Fourth-Order Conservative Transport on Overset Grids Using Multi-Moment Constrained Finite Volume Scheme for Oceanic Modeling

With an increase in model resolution,compact high-order numerical advection scheme can improve its effectiveness and competitiveness in oceanic modeling due to its high accuracy and scalability on massive-processor computers.To provide high-quality numerical ocean simulation on overset grids,we tried a novel formulation of the fourth-order multi-moment constrained finite volume scheme to simulate continuous and discontinuous problems in the Cartesian coordinate.Utilizing some degrees of freedom over each cell and derivatives at the cell center,we obtained a two-dimensional(2D)cubic polynomial from which point values on the extended overlap can achieve fourth-order accuracy.However,this interpolation causes a lack of conservation because the flux between the regions are no longer equal;thus,a flux correction is implemented to ensure conservation.A couple of numerical experiments are presented to evaluate the numerical scheme,which confirms its approximately fourth-order accuracy in conservative transportation on overset grid.The test cases reveal that the scheme is effective to suppress numerical oscillation in discontinuous problems,which may be powerful for salinity advection computing with a sharp gradient.

Hermite-Discontinuous Galerkin Overset Grid Methods for the Scalar Wave Equation

We present high order accurate numerical methods for the wave equation that combines efficient Hermite methods with geometrically flexible discontinuous Galerkin methods by using overset grids.Near boundaries we use thin boundary fitted curvilinear grids and in the volume we use Cartesian grids so that the computational complexity of the solvers approaches a structured Cartesian Hermite method.Unlike many other overset methods we do not need to add artificial dissipation but we find that the built-in dissipation of the Hermite and discontinuous Galerkin methods is sufficient to maintain the stability.By numerical experiments we demonstrate the stability,accuracy,efficiency,and the applicability of the methods to forward and inverse problems.

An Overset Grid Method for Fluid-Structure Interaction

An overset grid methodology is developed for the fully coupled analysis of fluid-structure interaction (FSI) problems. The overset grid approach alleviates some of the computational geometry difficulties traditionally associated with Arbitrary-Lagrangian-Eulerian (ALE) based, moving mesh methods for FSI. Our partitioned solution algorithm uses separate solvers for the fluid (finite volume method) and the structure (finite element method), with mesh motion computed only on a subset of component grids of our overset grid assembly. Our results indicate a significant reduction in computational cost for the mesh motion, and element quality is improved. Numerical studies of the benchmark test demonstrate the benefits of our overset mesh method over traditional approaches.

Enhancement on parallel unstructured overset grid method for complex aerospace engineering applications

In the present study,an efficient overset grid method by means of parallel implicit hole-cutting is proposed for the sake of simulating unsteady flows in aerospace engineering involving multiple bodies in relative movement.In view of the degraded computational efficiency and robustness for conventional overset grid assembly,several innovative techniques are developed within the overset grid assembly process,viz.,a bookkeeping alternative digital tree method to speed up the donor-cell searching,a fast parallel advancing front algorithm to accelerate the wall-distance calculation and a message-passing strategy with efficient information communication and lower storage expenditure within distributed computational architecture.The contribution of the developed techniques is evidenced by comparison with the existing alternative ways in terms of computing efficiency.Subsequently,the overset grid method is embedded into an inhouse programed URANS solver to examine its capability in predicting the flow field of complex applications such as helicopter,store separation and component deploying.Results show that the developed overset grid methodology is,in practice,able to resolve the aerodynamic characteristics of complex aerospace engineering with a high-fidelity flow topology and accuracy.

CFD simulations targeting the performance of the NTNU BT1 wind turbine using overset grids

In the present study, the performance of the NTNU Blind Test 1 wind turbine is analyzed in the computational fluid dynamics (CFD) simulations by using the CFD code FANS with structured overset grids. First, the numerical methods including the governing equations, the turbulence closure model, and the flow solver are introduced. In addition, the NTNU BT1 wind tunnel experiment is described. Then, structured overset grid blocks are generated in the computational domain with fully resolved wind turbine geometry, including the blades, hub, nacelle, and tower. Afterward, unsteady Reynolds averaged Navier-Stokes (RANS) simulations with the two-layer k - ε turbulence model are performed with an inlet velocity of 10 m/s and a tip-speed ratio (TSR) of 6. The overset-grid capability of FANS is leveraged to handle the rotation of the rotor. Finally, simulations are performed for a range of TSRs and a comparison is made among the present CFD results, other numerical results obtained from representative methods, and the experimental data. It is observed that the CFD-predicted thrust coefficients match the experimental measurement at low TSRs while under-predicting the values at high TSRs, and potential reasons for this deviation are discussed.

