Three-dimensional Large Eddy Simulation Research Articles

Effect of single vanes on turbulent flow

In response to growing environmental and ecological awareness, eco-friendly in-stream structures such as vanes have been implemented in different parts of the world to enhance stream conditions. FLUENT (ANSYS) was used to perform three-dimensional large eddy simulation to investigate the effect of the vanes permeability rate on flow characteristics. To evaluate the numerical model accuracy, numerical and experimental free surface profiles compared. It is observed that simulated free surface profiles agree reasonably well with measured values. The effect of different permeability rates is obvious in the flow characteristics such as depth-averaged velocity distribution, tip velocity variations, formation of the secondary flows in the flow field, turbulent kinetic energy, and mean kinetic energy contours. On average, maximum velocity values in the flow field is 1,54 times the approach velocity. Tip velocity decreases up to 30,6 % for the 70,0 % permeable vane. Maximum turbulent kinetic energy and mean kinetic energy for the 70,0 % permeable vane decrease up to 58,0 % and 43,3 %, respectively. Generally significant velocity and flow pattern variations around the impermeable vane can be attributed to the local effect of the vane structure and channel cross sectional constriction in comparison to the permeable vanes.

Aerodynamic analysis of building with zigzag-patterned façade: Insights from large eddy simulation

High-rise buildings have diverse shapes and configurations to harmonize with the architectural, engineering, environmental, economic, and functional considerations, and aspirations. However, the architectural shape and configuration plays a vital role in determining the aerodynamic characteristics of the surrounding flow field and wind forces acting on the building. This paper numerically investigates the aerodynamic characteristics of the zigzag-patterned building by adopting the three-dimensional large eddy simulation (LES). The flow around a square cylinder in high Reynolds number is used to valid the numerical method. The results show that the zigzag pattern has a little effect on the mean streamwise velocity but significantly influences the fluctuation of streamwise and vertical velocities in the wake region. It is observed that the zigzag pattern on the leeward wall results in a narrower wake region, while on the windward wall, leads to a thinner and longer shear layers and shorter recirculation region. As for the pedestrian level wind, the flow velocity within the separation area is noticeably reduced when incorporating a zigzag pattern on the windward wall. Moreover, the fluctuating pressure coefficient values at the sidewalls of a building with a zigzag windward wall is significantly smaller than that of a smooth configuration building due to the façade pattern's impact in diminishing the curvature of the nearby separation at the leading edge.

Fluid–structure interaction on vibrating square prisms considering interference effects

Existing research on interference effects predominantly focuses on rigid structures. However, studies based on rigid models tend to overlook the feedback of structural motions on the flow field, thus failing to capture the intrinsic dynamics of the interference effect induced by wind-induced structural vibrations. This paper provides a comprehensive analysis of the fluid–structure interaction mechanisms considering interference effects involving two parallel square prisms, employing large-eddy simulation (LES). Various factors, including wind speed, arrangement, and vibration amplitude, are meticulously considered in the analysis. The study utilized three-dimensional LES simulations, incorporating the narrowband synthesis random flow generator method for inlet turbulence generation and adjusted through the “feedback” approach to ensure accuracy and efficiency. The research highlighted different structural arrangements exhibited distinct interference effects, and the end effect of the structure could substantially modify the flow pattern at various heights. In the tandem arrangements, the study observed several flow phenomena, including early reattachment, attenuation of the end effect, premature formation of roll structures, increased turbulence in the flow field due to vibration, resulting in wider second leading-edge separation, and a fragmented wake flow on the downstream structure. For side-by-side arrangements, the “acceleration effect” was identified and found to be further intensified by structural vibrations. The vibration of the interfering structure was noted to cause changes in vortex shedding frequencies and alterations in the wake flow pattern. In addition, vibration would enhance the interference effect but increasing amplitude and wind speed might diminish the interference effect. Overall, this study offers valuable insight into the intricate interplay of factors influencing the aerodynamics of parallel structures across diverse arrangements and under varying conditions.

Quantitative evaluation on thermal seeing induced 2m ring solar telescope.

