Vortex Identification Methods Research Articles

New objective Liutex vector based on an optimization procedure

The objectivity of a vortex identification method, which is critical in capturing vortices in rotating machineries, requires that the visualized vortical structures are independent of the movement of the observer. However, the common methodology to objectivize an Eulerian vortex identification method with relative/net vorticity or relative/net velocity gradient suffers from the issues of requirement to select a spatial volume to do vorticity average and inconsistency with the non-objective counterpart in an inertial reference frame. In this regard, a new objective Liutex vector method to visualize vortices is developed in the current study, which is based on the observed intermittency of Liutex represented vortices in turbulence. The intermittency indicates that a large volume fraction of the considered domain is void of Liutex, and the rotating motion of the observer is very likely to decrease this volume fraction. Therefore, an optimization procedure is proposed to approximate the rotating angular velocity of the observer by maximizing the zero Liutex volume percentage. With the known angular velocity to offset the rotational effects of the observer’s reference frame, objective version of Eulerian vortex identification methods can be introduced. As an example, the new objective Liutex method has been applied to turbulent channel flow at Reτ=180 in arbitrarily rotating coordinate systems, and the results show that the new method is both objective, i.e. independent of observer’s motion including translation and rotation and consistent with the original Liutex method in an inertial reference frame.

Spacing Ratio Effects on the Evolution of the Flow Structure of Two Tandem Circular Cylinders in Proximity to a Wall

The flow around two tandem circular cylinders in proximity to a wall is investigated using particle image velocimetry (PIV) for Re = 2 × 103. The spacing ratios L/D are 1, 2, and 5, and the gap ratios G/D are 0.3, 0.6, and 1. The proper orthogonal decomposition (POD) method and λci vortex identification method are used to investigate the evolution of flow structure, and the influences of L/D and G/D on flow physics are shown. At L/D = 2 and G/D = 0.3, a “pairing” process occurs between the wall shear layer and the upstream cylinder’s lower shear layer, resulting in a small separation bubble behind the upstream cylinder. At L/D = 1, the Strouhal number (St) increases with decreasing G/D. At three gap ratios, the St gradually decreases as L/D increases. At G/D = 0.3, there is nearly a 49.98% decrease from St = 0.3295 at L/D = 1 to St = 0.1648 at L/D = 5, which is larger than the reductions in cases of G/D = 0.6 and G/D = 1. The effects of L/D on the evolution of flow structure at G/D = 0.6 are revealed in detail. At L/D = 1, the vortex shedding resembles that of the single cylinder. As L/D increases to 2, a squarish flow structure is formed between two cylinders, and a small secondary vortex is formed due to induction of the lower shear layer of the upstream cylinder. At L/D = 5, there is a vortex merging process between the upper wake vortices of the upstream and downstream cylinders, and the lower wake vortex of the upstream cylinder directly impinges the downstream cylinder. In addition, the shear layers and wake vortices of the upstream cylinder interact with the wake of the downstream cylinder as L/D increases, resulting in reductions in velocity fluctuations, and the production and turbulent diffusion of turbulent kinetic energy are decreased behind the downstream cylinder.

Laminar flow and convective heat transfer characteristics of plain and wavy wall cross confined circular cylinder

An attempt is made to identify the convective heat transfer characteristics of the plain wall (Pw) and wavy wall (Ww) cross confined circular cylinder in the laminar flow regime (20 ≤ Re ≤ 100). The study has been extended to identify the wake heat transfer characteristics for various confinements (L = 0.5D, 1D, 1.5D and 2D), such that the weaker and stronger heat transfer region in the downstream of cross-confined cylinder has been investigated. CFD solver Ansys Fluent (Version 18.0) is used for assessing the field variables, the results are analyzed using streamline plots, λ 2-criterion vortex identification method, Nusselt number and field synergy method. The wall confinement is highly significant on heat transfer, the maximum Nusselt number is observed at high confinement (L = 0.5D) for all the Reynolds number studied. The changes in Nusselt number with respect to confinements is more significant at high confinements (L = 0.5D and L = 1D), become less significant at low confinements (L = 1.5D, 2D). It is found around 73% and 50% rise in Nusselt number for the confinement height, L = 0.5D as compared to L = 1D at Reynolds number 20 and100, respectively. Besides, in order to identify the heat stagnation region in the wake, contour of zero temperature gradient along the flow direction is observed. From the contour, it is identified that more heat stagnation region in the wake of Pw confined cylinder as compared to Ww confinement which is due to the three-dimensional behavior of flow in the wake.

