Detached Eddy Simulation Method Research Articles

Aerodynamic performance and wake development of NACA 0018 airfoil with serrated gurney flaps

Improving the aerodynamic performance of the airfoil is important for optimising the rotor efficiency of the vertical axis wind turbines. As a simple passive control method, the Gurney flap is widely used to improve the aerodynamic performance of airfoils. In this paper, we study the impact of applying a novel serrated gurney flap with different heights on the NACA 0018 airfoil. An improved delayed detached eddy simulation method is adopted to investigate the lift-enhancing mechanism of the serrated gurney flap and the evolution of the downstream vortex system. The results show that the serrated gurney flap can effectively increase the airfoil lift coefficient and the lift-to-drag ratio. The improvement of the serrated gurney flap on the aerodynamic performance of the airfoil is more pronounced at moderate angles of attack. Further analysis of the downstream wake shows that a pair of vortices wraps over both sides of the airfoil and rotates perpendicular to the wake flow, which is produced by the columnar vortex upstream of the flap. These vortices mixed with the wake and accelerated the dissipation of the separated vortex on the suction surface of the airfoil.

Study on the distribution characteristics of TBL fluctuating pressure of a submerged conical-cylindrical structure

The fluctuating pressure within turbulent boundary layers (TBL) has garnered significant attention as a pivotal factor in generating hydrodynamic noise for underwater vehicles. This study employs numerical simulations to investigate the fluctuating pressure of a submerged conical-cylindrical structure, utilizing the Improved Delayed Detached Eddy Simulation (IDDES) method. This research delves into the spatiotemporal distribution characteristics of wall fluctuating pressure across various Reynolds numbers. Notably, the power spectral density (PSD) curve of TBL fluctuating pressure exhibits relative stability at lower frequencies, whereas it undergoes a rapid decline in the high-frequency range, following an inverse proportional trend with the power-law exponent of frequency. Moreover, the dimensionless migration velocity in the parallel mid-body segment is consistent with the TBL over a flat plate, while it is slightly smaller in the outflow segment.

Research on the drag reduction of high-speed train based on bottom two-multistage wing deflector

To address the challenge of reducing aerodynamic drag while further enhancing the speed of high-speed trains, this research employs the concept of flow control for the bottom parts and draws inspiration from the front wings of Formula 1 (F1) race cars. Three kinds of two-multistage wing deflectors are designed and systematically analyzed by unsteady Reynolds-averaged Navier–Stokes (URANS) turbulent model. The most suitable design is determined by the single bogie model with a simplified train body. Using the improved delayed detached eddy simulation method, the aerodynamic drag of 1:8 three-car train models with or without two-multistage wing deflector is studied at different operational speeds. The results present the total drag reduction is higher at higher speeds. The reductions of 4.26%, 3.92%, 3.63%, and 3.49% are obtained at the operating speeds of 400, 350, 300, and 200 km/h, respectively. The two-multistage wing deflector desirably improves the flow structure at the bottom of the train, which leads to the reduction of aerodynamic drag and a corresponding reduction in the positive pressure zones within the bogie area. Furthermore, the deflector restricts shedding vortices, effectively narrowing the interference range of airflow under the train, which will provide a potential drag reduction method for the next generation high-speed train.

Using leeward air-blowing to alleviate the aerodynamic lateral impact of trains at diverse yaw angles

The safety risks of high-speed trains in crosswind environments escalate with increasing train speeds. The present study employs the improved delayed detached eddy simulation method based on the shear stress transport k–ω turbulence model to evaluate an active control method targeting the reduction of lateral forces acting on the train. The effects of air blowing strategy on the leeward side of the train are examined considering different yaw angles and blowing speeds. The findings reveal that the active air blowing, mixed with the flow laterally downstream the train roof, induces the increase in the local turbulence and alters the surface pressure distribution. Within the investigated range of yaw angles, the active air blowing yields a lateral force reduction ranging from 1.0% to 8.8%. Varying the blowing speed can further decrease the lateral force of the entire train by 5.9% and 0.8% at yaw angles of 15° and 75°, respectively. The power invested in active blowing demonstrates maximum returns at a yaw angle near 45° while diminishing with increasing blowing speed.

