钝感炸药点火增长模型的欧拉数值模拟

在自主研发的二维多介质欧拉弹塑性流体力学程序中,通过引入点火增长的反应率模型以及炸药减敏模型,借助网格自适应技术,研究钝感炸药的冲击点火、直径效应以及死区形成等爆轰现象。数值模拟结果表明,该程序能够正确模拟平面爆轰波的爆速、CJ状态、von Neumann尖点等爆轰参数;并能够较好模拟炸药的直径效应。另外,通过引入考虑减敏效应的反应率模型,能较好地模拟钝感炸药的死区形成过程。  

自主推进俯仰震荡翼型的数值模拟研究

运用自适应多重网格法和内置边条法研究了俯仰震荡翼型的运动.通过研究翼型的原地摆动与自由游动,提出了一种确定自主推进俯仰震荡翼型推力的方法,得到了推力系数、功率系数和推进效率与Strouhal数的关系.与以往研究不同的是,我们还得到了Strouhal数随雷诺数的变化规律.此外,从自由游动翼型诱导出的涡量场中可以清楚的观察到旋涡的合并,这与实验研究非常吻合.  

自适应分析在确定裂纹尖端塑性区中的应用

在分析裂纹扩展及材料强度时对塑性区的估算是很重要的，本文提出采用自适应有限元分析来确定裂纹尖端塑性区的方法，计算结果表明这种方法通过网格自动加密能够有效地跟踪出弹塑性的交界面。  

Numerical investigation of the spray-mesh-turbulence interactions for high-pressure, evaporating sprays at engine conditions

This work presents a numerical methodology to simulate evaporating, high pressure Diesel sprays using the Eulerian-Lagrangian approach. Specific sub-models were developed to describe the liquid spray injection and breakup, and the influence of the liquid jet on the turbulence viscosity in the vicinity of the nozzle. To reduce the computational time and easily solve the problem of the grid dependency, the possibility to dynamically refine the grid where the fuel-air mixing process takes place was also included.The validity of the proposed approach was firstly verified simulating an evaporating spray in a constant-volume vessel at non-reacting conditions. The availability of a large quantity of experimental data allowed us to investigate in detail the effects of grid size, ambient diffusivity and used spray sub-models. In this way, different guidelines were derived for a successful simulation of the fuel-air mixture formation process. Finally, fuel injection and evaporation were simulated in an optical engine geometry and computed mixture fraction distributions were compared with experimental data.  

Adaptive thermo-fluid moving boundary computations for interfacial dynamics

In this study,we present adaptive moving boundary computation technique with parallel implementation on a distributed memory multi-processor system for large scale thermo-fluid and interfacial flow computations.The solver utilizes Eulerian-Lagrangian method to track moving(Lagrangian) interfaces explicitly on the stationary(Eulerian) Cartesian grid where the flow fields are computed.We address the domain decomposition strategies of Eulerian-Lagrangian method by illustrating its intricate complexity of the computation involved on two different spaces interactively and consequently,and then propose a trade-off approach aiming for parallel scalability.Spatial domain decomposition is adopted for both Eulerian and Lagrangian domain due to easy load balancing and data locality for minimum communication between processors.In addition,parallel cell-based unstructured adaptive mesh refinement(AMR) technique is implemented for the flexible local refinement and even-distributed computational workload among processors.Selected cases are presented to highlight the computational capabilities,including Faraday type interfacial waves with capillary and gravitational forcing,flows around varied geometric configurations and induced by boundary conditions and/or body forces,and thermo-fluid dynamics with phase change.With the aid of the present techniques,large scale challenging moving boundary problems can be effectively addressed.  

Modelling of high-pressure dense diesel sprays with adaptive local grid refinement

An Eulerian–Lagrangian fluid dynamics model simulating the development of dense liquid plumes formed during injection of fuels against compressed air is described and assessed against experimental data. The numerical model employs an adaptive local grid refinement methodology combined with a calculation procedure distributing the mass, momentum and energy exchanged between the liquid and gaseous phases in the numerical cells found in the vicinity of the moving droplets. The use of appropriate weighting functions resolves numerical as well as physical problems realised when the interaction volume available between the two phases is limited to the cell-containing parcel, whose volume may become comparable to that of the dispersed phase. Calculation of ‘virtual’ cell properties provide better estimates for the flow variables realised by the droplets crossing cells in the wake of those upstream and allows for larger time steps to be employed in the solution of the carrier phase conservation equations. The results suggest that the proposed methodology offers significant improvements compared to the standard Lagrangian one frequently adopted in simulation of combustion systems, without the need to use Eulerian flow models in dense spray regions.  

