Reattachment Phenomenon Research Articles

Investigation of longitudinal self-excited combustion instability in a micromix hydrogen combustor

The increasing emphasis on low-carbon energy has heightened investigations into hydrogen combustion. This study focuses on the critical yet underexplored topic of thermoacoustic instabilities in micromix hydrogen combustors. We employ a newly designed micromix burner to explore the intricate relationships among acoustic modes, flame dynamics, and flow-vortex interactions. High-speed particle image velocimetry (PIV) and OH* chemiluminescence, supplemented by dynamic pressure measurements, are employed to capture dynamic behaviors and instability modes of the flame. Through thermoacoustic network analysis, we can identify two primary self-excited modes: a dominant low-frequency mode oscillating between 520 Hz and 565 Hz, and a secondary high-frequency mode surpassing 4400 Hz, originating from the hydrodynamic instability of the nozzle's jet flow. The low-frequency mode significantly influences the flame dynamics, generating a distinct longitudinal flame oscillation behavior. In particular, the inner premixed flame exhibits a periodic pattern of lifting and re-attachment phenomena. The outer diffusion flame, however, dynamically interacts with the outer shear layer (OSL) and the shed outer vortex rings (OVRs), which gives rise to notable deformations in the flame surface, manifesting as neck and swell structures propagating downstream with the OVRs. Furthermore, the dynamics of the OSL and OVRs are attributed to the fluctuations in the bulk flow velocity and local equivalence ratio at the nozzle exit, which are sustained by pressure oscillations induced by thermoacoustics. This presents important insights into the interplay between hydrodynamics and thermoacoustics in the formation of combustion instability in the micromix hydrogen combustor.

Spectral Characteristics of Fluctuating Aerodynamic Forces Acting on Rectangular Prisms

The present work is devoted to the role of boundary layer turbulence on the spectral characteristics of fluctuating wind loads on large aspect-ratio rectangular prisms. Seven rectangular rigid models with different side ratios (1/4~4) were created, and simultaneous pressure experiments were conducted under the boundary layer turbulence flows. Using the measured data, the power spectrums of the fluctuating aerodynamic forces were calculated, and then, the spectral characteristics under different turbulent boundary layer flows were analyzed. In contrast to the typical power spectrum model, the main factors affecting the spectral characteristics of the fluctuating aerodynamic loads are presented and discussed in this study. The power spectrum of the rectangular prism was significantly impacted by the turbulent wind field, primarily because higher turbulence intensity levels result in a lower spectral peak and a wider spectral bandwidth, which also redistributes spectral energy. In particular, the effect on the spectral properties of across-wind fluctuating loads was stronger, and the turbulent disturbance modified the lateral separation flow structure, causing the reattachment phenomenon to occur earlier on rectangular prisms with small side ratios, which effectively altered the spectral properties. Thus, a normalized across-wind fluctuating wind load spectrum model is proposed, taking into account the effects of incoming turbulence, the side ratio, vortex shedding, separation reattachment flow-induced spectral bandwidth changes, and high-frequency fluctuations.

Aerodynamic characteristics simulation of helicopter rotor in hover using HeliX software

In order to predict hover performance of a new helicopter rotor, the aerodynamic performance and flow field of the rotor in hover is calculated by using HeliX software. The comparison between numerical results and wind tunnel test data of S-76 rotor as a validation case of CFD method demonstrate that HeliX software has the capability to consistently and reliably predict performance parameters for a helicopter rotor. The numerical results of the new rotor in hover suggest that the maximum hover efficiency of the new rotor is about 73% with collective pitch angle of 9.2°, while the shock induced separation and the reattachment phenomenon begin to appear on the upper surface near the blade tip. As the increase of collective pitch angle from 9.2° to 11.2°, hover efficiency decreases significantly due to the serious flow separation near the blade tip. The research in this paper can provide reference and guidance for aerodynamic optimization design of helicopter rotor.

