Large Eddy Simulation Method Research Articles

A study on characteristics of flow field around vehicle body

A numerical simulation calculation is performed using large eddy simulation method to investigate the characteristics of flow field around vehicle body, analyze the change rules of the velocity and pressure of the three-dimensional flow field around the vehicle body, as well as the frequency domain characteristics of the aerodynamic coefficients in the vehicle body. The results reveal that with the gradually expanding distance from the rear of the vehicle body, the velocity in the flow field around the vehicle body gradually recovers and reaches the incoming flow velocity at 0.9m from the rear of the vehicle body. Within the range of 0∼50HZ, the resistance coefficient and lift coefficient undergo a peak attenuation to about 20%, with a fluctuation of aerodynamic coefficient mainly within the range of 0∼500HZ. The research results are of certain significance and value for analyzing and applying the characteristics of flow field around vehicle body.

Flow noise and its suppression in exposed hydrophone

Underwater acoustic communications (UWAC) is an important technology in deep-sea research. In UWAC system, when the hydrophone as an acoustic receiver is exposed under sea and moves along with its attached objects, its performance will be affected by various ambient noises, among which its generated flow noise is the major source. This would influence the performance and shorten the communication distance of UWAC system. In this paper, we try to unveil how the flow field is correlated with the hydrophone flow noise. The large eddy simulation (LES) method and acoustic analogy are applied to simulate the flow field and the sound field around hydrophone, respectively. The hydrophone flow noise at different moving velocities with different attack angles is obtained. The flow noise result of experiments launched in an anechoic tank agree well with the flow noise simulation results, and both show that as the hydrophone moves faster, its sound pressure level climbs up higher. To suppress the flow noise, two kinds of cage to be installed on hydrophone are proposed, i.e. a cylinder cage and a serrated trailing edge cage. The simulation results show that the serrated trailing edge cage outperforms the cylinder cage. This study provides theoretical basis for improving the signal-to-noise ratio of low-frequency UWAC system.

Large-eddy simulation of droplet-laden decaying isotropic turbulence using artificial neural networks

We propose a model for large-eddy simulation (LES) of decaying isotropic turbulence laden with droplets with diameter of Taylor length-scale. The main challenge in creating LES models for such flow is that the presence of the droplets introduces additional subgrid-scale (SGS) closure terms to the filtered governing equations of motion of the flow. By processing available DNS data (Dodd & Ferrante, J. Fluid Mech. 806:356–412, 2016), we analyze these terms a priori to show that they are all significant enough to warrant modeling. Then, we propose a new modeling approach that we call mixed artificial neural network (MANN) LES because it is a mixed LES model that uses the standard Smagorinsky SGS stress model in the carrier fluid, and artificial neural networks to predict the SGS closure terms at the interface. Such an approach is justified because the SGS energy in the carrier flow away from the droplet interface is practically unaffected by the droplets, as we have previously shown using wavelet analysis of the DNS data (Freund & Ferrante, J. Fluid Mech. 875:914–928, 2019). Furthermore, we have performed the first a posteriori analysis of such flow for droplets of different Weber numbers, and show that our LES method closely reproduces the temporal decay of the filtered-velocity turbulence kinetic energy as well its p.d.f. of the filtered DNS, show that the modeling of the SGS terms at the interface is necessary for reproducing the results of the filtered DNS, and provide both physical- and spectral-space analysis of the LES results. Finally, the MANN LES approach could be applied to a variety of multiphase turbulent flows due to its ease of implementation, adaptability, and performance.

Numerical investigation of cavitation instability characteristics under the interference effects

The cavitation instability between two side-by-side cylinders at a constant Reynolds number of 64,000 and four typical cavitation numbers, σ = 1.5, 2.0, 2.5, and 3.0, are investigated in this study. Large eddy simulation (LES) method and Zwart-Gerber-Belamri cavitation model are used to capture the dynamic and unsteady cavitation behaviors. The results show that cavities around circular cylinders mainly originate in the free shear layers and can roughly be divided into band-like cavities and clump-like cavities. When cavitation occurs, the transition of shear layers is brought forward and the bi-stable regime is no longer existing. Based on the vorticity equation, the vortex dilatation is mainly concentrated in the shear layers of the two cylinders while the vortex stretching is concentrated in both shear layers and wakes. The Reynolds shear stress in spanwise for the down cylinder is significantly suppressed due to the interference effects while it is free from this suppression when cavitation occurs. It follows that the interference effects of the two side-by-side cylinders could be weakened when cavitation occurs. Besides, the mean drag coefficients will increase dramatically when cavitation occurs while the mean lift coefficients are little influenced.