Thermal seeing is one of the factors that affect solar telescope observations. A comprehensive analysis method is developed to quantify the thermal seeing effects. A three-dimensional Large Eddy Simulation (LES) turbulence model is used to obtain the transient flow fields around the primary mirror, the secondary mirror and the heat-stop. The thermal seeing is calculated based on the stochastic dynamic influence of turbulence on the light rays. The key parameters of the simulation were calibrated by experiments, and the simulation results were validated by empirical formulas. This method has been applied to evaluate the thermal seeing of the 2m Ring Solar Telescope (2m-RST). Error allocation is performed based on the research results to ensure the Observation effect of 2m-RST.

Numerical simulations on flow control of the long hanger around a bridge tower based on active suction and blowing method

Long hangers around the bridge tower are subjected to severe vibrations caused by the tower wakes. This study adopted an active suction/blowing control measure at the tower corners to control hanger vibrations and included four combination measures: upstream suction (US), upstream blowing (UB), downstream suction (DS), and downstream blowing (DB). The effects of control cases on the near-wake flow structures of the tower were first studied in two-dimensional RANS simulation, and the vibration behaviors of the hanger and control mechanism were further analyzed. The cases associated with UB and DS cannot effectively suppress hanger vibrations, and both lead to unfavorable upward trends in the aerodynamic coefficients of the tower. Especially for the cases associated with DS, at a specific control speed, the second-order frequency of the drag coefficient of the hanger is very close to its natural frequency, resulting in significant longitudinal vibration. The cases associated with US and DB are effective control schemes with longitudinal peak amplitudes of the hanger reduced by 94.1% and 94.5%, and lateral peak amplitudes reduced by 95.8% and 97.0%, respectively, compared with the case baseline. This is because the dominant frequency of the lift of the hanger is away from its natural frequency, and the fluctuating wind loads on the hanger are effectively suppressed. Finally, the control effect and vibration suppression mechanism for typical cases were further reproduced in three-dimensional large eddy simulations.

Numerical Study of the Flow and Blockage Ratio of Cylindrical Pier Local Scour

A three-dimensional large eddy simulation model is used to simulate the turbulent flow dynamics around a circular pier in live-bed and clear-water scour conditions. The Navier–Stokes equations are transformed into a σ-coordinate system and solved using a second-order unstructured triangular finite-volume method. We simulate the bed evolution by solving the Exner-Polya equation assisted by a sand-slide model as a correction method. The bedload transport rate is based on the model of Engelund and Fredsœ. The model was validated for live-bed conditions in a wide channel and clear-water conditions in a narrow channel against the experimental data found in the literature. The in-house model NSMP3D can successfully produce both the live-bed and clear-water scouring throughout a stable long-term simulation. The flow model was used to study the effects of the blockage ratio in the flow near the pier in clear-water conditions, particularly the contraction effect at the zone where the scour hole starts to form. The scour depth in the clear water simulations is generally deeper than the live-bed simulations. In clear-water, the results show that the present model is able to qualitatively and quantitatively capture the hydrodynamic and morphodynamic processes near the bed. In comparison to the wide channel situation, the simulations indicate that the scour rate is faster in the narrow channel case.

Effects of taper ratio on the aerodynamic forces and flow field of two tandem square cylinders

To explore the influence of taper ratio on aerodynamic characteristics of tandem square cylinders, three-dimensional (3D) large eddy simulations of flow around two tandem square cylinders at the Reynolds number (Re) = 2 × 103 are carried out with a spacing ratio G/D = 4, where G is the cylinder center-to-center distance and D is the cylinder width. Different taper ratios of ξ = 0%, 5%, 10%, and 15% are considered. The influence of taper ratio on aerodynamic coefficient, wind pressure coefficient, mean, and instantaneous flow fields are comprehensively studied. The mechanism of variation in flow fields is revealed, and the mathematical relationship between the taper ratio and aerodynamic characteristics is established, which can provide theoretical reference for design and construction of tandem structures. The results indicate that the taper ratio has significant influence on the mean force coefficient, fluctuating force coefficient, surface mean and fluctuating pressure coefficients, and vortex shedding frequency (fvs). As increase in the taper ratio, the mean force in the along-wind direction, fluctuating force in the across-wind direction, and surface pressure of the two cylinders will be decreased, but the vortex shedding frequency will be increased. The taper ratio has negligible influence on the flow separation location of the upstream cylinder. However, reattachment location of the shear layer moves backward along the leeward surface of the downstream cylinder, and width of the shear layer gradually becomes narrower and closer to surface of the two cylinders. The vortex shedding strength and vortex energy distribution of the two cylinders will be reduced as a result of the narrower shear layer. Meanwhile, coherence and periodicity of the vortex shedding will also be weakened, which results in reduction in the aerodynamic forces and increase in the vortex shedding frequency.

