Cylinders In Tandem Arrangement Research Articles

On the flow around two circular cylinders in tandem arrangement at high Reynolds numbers

The flows around two tandem circular cylinders in the subcritical and supercritical flow regimes are studied through three-dimensional numerical simulations. Different spacing ratios L/D (where L is the center-to-center distance between the two cylinders with D being the diameter of the cylinders) from 2.0 to 5.0 is considered. The instantaneous flow structures, pressure distributions and hydrodynamic forces on two tandem cylinders are analyzed at subcritical (Re=2.2×104) and supercritical (Re=3.0×106) Reynolds numbers. The present numerical solutions demonstrated that at the two Re, for L/D<3.5, the shear layer from the upstream cylinder reattaches on the surface of the downstream cylinder and vortex street is only formed behind the downstream cylinder, while the shear layer from the upstream cylinder rolls up alternately for L/D≥3.5, and vortex shedding occurs from both upstream and downstream cylinders. Compared with the flow characteristics at Re=2.2×104, for Re=3.0×106, the flow separation positions on the upstream cylinder move backward and the width of the wake behind the upstream cylinder becomes narrower, which leads to forward movement of the reattachment position. At the supercritical Reynolds number, the mean drag and fluctuating lift coefficients of the upstream cylinder are nearly independent of L/D while the sharp increase of force coefficients of the downstream cylinder occurs for L/D=3.5. Comparing to the case at the subcritical Reynolds number, higher vortex-shedding frequencies from the upstream and downstream cylinders are identified. It is also revealed that the slope of the best fit line for φ (the phase lag of the fluctuating lift force between two cylinders) and L/D increases at the supercritical Reynolds number. In addition, for L/D=2.5, the mean drag coefficients of the upstream cylinder decrease sharply for Re>1.0×105, while those of the downstream cylinder rise slightly with the growing Re. Moreover, the fluctuating lift coefficients of the upstream cylinder are almost independent of Re, but those of downstream cylinder drop as Re increases.

Numerical analysis of the interference between two elastically mounted cylinders in tandem subject to flows at low Reynolds numbers

Fluid-dynamic interference among cylindrical structures in arrays, frequently used in engineering, is of fundamental and practical interests since it may considerably change fluid–structure interactions and produce large fluctuating forces on structures that cause vibrations, lock-in and important motions. This paper aims to study the mutual interference between two circular cylinders in tandem arrangement, which is considered the simplest configuration of one array cylinders, elastically mounted in transversal direction and subject to a bi-dimensional uniform laminar flow at low Reynolds numbers. Both cylinders have the same diameter D, and the center-to-center spacing is equal to L = 5.25D which corresponds to co-shedding regime. The academic numerical model Ifeinco, which is based on the finite element method and uses a partitioned scheme that considers two-way interaction of fluid flow and structure, has been employed to the analysis. Combinations of upstream and downstream stationary and elastically mounted cylinders are investigated for Reynolds number range from 90 to 140. Differences of flow behaviors drag and lift forces and resonance between single cylinder and cylinders in tandem arrangement were found. Mutual and respective influences of upstream cylinder motion on the downstream cylinder and vice versa are analyzed. Results show that, in some engineering studies, the consideration of the flexibility of cylindrical structures in arrays is fundamental since it influences significantly the interference between cylinders, mainly the magnitude and direction of forces and characteristics of the resonance range, and, consequently, structural and dynamical behaviors.

Steady separated flow around a pair of identical square cylinders in tandem array at low Reynolds numbers

The steady separated flow past two identical square cylinders in tandem arrangement is studied numerically. For a Reynolds number of 40, flow features are investigated for varied centre-to-centre distance, S between cylinders. The range of normalized spacing, SD is 2–30 where D denotes edge length of cylinders. Based on separation topology, four distinct flow regimes are identified. In regime I, the cylinders are closely spaced and gap between the cylinders is occupied by a pair of eddies and no wake forms behind the upstream cylinder. In regimes I and II, the cylinders are bridged by zero streamlines enveloping the gap flow. Thus, the cylinders along with the enclosed fluid, act as a single obstacle. Regime II forms as a manifestation of the first bifurcation of flow; the twin eddies in the gap split into four vortices. In regime III, the second bifurcation of flow occurs. The gap recirculation with four eddies splits; the first pair of counterrotating eddies contribute to wake of the upstream cylinder whereas the other pair adheres as a tiny vortical structure to the front part of downstream cylinder. In regime IV, separation topology is characterized by individual cylinder wakes. Four separation topologies, one corresponding to each regime, are proposed. The maximum width of the recirculation zone/wake of the upstream cylinder always exceeds the width of its downstream counterpart signifying higher drag for the former. For most of regime I, the downstream cylinder acts as a streamlined body.