An extensive numerical benchmark of the various magnetohydrodynamic flows

There is a continuous need for an updated series of numerical benchmarks dealing with various aspects of the magnetohydrodynamics (MHD) phenomena (i.e. interactions of the flow of an electrically conducting fluid and an externally imposed magnetic field). The focus of the present study is numerical magnetohydrodynamics (MHD) where we have performed an extensive series of simulations for generic configurations, including: (i) a laminar conjugate MHD flow in a duct with varied electrical conductivity of the walls, (ii) a back-step flow, (iii) a multiphase cavity flow, (iv) a rising bubble in liquid metal and (v) a turbulent conjugate MHD flow in a duct with varied electrical conductivity of surrounding walls. All considered benchmark situations are for the one-way coupled MHD approach, where the induced magnetic field is negligible. The governing equations describing the one-way coupled MHD phenomena are numerically implemented in the open-source code OpenFOAM. The novel elements of the numerical algorithm include fully-conservative forms of the discretized Lorentz force in the momentum equation and divergence-free current density, the conjugate MHD (coupling of the wall/fluid domains), the multi-phase MHD, and, finally, the MHD turbulence. The multi-phase phenomena are simulated with the Volume of Fluid (VOF) approach, whereas the MHD turbulence is simulated with the dynamic Large-Eddy Simulation (LES) method. For all considered benchmark cases, a very good agreement is obtained with available analytical solutions and other numerical results in the literature. The presented extensive numerical benchmarks are expected to be potentially useful for developers of the numerical codes used to simulate various types of the complex MHD phenomena.

LES INVESTIGATION ON THE INFLUENCE OF CAVITATION ON THE EVOLUTION AND CHARACTERISTICS OF TIP LEAKAGE VORTEX

In the present paper, the tip-leakage vortex (TLV) cavitation around a NACA0009 hydrofoil is numerically investigated with large eddy simulation method combined with a new Euler-Lagrangian cavitation model, which takes into account the effect of nuclei on tip-leakage cavitating flow. A satisfying agreement between the numerical and experimental results is obtained. Based on the numerical data, the evolution behavior of TLV cavitation with different gap sizes and its influence on the strength and radius of TLV, the nuclei distribution in the vortex core and the distribution of tangential velocity are discussed in detail. The results show that once cavitation occurs, the intensity of TLV is mainly affected by the evolution behavior of the sheet cavitation, while the tip-leakage cavitating flow has little effect on the strength of TLV. In addition, the results suggest that a smaller gap size will result in a more unstable sheet cavity, and the strength of TLV in turn presents corresponding quasi-periodic fluctuation as influenced by the evolution of the sheet cavitation. As the tip clearance progressively increases, the strength of the sheet cavitation gradually decreases with the reduction of instability, and the intensity of TLV gradually returns to the level without cavitation, together with its fluctuation level. Cavitation has a more significant influence on the nuclei distribution in the vortex core, and its degree of influence depends on the spatial stability of TLV and the intensity of TLV cavitation after the occurrence of cavitation. Moreover, the numerical results obtained by our simulations also indicate that cavitation has a significant influence on the radius of TLV. After cavitation occurs, the radius of TLV increases to a certain extent, and a ``rigid body rotation'' tangential velocity distribution is formed on the periphery of cavitation region, which is mainly caused by the expansion process induced by cavitation growth and the viscous effect of flow.

