A numerical study on the fluid compressibility effects in strongly coupled fluid–solid interaction problems

Interactions between an incompressible fluid passing through a flexible tube and the elastic wall is one of the strongly coupled fluid–solid interaction (FSI) problems frequently studied in the literature due to its research importance and wide range of applications. Although incompressible fluid is a prevalent model in many simulation studies, the assumption of incompressibility may not be appropriate in strongly coupled FSI problems. This paper narrowly aims to study the effect of the fluid compressibility on the wave propagation and fluid–solid interactions in a flexible tube. A partitioned FSI solver is used which employs a finite volume-based fluid solver. For the sake of comparison, both traditional incompressible (ico) and weakly compressible (wco) fluid models are used in an Arbitrary Lagrangian–Eulerian (ALE) formulation and a PISO-like algorithm is used to solve the unsteady flow equations on a collocated mesh. The solid part is modeled as a simple hyperelastic material obeying the St-Venant constitutive relation. Computational results show that not only use of the weakly compressible fluid model makes the FSI solver in this case more efficient, but also the incompressible fluid model may produce largely unrealistic computational results. Therefore, the use of the weakly compressible fluid model is suggested for strongly coupled FSI problems involving seemingly incompressible fluids such as water especially in cases where wave propagation in the solid plays an important role.

     

A method to generate pressure gradients for molecular simulation of pressure-driven flows in nanochannels

One of the difficulties in molecular simulation of pressure-driven fluid flow in nanochannels is to find an appropriate pressure control method. When periodic boundary conditions (PBCs) are applied, a gravity-like field has been widely used to replace actual pressure gradients. The gravity-fed method is not only artificial, but not adequate for studying properties of fluid systems which are essentially inhomogeneous in the flow direction. In this paper, a method is proposed which can generate any desired pressure difference to drive the fluid flow by attaching a ??pump?? to the nanofluidic system, while the model is still compatible with PBCs. The molecular dynamics model based on the proposed method is applied to incompressible flows in smooth nanochannels, and the predicted velocity profiles are identical to those by the gravity-fed method, as expected. For compressible flows, the proposed model successfully predicts the changes of fluid density and velocity profile in the flow direction, while the gravity-fed method can only predict constant fluid properties. For fluid flows in nanochannels with a variable cross-sectional area, the proposed model predicts higher mass flow rates as compared to the gravity-fed method and possible reasons for the difference are discussed.     

Numerical simulation of a compressible turbulent boundary layer over a conductive wall with line heat source

A conjugated problem of supersonic turbulent flow over a conductive solid wall with an embedded line heat source has been investigated as a model of a separation detector and skin friction gage. The 2-D Navier-Stokes equations for compressible fluid, including a two layer eddy viscosity model, are solved simultaneously with the heat transfer equation for the solid, written in general coordinates. The effect of the interface boundary condition on the stability of the implicit scheme of the flow field has been checked. A careful investigation of the effect of heat source strength, solid and fluid conductivity and Mach and Reynolds numbers on flow and temperature fields has been performed. The results of this investigation may be used to design an optimal gage with a minimum influence on the flow field.     

Numerical solutions of unsteady flows with low inlet Mach numbers

This study deals with a numerical solution of a 2D unsteady flow of a compressible viscous fluid in a channel for low inlet airflow velocity. The unsteadiness of the flow is caused by a prescribed periodic motion of a part of the channel wall with large amplitudes, nearly closing the channel during oscillations. The channel is a simplified model of the glottal space in the human vocal tract and the flow can represent a model of airflow coming from the trachea, through the glottal region with periodically vibrating vocal folds, and to the human vocal tract.     

SPH simulation of selective withdrawal from microcavity

The selective withdrawal of weakly compressible fluids is investigated by smoothed particle hydrodynamics (SPH) with a revised model of surface tension. In our model problem, fluid is withdrawn from a two-dimensional microcavity through a narrow outlet above the interface of two immiscible fluids. The outflow boundary is implemented by a particular zone of fluid particles with prescribed velocity, together with the introduction of artificial boundary particles. Based on the average number density of fluid particles, the effective contribution of boundary particles is corrected for the compressible context. It is found that there exists a critical withdrawal rate for each initial interface height, beyond which the lower phase becomes entrained in a thin spout along with the upper phase. Besides, the Froude number with redefinition for this kind of multiphase flow could serve as a criterion of flow behavior. Furthermore, larger surface tension, smaller dynamical viscosity and density of the upper phase all lead to longer threshold time of formation of the spout state, and thus are favorable to the withdrawal of upper phase both in terms of higher efficiency and larger quantity.     

