Large-eddy simulation of vortex breakdown in compressible swirling jet flow

Vortex breakdown in a compressible swirling jet flow is investigated by large-eddy simulation (LES) using the approximate deconvolution model. Conditions are chosen similar to recent experimental investigations by Liang and Maxworthy [Liang H, Maxworthy T. An experimental investigation of swirling jets. J Fluid Mech 2005;525:115] for incompressible flow. LES results are presented for two simulations of a swirling jet at Mach number Ma = 0.6 with and without inflow forcing by imposed linear instability disturbances. Both the forced and the self-excited jet show three-dimensional helical waves developing in the jet breakdown zone. The features observed in the two simulations are compared to each other as well as to the experiments with respect to flow statistics and instability behaviour. Both simulations show favourable qualitative agreement with the experiment.     

Large-eddy simulation of streamwise-rotating turbulent channel flow

In many engineering and industrial applications the investigation of rotating turbulent flow is of great interest. Whereas some research has been done concerning channel flows with a spanwise rotation axis, only few investigations have been performed on channel flows with a rotation about the streamwise axis. In the present study an LES of a turbulent streamwise-rotating channel flow at Re_τ = 180 is performed using a moving grid method. The three-dimensional structures and the details of the secondary flow distribution are analyzed and compared with experimental data. The numerical-experimental comparison shows a convincing agreement as to the overall flow features. The results confirm the development of a secondary flow in the spanwise direction, which has been found to be correlated to the rotational speed. Furthermore, the findings show the distortion of the main flow velocity profile, the slight decrease of the streamwise Reynolds stresses in the vicinity of the walls, and the pronounced increase of the spanwise Reynolds stresses at higher rotation rates near the walls and particularly in the symmetry region. As to the numerical set-up it is shown that periodic boundary conditions in the spanwise direction suffice if the spanwise extent of the computational domain is larger than 10 times the channel half width.     

Large-eddy simulation of turbulent flow in a stirred tank driven by a Rushton turbine

Large-eddy simulation (LES) of mixing process in a baffled tank was presented. The impeller rotation was modeled using the sliding mesh technique. In this study the CFD code was used for simulation of a standard vessel agitated by a 6-blade Rushton turbine and results were evaluated in terms of the predicted flow field, power number, mean velocity components, mixing time, turbulent kinetic energy and turbulent dissipation rate using published experimental data. Subsequently, the effects of varying injection position of the passive scalar have been investigated. The results show that LES is a reliable tool to investigate the unsteady behavior of the turbulent flow in stirred tank.     

Large-eddy simulation of multi-component compressible turbulent flows using high resolution methods

The ability of a finite volume Godunov and a semi-Lagrangian large-eddy simulation (LES) method to predict shock induced turbulent mixing has been examined through simulations of the half-height experiment [Holder and Barton. In: Proceedings of the international workshop on the physics of compressible turbulent mixing, 2004]. Very good agreement is gained in qualitative comparisons with experimental results for combined Richtmyer-Meshkov and Kelvin-Helmholtz instabilities in compressible turbulent multi-component flows. It is shown that both numerical methods can capture the size, location and temporal growth of the main flow features. In comparing the methods, there is variability in the amount of resolved turbulent kinetic energy. The semi-Lagrangian method has constant dissipation at low Mach number, thus allowing the initially small perturbations to develop into Kelvin-Helmholtz instabilities. These are suppressed at the low Mach stage in the Godunov method. However, there is an excellent agreement in the final amount of fluid mixing when comparing both numerical methods at different grid resolutions.     

A computer simulation of turbulent jet flow

The evolution of a round turbulent jet is simulated numerically under the constraint of axial symmetry. The vortex sheet shed from the orifice is represented by vortex ring elements, with a velocity field cut-off to control close encounters. Large-scale vortex clusters form in the model jet, similar to those inferred from laboratory flow visualization experiments. However, comparisons of statistical properties reveal significant differences between the axisymmetric model flow and real turbulent jets.     