Numerical study on the flow and heat transfer of water-based Al2O3 forced pulsating nanofluids based on self-excited oscillation chamber structure

In this study, the flow and heat transfer characteristics of the forced pulsating Al2O3-water nanofluid were numerically studied. The pulsating excitation of the nanofluid is provided by the Helmhertz self-excited oscillating cavity. The large eddy simulation method is used to solve the equation, and the local Nusselt number and heat transfer performance index are used to analyze the heat transfer characteristics of the nanofluid in the self-excited oscillation heat exchange tube. In addition, the effect of different downstream tube diameters on heat transfer enhancement is discussed. The research results show that the existence of the countercurrent vortex can increase the disturbance of the near-wall fluid, thereby improving the mixing degree of the near-wall fluid and the central mainstream. As the countercurrent vortex migrates downstream, pulse enhanced heat transfer is realized. Furthermore, it was also found that when the downstream tube diameter d2 = 1.8d1, the periodic effect of the local Nusselt number of the wall is the best and the heat transfer performance index has the most stable pulsation effect within a pulsation cycle. But when d2 = 2.0d1, the change curve of heat transfer performance index in a pulsating period is the highest, the maximum value is 3.95.

Large eddy simulation on the mixing characteristics of liquid sodium at the core outlet of sodium cooled fast reactors (SFR)

It is a critical safety issue of thermal fatigue in the core outlet of sodium cooled fast reactors (SFRs) due to the mixing phenomenon of coolant with different temperatures flowing from different assemblies. In this paper, the large eddy simulation (LES) method was employed to study the coolant mixing phenomenon of a simplified assembly head model of SFR. The coolant velocity and temperature fields with vortices of different scales were obtained and analyzed. It shows that the lateral velocity is more sensitive to the inlet velocity than to the inlet temperature difference. In addition, the normalized parameters of temperature in axial and radial measuring lines and temperature fluctuations at specific points were obtained. Results show that there is no obvious dominant frequency of the temperature fluctuations with the energy of temperature fluctuations distributed in the frequency range of 0–100 Hz. The results would be beneficial for the design and structure optimization of SFRs.

Effect of airflow pressure on the droplet breakup in the shear breakup regime

In this paper, the coupled level set volume of fluid and the large eddy simulation methods are adopted to perform three-dimensional simulations of the shear breakup of a water droplet. We investigate the effect of airflow pressure (1–3 atm) on the temporary deformation and breakup characteristics, including the breakup initiations, the cross-stream, and streamwise deformations. In addition, special attention is paid to subsequent sub-droplet size distributions, which are generally ignored by many researchers. The results indicate that different morphologies on the surface of the droplets in the shear breakup regime are in relatively good agreement with the available experimental visualizations. Based on the present method, the physical mechanism for the variations in the wake recirculation with the development of Rayleigh–Taylor instability waves is discussed. Furthermore, higher airflow pressures can significantly increase cross-stream and streamwise deformations. However, the corresponding breakup initiations at high airflow pressures are much earlier than those of parent droplets at low airflow pressures. Specifically, a reduction of 12.17% in the mean sub-droplet sizes is obtained as the airflow pressure increases from 1 atm to 2 atm, while a reduction of less than 0.1% in the mean sub-droplet sizes is obtained at higher airflow pressures from 2 atm to 3 atm. Eventually, there are linear growths of the aggregate superficial area ratios (0.996–28.2) and the mass ratios (3.55%–64.29%) of the sub-droplets to the parent droplet.

Hydrodynamic performance of a Magnus anti-rolling device at zero and low ship speeds

Excellent seakeeping performance is the basis of safe ship operation. An anti-rolling device employing swinging rotating cylinders based on the Magnus effect can provide anti-rolling measures at any ship speed. In this study, the operating principle of a Magnus anti-rolling device was examined by studying its hydrodynamic performance using large eddy simulation method. A dimensionless parameter—the rotation to swing ratio (i.e., the ratio of the rotational speed to the swing speed of the cylinder)—was proposed for use in the hydrodynamic analysis. The changes in lift-drag characteristics and wake-flow field of the Magnus anti-rolling device were observed at zero and low ship speeds according to the swing angle, angular velocity, and rotation to swing ratio. The influence of the relative incoming flow velocity on the hydrodynamic characteristics of the Magnus anti-rolling device, when swinging upstream and downstream, was also studied at a low ship speed. The results show that an optimal rotation to swing ratio can be obtained at zero speed, and that the differential swing method can provide a large and consistent lift at low speeds. This analysis of the hydrodynamic characteristics of the Magnus anti-rolling device contributes to the study of ship seakeeping strategies.