Flow-induced vibrations of three and four long flexible cylinders in tandem arrangement: An experimental study

Multiple flexible cylindrical structures arranged in tandem are commonly used in various fields of engineering. Up to now, limited information is available about the flow-induced vibrations (FIVs) of more than two tandem flexible cylinders. Although some FIV characteristics of multiple tandem flexible cylinders can be studied by considering a tandem flexible cylinder pair, there are other characteristics remain unrevealed which require further investigations. This paper presents an experimental study on the FIVs of three and four flexible cylinders in tandem arrangement. The flexible cylinder models are identical with a mass ratio of 1.9 and a length-to-diameter ratio of 350. The centre-to-centre spacings between the cylinders are kept constant as 6D. The towing speed ranges from 0.05 m/s to 1 m/s and the corresponding Reynolds number lies in the range of 800–16 000. It is found that the presence and vibrations of the downstream cylinders have almost negligible effects on the responses of the most upstream cylinder. Wake-induced vibration (WIV) type responses are observed at high reduced velocities for the downstream cylinders. Due to the shielding effects of the upstream cylinder/cylinders and the influence of the second WIV regime, the transitions between adjacent frequency synchronisation regions and neighbouring vibration modes of the downstream cylinders are delayed to higher reduced velocities. Most of the cylinder responses are dual resonant except for the most downstream cylinder in the three-cylinder system which has identical dominant frequencies in the in-line and cross-flow directions. Combined standing wave and travelling wave responses and standing wave dominated responses are identified for three and four tandem flexible cylinders undergoing FIVs.

Flow-induced vibration of two elastically mounted tandem cylinders in cross-flow at subcritical Reynolds numbers

A series of experimental studies on flow-induced vibration (FIV) of two identical elastically mounted circular cylinders in tandem arrangement were carried out in a low turbulence surface water channel. The Reynolds number was between 2.86 × 104–1.14 × 105. Both upstream and downstream cylinders with diameter of 8.89 cm and mass ratio of 1.343 were allowed to oscillate in the transverse direction. Four spacing ratios T/D = 1.57, 2.57, 3.57 and 4.57 (where T is the center-to-center spacing and D is the cylinder diameter) were selected. Two virtual spring-damping oscillators were used to conduct the tests and measure the position signals of two tandem circular cylinders undergoing FIV. The vibrations and fluid forces of the cylinders were analyzed and discussed to illustrate the mechanisms of interaction between the two cylinders. The experimental results indicate that the FIV characteristics of the upstream cylinder are similar to those of a single cylinder for T/D = 2.57, 3.57 and 4.57 when the reduced velocity does not exceed 12.0. Moreover, the FIV response of the upstream cylinder in the two tandem cylinder system with T/D = 1.57 is significantly different from that of the isolated cylinder. The vibration of and the fluid force on the downstream cylinder are obviously influenced by the upstream cylinder for the spacing range tested. Vortex-induced vibration (VIV)-like response features are observed for the downstream cylinder.

Numerical investigation of vortex induced rotation of two square cylinders in tandem arrangement

This work performs a numerical investigation on the dynamics of the system with two square cylinders arranged in tandem at Reynolds number of 100. The influence of the vortex induced rotation and the spacing ratio (center-to-center distance over square width) between two cylinders is studied. Four system setups are considered. Case 1: Two stationary square cylinders in tandem; Case 2: Only the trailing square cylinder is allowed to rotate around its axis; Case 3: Only the leading square cylinder is allowed to rotate and Case 4: Both square cylinders are allowed to rotate around their own axes respectively. It shows that both the system setup and the spacing ratio can have significant influence on the dynamics of the system. When the leading cylinder is fixed, as in Case 1 and Case 2, the spacing ratio has the dominant effect. In these two cases, it can be very effective to suppress Cdmean/Clrms (mean drag coefficient/root mean square of lift coefficient) by keeping the spacing ratio below the critical value. In contrast, allowing the leading cylinder to rotate freely as in Case 3 and Case 4, changes the baseline of the dynamics of the system. It triggers formation of vortex shedding in the gap region with a smaller critical spacing ratio, compared with cases having a fixed leading cylinder. The force fluctuations and rotational amplitude are also much larger than those in Case 1 and Case 2. More complex flow patterns and interactions can be observed in these cases as well. It can be concluded that, keeping the leading cylinder stationary is crucial to suppress force fluctuations of the system under the conditions studied in this paper (at the Reynolds number of 100, ξ=0, Uθ=10 and I=1).

