Search Results (1,045)

This study employs the large-eddy simulation (LES) approach, together with the hybrid volume of fluid—discrete phase model, to examine the deformation and breakup of a water droplet impacted by a traveling shock wave. The research investigates the influence of Weber number on transient deformation and breakup characteristics. Particular focus is given to the detailed analysis of sub-droplet-size distributions, which are frequently overlooked in existing studies, providing a novel insight into droplet fragmentation dynamics. The predicted deformation and breakup patterns of droplets in the shear breakup regime align well with experimental data, validating the computational approach. Notably, LES is able to reproduce the underlying physical mechanisms, highlighting the significant role of recirculation zones and the progression of Kelvin–Helmholtz instabilities in droplet breakup. Additionally, it is shown that higher Mach numbers significantly amplify both cross-stream and streamwise deformations, leading to earlier breakup at higher airflow pressures. Increasing the Weber number from 205 to 7000 results in 25% reduction in the average size of the sub-droplets, indicating the strong influence of aerodynamic forces on droplet fragmentation. This comprehensive analysis, while aligning with experimental observations, also provides new insights into the complex dynamics of droplet breakup under post-shock conditions, highlighting the robustness and applicability of the proposed hybrid Eulerian–Lagrangian formulation for such advanced applications in fluid engineering. Full article

When an ice-class propeller is operating in an ice-covered environment, as some ice blocks slide along the ship hull in front of the propeller blades, the inflow ahead of the propeller will become non-uniform. Consequently, the excitation force applied to the blades will increase and massive cavitation bubbles will be generated. In this paper, a hybrid Reynolds-Averaged Navier–Stokes/Large Eddy Simulation method and Schnerr–Sauer cavitation model are used to investigate the hydrodynamics, excitation force, cavitation evolution and flow field characteristics of the propeller in ice blockage conditions. The results show that the numerical method adopted has a relatively high accuracy and the hydrodynamic error is controlled within 3.0%. At low cavitation numbers, although the blockage distance decreases, the cavitation phenomenon is still severe and the hydrodynamic coefficients hardly increase accordingly. Ice blockage causes a sharp increase in cavitation. When the distance is 0.15 times the diameter, the cavitation area amounts to 20% of the propeller blades. As the advance coefficient grows, the total cavitation area diminishes, while the cavitation area of the blade behind ice does not decrease, resulting in an increment in excitation force. Ice blockage also causes backflow in the wake. At this time, the largest backflow appears at the tip of the blade behind the ice. The higher the advance coefficient, the more significant the high-pressure area of the pressure side and the greater the pressure difference, causing the excitation force to rise sharply. This work offers a positive theoretical basis for the anti-cavitation design and excitation force suppression of propellers operating in icy regions. Full article

Two-staged fuel injection configuration for scramjet combustors has been shown to be effective in distributing heat release in the combustor for preventing the unstart transition of the engine by suppressing peak pressure while increasing the pressure thrust. In this study, the effect of fuel species on combustion characteristics in a two-staged fueled scramjet combustor was investigated. Wall pressure measurements in a two-staged fueled scramjet combustor were conducted in a combustion wind tunnel facility with fuels having different reactivity, such as H₂ and CH₄. Reynolds-Averaged Navier–Stokes/Large Eddy Simulation (RANS/LES) hybrid simulations were performed to verify the interaction characteristics between the primary and secondary combustion zones for different fuels. The experimental results confirmed that pressure peaks at injections were clearly separated in the hydrogen case, while these interacted with each other in the methane case with a lower reactivity than H₂. The RANS/LES Hybrid analysis predicted this effect of fuel reactivity on the pressure distribution, namely, the heat release delay of the first stage fuel caused the interaction with the second stage fuel heat release. The results indicate that the need to design the staged fueled combustor, i.e., the injection stage interval accordingly to the reactivity of the fuel. Full article