Study on the Tip Leakage Loss Mechanism of a Compressor Cascade Using the Enhanced Delay Detached Eddy Simulation Method.

The leakage flow has a significant impact on the aerodynamic losses and efficiency of the compressor. This paper investigates the loss mechanism in the tip region based on a high-load cantilevered stator cascade. Firstly, a high-fidelity flow field structure was obtained based on the Enhanced Delay Detached Eddy Simulation (EDDES) method. Subsequently, the Liutex method was employed to study the vortex structures in the tip region. The results indicate the presence of a tip leakage vortex (TLV), passage vortex (PV), and induced vortex (IV) in the tip region. At i=4°,8°, the induced vortex interacts with the PV and low-energy fluid, forming a "three-shape" mixed vortex. Finally, a qualitative and quantitative analysis of the loss sources in the tip flow field was conducted based on the entropy generation rate, and the impact of the incidence on the losses was explored. The loss sources in the tip flow field included endwall loss, blade profile loss, wake loss, and secondary flow loss. At i=0°, the loss primarily originated from the endwall and blade profile, accounting for 40% and 39%, respectively. As the incidence increased, the absolute value of losses increased, and the proportion of loss caused by secondary flow significantly increased. At i=8°, the proportion of secondary flow loss reached 47%, indicating the most significant impact.

On the airfoil leading-edge noise reduction using poro-wavy leading edges

This paper presents numerical studies on airfoil leading-edge turbulence interaction noise reduction using poro-wavy leading edges. Three different bionic treatments including wavy leading edges, porous leading edges, and a novel combined poro-wavy leading edges are modeled. The turbulent flow field is solved using the improved delayed detached eddy simulation method. The aerodynamic noise is predicted using the Ffowcs Williams and Hawkings acoustic analogy theory. The inflow Mach number is approximately 0.12 with an angle of attack of 0°, and the chord-based Reynolds number is 400 000. The present numerical method is first validated against experimental data and previous studies. Then the effects of the three bionic treatments on the aerodynamic performance and the aeroacoustic performance are analyzed. The results show that all the three bionic treatments will increase the mean drag of the airfoil, especially for the airfoils with porous treatment, while the lift and drag fluctuations are significantly reduced by the three bionic treatments. The wavy leading edges are found to be more effective for the reduction of broadband noise, while the porous leading edges are more effective for the reduction of the tonal noise. For the poro-wavy leading edges, both the tonal noise and broadband noise are significantly reduced, which means that the combined poro-wavy leading edges possess both the advantages of the wavy and porous treatments. The underlying flow mechanisms responsible for the noise reduction are finally analyzed in detail.

Research on the flywheel clearance flow and heat transfer characteristics in a horizontal canned motor reactor coolant pump

Flywheel is an important component for the canned motor reactor coolant pump (RCP). The flywheel of horizontal canned motor reactor coolant pump (HRCP) is different from RCP’s flywheel. Its clearance width or different thermal conductivity structure would affect the flow structure of flywheel clearance flow, and then affect temperature distribution of the clearance, which would further affects the life and stability of flywheel. Therefore, it is necessary to analyze the flow and heat transfer characteristics of flywheel clearance flow in detail. This paper applies a Detached Eddy Simulation (DES) method to study the flywheel clearance flow of HRCP. The results show that the DES method can capture the small-scale vortex structure in turbulence Taylor-Couette flow and then give a more refined flow field morphology. The authors further study the regularity of flywheel clearance flow with different thermal conductivity structure, and the results indicate that optimizing thermal conductivity structure can effectively reduce flywheel’s temperature. The authors also investigate the heat dissipation mechanism of different flywheel clearance structures.

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.

