Search Results (49)

The extended operation of high-speed railways has led to an increased incidence of tunnel lining defects, with falling debris posing a significant safety threat. Within tunnels, single-train passage and trains-crossing events constitute the most frequent operational scenarios, both generating extreme aerodynamic environments that alter debris trajectories from free fall. To systematically investigate the aerodynamic differences and underlying mechanisms governing falling debris behavior under these two distinct conditions, a three-dimensional computational fluid dynamics (CFD) model (debris–air–tunnel–train) was developed using an improved delayed detached eddy simulation (IDDES) turbulence model. Comparative analyses focused on the translational and rotational motions as well as the aerodynamic load coefficients of the debris in both single-train and trains-crossing scenarios. The mechanisms driving the changes in debris aerodynamic behavior are elucidated. Findings reveal that under single-train operation, falling debris travels a greater distance compared with trains-crossing conditions. Specifically, at train speeds ranging from 250–350 km/h, the average flight distances of falling debris in the X and Z directions under single-train conditions surpass those under trains crossing conditions by 10.3 and 5.5 times, respectively. At a train speed of 300 km/h, the impulse of C_Fx and C_Fz under single-train conditions is 8.6 and 4.5 times greater than under trains-crossing conditions, consequently leading to the observed reduction in flight distance. Under the conditions of trains crossing, the falling debris is situated between the two trains, and although the wind speed is low, the flow field exhibits instability. This is the primary factor contributing to the reduced flight distance of the falling debris. However, it also leads to more pronounced trajectory deviations and increased speed fluctuations under intersection conditions. The relative velocity (CRV) on the falling debris surface is diminished, resulting in smaller-scale vortex structures that are more numerous. Consequently, the aerodynamic load coefficient is reduced, while the fluctuation range experiences an increase. Full article

This study investigates dynamic stall mechanisms of a pitching NACA 0012 airfoil through high-fidelity computational fluid dynamics (CFD) simulations. The improved delayed detached eddy simulation (IDDES) method based on a sliding mesh system is constructed and validated against experimental airload measurements. The results demonstrate a good agreement and the capability to capture three-dimensional flow structures. Comparative analyses at two Mach numbers of 0.283 and 0.5 reveal distinct stall physics. At the Mach number of 0.283, a notable 9.7° delay is observed between the static and dynamic stall. The airfoil experiences a leading-edge stall dominated by a strong adverse pressure gradient and generates rapid airload variations. In addition, trailing-edge vortex (TEV) and secondary leading-edge vortices (LEVs) induce distinct airload fluctuations. After the shedding of primary vortices, secondary vortices develop. In contrast, the airfoil at the Mach number of 0.5 presents a reduced stall delay of 6.4° and a shock-induced dynamic stall characterized by dispersed, smaller vortices, which results in mild airload variations during stall. Aerodynamic damping analysis identifies stall delay as a primary contributor to negative damping. Enhanced pitching stability at the higher Mach number correlates with reduced stall delay and different LEV development characteristics. Results across varying reduced frequencies show that increasing reduced frequency delays the aerodynamic response and stall onset. At Ma = 0.283, this increasement promotes a divergent tendency in pitching motion, whereas at Ma = 0.5, it induces greater oscillatory stability attributed to distinct stall characteristics. Full article

Curved surfaces are a feature of many engineering applications, and as such, the accurate prediction of separation and reattachment from a curved surface is of great engineering importance. In this study, improved delayed detached eddy simulation (IDDES) is used, in conjunction with synthetic turbulence injection using the synthetic eddy method (SEM), to investigate the boundary layer separation from a curved backward-facing step for which large eddy simulation (LES) results are available. The commercial code Star CCM+ was used with the k-ω shear stress transport (SST) variation of the IDDES model to assess the accuracy of the code for this class of problem. The IDDES model predicted the separation length within 10.4% of the LES value for the finest mesh and 25.5% for the coarsest mesh, compared to 36.2% for the RANS simulation. Good agreement between the IDDES and LES was also found in terms of the distribution of skin friction, velocity, and Reynolds stress, demonstrating an acceptable level of accuracy, as has the prediction of the separation and reattachment location. The model has, however, found it difficult to capture the pressure coefficient accurately in the region of separation and reattachment. Overall, the IDDES model has performed well against a type of geometry that is typically a challenge to the hybrid RANS-LES method (HRLM). Full article