Large Eddy Simulation and Flow Field Analysis of Car on the Bridge under Turbulent Crosswind

As more long-span bridges continue to be completed and opened to traffic, the safety of cars driving across the bridge has attracted more and more attention, especially when the car is suddenly affected by the crosswind, the car is likely to have direction deviation or even a rollover accident. In this paper, the large eddy simulation method is used to study the flow field characteristics and safety of the car on the bridge under the turbulent crosswind. The numerical simulation model is established by referring to the Donghai Bridge, and the correctness of the car model is validated by combining with the data of wind tunnel test. The influence of factors such as the porosity and height of the bridge guardrail and the Reynolds number of airflow on the flow field characteristics is analyzed. The study shows that, in order to ensure the safety of cars on the bridge, the bridge guardrail porosity should be small, 35.8% is more suitable, the guardrail height should be more suitable within the range of 1.5–1.625 meters, and the Reynolds number should not be 3.51e + 5. The research results of this paper will provide reference for the optimal design of bridge guardrail.

A Numerical Study of the Global Instability in Counter-Current Homogeneous Density Incompressible Round Jets

The paper focuses on a global instability phenomenon in counter-current round jets issuing from co-axial nozzles. Three different configurations that differ in a way of the counter flow generation are investigated. Besides typical configurations used in experimental and numerical research performed so far, in which suction applied in an annular nozzle is a driving force for the counterflow, a novel set-up is proposed where the annular nozzle is oriented in the opposite direction and placed above the main one. Such a configuration eliminates the suction of fluid from the main jet, which in previous research was found to have a destructive impact on the occurrence of the global flow instability. The research is performed using a large eddy simulation (LES) method and the computations are carried out applying a high-order numerical code, the accuracy of which has been proven in previous works and also in the present research through comparisons with available experimental data. The research is complemented by the linear stability analysis which supports the LES results and formulated conclusions. In agreement with a number of the previous works it has been shown that the global modes can be triggered only when the velocity ratio (I) between the main jet velocity and the velocity of the jet issuing from the annular nozzle is above a certain threshold level (I_{text {cr}}). It has been shown that in the classical configurations of the co-axial nozzles the range of Ige I_{text {cr}} for which the global instability phenomenon exists is very narrow and it disappears for larger velocity ratios. Reasons for that have been identified through detailed scrutiny of instantaneous flow pictures. In the new set-up of the nozzles the global instability persists for a significantly wider range of I. It has been shown that I_{text {cr}} depends on both the momentum thickness of the mixing layer formed between the counter-current streams and the applied configuration of the nozzles. The LES results univocally showed that the latter factor decides on the type of the instability mode (Mode I or Mode II) that emerges in the flow, as it directly influences on a length of the region where the counter-current streams are parallel allowing the growth of short or long wave disturbances characteristic for Mode I and Mode II, respectively.

Numerical simulation of stratified intrusive gravity current in three-dimensional state due to the presence of particles using large eddy simulation method

The motion of gravity current is attributed to the difference in density of current and the surrounding environment. If a fluid with a certain density enters the stratified environment so that its density is less than the lower layers and higher than the upper layers, the gravity current would be of intrusive type. Turbidity current is known as an important class of density currents, where the density difference is formed due to the presence of suspended solid particles in the fluid. In this paper, a three-dimensional numerical simulation of stratified intrusive gravity current in the presence of particles is investigated. For this purpose, the open-source OpenFOAM code is implemented for large eddy simulation to model the flow turbulence. According to the obtained results, the addition of particle mass to the intrusive gravity flow in dual symmetric state and flat bed would be followed by reduced velocities of intrusive gravity forward advance and ambient return flow at the bottom of the channel. Moreover, the flow behavior for the case of reverse slopes of 2, 4 and 6° in dual symmetric state in the absence of particles and at a 4° slope in the presence of particles with diameters of 20 and 30 microns has been investigated. Finally, the effect of bed slope on the instability of ambient gravity flow in the absence and presence of particles has explored with the help of local Richardson number. Faster instability in gravity current was achieved as a result of an increase in the bed slope in the absence of particles. Moreover, examining the instability in the 4° bed slope in the presence of particle with diameters of 20 and 30 microns revealed a reduction in local Richardson. Hence, the higher stability of the flow in the presence of particles compared to their absence can be inferred.