Large-eddy simulation in the near-field of a transient multi-component gas jet with density gradients

Large-eddy simulation (LES) is a research tool that is increasingly being used to study practical engineering flows because of continuous improvements in computational power. This paper outlines an LES model developed for the study of multi-component transient gas jets with density gradients. The compressible LES formulation together with the numerical model, boundary conditions, perturbation, and parallelization are discussed. A non-dissipative sixth-order finite difference scheme is used to discretize the governing equations, and a low-pass sixth-order spatial filtering scheme is employed to avoid the growth of high-frequency modes. The conditions at the boundaries are implemented using Navier–Stokes characteristic boundary conditions. In addition to code performance, results are presented from a study of an impulsively started jet at high pressure and temperature with an injected to ambient gas density ratio of approximately 3.5.     

Numerical simulation of interaction between turbulent flow and a vibrating airfoil

The subject of this paper is the numerical simulation of the interaction of two-dimensional incompressible viscous flow and a vibrating airfoil. A solid elastically supported airfoil with two degrees of freedom, which can rotate around the elastic axis and oscillate in the vertical direction, is considered. The numerical simulation consists of the stabilized finite element treatment of the Reynolds averaged Navier–Stokes (RANS) approach, the use of turbulence models and the solution of the system of ordinary differential equations describing the airfoil motion. The time dependent computational domain and a moving grid are taken into account with the aid of the Arbitrary Lagrangian–Eulerian (ALE) formulation of the Navier–Stokes equations. High Reynolds numbers up to 10⁶ require to use a suitable stabilization of the finite element discretization and the application of a turbulence model. We apply the algebraic turbulence model, which was designed by Baldwin and Lomax and modified by Rostand. The developed technique was tested by the simulation of flow past a flat rigid plate and the computation of pressure distribution around a rotating airfoil with prescribed motion. Finally, the method was applied to the simulation of flow induced airfoil vibrations. This research was supported under the Grant No. IAA200760613 of the Grant Agency of Academy of Sciences of the Czech Republic. The research of M. Feistauer was partly supported by the research project MSM 0021620839 financed by the Ministry of Education of the Czech Republic and the research of L. Dubcová was partly supported by the grant No. 48607 of the Grant Agency of the Charles University. The authors acknowledge the support of these institutions.     

Numerical simulation of turbulent combustion of subsonic gas jet flows

A physical and mathematical model of turbulent combustion of subsonic gas fuel jet flows flowing into an air space is proposed. The processes are described by averaged equations of the boundary layer with a turbulent viscosity model and a combustion diffusion model. As turbulent viscosity models, the well-known two-parameter k-? standard and k-?? models are taken. The results of the averaged and pulsating flow characteristics?? comparison of numerical calculations with the experimental data are presented.     

Study of jet precession, recirculation and vortex breakdown in turbulent swirling jets using LES

Large eddy simulations (LES) are used to investigate turbulent isothermal swirling flows with a strong emphasis on vortex breakdown, recirculation and instability behaviour. The Sydney swirl burner configuration is used for all simulated test cases from low to high swirl and Reynolds numbers. The governing equations for continuity and momentum are solved on a structured Cartesian grid, and a Smagorinsky eddy viscosity model with the localised dynamic procedure is used as the sub-grid scale turbulence model. The LES successfully predicts both the upstream first recirculation zone generated by the bluff body and the downstream vortex breakdown bubble. The frequency spectrum indicates the presence of low frequency oscillations and the existence of a central jet precession as observed in experiments. The LES calculations well captured the distinct precession frequencies. The results also highlight the precession mode of instability in the center jet and the oscillations of the central jet precession, which forms a precessing vortex core. The study further highlights the predictive capabilities of LES on unsteady oscillations of turbulent swirling flow fields and provides a good framework for complex instability investigations.     