Implicit methods for viscous incompressible flows

Numerical methods and simulation tools for incompressible flows have been advanced largely as a subset of the computational fluid dynamics (CFD) discipline. Especially within the aerospace community, simulation of compressible flows has driven most of the development of computational algorithms and tools. This is due to the high level of accuracy desired for predicting aerodynamic performance of flight vehicles. Conversely, low-speed incompressible flow encountered in a wide range of fluid engineering problems has not typically required the same level of numerical accuracy. This practice of tolerating relatively low-fidelity solutions in engineering applications for incompressible flow has changed. As the design of flow devices becomes more sophisticated, a narrower margin of error is required. Accurate and robust CFD tools have become increasingly important in fluid engineering for incompressible and low-speed flow. Accuracy depends not only on numerical methods but also on flow physics and geometry modeling. For high-accuracy solutions, geometry modeling has to be very inclusive to capture the elliptic nature of incompressible flow resulting in large grid sizes. Therefore, in this article, implicit schemes or efficient time integration schemes for incompressible flow are reviewed from a CFD tool development point of view. Extension of the efficient solution procedures to arbitrary Mach number flows through a unified time-derivative preconditioning approach is also discussed. The unified implicit solution procedure is capable of solving low-speed compressible flows, transonic, as well as supersonic flows accurately and efficiently. Test cases demonstrating Mach-independent convergence are presented.     

Asymptotic and finite element approximations for heat transfer in rotating compressible flow over an infinite porous disk

The laminar boundary layer equations for the compressible flow due to the finite difference in rotation and temperature rates are solved for the case of uniform suction through the disk. The effects of viscous dissipation on the incompressible flow are taken into account for any rotation rate, whereas for a compressible fluid they are considered only for a disk rotating in a stationary fluid. For the general case, the governing equations are solved numerically using a standard finite element scheme. Series solutions are developed for those cases where the suction effect is dominant. Based on the above analytical and numerical solutions, a new asymptotic finite element scheme is presented. By using this scheme one can significantly improve the pointwise accuracy of the standard finite element scheme.     

Using exact Jacobians in an implicit Newton–Krylov method

In an implicit Newton–Krylov method for inviscid, compressible fluid flow, the derivation of the analytic flux Jacobian can become quite complicated depending on the complexity of the numerical flux calculation. Practically, the derivation of the exact Jacobian by hand is an unrealistic option because of the enormous man-hour investment needed. In this work, automatic differentiation is used to evaluate the exact Jacobian of upwind schemes implemented in the flow solver QUADFLOW. We compare the use of exact Jacobians and Jacobians numerically approximated by first-order forward differences. For a two-dimensional airfoil under three different flight conditions (quasi-incompressible flow, compressible subsonic flow, and transonic flow), we show that the robustness and performance of the present finite volume scheme is significantly improved by using exact Jacobians.     

A numerical modeling of multicomponent compressible flows in porous media with multiple wells by an Eulerian-Lagrangian method

We develop an Eulerian-Lagrangian numerical model for the simulation of fully miscible, highly compressible, multicomponent fluid flow processes through compressible porous media with multiple injection and production wells. We describe the numerical schemes, the treatment of the multiple injection and production wells, problems related to characteristic tracking, and other issues. We perform numerical experiments to investigate the performance of the numerical model. These results show that the numerical model generates robust, stable, and physically reasonable simulations without nonphysical oscillation or excessive numerical diffusion, even in the presence of multiple injection and production wells and the use of large time steps and coarse spatial grids. Finally, numerical experiments to well known test problems show that the numerical model does not generate noticeable grid orientation effect. Communicated by: G. Wittum     

Computational challenges of viscous incompressible flows

Over the past 30 years, numerical methods and simulation tools for incompressible flows have been advanced as a subset of the computational fluid dynamics (CFD) discipline. Although incompressible flows are encountered in many areas of engineering, simulation of compressible flow has been the major driver for developing computational algorithms and tools. This is probably due to the rather stringent requirements for predicting aerodynamic performance characteristics of flight vehicles, while flow devices involving low-speed or incompressible flow could be reasonably well designed without resorting to accurate numerical simulations. As flow devices are required to be more sophisticated and highly efficient, CFD tools become increasingly important in fluid engineering for incompressible and low-speed flow. This paper reviews some of the successes made possible by advances in computational technologies during the same period, and discusses some of the current challenges faced in computing incompressible flows.     

