fluent求解器资料.docx

压力基求解器在压力基求解器中，控制方程是依次求解的。压力基求解器是从原来的分离式求解器发展来的，按顺序仪次求解动量方程、压力修正方程、能量方程和组分方程及其他标量方程，如湍流方程等，和之前不同的是，压力基求解器还增加了耦合算法，可以自由在分离求解和耦合求解之间转换， 需要注意的是，在压力基求解器中提供的几个物理模型，在密度基求解器中是没有的。这些物理模型包括：流体体积模型(VOF)，多项混合模型，欧拉混合模型，PDF燃烧模型，预混合燃烧模型，部分预混合燃烧模型，烟灰和NOx模型，Rosseland辐射模型，熔化和凝固等相变模型，指定质量流量的周期流动模型，周期性热传导模型和壳传导模型等。 与密度基求解器的区别：区别1：压力基求解器主要用于低速不可压缩流动的求解，而密度基求解器则主要针对高速可压缩流动而设计，但是现在两种方法都已经拓展成为可以求解很大流动速度范围的求解方法。两种求解方法的共同点是都使用有限容积的离散方法，但线性化和求解离散方程的方法不同。 区别2：密度基求解器从原来的耦合求解器发展来的，同时求解连续性方程、动量方程、能量方程和组分方程。然后依次再求解标量方程。（注：密度基求解器不求解压力修正方程，因为其压力是由状态方程得出的）。密度基求解器收敛速度快，需要内存和计算量比压力基求解器要大！ 特点：适用于压力基但不适用于密度基的模型： （1） 空化模型 （2） VOF模型 （3） Mixture多相流模型 （4） Eulerian多相流模型 （5） 非预混燃烧模型 （6） 预混燃烧模型 （7） 部分预混燃烧模型 （8） 组合PDF传输模型密度基求解器(Coupled Sover）是同时fluent求解连续方程、动量方程、能量方程及组分输运方程的耦合方程组，然后逐一地求解湍流标量方程。由于控制方程是非线性的，且相互之间是耦合的，因此，在得到收敛解之前，要经过多轮迭代: 1)根据当前的解的结果，更新所有流动变量。如果计算刚刚开始，则用初始值来更新。 2)同时求解连续方程、动量方程、能量方程及组分输运方程的耦合方程组（后两个方程视需要进行求解） 3)根据需要，逐一地求解湍流、辐射等标量方程。注意在求解之前，方程中用到的有关变量要用到前面得到的结果更新。 4)对于包含离散相的模拟，当内部存在相间耦合时，根据离散相的轨迹计算结果更新连续相的源项。 5)检查方程组是否收敛，若不收敛，回到第1）步，重新计算。 格式：密度基求解器有两种格式：隐式和显式。密度基显式与隐式求解器依次求解额外的标量方程（如湍流和辐射等）。两种格式求解器的主要不同点在于对于耦合方程的线性化上。由于隐式格式具有很好的稳定性，因此使用隐式求解器能够比显式格式更快的获得收敛的稳定解。然而，隐式格式要比显式格式消耗更多的内存。具体情况可以查看Fluent理论手册。 特点：对于高速可压流动，由强体积力(如浮力或者旋转力)导致的强耦合流动，或者在非常精细的网格上求解的流动，需要考虑密度基求解器。密度基求解器耦合了流动和能量方程，常常很快便可以收敛。密度基求解器所需要的内存约是压力基求解器的1.5到2倍，选择时可以根据这一情况来权衡利弊。在需要耦合隐式的时候，如果计算机内存不够，就可以采用压力基或密度基显式。密度基显式虽然也耦合了流动和能量方程，但是它还是比密度基隐式需要的内存少，当然它的收敛性也相应差一些。FLUENT求解器设置主要包括：1、压力-速度耦合方程格式选择2、对流插值 3、梯度插值 4、压力插值下面对这几种设置做详细说明。一、压力-速度耦合方程求解算法FLUENT中主要有四种算法：SIMPLE，SIMPLEC，PISO，FSM(1)SIMPLE(semi-implicit method for pressure-linked equations)半隐式连接压力方程方法，是FLUENT的默认格式。(2)SIMPLEC(SIMPLE-consistent)。对于简单的问题收敛非常快速，不对压力进行修正，所以压力松弛因子可以设置为1(3)Pressure-Implicit with Splitting of Operators (PISO)。对非定常流动问题或者包含比平均网格倾斜度更高的网格适用(4)Fractional Step Method (FSM)对非定常流的分步方法。用于NITA格式，与PISO具有相同的特性。二、对流插值（动量方程）FLUENT有五种方法：一阶迎风格式、幂率格式、二阶迎风格式、MUSL三阶格式、QUICK格式（1）FLUENT默认采用一阶格式。容易收敛，但精度较差，主要用于初值计算。（2）Power Lar.幂率格式，当雷诺数低于5时，计算精度比一阶格式要高。（3）二阶迎风格式。二阶迎风格式相对于一阶格式来说，使用更小的截断误差，适用于三角形、四面体网格或流动与网格不在同一直线上；二阶格式收敛可 能比较慢。（4）MUSL(monotone upstream-centered schemes for conservation laws).当地3阶离散格式。主要用于非结构网格，在预测二次流，漩涡，力等时更精确。（5）QUICK（Quadratic upwind interpolation）格式。此格式用于四边形/六面体时具有三阶精度，用于杂交网格或三角形/四面体时只具有二阶精度。三、梯度插值梯度插值主要是针对扩散项。FLUENT有三种梯度插值方案：green-gauss cell-based，Green-gauss node-based，least-quares cell based.（1）格林-高斯基于单元体。求解方法可能会出现伪扩散。（2）格林-高斯基于节点。 求解更精确，最小化伪扩散，推荐用于三角形网格上（3）基于单元体的最小二乘法插值。推荐用于多面体网格，与基于节点的格林-高斯格式具有相同的精度和格式。四、压力插值压力基分离求解器主要有五种压力插值算法。（1）标准格式（Standard)。为FLUENT缺省格式，对大表妹边界层附近的曲线发现压力梯度流动求解精度会降低（但不能用于流动中压力急 剧变化的地方此时应该使用PRESTO!格式代替）（2）PRESTO!主要用于高旋流，压力急剧变化流（如多孔介质、风扇模型等），或剧烈弯曲的区域。（3）Linear（线性格式）。当其他选项导致收敛困难或出现非物理解时使用此格式。（4）second order（二阶格式）。用于可压缩流动，不能用于多孔介质、阶跃、风扇、VOF/MIXTURE多相流。（5）Body Force Weighted体积力。当体积力很大时，如高雷诺数自然对流或高回旋流动中采用此格式。+=首先，所谓的steady和unsteady就表述了流动状态是否随时间变化的含义，这是定性的问题，你自己建立模型一定要明确的。这在计算过程中就是体现在方程不同，unsteady流多了时间变量，那么unsteady流就要进行时间离散。其次，两种求解的结果对比而言：一种情况是你要求解的物理问题是steady的，从理论上来说那么两种求解方式收敛之后的结果都是一样或者近似的。只不过需要注意的是，在unsteady的求解中dt的选取会影响你的计算结果，有可能会计算发散，而且还必须要足够的计算步达到收敛才能和steady的结果进行比较。换句话说，如果你用unsteady的方法去求解steady流，如果计算本身就没收敛，就取结果进行比较，那么肯定是不行的。一般而言，在进行unsteady求解的时候，前面一段时间的计算结果基本上是不予采用的，因为有一个数值收敛的过程。另一种是你要求解的物理问题是unsteady的，那么你用steady的求解方法得出的结果就是一堆垃圾了，没有任何价值。看你的帖子里面说有物体的移动，我不清楚具体的物理模型，但估计应该是unsteady流。