Numerical investigation of the cavitation effects on the wake dynamics behind a blunt trailing edge hydrofoil

The influence of cavitation on the wake behind a NACA 0009 hydrofoil with a truncated trailing edge has been numerically investigated using a homogeneous mixture model coupled with a controlled decay SST γ-Reθt turbulence model. Using optimal definitions of the inlet turbulent intensity and the empirical condensation and vaporization coefficients of the cavitation model, simulated results have shown a good agreement with experimental data. Notably, the numerical results exhibit negligible deviations of less than 0.7% in vortex shedding frequencies for different cavitation levels. If the size of the vortex cavitation grows, a substantial increase in both lift and drag coefficients on the hydrofoil is observed. Furthermore, the influence of cavitation on the trajectories of vortex centers and the morphology of the primary shedding vortices has been revealed using a vortex identification method. The findings highlight that the cavitation development enhances the advected velocity of the shedding vortices while decreasing the streamwise inter-vortex spacing. Consequently, both factors are found to contribute to the increase of the vortex-shedding frequency behind the NACA 0009 hydrofoil with the truncated trailing edge when cavitation appears and develops.

An insight into the capability of the actuator line method to resolve tip vortices

Abstract. The actuator line method (ALM) is increasingly being preferred to the ubiquitous blade element momentum (BEM) approach in several applications related to wind turbine simulation, thanks to the higher level of fidelity required by the design and analysis of modern machines. Its capability to resolve blade tip vortices and their effect on the blade load profile is, however, still unsatisfactory, especially when compared to other medium-fidelity methodologies such as the lifting line theory (LLT). Despite the numerical strategies proposed so far to overcome this limitation, the reason for such behavior is still unclear. To investigate this aspect, the present study uses the ALM tool developed by the authors for the ANSYS® Fluent® solver (v. 20.2) to simulate a NACA0018 finite wing for different pitch angles. Three different test cases were considered: high-fidelity blade-resolved computational fluid dynamics (CFD) simulations (to be used as a benchmark), standard ALM, and ALM with the spanwise force distribution coming from blade-resolved data (frozen ALM). The last option was included to isolate the effect of force projection, using three different smearing functions. For the postprocessing of the results, two different techniques were applied: the LineAverage sampling of the local angle of attack along the blade and state-of-the-art vortex identification methods (VIMs) to outline the blade vortex system. The analysis showed that the ALM can account for tip effects without the need for additional corrections, provided that the correct angle of attack sampling and force projection strategies are adopted.

Suction and discharge flow controls of a co-rotating scroll hydrogen circulation pump

The purpose of this paper is to reveal the relationship between the suction, discharge flow control and self-designed co-rotating scroll hydrogen circulation pump (CSHCP) aerodynamic performance and irreversible flow loss. The effect of five suction angles and five discharge spoiler positions on the performance and flow loss of a CSHCP was firstly revealed using the computational fluid dynamics (CFD) three-dimensional numerical simulation method combined with entropy production (EP) theory and omega vortex identification methods. The optimized suction angle and discharge spoiler position of CSHCP were identified through comparing with isentropic and volumetric efficiency, EP, vortex distribution and flow pulsation intensity. Subsequently, the performance and flow loss difference of CSHCP are further distinguished before and after optimization of suction and discharge spoiler structure under designed or variable rotational velocities. The results demonstrate that the optimized suction angle and discharge spoiler position angle are 22.5° and 45°, respectively. After the combination optimization of suction and discharge spoiler structure, the total EP and discharge temperature pulsation intensity of CSHCP are reduced by about 13.2% and 67.5% respectively. The research results provide a reference for the efficient and stable operation of HCP for fuel cell system.

