Upstream Cylinder Research Articles

Effect of Upstream Cylinder on the Wake Structure and Evolution of Flow over a Finned Circular Cylinder

This paper reports the results of Computational Fluid Dynamics (CFD) simulations to investigate the effect of an upstream cylinder on the wake structure and evolution of flow over a finned circular cylinder evenly attached with rectangular ribs around its circumference and identifying the near-wall vortex structure are formed. Also examined the effect of the number of ribs (n=3, 5) in a low Reynolds number range of 60–180. The main concern in the aerospace domain is the vortex induced vibrations. It is necessary to find ways to reduce the fluid forces and stabilise the flow wake either actively or passively. Therefore, be it in the field of aerodynamics and in offshore wind turbines. The near-wall vortices and their evolution are sensitive to Re as well as n. The evolution of flow around the cylinder with 3 ribs (n = 3) QS are formed in the corners over which the boundary layers stride. The subordinate vortices disappear at Re=60, possibly attributed to the relatively great viscous force at such lower Re. The evolutions of flow over the finned cylinder with 5 ribs at three representative Re cases are compared. The number of Sub created in accompany with a main vortex grows with the increase in Re. and then generates the main vortices behind the associated ribs

Numerical study of the boundary layer flow past two wall mounted finite-length square cylinders in staggered arrangement

Flow past two wall-mounted square cylinders in staggered arrangement is simulated through direct numerical simulation to investigate the effects of the degree of staggering on the wake and forces compared to the tandem arrangement. Numerical simulations are conducted for a constant Reynolds number of 500, height to width ratio H = 4, staggered distance to diameter ratios S = 0.5 and 1, and gap to width ratios G in the range from 1 to 8. It is found that moving the downstream cylinder sideway for S = 0.5 and 1 changes flow features and force coefficient significantly. At small gaps of G = 1 to 3, the shear layer from one side of the upstream cylinder is trapped into the gap. In addition, a strong horseshoe vortex in front of the downstream cylinder is found for all the gaps at S = 0.5 and 1, but not found at S = 0. The friction velocity on the plane boundary caused by the horseshoe vortex of the downstream cylinder is greater than that of a single cylinder at S = 1 and G = 1 to 8 and at S = 0.5 and G = 2 to 5. At G = 1 to 3, vortex shedding is caused by the trapped flow into the gap on one side of the downstream cylinder and by the approaching flow on another side. The negative mean drag coefficient at small gaps was found in the tandem arrangement case, but not found at S = 0.5 and 1.

Three dimensional flow over elliptic cylinders arrays in octagonal arrangement

A numerical study of flow characteristics around eight elliptic cross-section cylinders in an octagonal configuration is presented. The axis ratio of elliptic cylinder is taken as 1:2. Three spacings between the cylinders (0.07 m, 0.14 m and 0.2 m), two angles of attack (α=0° and α=15°) and two Reynolds numbers (Re=4060 and Re=45800) are considered to investigate the parametric influences. The flow is modelled using three-dimensional large eddy simulation. Simulations are performed by utilizing ANSYS Fluent software. The results comprise of flow patterns and force coefficients (drag and lift). It is seen that both the drag and lift forces acting on the cylinders vary with the spacings values, angle of attack and Reynolds numbers. The lift force peaks at α=15° and Reynolds number 45800. The drag force on the upstream cylinder reaches its peak at α=0° and Reynolds number 4060. It is observed that the drag on the upstream cylinders decreases as the value of the Reynolds number increases. Contours of velocity and subgrid turbulent viscosity are also presented. Moreover, it is concluded from the present study that the effect of change in the Reynolds number on flow characteristics is more significant as compared to the change in the spacing between the cylinders.

Flows around two nonparallel tandem circular cylinders

The paper presents a systematic experimental study on the dynamics of flow around two nonparallel tandem cylinders at a Reynolds number of 5.6 × 104. Three generic configurations of the cylinders are considered: 1 the upstream cylinder yawed and downstream cylinder upright, 2 both cylinders yawed oppositely, and 3 the upstream cylinder upright and the downstream cylinder yawed. Extensive measurements are conducted of the surface flow pattern, fluid forces, shedding frequencies, and velocity fields around the cylinders. The flow is classified into three regimes based on the behaviors of shear layers around the cylinders for all configurations. The wake in each regime is characterized in detail, including its recirculation strength, vortex formation length, wake width, Strouhal numbers, and mean and fluctuating fluid forces on the cylinders. Conceptual flow structure models are proposed for each configuration. It has been found that the flow around two nonparallel cylinders is characterized by spiral vortices, with either upward or downward motion or both. These vortices play a crucial role in fluid dynamics, producing a profound impact on the fluid forces as well as flow three-dimensionality.

