Numerical simulation of phase separation and a priori two-phase LES filtering

This work reports on the potential application of Large Eddy Simulation (LES) in the calculation of turbulent isothermal two-phase flows, in the case where the large scales of each phase are resolved and small interface structures can be smaller than the mesh size. In comparison with single phase flows, application of LES to two-phase flow problems should account for the complex interaction between the interface and the turbulent motion. The complete filtered two-phase flow equations are formulated to deal with turbulence at the interface. Explicit filtering of 3D direct numerical simulations of a phase separation problem has been employed to evaluate the order of magnitude of the specific subgrid contributions. Analyses of the numerical results have been conducted to derive conclusions on the relative importance of the different subgrid scale contributions. Modeling issues and turbulent energy transfer across the interface are discussed.     

Developments in computational fluid dynamics-based modeling for disinfection technologies over the last two decades: A review

In the last two decades, Computational Fluid Dynamics (CFD) has shown great potential as a powerful and cost-efficient tool to troubleshoot existing disinfection contactors and improve future designs for water treatment industry. However, numerous challenges in the simulation of water disinfection processes still remain. This review summarizes past CFD studies of the hydraulic and associated disinfection efficiency of disinfection contactors. Hydraulic efficiency studies based on flow and tracer transport simulation were found to be the most common and successful. Challenges existing in flow and disinfection simulation are identified and discussed. These challenges can be overcome via advanced turbulent simulation approaches, such as Large Eddy Simulation and multi-phase resolving simulations. Although turbulence-chemistry interaction is found to be the most challenging problem for proper representation of the reaction system and inactivation kinetics, solutions to this challenge can be overshadowed unless errors induced by unresolved unsteady flow and multi-phase flow are reduced sufficiently.     

Normal shock boundary layer control with various vortex generator geometries

Various vortex generators which include ramp, split-ramp and a new hybrid concept “ramped-vane” are investigated under normal shock conditions with a diffuser at Mach number of 1.3. The dimensions of the computational domain were designed using Reynolds Average Navier–Stokes studies to be representative of the flow in an external-compression supersonic inlet. Using this flow geometry, various vortex generator concepts were studied with Implicit Large Eddy Simulation. In general, the ramped-vane provided increased vorticity compared to the other devices and reduced the separation length downstream of the device centerline. In addition, the size, edge gap and streamwise position respect to the shock were studied for the ramped-vane and it was found that a height of about half the boundary thickness and a large trailing edge gap yielded a fully attached flow downstream of the device. This ramped-vane also provided the largest reduction in the turbulent kinetic energy and pressure fluctuations. Additional benefits include negligible drag while the reductions in boundary layer displacement thickness and shape factor were seen compared to other devices.     

Assessment of multiscale resolution for hybrid turbulence model in unsteady separated transonic flows

This paper presents results of a computational study conducted to assess the multi-scale resolution capabilities of a hybrid two-equation turbulence model in predicting unsteady separated high speed flows. Numerical solutions are obtained using a third order Roe scheme and the SST (shear-stress-transport) two-equation-based hybrid turbulence model for three-dimensional transonic flow over an open cavity. A detailed assessment of the effects of the computational grid and the hybrid turbulence model coefficient is presented for the unsteady flow field. Computed results are presented for both the resolved and the modeled turbulent kinetic energy (TKE) and for the predicted sound pressure level (SPL) spectra, which are compared to available experimental data and large Eddy simulation (LES) results. The comparison shows that the predicted SPL spectra agree well with the experimental results over a frequency range up to 2500 Hz, and that hybrid turbulence effectively models the shorter wavelengths. The results demonstrate improved agreement with experimental SPL spectra with increased grid resolution and a reduced hybrid turbulence model coefficient. In addition, they show that energy dissipation of the unresolved scales is over-predicted at low resolutions and that the hybrid coefficient influences the grid resolution requirements.     