Over the past decade, several studies based on coupled ocean–atmosphere simulations have shown that the oceanic surface current feedback to the atmosphere (CFB) leads to a slow-down of the mean oceanic circulation and, overall, to the so-called eddy killing effect, i.e., a sink of kinetic energy from oceanic eddies to the atmosphere that damps the oceanic mesoscale activity by about 30%, with upscaling effects on large-scale currents. Despite significant improvements in the representation of western boundary currents and mesoscale eddies in numerical models, some discrepancies remain when comparing numerical simulations with satellite observations. These discrepancies include a stronger wind and wind stress response to surface currents and a larger air–sea kinetic energy flux from the ocean to the atmosphere in numerical simulations. However, altimetric gridded products are known to largely underestimate mesoscale activity, and the satellite observations operate at different spatial and temporal resolutions and do not simultaneously measure surface currents and wind stress, leading to large uncertainties in air–sea mechanical energy flux estimates. ODYSEA is a new satellite mission project that aims to simultaneously monitor total surface currents and wind stress with a spatial sampling interval of 5 km and 90% daily global coverage. This study evaluates the potential of ODYSEA to measure surface winds, currents, energy fluxes, and ocean–atmosphere coupling coefficients. To this end, we generated synthetic ODYSEA data from a high-resolution coupled ocean–wave–atmosphere simulation of the Gulf Stream using ODYSIM, the Doppler scatterometer simulator for ODYSEA. Our results indicate that ODYSEA would significantly improve the monitoring of eddy kinetic energy, the kinetic energy cascade, and air–sea kinetic energy flux in the Gulf Stream region. Despite the improvement over the current measurements, the estimates of the coupling coefficients between surface currents and wind stress may still have large uncertainties due to the noise inherent in ODYSEA, and also due to measurement capabilities related to wind stress. This study evidences that halving the measurement noise in surface currents would lead to a more accurate estimation of the surface eddy kinetic energy and wind stress coupling coefficients. Since measurement noise in surface currents strongly depends on the square root of the transmit power of the Doppler scatterometer antenna, noise levels can be reduced by increasing the antenna length. However, exploring other alternatives, such as the use of neural networks, could also be a promising approach. Additionally, the combination of wind stress estimation from ODYSEA with other satellite products and numerical simulations could improve the representation of wind stress in gridded products. Future efforts should focus on the assessment of the potential of ODYSEA in quantifying the production of eddy kinetic energy through horizontal energy fluxes and air–sea energy fluxes related to divergent and rotational motions. Full article

The main objective of the current study is a detailed (both numerical and experimental) investigation of the highly unsteady and complex swirl flow in a straight conical diffuser (with a total divergence angle of 8.6°) generated by an axial fan impeller. Pressure, and axial and tangential velocity profiles along several cross-sections were measured by original classical probes in two different flow regimes at the inlet: the modified solid body type of moderate swirl and the solid body type of strong swirl and reverse flow; they were additionally confirmed/validated by laser Doppler anemometry measurements. Computational studies of spatial, unsteady, viscous, compressible flows were performed in ANSYS Fluent by large eddy simulation. The fan was neglected, and its effect was replaced by the pressure and velocity profiles assigned along the inlet and outlet boundaries. The two sets of data obtained were compared, and several conclusions were drawn. In general, the relative errors of the pressure profiles (2–5%) were lower than the observed discrepancies in the axial velocity profiles (5–40% for the first and 15–50% for the second flow regime, respectively). The employed reduced numerical model can be considered acceptable since it provides insights into the complexity of the investigated swirl flow. Full article