Experimental investigation of downstream cylinder vibrations caused by FIV in the presence of upstream stationary cylinder

The mechanism of flow induced vibrations (FIV) exists in many engineering applications and plays a significant role. In offshore engineering, the risers subjected to flow induced vibrations. In this study, two circular cylinders of different diameters are arranged in tandem manner. In cross-flow transverse direction, the upstream cylinder is fixed, while the downstream cylinder is subjected to vibrations. This study carries out experiments in wind tunnel, which focuses on significant vibration-galloping. Variable mass-damping ratio is used to achieve the critical velocity corresponding to onset galloping vibration. In the present study, the diameter ratio is 0.4 and the gap L is 1.5D, and 3.5D. In the wind tunnel experiments, by changing the mass of the vibrating cylinder, the corresponding values of damping ratio, results in variable mass damping ratio m*ζ. For each value of m*ζ, the amplitude, frequency at every reduced velocity will be obtained at first, the critical velocity and stability map will be summarized. At static area between lock-in and galloping (desynchronization branch), a uniform initial displacement was set to get the total damping ratio. This study also investigates the relationship among the total damping ratio, m*ζ and critical velocity.

Mass ratio effect on vortex-induced vibration for two tandem square cylinders at a low Reynolds number

A numerical simulation study is conducted to investigate the effect of the mass ratio (m* = 3, 10, and 20) on vortex-induced vibration (VIV) of two tandem square cylinders at Re = 150. In this study, we mainly focus on the mass ratio effect on the vibration response, force characteristics, wake mode pattern, and fluid–structure-interaction (FSI) mechanism. The results show that mass ratio plays an important role in the VIV response of the two cylinders. With increasing reduced velocity, both the upstream cylinder (UC) and downstream cylinder (DC) at m* = 3 exhibit the soft-lock-in phenomenon (at a lock-in frequency ratio of fy/fn < 1) instead of the typical lock-in phenomenon (at a lock-in frequency ratio of fy/fn ≈ 1). With increase in the mass ratio to m* = 10 and 20, the soft-lock-in phenomenon disappears, while the DC exhibits the typical lock-in phenomenon. The maximum amplitudes of the two cylinders notably decrease with increasing mass ratio. Furthermore, the mass ratio exerts a major impact on the distance between the two cylinders, which may change the flow pattern. The distance sharply decreases in the synchronization region at m* = 3 but remains almost constant at m* = 10 and 20. In addition, the wake mode and FSI mechanism are more diverse at a low mass ratio (m* = 3).

Numerical investigation on the wave force characteristics of multi-cylinder with equilateral-triangular arrangement

Based on the open source program DualSPHysics, a numerical investigation is carried out on the hydrodynamic characteristics of multi-cylinder with equilateral-triangular arrangement and the wave field distribution characteristics under the action of regular waves. With the SPH method, the hydrodynamic response characteristics of multi-cylinder with equilateral-triangular arrangement are analyzed under different wave directions. The phase difference distribution characteristics of the wave force on multi-cylinder with equilateral-triangular arrangement are analyzed, as well as the wave field distribution characteristics under different wave angles. In addition, the wave force distribution characteristics and the phase difference distribution characteristics of multi-cylinder with equilateral-triangular arrangement are explored under different wave parameters (wave period and wave height). Through the observations of the hydrodynamic forces induced by the wave, the phase difference of these forces on upstream cylinder and downstream cylinder is the largest when the wave angle is 30°. The maximum wave force of the cylinder is roughly proportional to the distance between the center of the cylinder and the x-axis centerline of the wave field.

