Delayed Detached Eddy Simulation Model Research Articles

Delayed detached eddy simulation-based aerothermoelastic analysis of deployable control fin in supersonic and hypersonic flows

In the present work, nonlinear aerothermoelastic characteristics of a deployable control fin subjected to high supersonic and hypersonic flow are investigated. The delayed detached eddy simulation (DDES)-based computational fluid dynamics solver is strongly coupled to the finite element method-based structural dynamics and thermoelastic solver to perform coupled fluid-thermal-structural interaction analysis. A shear stress transport (SST) k−ω based DDES model is used for turbulence modeling, whereas the advection upstream splitting method scheme is used for flux calculation, and for dynamic meshing, a diffusion-based smoothing method is used. To solve the governing nonlinear structural dynamics equations of motion in the time domain, the Hilber–Hughes–Taylor (HHT)-α method is used with the Newton–Raphson linearization technique. Profile preserving and conservative mapping-based interfacing modules are used to couple the different solvers. For the validation of the methodology, two experimental test cases are considered, and the computations are in very good agreement with the experimental results. Furthermore, the effects of Mach number, angle of attack, and joint freeplay on the fin's structural and aerodynamic characteristics are investigated and presented. The results show a complex flow behavior over the fin including several separation and attachment zones because of the deployable joint arrangement. It is also observed that the temperature due to the severe aerodynamic heating effect is very high at the leading edge and increasing thickness zones at the joint. With increasing joint freeplay, the amplitude of the deformation response increases, indicating increased dynamic instability.

A Modified Shielding and Rapid Transition DDES Model for Separated Flows

In this paper, the major problems associated with detached eddy simulation (DES) (namely, modeled stress depletion (MSD) and slowing of the RANS to LES transition (RLT)) are discussed and reviewed, and relevant improvements are developed. A modified version for the delayed DES (DDES) method with adaptive modified adequate shielding and rapid transition is proposed; this is called MSRT DDES. The modified shielding strategy can be adjusted adaptively according to the local flow conditions: keeping the RANS behavior in the whole boundary layer when there is no resolved turbulence, and weakening the shielding function when resolved turbulence exists in the mainstream over the boundary layer. This strategy can significantly ameliorate the MSD in the RANS boundary layer, regardless of the mesh refinement, and avoid excessive shielding in the fully developed resolved turbulence that may otherwise delay the development of the separated and reattached flow. Three cases are designed to test the modified DDES, namely, complete shielding in the RANS zone of a boundary layer (the zero-pressure gradient turbulent boundary layer with the refined mesh), modified adaptive improved shielding with a rapid transition (the flow over a hump), and the overall performance in a complex 3D separation (the corner separation in a compressor cascade). The results show that the modified shielding function is more physical than earlier proposals compared to shielding functions, and according to detailed comparisons of the wall skin friction coefficients, velocity profiles, total pressure-loss coefficients, entropy production analyses, and so on, the MSD and RLT problems are moderately alleviated by the MSRT DDES.

A Delayed Detached Eddy Simulation Model for the Simulation of Complex Turbulent Flow

A new turbulent model based on Delayed Detached Eddy Simulation (DDES) with non-linear eddy viscosity model (NLEVM) was developed to predict the complex turbulent flow. The numerical simulation of the triangular cylinder and the centrifugal pump was carried out to investigate the ability and applicability of the DDES model based on NLEVM (DDES_NL). Compared to the turbulent model based on the eddy viscosity model, the computational results of the triangular cylinder showed the advantage of the non-linear eddy viscosity modification in the DDES_NL model which can improve the accuracy of the prediction in the flow phenomenon with a relatively simple turbulence structure. Regrettably, some small-scale turbulent structures among those still cannot be captured accurately. The numerical simulation of the centrifugal pump predicted by the DDES_NL model shows more abundant flow structures and gets close to the realistic statistical characteristics. It also proves the good applicability of the DDES_NL model in complex flow. This study aims to contribute to the growing area of turbulence modeling by exploring it.