连续热镀锌线气刀吹锌过程中锌波纹产生的大涡模拟

为寻求连续热镀锌线气刀吹锌过程中带钢上出现锌波纹的原因,采用大涡模拟（Large Eddy Simulation,LES）方法数值求解三维非稳态气刀射流湍流流场,并结合带钢表面的镀锌层厚度的零维预测模型研究锌波纹产生的机理．结果表明,在气刀射流冲击区域两侧有交替成对出现的漩涡,造成带钢表面的压强和切应力周期性波动分布,从而产生锌波纹．     

High-order Implicit Large Eddy Simulation of gaseous fuel injection and mixing of a bluff body burner

Implicit Large Eddy Simulation (ILES) with high-resolution and high-order computational modelling was applied to a turbulent mixing fuel injector flow. In the ILES calculation, the governing equations for three dimensional, non-reactive, multi-species compressible flows were solved using a finite volume Godunov-type method. Up to ninth-order spatial accurate reconstruction methods were examined with a second order explicit Runge–Kutta time integration. Mean and root mean square velocity and mixture fraction profiles showed good agreement with experimental data, which demonstrated that ILES using high-order methods successfully captured complex turbulent flow structure without using an explicit subgrid scale model. The effects of grid resolution and the influence of order of spatial accuracy on the resolution of the kinetic energy spectrum were investigated. An k^−5/3 decay of energy could be seen in a certain range and the cut-off wavenumbers increased with grid resolution or order of spatial accuracy. The effective cut-off wavenumbers are shown to be larger than the maximum wavenumbers appearing on the given grid for all test cases, implying that the numerical dissipation represents sufficiently the energy transport between resolved and unresolved eddies. The fifth-order limiter with a 0.6 million grid points was found to be optimal in terms of the resolution of kinetic energy and reasonable computational time.     

Segregated coupling of large-eddy simulations with downstream Reynolds-Averaged Navier-Stokes calculations

A technique is proposed for zonal coupling of Large Eddy Simulation (LES) with a downstream Reynolds-Averaged Navier-Stokes (RANS) calculation. At a pre-defined interface, mean velocities are coupled and velocity fluctuations of the LES zone are removed by employing a convective boundary condition. For incompressible flow, the handling of pressure at the interface is crucial to the success of the method. Global coupling as well as decoupling of the pressure are investigated. The latter is more robust and therefore more generally applicable, but it requires an additional mass flux correction at the interface. The resulting approach is used to explain observed short-comings of the so-called enrichment strategy when applied to downstream LES-RANS coupling and it represents an improved and more generally valid method without the need for calibration constants. The performance of the proposed method is scrutinised for turbulent flow in a channel and over periodic hills. The results corroborate the predicted increase in accuracy and robustness.     

DNS of the interaction between a deformable buoyant bubble and a spatially decaying turbulence: A priori tests for LES two-phase flow modelling

This paper reports on results of a 3D Direct Numerical Simulation (DNS) of a strongly deformable bubble in a spatially decaying turbulence. The complex interaction between interface and turbulence is fully resolved. This two-way coupling phenomenon is found to be of great importance for the flow dynamic. An explicit filtering of the simulation has been employed to evaluate the order of magnitude of the specific subgrid contributions in the Large Eddy Simulation (LES) modelling case. Subgrid closure models are proposed and evaluated thanks to a priori tests.     

Modelling of wind load on single and staggered dual buildings

A method of modelling numerically the wind loads on single and staggered dual buildings using Computational Fluid Dynamics is presented in this paper. Simulation of a turbulent boundary layer over test models was carried out at the Supercomputing and Visualization Unit, the National University of Singapore, using FLUENT 6.1.18. Turbulence was introduced at the inlet through a parallel auxiliary simulation and the computation of the flow advanced in time using Large Eddy Simulation with a ReNormalization Group subgrid-scale viscosity model. Wind velocities at different locations and wind pressures on the building faces were recorded. Subsequently the flow characteristics were examined and the force and moment spectra deduced. The results were compared with data from earlier wind tunnel experiments carried out at Virginia Polytechnic Institute and State University. It can be concluded from the study that numerical wind modelling on tall structures is a promising alternative to conventional tests in atmospheric boundary layer wind tunnels.     