generally, the default setting is choosing for solver：FLUENT provides three di erent solver formulations:segregatedcoupled implicitcoupled explicit(显式格式主要用于激波等波动解的捕捉问题)The segregated solver traditionally has been used for incompressible and mildly compressible flows. The coupled approach, on the other hand, was originally designed for high-speed compressible flows.By default, FLUENT uses the segregated solver, for high-speed compressible flows (as discussed above), highly coupled flows with strong body forces (e.g., buoyancy or rotational forces), or flows being solved on very fine meshes, you may want to consider the coupled implicit solver instead.For cases where the use of the coupled implicit solver is desirable, but your machine does not have sufficient memory, you can use the segregated solver or the coupled explicit solver instead.(explicit save memory use,but need more iterations for converged solution.Choosing the Discretization Scheme1)first-order upwind vs second-order upwindWhen the flow is aligned with the grid the first-order upwind discretization may be acceptable. For triangular and tetrahedral grids, since the flow is never aligned with the grid, you will generally obtain more accurate results by using the second-order discretization. For quad/hex grids, you will also obtain better results using the second-order discretization, especially for complex flows. For most cases, you will be able to use the second-order scheme from the start of the calculation. In some cases, however, you may need to start with the first-order scheme and then switch to the second-order scheme after a few iterations. For example, if you are running a high-Mach-number flow calculation that has an initial solution much different than the expected final solution, Finally, if you run into convergence diffculties with the second-order scheme, you should try the first-order scheme instead.2)Quick vs upwind(Quick适用于结构网格，流动方向与网格一致，对于非结构网格推荐用2阶迎风)The QUICK discretization scheme may provide better accuracy than the second-order scheme for rotating or swirling flows solved on quadrilateral or hexahedral meshes. For compressible flows with shocks, using the QUICK scheme for all variables, including density, is highly recommended for quadrilateral, hexahedral, or hybrid meshes.3)central-differencing scheme vs upwindThe central-differencing scheme is available only when you are using the LES turbulence model, and it should be used only when the mesh spacing(网格间距）is fine enough so that the magnitude of the local Peclet number (Equation 26.2-5) is less than 1.4)power law vs upwindA power law scheme is also available, but it will generally yield the same accuracy as the first-order scheme.Choosing the Pressure Interpolation Scheme(压力离散格式）a number of pressure interpolation schemes are available when the segregated solver is used in FLUENT. For most cases the standard(default) scheme is acceptable, but some types of models may benenit from one of the other schemes:For problems involving large body forces, the body-force-weighted scheme is recommended.For flows with high swirl numbers, high-Rayleigh-number natural convection, highspeed rotating flows, flows involving porous media, and flows in strongly curved domains, use the PRESTO! scheme. 