Transient Large Eddy Simulation of Slurry Erosion in Submerged Impinging Jets

Abstract Submerged impingement jets are widely used in erosion/corrosion experiments as it is easy to control jet standoff distance, jet angle, and flow velocities in experiments. In addition to experiments, typically computational fluid dynamics (CFD) technique has been used to simulate slurry flow in this geometry to investigate erosion process and develop erosion models or equations. The traditional CFD simulations of erosion in this geometry use the Reynolds-averaged Navier–Stokes (RANS) equations with turbulence models. By using this technique, time-averaged fluid flow is revealed, and thus, time-averaged erosion rate can be obtained by tracking particles in the fluid flow field. However, this seemingly simple flow displays unsteady flow structures in the stagnation zone of the flow field and its effects on the erosion process were previously unclear. In this study, large eddy simulation (LES) is used to simulate unsteady fluid flow in different impingement jets in an Eulerian scheme. Then, transient particle tracking is performed in a Lagrangian scheme. Particles are injected randomly at the inlet plane and tracked to simulate unsteady erosion that occurs on the target surface. Finally, an erosion equation is used to calculate solid particle erosion rates. The LES Eulerian–Lagrangian erosion modeling is further validated by available experimental data for fluid velocities and an erosion profile. The results show that the accuracy of erosion prediction of small particles is improved significantly by using the LES method. In addition, the unsteady particle motion and erosion process can be revealed by using this method.

Numerical convergence of volume of fluid based large eddy simulations of atomizing sprays

This paper presents three volume of fluid (VoF)-based methods for large eddy simulations of atomizing sprays with different treatments of the unresolved interface. The turbulent filtered VoF model uses conventional turbulent viscosity models to close the combined interfacial and turbulent sub-grid fluctuations. The hybrid turbulence filtering and artificial compression model includes an additional artificial compression term that is applied along regions where the liquid–air interface is continuous, while conventional turbulence filtering is activated in regions with discrete liquid objects. The new explicit volume diffusion model (EVD) is based on the concept of averaging the VoF equations over explicitly defined physical volumes that are independent of the numerical grid. Closure models of the sub-volume flux and stress terms introduce explicit volume diffusion and explicit volume viscosity that are physically defined and linked to the volume size. Numerical convergence is achieved by reducing the grid size while keeping the explicit volume size constant. The models are tested for two experimental atomizing spray cases with different Weber numbers. The superior numerical convergence of the EVD model is demonstrated by analysis of the mean and rms of the volume fraction and velocity fields. Two models for the surface tension force are investigated for the EVD simulations. Compared with the simple surface tension model which neglects sub-grid fluctuations, an improved volume-averaged model based on fractal properties of wrinkled sub-volume interfaces gives better predictions of the mean volume fraction relative to the experimental data but requires selection of a model constant.

Large eddy simulation of the turbulent mixing at an oblique interface induced by non-classical planar shock waves

The interaction between the shock wave and phase interface is one of the classic problems in aerospace and turbulent combustion engineering. In this paper, the instability of the gas–liquid interface driven by non-classical planar shock waves is studied. Based on the volume of fluid model and large eddy simulation method, the deformation process and the turbulent mixing phenomenon of the oblique interface induced by non-classical planar shock waves in a two-dimensional plane are numerically investigated by using a high-performance computer cluster. The effects of incident shock wave intensity, initial amplitude, initial wavelength, and inclined angle of the gas–liquid two-phase oblique interface are analyzed. The results show that the incident shock intensity has the most significant effect on the interface deformation and the development of turbulent mixing, and the initial amplitude and wavelength of the incident shock and the inclined interface angle also play a certain role in the deformation and development process. Overall, the width of the turbulent mixing zone increases with time under a given condition, and the convex structure will fall off and break up at the phase interface in the later stage of turbulent mixing.