Turbulence generation in the transitional wake flow behind a sphere

Turbulence generation in the transitional flow in the wake behind a sphere is studied with numerical simulations. The filtered Navier–Stokes equation and the large eddy simulation method are employed as the governing equation and the numerical method, respectively. The ΩR̃ vortex identification method is used to trace the evolution of vortices in the wake flow. The energy gradient theory is used to analyze the spike formation in the wake flow. The simulation results show that the vortex structure in the wake flow is the type of hairpin vortices, which is similar to that in a boundary layer flow. Ejection and sweep motions exist around the hairpin vortices. There are two most unstable regions in the wake where turbulence “burst” is first produced, one is near the center of the vortex head and the other is between the two vortex legs. There is a high-pressure zone above the vortex head due to the decrease in the streamwise velocity, and a soliton-like coherent structure exists in this area. The mechanism of turbulence generation in the wake is the discontinuity of the streamwise velocity, which makes the Navier–Stokes equation be singular. This singularity leads to the formation of the “negative spike” in the streamwise velocity. The amplitude of the “negative spike” reaches up to 60% of the incoming velocity, which is close to the situation in a boundary layer flow on a flat plate. It is concluded that the mechanism of turbulence generation in the wake flow is the same as that in the boundary layer flow.

Leakage vortices and energy characteristics of semi-open sewage pump with various blade tip alterations

As one of the most commonly used fluid machineries, sewage pumps play a critical role in various sectors, including urban construction, industry, and environmental protection. Considering the clog-resistant performance, the impeller usually uses a semi-open structure, giving rise to a tip structure that introduces leakage flow. This exacerbates the instability of flow within the pump, thereby impacting the overall performance and operational stability of sewage pumps. In this study, four distinct blade tip structure schemes are developed through a combination of numerical calculations and experimental validation. The Liutex–Omega vortex identification method is employed to scrutinize the initiation and development mechanisms of the leakage vortex system. A comprehensive analysis is conducted to explore the influence of the tip structure on both the energy characteristics and the internal flow dynamics of semi-open sewage pumps. It is found that the complex mixing effect of the leakage flow with the main flow is the primary source of energy loss within the semi-open impeller. Additionally, the characteristic angle of the leakage flow exhibits a high correlation with the energy dissipation characteristics of the impeller. Meanwhile, the impeller with a rounded tip exhibits the poorest performance, with the most substantial reductions in head and efficiency amounting to 44.17% and 36.11%, respectively, compared to the original impeller. Conversely, the impeller featuring a T-shaped tip demonstrates a maximum increase in the head of up to 7.25% under conditions of a large clearance size. This study not only contributes to the theoretical understanding of the unsteady flow within the semi-open impeller but also offers valuable insights for the optimized design of semi-open impeller sewage pumps, holding both theoretical and practical significance.

Analysis of vortex characteristics in hump region of reversible pump-turbine based on omega vortex identification method

