Large Eddy Simulation of Turbulent Attached Cavitating Flows around Different Twisted Hydrofoils

With the development of computational power, large eddy simulation (LES) method is increasingly used in simulating complex flow. However, there still exist many factors affecting the LES quality and appropriate mesh resolution is among one of them. This work aims to develop an automatic procedure to refine the LES mesh by combining adaptive mesh refinement (AMR) and LES quality criteria. An LES refinement criterion is developed by estimating the proper grid length scale which meets the accuracy requirement of LES method. With this criterion, the baseline mesh is automatically refined with the AMR method. In this work, an efficient one-shot refinement strategy is also proposed to reduce the overall simulation time. Current AMR-based LES method is verified with the typical LES test case about the flow past circular cylinder at ReD = 3900. Results show that the automatically refined mesh provides systematically better agreement with experimental results and with current method the balance between accuracy and computational expense for LES can be obtained.

The present paper describes a study that is aimed at establishing and quantifying the benefits of the Large Eddy Simulation (LES) method for predicting scalar turbulent transport in a combustor relevant jet-mixing problem. A non-reacting co-annular jet mixing configuration is considered for which comprehensive experimental data for both velocity and scalar fields have recently been obtained. Detailed comparisons are presented for the development of the axial velocity field in terms of both mean and turbulence intensity. Similarly, the mixing between the jets is examined by comparison with measurements for the mean concentration and the variance of concentration fluctuations. Agreement with these statistically averaged fields is demonstrated to be very good, and a considerable improvement over the standard eddy viscosity RANS approach. Illustrations are presented of the time-resolved information that LES provides such as time histories, and also conserved scalar pdf predictions. The LES results are shown, even using a simple Smagorinsky sub-grid-scale model, to predict correctly lower values of the turbulent Prandtl number (∼ 0.6) in the free shear regions of the flow, as well as higher values (∼ 1.0) in the wall-affected regions. The ability to predict turbulent Prandtl number variations (rather than input these as commonly done in most combustor RANS CFD models) is an important and promising feature of the LES approach for combustor simulation since it is known to be important in determining combustor exit temperature traverse.

Since 1960s, from experimental observation, there are several stages for liquid droplets in multi-phase combustion: pure heating, pure evaporation without individual droplet combustion, and individual droplet combustion (burning) with individual flame around which enhances evaporation. As for individual burning droplets, they have envelope flame or wake flame regimes. From experimental and theoretical research, in liquid fuel combustion chambers, according to the space between the droplets, there are four typical droplet group combustion modes. Generally, the external group combustion mode and the internal group combustion mode are the two main modes, and there are individual burning droplets in multiphase combustion. Up to now, based on the literature information, no matter in Reynolds Averaged Navier Stokes (RANS) method, Large Eddy Simulation (LES) method or Point Direct Numeral Simulation (PDNS) method, droplets in multi-phase combustion are treated as pure evaporation points, corresponding to the bright blue flame condition, i.e., small droplets in hot gas mixture with the pure evaporation stage, so in species equation and energy equation, the droplet points are mass source and heat sink. As for the individual droplets burning mode, in point source simulation method, corresponding to the big droplet in cold gas phase mixture with yellow trail flame condition, the source terms in the species equation and energy equation should be the mass sink and heat source. The individual burning droplets affects the NO formation because the flame around the droplet has higher temperature than the local gas phase mixture mostly for the quasi-stoichiometric ratio reaction in individual droplet flame. In this paper, a new spray combustion model with the consideration of individual droplets combustion mode is brought forward. The existing modeling approach is extended with an ignition criterion and a droplet combustion correlation. The ignition criterion, which judges the droplet stage between pure evaporation and burning, and the droplet combustion correlation, which integrates the micro-scales droplet burning effects into point source term frame, are both selected from pioneer’s experimental work and added to the present existing widely used multi-phase combustion model. Then a jet spray flame is tested by the new model with RANS and LES method. The prediction results are compared with the literature experimental results. New model gives better simulation results in downstream regions. According to the results, this model added the individual droplet combustion mode in multi-phase combustion model, while its combustion correlations are quite rough at present. More verify studies should be carried in the future.

Natural convection in a volumetrically heated fluid is of significant importance related to the In-Vessel Retention (IVR) issue. The COPRA facility is designed to simulate the lower plenum of reactor vessel at 1:1 scale for the Chinese advanced PWR, which is a two-dimensional 1/4 circular slice structure with an inner radius of 2.2 m. Water was used as simulant in COPRA-L1 tests. Due to the full scale geometry, The Rayleigh numbers within the corium pools could reach to 1016, matching those in the prototypical situation during severe accident. Due to the high Rayleigh numbers and high turbulence in the melt pool, we choose the Large-Eddy Simulation (LES) method to do the numerical calculations. The LES method can better capture the heat transfer characteristic in a high-Rayleigh number volumetrically heated liquid pool. The numerical results of pool temperature and heat flux distribution were compared with those from COPRA-L1 tests.