An adaptive multilevel wavelet framework for scale‐selective WENO reconstruction schemes

We put forth a dynamic computing framework for scale‐selective adaptation of weighted essential nonoscillatory (WENO) schemes for the simulation of hyperbolic conservation laws exhibiting strong discontinuities. A multilevel wavelet‐based multiresolution procedure, embedded in a conservative finite volume formulation, is used for a twofold purpose. (i) a dynamic grid adaptation of the solution field for redistributing grid points optimally (in some sense) according to the underlying flow structures, and (ii) a dynamic minimization of the in built artificial dissipation of WENO schemes. Taking advantage of the structure detection properties of this multiresolution algorithm, the nonlinear weights of the conventional WENO implementation are selectively modified to ensure lower dissipation in smoother areas. This modification is implemented through a linear transition from the fifth‐order upwind stencil at the coarsest regions of the adaptive grid to a fully nonlinear fifth‐order WENO scheme at areas of high irregularity. Therefore, our computing algorithm consists of a dynamic grid adaptation strategy, a scale‐selective state reconstruction, a conservative flux calculation, and a total variation diminishing Runge‐Kutta scheme for time advancement. Results are presented for canonical examples drawn from the inviscid Burgers, shallow water, Euler, and magnetohydrodynamic equations. Our findings represent a novel direction for providing a scale‐selective dissipation process without a compromise on shock capturing behavior for conservation laws, which would be a strong contender for dynamic implicit large eddy simulation approaches.  

Numerical solution scheme for inert,disperse, and dilute gas-particle flows

A multi-velocity formulation is proposed for the solution of an Eulerian representation of an inert, disperse, and dilute particle-phase of a gas-particle flow. Single-velocity formulations are capable of predicting regions of zero particle concentration but are problematic with crossing particle trajectories or compression waves. The multi-velocity formulation described here can account for crossing particle trajectories by splitting the particle-phase into distinct velocity families which are transported separately in the flow. Switching of the particle families at solid boundaries and due to momentum transfer with the gas-phase is conducted in a manner that enforces conservation of mass, momentum, and energy. This numerical method is combined with a parallel block-based adaptive mesh refinement algorithm that is very effective in treating problems with disparate length scales. The block-based data structure lends itself naturally to domain decomposition and thereby enables efficient and scalable implementations of the algorithm on distributed-memory multi-processor architectures. Numerical results are described to demonstrate the capabilities of the approach for predicting gas-particle flows.  

An immersed boundary solver for inviscid compressible flows

In this paper, a simple and efficient immersed boundary (IB) method is developed for the numerical simulation of inviscid compressible Euler equations. We propose a method based on coordinate transformation to calculate the unknowns of ghost points. In the present study, the body‐grid intercept points are used to build a complete bilinear (2‐D)/trilinear (3‐D) interpolation. A third‐order weighted essentially nonoscillation scheme with a new reference smoothness indicator is proposed to improve the accuracy at the extrema and discontinuity region. The dynamic blocked structured adaptive mesh is used to enhance the computational efficiency. The parallel computation with loading balance is applied to save the computational cost for 3‐D problems. Numerical tests show that the present method has second‐order overall spatial accuracy. The double Mach reflection test indicates that the present IB method gives almost identical solution as that of the boundary‐fitted method. The accuracy of the solver is further validated by subsonic and transonic flow past NACA2012 airfoil. Finally, the present IB method with adaptive mesh is validated by simulation of transonic flow past 3‐D ONERA M6 Wing. Global agreement with experimental and other numerical results are obtained.  

An adaptive finite element method for the modeling of the equilibrium of red blood cells

This contribution is concerned with the numerical modeling of an isolated red blood cell (RBC), and more generally of phospholipid membranes. We propose an adaptive Eulerian finite element approximation, based on the level set method, of a shape optimization problem arising in the study of RBCs. We simulate the equilibrium shapes that minimize the elastic bending energy under prescribed constraints of fixed volume and surface area. An anisotropic mesh adaptation technique is used in the vicinity of the cell membrane to enhance the robustness of the method. Efficient time and spatial discretizations are considered and implemented. We address in detail the main features of the proposed method, and finally we report several numerical experiments in the two‐dimensional and the three‐dimensional axisymmetric cases. The effectiveness of the numerical method is further demonstrated through numerical comparisons with semi‐analytical solutions provided by a reduced order model. Copyright © 2015 John Wiley & Sons, Ltd.