Three-dimensional Large Eddy Simulation Research Articles

Numerical study on dilution of an oscillating jet in current environments

The mixing behavior of an oscillating jet under the influence of currents remains incomprehensive. This study uses a three-dimensional large eddy simulation model to investigate the phase-averaged and time-averaged concentration distribution of three-dimensional scalar structures in the oscillating jet under a current environment. The effects of dimensionless parameters on dilution characteristics are also analyzed. The results indicate that increasing the jet-current velocity ratio (Rjc) and the amplitude-jet velocity ratio (Raj), while decreasing the Strouhal number (St), can enhance the dilution capacity of the receiving water. To quantify the oscillatory effect of jets on the initial dilution of wastewater discharge, semiempirical equations for the cross-sectional minimum dilution (Sc) and the visual area (A25%) of the oscillating jet in a current environment are developed using the least squares method. The oscillatory nature of the jets is found to behave similarly to wave effects. Furthermore, the empirical equations for the initial dilution of oscillating jets in current environments are structurally consistent with those for non-oscillating jets in wave-current coexisting environments. This study highlights the positive impact of oscillating jets on mixing and dilution.

Effect of gap width on flow structure and reattaching characteristic of two tandem 5:1 rectangular cylinders: Experimental and numerical studies

The flow structure and reattaching characteristic of two tandem rectangular cylinders with aspect ratio being 5:1 have been investigated through wind tunnel experiments and three-dimensional large eddy simulation (LES) methods. The gap width G between the two cylinders varies from 2 times of D to 20 times of D, where D represents the depth of the two cylinders. The surface pressure distribution and aerodynamic forces of each cylinder are obtained via wind tunnel experiments. Two distinct flow patterns are identified with the increasing G through three-dimensional LES methods, and the aerodynamic results are presented in good agreement with the experiments as well. The experimental and numerical results indicate that the flow structure is highly sensitive to the variation in G, leading to alterations in the aerodynamics and vortex-shedding characteristic of two cylinders. Furthermore, the simulation results also capture the shift in the reattaching points as G increases. Additionally, following the simulation findings, a proposed criterion based on the wind tunnel experimental data is presented for predicting the boundary layer reattachment points on two tandem 5:1 rectangular cylinders.

Numerical study on the flow and noise control mechanisms of a forced rotating cylinder

This numerical study proposes an active control method aiming to suppress aerodynamic noise from bluff bodies by employing a forced rotating cylinder and investigates its noise reduction effects and mechanisms. A three-dimensional large eddy simulation combined with the Ffowcs William–Hawkings equation was adopted to study the influence of different rotation ratios on the aerodynamic and aeroacoustic characteristics of a cylinder at a Reynolds number of 4.7×104, and to elucidate the primary mechanisms by which cylinder rotation reduces aerodynamic noise. The numerical method is validated through a comparison with previous numerical and experimental results of both flow field and far-field noise. The present numerical results indicate that cylinder rotation can not only effectively reduce aerodynamic drag but also significantly suppress aerodynamic noise across the entire frequency range, including vortex-shedding tonal noise and broadband noise. Two primary mechanisms of flow and noise control by the rotating cylinder are revealed within different ranges of rotation ratio. One mechanism stabilizes the shear layer, thereby suppressing vortex shedding. The other mechanism attenuates the Kelvin–Helmholtz instability on the upper side of the cylinder, leading to a transition into laminar flow which inhibits the formation of large-scale coherent turbulent structures.

3D LES numerical investigation of vertical vortex-induced vibrations of a 4:1 rectangular cylinder

The aerodynamic characteristics of rectangular cylinders provide valuable references for many engineering structures. Aiming to investigate the characteristics of vortex-induced vibration (VIV) of this kind of bluff section cylinder, the three-dimensional (3D) large-eddy simulations (LES) were conducted on the flow around an elastically mounted rectangular cylinder with an aspect ratio of 4. A novel method for creating 3D meshes was introduced and validated by comparing the aerodynamic parameters and vibration amplitudes with those of experimental data. The mechanism of VIV was then investigated by examining the evolution properties (including the mean and fluctuating pressure, predominant frequency, and spanwise correlation) of distributed pressure at various amplitude-dependent VIV stages. Additionally, the contributing/inhibiting effect of wind load acting on specific regions around the cylinder’s surface on VIV was clarified by investigating the energy input or output. The results show that the vibration amplitude affects both static and fluctuating pressures, with a significant variation in these pressures observed during the amplitude-ascent stage. The distributed aerodynamic force is mainly due to self-excited forces resulting from the cylinder vibration during amplitude-ascent stage, whereas both self-excited forces and forcing forces are prominent during the amplitude-descent stage. Notably, the spanwise correlation coefficient displays its minimum value at the peak amplitude, even lower than that in the non-lock-in region, with the highest value occurring at the onset of the amplitude-ascent stage. The energy analysis revealed that the wind loads acting on the upstream of the cylinder's surface tend to enhance VIV, while those acting downstream tend to inhibit it.