Numerical and Experimental Study on Flow Field around Slab-Type High-Rise Residential Buildings

High-rise residential buildings often adopt rectangular cross-sections with large depth-to-width ratios. Moreover, the cross-sections have many grooves and chamfers for better ventilation and lighting. However, related research is lacking. This study performed wind tunnel tests and large eddy simulations (LES) on two typical buildings to analyze the surface wind pressures and flow fields around the buildings. The base moment spectra, along with the wind pressure coefficients, demonstrate that numerical simulation is capable of accurately representing the magnitudes and variations in wind loads along the height of the building. Furthermore, numerical simulation effectively captures the dominant energy distribution characteristics of fluctuating wind loads in the frequency domain. The shear layer separations, vortex shedding and reattachment phenomenon were observed. It was found that in the middle and lower parts of the buildings, the shear layer separation changed dramatically. Buildings with depth-to-width ratios close to 2 are minimally affected by changes in wind direction. However, for buildings with larger depth-to-width ratios, especially when the short side faces the wind, the reattachment of the shear layer and the shedding of wake vortices become crucial factors in generating fluctuating cross-wind loads. This emphasizes the significant impact of wind direction and plan dimensions on flow characteristics and aerodynamic behavior. When the building contained corners and grooves, the low-wind-speed area induced by the shear layer separation shrank and the reattachment point shifted closer to the windward facade.

Flow-induced forces and vortex transportation characteristics of three circular cylinders at subcritical Reynolds number

Three-dimensional numerical simulation for the three circular cylinders at Re = 3900 is carried out by using the large eddy simulation (LES) in this paper. The present numerical model and parameters are verified by solving single cylinder and two cylinders cases. Then, the parameters consisted of spacing ratio (L/D) and incidence angle (α) are checked in three circular cylinders array. The fluid force and near-wake behavior of the two- and three-cylinder systems are analyzed with emphasis. Under a tight arrangement, there are three flow patterns: reattachment flow pattern, large-incidence-angle flow pattern, and biased flow pattern, corresponding to cases with incidence angles of 0, 45, and 90 degrees, respectively. The Strouhal number (St) of the three cylinders is essentially overlapping at α= 0° and α= 90°, the distinctive secondary peaks can be observed at the small spacing ratio in power spectral density (PSD). The mean pressure coefficient of midstream cylinder (MC) influenced by the number of cylinders is negative in the tandem arrangement at the stagnation point, accompanied by reattachment phenomenon the upstream surface of MC with L/D= 1.5. Placing the third cylinder between the two cylinders in a tandem arrangement significantly suppresses their lift oscillations. The phase difference (φum) between the fluctuating lift of the upstream two cylinders is highly sensitive to the incidence angle of the system.

Study on flow noise characteristic of transonic deep buffeting over an airfoil

Transonic buffeting can induce strong noise and reduce aircraft lifespan. In view of the complexity of the transonic buffeting flow, this study combines the highly accurate Delayed-Detached Eddy Simulation and Discrete Frequency Response method to analyze the flow field and sound propagation law in different buffeting states and also investigates its noise-generating characteristics by Dynamic Mode Decomposition and Pearson correlation. It is found that the low-frequency and small-amplitude shock oscillation of the light buffeting state is insufficient to trigger large separated flow. Besides, the reattachment phenomenon occurs in the trailing edge, which is the second mode of boundary layer separation, corresponding to the lower Sound Pressure Levels (SPL). In the deep buffeting state, however, the shock oscillates with high frequency and large amplitude, producing large separated bubbles without the reattachment phenomenon, which is the first mode of boundary layer separation. Moreover, there is a large-scale vortex structure with high energy content in the recirculation zone, which develops toward the trailing edge under the action of convection and produces strong Upstream Traveling Waves (UTWs). The collision occurs between UTWs and the shock wave oscillation. In this process, they promote each other, which increases the shock wave oscillation frequency and SPL. This state is not the superposition effect of buffeting and stall. And its main sound sources are shock oscillation and the von Kármán mode.