对于可压流，应该使用二阶格式Use the second-order scheme for improved accuracy when one of the other schemes is not applicable.Choosing the Density Interpolation Scheme which is available at solve a single-phase compressible flow.If you are calculating a compressible flow with shocks, the first-order upwind scheme may tend to smooth the shocks; you should use the second-order-upwind or QUICK scheme for such flows.Choosing the Pressure-Velocity Coupling Method(压力速度方程耦合方法）SIMPLE vs. SIMPLECSIMPLE is the default, but many problems will benenit from the use of SIMPLEC, For relatively uncomplicated problems (laminar ows with no additional models activated) in which convergence is limited by the pressure-velocity coupling, you can often obtain a converged solution more quickly using SIMPLEC. With SIMPLEC, the pressurecorrection under-relaxation factor is generally set to 1.0, which aids in convergence speedup. In some problems, however, increasing the pressure-correction under-relaxation to 1.0 can lead to instability due to high grid skewness. For such cases, you will need to use one or more skewness correction schemes, use a slightly more conservative under-relaxation value (up to 0.7), or use the SIMPLE algorithm. The SIMPLEC skewness correction allows FLUENT to obtain a solution on a highly skewed mesh in approximately the same number of iterations as required for a more orthogonal mesh.Pressure-Implicit with Splitting of Operators (PISO)The PISO algorithm with neighbor correction is highly recommended for all transient flow calculations, especially when you want to use a large time step. (For problems that use the LES turbulence model, which usually requires small time steps, using PISO may result in increased computational expense, so SIMPLE or SIMPLEC should be considered instead.) PISO can maintain a stable calculation with a larger time step and an under-relaxation factor of 1.0 for both momentum and pressure.For steady-state problems, PISO with neighbor correction does not provide any noticeable advantage over SIMPLE or SIMPLEC with optimal under-relaxation factors.When you use PISO neighbor correction, under-relaxation factors of 1.0 or near 1.0 are recommended for all equations.If you use just the PISO skewness correction for highly-distorted meshes (without neighbor correction), set the under-relaxation factors for momentum and pressure so that they sum to 1 (e.g., 0.3 for pressure and 0.7 for momentum). If you use both PISO methods, follow the under-relaxation recommendations for PISO neighbor correction, above.Fractional Step MethodThe Fractional Step method (FSM) is available when you choose to use the NITA scheme, the FSM is slightly less computationally expensive compared to the PISO algorithm. For some problems (e.g., simulations that use VOF), FSM could be less stable than PISO.In most cases, the default values for the solution controls are enough to set a robust convergence of the internal pressure correction sub-iterations due to skewness. Only very complex problems (e.g., moving deforming meshes, sliding interfaces, the VOF model) could require a reduction of relaxation for pressure up to a value of 0.7 or 0.8.Setting Under-Relaxation Factors-the most important is pressure and momentum Under-Relaxation FactorsUnder-Relaxation Factors control the change of variable value produced during each iteration.the smaller value of Under-Relaxation Factors is set, the more stable iteration is got, but the harder convergence.It is good practice to begin a calculation using the default under-relaxation factors. If the residuals continue to increase after the first 4 or 5 iterations, you