As the core equipment of pumped storage power stations, reversible pump-turbines will frequently switch between different working modes during operation, and it is easy to appear hump region under pump condition, which will greatly affect the performance of the pumped storage unit. Therefore, in order to explore the causes of the hump region, this paper takes the model reversible pump-turbine as the research object. First, the unit speed–unit flow characteristics are compared with the model test results under different working conditions. Then, based on the omega vortex identification method, the vortex distribution in the flow channel of the unit is analyzed. By analyzing the flow characteristics of the flow components of the unit under different flow rates in the hump region, the internal flow law of the hump region is revealed. It is found that when the flow rate decreases, the rising head cannot offset the head lost by the hydraulic loss, thus forming the hump region, and the unstable flow gradually appears in the guide vane and runner area. There are unstable phenomena, such as flow separation, in the guide vane area. There is a sudden change of radial pressure in the vaneless area between the guide vane and the runner; that is, the unstable vortex almost occupies the whole flow channel due to the change of pressure in this area, resulting in the deterioration of the instability of the unit operation. When the flow rate is large, the radial pressure mutation zone mainly exists between the guide vane cascades, making it easy to form an unstable vortex. The research results can provide a theoretical reference for improving the stability of reversible pump-turbines.

Detached-eddy simulation of the vortex system on the high-lift common research model

A complex vortex system exists on the high-lift transport aircraft with components of the nacelle, pylon, high-lift devices, strakes, and brackets, and the investigation of the vortex system is helpful in profoundly understanding the aerodynamic characteristics of actual high-lift aircraft. To investigate the vortex system on the high-lift transport aircraft and evaluate the simulation capacity of detached-eddy simulation (DES) for the vortex system on the high-lift aircraft configuration, a high-resolution DES numerical investigation is carried out on a standard high-lift transport aircraft configuration, the National Aeronautics and Space Administration (NASA) high-lift common research model (CRM-HL). Reynolds-averaged Navier–Stokes simulation (RANS) is also performed to investigate the differences between DES and RANS in predicting time-averaged aerodynamic data on the CRM-HL. After experimental validation of time-averaged surface load and flow patterns, three vortex identification methods are adopted to extract the vortex structures of the flow field. It is found that the Rortex-based vortex identification method can more distinctly extract the fine vortex structures. The DES method can reliably predict the time-averaged surface load while capturing the complex vortex system on the CRM-HL under the concerned case of this work with high resolution.

Numerical investigation of the effects of intake pipe deflection angles on the in-cylinder flow and vortex structures of a cycloidal rotary engine

Intake pipe structure has a pivotal impact on the distribution of vortices within the combustion chamber in a cycloidal rotary engine (CRE). Therefore, studying the influence of the intake pipe deflection angle (IPDA) on the in-cylinder airflow motion has significant importance for enhancing CRE performance. This study utilized computational fluid dynamics and chemical reaction kinetics methods to establish a numerical simulation model for the in-cylinder flow and combustion in the CRE. Subsequently, the Omega vortex identification method was employed to investigate the influence of IPDA on the vortex structures within the cylinder and to explore the relationship between CRE performance and the vortices. The research findings indicate that although the IPDA did not significantly alter the fuel mass injected into the cylinder, it increased the airflow velocity by 14.6% during the main intake stage and increased the mass fraction of the burned fuel at the compression top dead center by 19.1%. Additionally, the increased airflow velocity within the cylinder led to improvements in both the mean tumble ratio by 186.5% and the turbulent kinetic energy by 25.5%. Furthermore, the IPDA significantly changed the distribution of vortices within the cylinder, which is a key factor contributing to the combustion variation of the CRE. The case of IPDA = 16° provided the largest volume of the strong vortices and the highest mean in-cylinder pressure. Compared to the original design, the volume of strong vortices was 1323.6% greater, and the mean in-cylinder pressure was higher by 5.3%.