Heat and fluid flow around two co-rotating cylinders in tandem arrangement

This paper discusses on the heat and fluid flow around two co-rotating cylinders in the tandem arrangement. The non-dimensional rotating speed (R.S) varies in the range of 0 ≤ R.S ≤ 4 and different non-dimensional gap spaces such as G/D = 1.5, 2.0, and 3.0 are considered between the cylinders. Computations are performed at the Reynolds number of 200 with constant Prandtl number of 7.0. It is demonstrated that rotating the cylinders deforms the recirculating regions of both upstream and downstream cylinders in which the rate of this deformation changes as a function of the R.S and G/D. On the other hand, co-rotating the cylinders shows some additional events such as the azimuthal displacement of the front stagnation points and development of the negative lift coefficient for both cylinders. It is found that the instabilities of the shear layer for both upstream and downstream cylinders are maximum at R.S = 1 and with increasing the R.S, the vortex shedding suppresses around the cylinders due to dominating the fluid rotating zone. Finally, it is revealed that at higher R.S values, a uniform Nusselt number distribution can be observed on both cylinders regardless of the gap space between the upstream and downstream cylinders.

Direct and integral noise computation of two square cylinders in tandem arrangement

The present paper presents a comparison between two different approaches for calculating far-field noise of two square cylinders in tandem arrangement. Experimental data show that a flow past such an arrangement results in aeolian tones as well as broadband noise. The turbulent flow past the cylinders is computed using a compressible solver for low-Mach number flows. In order to validate the direct computation of acoustic waves, the complete span including endplates is calculated. Wind tunnel measurements show two different but stable flow states past the cylinders: a quiet state, where no vortex shedding is present in the wake of the upstream cylinder, and a loud state, where separated shear layers flow into the gap between the cylinders intermittently. Different initialisations of the velocity field are used in order to capture both flow states. Mean flow quantities as well as surface pressure spectra are compared to wind tunnel measurements. Far-field noise is calculated directly using the compressible solver, as well as indirectly using a hybrid method based on a Ffowcs Williams Hawkings formulation for stationary rigid surfaces. Far-field spectra and directivity patterns are compared to wind tunnel measurements. Results of both methods show good agreement with experimental data.

Influence of rounded corners on flow interference between two tandem cylinders using FVM and IB-LBM

PurposeThe purpose of this study is to numerically investigate the influence of corner radius on the flow around two square cylinders in tandem arrangements at a Reynolds number of 100.Design/methodology/approachSix models of square cylinders with corner radii R/D = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 (where R denotes the corner radius and D denotes the characteristic dimension of the body) were studied using an immersed boundary-lattice Boltzmann method, and the results were compared with those obtained using a two-dimensional unsteady finite volume method. The cylinders were mounted in a tandem configuration (1.5 ≤ L/D ≤ 10 where L denotes the in-line separation between the cylinder centers). The simulated models were quantitatively compared to the aerodynamic force coefficients and Strouhal number. Furthermore, qualitative analysis is presented in the form of flow streamlines and vorticity contours.FindingsThe R/D and L/D values were varied to observe the variation in the flow characteristics in the gap and wake regions. The numerical results revealed two different regimes over the spacing range. The drag force on the downstream cylinder was negative for all corner radii values when the cylinders were placed at L/D = 3.0 (a single-body system). Subsequently, a sudden increase was observed in the aerodynamic forces (drag and lift) when L/D increased. A different gap value was identified in the transformation from a single-body to a two-body system for different corner radii. To verify the single-body system, a simulation was carried out with a single cylinder having a longitudinal geometric dimension equal to the tandem arrangement (L/D + D). Furthermore, in a single-body regime, the total drag of a tandem cylinder was less than that of a single cylinder, thus demonstrating the benefits of using tandem structures. A significant reduction in the aerodynamic forces and drag force was achieved by rounding the sharp corners and placing the cylinders in close proximity. An appropriate configuration of the tandem cylinders with a rounded corner of R/D = 0.4 and 0.5 at L/D = 3.0 and the range is enhanced to L/D = 4.0 for 0.0 ≤ R/D < 0.4 to achieve adequate drag reduction.Originality/valueTo the best of the author’s knowledge, there is a paucity of studies examining the effect of corner radius on bluff bodies arranged in a tandem configuration.