The Influence of Boundary Conditions on Three-Dimensional Large Eddy Simulations of Calorically Perfect Gas Detonations

The Influence of Boundary Conditions on Three-Dimensional Large Eddy Simulations of Calorically Perfect Gas Detonations

Large Eddy Simulation of Cavitation Jets from an Organ-Pipe Nozzle: The Influence of Cavitation on the Vortex Coherent Structure

High-speed water jets are widely used in deep mining and the in-depth study of jet characteristics helps to improve drilling efficiency. Three-dimensional Large Eddy Simulation is used to simulate turbulent flows generated by an organ-pipe nozzle. The simulation is validated with existing experimental data and is focused on the evolution and interaction of cavitation bubbles and vortices. Dynamic mode decomposition is performed to extract structural information about the different motion modes and their stability. Results show that the dominant fluid frequency is positively correlated with inlet pressure while unrelated to the divergence angle. Meanwhile, jets’ oscillation is amplified by a large divergence angle, which facilitates the occurrence of cavitation. Results about the flow field outside of an organ-pipe nozzle advance the understanding of the basic mechanism of cavitation jets.

Streamwise sinusoidal flow over two identical tandem circular cylinders

This study conducts three-dimensional large eddy simulations at a subcritical Reynolds number of 103 to investigate the effects of sinusoidal streamwise flow on the flow structure, Strouhal number, and aerodynamic forces of two tandem circular cylinders. The center-to-center spacing L between the two cylinders is fixed at 3.5D, where D is the cylinder diameter. The velocity U of sinusoidal streamwise flow is set as U* = U/U0 = 1 + A*sin(2π(f*U0/D)t), where A* denotes the velocity amplitude ratio, and f* ((fuD/U0)/St0) denotes the non-dimensional frequency of sinusoidal flow, with fu being the frequency of sinusoidal flow and St0 being the Strouhal number of two tandem cylinders with L/D = 3.5 under the uniform approaching flow. The results show that the variations of f* (0–2.67) and A* (0–0.25) lead to the generation of not only the reattachment and coshedding flows independently but also their joint flow involving intermittent occurrences of the reattachment and coshedding flows. A map of the dependence of the flow structure on f* and A* is presented, marking the borders of various flow patterns in the f* − A* plane. Flow separation, Strouhal number, and aerodynamic forces on the two cylinders are found to be sensitive to f* and A*, exhibiting distinct variations following the flow structures. A time-frequency analysis of the bistable pattern is presented, identifying the two distinct vortex-shedding frequencies and confirming the time dependence and bi-stability of the bistable flow.

Large eddy simulation of combustion instability in a subcritical hydrogen peroxide/kerosene liquid rocket engine: Intermittency route to period-2 thermoacoustic instability

This paper presents the first numerical evidence of an intermittency route to period-2 thermoacoustic instability in a subcritical single-element liquid rocket engine burning hydrogen peroxide/kerosene as we decrease the equivalence ratio (ϕ) from fuel-rich to fuel-lean. To achieve this, three-dimensional compressible large eddy simulation algorithms combined with the Euler–Lagrangian framework are used. A one-equation eddy sub-grid turbulence model with a partially stirred reactor sub-grid combustion model is employed to simulate the spray turbulent combustion process in a high-pressure liquid-fueled combustor based on open-source platform OpenFOAM. This paper focuses on examining the transition process of the dynamical states in the thermoacoustic system and the synchronization between multiple subsystems. The results indicate that, as the equivalence ratio reduces continuously (1.5 ≤ ϕ ≤ 0.5), the system dynamics shift from period-1 oscillations (ϕ = 1.5) to period-2 oscillations (ϕ = 0.5) via intermittency (1.3 ≤ ϕ ≤ 0.9). Under the equivalence ratio of 0.7 (ϕ = 0.7), a transient mode switching between period-1 and period-2 was also observed. The synchronization processes between the pressure and combustion subsystems in terms of phase-locking and frequency-locking are responsible for the emergence of complex dynamical states. The cycle snapshots analysis also provides more details on the synchronization processes between the pressure and the multiple subsystems, such as vortex dynamics, mixture fraction, and combustion heat release. In summary, this paper sheds light on the complex non-linear thermoacoustic oscillations and the underlying physical mechanisms related to the two-phase flow of spray combustion in liquid rocket engines using three-dimensional large eddy simulations, paving the way for developing passive or active control methods.

