Vortex Intensity Research Articles

Large-scale dynamic processes during the minor and major sudden stratospheric warming events in January–February 2023

Using data from reanalyses of meteorological information, this study examines peculiarities of the thermodynamic regime of the Arctic stratosphere in the winter season of 2022–2023. For this purpose, components of the global meridional circulation, wave activity fluxes, volume of polar stratospheric clouds type I (PSC NAT) are analyzed, as well as changes in these indicators during sudden stratospheric warming (SSW) events formed in January–February. The main features of the 2022–2023 winter season were the following: (1) a persistent cold stratospheric polar vortex was observed until mid-January, which caused the PSC NAT volume growth to its maximum values since 1980; (2) enhanced wave activity from mid-January led to the formation of a minor SSW at the end of January, an increase in the temperature of the lower stratosphere and a sharp reduction in PSC NAT; (3) a week of intensification of the polar vortex was then observed, followed by a weakening due to the next burst of wave activity propagation during the second SSW event in mid-February. This second SSW event was major, accompanied by a reversal of the mean zonal wind, which lasted until early March; (4) meridional heat flux in January–February 2023 was the highest since 1948, resulting in unprecedented warming of the lower Arctic stratosphere and preventing ozone depletion in March; (5) the minor SSW at the end of January was accompanied by a more intense change in the meridional circulation than the major SSW in mid-February. Formation of ozone mini-hole over Northern and Central Europe in February 2023 was associated with the strengthening of the anticyclone, leading to a stratopause elevation and northward transport of ozone-poor air masses from the subtropics along the western periphery of the anticyclone additionally to related to the major SSW temperature changes affected the ozone concentration in the lower stratosphere.

Baroclinic vortex pulsars in unstable westward flows

We present a computational modeling study of geophysical coherent vortices embedded in horizontally homogeneous, baroclinically unstable, westward background flows with vertical shear. Within an idealized two-layer quasigeostrophic beta-plane model, we discovered two types of robust vortex-wave structures with distinct properties, which remain asymmetric and nonstationary in statistically-equilibrated turbulent flow regimes. The corresponding vortices, referred to as baroclinic vortex pulsars, are characterized by intense vorticity core coupled to the Rossby wave wake. The main conclusion — on the top of various analyses discussed in the paper — are that the vortex pulsars are fundamentally non-isolated coherent vortices, because they extract energy from the background circulation and expel excess potential vorticity, accumulating due to down-gradient material propagation, back into the environment. Both types may coexist as multiple statistically equilibrated states in some range of physical parameters, complicating any parameterization of eddy effects in climate-type models.

Numerical Aerodynamic and Aeroacoustic Analysis of Toroidal Propeller Designs

Drones have a major drawback which prevents them from being used extensively - the excessive noise they produce. This article presents an optimization process of aerodynamic noise reduction for a novel design called the Toroidal Propeller, which consists of two blades looping joined in a manner where the end of one blade arches back into the other, has been designed by the Aerospace Propulsion Systems (APSs) research team at School of Mechanical Engineering, Hanoi University of Science and Technology. In addition, four toroidal propellers with different curved shapes (pitch) and Number of Blades (NOB) are considered. The goal of this research is to evaluate the effects of modifying the geometry design of the Toroidal Propeller on the intensity of blade tip vortex and aerodynamic flow characteristics passing through the blade. The ultimate goal is to decrease blade tip vortex and turbulence produced by the blade, which can help estimate the sound pressure level and minimize it without causing significant performance losses. Based on the outcome results, the models of the four geometry studies are compared in terms of Acoustic Power Level (APL), Surface Acoustic Power Level (SAPL), Thrust, Torque, and Power. In general, the propeller model with NOB of 3 provides the most optimal efficiency in terms of both Thrust and APL. At the output cross-section, the APL dropped from nearly 139 dB to 121 dB, while thrust increased from almost 6.2N to 8.7N compared to the first version of the Toroidal Propeller model.