As wind turbines increase in size, blades become longer, thinner, and more flexible, making them more susceptible to large geometric nonlinear deformations, which pose challenges for aeroelastic simulations. This study presents a nonlinear aeroelastic model that accounts for large deformations of slender, flexible blades, coupled through the Actuator Line Method (ALM) and Geometrically Exact Beam Theory (GEBT). The accuracy of the model is validated by comparing it with established numerical methods, demonstrating its ability to capture the bending–torsional coupled nonlinear characteristics of highly flexible blades. A bidirectional fluid–structure coupling simulation of the IEA 15MW wind turbine under uniform flow conditions is conducted. The effect of blade nonlinear deformation on aeroelastic performance is compared with a linear model based on Euler–Bernoulli beam theory. The study finds that nonlinear deformations reduce predicted angle of attack, decrease aerodynamic load distribution, and lead to a noticeable decline in both wind turbine performance and blade deflection. The effects on thrust and edgewise deformation are particularly significant. Additionally, nonlinear deformations weaken the tip vortex strength, slow the momentum exchange in the wake region, reduce turbulence intensity, and delay wake recovery. This study highlights the importance of considering blade nonlinear deformations in large-scale wind turbines. Full article

In this work, we propose a deep learning-based model for mapping between the data of the flow field of the propeller generated by the Reynolds-averaged Navier–Stokes (RANS) and those generated by Large Eddy Simulation (LES). The goal of establishing the mapping model is to generate LES data, which needs higher computing power requirements, with the help of RANS data. The model utilizes a deep learning method for computer vision to handle three-dimensional data generated by RANS and those by LES. Firstly, the data samples of the RANS flow field and those of the LES flow field are processed to obtain their corresponding three-dimensional image data, respectively. Secondly, the two kinds of field flow images are used as the training data for constructing a mapping model between RANS flow field images and the corresponding LES flow field images. The obtained mapping model thus can be used to predict the LES flow field images. Thirdly, the regression module is employed to regress the three-dimensional LES image point-by-point to the velocities at the points of the LES flow field, thereby ultimately achieving the generation of LES flow field data from RANS data. The experimental results show that by applying this method to RANS data and LES data of propeller flow fields, the overall error rate of LES flow field prediction by this method is 17.68% compared to actual flow field data, which verifies the effectiveness and accuracy of the proposed model in LES flow field prediction. Full article

Tidal sites can present uneven seabed bathymetry features that induce favourable or adverse pressure gradients and are sources of turbulence, and so are likely to affect the operation, performance, and wake recovery dynamics of deployed tidal-stream turbines. Large-eddy simulations are conducted to analyse the unsteady loading of a tidal turbine subjected to the wake of an upstream turbine that interacts with a two-dimensional ridge located between the two turbines. Relative to an isolated turbine, blade fatigue loading is increased by up to 43% when subject to the wake of a turbine located 8 turbine diameters upstream interacting with a ridge located 2 turbine diameters upstream, whereas for the same spacing, the turbine wake led to a limited 6% reduction in loading and the ridge wake only caused a 79% increase. For larger spacings, the trends were similar, but the magnitude of difference reduced. Predictions of fatigue loads with a blade element momentum model (BEMT) provided a good agreement for flat bed conditions. However, the ridge-induced pressure gradient drives rapid spatial change of coherent flow structures, which limits the applicability of Taylor’s frozen turbulence hypothesis adopted in the BEMT. Reasonable prediction of rotor loading with BEMT was found to be obtained using the turbulent onset flow field at a plane one-diameter upstream of the turbine. This is more accurate than use of the planes at the rotor plane or two-diameters upstream, as coherent structures represent those modified by wake recovery and rotor induction in the approach flow to the turbine. Full article

Numerical simulations of sub-critical flow past a circular cylinder at Reynolds number $\mathrm{R}\mathrm{e}$ = 130,000 are performed using two numerical platforms: the commercial, Ansys Fluent, and the open-source, OpenFOAM (finite volume method and large-eddy simulation based on a differential equation for the sub-grid kinetic energy). An overview of the available experimental data and similar large-eddy simulation studies is presented. A detailed analysis of all accumulated data demonstrates satisfactory agreement between them with a dispersion of approximately $20\%$ for the main integral flow parameters. A detailed comparison of the results obtained using single- and double-precision numerical methods in Ansys Fluent did not reveal any noticeable discrepancies in the integral and local flow parameters as well as the spectral characteristics. It is shown that the behavior of the dynamic system of the fluid dynamic equations computed with single precision is stable by Lyapunov and does not lead to any loss of accuracy. The reconstructed attractors of the dynamic systems in phase space are limited by an ellipsoid. Full article