Component decomposition and spatial distribution of unsteady forces on a circular cylinder oscillated under various amplitudes at Re = 2.0×104

Vortex-induced vibrations (VIVs) are of practical significance in the design of engineering structures, and the dynamic loads responsible for VIVs are thus crucial. In this study, the force characteristics of a circular cylinder undergoing forced oscillations at vortex lock-in states are investigated by employing three-dimensional large eddy simulations (LES) at a moderate subcritical Reynolds number of 2.0 × 104. Simulations of the oscillating cylinder are performed under a series of normalized amplitudes ranging from 0.05 to 0.60 at a fixed reduced velocity of 6.0. Moreover, a force spectrum-based approach is proposed to decompose the overall force into motion-induced and vortex-shedding components. Alternative insights into the spatial distribution of the force are also obtained by analysing the spanwise correlation, pressure field, and flow feature. The numerical results show that the motion-induced lifts experience an abrupt jump in both magnitude and phase as the oscillation amplitude increases. Furthermore, the motion-induced forces are almost fully correlated except in the phase-jump regime, although the correlation of the overall forces is relatively weak. Finally, the identified force parameters are successfully applied to predict the VIV amplitudes of the cylinder under various mass-damping conditions.

Large eddy simulation of flows past an array of square cylinders

ABSTRACT The flows past an array of square cylinders occur in wide range of applications. To study these flows, we conducted three-dimensional large-eddy simulations for flows past an array of the circular shape formed by multiple smaller square cylinders, where the Reynolds number defined by the array diameter was 1994. We conducted simulations with different solid volume fractions (SVFs) of the square cylinder array (). The effects of these configurations on flow characteristics were analyzed. Periodic drag fluctuation was observed when the SVF was small since the distance between the square cylinder was large, and the vortex shedding occurred with impact from other cylinders. No significant fluctuation in the drag was observed over time when the SVF was large. The minimum distance between the square cylinders was used to measure the degree of vortex shedding and reflect the relationship between the square cylinder position and the drag fluctuation.

Controls on Surface Warming by Winter Arctic Moist Intrusions in Idealized Large-Eddy Simulations

Abstract The main energy input to the polar regions in winter is the advection of warm, moist air from lower latitudes. This makes the polar climate sensitive to the temperature and moisture of extrapolar air. Here, we study this sensitivity from an air-mass transformation perspective. We perform simulations of an idealized maritime air mass brought into contact with sea ice employing a three-dimensional large-eddy simulation model coupled to a one-dimensional multilayer sea ice model. We study the response of cloud dynamics and surface warming during the air-mass transformation process to varying initial temperature and humidity conditions of the air mass. We find in all cases that a mixed-phase cloud is formed, initially near the surface but rising continuously with time. Surface warming of the sea ice is driven by downward longwave surface fluxes, which are largely controlled by the temperature and optical depth of the cloud. Cloud temperature, in turn, is robustly constrained by the initial dewpoint temperature of the air mass. Since dewpoint only depends on moisture, the overall result is that surface warming depends almost exclusively on initial humidity and is largely independent of initial temperature. We discuss possible climate implications of this result—in particular, for polar amplification of surface warming and the role played by atmospheric energy transports.