Analysis of mixing enhancement ability of bypass dual throat nozzle with single/multi-tabs

Bypass dual throat nozzle (BDTN) represents an innovative fluidic thrust vectoring nozzle capable of adjusting thrust vectoring angles through self-adaptive flow control. This study explores the BDTN-single/multi-tabs configurations, ensuring that the nozzle’s outlet projected area remains constant while introducing one or more pairs of tabs coplanar with the convergent section of the cavity. This design accounts for aerodynamic performance, thrust vectoring capabilities, and mixing abilities of the BDTN. Through theoretical analysis, design implementation, numerical simulations, and wind tunnel experiments, findings reveal that, in the case of BDTN-single-tab configuration, an increase in tab angle (α) leads to heightened streamwise vortex intensity at the nozzle outlet, reduced high-temperature core area, and enhanced jet centerline velocity and temperature attenuation rate. BDTN-single-tab achieves up to a 90% reduction in centerline-temperature/velocity decay rate, implying stronger mixing enhancement with higher tab angles. Compared to α = 10° BDTN-single-tab, the area of the high-temperature core area decays by 75%. For the BDTN-multi-tab configuration, an increase in the number of tabs results in increased thrust coefficient and vectoring angle, alongside diminished streamwise vortex strength at the nozzle outlet, and a more complex streamwise vortex structure. However, the jet’s centerline velocity and temperature decay rate decreased as a consequence.

Effects of the Slotted-Cavity Tip on the Vortices and Aerothermal Performance in the High-Pressure Turbine

To enhance aerodynamic performance in high-pressure turbines, cavity tips are commonly employed. However, these cavity tips tend to generate more vortices than flat tips, leading to an increased thermal load on the blade tips. This study introduces a novel configuration aimed at suppressing vortex intensity by incorporating a slot at the cavity bottom. Through numerical simulations, the impact of slot geometry parameters on vortices and aerothermal performance is investigated. The findings reveal that a slot height of 0–0.1 mm effectively suppresses the pressure side corner vortex within the cavity, while a slot height of 0.2–0.3 mm eliminates it. A slot height of 0.3 mm reduced average heat transfer and the high heat transfer region at the cavity bottom by 22.6% and 92.6%, respectively; however, it was observed that the leakage flow increased by 15.9%. Therefore, this study further combines the slot structure with a winglet to control the leakage flow. The combined structures exhibit superior overall performance compared to the single-slot cavity tip. Compared to the conventional cavity tips, the combined structure achieves a maximum reduction of 27.3% in average heat transfer and 7.9% in leakage flow, along with a decline of 1.9% in total pressure loss.

Effect of Misalignment at the Flat and Profiled Endwall of Nozzle Guide Vane on Heat Transfer

Misalignment of gas turbine engines is caused by the difference in thermal expansion from the design point when starting or stopping the engine, or when operating at partial load. This study evaluated the differences in heat transfer characteristics between a flat endwall and a profiled endwall with misalignment. To ensure consistency with the flow conditions, experiments were conducted at an outlet Reynolds number of 300,000 for both geometries. The experiment was performed under three different conditions: a case without step, a case with a forward-facing step (h/Cx= -0.1), and a case with a backward-facing step (h/Cx= 0.1). In the flat endwall, the thermal load of the upstream region increases significantly with both step circumstances compared to the case without a step. The misalignment causes a recirculation flow to occur between the interface of the combustor and turbine. This leads to the mainstream reattaching upstream of the endwall. Nevertheless, the heat transfer characteristics of the profiled endwall differ from those of the flat endwall under step conditions. Most locations do not experience significant changes, with only a slight increase observed in the upstream region. The recirculation flow is crucially reduced due to the intensity of the secondary vortex in the profiled shape. Hence, the increase in the thermal load on the profiled endwall with step is considerably lower than that on the flat endwall. As a result, the thermal load on the profiled endwall has decreased by 25 % for the forward-facing step and 20.7 % for the backward-facing step compared to the flat endwall.

Numerical Simulation of Unsteady Cavitation at the Tongue of a Centrifugal Pump

Abstract The flow separation at the tongue of the centrifugal pump will cause a sharp reduction of local pressure, thus inducing the cavitation phenomenon, which hurts the stable operation of the pump. This article focuses on the centrifugal pump as the research object. The Delayed Detached Eddy Simulation (DDES) method based on the SST turbulence model was employed to numerically simulate the cavitation at the tongue of the centrifugal pump. The typical characteristics and unsteady changes of cavitation were observed and studied, and the pressure at the tongue position was monitored for flow rates of 1.36 Q 0 and 1.54 Q 0. The results show that, in the cloud cavitation stage, the area of the cavitation bubble presents periodic changes, the main frequency of the cavitation area pulsation is twice the axial frequency, the periodic shedding of the cavitation bubble is caused by the attached vortex (re-entrant jet) and the pressure gradient, and the wall attached vortex will periodically shed and break. The high intensity vortices generated by flow separation at the tongue will change periodically with the rotation of the blade. With the increase of inlet flow, the pressure pulsation at the tongue increases, and the shedding and collapse of the cavitation bubble will enhance the pressure pulsation.