The effect of neighbors on the effective inertial collision efficiency of cylindrical collectors

We investigate the effect that an upstream cylinder has on the effective inertial collision efficiency of a downstream cylinder. We consider an initially uniform distribution of particles moving past a cylindrical fiber in a potential crossflow and numerically calculate the resulting particle distribution far downstream of the fiber using finite differences. We then investigate the effect that the downstream distribution of particles has on the effective inertial collision efficiency of a downstream test cylinder oriented parallel to the first. We show that the effective collision efficiency of the test cylinder depends on its relative offset in the transverse direction (normal to the flow) with respect to the upstream cylinder. For relatively small offsets there is complete shielding, and no particles collide with the test cylinder. As the offset increases, a growing effective collision efficiency is observed leading to a significant enhancement relative to the collision efficiency of an isolated cylinder for intermediate offsets and Stokes numbers close to unity. Importantly, we observe relative gains in effective collision efficiency above 100% for a range of Stokes numbers and offset values. For larger offsets and Stokes numbers, the effect of the upstream cylinder on the effective collision efficiency of the test cylinder decreases and eventually becomes negligible. Detailed results are provided for Stokes drag acting on the particles and we also show that a non-linear drag has a negligible effect on the observed trends for the conditions relevant to air filtration systems. These results suggest that the arrangement of fibers has a substantial effect on inertial particle impaction even when neighboring fibers do not alter the flow field around each individual collector. Moreover, periodic arrangements with specific orientations with respect of the flow could lead to a significant enhancement of the inertial collision of particles on an array of collectors, thus contributing towards a better filtration performance.

Wake evolution and vortex structure characteristics of flow over two tandem semi-circular cylinders with flat surfaces facing each other

This paper reports the results of a numerical investigation into the flow over two tandem semi-circular cylinders with flat surfaces facing each other and the associated wake flow characteristics. The effect of spacing ratio (L/D, L is the in-line spacing between two flat surfaces and D is the diameter of cylinders) ranging from 1 to 16 is examined in a low Reynolds number range of Re = 60–180. Five flow patterns are identified including the over-shoot, continuous reattachment, alternate reattachment, quasi-co-shedding, and co-shedding regimes. The difference between quasi-co-shedding and co-shedding regimes depends on the origin of the vortices shed from the downstream cylinder. Unlike the vortices generated due to the roll-up of shear layers detached from both upstream and downstream cylinders in the co-shedding regime, the vortices shed from the downstream cylinder come from upstream instead of the roll-up of shear layers for the quasi-co-shedding regime. The wake flow characteristics are discussed in terms of the vortex formation and shedding, wake structure and evolution, boundary layer separation and shear layer development, the occurrence of secondary vortex street, and the associated migrating vortex modes. The variations of hydrodynamic coefficients and Strouhal number are associated with the flow regime transition. When the alternate reattachment and quasi-co-shedding regimes emerge, a small pair of peaks of the root-mean-squared streamwise flow velocity (u*rms) fixedly occurs at the two corners of the downstream cylinder, illustrating the strong velocity fluctuation caused by vortices impingement and the secondary separation. Three types of vortex street are observed in the wake, where the secondary vortex street region is further divided into three parts in terms of the vortex structure, including S + P (a single vortex and a pair of vortices are formed per cycle in the secondary vortex street), P (a pair of vortices is formed per cycle in the secondary vortex street), and 2S (two vortices in opposite directions are formed per cycle in the secondary vortex street) modes. Two partitions in Re–L/D map (Re and L/D as the vertical and horizontal ordinates, respectively) are proposed in this work for the flow regime as well as the vortex street.

A Numerical Investigation of Turbulent Flow Over Single and Tandem Square Cylinders

This paper presents the three-dimensional flow investigations over the square cylinders placed in the tandem arrangement. Two different flow configurations were investigated in detail; one comprising of a single square cylinder and the other comprising of three square cylinders placed in a tandem arrangement with the spacing of six times the width (w) of each square cylinder. The Reynolds number based on the width of the square cylinder (w) and free stream velocity (Uo) is 22,000. The problem was solved numerically using an Unsteady Reynolds-Averaged Navier-Stokes (URANS) based model and Large Eddy Simulation (LES) based model. Strouhal Number, lift, and drag coefficients were computed for each configuration. By comparing both the models using contour plots of pressure, velocity and vorticity it is found that the LES model is more accurate to capture the turbulence around single and tandem square cylinders. In the tandem arrangement, complex periodic vortex shedding was observed in the wake of each square cylinder. The production of turbulent kinetic energy was also investigated to understand the roles of stresses during flow over the cylinders. The analysis showed that the production of turbulence by normal stresses is higher than that of shear stresses. Furthermore, it was observed that the flow over the first cylinder arranged in tandem is quite identical to that of the single square cylinder. Moreover, the upstream cylinder experienced a higher lift in comparison to the downstream cylinders.

