Improved Delayed Detached Eddy Simulation Research Articles

Self-Adaptive Turbulence Eddy Simulation with a high-order finite differencing method for high Reynolds number complex flows

Accurate prediction of high Reynolds number wall-bounded flows is a challenge for turbulence modeling and numerical discretization. Moreover, complex engineering applications increase the difficulties in implementing robust and low-dissipation numerical schemes. A newly developed unified turbulence approach, Self-Adaptive Turbulence Eddy Simulation (SATES), is assessed in the simulation of a multi-element 30P30N airfoil at a high Reynolds number of 1.71 × 106. To reduce the numerical error, high-order finite differencing-based weighted compact nonlinear schemes (WCNSs) are successfully applied. The effects of grid resolution, numerical schemes and turbulence model are investigated by grouping studies. The SATES shows low sensitivity to grid resolution and predicts satisfactory results even on a coarse grid containing approximately 7.2 million cells. Compared with second-order accurate central differencing, the high-order accurate WCNS has less numerical dissipation and significantly improves the resolution of turbulence structures and the accuracy of numerical results. Z-type nonlinear weighted averaging interpolation, designed for shock capturing property in the WCNS scheme, leads to extra numerical dissipation and affects the accuracy of the results. The numerical results of the SATES are more accurate than those of the Smagorinsky Large Eddy Simulation (LES) and the Improved Delayed Detached Eddy Simulation (IDDES). The SATES provides sufficient modeling of the boundary layer on the coarse grid and a fast ‘transition’ from the RANS to the LES mode for accurate prediction of the separated shear layer and flap boundary layer separation. These results confirm that the SATES turbulence modeling approach is efficient for high Reynolds number flows and that the high-order accurate WCNS schemes show sufficient robustness and accuracy for engineering applications.

Impact of the train heights on the aerodynamic behaviour of a high-speed train

The impact of train heights on train aerodynamic performance is studied by using an improved delayed detached-eddy simulation (IDDES) method. The correctness of the numerical method has been verified by the existing wind tunnel and moving model experiments data. The aerodynamic drag, lift, slipstream, and wake flow are compared for three train heights. The results presented that the drag and lift increased by 6.2% and 23.8% respectively, with an increase in train height from 3.89 m to 4.19 m. Compared with the 3.89 m case, the maximum time-averaged slipstream at the platform location for 4.04 and 4.19 m cases are increased by 2.0% and 4.3% respectively. Meanwhile, the wake topology for three cases is described and analyzed quantitatively. The downwash angle of the wake longitudinal flow is increased with the increasing train height, resulting in the mixing of the downwash flow and the ground flow in advance. The wake in the higher trains tends to develop outward and downward. Besides, the higher trains will also bring greater transient aerodynamic loads to the equipment above the train. It’s recommended to shorten the maintenance period of the electrical equipment above the higher trains to ensure the devices’ safety.Abbreviations: CFL: Courant–Friedrichs-Lewy; COT: Center of the track; FDR: Flow development region; FFT: Fast Fourier transform; GF: Ground-fixed reference system; ICE3: Intercity Express 3; IDDES: Improved delayed detached-eddy simulation; LES: Large-eddy simulation; LV: Longitudinal vortex; MME: Moving model experiments; NBL: Negative bifurcation line; PBL: Positive bifurcation line; PSD: Power spectral density; RANS: Reynolds averaged Navier – Stokes; SF: Stable focus; SP: Saddle point; STBR: Single-track ballast and rails; SV: Spanwise vortex; TF: Train-fixed reference system; TOR: Top of the track; TSI: Technical specification for interoperability; UN: Unstable node; WPR: Wake propagation region

Improved delayed detached eddy simulation-based investigation of aerodynamic performance and flow field characteristics of high-speed trains with plate brakes

