Investigating the Effects of Injection Pipe Orientation on Mixing and Heat Transfer for Fluid Flow Downstream a T-Junction

At T-junctions, where hot and cold streams flowing in pipes join and mix, significant temperature fluctuations can be created in very close neighborhood of the pipe walls. The wall temperature fluctuations cause cyclical thermal stresses which may induce fatigue cracking. Temperature fluctuation is of crucial importance in many engineering applications and especially in nuclear power plants. This is because the phenomenon leads to thermal fatigue and might subsequently result in failure of structural material. Therefore, the effects of temperature fluctuation in piping structure at mixing junctions in nuclear power systems cannot be neglected. In nuclear power plant, piping structure is exposed to unavoidable temperature differences in a bid to maintain plant operational capacity. Tightly coupled to temperature fluctuation is flow turbulence, which has attracted extensive attention and has been investigated worldwide since several decades. The focus of this study is to investigate the effects of injection pipe orientation on flow mixing and temperature fluctuation for fluid flow downstream a T-junction. Computational fluid dynamics (CFD) approach was applied using STAR CCM+ code. Four inclination angles including 0 (90), 15, 30 and 45 degrees were studied and the mixing intensity and effective mixing zone were investigated. K-omega SST turbulence model was adopted for the simulations. Results of the analysis suggest that, effective mixing of cold and hot fluid which leads to reduced and uniform temperature field at the pipe wall boundary, is achieved at 0 (90) degree inclination of the branch pipe and hence may lower thermal stress levels in the structural material of the pipe. Turbulence mixing, pressure drop and velocity distribution were also found to be more appreciable at 0 (90) degree inclination angle of the branch pipe relative to the other orientations studied.

Towards the Deep Convergence of Communication and Computing in RAN: Scenarios, Architecture, Key Technologies, Challenges and Future Trends

To accommodate the diversified emerging use cases in 5G,radio access networks(RAN)is required to be more flexible,open,and versatile.It is evolving towards cloudification,intelligence and openness.By embedding computing capabilities within RAN,it helps to transform RAN into a natural cost effective radio edge computing platform,offering great opportunity to further enhance RAN agility for diversified services and improve users’quality of experience(Qo E).In this article,a logical architecture enabling deep convergence of communication and computing in RAN is proposed based on O-RAN.The scenarios and potential benefits of sharing RAN computing resources are first analyzed.Then,the requirements,design principles and logical architecture are introduced.Involved key technologies are also discussed,including heterogeneous computing infrastructure,unified computing and communication task modeling,joint communication and computing orchestration and RAN computing data routing.Followed by that,a VR use case is studied to illustrate the superiority of the joint communication and computing optimization.Finally,challenges and future trends are highlighted to provide some insights on the potential future work for researchers in this field.

Navier-Stokes Computation of Grid Fin Missile Using Hybrid Structured-Unstructured Grids

Viscous CFD computations are performed to predict the aerodynamic coefficients and flowfield for a missile with grid fins by using hybrid grids. The computations arc made at Math numbvrs of 0.7 and 2.0. Full N-S equations arc discretized into finite volume form and solved by an algorithm in LU-SGS. The comparisons between computation and experimental data are made, and the detailed flow structure near grid fin is shown and examined.

宽速域RANS-LES混合方法的发展及应用

传统的雷诺平均方法(RANS)已经不能满足大范围分离、激波振荡、压力脉动、动载荷等极端工况下的流动预测需求;大涡模拟(LES)、直接数值模拟(DNS)等方法资源耗费多、效率低,离工程湍流问题仍较为遥远。RANSLES混合方法结合了RANS高效率和LES高精度的特点,近期有望大规模应用到工程湍流问题中。首先对现有的RANS-LES混合方法进行了归类,对各自的构造思想、特点进行了分析。然后报告了脱体涡模拟(DES)类方法的发展历程和现状,讨论了使用DES类方法计算分离流动时,对流项离散格式对分离特性、小尺度结构及频谱特性等的影响,并构造了自适应耗散函数。最后介绍了近年来国内外RANS-LES混合方法在宽马赫数范围(马赫数从0.1到20)内的机理研究和工程应用。现有的以DES类方法为代表的RANS-LES混合方法能够较为精细地模拟非定常大分离流动中的复杂现象,但在计算效率等方面还有较大的改进空间;植入式DES方法在模拟全机带部件流动上具有较高的效率和模拟精度,是重要的发展方向。RANS-LES混合方法在动态失速、燃烧、气动弹性、气动噪声、气动光学等与非定常流动密切相关的方面也有广阔的应用前景。