Numerical investigation of turbulent flow and mixed convection around a wavy cylinder

The performance of different scale-resolving simulation (SRS) models in resolving the turbulent flow structures and forced convection around a wavy cylinder (TFWC) at subcritical Re = 3000 is compared firstly. The SRS models include the SST scale adaptive simulation (SAS), the SST delayed detached-eddy simulation (DDES), and the production-limited DDES (PL-DDES) models. The simulation results on a coarse grid resolution testify that the PL-DDES model is superior to the SAS and DDES models. Another primary purpose is exploring the turbulent flow and mixed convection around a wavy cylinder using the PL-DDES model. For the contra flow (Richardson number = −0.3), the Strouhal number, the drag coefficient, and the Root-Mean-Square (RMS) lift coefficient decrease, but the length of the recirculation zone increases. In contrast, it is inverse results in the parallel flow (Richardson number = 0.3). Due to the buoyancy effect, the region of the 3 d flow structures in the contra flow and the parallel flow are respectively larger and smaller than those in the non-buoyancy flow. The laminar mixed convection is dominant around the cylinder in the contra flow. The turbulent mixed convection plays an essential role in the wake region for the parallel flow. In the windward region, the laminar mixed convection is dominant for both flows. The Nusselt number decreases and increases in the contra flow and the parallel flow, respectively.

New Strategies for Mitigating the Gray Area in Delayed-Detached Eddy Simulation Models

This paper presents a new approach for mitigating the unphysical delay in the Reynolds-averaged Navier–Stokes (RANS) to large-eddy simulation (LES) transition, often referred to as the gray area, which is a common issue for hybrid RANS–LES turbulence models such as delayed-detached eddy simulation. An existing methodology designed for improving the LES performance in complex flows is adapted and tested. This is based on reducing the numerical diffusion in critical areas for permitting a more accurate development of turbulence. The new formulation comprises both a two-dimensional sensitive velocity gradient model and an alternative definition of the subgrid length scale, which are tested both individually and in tandem, and compared with the other formulations commonly used for addressing the gray area. Four test cases are examined, a flat plate, two variants of the incompressible backward-facing step, and an open jet compressible case, all of which are selected to expose the adverse impact of numerical diffusion that this study seeks to address. Furthermore, the proposed changes are implemented in two different codes for the purpose of cross-validation. Encouraging results are observed, supporting the suitability of the new approach as a candidate for addressing the gray area issue in flows of this kind.

Modeling of Axial Compressor With Large Tip Clearances

Abstract In the drive for lower fuel consumption through increased bypass ratio and increased overall pressure ratio (OPR), engine designs for the next generation of single-aisle aircraft will require core sizes below 3 lb/s and OPRs above 50. Traditionally, these core sizes are the domain of centrifugal compressors, but materials limit pressure ratio in these machines to well below 50. An all-axial high-pressure compressor (HPC) at this core size, however, comes with limitations associated with the small blade spans at the back of the HPC, as clearances, fillets, and leading edges do not scale with the core size. The result is a substantial efficiency penalty, driven primarily by the tip leakage flow produced by the larger clearance-to-span ratio, which negates the cycle efficiency benefits of the high OPR. In order to enable small-core, high-OPR, all-axial compressors mitigating technologies need to be developed and implemented to reduce the large clearance-to-span efficiency penalty. However, for this technology development to be successful, it is imperative that predictive design tools accurately model the overall flow physics and trends of the technologies developed. In this paper, we describe an effort to determine whether different modeling standards are required for a large clearance-to-span ratio, and if so, identify criteria for an appropriate solver and/or mesh. Multiple models are run and results compared with data collected in the NASA Glenn Research Center’s low-speed axial compressor. These comparisons show that steady Reynolds-averaged Navier–Stokes (RANS) solvers can predict the pressure-rise characteristic to an acceptable level of accuracy, if careful attention is paid to mesh topology in the tip region. However, unsteady tools are necessary to accurately capture radial profiles of blockage and total pressure. The Delayed-Detached Eddy Simulation model was also used to run this geometry, but did not resolve any additional features not captured by the unsteady RANS simulation near stall.