Enhancement and Application of Overset Grid Assembly

In this article, the overset grid assembly method is improved to efficiently solve several critical problems that occur when applying overset grids to the complicated geometries and moving body. First, instead of using the two-step searching (i.e. cut and paste), a single-step searching method based on the grid cell size is proposed to modify holes and optimize the grid overlapping automatically. Second, discrepancies between the wall surface representations, where the grids overlap, are handled by introducing the wall's normal directions to the hole-map and projecting the interpolated points on the recipient mesh into the donor mesh. Finally, the dynamic overset method is modified to address the complex moving body problem. At every time step, the initial hole surface of the previous time step is dynamically adjusted to accomplish hole cutting and avoid the time consuming hole-map procedure. Numerical experiments show that the enhanced overset grid assembly method obtains satisfactory results.

Numerical Simulation of Rotor Flow Field Based on Overset Grids and Several Spatial and Temporal Discretization Schemes

A numerical method based on solutions of Euler/Navier-Stokes （N-S） equations is developed for calculating the flow field over a rotor in hover. Jameson central scheme, van Leer flux-vector splitting scheme, advection upwind splitting method （AUSM） scheme, upwind AUSM/van Leer scheme, AUSM＋ scheme and AUSMDV scheme are implemented for spatial discretization, and van Albada limiter is also applied. For temporal discretization, both explicit Runge-Kutta method and implicit lower-upper symmetric Gauss-Seidel （LU-SGS） method are attempted. Simultaneously, overset grid technique is adopted. In detail, hole-map method is utilized to identify intergrid boundary points （IGBPs）. Furthermore, aimed at identification issue of donor elements, inverse-map method is implemented. Eventually, blade surface pressure distributions derived from numerical simulation are validated compared with experimental data, showing that all the schemes mentioned above have the capability to predict the rotor flow field accurately. At the same time, vorticity contours are illustrated for analysis, and other characteristics are also analyzed.

Numerical simulation of rotor-airframe aerodynamic interaction based on unstructured dynamic overset grids

A method of unstructured dynamic overset grids is developed for the numerical simulation of helicopter unsteady rotorairframe aerodynamic interaction.For the effective treatment of the relative motion between the rotor and the airframe,the domain of flowfield is divided into two overset subzones,namely,a rotational subzone containing the blades and a stationary subzone containing the airframe.The overset part of two subzones is used to convect the flow variables of the two zones.The Taylor series expansion is used to obtain a second-order spatial accuracy,and dual-time stepping is adopted to improve the solution accuracy.Mesh deformation from the blade motion in forward flight is treated by using a spring analogy.Validation is made by numerically simulating the flows around a wind tunnel configuration and comparing the predicted time-averaged and instantaneous inflow and airframe surface pressure distributions with the experimental data.It shows that the present method is efficient and robust for the prediction of complicated unsteady rotor-airframe aerodynamic interaction phenomena.

Structure overset grid method and its applications to simulation of multi-body separation

This paper proposes an automatic structure overset grid method, which utilizes the hole-surface optimization with one-step searching, wall-surface grid oversetting, and dynamic overset grid approaches to achieve the high adaptability of overset grids for complex multi-body aircrafts. Specifically, based on the automatic structure overset grids, the method first solves the coupling of Navier-Stokes(N-S) unsteady flow equation and 6DOF motion equation, and establishes the multi-body collision model. Then, the numerical simulation of unsteady flow for complex aircrafts' multi-body separation, the simulation of multi-body separating trajectory and the separation safety analysis are accomplished. Thus, the method can properly handle practical engineering problems including the wing/drop tank separation, aircraft/mount separation, and cluster bomb projection. Experiments show that our numerical results match well with experimental results, which demonstrates the effectiveness of our methods in solving the multi-body separation problem for aircrafts with complex shapes.