Large Eddy simulations of flow around tandem circular cylinders in the vicinity of a plane wall

To investigate the three-dimensional flow around free-spanning, tandem marine pipelines, Large Eddy Simulations (LES) with Smagorinsky subgrid scale model are performed using the open-source code OpenFOAM. Two circular cylinders in tandem arrangement are placed in the vicinity of a rigid, horizontal, plane wall. The cylinders are immersed in a steady current with a logarithmic boundary layer profile at an intermediate, subcritical Reynolds number (Re = 1.31 × 104). The non-dimensional distances between the cylinder centres are 2 and 5 (L/D = 2 and 5) and the gap to diameter ratios are G/D = 0.6 and 1, where gap G is the distance between the bottom of the cylinders and the wall. The present results are analysed through the values of drag and lift coefficients, as well as by the details of the flow fields in the wake of the cylinders. The results are compared with experimental results of the flow around tandem cylinders in an unlimited fluid for both L/D = 2 and 5, showing that, at chosen gaps, at L/D = 2, the flow belongs to the reattachment regime, and at L/D = 5 to the co-shedding regime. Compared with the case of a single cylinder near a plane wall, the flow around the tandem cylinders at G/D = 1 belongs to the wide gap regime, while at G/D = 0.6, tandem yields a stronger interaction with the wall than in the case of one cylinder. The presence of the wall modifies the flow in the spacing between the two cylinders, giving it some characteristics of the extended body flow regime.

Effect of spacing on a pair of naturally oscillating circular cylinders in tandem arrangements employing IB-LB methods: Crossflow-induced vibrations

An efficient velocity-correction based immersed boundary-lattice Boltzmann method (IB-LBM) is employed and coupled with structural equations to investigate the vortex-induced vibration characteristics of two identical circular cylinders in various tandem arrangements at a low Reynolds number of 100 with 1.0 < Sx/D ≤ 12.0 and frequency ratio Fr=1.0; here, Sx is the streamwise displacement between the cylinder centers, D is the cylinder diameter, and Fr is the ratio of the vortex shedding frequency to the natural frequency of the oscillator. The cylinders are set to oscillate naturally in the transverse direction only, and a mass-spring system characterizes their motion. Considering a similar flow and structural parameters, a single cylinder is also studied for reference and comparison. Furthermore, a fixed cylinder and two stationary cylinders in tandem arrangements with same spacing ratios are also examined. Four distinct regimes of cylinder arrangements have been identified based on the cylinder maximum, gradual decrease to a minimum, persistence, and single cylinder response. At Sx/D=1.5, the cylinders vibrate violently with overall maximum amplitude. A critical spacing at (Sx/D)c=3.5 is observed both in the stationary and oscillated cylinders, where the forces and amplitude have the minimum values. The response amplitude and force coefficients remain constant over a range of spacing, i.e., Sx/D=4.0−8.0. The cylinders behave as single cylinders when placed more than eight-diameters apart as the interaction between the fluid and structure are decoupled. The interference and wake effects are diminished in this regime. The hydrodynamic force coefficients and flow physics of the oscillating tandem cylinders are also discussed. The results reveal that the flow in the gap between the cylinders and in the wake significantly affects the forces and hence, the dynamic response of the tandem structure.

Numerical investigation of wake induced vibrations of cylinders in tandem arrangement at subcritical Reynolds numbers

Wake-induced vibrations are considered distinctively different from well-known vortex induced vibrations where bluff body structures vibrate under vortex shedding in the wake of flows passing through them. Understanding characteristics of wake-induced vibrations plays an important role in design of marine-offshore structures such as platform columns, risers operating at high Reynolds number currents. This work presents an extensive study of wake-induced vibrations using a computational fluid dynamics approach. The numerical model is based on an incompressible Navier-Stokes flow solver with a hybrid detached eddy simulation (DES) approach for turbulence modelling of high Reynold number flows. The hybrid DES approach is first validated for wake-induced vibrations at subcritical Reynolds number ranging from 103−105. Comparison between numerical results and experiment shows very good agreement in prediction of downstream cylinder responses including amplitudes, frequency and phase angle. Data from numerical simulations is then used to characterize and study mechanisms of wake-induced vibrations by looking at detailed force components and their frequencies in relation to cylinder responses. The effect of different Reynolds number flow conditions in tandem cylinder responses are also analysed in the present work. It is found that Reynolds number has strong influence in changing response amplitude and frequency of cylinders in wake-induced vibrations.