Aerodynamic performance analysis of a floating wind turbine with coupled blade rotation and surge motion

The unsteady aerodynamic characteristics and interference effects of a floating wind turbine are significantly influenced by the six-degree-of-freedom (6DoF) motions. In this paper, the computational fluid dynamics (CFD) method using the overset grid technique and the improved delayed detached eddy simulation (IDDES) model was adopted to investigate the effects of harmonic enforced surge motions on the aerodynamic performance and wake characteristics of wind turbines. The aerodynamic load coefficients including the constant term, damping coefficients (surge velocity-induced), and additional mass coefficients (surge acceleration-induced), were fitted by using the least squares method at different surge frequencies and surge amplitudes. The blade pressure distributions and wake characteristics were analysed in detail. The Ω ~ R of the third-generation vortex identification method was applied to visualize vortex evolution. The numerical results are evaluated against the data obtained by wind tunnel experiments, indicating an acceptable relative error of 3.45 % for the power coefficient, C p , at the rated tip speed ratio (TSR). The higher frequency and greater amplitude surge motions lead to more dramatic fluctuations for the power coefficient than the axial load coefficient. The neglect of the additional mass coefficient only results in an acceptable error. Furthermore, the mean damping coefficient is on average approximately 3.5 times the mean constant term coefficient for C p , approximately 1.5 times for C z . In terms of the blade pressure distributions and wake characteristics, a higher comprehensive induced velocity, V c , is induced to expand the wider high-pressure area and shrink the low-pressure area at the pressure surface. Meanwhile, toroidal vortex ropes were induced by surge motion. Besides, the surge motion at high frequency and greater amplitude positively impacts enhancing wake deficit recovery.

Vorticity dynamics in transcritical liquid jet breakup

A transcritical domain with a sharp two-phase interface may exist during the early times of liquid hydrocarbon fuel injection at supercritical pressure. Thus, two-phase dynamics are sustained before substantial heating of the liquid and drive the early three-dimensional deformation and atomisation. A recent study of a transcritical liquid jet showed distinct deformation features caused by interface thermodynamics, low surface tension and intraphase diffusive mixing. In the present work, the compressible vortex identification method $\lambda _\rho$ is used to study the vortex dynamics in a cool liquid n-decane transcritical jet surrounded by a hotter oxygen gaseous stream at supercritical pressures. The relationship between vortical structures and the liquid surface evolution is detailed, along with the vorticity generation mechanisms, including variable-density effects. The roles of hairpin and roller vortices in the early deformation of lobes, the layering and tearing of liquid sheets and the formation of fuel-rich gaseous blobs are analysed. At these high pressures, enhanced intraphase mixing and ambient gas dissolution affect the local liquid structures (i.e. lobes). Thus, liquid breakup differs from classical sub-critical atomisation. Near the interface, liquid density and viscosity drop by up to 10 % and 70 %, respectively, and the liquid is more easily affected by the vortical motion (e.g. liquid sheets wrap around vortices). Despite the variable density, compressible vorticity generation terms are smaller than the vortex stretching and tilting. Layering traps and aligns the vortices along the streamwise direction while mitigating the generation of new rollers.

Experimental study on the wave-induced fluid resonance and suppression scheme in the narrow gap of two floating rectangular structures

An experimental study on the wave-induced gap resonance and resonant suppression scheme for two floating rectangular structures (TFRSs) is systematically presented. The instantaneous velocity fields for the present problem are measured using an enhanced underwater particle image velocimetry measurement system. Detailed experimental results have been presented to study the influences of the wave and structural parameters on the gap resonant characteristics. The resonant wave height and wave frequency were sensitive to the variation of the gap width but not very sensitive to the variation of the surface roughness on the rounded corner of TFRSs. The nonlinear effects of harmonic components on resonant free surface elevation are estimated. To analyze the vortex structures, different vortex identification methods in literature are analyzed and their merits and demerits are summarized based on a comparative analysis of the measured velocity data. The Ω-criterion vortex identification approach is then selected to evaluate the vortex intensity and capture the main vortex cores. Thanks to the adopted technique, the subsequent separation of the two quasi-symmetric vortex structures is considered to be a significant contribution to energy dissipation. The vortex shedding and dissipation modes of different TFRSs are very similar and exhibit slight variations in resonant responses. A formula of the modified Reynolds number for the gap resonance problem of TFRSs is proposed. The non-dimensional vortex strengths are nonlinearly dependent on the newly modified Reynolds number and linearly dependent on the Keulegan–Carpenter (KC) number. Finally, an optimal suppression scheme for the gap resonance problem of TFRSs is suggested.