As a key component, the braking system is vital for ensuring safe and comfortable operation of trains moving at high speeds. Because the aerodynamic drag depends linearly on the square of the vehicle speed, plate braking has broad prospects for high-speed trains. The train body interface is altered by the plate opening, which causes a sudden change in the flow field around the train and deteriorates its aerodynamic performance. In this study, dynamic grid technology was used to realize plate opening, and an improved delayed detached eddy simulation (IDDES) was used to simulate the unsteady aerodynamic characteristics of trains during plate braking. The simulations and settings were verified using wind-tunnel test data. The effects of the plate motion on the aerodynamic performance of the train were clarified by analyzing the changes of the aerodynamic loads of the plate and the train body within 0.2 s for an opening plate and 1.8 s for an opened plate. The aerodynamic forces of the different cars were significantly different during the plate-opening process, especially near the maximal opening angle. In particular, for the middle car, the aerodynamic force depended on the number of plates. The plate opening caused lateral aerodynamic fluctuations in the car body, which caused the car body to shake laterally, especially the downstream car, and the degree of the fluctuation of the lateral force of the car body also depended on the number of plates. Owing to the airflow inertia, the aerodynamic drag of the plate at the end of the motion appeared as a pulse peak, and the peak value decreased downstream of the train. The opening of the plates caused the flow field above the roof to develop disordered downstream. Owing to the coupling and superposition of the downstream flow field, the aerodynamic drag of the first plate fluctuated slightly, whereas that of the downstream plate fluctuated significantly, likely affecting the normal operation of the equipment on the top of the train, such as air conditioning and pantograph.

A novel hybrid method for aerodynamic noise prediction of high-lift devices

Aerodynamic noise of High-Lift Devices (HLDs) is one of the main sources of airframe noise, and has immediate impacts on the airworthiness certification, environmental protection and security of commercial aircraft. In this study, a novel hybrid method is proposed for the aerodynamic noise prediction of HLD. A negative Spalart-Allmaras (S-A) turbulence model based Improved Delayed Detached Eddy Simulation (IDDES) method coupling with AFT-2017b transition model is developed, in order to elaborately simulate the complex flow field around the HLD and thus obtain the information of acoustic sources. A Farassat-Kirchhoff hybrid method is developed to filter the spurious noise sources caused by the vortex motions in solving the Ffowcs Williams-Hawkings (FW-H) equation with permeable integral surfaces, and accurately predict the far-field noise radiation of the HLD. The results of the 30P30N HLD indicate that, the computational Sound Pressure Levels (SPLs) obtained by the Farassat-Kirchhoff hybrid method conform well with the experimental ones in the spectrum for the given observation point, and are more accurate than those obtained by the Farassat 1A method. Based on the hybrid method, the acoustic directivity of the HLD of a commercial aircraft is obtained, and the variation of the SPLs in the spectrum with the deflection angle of the slat is analyzed.

Spiral flow instability between a rotor and a stator in high-speed turbomachinery and its relation to fan noise

Hydrodynamic instabilities in the interstage of turbomachinery and their relation to fan broadband noise are investigated over wide operating conditions. By applying compressible linear stability analysis to the velocity profiles of NASA's high-speed fan rig, source diagnostic test (SDT), three distinct unstable regimes are found: the first mode amplifies disturbances over a wide frequency range including the rotor speed. The second mode rotates much slower and develops inside the outer-wall boundary layer. The third mode spins in the opposite direction. An approach equivalent to dynamic mode decomposition (DMD) successfully extracts these modes from a database of improved delayed detached-eddy simulation (IDDES) solving the SDT geometry at approach and cut-back conditions: the extracted DMD modes generally capture the characteristics of eigenfunctions predicted by the linear stability analysis. At the approach condition, the first two unstable modes seem to interact inside the outer-wall boundary layer, while at the cut-back condition, disturbances associated with the first regime migrate away from the wall. Moreover, outer-wall pressure fluctuations of the IDDES database are Fourier decomposed in the interstage: coherent structures extracted using proper orthogonal decomposition are found to be dominated by duct acoustic modes, and they can be mapped not only to interaction tones but also to broadband noise likely associated with the interaction between disturbances in the rotor wakes and quasi-stationary structures near the outer wall. This interaction noise appears to be radiated to both inlet and exhaust with similar azimuthal-mode distributions; however, the importance relative to other noise sources decreases with the engine speed.