Method of Unsteady Hydrodynamic Characteristics Determination in Turbulent Boundary Layer

This paper presents the method of the turbulent flow simulation. The method may be used to address the computational aeroacoustics (CAA) problems, where the vortex noise’s sources have to be determined. This method is an alternative to both large-eddy simulation (LES) methods and stochastics turbulence simulation techniques. The proposed method is more computationally efficient compared to LES and, unlike stochastic approaches, it does not require empirical constants. The simulation according to this method is achieved in two main stages. During the first step the averaged flow’s properties are obtained using the RANS simulation. These properties are used for the formulation of the discrete vortex model on the second step. Vortices’ intensities are oscillating with amplitudes and frequencies obtained from the RANS simulation with random phase shifts. Turbulent velocity field is then determined as the sum of averaged flow velocities, velocities induced by the pulsing vortices and velocities induced by the trailing vortices (Kelvin circulation theorem). The method is verified by considering the test problem. The developed turbulent boundary layer near the horizontal wall is simulated by means of both the presented method and the LES method. A good agreement between these two methods indicates on the viability of the approach presented in this paper. However, a thorough investigation of the method is still yet to be accomplished.

Unified wall-resolved and wall-modeled method for large-eddy simulations of compressible wall-bounded flows

Wall-resolved large eddy simulations and wall-modeled large eddy simulations are still separate strategies in the field of wall-turbulence applications, and no significant attempts have been documented to blend them in order to exploit the full potential of the two methods. This work enables a smooth transition between these two techniques, designing a robust algorithm that dynamically adapts the wall treatment. In particular, we propose a unified method that employs augmented turbulent viscosity and diffusivity at the wall location and allows for preservation of both the no-slip and isothermal/adiabatic conditions.

A discontinuous Galerkin method for wall-modeled large-eddy simulations

We develop an augmented discontinuous Galerkin method for wall-modeled large-eddy simulations. This method is motivated by the enrichment method that has been formulated for finite-element method, by statistically augmenting the variational solution of the near-wall element with a simple wall function to model the effects of the unresolved momentum-carrying near-wall eddies on the resolved scales. An explicit model is employed to model the subgrid-scale stresses. It is shown that this approach improves the solution accuracy on significantly underresolved grids and reduces spurious oscillations that occur in the presence of strong wall gradients on coarse grids with high-order DG methods. The proposed method is applied in low-Mach-number plane channel flows over a range of Reynolds numbers to examine effects of mesh resolution, polynomial order, solution-augmentation, and the SGS closure on the model performance. The results show that the proposed approach reproduces the log-law profile for different flow conditions, even at extreme Reynolds numbers, and yields improved predictions with increasing polynomial order. In addition, the capability of the present DG-based WMLES framework for computations of complex flows is demonstrated in LES of a shock-induced boundary-layer separation for the transonic bump experiment by Bachalo & Johnson (AIAA J., 24, 1986).

Multi-field coupling prediction for improving aeroacoustic performance of muffler based on LES and FW-H acoustic analogy methods

Based on the large eddy simulation (LES) and Ffowcs Williams and Hawkings (FW-H) equation, a multi-field coupling method is presented for aeroacoustic prediction of a muffler with high-speed and high-temperature exhaust gasflow. A three-dimensional finite-volume model of the muffler is established by using the LES and FW-H acoustic analogy (FW-H-AA) methods. Experimental validations of the simulated results suggest a good accuracy of the combined LES and FW-H-AA approach. Some factors influencing on noise attenuation, such as the gasflow velocity, temperature and the structural parameters of the muffler are analyzed. The results show that the aerodynamic noise and turbulent kinetic energy (TKE) are mainly attributed to the structural mutations in the muffler. The outlet sound pressure level (SPL) increases with the inlet gasflow velocity and decreases with temperature. According to the factor analysis results, the target muffler is modified by adding a fillet transition to the end of inserted tube and redesigning the structures where the TKE concentrated for improving the aerodynamic performance. In terms of the outlet SPL, the inner TKE and the backpressure of the muffler, the modified muffler is significantly improved by the maximum reductions of 3-5dB in SPL, 10–20% in TKE and 0.5–2.5 kPa in backpressure. The presented method might be extended to other kinds of muffler for aeroacoustic calculation and improvement design.