嵌入式RANS/LES混合方法在槽道流动数值模拟中的应用

与传统整体式RANS/LES混合方法相比,嵌入式RANS/LES混合方法不再依靠改进后的湍流模型本身或网格疏密自动划分流场中的RANS区域和LES区域,而是通过对流动现象的预估,人为将LES区域嵌入到RANS区域,从而使RANS区过渡到LES区的交界面尽可能与流向垂直,方便合理附加额外的湍流脉动信息。这样做不但确保了LES在某些复杂流动区域彻底支配地位,更重要的是能够避免RANS区对LES区解析湍流发展的污染和抑制。将槽道流场划分为上游RANS区和下游LES区,在经典的SST-DES模型基础上,采用合成涡方法生成交界面的湍流信息,取得了良好的模拟结果。

高速滑行艇阻力性能RANS计算中网格影响因素

为了准确预报高速滑行艇的水动力与阻力特性,本文基于RANS方法和重叠网格技术针对某50kn高速单体滑行艇的缩比船模阻力特性开展了数值计算研究。分析了网格类型、船体边界层网格分布、船体表面网格尺度、流场背景域网格加密等网格相关因素对艇底气水混合物分布、兴波形状、船体姿态以及船体阻力预报结果的影响,并与船模阻力试验结果进行了比较。通过研究认为:开展高速滑行艇的运动与阻力性能RANS预报时,网格因素对模拟高速滑行艇边界层流动、艇体周围湍流流动和兴波特性,以及预报艇底气水分布、艇体航行姿态与阻力性能具有显著影响。

Stress-Blended Eddy Simulation of the Turbulent Flow Around Circular Cylinder with Dynamic Hybrid RANS/LES Model

In order to verify the effectiveness and superiority of the dynamic hybrid RANS/LES(DHRL)model,the flow around a cylinder with sinusoidal fluctuating velocity at the inlet was used as the test case.The latest computational fluid dynamics(CFD)model can flexibly choose any existing large-eddy simulation(LES)method combined with RANS method to calculate the flow field.In addition,the DLES model and DDES model are selected as typical representatives of the turbulence model to compare the capture ability of the flow field mechanism.The internal flow field including the y+value,velocity distribution,turbulent kinetic energy and vortex structures is comprehensively analyzed.Finally,the results show that the new model has enough sensitivity to capture the information of the flow field and has more consistent velocity distribution with the experimental value,which shows its potential in practical engineering applications to some extent.

Application of a RaNS and PF-Based Method to Study the Resistance and Motion of a Bulk Carrier

Resistance prediction of ships using computational fluid dynamics has gained popularity over the years because of its high accuracy and low cost. This paper conducts numerical estimations of the ship resistance and motion of a Japan bulk carrier model using SHIP_Motion, a Reynolds-averaged Navier–Stokes (RaNS)-based solver, and HydroSTAR, a commercial potential flow (PF)-based solver. The RaNS solver uses an overset-structured mesh and discretizes the flow field using the finite volume method, while the PF-based solver applies the three-dimensional panel method. In the calm water test, the total drag coefficient, sinkage, and trim were predicted using the RaNS solver following mesh dependency analysis, and the results were compared with the available experimental data. Next, calm water resistance was investigated for a range of Froude numbers. The added resistance in short-wave cases was simulated using both RaNS and PF solvers, and the results were compared. The PF solver showed better agreement with the RaNS solver for predicting motion responses than for predicting added resistance. While the added resistance results could not be directly validated because of the absence of experimental data, considering the previous accuracy of the RaNS solver in added resistance prediction and general added resistance profile of similar hull forms (bulk carriers), the prediction results could be concluded to be reliable.

RANS方法预报对转桨非定常水动力性能研究

对转螺旋桨的非定常激振力直接影响着对转桨的噪声性能及振动特性,准确预报对转桨的非定常性能可以更好地为对转桨设计工作服务。基于计算流体力学(CFD)中雷诺平均纳维-斯托克斯(RANS)方法,对对转桨非定常水动力性能预报方法进行研究。通过对前后桨区域及交界面处网格精细划分,结合滑移网格特点,考虑每一时间步内旋转角度的对应性,进行合理的计算设置,提高对转桨的非定常性能预报精度。进行了2对对转桨方案的非定常水动力性能数值模拟,与试验数据的比较分析验证了预报方法的准确性和可行性。