On the residence enhancement mechanism of the cavity-based dual-mode scramjet combustor through the Eulerian and Lagrangian analysis

The lower total temperature and pressure in a dual-mode scramjet engine lead to slower rates of evaporation and chemical reactions, while the inflow velocity is higher than that in a ramjet engine. Consequently, enhancing fuel residence time becomes a more critical challenge. The cavity is a crucial device for enhancing the residence time. However, the quantitative residence capacity and the mechanism have not yet been revealed, especially in wide-range speed inflow conditions. This work employs the delayed detached eddy simulation method to investigate the mass transport and fluid residence characteristics of the wide-range subsonic flow (Ma = 0.3, 0.4, 0.5, 0.6, 0.7) over the cavity. The Lagrangian coherent structure is utilized to characterize the dynamic evolution of the large-scale vortex in the cavity shear layer. Particle tracking is employed to delicately determine the net mass exchange rate and the quantitative cavity residence time. Based on the entrainment process of the large-scale vortex and mass exchange between the mainstream and cavity, this paper proposes a novel theoretical entrainment-impinging model of the large-scale vortex for calculating the residence enhancement coefficient (τr). The theoretical model demonstrates that the residence enhancement coefficient is a function of the cavity geometry (L, D), the vortex radius (rv), the shedding Strouhal number of the vortex (St), and the vortex/trailing edge interaction coefficient (η). Furthermore, it has been proven that the model proposed in this paper is applicable to a wide range of inflow turbulent conditions and cavity geometric configurations.

Serration of the duct trailing edge to affect the hydrodynamics and noise generation for a pump-jet propulsor

Inspired by the silent gliding feather of owl wings, the trailing edge of the duct of a pump-jet propulsor was designed with a similar serrated structure in order to reduce noise generation. Two distinct serrated structures were proposed and evaluated using the detached eddy simulation method with the shear stress transport k−ω turbulence model. The findings indicated that while the hydrodynamic efficiency changed within 1% upon the inclusion of the serrated trailing edge, a significant alteration existed in vortex structures of the wake. More horseshoe and secondary vortices were generated since large-scale vortices induced by the duct were disrupted circumferentially. This phenomenon expedited the distortion and mixing of trailing-edge vortices, causing flow instability. Furthermore, the serrated trailing-edge structure led to noise reduction. Particularly in the 0–1000 Hz range, the sound pressure level behind the duct showed a maximum reduction of 4.43 dB.

Numerical study on wind-loading characteristics of a high-speed train running over the bridge under tornado-like vortices

With global warming intensifying, weather patterns become more volatile and extremes more common. Tornadoes are the most destructive natural disasters causing significant damage to infrastructure. Meanwhile, high-speed railways now face greater risks from tornado events as the national railway network and mass transit trains expand. Thus, studying the tornado flow characteristics and associated effects on high-speed trains is necessary. A study is presented regarding the wind-loading characteristics of a high-speed train running over a railway bridge induced by a tornado belonging to the future railway network. The wind-loading characteristics analyses are performed using the improved delayed detached eddy simulation method. After verifying the numerical approach and mesh strategy, computational studies are conducted to produce a tornado-like vortex and investigate the tornado-induced wind-loading characteristics of a high-speed train running on the bridge by combining a tornado simulation with a moving mesh technique. For the wind-loading parameters studied herein, the selected train's velocity range is between 50 and 350 km/h, the typical operation speed of either regular or high-speed trains. The numerical results show that the time histories of aerodynamic forces on the train revealed a pattern in tornadic flow variability, the time evolutions of the wind loads on the train were affected by train speeds, and the fluctuation was the greatest when the train ran at 50 km/h. Moreover, the train is subjected to larger aerodynamic forces and moments when it operates along with the rotating vortex flow, especially in the core region, and the train is more dangerous when it runs at a lower speed. The results in this study provide references for assessing operation safety, while a train running on the bridge encounters tornadoes.

