Richardson number for the onset of vortex shedding decreases and the corresponding critical Strouhal number increases with increasing Reynolds number in the chosen range.

Mixed convective heat transfer from two identical square cylinders in cross flow at Re = 100

Numerical results of mixed convective heat transfer from two identical cylinders in a uniform upward flow has been demonstrated at Re = 100. The effect of aiding and opposing buoyancy is brought about by varying Richardson numbers. A stabilized SUPG based finite element technique has been used. Results are discussed for 20 cases by varying the locations of second cylinder with respect to the fixed location of the first cylinder. The effect of buoyancy on force coefficients, Strouhal number and Nusselt number is investigated. Under the same buoyancy induced field the cylinders are found to oscillate at the same frequency. For a particular cylinder spacing, vortex shedding is observed up to Ri = 0.25. Maximum heat transfer is found at the front face of second cylinder. It is found that hydrodynamic instabilities grow and flow shows chaotic phenomena when the system is severely influenced by thermal buoyancy. It is for the first time that such behavior for the Richardson number at this tandem configuration is being reported.

Time-averaged topological flow patterns and their influence on vortex shedding of a square cylinder in crossflow at incidence

Flow characteristics around the square cylinder and their influence on the wake properties are studied. Time-averaged flow patterns on the surfaces of square cylinder in a cross-stream at incidence are experimentally probed by surface-oil flow technique and analyzed by flow topology for Reynolds numbers between 3.9×10 4 and 9.4×10 4 as the incidence angle changes from 0° to 45°. Vortex shedding characteristics are measured by a single-wire hot-wire anemometer for Reynolds numbers between 5×10 3 and 1.2×10 5. The effects of topological flow patterns on the wake properties then are revealed and discussed. Flows around the square cylinder are identified as three categories: the subcritical, supercritical, and wedge flows according to the prominently different features of the topological flow patterns. The Strouhal number of vortex shedding, turbulence in the wake, and wake width present drastically different behaviors in different characteristic flow regimes. A critical incidence angle of 15° separates the subcritical and supercritical regimes. At the critical incidence angle the wake width and shear-layer turbulence present minimum values. The minimum wake width appearing at the critical incidence angle, which leads to the maximum Strouhal number, is due to the reattachment of one of the separated boundary layer to the lateral face of the square cylinder. If the Strouhal numbers are calculated based on the wake width instead of the cross-stream projection width of cylinder, the data in the subcritical and supercritical regimes are well correlated into two groups, which would approach constants at high Reynolds numbers.

Forced‐convection heat transfer from tandem square cylinders in cross flow at low Reynolds numbers

AbstractThis paper presents the results of a numerical study on the flow characteristics and heat transfer over two equal square cylinders in a tandem arrangement. Spacing between the cylinders is five widths of the cylinder and the Reynolds number ranges from 1 to 200, Pr=0.71. Both steady and unsteady incompressible laminar flow in the 2D regime are performed with a finite volume code based on the SIMPLEC algorithm and non‐staggered grid. A study of the effects of spatial resolution and blockage on the results is provided. In this study, the instantaneous and mean streamlines, vorticity and isotherm patterns for different Reynolds numbers are presented and discussed. In addition, the global quantities such as pressure and viscous drag coefficients, RMS lift and drag coefficients, recirculation length, Strouhal number and Nusselt number are determined and discussed for various Reynolds numbers. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Flow-Induced Sound of Wall-Mounted Cylinders with Different Geometries

The objective of this work was to investigate the influence of changes in geometry of a wall-mounted square cylinder in crossflow on the generated aerodynamic noise. The flowfield and acoustic field of several geometries are obtained and their interaction is analyzed. The results of the subset of geometries investigated indicate that the reduction in turbulent kinetic energy does not necessarily also lead to a reduction in radiated noise. Reducing the fluctuations of the stagnation point in front of the cylinder, maintaining a fixed separation line, and reducing the symmetry of the vortex street and its stability seem to be effective measures for reducing aerodynamic noise, among other known approaches.

Flow and heat transfer across a confined square cylinder in the steady flow regime: Effect of Peclet number

The flow and heat transfer characteristics of an isolated square cylinder in crossflow placed symmetrically in a planar slit have been investigated for the range of conditions as 1 ⩽ Re ⩽ 45, 0.7 ⩽ Pr ⩽ 4000 ( Pe ⩽ 4000) and β = 1/8, 1/6 and 1/4. Heat transfer correlations have been obtained in the steady flow regime for the constant temperature and constant heat flux boundary conditions on the solid square cylinder in crossflow. In addition, variation of the local Nusselt number on each face of the obstacle and representative isotherm plots are presented to elucidate the role of Prandtl number and blockage ratio on drag coefficient and heat transfer.

EFFECT OF CHANNEL CONFINEMENT ON THE TWO-DIMENSIONAL LAMINAR FLOW AND HEAT TRANSFER ACROSS A SQUARE CYLINDER

ABSTRACT The flow structure and heat transfer characteristics of a square cylinder in cross flow are investigated numerically for both unconfined and channel-confined (blockage ratios, 10–50% in steps of 10%) flow situations at Reynolds numbers of 50, 100, and 150, and a Prandtl number of 0.7. As the blockage ratio is increased, the Reynolds number for the onset of vortex shedding increases and then decreases. Increases in Strouhal number, drag coefficient, pumping power, and cylinder Nusselt number are also observed with increasing blockage ratio. The enhancement of cylinder Nusselt number due to channel confinement is investigated for uniform heat flux and constant cylinder temperature boundary conditions.

Closed-loop-controlled vortex shedding and vibration of a flexibly supported square cylinder under different schemes

Different schemes were experimentally investigated of the closed-loop control of vortex shedding from a spring-supported square cylinder in cross flow. The control action was implemented through the perturbation of one cylinder surface, which was generated by three piezoelectric ceramic actuators, embedded underneath the surface and controlled by a proportional-integral-derivative controller. Three control schemes were investigated using different feedback signals, including the turbulent flow signal measured by a hot wire, flow-induced structural oscillation signal obtained by a laser vibrometer, and a combination of both signals. An investigation was conducted at the resonance condition, when the vortex-shedding frequency coincided with the natural frequency of the fluid-structure system. The flow and structural vibration were measured using particle image velocimetry, laser-induced fluorescence flow visualization, a laser Doppler anemometer, and a laser vibrometer. It was observed that the control scheme based on the feedback of both flow and structural oscillation led to the almost complete destruction of the Kármán vortex street and a reduction in the structural vibration, vortex shedding strength, and drag coefficient by 82%, 65%, and 35%, respectively, outperforming by far an open-loop control as well as the other two closed-loop schemes.

HEAT AND FLUID FLOW ACROSS A SQUARE CYLINDER IN THE TWO-DIMENSIONAL LAMINAR FLOW REGIME

The flow structure and heat transfer characteristics of an isolated square cylinder in cross flow are investigated numerically for both steady and unsteady periodic laminar flow in the two-dimensional regime, for Reynolds numbers of 1 to 160 and a Prandtl number of 0.7. The effect of vortex shedding on the isotherm patterns and heat transfer from the cylinder is discussed. Heat transfer correlations between Nusselt number and Reynolds number are presented for uniform heat flux and constant cylinder temperature boundary conditions.

Autocorrelation Measurements in the Near Wake of Square Cylinders in Turbulent Free Streams

Eddy-shedding frequencies were measured for square cylinders in cross flow of turbulent air streams of different intensities and scales. The Strouhal number was found to remain almost constant at 0.130 in the Reynolds number range studied.