Drag coefficient prediction of complex-shaped snow particles falling in air beyond the Stokes regime

This study considers complex ice particles falling in the atmosphere: predicting the drag of such particles is important for developing of climate models parameterizations. A Delayed-Detached Eddy Simulation model is developed to predict the drag coefficient of snowflakes falling at Reynolds number between 50 and 2200. We first consider the case where the orientation of the particle is known a posteriori, and evaluate our results against laboratory experiments using 3D-printed particles of the same shape, falling at the same Reynolds number. Close agreement is found in cases where the particles fall stably, while a more complex behavior is observed in cases where the flow is unsteady. The second objective of this study is to evaluate methods for estimating the drag coefficient when the orientation of the particles is not known a posteriori. We find that a suitable average of two orientations corresponding to the minimum and maximum eigenvalues of the inertia tensor provides a good estimate of the particle drag coefficient. Meanwhile, existing correlations for the drag on non-spherical particles produce large errors (≈ 50%). A new formula to estimate snow particles settling velocity is also proposed. Our approach provides a framework to investigate the aerodynamics of complex snowflakes and is relevant to other problems that involve the sedimentation of irregular particles in viscous fluids.

Verification and validation of Delayed Detached Eddy Simulation for cavitating turbulent flow around a hydrofoil and a marine propeller behind the hull

The Delayed Detached Eddy Simulation (DDES) was used to study unsteady cavitating turbulent flow around a hydrofoil and a highly skewed marine propeller behind a ship hull. The numerical results reproduced the transient cavitating flow around the hydrofoil and marine propeller, which agreed fairly well with the available experimental data. To further evaluate the accuracy of these DDES results quantitatively, the verification and validation methodologies and procedures were firstly applied in DDES model. The modified three-equation method was derived and illustrated to do verification and validation in DDES model. The hydrofoil cavitation simulation results indicated that the unsteady cavitating flow significantly influenced the distribution of the DDES errors. The periodic cavity shedding and large-scale vortex increased the DDES errors with both being the main source of the large errors. Similar verification and validation methodologies and procedures were then used to analyze propeller cavitation. These results showed that the errors in the predicted thrust coefficient were much smaller than the errors in the total cavity volume. Further analyses with different mesh resolutions showed that the finer flow structures were reproduced with better mesh resolution. Two small peaks between the crest and trough on the fluctuating pressure curves were induced in the predictions using Mesh 6 and 7 due to the area change of the shedding cavity, which indicated that the pressure fluctuations was closely correlated to the cavitation.

Numerical Investigation of Fixed and Non-fixed Separation with Shear Layer Adapted Delayed Detached Eddy Simulation

Delayed detached eddy simulation (DDES) has been proved to be suitable for the numerical simulation of massively separated flow. Whereas, there are still some drawbacks in the treatment of gray area, which is the transition zone between Reynolds-Averaged Navier–Stokes (RANS) and large eddy simulation (LES). In this paper, a modified DDES with shear layer adapted (SLA) subgrid length scale was employed, which takes advantage of the peculiarities of flow and grid topology in the initial shear layer, it can rapidly destabilize the separated shear layer and accelerate RANS to LES transition. To evaluate the performance of modified DDES versus conventional DDES, two typical separated flows are considered, they are the flow over backward-facing step with fixed geometry-induced separation and wall-mounted hump with non-fixed pressure-induced separation. The fifth-order Adaptive Dissipative Compact Scheme (ADCS) is also formulated to reduce numerical dissipation in grey area. The results show that the gray area can be slightly alleviated by ADCS, but it cannot be effectively mitigated with conventional DDES model. The visualizations of instantaneous flow reveal that the modified DDES is capable of unlocking the Kelvin–Helmholtz instability rapidly and accelerating the transition to resolved turbulence in the initial shear layer, which is strongly delayed by conventional DDES. The time-averaged pressure and skin friction coefficients show the mitigation of delayed transition as well. The distributions of mean velocity and Reynolds stress of modified DDES exhibit a rapid development in the initial shear layer; thus, more turbulent structures can be distinguished and the accuracy of results can be enhanced.

Comparison of different turbulence models in predicting cohesive fluid mud gravity current propagation