Investigation of the derivation and consistency of the quasi-two-dimensional flamelet models for non-premixed flames

Three non-premixed quasi-two-dimensional flamelet (Q2DF) models can be derived via integrating one-dimensional flamelet libraries, which are generated by premixing the third stream with the fuel stream or (and) the oxidizer stream as the premix fraction (η) varied from zero to unity. These models are attractive and considered effective substitutes for two-dimensional flamelet (2DF) formulations; however, the connections between these Q2DF models and 2DF equations have not been clearly illustrated formerly. In this work, the 2DF formulations are converted from the space defined by two mixture fractions (Z1×Z2) to that used for each Q2DF model (ξ×η) in which ξ designates a newly defined mixture fraction. For moderate Reynolds number conditions, it is considered that the Q2DF models and the 2DF formulations are equivalents as the one-dimensional (1D) flamelet elemental libraries are integrated upon sufficient grids of η. To confirm this, the two-dimensional (2D) direct numerical simulations (DNS) and three-dimensional (3D) large-eddy simulation (LES) are performed to provide reference data since it is readily to associate the 2DF formulations with variable transports in physical space. The 2D DNS cases present various three-feed systems characterized by different third streams and ensure a well-mixed mixture in moderate η, while the 3D LES case is performed with a large flux of pure CO2 as the third stream. Three Q2DF models are compared by using the same tracking parameters obtained from, respectively, the DNSs and LES. The comparisons based on the DNS cases demonstrate that the Q2DF models can reproduce the DNS data with quite satisfying accuracy, and it is confirmed that the accuracy has nothing to do with the type of the third stream. Through the LES results and the related comparisons, it can be concluded that the Q2DF models are reliable to predict the experimental measurements, and on the condition of a sufficient η resolution, the three Q2DF models can give the same prediction in the whole range of η.

LES flamelet modeling of hydrogen combustion considering preferential diffusion effect

A flamelet-generated manifold (FGM) method that explicitly considers the preferential diffusion effect, referred to as FGM-PD method, is employed for large-eddy simulations (LESs) of a lean-premixed H2/air low-swirl lifted flame, and the validity is examined by comparing with the experiment. First, the applicability of the FGM-PD method is investigated by one-dimensional numerical simulations of planar laminar premixed H2/air flames. Next, LESs of a lean-premixed H2/air low-swirl lifted flame are performed employing the FGM-PD and conventional FGM methods. Results of the one-dimensional numerical simulations show the importance of considering preferential diffusion to accurately predict species concentrations near the flame front. The FGM-PD method accurately predicts this, and therefore, reproduces the laminar burning velocity and spatial distributions of temperature and mixture fraction. Three-dimensional LES results confirm that the prediction accuracy of the velocities near the flame front is improved by employing this FGM-PD method. Additionally, the OH mass fraction distribution predicted by the FGM-PD method exhibits the inhomogeneous finger-like structure, which has been observed in previous experiments. This inhomogeneity of OH mass fraction distribution, which corresponds to that of the reaction rate, predicted by the FGM-PD method, strongly affects the flame front structure.

Development of Three-Dimensional LES Based Meshless Model of Continuous Casting of Steel

A large-eddy simulation (LES) based meshless model is developed for the three-dimensional (3D) problem of continuous casting (CC) of steel billet. The local collocation meshless method based on radial basis functions (RBF) is applied in 3D. The method applies scaled multiquadric (MQ) RBF with a shape parameter on seven nodded local sub-domains. The incompressible turbulent fluid flow is described using mass, energy, and momentum conservation equations and the LES turbulence model. The solidification system is solved with the mixture continuum model. The Boussinesq approximation for buoyancy and the Darcy approximation for porous media are used. Chorin’s fractional step method is used to couple velocity and pressure. The microscopic model is closed with the lever rule model. The LES model is compared to the two-equation Low Re k−ε turbulence Reynolds Averaged Navier–Stokes (RANS) model in terms of temperature, velocity and computational times. The LES model resolves transient character of vortices which RANS-type turbulence models are unable to tackle. The computational cost of LES models is considerably higher than in RANS. On the other hand, it results in a much lower computational cost than the direct numerical simulation (DNS). The paper demonstrates the ability of the method to solve realistic industrial 3D examples. Trivial adjustment of nodal densities, high accuracy, and low numerical diffusivity are the main advantages of this meshless method.