Assessment of nonlinear-low Reynolds number/detached eddy simulation turbulence model for wake flow field simulation of a realistic automotive model

Since the results of wake flow simulation with commonly used turbulence models are unsatisfactory, by introducing a nonlinear Reynolds stress term and combining the DES (Detached Eddy Simulation) model, this paper further validates the nonlinear-LRN (Low Reynolds Number)/DES turbulence model which can predict the flow separation and the reattachment phenomenon more accurately. This model was verified by a wall-mounted hump flow case and was applied to the time-averaged and transient flow field structure analysis of a realistic automotive model with several widely used turbulence models. These simulation results were compared with experimental data, indicating that the nonlinear-LRN/DES model gives better agreement with the experiment and can predict the automobile wake flow structures and aerodynamic characteristics more accurately. Furthermore, the performance of the nonlinear-LRN/DES model in mesh with different refinements is compared, concluding that the new proposed model can obtain high accuracy in the coarse mesh.

Characteristics of Extreme Wind Pressure on the Open Prefabricated Spatial Grid Structure of Evergrande Stadium

Large-span open prefabricated spatial grid structures are characterized by light mass, high flexibility, low self-oscillation frequency, and low damping, resulting in wind-sensitive structures. Meanwhile, their height tends to be relatively low, located in the wind field with a large wind speed gradient and high turbulence area. Therefore, surface airflow is complex, and many flow separations, reattachment, eddy shedding, and other phenomena occur, causing damage to local areas. This paper took the Evergrande Stadium in Guiyang, China, as the research object and used the random number cyclic pre-simulation method to study its surface extreme wind pressure. Firstly, five conventional distributions (Gaussian, Weibull, three-parameter gamma, generalized extreme value, and lognormal distribution) were fitted to the wind pressure probability densities at different measurement points on the surface of the open stadium. It is found that the same distribution could not be chosen to describe the probability density distribution of wind pressure at all measurement points. Hence, based on the simulation results, the Gaussian and non-Gaussian regions of this structure were divided to determine where to apply which distribution. Additionally, the accuracy of the peak factor, improved peak factor, and modified Hermite moment model method were compared to check their applicability. Finally, the effect of roughness on the extreme wind pressure distribution on the open stadium surface was also investigated according to the highest accuracy method above. The findings of this study will provide a reference for engineers in designing large-span open stadiums for wind resistance to minimize the occurrence of wind damage.

Investigation on Film Cooling Performance of a Novel Hole Arrangement Based on Combined-Hole Design

Abstract Based on a combined-hole design, a novel hole arrangement was proposed and tested on a flat plate model by numerical simulation. Replacing one row in two rows of staggered holes with a row of smaller holes, the bigger holes in one row and smaller holes in the other row will form combined-hole units. The kidney–vortex pairs in the combined-hole unit will interact with each other and get weakened, finally enhancing film cooling performance. Compared to the original two rows of staggered holes, the coolant mass flowrate will be reduced by 25% for the new hole arrangements. Results show that the area-averaged film cooling effectiveness of the new arrangements is similar or even higher than that of the original one at different blowing ratios. Replacing the second row of holes with smaller holes provide better cooling performance than replacing the first row of holes. The overall film cooling performance is not sensitive to the row space. The highest improvement of film cooling effectiveness is 24% when the blowing ratio is 1. However, as the coolant accumulation and reattachment phenomenon is weakened by the new arrangements, the film cooling performance is not as good as the original one when the blowing ratio is 1.5.

Interactive flow around a circular cylinder controlled by a small rod

In this paper, a unique flow control method using a small rod, called the forced reattachment method, is proposed for the control of the flow around a bluff body. As described in previous papers, the forced reattachment method is a type of separated shear layer control that reduces drag and generates lift, and it can only occur under certain conditions. The forced reattachment phenomenon occurs as a result of an interactive flow between a circular cylinder and a small rod. This phenomenon is characterized by many special features, such as a large stagnant region behind the circular cylinder and the reattachment flow, which adheres to the rear face of the circular cylinder. It is also interesting how it reduces the total drag, including that of the small rod. The aim of this study was to elucidate the details of the stagnant region, the properties of the reattachment flow, and the reduction of the total drag.