Numerical simulation of the interaction of a circular synthetic jet with a boundary layer

In this work, a 3D numerical simulation of circular synthetic jets issued into a laminar boundary layer developing over a flat plate was undertaken in a complementary manner alongside with an experimental study with the aim of achieving an improved understanding of the fluid mechanics underlying the interaction process between the synthetic jets and the boundary layer. The simulation was carried out in FLUENT at two diaphragm operating conditions, which produced two distinctly different vortical structures and shear stress footprints on the wall. The simulation results were validated using experimental data and a good agreement was achieved. The temporal evolution of coherent structures formed as the result of this interaction was examined using the Q-criterion. The hierarchy of the coherent structures was established which provided a credible explanation of the wall shear stress pattern observed in both the experiment and the simulation. The high spatial resolution in the near-wall region and 3D nature of the simulation results provide the information about the flow field which is not only consistent with but also additional to that from the experiment, leading to an improved understanding of the interaction process between the synthetic jets and the boundary layer and its resultant structures.     

Simulations of confined turbulent vortex flow

Large-eddy simulations (LES) of the turbulent flow in a swirl tube with a tangential inlet have been performed. The geometry, and flow conditions were chosen according to an experimental study by [Escudier MP, Bornstein J, Zehnder N. Observations and LDA measurements of confined turbulent vortex flow. J Fluid Mech 1980;98:49-63]. Lattice-Boltzmann discretization was used to numerically solve the Navier-Stokes equations in the incompressible limit. Effects of spatial resolution and choices in subgrid-scale modeling were explicitly investigated with the experimental data set as the testing ground. Experimentally observed flow features, such as vortex breakdown and laminarization of the vortex core were well represented by the LES. The simulations confirmed the experimental observations that the average velocity profiles in the entire vortex tube are extremely sensitivity to the exit pipe diameter. For the narrowest exit pipe considered in the simulations, very high average velocity gradients are encountered. In this situation, the LES shows the most pronounced effects of spatial resolution and subgrid-scale modeling.     

喷射反应器内液-液湍流微观混合的研究

通过定义局部分离度,来定量表征喷射器内液-液湍流微观混合规律。其定义式为:(实验所得H~+浓度-模拟所得H~+浓度)/模拟所得H~+浓度。选用酸碱反应体系,利用PLIF技术得到沿喷射器轴线的H~+浓度;采用Fluent软件,利用Standard k-ε模型,模拟获得相同操作条件下的H~+浓度变化趋势。得到不同操作条件下,分离度沿喷射器轴线的变化,结果表明:(1)引流速度不变,喷嘴速度越大,两流体越容易达到微观尺度的均匀混合;(2)喷嘴速度不变,引流速度越大,两流体反而不容易达到微观尺度上的均匀混合;(3)速度比一定的情况下,喷嘴速度越大,越有利于流体的微观混合。     

A numerical analysis of vortex growth in a two-dimensional jet

The vortex growth in a two-dimensional jet is simulated by solving numerically full Navier-Stokes equations with primitive variables. The streakline structures visualized with marker particles are well related to the fluctuations of velocity and pressure. The mechanism of vortex growth is also explained by using the visualized structure. The predicted features will agree with the experimental fact that, for example, the dominant frequency of fluctuation exists and the longitudinal velocity fluctuation along the jet axis increases exponentially in the zone of linear growth, while in the zone of non-linear growth the increasing rate decreases.     

Large-eddy simulation of the flow around a freight wagon subjected to a crosswind

The purpose of the research reported in this paper is to investigate the aerodynamic response of a freight train subjected to a crosswind at 90° yaw angle using large-eddy simulation (LES). The freight train under investigation consists of many identical wagons. The wagons of the train are of the single-stacked container type that is having a box-like shape. In order to reduce the computational cost, attention is confined to the aerodynamic loads and flow structures around a selected single wagon from the middle of the train. The influences of the neighbouring wagons are simulated by imposing spanwise periodicity. The Reynolds number of the flow, based on the time-averaged speed of the crosswind and the height of the wagon from the ground, is 300,000. The standard Smagorinsky model with model constant of 0.1 and the Van Driest damping function, implemented in the commercial CFD package CFX, are used. The assessment of LES accuracy was carried out by performing three different computations using three different meshes; coarse, medium and fine. The fine mesh simulation gives results similar to that of the medium mesh simulation. The LES results showed that the flow moves in the gaps between the containers and under the wagon with high speed resulting in a much more complex flow topology in the wake of the wagon compared to that previously published around passenger trains. The flow separates at the sharp windward edge of the container to form a large separated flow region on the roof of the container. Aerodynamic forces on the container and freight wagon were computed and their time history was used to reveal the characteristic frequencies of the flow motion around the body. Further, the effect of the moving ground on aerodynamic coefficients is investigated by performing simulation on a moving ground. As a result of the floor motion, the side force coefficient was reduced by about 2.5% while the lift force was increased by about 11%.     