Effects of upstream sharp edge square and diamond cylinders on the FIV of a circular cylinder

This study deals with the flow induced vibrations of a freely vibrating circular cylinder in tandem arrangement with sharp edge square and diamond upstream cylinders. Spacing ratios of 4, 8 and 11 are examined. The lateral displacements, lift forces and the pressure data in the wake of the oscillating cylinder are recorded and analyzed. The single degree of freedom vibrating system has a low mass-damping parameter and the Reynolds number ranges from 1.5 × 103 to 3.7 × 104.The overall transverse response of a circular cylinder in the wake of a sharp edge cylinder look like a VIV type of response. The ever increasing type of transverse oscillations for tandem circular cylinders at low spacing ratios no longer exists. It is concluded that in general, the wake from the upstream sharp edge cylinder has mitigated (around 40%) the lift force coefficient on the downstream cylinder. The oscillations and pressure frequencies occur primarily at the upstream sharp edge cylinder shedding frequency. It appears that the upstream wake is much stronger than the vortices past the downstream cylinder, so they take control of the downstream circular cylinder oscillations and shedding.

Numerical Investigation on Vortex-Induced Vibration Energy Extraction Efficiency of Double Circular Cylinders In Tandem Arrangement at Low Reynolds Number

Vortex shedding from a bluff body results in fluctuating forces acting on the bluff body, which may induce vibration of the bluff body when the bluff body is elastically mounted or deformable. Researchers put forward an idea that we can ex-tract energy from the water flow based on VIV at low flow velocity. Although plenty of researches on parameters of VIV are already presented, however, the improvement of energy extraction efficiency still needs further study. According to the previous research, this essay has simulated flow-induced vibration of tandem double circular cylinders when Reynolds number is 100. Working condition has been considered as the fixed upstream cylinder and the free vibration of the downstream cylinder. The influence of the mass coefficient and the two cylinders spacing ratio on the downstream cylinder’s energy obtained from the fluid is studied. Analysis results show that, the maximum value of the energy extraction efficiency is before the frequency locked range. In the case of large spacing ratio (L/D=7~9), the phenomenon of "beat vibration" appears on the downstream cylinder. The results of this work could provide reference for the improvement of energy extraction efficiency and the design of VIV converter.

A Coupled Lattice Boltzmann-Volume Penalization for Flows Past Fixed Solid Obstacles with Local Mesh Refinement

A coupled Lattice Boltzmann-Volume Penalization (LBM-VP) with local mesh refinement is presented to simulate flows past obstacles in this article. Based on the finite-difference LBM, the local mesh refinement is incorporated into the LBM to improve computing efficiency. The volume penalization method is introduced into the LBM by an external forcing term. In the LBM-VP method, the processes of interpolating velocities on the boundaries points and distributing the force density to the Eulerian points near the boundaries are unnecessary. Performing the LBM-VP on a certain point, only the variables of this point are needed, which means the whole procedure can be conducted parallelly. As a consequence, the whole computing efficiency can be improved. To verify the presented method, flows past a single circular cylinder, a pair of cylinders in tandem arrangement, and a NACA-0012 are investigated. A good agreement between the present results and the data in the previous literatures is achieved, which demonstrates the accuracy and effectiveness of the present method to solve the flows past obstacle problems.

Convective heat transfer and fluid flow of two counter-rotating cylinders in tandem arrangement

This paper discusses the forced convective heat transfer and fluid flow around two counter-rotating cylinders in tandem arrangement at a constant Reynolds number of 200. The upstream and downstream cylinders rotate in counterclockwise and clockwise directions, respectively, with an identical non-dimensional rotating speed (RS) in the range of $$0\le \hbox {RS}\le 4$$ . Computations are carried out at various non-dimensional gap spaces between the cylinders such as the $${G{/}D}=1.5$$ , 2.0, and 3.0. It is found that counter-rotating the tandem cylinders deforms the wake region downstream of both cylinders in which the vortex strength of the upstream cylinder is realized to be stronger at larger gap spaces. On the other hand, it is stated that the instabilities of the shear layer of both cylinders become maximum and minimum at $$\hbox {RS}=1$$ and $$\hbox {RS}=2$$ , respectively. Examination of the Nusselt number distributions on the cylinders indicates that at the high RS values, more or less, all points on the each individual cylinder have identical roles in the heat dissipation rate. Finally, it is concluded that the maximum heat transfer occurs at $$\hbox {RS}=1$$ for both cylinders.