Aerodynamic Characteristics Analysis of Rectifier Drum of High-Speed Train Environmental Monitoring Devices

To study the aerodynamic characteristics of the convex structure of a surface-monitoring device on a high-speed train and to evaluate its impact on the aerodynamic performance of the high-speed train, numerical simulation research was conducted on three different layouts of the monitoring device. The computational fluid dynamics (CFD) method was used for the simulation study, and the unsteady compressible NS equation was used as the control equation. Hexagonal grid technology was used to reduce the demand for the grid quantity. The rationality of the grid size and layout was verified through grid independence research. To increase the accuracy of the numerical simulation, the γ-Reθ transition model and improved delayed detached eddy simulation (IDDES) method were coupled for the simulation research. The aerodynamic characteristics of the different operation directions and configurations were compared and analyzed. The research results showed that the windward side of the single pantograph detection device experienced positive pressure, and the sideline and leeward sides experienced negative pressure. Increasing the fillet radius of the sideline could appropriately reduce the aerodynamic resistance. When the speed was about 110 m/s, the drag force coefficient of the detection device was 210~410 N, and the lateral force was small, which means that it had little impact on the overall aerodynamic force of the train. According to the results of the unsteady analysis of the layout with a large space, the resistance during forward travel was greater than that during negative travel. The streamlined upwind surface was conducive to reducing the scope of the leeward separation zone and the amplitude of the pressure fluctuation in the leeward zone, and it thus reduced the resistance. For the running trains, a vortex was formed on their leeward surface. The pressure monitoring results showed that the separated airflow had no dominant frequency or energy peak. The possibility of the following train top and other components experiencing resonance damage is low.

Mesh and model requirements for capturing deep-stall aerodynamics in low-Mach-number flows

The paper presents a comprehensive computational fluid dynamics investigation of the effects of grid resolution and turbulence-model choice for capturing the unsteady three-dimensional aerodynamic performance of NACA 0012 and 0021 airfoils, with specific focus on the deep-stall regime. At high angles of attack (α), wind turbine blades routinely experience vortex-induced vibrations, which can cause significant structural damages. Accurate predictions of post-stall aerodynamics can identify the frequencies at which such vibrations maybe triggered. In this context, the NACA 0012 airfoil simulations are conducted at a chord-based Reynolds number, , with the k-ω Shear-Stress Transport Reynolds-Averaged Navier-Stokes (RANS) and Improved Delayed Detached Eddy Simulation (IDDES) hybrid RANS-Large Eddy Simulation turbulence models. The effect of mesh resolution both in the wall-normal and spanwise directions is investigated. Only the IDDES model with a minimum spanwise resolution of 24 cells per chord length correctly predicts the aerodynamic forces. Spectral analysis shows the peak primary shedding frequency at , which signifies the end of the stall region. In the post-stall regime, both lift and drag frequencies drop asymptotically with increasing α. The Strouhal number, based on normalised chord length, remains nearly constant in this region. Based on this study, NACA 0021 airfoil runs are performed with IDDES for and on the finest wall-normal mesh and three spanwise grids. Simulations conducted on the finer spanwise grids demonstrate grid independence and show good agreement with experiments. The effect of varying on the airfoil frequency statistics is investigated. Additionally, comparison studies are presented to investigate the impact of airfoil thickness on the frequency content at . The results from the study provide guidance on the choice of mesh resolution with the IDDES model to accurately capture aerodynamic quantities for complex industrial applications.

An investigation on the wake vortex structure and trajectory characteristics of the vehicle launched underwater

ABSTRACT The Improved Delayed Detached Eddy Simulation (IDDES), VOF (Volume of Fluid) multiphase flow model, and overlapping grid technology were adopted in this paper. The results show that the interaction between the high-speed regions and the low-speed flow increases, as the Kelvin–Helmholtz instability phenomenon occurs in the mixing fluid during traveling underwater. Under the effect of the inertial force, the tail bubble expands successively, resulting in an increase in cavity volume and a decrease in pressure. In addition, the wake vortex is composed of the main vortex and secondary vortex, which is a cone of the major influencing factors causing the disturbance of the flow field. As the vehicle continuously passes through the free surface, the buoyancy center of the vehicle successively moves downward, resulting in a change in the direction of buoyancy moment and aggravating the deflection amplitude of the vehicle.