For this study, Improved Delayed Detached-Eddy Simulations (IDDES) were used to analyze the wake of a modified Ahmed body with varying upper and lower slants. The modified geometry produced a constant projected vertical base area, ensuring that the base and slant drag were a function of the pressure caused by the wake structures. Except at extreme slant angles, the general structures of the wake were a base torus with two pairs of streamwise-oriented vortices on each slant. These structures strongly correlated with the drag contribution of the rear surfaces: the torus with the vertical base and the streamwise-oriented vortices with the slants. As such, the base drag was minimized when the torus was most centrally aligned with the base, producing the largest stagnation region. Two slant-drag minima developed corresponding to two regimes of vortical flow on opposing slants. On one slant, the vortices were attached, and the drag correlated with the size and strength of the vortices. On the other slant, the vortices separated, and the drag correlated with the slant normal due to a more uniform pressure. This demonstrates a rich and complex set of interactions that must be managed in the development of base drag caused by wake flows. Full article

This study provides a detailed analysis of the convergence criteria for dynamic mode decomposition (DMD) parameters, with a focus on sampling frequency and period in high-Reynolds-number flows. The analysis is based on flow over an idealized road vehicle, the Ahmed body ($Re=7.7\times {10}^5$), using computational fluid dynamics (CFD) data from improved delayed detached eddy simulation (IDDES). The pressure and velocity spectrum analysis validated IDDES’s ability to capture system dynamics, consistent with existing studies. For a comprehensive understanding of the contributions of different components of the circle, the Ahmed body was divided into three regions: (a) front; (b) side, lower, and upper surfaces; and (c) rear fascia. Both pressure and skin-friction drag were analyzed in terms of frequency spectra and cumulative energy. Key findings show that a 90% contribution to the pressure drag comes from modes with a frequency of less than 26 Hz ($St$ = 0.187), while the friction drag requires 84 Hz ($St$ = 0.604) for similar energy capture. This study highlights the significance of accounting for intermittency and non-stationary behavior in turbulent flows for DMD convergence. A minimum of 3000 snapshots is necessary for the convergence of DMD eigenvalues, and sampling frequency ratios between 5 and 10 are needed to achieve a reconstruction error of less than 1%. The sampling period’s convergence showed that $T^{*}=250$ (equivalent to 20 cycles of the slowest coherent structures) stabilizes coherent mode shapes and energy levels. Beyond this, DMD may become unstable. Additionally, mean subtraction was found to improve DMD stability. These results offer critical insights into the effective application of DMD in analyzing complex vehicle flow fields. Full article

A transonic hump with a Mach number (M∞) of 0.875 was utilized to compare the prediction capabilities of RANS, URANS, IDDES, and IDDES-SLA for shock-induced separation. The results matched the experimental data concerning pressure, friction coefficients, velocity distribution, and other parameters. The improved delayed detached eddy simulation with a modified sub-grid scale (IDDES-SLA) method performed the best amongst tested numerical methods, demonstrating differences of 1.3% and 4.1% from the experimental results for separation and reattachment locations, respectively. The primary difference between IDDES and IDDES-SLA is reflected in detailed data, such as velocity and high-frequency characteristics, although both methods exhibit similar simulation capabilities for the main structural features of the flow field. The POD and PSD analyses of the flow field results demonstrated that the IDDES-SLA method was more capable of resolving higher modes of flow field. Full article

The Improved Delayed Detached-Eddy Simulation (IDDES) is a modification of the original Detached-Eddy Simulation (DES) design to incorporate Wall Modeled Large Eddy Simulation (WMLES) capabilities and to extend the class of flows suitable for this methodology. For thin attached boundary layers, typically seen in external aerodynamic flows, the DES branch of the model is active, whereas with thick boundary layers, typically seen in internal flows and also wake flows, the WMLES branch is active, thus providing a numeric method suited to handling most flow cases automatically. The flow over a triangular bluff body is used to validate the suitability of the IDDES model and compare the results with experimental, DDES, and LES data. The IDDES model is found to be relatively accurate when compared with the experimental results, with recirculation length, streamwise velocity, and Reynolds stresses all showing good agreement with the experimental data. However, when compared with the DDES model, there is a ~4% overprediction of the recirculation length using the same mesh and numerical scheme. The code, with its extra complexity, is also ~3% slower to solve. The IDDES model has also been tested against different meshes, and the results show that even for a coarse mesh, there is still good agreement with the experimental data. Full article