On a mechanism of stabilizing turbulent free shear layers in cavity flows

Turbulent free shear flows are subject to the well-known Kelvin–Helmholtz type [Panton RL. Incompressible flow. John Wiley and Sons; 1984. p. 675] instability, and it is well-known that any free shear flow which approximates a thin vorticity layer will be unstable to a wide range of amplitudes and frequencies of disturbance. In fact, much of what constitutes flow control in turbulent free shear layers consists of feeding a prescribed destabilizing disturbance to these layers. The question in the control of free shear flows is not whether the shear layer will be stable, but whether you can influence how the layer becomes unstable. In most cases, since these flows are so receptive to forcing input, and naturally tend toward instability, large changes in flow conditions can be achieved with very small amplitude periodic inputs.

Recently, it has been discovered that turbulent free shear flows can also be stabilized using periodic forcing. This is, at first glance, counter-intuitive, considering our long history of considering these flows to be very unstable to forcing input. It is a phenomenon not described in modern fluid dynamic text books. The forcing required to achieve this effect (which we will call turbulent shear layer stabilization) is of a much higher amplitude and frequency than the more traditional type of shear layer flow control effect seen in the literature (which we will call turbulent shear layer destabilization).

A numerical study is undertaken to investigate the effect of frequency of pulsed mass injection on the nature of stabilization, destabilization and acoustic suppression in high speed cavity flows. An implicit, 2nd-order in space and time flow solver, coupled with a recently developed hybrid RANS-LES (Reynolds Averaged Navier Stokes-Large Eddy Simulation) turbulence model by Nichols and Nelson [Nichols RH, Nelson CC. Weapons bay acoustic predictions using a multi-scale turbulence model. In: Proceedings of the ITEA 2001 aircraft-stores compatibility symposium, March 2001], is utilized in a Chimera-based parallel format. This tool is used to numerically simulate both an unsuppressed cavity in resonance, as well as the effect of mass-addition pulsed jet flow control on cavity flow physics and ultimately, cavity acoustic levels.

Frequency (and in a limited number of cases, amplitude) of pulse is varied, from 0 Hz (steady) up to 5000 Hz. The change in the character of the flow control effect as pulsing frequency is changed is described, and linked to changes in acoustic levels. Limited comparison to 1/10th scale experiments is presented.

The observed local stabilization of the cavity turbulent shear layer, when subjected to high frequency pulsed blowing, is shown in simulation to be the result of a violent instability and breakdown of a pair of opposite sign vortical structures created with each high frequency “pulse”. This unique shear layer stabilization behavior is only observed in simulation above a certain critical pulsing frequency. Below this critical frequency, pulsing is shown in simulation to provide little benefit with respect to suppression of high cavity acoustic levels.     

Numerical simulation of the transonic turbulent flow around a wedge-shaped body with a backward-facing step

A fully three-dimensional near-wall complex turbulent flow around a wedge-shaped body with a backward-facing step is considered with the transonic flow regime (Mach number M = 0.913) at the Reynolds number Re = 7.2 × 10⁶. The technology of the numerical simulation of problems of the class under study is represented in detail. A series of preliminary auxiliary calculations is carried out for choosing the optimal computational algorithm. The numerical results of the problem simulation based on the eddy-resolving hybrid RANS-LES approach IDDES are finally given for the full configuration. The validity of the results obtained is confirmed by comparing them to the corresponding experimental data.     

Study of hydrogen auto-ignition in a turbulent air co-flow using a Large Eddy Simulation approach

Large Eddy Simulation (LES) is applied to the auto-ignition of an hydrogen jet issuing into a turbulent co-flowing air stream. A 19 step, 9 species detailed mechanism is used for modelling the chemical reactions. The influence of sub-grid fluctuations is accounted for by a sub-grid joint probability density function (PDF) for the reactive scalars. A Eulerian Stochastic Field method is used to solve the modelled form of the PDF transport equation. The model is able to reproduce ignition lengths and different regimes observed experimentally without adjustment of the sub-grid scale model parameters.     