Numerical analysis for wake flow field of Ahmed model based on a nonlinear-LRN/DES turbulence model

PurposeThis paper aims to propose a precise turbulence model for automobile aerodynamics simulation, which can predict flow separation and reattachment phenomena more accurately.Design/methodology/approachAs the results of wake flow simulation with commonly used turbulence models are unsatisfactory, by introducing a nonlinear Reynolds stress term and combining the detached Eddy simulation (DES) model, this paper proposes a nonlinear-low-Reynolds number (LRN)/DES turbulence model. The turbulence model is verified in a backward-facing step case and applied in the flow field analysis of the Ahmed model. Several widely applied turbulence models are compared with the nonlinear-LRN/DES model and the experimental data of the above cases.FindingsCompared with the experimental data and several turbulence models, the nonlinear-LRN/DES model gives better agreement with the experiment and can predict the automobile wake flow structures and aerodynamic characteristics more accurately.Research limitations/implicationsThe nonlinear-LRN/DES model proposed in this paper suffers from separation delays when simulating the separation flows above the rear slant of the Ahmed body. Therefore, more factors need to be considered to further improve the accuracy of the model.Practical implicationsThis paper proposes a turbulence model that can more accurately simulate the wake flow field structure of automobiles, which is valuable for improving the calculation accuracy of the aerodynamic characteristics of automobiles.Originality/valueBased on the nonlinear eddy viscosity method and the scale resolved simulation, a nonlinear-LRN/DES turbulence model including the nonlinear Reynolds stress terms for separation and reattachment prediction, as well as the wake vortex structure prediction is first proposed.

Analysis of Adverse Influence of Metal Vapor to Arc Movement Between Electrodes with External Magnetic Field Applied by Numerical Simulation

The demand and sustainable development goals for electric vehicles (EVs) have been increasing. A quick charging circuit is an essential technology for EVs. To protect this circuit from faults or accidents, rapid interruption of the arc generated by the switching of direct current contactors (DCCs) is required. However, the DCCs have small sizes and close boundaries against an arc stretch toward the extinguishing chamber. The metal vapor generated by the electrodes causes the arc to remain between the electrodes instead of stretching forward, because the gas electrical conductivity increases when the metal vapor contaminates it. In this study, the arc movements with and without consideration of the metal vapor were simulated numerically to elucidate the influence of metal vapor on arc mobility. It was observed that the metal vapor generated from the electrodes causes the reattachment phenomenon to occur several times on the anode side, which increases the time of arc stretching into the extinguishing chamber. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Cut-corner prism piezoelectric energy harvester based on galloping enhancement mechanism

To improve the efficiency of harvesting energy, a cut-corner prism piezoelectric energy harvester based on galloping enhancement is designed in this study. Galloping is caused by the pressure difference between the upper and lower surfaces of the prism. Numerical simulations indicate that the cut-corner can reduce the occurrence of the shear layer reattachment phenomenon and also increase the strength of the secondary backflow between the shear layer and the lateral side of the prism, thereby raising the pressure difference between the upper and lower surfaces of the prism. The effect of cut-corner size on the performance of cut-corner prism energy harvesters is investigated experimentally. The results show that the optimal range for the prism is when the lengths of the cut-corner windward side and parallel side are 0.2B–0.4B (B is the length of the prism side) and 0.5B–0.6B, respectively. At a wind speed of 6.24 m/s and a resistance of 1 × 105Ω, the maximum output power of the cut-corner prism energy harvester with a windward side length of 0.4B and a parallel side length of 0.6B can reach 47.5 mW, which is 261% higher than that of the reference square prism, effectively raising the performance of the piezoelectric energy harvester.

Compressible large eddy simulation of the boundary layer evolution in a low-pressure turbine cascade at different Reynolds numbers

An in-house large eddy simulation code based on three-dimensional compressible N-S equations was used to research the evolution process of the boundary layer in the low-pressure turbine T106A cascade. Analyses of the separation and reattachment phenomenon as well as the unsteady aerodynamic characteristic on the suction surface at different Reynolds numbers were performed. A variation range of Reynolds number which is based on the exit isentropic velocity and axial chord length extends from 0.6 × 105 to 2.0 × 105. The time-averaged wall-static pressure coefficient compares well with the experimental and DNS data. The effect of Reynolds number on the flow field is mainly embodied at the rear part of the suction side. At low Reynolds number condition, the value of the total pressure loss and the width of the wake are significantly large. With the increase of Reynolds number, the wake zone becomes narrower and the wake centerline moves to the pressure side. The size of separation bubble at the leading edge grows slightly and the coherent structures are more close to the cascade surface. At the case of Re = 2.0 × 105, the two-dimensional shear layer evolves directly into small-scale vortices, whereas the rolling of spanwise vortex and hairpin vortex generation do not appear.