Performance of helical strakes in suppressing the FIV fatigue damage of two long flexible cylinders in a tandem configuration

Helical strakes are the most common vibration suppression devices for cylindrical structures undergoing vortex-shedding. Many problems with the performance of helical strakes on flow-induced vibration (FIV) fatigue damage suppression of two tandem cylinders have not been solved yet. The effectiveness of three-strand helical strakes with a pitch of 17.5D and a height of 0.25D (where D is the structural diameter) in suppressing fatigue damage of two tandem long flexible cylinders undergoing FIV was investigated in this paper. A constant spacing ratio (center-to-center distance to cylinder diameter) of 8.0 was selected. Four cases of the two cylinders, i.e., naked-naked, naked-straked, straked-naked and straked-straked, were executed under subcritical Reynolds number (Re ≤ 1.6 × 104). Based on the strain responses from the experimental tests, fatigue damage was estimated by using S–N curves and Miner's rule. To highlight the role of helical strakes in FIV fatigue damage suppression, an isolated naked cylinder was considered as a benchmark. It is shown that helical strakes exert substantially the same effect on FIV fatigue suppression of the upstream cylinder as they do on an isolated cylinder. The upstream cylinder significantly affects the performance of helical strakes on FIV fatigue suppression of the downstream cylinder. Both vortex breaking and prevention of interaction of two shear layers predominate the primary mechanism of the downstream straked cylinder when the upstream one is naked, which reinforces its cross-flow fatigue damage and hereby lowers the performance of helical strakes. In contrast, due to wake-induced vibration (WIV), the downstream straked cylinder laid behind a straked one might encounter much more serious fatigue damages in both the cross-flow and in-line directions than the case when the upstream cylinder is naked.

직렬로 배열된 두 원형 실린더 주위를 지나는 입자가 포함된 유동

Numerical simulations are performed for particle-laden flow around two circular cylinders in tandem arrangements. The present study aims to understand the effects of cylinder spacing and particle size on particle dispersion and deposition characteristics. We consider the Reynolds number of 100 at which vortex shedding takes place, and the density ratio of particle to fluid is fixed as 1,000. For a parametric study, we vary the Stokes number from 0.1 to 10 for the center-to-center spacing of 2D, 3D, and 4D, where D is the cylinder diameter. As the spacing decreases from 4D to 2D, vortex shedding becomes weaker and lift fluctuations are reduced, agreeing well with the previous results. The dispersion pattern of particles around the cylinder changes significantly depending on the spacing. The overall deposition efficiency of the upstream cylinder is very similar to that of a single cylinder. However, the efficiency of the downstream cylinder has much smaller values with a strong dependency on the spacing. The local deposition efficiency and the deposition location are also analyzed in detail.

Prediction of wind-induced vibrations of twin circular cylinders based on machine learning

This paper investigates vortex-induced vibrations (VIV) of a pair of circular cylinders with identical diameters at tandem and staggered configurations. The lattice Boltzmann method was used for two-dimensional computational fluid dynamics (CFD) simulations. Both cylinders were installed elastically and vibrate transversely and the Reynolds number Re = 150. The computation results reveal that mass ratio M∗, the angle between the centerline of two cylinders and the fluid flow direction α, the reduced velocity U∗ and the ratio of the distance between the center lines of the two cylinders L∗ play key roles in the amplitude of the upstream cylinder and downstream cylinder. Subsequently, this study selected the above four parameters as input feature and trained two machine learning models to predict the amplitude of the upstream cylinder and the downstream cylinder, respectively. Three machine learning algorithms, namely decision tree regressor (DTR), random forest (RF), and gradient boosting regression trees (GBRT), were tested. Among them, the GBRT model performed optimally in predicting the amplitude of both the upstream and downstream cylinders. The GBRT model is capable of predicting the amplitude of the upstream and downstream cylinders precisely within the test range of M∗, α, U∗ and L∗.