Accuracy Investigations of Dynamic Characteristic Predictions of Tip Leakage Flow Using Detached Eddy Simulation

The accurate prediction of tip leakage flow is the premise for flow mechanism analysis and compressor performance optimization. The detached eddy simulation (DES) method, which compromises cost and accuracy, has excellent potential for a high Reynolds flow, like a compressor.However, in the case of tip leakage flow, especially when there are multiple wall boundary layers and strong shear between the mainstream and leakage flow, the DES method exhibits accuracy deficiencies. This paper explores the resolution of the critical detailed structures using the DES method and its correlation with the accuracy of time-averaged aerodynamic parameter predictions. Based on this, we propose the necessary conditions for the DES method to accurately predict the leakage flow from the perspective of the detailed structure of the flow field. A simplified model is proposed to emphasize the characteristics of tip leakage flow with “multiple walls + narrow tip gap”, and the high-fidelity flow field of the WALE LES method is used as a benchmark. With the main fluctuation structures obtained by the SPOD method, it is concluded that the DES method is unable to resolve the Kelvin–Helmholtz instability at the initial position of the leakage, which leads to the generation of the secondary leakage vortex upstream of the leakage and the breakdown of the induced vortex, two critical flow structures, being incorrectly estimated. This can lead to misestimationsof the force direction on the tip leakage vortex and the main fluctuation on the flow field. As a result, the tip leakage vortex trajectory evolves toward the middle of the passage along the tangential direction and away from the upper wall downstream of the leakage compared with the LES results. Predictions of losses in the upstream and midstream regions are underestimated, whereas they are overestimated downstream of the leakage and outside the passage.Therefore, the accurate resolution of these two critical detailed structures is an essential prerequisite for the precise prediction of tip leakage flow using DES series methods.

Aerodynamic noise from a high-speed train bogie with complex geometry under a leading car

Among the various aerodynamic noise sources associated with high-speed trains, the bogie regions, and particularly the leading bogie, contribute most to the overall noise. To control this noise, insight into the noise generation mechanisms is imperative but relevant studies are limited due to the difficulties of simulating the flow in regions with complex geometry. An investigation is presented of the aerodynamic noise from a bogie under the leading car of a high-speed train using Delayed Detached Eddy Simulation method. A hybrid grid system is utilized to simulate the flow field, which provides input for the far-field noise calculations based on the Ffowcs Williams and Hawkings equation. Analysis shows that the outer bogie components experience large pressure fluctuations as they are immersed in the shear layer detached from upstream, which is therefore identified as playing a critical role in the noise generation. The resulting noise spectra are broadband and dominated by low frequencies. Compared with the bogie, the bogie cavity and train nose emit 5 dB greater sound power for the current geometry. This study improves understanding of the mechanisms behind the generation of aerodynamic noise in train bogies and lays the foundations for future studies of train bogie noise reduction.

Effect of upstream flow characteristics on the wake topology of a square-back truck

The influence of upstream flow characteristics on the bi-stable flow structure in the wake region of a simplified square-back heavy vehicle model at a Reynolds number of 2.7 × 104 was investigated by using the improved delayed detached eddy simulation method. The asymmetric wake structure of this model and its corresponding aerodynamic response were examined, aiming to identify the effect mechanism of three inlet profiles on the asymmetric wake structure of the named ground transportation system (GTS) model in simulations. The accuracy of the numerical method used in this study was validated by comparison with wake structure data, including the flow states, vortex core's location, and aerodynamic drag obtained from previous large eddy simulations and water channel experiments. The numerical results show that different turbulent inlet velocity profiles lead to different wake topologies. When the turbulent velocity profile with a turbulence intensity of 15% generated by TurbSim, a stochastic inflow turbulence tool for generating turbulent velocity inlet on an atmospheric boundary layer profile, is used, the expected bi-stable flow topology is still observed, but it is not shown in the case by means of the turbulence generator incorporated into ANSYS Fluent. Those turbulent inlet velocity profiles contribute to the increase in GTS model's aerodynamic drag forces. Compared to the uniform velocity profile, the TurbSim velocity profile can achieve a drag increase in 7.23%. In addition, this turbulent profile intensifies the flow fluctuations in the wake region and enhances the transient response frequency of the wake region. Thus, when assessing the vehicle aerodynamic performance in open air, especially under crosswinds, the real turbulence velocity profile, e.g., the profile generated by TurbSim in the current study, is recommended to be used for a more accurate prediction in numerical simulations.