Large Eddy Simulation of a turbulent non-premixed propane-air reacting flame in a cylindrical combustor

Large Eddy Simulation (LES) is applied to investigate the turbulent non-premixed combustion flow, including species concentrations and temperature, in a cylindrical combustor. Gaseous propane (C₃H₈) is injected through a circular nozzle which is attached at the centre of the combustor inlet. Preheated air with a temperature of 773 K is supplied through the annulus surrounding of this fuel nozzle. In LES a spatial filtering is applied to the governing equations to separate the flow field into large-scale and small-scale eddies. The large-scale eddies which carry most of the turbulent energy are resolved explicitly, while the unresolved small-scale eddies are modelled using the Smagorinsky model with C_s = 0.1 as well as dynamically calibrated C_s. The filtered values of the species mass fraction, temperature and density, which are the functions of the mixture fraction (conserved scalar), are determined by integration over a beta probability density function (β-PDF). The computational results are compared with those of the experimental investigation conducted by Nishida and Mukohara [1]. According to this experiment, the overall equivalence ratio of 0.6, which is calculated from the ratio of the air flow rate supplied to the combustion chamber to that of the stoichiometric reaction, is kept constant so that the turbulent combustion at the fuel nozzle exit starts under the fuel-rich conditions.     

Implicit Large Eddy Simulation of non-wall-bounded turbulent flows based on the multiscale properties of a high-order finite volume method

In this paper it is presented the application of a higher-order finite volume method based on Moving Least Squares approximations (FV-MLS) to the resolution of non-wall-bounded compressible turbulent flows. Our approach is based on the monotonically implicit Large Eddy Simulation (MILES). The main idea of MILES methodology is the absence of any explicit subgrid scale (SGS) model in the numerical algorithm to solve turbulent flows. In the case of the FV-MLS method, we take advantage of the multiresolution properties of Moving Least Squares Approximations, and we show that they can be used as an implicit SGS model. The numerical results are encouraging. The third-order FV-MLS method is able to reproduce the inertial subrange, and it obtains better results than other usual numerical schemes in LES computations, such as the MUSCL scheme. We note that in the present state of this research, the numerical method is not yet suited for wall-bounded flows. This paper is the first step in the application of the FV-MLS method to general turbulent flows.     

Studies of shock/turbulent shear layer interaction using Large-Eddy Simulation

Shock/shear/turbulence interactions are simulated using Large-Eddy Simulation (LES) with a new localized subgrid closure approach. Both normal and oblique shocks interactions with turbulence are considered. The LES methodology adopted here combines a hybrid numerical scheme that switches automatically and locally between a shock-capturing scheme and a low-dissipation high-order central scheme.The fundamental role of the diffusion of turbulent kinetic energy by pressure fluctuations in the problem of normal shock/isotropic turbulence interaction is stressed in the DNS study, and accounted for in the closure model. The study of the interaction between two oblique shocks and a turbulent shear layer shows that the turbulence evolution is mostly affected by two competing phenomena. An amplification of the turbulent levels occurs downstream of the interaction, and the mixing layer growth rate is significantly increased. However, the integrated production of turbulent energy across the mixing layer is reduced, and the increase in mixing is found to be localized in space, the turbulent statistics quickly relaxing to their undisturbed levels. Furthermore, the increase in vorticity from the compression of the mixing layer remains small, unaffected by the presence of turbulent and coherent structures.     

Parallel Simulations of Swirling Turbulent Flames

The feasibility of using massively paralleled computations as an engineering design tool is evaluated. A parallel Large-Eddy Simulation (LES) algorithm which simulates turbulent reacting flows using a space and time-accurate method, is used to model the complex flow found inside a realistic gas-turbine combustor. The parallelization philosophy and its implementation as a platform-independent solver is discussed. A performance analysis is carried out to determine the communication and storage requirements, and the associated overhead. As a case study, the LES methodology is used for a parametric investigation of swirl effects on the turbulent reacting flow in the gas-turbine.     