Numerical investigation of acoustic streaming vortices in cylindrical tube arrays

Abstract Acoustic streaming has a significant effect on accelerating material mixing and flow field disturbance. To explore the characteristics of acoustic streaming in the cylindrical tube array field under the action of an acoustic wave, we derive the dimensionless acoustic streaming control equation and establish a numerical calculation model of acoustic streaming. The effects of acoustic incidence angle, acoustic Reynolds number, and Strouhal number on the acoustic streaming vortex flow field in the tube array were investigated. The numerical results show that with the change in acoustic parameters, the acoustic streaming in the tube array presents rich changes in the vortex flow field, and there are flow field phenomena such as shrinking, merging, tearing, and splitting of the vortex structure. Toward the walls of each tube, there is a strong acoustic streaming flow velocity. Besides, there is also a large streaming velocity on the interface of the adjacent acoustic streaming vortices. The inner streaming vortex structure in the acoustic boundary layer decreases with the increase in the acoustic Reynolds number, but the intensity of the inner streaming vortex and outer streaming vortex increases rapidly, and the disturbance effect of the flow field is enhanced. With the increase in the dimensionless acoustic frequency (or Strouhal number), although the structure and intensity of the inner streaming vortex decrease, the velocity gradient on the wall of the cylindrical tube increases, which is beneficial to destroy the flow boundary layer of the cylindrical tube wall and accelerate the instability of the wall flow field.

Ultrabright attosecond gamma ray from irradiating solid foil with tailored vortex laser pulse

Intense attosecond light sources are required for investigating ultrafast electron dynamics, such as that in molecules and atoms, etc. However, so far the achievable photon energy remains at the keV level, thereby limiting their range of applications. For instance, probing intranuclear dynamics requires photon energies in the gamma-ray regime. We propose here a scheme for generating intense attosecond gamma-ray pulses by irradiating a thin solid-density foil with intense vortex laser. In the interaction, the laser driven-foil electrons are differentially accelerated within a quarter-wave field and become highly bunched. Three-dimensional particle-in-cell simulations show that they can efficiently (with ∼30% energy conversion efficiency) radiate ultrashort ( ∼400 as), high-flux ( >1010 photons per pulse), and ultrabrilliant (up to 1027 photons s−1 mm−2 mrad−2 per 0.1% BW at 1 MeV photon energy) gamma rays. With an x-ray driving laser, the scheme can even produce isolated bright sub-attosecond gamma-ray pulses that are useful for time-resolved nuclear spectroscopy and many other areas.

The impact of vertical mixing on the variability of salinity in the Black Sea pycnocline: Role of winter convection, vertical shear and mediterranean waters injections.

Rapid rise of salinity is observed in the Black Sea in recent years, with the largest positive trend (0.07 psμ per 10 years) detected in the pycnocline. We use long-term hydrological measurements for 1985–2019 to show that salinity of pycnocline has intense seasonal and interannual variability modulated by the mechanical and convective mixing. In the warm period of a year, shear turbulence driven by strong winds and intense geostrophic currents causes the penetration of warm waters into the lower density layers. This is accompanied by the rise in their salinity, the source of which is the deep saline waters situated below the halocline. This process is most intense in the areas of downwelling and intensifies in the autumn period, when thermal stratification is relatively weak. Another important reason is the entrainment of salty Mediterranean waters in the upper part of the Black Sea halocline, which is modulated by the deepening of the seasonal thermocline near the Bosphorus strait and mechanical mixing.The increase of salinity is compensated during cold winters, when convective mixing transports fresher water influenced by river discharge into lower density layers of the basin and causes a decrease in pycnocline salinity. This process is most intense in the center of the cyclonic gyres, where pycnocline is located closer to the surface and winter temperature reaches minimal values.Due to the long-term warming of the Black Sea, the process of freshening of deep layers now is observed only in rare cold years. At the same time, an intensification of wind speed, vorticity, and geostrophic circulation processes promote the blurring of the halocline and the rise of the salinity of the Black Sea upper layers. Such rise begins after 2007 in the upper part of Black Sea halocline (depth 50–100 m) and is traced down to 250 m by 2020.