A production term study of delayed detached eddy simulation for turbulent near wake based on proper orthogonal decomposition

In this study, we investigate the distribution of turbulent kinetic energy (TKE) in the near wake of a circular cylinder in a turbulent flow. Numerical calculations were performed using the delayed detached eddy simulation method, incorporating two different production terms for TKE: one in its original form and the other with the Kato–Launder correction term. Our results demonstrate that the turbulence model utilizing the Kato–Launder correction term exhibits a strong correlation between TKE and the vorticity field, which is related to the calculations of mean velocity, velocity fluctuation, and other parameters that are in closer agreement with direct numerical simulation and experimental values. By employing the proper orthogonal decomposition technique, we extract and reconstruct three significant modes within the flow: the shear layer mode, vortex shedding mode, and near-wake bubble mode. The findings reveal that the Kato–Launder correction term offers a more detailed portrayal closer to the real flow physics. Conversely, the original form of the TKE production term exhibits an uneven energy distribution among the three modes and affects the role of the diffusion term within the flow. This leads to a less accurate representation of the vortex shedding mode and an excessive diffusion effect in the near-wake bubble mode. Finally, possible modifications of the turbulence model in this problem are given to enhance the portrayal of these characteristics. This work presents an analytical framework that enables a comprehensive analysis of turbulence models, providing valuable physical insights and guidance for improvement.

Numerical study on the effect of pantograph fairing on aerodynamic performance at various train heights

The pantograph is one of the primary sources of aerodynamic drag and noise during the operation of a high-speed train. To investigate the impact of the pantograph fairing on the aerodynamic performance at various train heights, six computational models are established, and the Improved Delayed Detached Eddy Simulation method is employed. The results indicate that the installation of the fairing can effectively reduce the aerodynamic drag coefficient of the high-speed train and pantograph at different train heights. As the train height increases, the drag reduction effect of the fairing enhances when the sinking platform coexists. The overall drag reduction rate of up to 4.30% can be achieved at a train height of 4000 mm. However, the larger reduction rate of 58.32% in the pantograph's drag coefficient at a train height of 3700 mm indicates that the fairing could make a greater effect on the pantograph at a smaller train height. The reduction in train height and pantograph sinking height leads to an increase in drag coefficient on the tail car, and the implementation of the fairing further amplifies the drag. This work offers a reference for optimizing the aerodynamic performance of high-speed train.

Mixing enhancement of a compressible jet over a convex wall

A highly compressive effect would suppress the mixing of the shear layer in a convex wall jet. The spanwise distributed protrusions at the nozzle lip are employed to achieve mixing enhancement in this study. The mixing characteristics and enhancement mechanisms are numerically investigated by the delayed detached-eddy simulation method based on the two-equation shear-stress transport model. A widely applicable flow spatiotemporal analysis method, called proper orthogonal decomposition (POD), is used to gain further insight into the dynamical behaviours of the flow instability mode. The results reveal that the centrifugal effect maintains and amplifies the initial perturbations induced by the spanwise distributed heterogeneities, resulting in forced streamwise vortices. The instabilities induced by the streamwise vortices significantly increase the growth rate of the jet half-width and the shear layer vorticity thickness. The spanwise wavelength of the streamwise vortices is consistent with the spanwise distributed forced excitation. In addition, the spanwise meandering motion of the streamwise vortices is observed, which is usually associated with the streamwise travelling wave. This is further confirmed by the POD analysis of the spanwise velocity fluctuation in both stream-radial and stream-span sections. Also, the spatial distributions of the POD modes with the highest energy provide information on the secondary instability modes. Both sinuous and varicose types of disturbances are observed in the unforced jet, whereas the forced jet seems to be dominated by the sinuous type instability, which is more easily excited than the varicose type instability. Moreover, the turbulence intensity in the forced jet is also significantly enhanced as expected due to the earlier and stronger streamwise vortices and associated instabilities. The enhanced turbulent characteristics of the highly compressible condition tend to be isotropic, whereas in the unforced jet, it is anisotropic due to the strong compressibility suppressing the spanwise turbulent fluctuations.