In this work, we present the first three-dimensional (3D) computational investigation of wind turbine airfoils over ${360}^{°}$ angles of attack to predict unsteady aerodynamic loads and vortex-shedding characteristics. To this end, static–airfoil simulations are performed for the FFA-W3 airfoil family at a Reynolds number of ${10}^7$ with the Improved Delayed Detached Eddy Simulation turbulence model. Aerodynamic forces reveal that the onset of boundary-layer instabilities and flow separation does not necessarily coincide with the onset of stall. In addition, a comparison with two-dimensional simulation data and flat plate theory extension of airfoil polars, suggest that, in the deep stall regime, 3D effects remain critical for predicting both the unsteady loads and the vortex-shedding dynamics. For all airfoils, the vortex-shedding frequencies are found to be inversely proportional to the wake width. In the case of slender airfoils, the frequencies are nearly independent of the airfoil thickness, and their corresponding Strouhal number $St$ is approximately $0.15$. Based on the calculated $St$, the potential for shedding frequencies to coincide with the natural frequencies of the International Energy Agency 15 MW reference wind turbine blades is investigated. The analysis shows that vortex-induced vibrations occur primarily at angles of attack of around $\pm {90}^{°}$ for all airfoils. Full article

The aerodynamic loads of a sedan experience significant fluctuations when passing by a sand dune at the roadside under crosswinds, which can easily cause yawing and overturning. Computational fluid dynamics (CFD) methods, based on different turbulence modeling approaches, yield different aerodynamic results for sedans. This study aims to investigate the effects of three prevailing turbulence modeling approaches (renormalization group (RNG) k-ε, large eddy simulation (LES), and improved delayed detached eddy simulation (IDDES)) on the aerodynamic characteristics of a sedan passing by a sand dune under crosswinds. The CFD dynamic mesh models are constructed using the “mosaic” mesh technique to account for the dune–air–sedan interaction. The reliability of the CFD prediction method is verified by comparing it with field test results. The predictive capabilities of the three turbulence modeling approaches are compared in terms of aerodynamic loads and flow field characteristics. The simulation of sand particle movement is conducted through the discrete phase model, aiming to assess the impact of wind–sand flow on the aerodynamic properties of sedans. Corresponding results show that the aerodynamic loads predicted by the LES model closely match (within 4.4–7.5%) the corresponding data obtained from field tests. While the IDDES and LES models demonstrate similar abilities in characterizing the wind field details, and their results exhibit maximum differences of 8.3–15.7%. Meanwhile, the maximum difference between the results obtained by the RNG k-ε and LES models ranges from 14.8% to 18.4%, attributed to its inability to capture subtle changes in the vortex structure within the flow field. This work will provide a numerical modeling reference for studies on the wind–sand flow and the aerodynamic characteristics of sedans running through the desert, and it has implications for the safe driving of sedans under extreme conditions. Full article

Floating offshore wind turbines (FOWTs) exhibit complex motion with multiple degrees of freedom due to the interaction of wind and waves. The aerodynamic performance and wake characteristics of these turbines are highly intricate and challenging to accurately capture. In this study, dynamic fluid body interaction (DFBI) and overset grid technology are employed to investigate the dynamic motion of a 5 MW FOWT. We use the volume of fluid (VOF) method and improved delayed detached eddy simulation (IDDES) model to investigate the aerodynamic performance and wake evolution mechanism for various wave periods and heights. According to the findings, the magnitude of the pitch motion increases with the period and height of the waves, leading to a decrease in both the power output and thrust; the maximum power was reduced by nearly 6.8% compared to a wind turbine without motion. The value of power and thrust reduction varies for different wave periods and heights, and is influenced by the relative speed and pitch angle, which play a crucial role. Wind–wave coupling has a significant impact on the evolution of both wake and vortex structures for FOWT. The wake shape downstream is also dynamically influenced by the waves. In the presence of wind and wave coupling, the interaction between the wind turbine and the wake is heightened, leading to the merger of two unstable vortex rings into a single, larger vortex ring. The research unveils a comprehensive picture of the offshore wind energy dynamics and wake field, which holds immense significance for the design of floating wind turbines and the optimization of wind farm layout. Full article