Three-dimensional particle image velocimetry experimental study of transient motion characteristics of different scale vortices in gasoline engine

The paper adopted three-dimensional (3D) Particle Image Velocimetry (PIV) combined with high-speed measurement technology to study the transient motion characteristics of tumble flow induced by small and medium-sized vortices on an optical gasoline engine. Meanwhile, the Proper Orthogonal Decomposition (POD) method was introduced to analyze the evolution of transient vortices, revealing the three-dimensional motion principle of vortices with different spatial scales within the cylinder. The results showed that increasing the engine speed can cause boosted in-plane and out-of-plane velocity, especially for the in-plane velocity. However, the average velocity in the plane was higher than that out of the plane with the difference generally within an order of magnitude. The strong intake jet was the main reason for bringing turbulent kinetic energy in the cylinder. The average kinetic energy and turbulent kinetic energy in the cylinder are both significantly improved at high engine speed. From the results of this experiment, it was revealed that there exists isotropic turbulent pulsation characteristics. Overall, the corresponding flow characteristics of different decomposed modes were different. The first mode section had the lowest average velocity ratio, while the other two had the highest. Due to the dissipation of large vortices into small vortices or the shear action between flow layers, new vortices were generated, leading to the intermittent phenomenon of the motion path of the vortex cluster in the target plane. The average vortex size and velocity circularity corresponding to two varied engine speeds presented a downward trend. The average vortex size was smaller at high engine speed, while the vortex intensity was higher. In addition, increasing the engine speed would decrease the vortex scale but increase the vortex strength. This work provided new fundamental insights into analyzing the characteristics of micro-scaled vortices in turbulent flow, as well as the organization of intake in the gasoline engine.

Influence of the tip speed ratio on the wake dynamics and recovery of axial-flow turbines

Detached eddy simulation is employed to investigate the wake development downstream of the rotor of an axial-flow turbine and its dependence on the tip speed ratio. In this study, we found that the trend of the momentum deficit as a function of the rotational speed shows opposite directions in the near wake and further downstream. While the momentum deficit in the near wake increases with the rotational speed, it decreases further downstream. For instance, we found that at six diameters downstream of the rotor the streamwise velocity in its wake recovered to about 30% of its free-stream value at the lowest simulated tip speed ratio of 4, while its recovery was equal to about 65% at the largest tip speed ratio of 10. This is due to the earlier breakdown of the tip vortices. The results of the computations demonstrate indeed that mutual inductance phenomena between tip vortices, promoting pairing events and the eventual instability of the helical structures, occur at shorter downstream distances for higher values of tip speed ratio. Wake instability enhances the process of wake recovery, especially due to radial advection. Therefore, higher rotational speeds do not promote wake recovery through more intense tip vortices, but through their greater instability. Implications are important, affecting the optimal distance between rows of axial-flow turbines in array configurations: the operation at higher rotational speeds allows for smaller distances between turbines, decreasing the cost and environmental impact of farms consisting of several devices.

The lift enhancement mechanism caused by the deformation of the surface of the wide-speed waverider

The optimization method provides an effective approach to enhance the low-speed lift of the vortex lift waveriders by deforming the aerodynamic shape refinedly. However, the vortex lift enhancement mechanism of the optimized configuration is unclear. In this study, the flow evolutions of the original and the optimized configurations are studied by employing the delayed detached-eddy simulation. Results indicate that the convex deformation of the leeward surface plays a dominant role in enhancing the vortex lift by enhancing the low-pressure suction at the upstream breakdown location and delaying the vortex breakdown. For the enhancement of the low-pressure suction, the convex deformation intensifies the streamwise vorticity below the axis of the primary vortex of the leading-edge vortex, in turn enhancing the downwash effect and causing the primary vortex to move downward. This reduces the pressure coefficient induced by the primary vortex on the leeward surface, thus enhancing the vortex lift. In terms of the delay of the vortex breakdown, the convex deformation compresses and accelerates the flow between the spanwise convex and the leading edge. These intensities enhance the washing effect along the spanwise direction on the outward wing and cause the primary vortex to deflect toward the outboard wing. Subsequently, the primary vortex and the shedding vortices generated by the shear layer instability merge, which increase the primary vortex intensity, and enhance the streamwise velocity in the vortex axis. Correspondingly, the primary vortex breakdown is delayed. Ultimately, the increased low-pressure region caused by the delay of the vortex breakdown enhances the vortex lift.