Flow Characteristics and Fluid Forces Reduction of Flow Past Two Tandem Cylinders in Presence of Attached Splitter Plate

Numerical simulations are carried out to study the flow around two tandem square cylinders (SC) under the effect of spacing ratio(g/D) and splitter plate length (l/D) for a fixed Reynolds number (Re) = 100. The g/D is varied from 0 to 10 and l/D is varied from 0.5 to 10. The splitter plate length is found to have strong effect on vortex shedding and fluid forces. The maximum reduction in mean drag coefficient is observed at l/D = 8, that is 15% and 78% for upstream and downstream cylinders, respectively. The maximum reduction in root-mean-square value of lift coefficient is found at l/D = 10, that is 99%. The flow pattern at both of these points is steady flow. There is 100% vortex shedding suppression for l/D > 5. The observed flow patterns for flow past tandem cylinders without splitter plate are; single bluff body (SBB), steady flow (SF), quasi-steady flow (QSF), fully developed flow (FDF) and fully developed two-row vortex street flow (FDTRVS) regimes. SBB, QSF and SF regimes were observed in presence of splitter plate.

Numerical study of the flow past two wall-mounted finite-length square cylinders in tandem arrangement

Flow past two wall-mounted square cylinders in a tandem arrangement are simulated through direct numerical simulation to investigate the effects of the gap between the two cylinders on the wake. Numerical simulations are conducted for a constant Reynolds number of 500, height to width length ratio H = 4, and gap to width ratios of G = 1 to 8 with an interval of 1. The flow in the wake of the downstream cylinder is found to be significantly affected by the free shear layers from the top and sides of the upstream cylinder. At G = 1 and 2, the free-shear layer generated from the upstream cylinder reattaches the top surface of the downstream cylinder and further develops into a downwash behind the downstream cylinder. At G = 3 to 8, the downwash behind the downstream cylinder disappears because flow separation from the top upstream edge of the downstream cylinder does not occur for G = 3 to 6 and is very weak for G = 7 and 8. The disappearance of downwash in the wake of the downstream cylinder further results in very weak variation of flow along the span of the downstream cylinder. The single, reattachment, and binary wake modes at the mid-span of the cylinder occur at G = (1 and 2), (3 and 4), and (5 and above), respectively.

Numerical simulation of the VIV of twin tandem diamond cylinders at low Reynolds numbers

To further reveal vibration interference of a multicylinder system, the vortex-induced vibration of two-degree-of-freedom upstream and downstream diamond cylinders with a center-to-center distance L = 5B (B is characteristic length of cylinders) and a mass ratio of the oscillator fixed at m* = 2 was numerically investigated. The influences of the Reynolds number (Re = 40–200) and the reduced velocity (Ur = 3–12) on the dynamic characteristics were considered. The results show that Re has a remarkable effect on the VIV characteristics of twin cylinders, especially on the downstream cylinder. With increasing Re numbers, the lock-in region for the upstream cylinder remains almost constant, while that of the downstream cylinder increases. Unlike the low Re numbers (Re = 40–80), the trajectory of the upstream cylinder at high Re numbers is a closed ring shape rather than a figure “8” within the lock-in region. The downstream cylinder is dominated by transverse vibration in most cases of Re = 40, while it exhibits a significant vibration in both the streamwise and transverse directions at Re = 80–200. Additionally, the wake of twin cylinders at Re = 40 and Ur = 3–4 displays a steady state. With increasing Re numbers, the steady state disappears, and the wake modes with various reduced velocities at high Re (Re = 120–200) are more complicated and disordered.

Analysis of the thermo-fluidic transport around counter-rotating tandem circular cylinders

The work physically relates to the influence of thermal buoyancy on the flow and heat transfer of an incompressible fluid around two counter-rotating circular cylinders arranged in tandem configuration within an unconfined domain. Two-dimensional numerical simulations are conducted using a finite volume based computational fluid dynamics tool to explore the problem. The Reynolds number is taken as 100 with Prandtl number 0.71, keeping the non-dimensional spacing between the cylinders fixed at 1.5. The cylinder rotations are considered in the range of a dimensionless speed of 0 to 5. The upstream cylinder is rotating in the clockwise sense, whereas, the downstream one in the counter-clockwise sense. The buoyancy effect is analyzed for the Richardson number range 0 to 1. The flow is unsteady periodic characterized by vortex shedding around the stationary cylinders at the chosen value of the Reynolds number. The flow shows unsteadiness with vortex shedding initially with increasing rotational speed; however, at a critical value of the rotation, the flow becomes stabilized with suppression of vortex shedding. On the contrary, the cross thermal buoyancy effect destabilizes the flow into an unsteady periodic pattern. This complex interplay among the free stream flow, cross buoyancy, and counter-rotation produces intriguing fluid dynamic and thermal phenomena. The critical rotational speeds for the range of Richardson numbers are obtained as [Formula: see text] respectively for Ri = 0, 0.25, 0.5 and 1. A corresponding regime diagram is also constructed to depict the unsteady and steady zones of operation.