Prediction of sound generated by a rod-airfoil configuration using EASM DES and the generalised Lighthill/FW-H analogy

Sound generated by an airfoil in the wake of a rod is predicted numerically by using a Detached-Eddy Simulation (DES) unsteady flow field and a Ffowcs Williams and Hawkings acoustic analogy formulation for the far field computation. Volume sources from the rod wake are found to play a non-negligible role at high frequencies and surface contributions might be flawed if the surfaces cross highly turbulent flow regions even if surrounding volume terms are accounted for. The DES approach is based on a novel cubic explicit algebraic stress turbulence model which is built on a two-equation k-ε model from Lien and Lechziner. This DES has been recently implemented at the Berlin University of Technology in the compressible Navier-Stokes flow solver ELAN. The aerodynamic results are compared to experimental data obtained at the ECL by Jacob et al., as well as to previous Large Eddy Simulations results from the Proust/Turbflow code by Boudet et al. and DES simulations from Greschner et al. based on standard turbulence models. The acoustic analogy is applied both with and without volume terms to rigid and permeable control surfaces surrounding the rod-airfoil system. Aeroacoustic results are compared to experimental data from the literature, showing that the inclusion of volume terms improves the aeroacoustic prediction in the broadband high frequency range.     

Small scale localization in turbulent flows. A priori tests applied to a possible Large Eddy Simulation of compressible turbulent flows

A method for the localization of small scales in turbulent velocity fields is proposed. The method is based on the definition of a function f of the velocity and vorticity fields that reproduces a normalized form of the twisting-stretching term of the Helmholtz equation. By means of this method the equations of motion can be selectively filtered in regions that are rich in small scale motions. The method is applied through a criterion built on a statistical link between the function f and a local property of the turbulence that was derived from the analysis a homogeneous and isotropic high Reynolds number (Reλ=280) turbulence field. The localization criterion is independent of the subgrid scale model used in a possible Large Eddy Simulation carried out after the small scale localization is obtained. This extends the typology of possible applications to the analysis of experimental laboratory data. In case of compressible regimes, a second sensor that depends on the local pressure variation and divergence can be associated to the previous one in order to determine the eventual emergence of shocks. The capture of shocks is made possible by suppressing the subgrid terms where this second sensor indicates the presence of a shock.A priori tests were carried out on the turbulent channel flow, Reλ=180 and 590, to validate the localization procedure in a highly inhomogeneous flow configuration. A second set of a priori test was carried out on a turbulent time decaying jet which initiates its evolution at Mach 5 and which reproduces a few hydrodynamical properties of high Reynolds number hypersonic jets which exist in the Universe.     

Development of Roe-type scheme for all-speed flows based on preconditioning method

The problems of mixed low-speed/high-speed flows are not solved properly with existing compressible method. Based on preconditioning technology, a new scheme, All-Speed-Roe scheme, is proposed. Compared with Roe scheme, it decreases the effect of acoustic speed in its numerical dissipation when Mach numbers decrease. Compared with traditional preconditioned Roe scheme, it overcomes the limit of being cut-off by the global Mach number through modifying the eigenvalues only and has the good convergence acceleration rate for low-Mach-number flows through multiplying the spatial residual by the preconditioner. From another perspective, All-Speed-Roe scheme suggests that there is no necessity to replace the physical acoustic speed in denominator with pseudo-acoustic speed. In theory, All-Speed-Roe scheme is suitable for all speed flow calculations with capturing shock and simulating low-Mach-number flows. The numerical results of Euler nozzle flow, RANS of NASA rotor 37 flow, Euler simulation, Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) of high-loaded blade T106 flow show that All-Speed-Roe scheme can replace traditional preconditioned Roe scheme because it is easier for programming, more robust in computations, more accurate in spatial, and has good convergence rate.     

基于格子Boltzmann方法和大涡模拟的颈动脉分叉狭窄流动并行计算

颈动脉斑块的形成与复杂的血流动力学因素密切相关,血液流动状况的精确模拟对颈动脉斑块的临床诊断具有重要意义。为了精确模拟脉动流场,在格子Boltzmann方法（LBM）的基础上,添加大涡模拟（LES）模型,建立了LBM-LES颈动脉模拟算法。利用医学图像重构软件,建立颈动脉狭窄真实几何模型,对颈动脉狭窄脉动流动进行了数值模拟,通过计算血液流动速度、壁面剪切应力（WSS）等,得出了有意义的流动结果,验证了LBM-LES对颈动脉狭窄后段血液流动研究的有效性。基于OpenMP编程环境,在高性能集群机全互联胖节点上进行了千万量级网格的并行计算,结果表明LBM-LES颈动脉模拟算法具有较好的并行性能。