Numerical Comparison in Aerodynamic Drag and Noise of High-Speed Pantographs with or without Platform Sinking

Flat roofs and platform sinking are two common installation configurations for high-speed pantographs. The cavity formed by the platform sinking is a potential source of aerodynamic drag and noise. In this paper, the shape of the rectangular cavity is optimized, and the aerodynamic performance of the high-speed pantograph with or without platform sinking is compared and discussed based on the optimized cavity results. The flow field and sound propagation are predicted by the improved delayed detached eddy simulation (IDDES) method and the FW-H equation. The results show that the rectangular cavity produces the largest aerodynamic drag and radiation noise. The upstream, downstream, and bottom surfaces of the cavity can be optimized by rounded and sloped edges to reduce aerodynamic drag and noise. The unstable shear flow and recirculation zone formed by flow separation and reattachment can be reduced by modifying the upstream and downstream surfaces of the cavity. In addition, the vortex in front of the downstream surface of the cavity can be reduced or even eliminated by modifying the bottom surface. When the upstream and downstream surfaces of the cavity are rounded and the bottom surface is sloped (R/H = 0.8), the aerodynamic performance of the cavity is better. Compared with the pantograph installed on the flat roof, the aerodynamic drag and noise of the pantograph with platform sinking are significantly reduced due to the shielding of the lower structure by the cavity, and the total drag and noise are reduced by 5.22% and 1.45 dBA, respectively.

Simulation and experimental research on hydrodynamic performance of fully attached submarine in the surface and near the surface modes

The switch between the surface and near-surface modes is critical to the hydrodynamic performance of the submarines. In this study, based on tank experiments, an improved delayed detached-eddy simulation (IDDES) method based on the shear stress transport (SST) K-Omega was tested for its ability to predict the hydrodynamic coefficients of submarines with satisfactory results. By expanding the simulation range, the changes in the hydrodynamic performance of the submarine on and near the water surface were analyzed with respect to force coefficient, flow field, maximum wave height, and maximum ray angle. The change in the drag coefficient of the hull has a water surface mode and near-water surface mode, which leads to a peak value of the drag coefficient at different submerged depths. When a "hump" phenomenon occurs at high Froude number(Fr = 0.5), the flow separation at the stern shoulder of the hull weakens, resulting in an increase in wake velocity and ray angle. The numerical simulation results showed that the Rabaud and Moisy model has better applicability, but when Fr > 0.8 and Hd > 1.5, the theory underestimates the maximum ray angle.

On the Effectiveness of Scale-Averaged RANS and Scale-Resolved IDDES Turbulence Simulation Approaches in Predicting the Pressure Field over a NASCAR Racecar

Racecar aerodynamic development requires well-correlated simulation data for rapid and incremental development cycles. Computational Fluid Dynamics (CFD) simulations and wind tunnel testing are industry-wide tools to perform such development, and the best use of these tools can define a race team’s ability to compete. With CFD usage being limited by the sanctioning bodies, large-scale mesh and large-time-step CFD simulations based on Reynolds-Averaged Navier–Stokes (RANS) approaches are popular. In order to provide the necessary aerodynamic performance advantages sought by CFD development, increasing confidence in the validity of CFD simulations is required. A previous study on a Scale-Averaged Simulation (SAS) approach using RANS simulations of a Gen-6 NASCAR, validated against moving-ground, open-jet wind tunnel data at multiple configurations, produced a framework with good wind tunnel correlation (within 2%) in aerodynamic coefficients of lift and drag predictions, but significant error in front-to-rear downforce balance (negative lift) predictions. A subsequent author’s publication on a Scale-Resolved Simulation (SRS) approach using Improved Delayed Detached Eddy Simulation (IDDES) for the same geometry showed a good correlation in front-to-rear downforce balance, but lift and drag were overpredicted relative to wind tunnel data. The current study compares the surface pressure distribution collected from a full-scale wind tunnel test on a Gen-6 NASCAR to the SAS and SRS predictions (both utilizing SST k−ω turbulence models). CFD simulations were performed with a finite-volume commercial CFD code, Star-CCM+ by Siemens, utilizing a high-resolution CAD model of the same vehicle. A direct comparison of the surface pressure distributions from the wind tunnel and CFD data clearly showed regions of high and low correlations. The associated flow features were studied to further explore the strengths and areas of improvement needed in the CFD predictions. While RANS was seen to be more accurate in terms of lift and drag, it was a result of the cancellation of positive and negative errors. Whereas IDDES overpredicted lift and drag and requires an order of magnitude more computational resources, it was able to capture the trend of surface pressure seen in the wind tunnel measurements.