基于新型智能探头的大气数据系统算法研究

开展了基于一种新型半圆柱形探头的大气数据系统算法研究工作。通过计算流体力学(CFD)软件计算获得了半圆柱形探头本体和探头所在处的局部流场信息,建立了该种大气数据系统的气动模型和系统算法模型,并通过仿真验证了算法模型的精度。该项研究工作对大气数据系统及大气数据系统传感器的设计开发具有较高的借鉴和参考价值。     

Thermal Investigation for Solid Rocket Motor Nozzle Inserts Material

A time-resolved numerical computational approach, involving the combustion of double-base propellant is performed on thermal protection material for SRM nozzle. An implicit Navier-Stokes Solver is selected to simulate two-dimensional axial-symmetric unsteady turbulent flow of compressible fluid. The governing equations are discredited by using the finite Volume method. S-A turbulence model is employed. CFD scheme is implemented to investigate the temperature distribution causes at nozzle throat inserts comp...     

Two dimensional shape optimization using partial control and finite element method for compressible flows

The objective of this study is to determine the two dimensional shape of a body located in a compressible viscous flow, where the applied fluid force is minimized. The formulation to obtain the optimal shape is based on an optimal control theory. An optimal state is defined as a state, in which the performance function defined as the integration of the square sum of the applied fluid forces is minimized due to a reduction in the applied fluid forces. Compressible Navier–Stokes equations are treated as constraint equations. In other words, the body is considered to have a shape that minimizes the fluid forces under the constraint of the Navier–Stokes equations. The gradient of the performance function is computed using the adjoint variables. A weighted gradient method is used as the minimization algorithm. The volume of the body is assumed to be the same as that of the initial body. In the case of the algorithm used in this study, both the creation of a structured mesh around the surface of the body and the smoothing procedure are employed for the computation of gradient. In this study, a remeshing technique based on the structured mesh around the body changing its configuration in the iteration cycle is employed. For the correction to keep the volume constant, the surface coordinates are moved along the radial direction. For the discretization of both the state and adjoint equations, the efficient bubble function interpolation presented previously by the authors [18] is employed. The algorithm, which is known as the partial control algorithm, is applied to the numerical procedure to determine the movement of the coordinates. In the case of the gradient method, in order to avoid the convergence of the final shape to the local minimum shape, the new algorithm, which is called the partial control algorithm, is presented in this study. In numerical studies, the shape determination of a body in a uniform flow field is carried out in 2D domains. The initial shape of the body is assumed to be an elliptical cylinder. The shape is modified by minimizing the applied fluid forces. Finally, the desired shape of a body, whose performance function is reduced and converged to a constant value, is obtained. By carrying out a procedure that involves the use of the partial control algorithm, the desired shape of a body, whose performance function is reduced further, is obtained. Stable shape determination of a body in a compressible viscous flow is carried out by using the presented method. It is indicated that the optimal shape can be obtained by using the partial control algorithm.     

湍流燃烧反应数值模拟的研究与实现

基于开源计算流体力学平台OpenFOAM和化学动力反应模型库Cantera设计出定常可压缩的湍流燃烧反应解算器,使用该解算器对Sydney钝体驻定火焰HM1进行数值模拟,模拟采用煤气和空气的详细反应机理,并根据计算结果得到燃烧流动组分浓度分布图和温度曲线变化图.通过计算结果与实验数据对比分析表明,模拟效果较好的符合燃烧组分变化的研究要求,这说明设计的解算器对定长可压缩燃烧流动问题有很好的计算仿真效果,体现了其可行性.湍流燃烧流动解算器的设计对于燃烧室性能预估有一定的参考价值.     