A numerical study of propagation of cohesive fluid mud gravity currents in the form of lock-exchange was done using the OpenFOAM open source toolbox. An Eulerian approach solution for three separate phases was developed by incorporating a rheological model to predict the front position of cohesive fluid mud gravity currents. The model also simulates features in the complete movement phases including slumping, self-similar, and viscous in which the dynamics of propagation are affected by the balance of viscous and buoyancy forces, and the inertia force is negligible. The influence of using different turbulence models containing sub-grid scale (SGS), modified SGS, detached eddy simulation (DES), delayed-detached eddy simulation (DDES), Launder-R eece-Rodi (LRR), and k-ɛ models on the accuracy of simulation results was evaluated by comparing with available experimental data. The results show that the selection of the proper turbulence model is one of the most important issues for this type of the numerical modeling. The more efficient turbulence model was suggested and tabulated for each stage of propagation and different selected concentrations of 1,045, 1,140, and 1,214 g/L. Although different turbulence models (except k-ɛ) lead to front propagation dynamic simulation results that are in good agreement with the experimental measurements in the early stage of propagation for low concentrations, only DES, SGS, and modified SGS are able to capture the Kelvin-Helmholtz instability vortex shapes at the dense fluid interface, which is the main characteristic of the gravity current through the slumping phase. The calculated accuracies of SGS and modified SGS in predicting gravity current propagation for the both self-similar and viscous phases also are slightly better than DES, DDES, and LRR model results. The results of this study confirmed the performance and efficiency of the modified SGS model in which the interaction coefficients between phases are calibrated for the numerical modeling of fluid mud gravity current propagation.

Numerical investigation of turbulent shear flows using production‐limited delayed detached‐eddy simulation

AbstractIn chemical engineering, turbulent shear flows are often encountered. The grid induced separation (GIS) and the slow RANS‐LES transition issues should be alleviated when the delayed detached‐eddy simulation (DDES) is used to simulate turbulent shear flows. This paper studies the performance of the production‐limited DDES (PL‐DDES) model in improving the GIS and the slow RANS‐LES transition issues. Since the simplified IDDES (S‐IDDES) model is proposed to improve the GIS issue, the S‐IDDES model is chosen as the model for comparison. The simulation results show that the PL‐DDES model with constant C d1 = 14 alleviates the GIS issue better than the S‐IDDES model and the PL‐DDES model with C d1 = 8. The results of the free shear layer show that the PL‐DDES model can switch RANS to LES more rapidly and unlock the Kelvin‐Helmholtz instability more effectively than the S‐DDES model. For the backward‐facing step flow, the S‐IDDES model performs poorly when unlocking the Kelvin‐Helmholtz instability in the separation zone. On the other hand, the PL‐DDES model has a rapid RANS‐LES transition after the step and produces a significant transport of momentum in the shear layer, leading to reasonable separation distance and flow structures.

Effect of RANS Model on the Aerodynamic Characteristics of a Train in Crosswinds Using DDES

Detached eddy simulation has been widely applied to simulate the flow around trains in recent years. The Reynolds-averaged Navier-Stokes (RANS) model for delayed detached eddy simulation (DDES) is an essential user input. The effect of the RANS model for DDES on the aerodynamic characteristics of a train in crosswinds is investigated in this study. Three different DDES models are used, based on the Spalart-Allmaras model (SA), the realizable k-ε model (RKE), and the shear stress transport k-ω model (SST). Results show that all DDES models can give relatively accurate predictions of pressure coefficient on almost all surfaces. There are only some specific differences in the small vortices, while similar flow patterns around trains could be predicted. The SST based DDES model (SSTDDES) gives the most accurate numerical results among the three models for the surface pressure. The variations in pressure on the leeward face play a key role in the variation of the side force.

Enhanced Delayed Detached Eddy Simulation for Cavities and Labyrinth Seals

Abstract A range of popular hybrid Reynolds-averaged Navier–Stokes -large eddy simulation (RANS-LES) methods are tested for a cavity and two labyrinth seal geometries using an in-house high-order computational fluid dynamics (CFD) code and a commercial CFD code. The models include the standard Spalart–Allmaras (SA) and Menter shear stress transport (SST) versions of delayed detached eddy simulation (DDES) and the Menter scale adaptive simulation (SAS) model. A recently formulated, enhanced, variant of SA-DDES presented in the literature and a new variant using the Menter SST model are also investigated. The latter modify the original definition of the subgrid length scale used in standard DDES based on local vorticity and strain. For all geometries, the meshes are considered to be hybrid RANS-LES adequate. Very low levels of resolved turbulence and quasi-two-dimensional flow fields are observed for the standard DDES and SAS models even for the test cases here that contain obstacles, sharp edges, and swirling flow. Similar findings are observed for both the commercial and in-house high-order CFD codes. For the cavity simulations, when using standard DDES and SAS, there is a significant under prediction of turbulent statistics compared with experimental measurements. The enhanced versions of DDES, on the other hand, show a significant improvement and resolve turbulent content over a wide range of scales. Improved agreement with experimental measurements is also observed for profiles of the vertical velocity component. For the first labyrinth seal geometry swirl velocities are more accurately captured by the enhanced DDES versions. For the second labyrinth seal geometry, the mass flow coefficient prediction using the enhanced models is significantly improved (up to 30%). Standard, industrially available hybrid RANS-LES models, when applied to the present canonical cases can produce little to no resolved turbulent content. The standard SA- and Menter-based DDES models can yield lower levels of eddy viscosity when compared to equivilent steady RANS simulations which means that they are not operating as RANS or LES. It is recommended that hybrid RANS-LES models should be extensively tested for specific flow configurations and that special care is exercised by CFD practitioners when using many of the popular hybrid RANS-LES models that are currently available in commercial CFD packages.