Hybrid LES/RANS simulations of a 90° pipe bend using different CFD solvers

In this study, the performance of several RANS and hybrid LES/RANS turbulence models along with different CFD solvers (ANSYS Fluent, OpenFOAM and RapidCFD) is tested in terms of their ability to reproduce the flow downstream of a 90° pipe bend. The focus is on the temporal mean profiles downstream of the bend up to approximately 31 times the pipe's diameter D. The turbulence models' accuracy is evaluated by means of performance indicators and flow profiles in comparison to LDA measurements. The results demonstrate that the use of RANS models is suitable as long as the location of interest is in the vicinity of the bend up to approximately 10 D downstream of the bend. With increasing distance, the accuracy of the RANS models decreases, and the use of hybrid LES/RANS models is recommended.Three hybrid LES/RANS models are tested: the stress-blended eddy simulation (SBES) model as well as the Spalart-Allmaras-based and the k-omega SST-based improved delayed detached eddy simulation (IDDES) models. All three models show enhanced results for a distance to the bend >10 D. Additionally, it is found that in this geometry configuration, the IDDES models require a transient inlet. The flow separation due to the redirection inside the bend is too weak to generate turbulent fluctuations and a fast transition to full LES mode in the separation region. Due to the turbulent excitation at the inlet, the RANS region downstream of the bend is distinctly reduced, resulting in an enhanced performance of the hybrid IDDES models.

Investigation of Very Large Eddy Simulation Method for Applications of Supersonic Turbulent Combustion

The very large eddy simulation (VLES) method was investigated for supersonic reacting flows in the present work. The advantages and characteristics of the VLES model and the widely used improved delayed detached eddy simulation (IDDES) method were revealed through a supersonic ramped-cavity cold flow. Compared to the IDDES model, the VLES model transformed from RANS mode to LES mode faster, resulting in a smaller gray region caused by the mode transition. However, the original volume-averaging truncation length scale could lead to poor predictions of the velocity profiles and wall pressure distribution. By introducing a hybrid truncation length scale combining the maximum grid length and the shear layer adaptive (SLA) length with different coefficients, the accuracy of the VLES method was significantly improved, and the issue of the low shear layer position was solved. Moreover, owing to the resolution control function, the VLES method could adaptively model more turbulent kinetic energy and maintain a good accuracy in a coarser mesh. Finally, the modified VLES method was applied in conjunction with a hybrid combustion model constructed by the partially stirred reactor (PaSR) model and the Ingenito supersonic combustion model (ISCM) in simulations of the supersonic flame in the DLR scramjet combustor. After introducing the correction of the molecular collision frequency by the ISCM, the results obtained by the hybrid combustion model were more consistent with the experimental results, especially for the time-averaging temperature profile in the ignition zone.

Self-adaptive turbulence eddy simulation of flow control for drag reduction around a square cylinder with an upstream rod

The use of an upstream circular rod to control the flow around a downstream square cylinder is found to efficiently reduce the drag of the square cylinder. The complex flow characteristics and mechanism of the rod-square system in tandem arrangement as a passive flow control method for various centre-to- centre gap ratios (L/D) are investigated by numerical simulation. Turbulence modelling of the rod-square cylinder system is challenging. To select a suitable turbulence modelling, the recently developed Self-Adaptive Turbulence Eddy Simulations (SATES), as well as IDDES (Improved Delayed Detached Eddy Simulation) and SBES (Stress Blended Eddy Simulation) are first assessed for single circular and single square cylinder flow test cases. The SATES method performs best among the three models for bluff body flow simulation and is thus used in the drag flow control study. For the rod-square flow control system, the Reynolds number is 34000 based on freestream velocity U0 and the edge length of the square cylinder D. The diameter of the control rod is kept constant at d/D = 0.18. The flow characteristics and mechanism are explored in detail, varying the rod and square cylinder distance of L/D = 1.5, 2.0, 2.5and 3.0. According to the absence or presence of vortex shedding from the upstream rod cylinder, two flow patterns (pattern I and pattern II) are observed. Large drag reduction is observed for all the simulation cases the maximum drag reduction is approximately 63% with L/D = 2.0, and the minimum drag reduction approximately 53% with L/D = 3.0. The secondary lock-frequency, which presents as pattern II, is caused by the excitation of upstream vortices. It is found that the side recirculation is associated with the wake recirculation length of the square cylinder, which is similar to the elongated cylinder in the flow pattern I. The flow mechanism of suppression of the vortices from the rod can be attributed to the effective diffusion of vorticity in the separating shear layer caused by the presence of the front wall of the square cylinder.