Effect of the presence of end plates and aspect ratio on the aerodynamic forces on circular cylinders in various flow regimes

The presence of end plates and the cylinder aspect ratio significantly influence the aerodynamics of circular cylinders in wind tunnel tests. Although a large aspect ratio sufficiently reduces the effects of the free end and boundary layer of the tunnel walls, smaller end plates and aspect ratios are required for practical reasons. Thus, the aerodynamic characteristics of circular cylinders with small end plates and aspect ratios in the critical Reynolds number range are worthy of investigating because of the complexity of the flow states in these regimes. The wind pressure distribution on the circular cylinder is tested in the wind tunnel for Reynolds numbers of Re = (0.47–5.07) × 105, which cover the entire range of the critical Reynolds numbers. The effects of the existence of end plates and the aspect ratio of the cylinder on the aerodynamic forces and wind pressures at the middle of the cylinder and along the length of the cylinder in different flow regimes are discussed. The phenomenon of flow reattachment in the critical Reynolds number range is focused on to reveal the effects of the cylinder aspect ratio on the aerodynamics related to dry galloping for a cylinder with small end plates. The results show the necessity of including end plates and assuming a large aspect ratio to simulate a two-dimensional flow in the subcritical range. In the critical range, the less significant influences of the end plate diameter and cylinder aspect ratio strongly depend on the development of attachment along the length of the cylinder, which creates a three-dimensional flow and mitigates the spanwise correlation.

Electrical conductivity effect on MHD mixed convection of nanofluid flow over a backward-facing step

In this study, magneto-hydrodynamics (MHD) mixed convection effects of Al2O3-water nanofluid flow over a backward-facing step were investigated numerically for various electrical conductivity models of nanofluids. A uniform external magnetic field was applied to the flow and strength of magnetic field was varied with different values of dimensionless parameter Hartmann number (Ha=0, 10, 20, 30, 40). Three different electrical conductivity models were used to see the effects of MHD nanofluid flow. Besides, five different inclination angles between 0o~90o is used for the external magnetic field. The problem geometry is a backward-facing step which is used in many engineering applications where flow separation and reattachment phenomenon occurs. Mixed type convective heat transfer of backward-facing step was examined with various values of Richardson number (Ri=0.01, 0.1, 1, 10) and four different nanoparticle volume fractions (o=0.01, 0.015, 0.020, 0.025) considering different electrical conductivity models. Finite element method via commercial code COMSOL was used for computations. Results indicate that the addition of nanoparticles enhanced heat transfer significantly. Also increasing magnetic field strength and inclination angle increased heat transfer rate. Effects of different electrical conductivity models were also investigated and it was observed that they have significant effects on the fluid flow and heat transfer characteristics in the presence of magnetic field.

Numerical investigation of attached cavitating flow in thermo-sensitive fluid with special emphasis on thermal effect and shedding dynamics

This study aims to investigate the unsteady cavitation shedding dynamics flow around a NACA 0015 hydrofoil in thermo-sensitive fluid with thermal effect. The thermal effects are captured by a coupled solution of the continuity, momentum and energy equations, and the numerical results show a reasonable agreement with the available experiments. Time evolution process of the vortex structure is investigated. The ability and limitation of B-factor is analysed to evaluate the thermal effect, which can provide guidance for the further improvement of B-factor. The re-entrant jet and vortex structure show strong coherent relationship with the flow separation. The re-entrant jet promotes the formation of vortex structure, which in turn affects the separation flow of the flow field. Skin friction coefficient and boundary vorticity flux are applied to displace the flow separation on the hydrofoil surface. The results showed that the streamwise velocity decreases sharply in the vicinity of the collision between the re-entrant jet and the main stream, the skin friction streamline suddenly breaks off and separation or re-attachment line occurs. The intensity of the re-entrant jet inside the cavity becomes gradually weaker, the strength of the vortex is also weakened, which causes the skin friction coefficient in this region to be almost zero, and the phenomenon of flow separation and re-attachment is indistinct.