Application of the Fictitious Domain Method for Navier-Stokes Equations

To apply the fictitious domain method and conduct numericalexperiments, a boundary value problem for an ordinary differential equation is considered. The results of numerical calculations for different valuesof the iterative parameter τ and the small parameter ε are presented. Astudy of the auxiliary problem of the fictitious domain method for NavierStokes equations with continuation into a fictitious subdomain by highercoefficients with a small parameter is carried out. A generalized solutionof the auxiliary problem of the fictitious domain method with continuationby higher coefficients with a small parameter is determined. After all theabove mathematical studies, a computational algorithm has been developedfor the numerical solution of the problem. Two methods were used to solvethe problem numerically. The first variant is the fictitious domain methodassociated with the modification of nonlinear terms in a fictitious subdomain.The model problem shows the effectiveness of using such a modification. Theproposed version of the method is used to solve two problems at once that arisewhile numerically solving systems of Navier-Stokes equations: the problem ofa curved boundary of an arbitrary domain and the problem of absence of aboundary condition for pressure in physical formulation of the internal flowproblem. The main advantage of this method is its universality in developmentof computer programs. The second method used calculation on a uniform gridinside the area. When numerically implementing the solution on a uniformgrid inside the domain, using this method it’s possible to accurately take intoaccount the boundaries of the curved domain and ensure the accuracy of thevalue of the function at the boundaries of the domain. Methodical calculationswere carried out, the results of numerical calculations were obtained. Whenconducting numerical experiments in both cases, quantitative and qualitativeindicators of numerical results coincide.

Application of the Hierarchical Functions Expansion Method for the Solution of the Two Dimensional Navier-Stokes Equations for Compressible Fluids in High Velocity

This work presents a new application for the Hierarchical Function Expansion Method for the solution of the Navier-Stokes equations for compressible fluids in two dimensions and in high velocity. This method is based on the finite elements method using the Petrov-Galerkin formulation, know as SUPG (Streamline Upwind Petrov-Galerkin), applied with the expansion of the variables into hierarchical functions. To test and validate the numerical method proposed as well as the computational program developed simulations are performed for some cases whose theoretical solutions are known. These cases are the following: continuity test, stability and convergence test, temperature step problem, and several oblique shocks. The objective of the last cases is basically to verify the capture of the shock wave by the method developed. The results obtained in the simulations with the proposed method were good both qualitatively and quantitatively when compared with the theoretical solutions. This allows concluding that the objectives of this work are reached.

A Parallel Finite Element Algorithm for Navier-Stokes Equations

The authors propose a numerical algorithm for the two-dimensional Navier-Stokes equations written in stream function-vorticity formulation. The total time derivative term is treated with a first order characteristics method. The space approximation is based on a piecewise continuous finite element method. The proposed algorithm is used to simulate the mechanical aeration process in lakes. Such process is used to combat the degradation of the water quality due to the eutrophication phenomena. For this application high computing facilities and capacities are required. In order to optimize the computing time and make possible the simulation of real applications, the authors propose a parallel implementation of the numerical algorithm. The parallelization technique is performed using the Message Passing Interface. The efficiency of the proposed numerical algorithm is illustrated by some numerical results.

C-RAN为移动通信带来的机遇和挑战

C-RAN技术概念,在某种程度上"颠覆"了传统的移动通信网络架构,并由此产生新的网络建设模式和设备形态,为移动通信设备市场开辟了新的发展机遇。但也需要看到,C-RAN在具体技术环节上仍存在一些关键技术问题仍待攻克,这些问题解决的进度将直接影响C-RAN技术的应用和推广。

C-RAN系统中的软件基带信号处理技术

云无线接入网(C-RAN)是融合了集中化处理、协作无线电和实时云计算的无线接入网架构,不同于传统无线通信网络架构,其基带处理单元基于统一开放的软件无线电平台。本文从灵活性、实现复杂度、运算精确度、处理时延、吞吐量、资源利用率、能耗等方面对比C-RAN系统中的软件发送接收技术与传统基带信号处理技术,并以低密度奇偶校验码(LDPC)编码载波传输系统为典型案例,展示软件技术运算精确度高、性能优良,而传统技术处理时延低的特点。

Joint Resource Allocation and Coordinated Computation Offloading for Fog Radio Access Networks

The cloud radio access network(C-RAN) and the fog computing have been recently proposed to tackle the dramatically increasing traffic demands and to provide better quality of service(QoS) to user equipment(UE).Considering the better computation capability of the cloud RAN(10 times larger than that of the fog RAN) and the lower transmission delay of the fog computing,we propose a joint resource allocation and coordinated computation offloading algorithm for the fog RAN(F-RAN),which takes the advantage of C-RAN and fog computing.Specifically,the F-RAN splits a computation task into the fog computing part and the cloud computing part.Based on the constraints of maximum transmission delay tolerance,fronthaul and backhaul capacity limits,we minimize the energy cost and obtain optimal computational resource allocation for multiple UE,transmission power allocation of each UE and the event splitting factor.Numerical results have been proposed with the comparison of existing methods.