Computational studies on Darrieus VAWT noise mechanisms employing a high order DDES model

The aerodynamic and aeroacoustic behaviors of the Darrieus vertical axis wind turbine are very complicated. Hence, it is challenging to simulate the real flow behavior using the computational fluid dynamics approach. In the present study, the behavior of two different numerical techniques, Unsteady Reynolds Averaged Navier Stokes and Delayed Detached-Eddy Simulation (DDES), are compared to predict the aerodynamic and aeroacoustic performance. The DDES approach with the higher order scheme is used to investigate the noise mechanisms of Darrieus VAWT at several operating conditions. It is concluded that the dominant noise mechanism, when the turbine operates at low and high speed ratios, is a dipole noise, without any contribution from the monopole sound in the time averaged signal. Moreover, all the blade segments along the chord have a significant contribution to noise production. The leading edge segment is responsible for the noise reflects the load fluctuations by the variation of the effective angle of attack, flow blade relative velocity, and Blade vortex interaction. On the other hand, the rest of the blade segments reflect the load fluctuations by the massively separated flow at low speed ratio, while at high speed ratio it is as the same as the leading edge.

Numerical study on turbulent mixed convection in a vertical plane channel using hybrid RANS/LES and LES models

Abstract Two Delayed-Detached Eddy Simulation (DDES) models, and a Large-Eddy Simulation (LES) model are used to investigate the turbulent flows and mixed convection between a hot plate and a cold plate via the software FLUENT. The two DDES models include Production-limited DDES (PL-DDES) and Improved DDES (IDDES) models. The Wall-Adapting Local Eddy-Viscosity (WALE) model is the used LES model. The numerical computations are performed at Reynolds number Reb = 4494 and different Richardson numbers Ri = 0.025, 0.048, 0.1. The comparing data is from the Direct Numerical Simulation (DNS) at Reb = 4494 and Ri = 0.048. The comparison reveals that the two DDES models have better performance in predicting time-averaged parameters than the WALE model in the aiding flow. The best predicted time-averaged results are obtained by the PL-DDES model in the opposing flow. Furthermore, the results of different Ri obtained by the PL-DDES model agree well with the DNS data.

DDES analysis of the unsteady wake flow and its evolution of a centrifugal pump

In the present paper, to investigate the unsteady wake flow and its evolution at different moments when the blade sweeps the volute tongue, numerical simulation method is applied to discuss the flow field in detail based on the DDES (Delayed Detached Eddy Simulation) model when the model pump works at the nominal flow rate. Emphasis is attracted on the relative velocity distribution, besides the typical vorticity and TKE distributions at various moments are also analyzed to obtain the turbulent characteristics of the model pump. Results show that the current method has the ability to capture the main flow structure of the model pump, especially for the jet flow. From the blade pressure side to the suction side, the relative velocity distribution shows typical jet-wake flow pattern. With the impeller rotating, the reflection point of the jet-wake flow pattern changes at the mid span of the impeller. From flow distributions on different spans of the impeller, it is noted that the relative velocity decreases from the front chamber to the back chamber at major region of the blade channel. In the model pump, it is observed that several typical high vorticity regions are generated, where the high TKE values are also expected, especially at the blade trailing edge and in the volute zone caused by the wake flow. So it is inferred that for the unsteady flow in the pump and the corresponding induced pressure pulsation, controlling the wake flow is an effective approach to lower the unsteady flow pulsation in the centrifugal pump.