Development of a MEMS-based control system for compressible flow separation

A MEMS-based sensor and actuator system has been designed and fabricated for separation control in the compressible flow regime. The MEMS sensors in the system are surface-micromachined shear stress sensors and the actuators are bulk-micromachined balloon vortex generators (VGs). A three-dimensional (3-D) wing model embedded with the shear stress sensors and balloon VGs was tested in a transonic wind tunnel to evaluate the performance of the control system in a range of Mach number between 0.2 and 0.6. At each Mach number tested, the shear stress sensors quantify the boundary layer on the surface of the wing model while the balloon VGs interact with the boundary layer in an attempt to provide flow control. The shear stress measurements indicate the presence of a separated flow on the trailing ramp section of the wing model at all Mach numbers tested when the balloon VGs are not activated. This result is confirmed by total pressure measurements downstream from the wing model where a wake profile is observed. When the balloon VGs are activated, the shear stress level on the trailing ramp increases with the Mach number. At the highest Mach number tested, this increase elevates the shear stress on the ramp to almost the same level as the unseparated flow, suggesting the possibility of a boundary layer reattachment. This result is supported by the downstream pressure measurements which show a large pressure recovery when the balloon VGs are activated. The wind tunnel experiment successfully demonstrated two aspects of the MEMS flow control system: the effectiveness of the microshear stress sensors in measuring the separation characteristics of a high-speed compressible flow and the ability of the microballoons in positively enhancing the aerodynamic performance of a high-speed wing through boundary layer modification.     

分布式功能块控制应用的性能分析

IEC61499功能块逐渐被工业采纳。本文针对分布式功能块控制应用（DFBCA）缺乏性能分析方法的情况，提出了一种基于随机Petri网的DFBCA性能分析方法。该方法以DFBCA的运行状态为着手点，利用Petri网易于表示系统中可能发生的各种状态变化及其关系的特点，将DFBCA转换为随机Petri网模型。再利用随机Petri网模型与马尔可夫链（MC）同构的特征，将随机Petri网模型转换为MC。得到的MC为DFBCA的性能分析提供了数学基础。最后基于MC的状态转移矩阵和稳态概率，对在每个状态中的驻留时间、变迁的利用率、变迁的标记流速、子系统延时时间等性能指标进行了分析。通过具体的示例说明了这种性能分析方法的可行性。     

A generalized macroscopic model for sound-absorbing poroelastic media using the homogenization method

This paper proposes a new macroscopic model for sound-absorbing poroelastic media which is derived by using the homogenization theory based on the method of asymptotic expansions. The derivation of the macroscopic properties and governing equations takes into account the multiphysics occurring in poroelastic media for sound absorption, including elastic motions of the solid phase, compressible viscous fluid flow, and the distributions of pressure and temperature in the fluid phase. The coupled effects between the elastic solid and the fluid pressure, and the temperature and the fluid pressure are also considered. In contrast to the conventional Biot’s model, which includes heuristic formulae, the proposed method yields a rigorous model that is consistent with the principal governing equations on the microscopic scale. Utilizing several models that have simple microscopic geometry and comparing the numerical solutions obtained using the proposed method with corresponding analytical solutions, we demonstrate that the derived macroscopic governing equations can provide accurate and effective predictions.     

Parallel simulation of three-dimensional complex flows: Application to two-phase compressible flows and turbulent wakes

In this paper, we present parallel simulations of three-dimensional complex flows obtained on an ORIGIN 3800 computer and on homogeneous and heterogeneous (processors of different speeds and RAM) computational grids. The solver under consideration, which is representative of modern numerics used in industrial computational fluid dynamics (CFD) software, is based on a mixed element-volume method on unstructured tedrahedrisations. The parallelisation strategy combines mesh partitioning techniques, a message-passing programming model and an additive Schwarz algorithm. The parallelisation performances are analysed on a two-phase compressible flow and a turbulent flow past a square cylinder.     

Topology optimization applied to the design of 2D swirl flow devices

The design of fluid devices, such as flow machines, mixers, separators, and valves, with the aim to improve performance is of high interest. One way to achieve it is by designing them through the topology optimization method. However, there is a specific large class of fluid flow problems called 2D swirl flow problems which presents an axisymmetric flow with (or without) flow rotation around the axisymmetric axis. Some devices which allow such simplification are hydrocyclones, some pumps and turbines, fluid separators, etc. Once solving a topology optimization problem for this class of problems using a 3D domain results in a quite high computational cost, the development and use of 2D swirl models is of high interest. Thus, the main objective of this work is to propose a topology optimization formulation for 2D swirl flow fluid problem to design these kinds of fluid devices. The objective is to minimize the relative energy dissipation considering the viscous and porous effects. The 2D swirl laminar fluid flow modelling is solved by using the finite element method. A traditional material model is adopted by considering nodal design variables. An interior point optimization (IPOPT) algorithm is applied to solve the optimization problem. Numerical examples are presented to illustrate the application of this model for various 2D swirl flow cases.     

可压缩流体网络一种改进的建模方法

给出了可压缩流体网络支路特性的通用表现形式，进而在已有方法的基础上给出了一种改进的可压缩流体网络的整体模型及其求解方法。