Three-Dimensional Large Eddy Simulations and Proper Orthogonal Decomposition Analysis of Flow Around a Flexibly Supported Circular Cylinder

Abstract Three-dimensional (3D) numerical simulations of the flow around a cylinder undergoing vortex-induced vibration (VIV) at Re = 3900 (defined as Re = U∞D/ν, where U∞ is the inlet flow velocity, D is the diameter of the cylinder, and ν is the fluid kinematic viscosity) are performed using large eddy simulations (LES). Detailed mesh and time-step convergence studies are carried out for the flow past a stationary cylinder to obtain the optimal mesh and time-step. The validation studies are performed by comparing the present results with the previous published experimental data and numerical results to confirm feasibility of the present numerical model for predicting the wake characteristics. The numerical model is then applied to investigate the flow around a self-excited cylinder vibrating in the cross-flow direction at three different reduced velocities. The effects of the reduced velocity are analyzed based on the cross-flow vibration amplitudes, the power spectral densities of lift and drag coefficients, and the wake flow coherent structures. Different vortex shedding modes are identified and categorized using the iso-surfaces of the pressure coefficient and the Q-criterion. The dominant 3D wake flow features are extracted and discussed by carrying out the proper orthogonal decomposition (POD) for flow velocities on multiple two-dimensional (2D) sampling planes in the wake region. The modes on the XY planes display a traveling-wave behavior and the modes on the XZ planes show cells of streamwise vortices, which display the three-dimensionality of the wake flow.

Numerical study on the flow characteristics of pressurized hydrogen leaking into the confined space through different shaped orifices

The safety concerns associated with accidental hydrogen leakage remain a challenge. Although numerous studies have been carried out on the flow behavior of compressed hydrogen leaking in unconfined spaces, the knowledge of the flow characteristics when it leaks into confined spaces through irregular holes is lacking. Therefore, this study uses the three-dimensional Large-Eddy Simulation (LES) model to simulate high-pressure hydrogen leakage in a confined space. Effects of the leakage hole shape on the flow features are investigated, including the circle, square, and triangle with the same equivalent area. Results indicate that the Mach disk shows circular, hexagonal, and octagonal shapes depending on the leakage port geometry. When the leak hole shape is square and triangular, the distribution of pressure waves deflects 45° and 60° near the hole, respectively. It means that non-circular holes affect the pressure distribution after hydrogen leakage. In addition, changing the shape of the leak hole results in a difference in hydrogen jet morphology. Compared to circular jets, in square and triangular jets, the jet boundary narrows along the diagonal plane and the gravity plane, respectively. It indicates that leakage ports with sharp corners can complicate the flow. Therefore, it is essential to consider the influence of the leakage port shape when performing a risk assessment of high-pressure hydrogen leaks in confined spaces.

Effects of streamwise gust amplitude on the flow around and forces on two tandem circular cylinders

Three-dimensional large eddy simulations are carried out to investigate the effect of streamwise gust amplitude ratio AR on the flow structure around and dynamic forces on two tandem circular cylinders at a subcritical Reynolds number of Re = 1 × 103. The center-to-center space L between the two cylinders is fixed at 3.5D, where D is the cylinder diameter. The velocity U of sinusoidal streamwise gust is set as U = U0 (1+ARsin2πfut), where the velocity amplitude ratio AR is varied from 0 to 0.25, U0 is the mean velocity, fu is the frequency of the sinusoidal streamwise gust, and t is the time. In order to enhance the understanding of the streamwise gust effect on the physical properties of the flow around the two tandem cylinders, the shear-layer reattachment, flow separation, wake recirculation, Strouhal number (St), unsteady forces, and phase lag (φ) between the fluctuating lift forces of the two cylinders are explicated. The results show that the flow structure around the cylinders is sensitive to AR. A change in AR thus leads to modification of reattaching flow (AR ≤ 0.15) to co-shedding flow (AR ≥ 0.20). The St, fluctuating forces and φ thus jumps at the boundary between the reattaching and co-shedding regimes. A stronger gust effect appears on the fluctuating drag of the upstream cylinder than that of the downstream cylinder. Besides the dominant component of the fluctuating lift induced by the vortex shedding, two other components are observed with the frequencies equaling to the superposition and subtraction of the gust and vortex-shedding frequencies, respectively. In addition, two more cases with L/D = 1.5 and 6.0 are conducted for deeper understanding of AR effects.