Inelastic two-wave mixing induced high-efficiency transfer of optical vortices

A scheme for high-efficiency transfer of optical vortices is proposed by an inelastic two-wave mixing (ITWM) process in an inverted-Y four-level atomic medium, which is originally prepared in a coherent superposition of two ground states. The orbital angular momentum (OAM) information in the incident vortex probe field can be transferred to the generated signal field through the ITWM process. Choosing reasonable experimentally realizable parameters, we find that the presence of the off-resonance control field can greatly improve the conversion efficiency of optical vortices, rather than in the absence of a control field. This is caused by the broken of the destructive interference between two one-photon excitation pathways. Furthermore, we also extend our model to an inelastic multi-wave mixing process and demonstrate that the transfer efficiency between multiple optical vortices strongly depends on the superposition of the ground states. Finally, we explore the composite vortex beam generated by collinear superposition of the incident vortex probe and signal fields. It is obvious that the intensity and phase profiles of the composite vortex can be effectively controlled via adjusting the intensity of the control field. Potential applications of our scheme may exist in OAM-based optical communications and optical information processing.

Analysis of unsteady flow and pressure pulsation of multistage centrifugal pump based on actual size

In this paper, the unsteady flow and pressure pulsation in 11-stage centrifugal pump were analyzed based on large eddy simulation. The performances from the numerical simulation agreed with the ones from the experiment. The numerical results show that large pressure gradient area appears on trailing edge of blade pressure surface, and large vorticity intensity area appears on blade suction surface and trailing edge. The vortexes in the impeller passage decrease obviously with the increase of flow rate from 0.4 Q to 1.2 Q. As the stage increase, there is a local high pressure area at the tongue and it affects the fluid backflow. The main frequency of the pulsation at the impeller inlet is the blade passing frequency ( fBPF), and the main frequency at the impeller outlet and the interface is 2 fBPF. Due to interstage interference, the main frequency of the second impeller includes blade passing frequency of the first stage impeller (5 fn) and its own blade passing frequency (7 fn). With the increase of the stages, the rule of pulsation gradually tends to be consistent.

Characteristics and evolution of ground vortex under heaving ground conditions: Insights from large eddy simulation (LES) analysis

When an aircraft engine operates near the ground, the ambient vortices generated by the freestream velocity boundary layer and engine suction will gather and lift, forming ground vortices which may result in intake distortion, consequently affecting the engine operation stability. To develop a shipborne test platform, this study conducted large eddy simulation calculations using the ANSYS Fluent and based on the assumption of incompressible flow. The 3-D unsteady Navier-Stokes (N-S) equations were solved using the WALE sub-grid model. The analysis focused on the characteristics and evolution of ground vortices under heaving ground conditions within 1 s. The results indicate that the ground vortex exhibits strong unsteady characteristics. During the descending phase of the platform, the quantity of vortex formed beneath the intake duct would decrease while the trailing vortex becomes more pronounced. However, the structure of the inlet vortex remains relatively unchanged at each time step. The intensity of the vortex and pressure distortion coefficient exhibit significant fluctuations over time, and their temporal variations approximately follow a sinusoidal function, which is also the heaving motion of the platform. Generally, when the velocity of the platform reaches its maximum downward vertical value, the average vortex intensity of the ground vortex tends to be minimal while the pressure distortion coefficient approaches its maximum value. Conversely, when the velocity of the platform reaches its maximum upward vertical value, the opposite trend can be observed.

Wind Pressure of Low-rise Buildings Based on Wind-tunnel Numerical-simulation Test

Based on the numerical-simulation test data from the wind tunnel, the wind-pressure characteristics of low-rise buildings with gable roofs are examined and compared to standard values under different roof slopes, eave heights and terrains. To further investigate the underlying mechanisms of variation, a numerical-simulation study was conducted for selected conditions. The results indicate that when the roof slope is less than 3:12, the negative value of the shape coefficient for the windward roof of the building can reach up to -0.89, far exceeding the code value of -0.5. It is suggested that when the slope is not larger than 3:12, the shape coefficient for the windward roof should be -0.9 and the coefficient for the leeward roof should be determined through linear interpolation within the range from -0.4 to -0.75 based on the roof's slope size. With the increase of eave height and the decrease of ground roughness, the vortex intensity of the roof and the leeward surface increases and the area proportion of high negative pressure area in the vortex influence range increases. Thus, the negative value of the shape coefficient for the roof and the leeward surface becomes larger, which is higher than the code value. It is proposed that under an eave height of 12 m, the shape coefficient for the windward roof should be -0.95. Meanwhile, those of the leeward roof, leeward surface and sidewall 2 should be -0.6 under open terrain. Keywords: Low-rise buildings, Wind-tunnel test, Wind load, Shape coefficient, Numerical simulation

Effect of local mechanical oscillation on the performance of PCM thermal energy storage with various unit configurations