Orbitally compact and loose vortex regions

The measure of orbital compactness of the motion of swirling material points inside a vortex is formulated without the assumption of incompressibility. Orbitally compact and loose vortex regions are distinguished. Their boundary is set very permissively and expressed through the introduced measure of orbital compactness. The present analysis is associated with the vortex-identification local trace criterion and explains its limited applicability. The comparison of compact and loose volumetric ratios for selected vortex-identification methods employs numerical data of four flow situations. The investigation shows that some vortex-identification criteria are too permissive, more or less ignoring the inherent vortex property of orbital compactness.

Experimental investigation of characteristics and influence of tip leakage vortex wandering in an axial compressor cascade

Vortex wandering is one of the most basic unsteady flow characteristics of the tip leakage vortex (TLV) in compressors. In this study, stereo particle image velocimetry (stereo-PIV) has been conducted in compressor cascades with various tip clearances to investigate the characteristics and influence of TLV wandering. The most effective vortex identification method for stereo-PIV data has been clarified. The wandering characteristics of the TLV are statistically analyzed, and the relationship between the vortex wandering and the dominant proper orthogonal decomposition mode is identified. The results reveal that TLV preferentially wanders along the pitchwise direction in the blade channel and gradually loses the dominant wandering direction downstream of the blade. The large displacement of the vortex center around its mean location is characterized by higher probabilities at small tip clearance size (1% chord length C). The spatial distribution characteristics demonstrate similarly concentric isocontour around the mean vortex center location for large tip clearances (3% and 5% chord length C). The effect of TLV wandering on the secondary velocity distribution, the tip flow blockage, and the distribution of Reynolds stress is explored using the vortex wandering corrected technique. The analysis verifies that the elevated turbulence kinetic energy in the core of the time-averaged uncorrected TLV is caused by vortex wandering rather than vortex deformation. The discussions of the current paper will enhance our knowledge of TLV wandering. Regardless of the reference to TLV, the interpretation of other swirling flows can benefit from the discussions presented here.

Vortex-induced energy loss and pressure fluctuations in a closed sump under various conditions: An experimental investigation

The roof-attached vortices (RAVs), floor-attached vortices (FAVs), and complex combined submerged vortices (CSVs) excited in a closed pump sump will induce irregular pressure fluctuations that can decrease the energy efficiency of the pump and the stability of its input flow. This article presents an experimental investigation of the energy loss and pressure fluctuations induced by these vortices under various conditions. A transparent closed-loop test rig was used for high-speed visualization and pressure-fluctuation tests. The findings demonstrate that as the flow rate is increased, the head and efficiency of the pump device significantly decrease. A CSV, which is composed of an RAV and an FAV, has the highest head-loss value, reaching 0.21 m, with an efficiency decrease in up to 2.8%. The typical evolution stages of RAV are divided into newborn, developing, fully developed, and dissipative collapsing. The typical evolution stages of FAV are divided into germinating, growing, maintaining, decaying, and disappearance. The maximum diameter of the FAV core is approximately 0.05 times the maximum vortex-core scale of the RAV. Based on the proposed vortex-identification method, as the bellmouth height decreases, the strength of the FAV increases during the CSV period, and the degree of pressure fluctuation becomes more severe. In addition, regardless of the flow rate, the strength of the FAV structure is always much greater than that of the RAV.