A Computing Resource Adjustment Mechanism for Communication Protocol Processing in Centralized Radio Access Networks

The centralized radio access cellular network infrastructure based on centralized Super Base Station(CSBS) is a promising solution to reduce the high construction cost and energy consumption of conventional cellular networks. With CSBS, the computing resource for communication protocol processing could be managed flexibly according the protocol load to improve the resource efficiency. Since the protocol load changes frequently and may exceed the capacity of processors, load balancing is needed. However, existing load balancing mechanisms used in data centers cannot satisfy the real-time requirement of the communication protocol processing. Therefore, a new computing resource adjustment scheme is proposed for communication protocol processing in the CSBS architecture. First of all, the main principles of protocol processing resource adjustment is concluded, followed by the analysis on the processing resource outage probability that the computing resource becomes inadequate for protocol processing as load changes. Following the adjustment principles, the proposed scheme is designed to reduce the processing resource outage probability based onthe optimized connected graph which is constructed by the approximate Kruskal algorithm. Simulation re-sults show that compared with the conventional load balancing mechanisms, the proposed scheme can reduce the occurrence number of inadequate processing resource and the additional resource consumption of adjustment greatly.

Virtualization Technology in Cloud Computing Based Radio Access Networks:A Primer

Since virtualization technology enables the abstraction and sharing of resources in a flexible management way, the overall expenses of network deployment can be significantly reduced. Therefore, the technology has been widely applied in the core network. With the tremendous growth in mobile traffic and services, it is natural to extend virtualization technology to the cloud computing based radio access networks(CCRANs) for achieving high spectral efficiency with low cost.In this paper, the virtualization technologies in CC-RANs are surveyed, including the system architecture, key enabling techniques, challenges, and open issues. The enabling key technologies for virtualization in CC-RANs mainly including virtual resource allocation, radio access network(RAN) slicing, mobility management, and social-awareness have been comprehensively surveyed to satisfy the isolation, customization and high-efficiency utilization of radio resources. The challenges and open issues mainly focus on virtualization levels for CC-RANs, signaling design for CC-RAN virtualization, performance analysis for CC-RAN virtualization, and network security for virtualized CC-RANs.

C-RAN的应用和发展趋势分析

C-RAN通过结合集中化的基带处理、高速的光传输网络和分布式的远端无线模块,形成绿色清洁、集中化处理、协作化无线电、云计算化的无线接入网构架。其在某种程度上"颠覆"了传统的移动通信网络架构,为移动通信设备市场开辟了新的发展机遇。通过对C-RAN的应用优势以及其发展趋势分析,为未来相关技术的应用与推广提供借鉴作用。

基于NOMA的F-RAN下的无线资源分配优化

在4G到5G过渡阶段,射频拉远头(remote radio head,RRH)和雾节点(fog access point,F-AP)并存的情况将越来越普遍,为了充分利用RRH和F-AP的通信资源,本文提出了一种基于非正交多址接入(non-orthogonal multiple access,NOMA)的雾无线接入网(fog radio access networks,F-RAN)下的新型卸载方案.该方案以最小化所有用户的最大任务卸载时延为优化目标,联合优化了资源块(resource block,RB)分配和上行NOMA用户发射功率.本文对优化问题进行解耦,首先基于超图的匹配算法解决了RB分配问题,然后利用序列二次规划(sequential quadratic programing,SQP)算法解决非凸的功率优化问题,最后通过对两算法进行迭代提出一种资源分配算法.仿真结果表明,本文提出的卸载方案及联合资源分配算法相比于已有的方案在用户时延上具有更好的性能.

斜流中船后螺旋桨水动力载荷实尺度计算分析

斜流条件下螺旋桨可能受到较大的侧向载荷,会严重影响船舶推进效率和尾轴的安全。本文采用RANS(Reynolds-Averaged Navier Stokes)方法,结合SST k-ω两方程湍流模型和滑动网格方法,对国际标模KCS斜流中船后螺旋桨水动力载荷进行实尺度数值计算,分析不同漂角时螺旋桨在纵向、横向、垂向3个方向上的力和力矩的变化规律。计算考虑±20°,±15°,±10°,±5°以及0°漂角,结果表明斜流对螺旋桨推进载荷和侧向载荷都有较大影响,且相对漂角具有不对称特点。通过单独桨叶的推力变化曲线分析漂角对不同相位角度处螺旋桨载荷的影响规律,结果显示在桨盘面下端附近桨叶载荷更为敏感于漂角变化。通过与其他文献数据对比,分析斜流下螺旋桨载荷的尺度效应,结果表明轴向载荷和垂向载荷系数的尺度效应较大。