The low thermal conductivity of phase change materials is an essential restriction to their usage in thermal energy storage units. In this research, applying local oscillation by an oscillator on the portion of a side wall of RT35 enclosure is presented as a novel method to enhance the charging rate. The finite volume method is conducted for the numerical study on the effect of different configuration parameters including enclosure aspect ratios in the range of 1: 6, 1:2, 1:1, and 2:1, various oscillator lengths of 5, 10, 15, and 20 mm, as well as different oscillation frequencies of 10, 100 and 1000 Hz on the melting performance. Results indicate that applying local oscillation enhances the melting rate by more than 50 % as compared to without using oscillator because of enhancing convection heat transfer originating from vorticity intensification. The improvement of required time for melting the PCM is 91.55 %, 54.66 %, 67.7 % and 71.4 % for aspect ratios of 1:6, 1:2, 1:1 and 2:1, respectively.Results show that the melting performance is enhanced as the length of the oscillator increases. The required time for complete melting is obtained equal to 24, 26, 28, and 33 min at the oscillator length of 20, 15, 10, and 5 mm, respectively. The finding on the effect of oscillation frequency reveals that the liquid fraction, mean temperature, and heat flux at a specified time of melting process becomes higher as the oscillation frequency increases with significant enhancement of vorticity around the oscillator. At the frequency of 1000 Hz, the required time for melting is reduced by 9.5 % and 20.8 % as compared to frequencies of 100 Hz and 10 Hz, respectively. Other results also indicate that the higher Nusselt number is obtained for a higher enclosure aspect ratio.

Prediction of the Influence of the Inlet End-Wall Boundary Layer on the Secondary Flow of a Low Pressure Turbine Airfoil Using RANS and Large-Eddy Simulations

Abstract This paper analyzes the effect of the inlet end-wall boundary layer on the secondary flow of a low pressure turbine airfoil cascade at Reynolds number 2 × 105 using RANS and implicit large-eddy simulations (LES). The results are compared against experimental data obtained at two low-speed linear cascade facilities, one located at the Whittle Laboratory of the University of Cambridge and the other at the Polytechnic University of Madrid. The RANS turbulence model is the k−ω−γ−Reθt and no sub-grid scale model has been used in the LES. An unstructured mesh of hexahedra and prisms is used, with high order elements used in the boundary layer region to better describe the airfoil shape in the LES. Two inlet end-wall boundary layers that produce different secondary flow patterns are analyzed: a laminar thin velocity profile and a turbulent thick velocity profile with several inlet turbulent intensities. The agreement between LES numerical predictions and experimental measurements of the position and intensity of the secondary vortices is very good for both cases. RANS simulations are much cheaper in terms of computational cost and reasonably predict most of the flow features, except when the inlet turbulence is low and turbulent transition prediction becomes critical. The effect of the inlet velocity profiles and inlet turbulence on the secondary flow structure is quite pronounced. The velocity profile thickness determines the spanwise penetration of the passage vortex, and different inlet turbulence intensities modify its mixing. Higher inlet turbulence intensities lead to a decrease of the secondary losses due to the passage vortex and an increase of end-wall losses.

Fluid dynamics of a flapping wing interacting with the boundary layer at a flat wall

In this paper, we consider the fluid dynamics of a flapping wing interacting with a boundary layer developed at a no-slip flat wall. Direct numerical simulations are carried out via implementing the non-iterative immersed boundary-lattice Boltzmann method, over a Reynolds number range of 10≤Re≤1000, for a fixed Strouhal number of St = 0.3 and for a given symmetric plunging and pitching flapping motion. The interactions between the wing and the boundary layer are modulated by varying the mean distance of the wing to the wall H0. The results indicate that the presence of the boundary layer at the wall amplifies the fluctuations in both lift and drag due to the boundary layer separation, in contrast to the pure ground effect. This separation also leads to the decrease in both average lift and average drag over one flapping cycle when H0 is low. When it comes to the flow patterns in the wake, it generally gets more complex for a low H0 and/or a high Re. Secondary vortices can be observed for Re≥500 in the present configuration, which either evolve by themselves or interact with the vortices in the wake while being convected downstream and dissipated via viscosity. In the end, a dynamic mode decomposition analysis is performed to explore further the flow structures in the wake. One observes the sheltering effect of the boundary layer that the vortices in the wake are prevented from penetrating the boundary layer, while this effect will not hold if the vortex intensity is sufficiently high, such as the low order mode of the case for Re≥1000 in this study.