A two-level machine learning approach for predicting thermal striping in T-junctions with upstream elbow

Thermal striping is a phenomenon characterized by oscillatory mixing of non-isothermal streams, which is commonly seen in industrial processes such as nuclear coolant piping, petrochemical plants and liquefied natural gas transportation. The oscillatory mixing of hot and cold fluid can produce thermal field fluctuations and pose a potential risk of high-cycle thermal fatigue failures. Predicting and evaluating spatiotemporal fluctuations in thermal striping often requires high resolution and massive computational power. Although there have been extensive studies using machine learning algorithms on surrogate modeling, research focused on spatiotemporal fluctuation predictions is very limited. Due to the high dimensionality, it often requires complex algorithms with a large amount of high-fidelity training data, which limits the adoption of such methods for industrial applications. In this research, a two-level machine learning framework based on turbulence coherent structures is proposed and its application to a practical problem is demonstrated. The two-level design leverages vortex identification and local bias correction techniques, efficiently reducing the number of full-order simulations required for training. In the first level, well-organized coherent structures are extracted by performing Proper Orthogonal Decomposition on local parameters and then a tree-based machine-learning model is used to down-select the reference structures for the field reconstruction. In the second level, a parameterized convolution neural network is trained to predict the bias introduced by reference structures approximation. The demonstration of the methodology shows that the method can accurately capture the fluctuation frequencies and amplitudes of the spatiotemporal fields in a highly variational setting. Based on the vortex identification method, the methodology is expected to be applicable to general phenomenon driven by large coherent structures.

Analysis of the vortical flow in a cyclone using four vortex identification methods

This paper analyzes the vortex flow field and performance of a solid-gas cyclone separator using computational fluid dynamics (CFD) by employing the Reynolds stress turbulence model (RSM) and Eulerian-Lagrangian particle tracking scheme for particle diameter (d) range of 0.1 to 8 μm. The complex structure of vortices is analyzed using the vorticity method, λ2-criterion, Q-criterion, and Ω-method. The results demonstrate that the presence of helical guide vanes (HGVs) enhances the centrifugal force applied to the solid particles and fluid velocity up to 4.4 times compared to the conventional cyclone. It is revealed that the λ2, Q, and Ω methods provide more details of eddy structure in the inner and outer layers of the isovortex surfaces than the vorticity method. Besides, the vortex core diameter can be estimated by all vortex identification methods.

Experimental research on wake characteristics and vortex evolution of side-by-side circular cylinders placed near a wall

The wake characteristics and the vortex evolution in flow past the side-by-side circular cylinders placed near the wall are investigated with particle image velocimetry (PIV), with the objective of clarifying the interaction between the side-by-side cylinder wake and the wall boundary layer flow. The Reynolds number based on the cylinder diameter D is 1.5×103, and the gap ratio is G/D=1 where G is the gap size, and three spacing ratios (T/D=3, 1.5, and 1.1) are selected for comparison at δ/D=0.63 where δ is the boundary layer thickness without the cylinder. The vortex identification method λci and the virtual dye visualisation are applied to capture vortex evolution. Firstly, the wake characteristics of the side-by-side cylinder without the effects of wall are investigated, and the cylinder wake can be classified into three typical flow states: double wake for T/D=3, deflected gap flow for T/D=1.5, and micro gap flow for T/D=1.1. Secondly, the wake characteristics and vortex evolution of the side-by-side cylinders placed near the wall are investigated in detail. At T/D=3 and G/D=1, the interaction between the wakes of the two cylinders is weak, and two Karman vortex streets are formed, and the secondary vortex is induced periodically by the wake vortex of lower cylinder. As T/D decreases, the lower wake vortex of upper cylinder directly interacts with the lower cylinder wake vortices, and there is the vortex merging occurred in lower wake vortices of two cylinders, meanwhile, the lifting up of the secondary vortex is delayed for small T/D. In addition, the wake vortex shedding is affected by the wall and the T/D, and the deflection of the gap flow between the two cylinders is the reason for the variation of Strouhal number. As T/D decreases from 3 to 1.5, the magnitudes of Reynolds stresses are decreased, and the distributions of Reynolds stresses are asymmetric about the centreline of cylinder due to the deflection of the gap flow.