The effects of leading edge radius on the static and dynamic stall characteristics of rotor airfoils are investigated. Initially, a parametric airfoil (PARFOIL) method is employed to generate four morphed airfoils with different leading edge radii based on a NACA 0012 airfoil. Subsequently, the Reynolds-averaged Navier–Stokes (RANS) method is employed to simulate the aerodynamic characteristics of static airfoils, while the improved delayed detached-eddy simulation (IDDES) method is employed for pitching airfoils. The effectiveness and accuracy of the computational fluid dynamics (CFD) methods are demonstrated through favorable agreement between the numerical and experimental results. Finally, both the static and dynamic aerodynamic characteristics are simulated and analyzed for the airfoils with varying leading edge radii. Comparative analyses indicate that at low Mach numbers, the high adverse pressure gradient near the leading edge is the primary cause of leading edge separation and stall. A larger leading edge radius helps to reduce the suction pressure peak and adverse pressure gradients, thus delaying the leading edge separation and stall of airfoil. At high Mach numbers, the leading edge separation and stall are mainly induced by the shock wave. Variations in leading edge radius have minimal impacts on the high adverse pressure gradient induced by the shock wave, thus making the stall characteristics of airfoils almost unaffected at high Mach numbers. Full article

The improved delayed detached Eddy simulation (IDDES) approach used in the part I of this investigation to study the self-ignition and combustion of hydrogen jets in a high-speed transverse flow of hot vitiated air in a duct is extended in the following directions: (i) the wall boundary conditions are modified to take into account the optical windows employed in the experiments; (ii) the detailed chemical kinetic model with 19 reactions is used; (iii) a nonlinear turbulence model is implemented in the code to capture the secondary flows in the duct corners; (iv) the wall roughness model is adapted; (v) the synthetic turbulence generator is imposed upstream of the fuel injection. As a result of improving the mathematical and physical problem statements, a good agreement between the simulation and the experimental database obtained at the LAERTE workbench (ONERA) is achieved. Full article

Shock wave/turbulent boundary layer interaction (SBLI) is one of the most common physical phenomena in transonic wing and supersonic aircraft. In this study, the compression ramp SBLI (CR-SBLI) was simulated at a 24° corner at Mach 2.84 using the open-source OpenFOAM improved delayed detached eddy simulation (IDDES) turbulence model and the “Rescaling and Recycling” method at high Reynolds number $1.57\times {10}^6$. The results of the control effect of the jet vortex generator on CR-SBLI showed that the jet array can effectively reduce the length of the separation zone. The simulation results of different jet parameters are obtained. With the increasing jet angle, the reduction in the length of the separation zone first increased and then decreased. In this work, when the jet angle was 60°, the location of the separation point was $x/\delta =-1.48$, which was smaller than other jet angles. The different distances of the jet array also had a great influence. When the distance between the jet and the corner $d_{jet}=70 \mathrm{m}\mathrm{m}$, the location of the separation point $x/\delta =-1.48$ was smaller than that when $d_{jet}=65/60 \mathrm{m}\mathrm{m}$. A closer distance between the jet hole and the corner caused the vortex structures to squeeze each other, preventing the formation of a complete vortex structure. On the other hand, when the jet was farther away, the vortex structures could separate effectively before reaching the shock wave, resulting in a better inhibition of SBLI. The simulation primarily focused on exploring the effects of the jet angle and distance, and we obtained the jet parameters that provided the best control effect, effectively reducing the length of the CR-SBLI separation zone. Full article

In the current paper, high-fidelity improved delayed detached-eddy simulations (IDDES) using the OpenFOAM library are employed to characterize the performances and the wake dynamics of RIM-driven thrusters. Despite their relatively old conception, this type of propulsors has gained attention only recently thanks to innovative manufacturing technologies and materials, which finally have made possible their practical implementation. Fostered by stringent regulations on radiated noise and emissions enforced in protected areas, they are also replacing conventional thrusters and main propulsors. By mitigating the tip vortex cavitation and, more in general, by reducing the strength of tip vortices, indeed, these propulsors may grant a reduction in the induced pressure pulses and of the radiated noise without excessively sacrificing the efficiency of the equivalent ducted propellers they replace. To provide proof of this, three different RIM-driven thrusters (one four-bladed, two six-bladed) are analyzed and compared to a reference decelerating ducted propeller delivering the same thrust at identical functioning conditions. The evolution of the trailing wakes of the propulsors, the role of the leakage vortex, and the nozzle wake destabilizing effects, are highlighted, and the superior performances of RIM-driven thrusters, in terms of less intense tip vortices, are discussed. Near-field pressure pulses, as a measure of the radiated noise, are compared, showing a reduction in the sound pressure levels of the selected RIM propulsors up to 15 dB (non-cavitating case) with respect to the reference ducted propeller. Full article

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-Re_θ 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. Full article