CFD Validation of a Model Wind Turbine by Means of Improved and Delayed Detached Eddy Simulation in OpenFOAM

With the ongoing increase in the size of wind turbines, experimental investigations have become more complicated and expensive. Therefore, computational models have proven to be a viable solution for design purposes. This article aims to validate CFD simulations of an experimental model wind turbine (MoWiTO 1.8) using Delayed Detached Eddy Simulation (DDES) and Improved DDES (IDDES) turbulence modelling approaches. For the purpose of validation, integral quantities (such as power, thrust, torque and blade-root bending moment in the flapwise direction) measured in the wind tunnel are compared with numerical results obtained with OpenFOAM. In general, the computational results show a very good agreement with the measurements for most of the monitored quantities. In particular, the blade-root bending moment presents the largest difference, taking into account that the simulation assumes the turbine blades are rigid. Nevertheless, the simulation does achieve in recreating the turbulent behavior as can be evidenced by the Power Spectral Density graphs, and the wake’s velocity measurements. In general, the IDDES turbulent model achieves a better agreement to the experimental results, while maintaining a very similar computational time as the DDES model.

Numerical investigations of hypersonic transition and massive separation past Orion capsule by DDES-Tr

The hypersonic flow at high angle of attack past the Orion capsule was predicted by a delayed detached eddy simulation (DDES), based on a three-equation k-ω-γ transition/turbulence integrated model (Tr). The computational results of the mean and instantaneous heat rates by DDES-Tr are compared with the available measurements and DDES-FT (full turbulence). As expected, the massive separation on the leeward side can be well predicted by both DDES models, but DDES-FT cannot model the transition and over-predicts the heat flux on the windward side. Fortunately, the DDES-Tr model can not only resolve the massive separation on the leeward side, but also model the transition on the windward side. Furthermore, the contributions of the first, second, and crossflow modes on the transition are quantitatively explored. Although the freestream is hypersonic, the second Mack mode is very weak owing to the low supersonic speed after the strong bow shock wave. The transition is mainly dominated by the first and rarely by the crossflow mode.

Delayed detached eddy simulation of pedestrian-level wind around a building array – The potential to save computing resources

The appropriate selection of a turbulence model directly affects the prediction of pedestrian-level wind (PLW) around buildings. Delayed detached eddy simulation (DDES) model, has been reported to be able to predict airflow around a building as good as large eddy simulation (LES) does, but with a lower mesh requirement and much less computing time. This study aims to see if DDES model can perform similarly when simulating the wind flow around a building array. This hypothesis is tested by comparing wind velocities of DDES, LES, and a benchmark wind tunnel experiment. Sensitivity assessments of DDES model are conducted, including the mesh resolution and the choice of an unsteady Reynolds Averaged Navier-Stokes (URANS) model used in conjunction. The normalized minimal grid sizes (0.005 for the building array's windward side and 0.0025 for the lateral and leeward sides) and the unsteady k−ω shear stress transport (SST) model are the most economical and effective. Simulated results are further quantified using four validation indices. Specifically, the correlation coefficient R between the predicted mean velocities using DDES and LES is 0.90, which basically proves our hypothesis in the mean flow field; but DDES only takes 80.3% of the computing time using LES. The time histories and spectrums of instantaneous velocities are also analyzed, indicating that DDES performs the similar predictions as LES of the unsteady flow fluctuations, while it has the potential to save computing time and mesh numbers.

Three-dimensional delayed detached-eddy simulation of wind flow and particle dispersion in the urban environment

Recently, Haze-fog as an atmospheric phenomenon associated with high level of air pollutant has negative impacts on human health and environment, which arises the society attention. This paper is aimed at investigating the haze-fog dispersion in the urban environment from the physical view. The wind flow and particle dispersion in the urban environment are simulated by using the Delayed Detached-eddy Simulation (DDES) turbulence model and the Discrete Phase Model (DPM) under the atmospheric boundary layer flow. The results show that the particle dispersion is dominated by the wind flow including the velocity and vortex distribution and the gravity can slightly influence the particle deposition. The particle-empty areas around the buildings are identified during the dispersion and then they disappear gradually mainly through the recirculation function. When the wind direction changes, the blockage cross-section area derived from the building group becomes larger resulting in the larger low wind velocity area. Furthermore, the different reference inlet velocities directly affect the vorticity magnitude and the particle dispersion periods. It is concluded that the DDES model is a promising approach to simulate the haze-fog physical development in the urban environment.