should reduce the under-relaxation factors.For most flows, the default under-relaxation factors do not usually require modification. If unstable or divergent behavior is observed, however, you need to reduce the underrelaxation factors for pressure, momentum, k, and from their default values to about 0.2, 0.5, 0.5, and 0.5.In problems where density is strongly coupled with temperature, as in very-high-Rayleigh-number natural- or mixed-convection flows, it is wise to also underrelax the temperature equation and/or density (i.e., use an under-relaxation factor less than 1.0).Conversely, when temperature is not coupled with the momentum equations (or when it is weakly coupled), as in flows with constant density, the under-relaxation factor for temperature can be set to 1.0.For other scalar equations (e.g., swirl, species, mixture fraction and variance) the default Setting Solution Controls for the Non-Iterative Solver under-relaxation may be too aggressive for some problems, especially at the start of the calculation. You may wish to reduce the factors to 0.8 to facilitate convergence.other usesChanging the Courant Number1)Courant Numbers for the Coupled Explicit Solver:in general, you can assume that the multi-stage scheme is stable for Courant numbers up to 2.5. The default CFL for the coupled explicit solver is 1.0, but you may be able to increase it for some 2D problems. You should generally not use a value higher than 2.0. If your solution is diverging, and your problemis properly set up and initialized, this is usually a good sign that the Courant number needs to be lowered. Depending on the severity of the startup conditions, you may need to decrease the CFL to a value as low as 0.1 to 0.5 to get started.2)Courant Numbers for the Coupled Implicit Solver:The default CFL for the coupled implicit solver is 5.0. It is often possible to increase the CFL to 10, 20, 100, or even higher, depending on the complexity of your problem.多重网格基本原理：微分方程的误差分量可以分为两大类，一类是频率变化较缓慢的低频分量；另一类是频率高，摆动快的高频分量。一般的迭代方法可以迅速地将摆动误差衰减，但对那些低频分量，迭代法的效果不是很显著。高频分量和低频分量是相对的，与网格尺度有关，在细网格上被视为低频的分量，在粗网格上可能为高频分量。多重网格方法作为一种快速计算方法，迭代求解由偏微分方程组离散以后组成的代数方程组，其基本原理在于一定的网格最容易消除波长与网格步长相对应的误差分量。该方法采用不同尺度的网格，不同疏密的网格消除不同波长的误差分量，首先在细网格上采用迭代法，当收敛速度变缓慢时暗示误差已经光滑，则转移到较粗的网格上消除与该层网格上相对应的较易消除的那些误差分量，这样逐层进行下去直到消除各种误差分量，再逐层返回到细网格上。FLUENT 中有四种多重网格循环：V，W，F 以及灵活(flex)循环。V 和W 循环可以用在AMG 和FAS 中，F 和灵活循环只限用于AMG 方法。（W 和灵活AMG 循环由于要花费大量的计算而不可用于解耦合方程组。），F 循环比V 循环需要更多的计算，但是比W 循环花费要少一些。但是它的收敛性比V 循环要好，大致和W 循环的收敛性差不多。对于耦合求解器设置来说，F 循环是默认的AMG 循环类型。灵活循环和V，W 循环之间的主要区别是：灵活循环会通过残差减小的公差和终止判据的满足情况来确定什么时候，按什么样的频率来处理每一层网格，而V 和W 循环则明确定义了各个层面之间的转换模式。灵活循环：当当前层面的误差减小速度不够快时，多重网格程序就会调用下一个网格层面的计算（restriction），。B 的值控制了处理的粗化网格层面的频率。默认值是0.7。如果b 的值较大，就会处理较小的频率，反之亦然。当校正解的误差减小到该网格层初始误差的某一分数a（在0 和1 之间）时，当前网格层上的校正方程就可以被认为是充分收敛了。参数a 被称为终止判据（termination）。默认值是0.1。FAS 优于AMG 方法的地方在于，对于非线性问题前者可以做得更好，这是因为系统的非线性可以通过重新离散传到粗糙层面；当使用AMG 时，一旦系统被线化，直到细化层面算子被更新，求解器才会“感觉到”非线性。Turning On FAS MultigridFAS multigrid is an optional component of the coupled explicit solver. For most problems, you can start out with 4 or 5 levels. For large 3D problems, you may want to add more levels. If you believe that multigrid is causing convergence trouble, you can decrease the number of levels.Initializing the Solutionyou can initialize the entire flow domain, also you can Patching Values in Selected Cells.Special Treatment for Strong Body Forces in Multiphase Flows