Empirical model of the wake-induced lift force on a cylinder with low mass ratio

The vortex-induced vibrations (VIV) of two flexible circular cylinders in a tandem configuration were studied numerically for spacing ratios ranging from 6 to 18 and the reduced velocities ranging from 2.35 to 12.59. The VIV response amplitude, response frequency, fluid force, pressure distribution and vortex structure of the tandem cylinders with different spacing ratios under different reduced velocities were compared. The results indicate that there is a great difference between the lift forces on the downstream and upstream cylinders. The lift coefficient of the downstream cylinder undergoing the wake-induced vibrations (WIV) is larger than that of the upstream cylinder, and the dominant frequency curves of the lift coefficients of the upstream and downstream cylinders separate. It can be found that the length and intensity of the wake are quite different under different reduced velocities and spacing ratios, and the reattachment positions between the wake and the downstream cylinder are different, which leads to a great change in the flow around the downstream cylinder and have a great effect on the wake-induced lift force on the downstream cylinder. Considering these factors, an empirical model for the wake-induced lift force on a cylinder with low mass ratio was proposed and verified.

Effects of obstacle's curvature on shock dynamics of gravity-driven granular flows impacting a circular cylinder

Granular flows, such as rock avalanches and debris flows, are gravity-driven and often hit engineering structures that are placed in their paths, generating dynamic impact pressures and potentially causing hazard to infrastructures. To improve the design of protective structures and the hazard assessment, it is important to understand the physical mechanism of such granular flows impacting on obstacles with variable sizes and shapes. In this study, the dynamics of granular shock waves generated by experimental flows impacting on a circular cylinder varying in diameter and on a closed barrier were investigated by conducting systematic laboratory experiments. Pressure sensors were mounted at the bottom of the experimental chute and the upstream cylinder surface to measure the dynamic impact pressures in the granular shock area. Accelerometers that were installed at the underside of the chute bed were used to record the seismic signals of granular flows during the entire impacting process. The velocity and depth of a granular flow impacting (or just before impacting) on a cylinder were estimated using an image processing method. The results showed that the dimensionless standoff distance DDstandoff of the granular shock wave decreases nonlinearly with increasing Froude number ranging from 5.8 to 12.3. The values of DDstandoff tend to change less for the larger investigated Froude numbers, approaching a limit of about 0.19. The dimensionless runup and the pinch-off distance correlated linearly with increasing Froude number and can be affected by the radius of curvature (RC) of the obstacle at the stagnation point. The impact pressures grow linearly as Froude number increases, and the RC with a smaller value usually excites a larger impact pressure. Additionally, the effect of RC on the dimensionless pressures was observed to be weakened by an increase in Froude number. The mean frequency of the seismic signals produced by the impacts of a granular flow also indicates a dependency on RC. The findings of this study contribute to the understanding of the dynamic signal response generated by a granular flow, which may result in improving the design of the protective structures.

Flow-induced vibration of two tandemly arranged circular cylinders with attached splitter plates

In this numerical study, vortex-induced vibrations (VIVs) of two elastically mounted tandem circular cylinders with attached splitter plate are simulated at Re=150, mr=2 and damping ratio of ζ = 0. The cylinders are allowed to vibrate in both streamwise and transverse directions. Three different configurations including splitter plate attached to only upstream cylinder, attached to only downstream cylinder and attached to both cylinders are investigated. Results show that in many of the examined cases the galloping response occurs which led to large values of vibration amplitudes. Moreover, in tandem cylinders with a large spacing ratio, adding a plate to the downstream cylinder does not change the vibrations of the upstream cylinder at small to medium reduced velocities. For this case, the galloping vibration occurs at medium reduced velocities for downstream cylinder-plate assembly (CP) and at high reduced velocity for downstream cylinder. Furthermore, adding a plate to the upstream cylinder greatly affects the vibrations of both cylinders, causing the galloping vibration of the upstream cylinder at low reduced velocity and the galloping vibration of upstream CP at medium reduced velocity. During the galloping response, various wake patterns behind downstream cylinder-plate body including P + S, 2P, 2D, T + P, 2T, Q + T and 2Q are observed.