Numerical Investigation of Stall Characteristics of Common Research Model Configuration Based on Zonal Detached Eddy Simulation Method

A zonal detached eddy simulation (ZDES) method, based on the two-equation k-ω SST turbulence model, was employed to predict stall characteristics and capture small-scale vortex structures in the wake region of the main wing under the post-stall condition of the Common Research Model (CRM) configuration. Additionally, the unsteady Reynolds-averaged Navier–Stokes (URANS) method was utilized for performance comparison in resolving small-scale vortices with ZDES. The results revealed a pronounced lateral flow on the wing, induced by the low-pressure region of the inner wing at post-stall angles of attack. Due to the downwash effect, the horizontal tail was influenced by the vortices in the wake region of the main wing, which the URANS method did not capture adequately. As the angle of attack increased, the separation area on the main wing expanded from the middle of the wing towards the inner wing. Consequently, the vortex structures in the wake region of the main wing became more intricate, and the primary peak of the lift coefficient spectrum shifted to the low-frequency region.

Supersonic Base Flow With and Without Propulsive Jets: A Brief Review

Supersonic base flow is encountered in many aerospace applications related to satellite launch vehicle, missiles and projectiles. The origin and evolution of large scale structures which govern the dynamics of the base flow is not properly understood. The presence of central propulsive jet further complicates the base flow characteristics. Different semi-empirical, experimental and numerical studies carried out in supersonic base flow is briefly reviewed. Various issues and challenges faced in CFD simulation of base flow problems are described. Apart from resolution of numerics, proper choice of turbulence model remains the key aspect in the Reynolds Averaged Navier Stokes (RANS) simulations. Prediction of axial distribution of centerline velocities and recirculation bubble size remain problematic in RANS simulation. Detached Eddy Simulation (DES) results show dependence on model constants. Large Eddy Simulation (LES) studies with proper filter and Sub Grid Scale (SGS) model is promising, but its applications are limited to canonical problems due to very high computational requirements. Direct Numerical Simulation (DNS) studies reported in the literature consider much lower Reynolds number and hence could not be compared with the experimental data. It is suggested to use RANS and DES methods with anchored turbulence model from well-designed experiments for design data generation. Efforts should be made to use higher order numerical methods (LES and DNS) to explore the origin and evolution of large scale structures observed in compressible base flow.

Performance evaluation and shape improvement of a novel vertical-axis autorotation current turbine

A new type of vertical-axis turbine called the vertical-axis autorotation current turbine (VAACT) has been investigated experimentally and numerically. This novel turbine utilizes the autorotation phenomenon to extract kinetic energy and is characterized by a higher moment of inertia and a flapped blade. The main objective of the present work is to further improve the efficiency of this turbine type by improving the original shape. First, through a comparison of experimental and numerical results under specific conditions, the accuracy of the three-dimensional (3D) detached eddy simulation (DES) used in the present study is validated. The free surface effect on the average rotation velocity and efficiency is found to be insignificant when the fluctuation of the free surface is small. Then, shape improvement based on response surface methodology (RSM) incorporated into computational fluid dynamics (CFD) simulations is performed. The inner length and flapped angle are selected as shape variation parameters. Finally, the optimal parameters of an inner length of 10.8 cm and a flapped angle of 47.2°, corresponding to a maximum mean power coefficient of 0.1545, are found and verified. The improvement methodology adopted in this paper greatly reduces the number of required sampling points compared to other optimization algorithms, such as genetic algorithms, which confirms the potential of the resource-intensive 3D DES method as a computational tool.