Numerical Study on Turbulent Separation Reattachment Flow in Pipe Bends with Different Small Curvature Ratio

A turbulent separation flow is one of the most complex flows in existence. Flow separation and reattachment phenomena take place on the lower curved wall under high-pressure gradient for high Reynolds number in different pipe bends. The present study deals with the numerical simulation of the turbulent separation reattachment flow under high Reynolds number in different curvature ratios. Through making use of k–e turbulence model, the turbulent single-phase flow via a 90-degree bend pipe is studied numerically. A more precise research has been conducted to discover the impact of Reynolds number (Re) as well as curvature ratio (Rc/D) on the reattachment and separation of flow inside the pipe bend. The separation region and the separation–reattachment points are reported here. Additionally, mean statistics of the primary and secondary velocity field flows in diverse sections and the occurrence of Dean vortices have been shown. It has been observed that bends with low curvature ratio enable flow separation to be clearly visualized. Distribution of velocity depicted that the secondary motion is clearly encouraged by the progress of fluid from the inner to the outer wall region of the bend resulting in the separation of flow. In general, this numerical study analyses the flow separation phenomenon and predicts the separation and reattachment locations in 90° bend pipe for different Reynolds numbers and bend curvatures.

Turbulent forced convection flow of nanofluids over triple forward facing step

PurposeThe purpose of this paper is to numerically study the phenomenon of separation and subsequent reattachment that happens due to a sudden contraction or expansion in flow geometry, in addition, to investigating the effect of nanoparticles suspended in water on heat transfer enhancement and fluid flow characteristics.Design/methodology/approachTurbulent forced convection flow over triple forward facing step (FFS) in a duct is numerically studied by using different types of nanofluids. Finite volume method is employed to carry out the numerical investigations. with nanoparticles volume fraction in the range of 1-4 per cent and nanoparticles diameter in the range 30-75 nm, suspended in water. Several parameters were studied, such as the geometrical specification (different step heights), boundary conditions (different Reynolds [Re] numbers), types of fluids (base fluid with different types of nanoparticles), nanoparticle concentration (different volume fractions) and nanoparticle size.FindingsThe numerical results indicate that the Nusselt number increases as the volume fraction increases, but it decreases as the diameter of the nanoparticles of nanofluids increases. The turbulent kinetic energy and its dissipation rate increase as Re number increases. The velocity magnitude increases as the density of nanofluids decreases. No significant effect of increasing the three steps heights on Nusselt along the heated wall, except in front of first step where increasing the first step height leads to an increase in the recirculation zone size adjacent to it.Research limitations/implicationsThe phenomenon of separation and subsequent reattachment happened due to a sudden contraction or expansion in flow geometry, such as forward facing and backward facing steps, respectively, can be recognized in many engineering applications where heat transfer enhancement is required. Some examples include cooling systems for electronic equipment, heat exchanger, diffusers and chemical process. Understanding the concept of these devices is very important from the engineering point of view.Originality/valueConvective heat transfer can be enhanced passively by changing flow geometry, boundary conditions, the traditional fluids or by enhancing thermal conductivity of the fluid. Great attention has been paid to increase the thermal conductivity of base fluid by suspending nano-, micro- or larger-sized particles in fluid. The products from suspending these particles in the base fluid are called nanofluids. Many studies have been conducted to investigate the heat transfer and fluid flow characteristics over FFS. This study is the first where nanofluids are employed as working fluids for flow over triple FFS.

Numerical study of convective heat transfer in T‐bifurcating channel

AbstractA numerical study of a three‐dimensional turbulent flow in a rectangular T‐bifurcating duct was performed. It focused on the analysis of heat transfer in the branching duct at 90 to the main flow. Including separation and reattachment phenomena, the flow seemed to be anisotropic. The closure system of the full set of Navier–Stokes equations governing the flow was based on the on one point statistical modeling using a low Reynolds number second‐order full stress transport model. For several aspect ratios, results show that in addition to the recirculation zone in the branching duct close to the upstream side; pairs of streamwise vortices were generated downstream of the junction zone with their centers moving towards the symmetry plane. The effect of the aspect ratio of the branching section in enhancing this phenomenon and flow rate effect on the heat transfer were particularly analyzed in this paper.