The pressure oscillation caused by vortex–acoustic coupling is one of the main gain factors that results in the combustion instability of motors. Focusing on a solid rocket motor with a backward step configuration that can generate a corner vortex, this study aims to investigate the pressure oscillation characteristics in a combustion chamber under two-phase flow interactions through numerical simulations. The two-phase flow discrete phase model (DPM) was chosen to study particle motion and two-phase interactions. The numerical methodology was hence established by coupling the DPM with the large eddy simulation (LES) method. Taking the Clx motor as a reference and introducing aluminum oxide particles, two important particle parameters (diameter and concentration) and the key geometric parameters of the backward step were numerically studied. The numerical results show that both increased particle diameter and concentration can decrease the frequency and amplitude of pressure oscillations; additionally, the effects of geometric parameters on the pressure oscillations of the backward step, such as the downstream aspect ratio, the expansion ratio, and the step position, are basically consistent under both pure gas and two-phase flows. The influences of those geometric parameters are mainly reflected in defining the space for the development of upstream flow instability and the motion of downstream vortices. Compared with the pure-gas flow, the presence of aluminum oxide particles in two-phase flow globally decreases the vortex shedding frequency, the primary frequency, and the amplitude of pressure oscillations. It can also weaken the effects of vortex–acoustic coupling due to increased turbulent viscosity, which hinders the orderly development of vortices. Full article

The aerodynamic performance of twin tall buildings immersed in the atmospheric boundary layer was numerically investigated by adopting the spatial-averaged large eddy simulation (LES) method. This study focused on the effects of corner cutting and chamfering. The buildings were both square and sectional with a width-to-height ratio of 1:6, and were arranged in a tandem configuration with a spacing ratio of 2.0. The corner-cutting and chamfering measures were only applied to the upstream cylinder, with a corner modification rate of 10%. To generate the turbulent inflow boundary condition (IBC) for LES, steady-state equilibrium IBC expressions were introduced into the vortex method, which were implemented in the commercial code Ansys Fluent. The present simulation method and solution parameters were first verified by comparing the simulated wind field and the wind pressure distribution on a single tall building with those of the wind tunnel test. The influences of the corner-cutting and chamfering measures on the wind load of the tandem buildings were then comparatively studied concerning the statistical values of their aerodynamic force coefficients and wind pressure coefficients. The influence mechanism was analyzed based on the simulated time-averaged flow field and the instantaneous vortex structure around the buildings. The results indicated that upstream corner-cutting and chamfering measures can induce a diffusion angle shift in the separated shear flow from the leading edge of the upstream building, thus affecting the separation and reattachment of the separated upstream flow on the downstream building. Among the measures studied, upstream corner cutting is more effective in reducing wind pressure and aerodynamic force coefficients. Full article

We analyzed the planetary boundary layer (PBL) characteristics in Warsaw, Poland for a day of summer, autumn, winter, and spring of 2021 by integrating and comparing measured and simulated data. Using remote sensing lidar sensor data, the PBLH was calculated using wavelet covariance transform (WCT) and the gradient method (GM). Also, simulations of turbulent fluxes were performed utilizing the large eddy simulation (LES) from the Parallel Large Eddy Simulation Model (PALM) to better understand how turbulence and convection behave across different seasons in Warsaw. The PBLH diurnal cycles showed pronounced changes in their vertical structure as a function of the season: the winter heights were shallow (~0.7 km), while summer heights were deeper (~1.7 km). The spring and autumn presented transient characteristics of PBLH around 1.0 km. This study is crucial for enhancing urban air quality and climate modeling. The PBLH simulations from PALM showed agreement with the measured data, with an underestimation of approximately 10% in both methods. Through PALM, it was possible to observe that summer exhibited increased convection, enhanced mixing efficiency, and a deeper boundary layer compared to other seasons throughout the daily cycle. Winter has a lower sensible heat flux and little convection throughout the day. Spring and autumn showed intermediate characteristics. In this way, the effectiveness of the applicability of the PALM model to obtain flows within the PBL and their heights is highlighted, because correlations ranged from strong to very strong (r ≥ 0.70). Full article