Numerical investigation of the evolution of aerodynamic behaviour when a high-speed train accelerates under crosswind conditions

Crosswind degrades the aerodynamic behaviour of trains, especially during train acceleration, and an understanding of the flow field and train safety details requires more research attention. In the present study, the improved delayed detached eddy simulation (IDDES) method is applied to investigate the aerodynamic behaviour when a high-speed train accelerates under crosswind conditions. The spatial–temporal evolution of eddies and pressure distributions on the train surface based on eight discrete moments are explored. Moreover, the effects of three acceleration values on the aerodynamic force/moment coefficients and safety indicators are evaluated. The results show that the distance between the eddy shedding from the head car and the train body becomes smaller and the negative pressure of the eddy core increases in the process of acceleration. The pressure on the windward side of the train is stable, while the pressure on the leeward side shows an increasing trend with increasing train speed. An increase in the acceleration value increases the maximum side force and rolling moment coefficients, most prominently for the head car. In addition, a larger acceleration value may cause stronger pulsation of the train. Although the acceleration values do not increase the maximum values of the safety indicators, they have a slight effect on the local maximum values.

Effect of Wavy Leading Edges on Airfoil Trailing-Edge Bluntness Noise

Among the several noise-generation mechanisms of airfoil self-noise, trailing-edge bluntness noise is an important noise source, which is caused by the vortex shedding at blunt trailing edges. A numerical study on airfoil trailing-edge bluntness noise control using the bio-inspired wavy leading edges is presented in this paper. The high-fidelity, improved, delayed, detached eddy simulation (IDDES) method was used to calculate the flow field, and the acoustic analogy method was used for noise prediction. For both the blunt-trailing-edge airfoils, a baseline airfoil with a straight leading edge and a bio-inspired airfoil with a wavy leading edge were used in this study. The chord-based Reynolds number was 400,000, and there was no angle of attack. The numerical results show that the trailing-edge bluntness noise of the baseline airfoil was significantly reduced by the wavy leading edges. The sound pressure level reduction was about 3.7 dB at the characteristic frequency, and the maximum sound pressure level reduction was as high as 35 dB. The trailing-edge bluntness noise was decreased at all directional angles. The maximum overall sound pressure level reduction was 6.3 dB at 0°. In addition, by analyzing the pressure fluctuations, wake characteristics, turbulent vortex structures and spanwise correlation and coherence of the flow field, the noise-reduction mechanisms of the bio-inspired airfoil are deeply revealed.

Experimental and numerical investigations of effects of ship superstructures on wind-induced loads for benchmarking

For a representative large modern containership, the effects of a deck container arrangement on the wind-induced loads were systematically investigated using physical model tests and numerical computations. Numerical simulations based on various turbulence models were performed to validate our predictions against comparative wind tunnel measurements. Not only standard two-equation turbulence models of the unsteady Reynolds-Averaged Navier–Stokes (URANS) equations solver but also the improved delayed detached eddy simulation (IDDES) and large eddy simulation (LES) turbulence models were used to determine their limits in the prediction of aerodynamic loads. Systematic discretization studies ensured adequate discretization independent predictions. With URANS, numerically predicted wind forces and moments in near-head and near-tail winds were compared favorably with the measured data. However, in oblique winds, URANS predictions deviated from measurements. In oblique winds, flow separations were pronounced; therefore, the flow was strongly transient. Consequently, a two-equation turbulence model was inappropriate. With IDDES, more accurate predictions were achieved, especially in oblique winds. With LES, although the computational effort was high, the agreement of the computed forces and moments with the measured values was superior. Flow details were also presented and discussed. The container arrangement on deck showed major effects on aerodynamic forces and moments. A tarpaulin covering the containers on deck reduced wind resistance by up to 70%.