Numerical study of helicopter blade–vortex mechanism of interaction using large-eddy simulation

A novel approach, large-eddy simulation, is proposed for the numerical investigation of helicopter blade–vortex mechanism of interaction. The novel approach overcomes all the issues, posed by the other CFD approaches, associated with the vortex dissipation due to the turbulence modeling (RANS, URANS) and computational limitations of DNS. The influence of vertical miss distance and vortex core size on the helicopter blade–vortex mechanism of interaction is subject of investigation. It was observed that the magnitude of the aerodynamic coefficients decreases with the increase of vertical miss distance and the decrease of vortex core size.     

A numerical analysis of vortex growth in a bounded rectangular jet

A low apsect ratio bounded rectangular jet flow with an artificial disturbance is numerically analyzed by solving three-dimensional Navier-Stokes equations with primitive variables. The streakline structures are related to the velocity and pressure fields. To explain the mechanism initiating a secondary flow in the initial field where the flow shows a well ordered structure, the streakline distortions are discussed by the velocity fluctuations in the flow field. The results advocate the existing relations between the secondary flow and the vortex growth.     

Noise prediction for a turbulent jet using different hybrid methods

The acoustic field of a cold single stream jet at Mach number 0.9 and Reynolds number 3600 is determined via computational aeroacoustics (CAA) methods. The jet computation of the acoustical field is performed by two hybrid approaches using a large-eddy simulation (LES) for the flow field and various systems of equations for the acoustical field to construct a robust, efficient, and reliable LES/CAA solver. The acoustic equations are the Ffowcs Williams-Hawkings equation (FWH) in the frequency domain and the acoustic perturbation equations (APE). The pronounced impact of the data windowing and the radial and streamwise extension of the integration surface on the directivity of the FWH solution is discussed at length. The comparison with available experimental and numerical results at similar flow conditions based on the noise characteristics in the near field shows the solution of the APE system to match the results of the direct LES more accurately than the FWH approach. The APE solution is less susceptible to the size of the source term region than the FWH approach to the location of the source surface. In conjunction with the APE formulation the LES domain can be chosen smaller than for the FWH ansatz resulting in less computational cost for the jet flow. The dominant source term in the APE system for cold jet noise is shown to be the Lamb vector.     

LES of intermittency in a turbulent round jet with different inlet conditions

Large eddy simulation (LES) is a promising technique for accurate prediction of turbulent free shear flows in a wide range of applications. Here the LES technique has been applied to study the intermittency in a high Reynolds number turbulent jet with and without a bluff body. The objective of this work is to study the turbulence intermittency of velocity and scalar fields and its variation with respect to different inlet conditions. Probability density function distributions (pdf) of instantaneous mixture fraction and velocity have been created from which the intermittency has been calculated. The time averaged statistical results for a round jet are first discussed and comparisons of velocity and passive scalar fields between LES calculations and experimental measurements are seen to be good. The calculated probability density distributions show changes from a Gaussian to a delta function with increased radial distance from the jet centreline. The effect of introducing a bluff body into the core flow at the inlet changes the structure of pdfs, but the variation from Gaussian to delta distribution is similar to the jet case. However, the radial variation of the intermittency indicates differences between the results with and without a bluff body at axial locations due the recirculation zone created by the bluff body.