To explore the effect of different explosive charge height parameters on the bonding interface of titanium–aluminum multilayer composite plates during explosion welding, the smooth particle dynamics method (SPH method) was used to simulate the explosion welding of titanium–aluminum multilayer plates, reproducing the formation process of plasma jet and waveform bonding interface and obtaining the bonding surface conditions at various charge heights. Based on the simulation, experiments were conducted, and the bonding surface quality was verified through scanning electron microscopy (SEM). The elemental distribution of the binding interface was analyzed using an energy-dispersive spectrometer (EDS). The results show that the welding effect of the plate closer to the explosive is better during explosion welding. Within the weldable window, as the charge height increases, the waviness of the bonding interface transitions towards smaller and more continuous ripples, with continuous small ripples accompanied by vortex-like eddies indicating good welding conditions. When the charge height is too large, the plate may experience a brittle fracture, reducing the strength of the bonding interface. The welding effect is best when the charge height is 24 mm. Under a certain distance between the base and overlay plates, with the increase in charge height, the collision speed of the base plate also increases, increasing the pressure between the plates, causing changes in the shapes of the bonding interface ripples, and expanding the melting zone. Excessive collision speed and pressure also promote the generation of cracks, leading to a decrease in the strength of the composite material. Full article

In this paper, the macroscopic and microscopic characteristics of diesel spray with elliptical and circular nozzles were investigated under an ambient density of 65.6 kg/m³ by combining an optical test and numerical simulation method of VOF-Spray One-Way Coupling and Large Eddy Simulation. Two elliptical nozzles with varying aspect ratios (1.25 and 1.5) and a circular nozzle were employed for comparison, with the same cross-sectional area. The results demonstrated that the spray tip penetration (STP) of elliptical nozzles was significantly diminished in comparison to that of the circular nozzle and that STP for the elliptical nozzle with a larger aspect ratio was observed to be smaller, primarily due to the elevated aerodynamic drag and accelerated kinetic energy dissipation. The spray cone angle (SCA) of elliptical nozzles was greater than that of the circular nozzle. The average SCA of the elliptical nozzle with a larger aspect ratio was the greatest in both planes. The spray asymmetry with elliptical nozzles resulted in the instability of the spray boundary, leading to the earlier fragmentation and atomization of the spray and faster radial diffusion. For the same STP, the elliptical nozzle with a larger aspect ratio exhibited the greatest spray area in both planes. Elliptical nozzles are subject to a greater degree of inhomogeneous shear than circular nozzles, which results in an accelerated rate of droplet breakage and a concomitant decrease in Sauter Mean Diameter. Full article

Attempts to mitigate the computational cost of fully resolved large-eddy simulation (LES) in the near-wall region include both the hybrid Reynolds-averaged Navier–Stokes/LES (HRL) and wall-modeled LES (WMLES) approaches. This paper presents an LES wall treatment method that combines key attributes of the two, in which the boundary layer mesh is sized in the streamwise and spanwise directions comparable to WMLES, and the wall-normal mesh is comparable to a RANS simulation without wall functions. A mixing length model is used to prescribe an eddy viscosity in the near-wall region, with the mixing length scale limited based on local mesh size. The RANS and LES regions are smoothly blended using the dynamic hybrid RANS-LES (DHRL) framework. The results are presented for the turbulent channel flow at two Reynolds numbers, and comparison to the DNS results shows that the mean and fluctuating quantities are reasonably well predicted with no apparent log-layer mismatch. A detailed near-wall meshing strategy for the proposed method is presented, and estimates indicate that it can be implemented with approximately twice the number of grid points as traditional WMLES, while avoiding the difficulties associated with analytical or numerical wall functions and modified wall boundary conditions. Full article