Numerical simulation of bluff body turbulent flows using hybrid RANS/LES turbulence models

Many engineering applications involve turbulent flows around bluff bodies. Because of their intrinsically unsteady dynamics, bluff body characteristic flows feature unique turbulence-related phenomena, which makes their numerical modeling challenging. Accordingly, accounting for a circular bluff body flow configuration, three different turbulence modeling approaches are investigated in this work, (i) Reynolds-averaged Navier–Stokes (RANS), (ii) large eddy simulation (LES), and (iii) hybrid RANS/LES. Regarding the hybrid approaches, two variants of the detached eddy simulation (DES) one, delayed DES (DDES) and improved delayed DES (IDDES), are studied. As RANS model, the $$\mathrm{k}-\mathrm{\omega SST}$$ is utilized here. This RANS model is also used as the background one for both DDES and IDDES. Wall-adaptive local eddy viscosity (WALE) is used in turn as the sub-grid scale (SGS) model for LES. The velocity two-point correlation function is used to assess the mesh size requirements. When compared to experimental data, the obtained numerical results indicate that RANS overestimates the recirculating bubble length by over 18% and is not capable of describing the turbulent kinetic energy and the flow anisotropy in agreement with the experimental data. In contrast, LES, DDES, and IDDES are all within 1% of the recirculating bubble length while predicting both the Reynolds stress tensor components and the corresponding flow anisotropy in agreement with the measurements. Besides, normalized anisotropy tensor invariants maxima in the shear layer were reproduced by all scale resolving models studied here, but they failed to yield the local extrema measured within the wake recirculation region. A comparative analysis of the anisotropic Reynolds stress tensor invariances underscores the adequacy of the scale resolving models.

Reducing slipstream velocities around high-speed trains through retractable stationary surfaces

The slipstream wake structures generated by the passing of high-speed rail vehicles represents a hazard to passengers and workers in close proximity, and as such, maximum allowable slipstream velocities are prescribed in regulations. In this article, the possibility of reducing peak slipstream velocities using retractable or foldable local surface modifications on a generic high-speed train is assessed using Improved Delayed Detached Eddy Simulation (IDDES) modelling. It is demonstrated that the danger imposed by the slipstream wake structures, as measured by the induced velocities recorded at a pair of parallel test lines, as dictated in the standards, could be reduced by designs employing each of three methods examined. The most dramatic reductions were generated by large fins at the train rear, which redirected the time-mean trailing vortex structures to one side of the vehicle. However, the most feasible control method for symmetrical slipstream reduction consisted of placing small rectangular plates perpendicular to the flow near the nose section of the train. The induced disruption to the flow was observed to increase wake turbulence in a region above the train, which appeared to interfere with the formation of the trailing vortex structures and reduce the intensity of the cross-stream oscillation.

Improved Delayed Detached Eddy Simulation of Combustion of Hydrogen Jets in a High-Speed Confined Hot Air Cross Flow

The paper deals with the self-ignition and combustion of hydrogen jets in a high-speed transverse flow of hot vitiated air in a duct. The Improved Delayed Detached Eddy Simulation (IDDES) approach based on the Shear Stress Transport (SST) model is used, which in this paper is applied to a turbulent reacting flow with finite rate chemical reactions. An original Adaptive Implicit Scheme for unsteady simulations of turbulent flows with combustion, which was successfully used in IDDES simulation, is described. The simulation results are compared with the experimental database obtained at the LAERTE experimental workbench of the ONERA—The French Aerospace Laboratory. Comparison of IDDES with experimental results shows a strong sensitivity of the simulation results to the surface roughness and temperature of the duct walls. The results of IDDES modeling are in good agreement with experimental pressure distributions along the wall and with the results of videoregistration of the excited radical chemiluminescence.