Large eddy simulations of mixing effects on the course of precipitation process

The explicit filtering method for large eddy simulations of a turbulent premixed flame

Convergence of cognitive and adaptive decline in chronic schizophrenia

Computational analysis of locally forced flow over a wall-mounted hump at high- Re number

In order to validate the application of Large eddy simulation in the field of incompressible flow, the author used Large eddy simulation to compute 2-dimensional incompressible flows around an airfoil by software Fluent. In the beginning of computation, the paper prepared five types of mesh in order to check up the LES requiring for mesh. Then the author monitored lift coefficient and drag coefficient on the airfoil. The present results are compared with the experimental results. When the section angle of attack is low, the results of numerical simulation are very close to the experimental. However, at the attack angle of near stall condition there is some error. Large eddy simulation can simulate the origination and breaking off of the eddy very well. Strouhal number obtained is close to the theory. Validate the method of large eddy simulation by numerical simulation.

The paper is to integrate aerodynamic and aero-acoustic optimizatiom design of high lift devices, especially for two-element airfoils with slat. Aerodynamic analysis on flow field utilizes a high-order, high-resolution spatial differential method for large eddy simulation (LES), which can guarantee accuracy and efficiency. The aero-acoustic analysis for noise level is calculated with Ffowcs Williams-Hawkings (FW-H) integration formula. Fidelity of calculation is verified by standard models. Method of streamline-based Euler simulation (MSES) is used to obtain the aerodynamic characters. Based on the confirmation of numerical methods, detailed research has been conducted for the leading edge slat on multi-element airfoils. Various slot parameter influences on noise are analyzed.The results of the slot optimization parameters can be used in multi-element airfoil design.

This work focuses on the development of efficient computational tools for the simulation of turbulent multiphase polydispersed flows. In terms of methodologies we focus here on the use of Large Eddy Simulation (LES) and Quadrature-Based Methods of Moments (QBMM). In terms of applications the work is finalised, in order to be applied in the future, to particle production processes (precipitation and crystallisation in particular). An important part of the work concerns the study of the flow field in a Confined Impinging Jets Reactor (CIJR), frequently used in particle production processes. The first part is limited to the comparison and analysis of micro Particle Image Velocimetry (μPIV) experiments, carried out in a previous work, and Direct Numerical Simulation (DNS), carried out in this thesis. In particular the effects of boundary and operating conditions are studied and the numerical simulations are used to understand the experimental predictions and demonstrate the importance of unavoidable fluctuations in the experimental inlets. This represents a preparatory work for the LES modelling of the CIJR. Before investigating the accuracy of LES predictions for this particular application, the model and the implementation are studied in a more general context, represented by a well-known test case such as the periodic turbulent channel flow: the LES model implementation in TransAT, the code used in this work, is compared with DNS data and with predictions of other codes. LES simulations for the CIJR, provided with the proper boundary conditions obtained by the previous DNS/μPIV study, are then performed and compared with experiments, validating the model in a more realistic test case. Since particle precipitation and crystallization often result in complex interactions between particles and the continuous phase, in the second part of the work particular attention has been paid in the modelling of the momentum transfer and the resulting velocity of the particles (relative to the fluid). In particular the possibility of describing poly-disperse fluid-solid systems with QBMM together with LES and Equilibrium Eulerian Model (EEM) is assessed. The study is performed by comparing our predictions with DNS Lagrangian data in the turbulent channel flow previously described, seeded with particles corresponding to a realistic Particle Size Distribution (PSD). The last part of the work deals with particle collisions, extending QBMM to the investigation of non-equilibrium flows governed by the Boltzmann Equation with a hard-sphere collision kernel. The evolution of the particle velocity distribution is predicted and compared with other methods for kinetic equations such as Lattice Boltzmann Method (LBM), Discrete Velocity Method (DVM) and Grad's Moment Method (GM). The overall results of this thesis can be extended to a broad range of other applications of single-phase, dispersed multiphase and non-equilibrium flows

The critical review discusses the most accurate methods for description of turbulent flows: the computationally very expensive direct numerical simulation (DNS) and slightly less accurate and slightly less expensive large eddy simulation (LES) methods. Both methods have found their way into nuclear thermal hydraulics as tools for studies of the fundamental mechanisms of turbulence and turbulent heat transfer. In the first section of this critical review, both methods are briefly introduced in parallel with the basic properties of the turbulent flows. The focus is on the DNS method, the so-called quasi-DNS approach, and the coarsest turbulence modeling approach discussed in this work, which is still on the very small-scale, wall-resolved LES. Other, coarser turbulence modeling approaches (such as wall-modeled LES, Reynolds Averaged Navier-Stokes (RANS)/LES hybrids, or RANS) are beyond the scope of the present work. Section II answers the question: “How do the DNS and LES methods work?” A short discussion of the computational requirements, numerical approaches, and computational tools is included. Section III is about the interpretation of the DNS and LES results and statistical uncertainties. Sections IV and V give some examples of the DNS and wall-resolved LES results relevant for nuclear thermal hydraulics. The last section lists the conclusions and some of the challenges that might be tackled with the most accurate techniques like DNS and LES.

A coupled MMC-LES and sectional kinetic scheme for soot formation in a turbulent flame

Regularization method for large eddy simulations of shock-turbulence interactions

Numerical Simulation of Two-Dimensional Parallel Blade-Vortex Interactions Using Large Eddy Simulation