Numerical simulation of the vortex-induced vibrations of a 4:1 rectangular cylinder subjected to yaw uniform currents

The practical engineering structures are not always perpendicular to the wind but often in skew positions. This yawed configuration may lead to more complex three-dimensional flow characteristics and different vortex-induced vibration (VIV) responses. However, the related study is very inadequate. Therefore, comprehensive numerical investigations of the VIV characteristics of a 4:1 rectangular cylinder have been conducted using the three-dimensional (3D) Large-Eddy Simulation (LES) at five different yaw angles (β = 0°, 5°, 10°, 20°, 30°). An innovative method for creating 3D meshes was introduced, demonstrating a good agreement with the experimental results while significantly reducing the mesh number, and consequently the computation time. The VIV response, pressure distribution, aerodynamic force, flow field, and energy input/output were analyzed for various yaw angles. The results demonstrate that the periodic boundary conditions on the spanwise domain side is more reliable than those of the wall boundary conditions. The velocity component parallel to the cylinder axis can be ignored in the analysis of elastically-mounted rectangular rigid cylinders for β up to 30°. If only considering the perpendicular axis velocity component, the amplitudes and lock-in regions for β = 5°, 10°, 20°, and 30° are highly consistent with those of β = 0°, which have effectively validated the accuracy of Independence Principle in evaluating the VIV of a 4:1 rectangular cylinder.

Three-dimensional large eddy simulation urban neighborhood model with updated building drag coefficient and universal multiscale Smagorinsky model

Three-dimensional large eddy simulation urban neighborhood model with updated building drag coefficient and universal multiscale Smagorinsky model

Reynolds number sensitivity of the vortex dynamics around a long-span rail-cum-road bridge girder with three separated boxes

The novel separated three-box girder is gradually adopted for very long-span cable-stayed bridges due to its excellent flutter stability. Due to its special design of cross section, it exhibits notable Reynolds number effects in the flow patterns and vortex dynamics when subject to incoming flow. To explore its Reynolds number sensitivity, this paper performs a three-dimensional large eddy simulation (LES) on the flow around a static long-span rail-cum-road bridge girder with three separated boxes at different Reynolds numbers. The overall centre height H of the bridge model is chosen as the characteristic length, and the investigated Reynolds number regime falls into the regime of 1e2≤Re≤2.8e4. Through the analysis of the flow characteristics of three distinct regions in the flow field(the windward leading edge region, the gap region and the wake region), a successive transition of the wake flow, gap flow, and leading edge shear flow is observed with increasing Reynolds number. The surrounding flow patterns are divided by different Reynolds number regimes as: laminar separation at the trailing edge (Re<5e2); simultaneous instability of the gap shear layer and the wake (5e2≤Re<1e3); restabilisation of the upstream gap shear layer (1e3≤Re<3e3); secondary instability of the upstream gap shear layer and transition of the wake (3e3≤Re<8e3); transition of the separated shear layer at the leading edge (8e3≤Re≤2.8e4). This paper reveals in detail the complex vortex dynamics in the surrounding flow field of the special cross section with three separated boxes.

Active Control of Longitudinal Combustion Instability in Bluff-Body Stabilized Premixed Flames with Multiple Neural Network Controller

ABSTRACT The suppression performance of multiple neural network controller on the longitudinal combustion instability is investigated in the Volvo bluff-body stabilized premixed flame. Two controlled plant models, e.g. one-dimensional (1D) reduced order acoustic network model and three-dimensional (3D) large eddy simulation model are adopted to verify this control strategy. In order to build the reduced order Volvo acoustic network model, large eddy simulation is adopted to extract the characteristics of system heat release under wide bandwidth and construct the flame transfer functions. For the 1D acoustic network model, this controller can completely eliminate the pressure oscillation using fuel valve as actuator. For the 3D large eddy simulation model that mimics the real-time oscillating characteristics, clustered prediction method and amplitude limiting method are adopted in multiple neural network control strategy to better account for the nonlinear characteristics of the 3D Volvo flame. This control strategy enables the modulation of the phase relationship between the heat release rate oscillation and pressure oscillation by introducing small perturbations to the inlet equivalence ratio, and attenuates the amplitude of the second harmonic of the unstable mode effectively.

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.