A fast and reliable overset unstructured grids approach

A cell-centred overset unstructured grids approach is developed.In this approach,the intergrid boundary is initially established based on the wall distance from the cell centre,and is then optimized.To accelerate the intergrid-boundary definition much more,a neighbor-toneighbor donor search algorithm based on advancing-front method is modified with the help of minimum cuboid boxes.To simplify the communications between different grid cell types and to obtain second-order spatial accuracy,a new interpolation method is constructed based on linear reconstruction,which employs only one layer of fringe cells along the intergrid boundary.For unsteady flows with relative motion,the intergrid boundary can be redefined fast and automatically.Several numerical results show that the present dynamic overset unstructured grids approach is accurate and reliable.

基于计算流体动力学方法的两栖飞机规则波中运动特性研究

针对水陆两栖飞机加速起飞运动过程受波浪干扰问题,本文基于Star-CCM+平台数值模拟技术,引入SST k-ε湍流模型求解RANS方程,VOF方法捕捉自由液面,DFBI模块以及重叠网格技术模拟水上飞机六自由度运动,建立了一套波浪中水陆两栖飞机加速滑行起飞运动特性研究方案。以波长和波高为变量,探讨了加速滑行过程中上述变量对水上飞机升沉、纵摇运动响应以及机身重心处垂向加速度的影响规律。结果表明:随着波长增加,飞机升沉运动先增加后缓慢变化,飞机重心垂向加速度、纵摇角加速度和纵摇峰值集中在波长为2~3倍机身长度海况范围内;随着波高增大,飞机升沉、纵摇运动及重心垂向加速度最大值随之增大,且长波中的运动变化量较短波中更大;机翼气动升力数值模拟结果与计算值一致,该方案可用于水陆两栖飞机气动力相关计算。

基于重叠网格的固定翼无人机尾流冲浪参数研究

在编队飞行中合理利用尾流冲浪技术,可使得后机获取明显的增升性能。基于该原理,按照候鸟迁徙飞行时所排成“V”字形设计了两架固定翼无人机组成的编队;利用CFD绘制无人机与流体域的重叠网格,对该飞行编队进行了数值模拟;并对飞行编队的气动参数进行了分析。研究结果表明:在气动作用下,尾流冲浪效应对前机的影响很小,对后机的影响明显;双机纵向间隔和垂直间隔对后机的升力系数、阻力系数、升阻比、俯仰力矩等参数均有影响;后机在合适的位置和迎角下,升力系数可提升11.7%。

海洋盐度对水下航行器热尾流行为的影响

针对海洋中的盐度环境对于水下航行器热尾流浮升扩散的影响,本文基于重叠网格技术,建立了水下航行器热尾流三维模型,分别对热尾流在均匀盐度与分层盐度环境下的浮升扩散过程进行了数值模拟,采用浮频率数表征盐度环境特征,研究了海洋盐度对于热尾流的影响。结果表明:盐度分层环境对潜艇冷却水浮升具有明显抑制作用,该环境下海面温度特征随盐度梯度的增大而减弱。

基于重叠网格的滑行艇水动力性能预报

滑行艇航行时重力主要靠水动力升力支撑,其具有阻力小、速度快等优点,目前在娱乐、救援、军事等多种场景有着广泛应用。滑行艇在航行过程中处在滑行状态,纵摇、升沉运动幅值较常规排水型船舶高,尤其是在波浪中航行时会出现周期性出水、入水类似跳跃的现象。该文使用自主开发动态重叠网格技术,结合开源计算流体力学库OpenFOAM平台对标准的深V型滑行艇(generic prismatic planing hull,GPPH)进行了快速性、耐波性数值模拟,并将数值计算结果与模型试验结果进行对比,证明了使用动态重叠网格技术能较好地捕捉滑行艇大幅纵摇、升沉运动;艇体周期性出水、入水会在船底部产生周期性的大幅值抨击压力,文中的数值计算也可以得出合理的压力预报结果。

基于重叠网格法的圆柱涡激振动非线性特性研究

该文基于重叠网格法对在不同约化速度下质量比和阻尼比分别为2.6和0.0036的圆柱涡激振动进行数值模拟研究,系统地分析了其响应幅值、频率比、尾流漩涡脱落模式及其非线性特性。随着约化速度的增大,圆柱涡激振动响应共发生4次切换,但只有其中1次切换时横流向振动响应与涡流力系数的相位差时发生了突增变化,这次突增促使尾流漩涡由“2T”模式转变为“2P”模式。此外,任何约化速度下的圆柱涡激振动速度变化范围均小于其位移变化范围。当约化速度为4.0时,圆柱来流向和横流向的涡激振动均处于混沌状态;当约化速度为8.0时,圆柱横流向涡激振动同样处于混沌状态,但仍然具有自限性;在其他约化速度下,圆柱来流向和横流向的振动稳定且规律。