Wake-Induced Vibrations of Tandem Circular Cylinders of Different Diameters

Wake-induced vibrations (WIV) of two circular cylinders in tandem arrangement of different diameters (larger downstream cylinder) has been analyzed in the present work. The ratio of the diameter of first cylinder to that of the second cylinder (D1/D2) is kept constant at 0.8. The cylinder centers are separated by a distance S (= 5D1). The computations were carried out at Reynolds number (Re) = 100 over a range of reduced speed (U*). The fluid was assumed to be incompressible and flow to be two-dimensional. The flow equations were discretized using stabilized finite-element method and solved iteratively by Generalized Minimal RESidual (GMRES) technique. In the present work, the upstream cylinder is kept stationary while the downstream cylinder is allowed to traverse only across the flow. The observations focus the response, aerodynamic coefficients and flow features.

Wake-Induced-Vibrations of Circular Cylinders in Tandem Arrangement

Stabilized space-time finite element technique is utilized to perform numerical simulation on the wake-induced-vibrations of two identical tandem circular cylinders placed in the flow at Reynolds number 100. Cylinders are separated by a center to center distance of 5 times the diameter of the cylinder. Results are computed for various values of reduced velocities (1 to 8). The downstream cylinder, of mass ratio 10, is allowed to execute oscillations in both in-line and transverse directions, whereas the upstream cylinder is stationary. Response curve of downstream cylinder is observed to be composed of initial and lower branches. Phenomenon of lock-in is identified at a reduced velocity of 5. Nature of flow exhibits periodicity for the entire range of initial branch with single dominant lift frequency. For reduced velocity near the end of lock-in, flow and body motion shows quasi-periodic behavior with complicated non-overlapping loops in the cylinder trajectory as well as in the phase plots of drag and lift coefficients.

Flow induced vibrations of a sharp edge square cylinder in the wake of a circular cylinder

The response of an oscillating circular cylinder at the wake of an upstream fixed circular cylinder was classified by different researchers as galloping, wake induced galloping or wake induced vibration. Furthermore it is already known that a sharp edge square cylinder would undergo galloping if it is subjected to uniform flow. In this study the influence of the wake of a fixed circular cylinder on the response of a downstream square cylinder at different spacing ratios (S/D=4, 8, 11) is experimentally investigated. The subject appears not to have received previous attention. The lateral displacements, lift forces and the pressure data from gauges mounted in the wake of the oscillating cylinder are recorded and analyzed. The single degree of freedom vibrating system has a low mass-damping parameter and the Reynolds number ranges from 7.7×102 to 3.7×104.In contrast to that for two circular cylinders in tandem arrangement, the freely mounted downstream square cylinder displays a VIV type of response at all spacing ratios tested. There is no sign of galloping or wake induced galloping with the square cylinder. With increase at the spacing ratio the cross-flow oscillations decrease. It is shown that the vortices arriving from the upstream fixed circular cylinder play a major role on the shedding mechanism behind the downstream square cylinder and cause the square cylinder to shed vortices with frequencies above Strouhal frequency of the fixed square cylinder (St=0.13). The VIV type of oscillations in the downstream square cylinder is most probably caused by the vortices newly generated behind the square cylinder.

Flow pattern and heat transfer past two tandem arranged cylinders with oscillating inlet velocity

Unsteady laminar convective fluid flow and heat transfer past two cylinders in tandem arrangement is investigated. Besides forced mixing by the natural vortex streets, extra imposed oscillating inlet velocity are considered. The lift and drag coefficients and Strouhal number for a single cylinder and their dependence on Reynolds number are also provided to validate the calculations with previous experiments. Different frequencies and amplitudes of the oscillating flows are set as inlet boundary conditions. For the cycle effect, at some specified points of the cycle ratios, the heat transfer enhancement of the upstream cylinder and the downstream cylinder are relatively larger due to the combined-effect of the natural vortex streets and the imposed oscillating inlet flows. The interaction between the natural vortex streets and imposed oscillating inlet flow leads to non-regular shaped vortices with increased flow mixing. For the amplitude effect, Nusselt numbers of the two cylinders are both greatly increased with increased amplitude ratios. With increased amplitude ratios, the vortex street expands and vibrates in a larger region which leads to stronger flow mixing and higher heat transfer. Generally, oscillating inlet flows can efficiently increase the heat transfer of two cylinders in crossflow without increasing the flow rate of the coolants.