基于嵌套网格的对称水翼流致振动数值模拟

在锁频工况下,水力机械叶片/导叶涡激振动幅值急剧增强,可达到非锁频工况时幅值百倍量级.以对称水翼为研究对象,基于嵌套网格的分离式双向流固耦合计算策略,开展了锁频和非锁频工况下流致振动的数值模拟.结果表明,锁频工况,水翼呈扭转振型,尾迹区涡脱落展向呈同相位平行脱落;非锁频工况,水翼以拍频形式振动.预测得到的锁频与非锁频区水翼振动振幅的相对差异与实验情况基本一致,可为大变形工况下的振动特性评估提供指导.

Numerical Simulation about the Characteristics of the Store Released from the Internal Bay in Supersonic Flow

To understand the influence of the initial release conditions on the separation characteristics of the store and improve it under high Mach number(Ma=4)flight conditions,the overset grid method and the Realizable k−εturbulence model coupled with an equation with six degrees of freedom are used to simulate the store released from the internal bay.The motion trajectory and the attitude angle of the store separation under the conditions of different centroid,velocity,height and control measures are given by the calculated result.Through analysis,the position of the centroid will affect the separation of the store,which needs to be considered in the design.Increasing the launching height is conducive to the separation of the store.If the store has an initial velocity,it can leave the internal bay more quickly and reduce the probability of collision with the wall.Cylindrical rod and slanted aft wall control measures can improve the attitude of the store and make the store fall more smoothly.

基于重叠网格的桥墩防撞浮箱流场特性数值研究

为了揭示桥墩加装防撞浮箱后的水动力性能及流场特性,基于重叠网格方法,以典型的圆形自浮式浮箱-桥墩系统为例,采用雷诺应力(RSM)湍流模型,利用VOF方法捕捉自由液面,同时考虑浮箱升沉位移,建立了三维数值计算模型。通过与实验数据对比,验证了数值仿真方法的合理性和结果精确度;以重叠网格技术实现浮箱的升沉垂荡运动,研究了水流流速和水深变化对防撞浮箱的升沉垂荡、动力响应和流场特性的影响。研究结果表明:水流流速主要影响防撞浮箱的升沉位移值,而水深主要影响防撞浮箱的垂荡历程;防撞浮箱在3个方向上的力系数基本都是随水深的增加而下降,但较低流速时,力系数随水深的增加下降趋势较为平缓,纵向力系数随水深的增加先略微上升后再下降;防撞浮箱的垂荡与垂向力有关,较低流速下浮箱垂向力的变化周期随水深的增加而增大,较高流速下,防撞浮箱产生强烈的垂荡,周期变化规律完全消失。

基于静态重叠网格法的全附体潜艇的斜航仿真

潜艇在斜航状态下的水动力特性对于潜艇的水下作战性能有着重要的影响,为提高潜艇的作战性能,有必要对此进行分析。有鉴于此,结合SUBOFF潜艇模型的具体参数,基于静态重叠网格法,对全附体潜艇进行了斜航仿真,得到了斜航状态下的潜艇水动力特性,并将计算结果与实验进行了对比。对比结果显示,上述方法计算得到的位置导数与试验相比误差在允许范围之内,满足工程应用需求。得到了一些有价值的结论,对于指导真实潜艇水下斜航有一定的借鉴与参考价值。

基于重叠网格的翼型动态失速数值模拟

由于动态失速的相关特性及网格变形方法的限制性,采用重叠网格方法及广泛应用在分离流动问题中的SST k-ω湍流模型对NACA0012翼型开展了非定常动态失速的数值模拟研究,得到了动态运动过程中的气动力系数及翼型周围的流线图。研究结果表明采用的计算网格及方法不仅可以有效模拟翼型小幅振荡的动态特性,对大迎角俯仰运动的计算也具有良好的效果。同时能够精确的模拟出俯仰振荡过程中的气动力迟滞效应及大迎角动态失速时翼型前缘涡产生、脱落到再附的运动过程。