generally, the default setting is
choosing for solver：
FLUENT provides three di erent solver formulations:
segregated
coupled implicit
coupled explicit(顯式格式主要用于激波等波動解的捕捉問題)
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 Scheme
1)first-order upwind vs second-order upwind
When 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適用于結構網(wǎng)格，流動方向與網(wǎng)格一致，對于非結構網(wǎng)格推薦用2階迎風)
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 upwind
The central-differencing scheme is available only when you are using the LES turbulence model, and it should be used only when the mesh spacing(網(wǎng)格間距）is fine enough so that the magnitude of the local Peclet number (Equation 26.2-5) is less than 1.
4)power law vs upwind
A power law scheme is also available, but it will generally yield the same accuracy as the first-order scheme.
Choosing the Pressure Interpolation Scheme(壓力離散格式）
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.
對于可壓流，應該使用二階格式
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(壓力－速度方程耦合方法）
SIMPLE vs. SIMPLEC
SIMPLE 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 Method
The 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 Factors
Under-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 εfrom 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 uses
Changing the Courant Number
1)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 problem
is 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.
多重網(wǎng)格
基本原理：微分方程的誤差分量可以分為兩大類，一類是頻率變化較緩慢的低頻分量；另一類是頻率高，擺動快的高頻分量。一般的迭代方法可以迅速地將擺動誤差衰減，但對那些低頻分量，迭代法的效果不是很顯著。高頻分量和低頻分量是相對的，與網(wǎng)格尺度有關，在細網(wǎng)格上被
視為低頻的分量，在粗網(wǎng)格上可能為高頻分量。
多重網(wǎng)格方法作為一種快速計算方法，迭代求解由偏微分方程組離散以后組成的代數(shù)方程組，其基本原理在于一定的網(wǎng)格最容易消除波長與網(wǎng)格步長相對應的誤差分量。該方法采用不同尺度的網(wǎng)格，不同疏密的網(wǎng)格消除不同波長的誤差分量，首先在細網(wǎng)格上采用迭代法，當收斂速度變緩慢時暗示誤差已經(jīng)光滑，則轉移到較粗的網(wǎng)格上消除與該層網(wǎng)格上相對應的較易消除的那些誤差分量，這樣逐層進行下去直到消除各種誤差分量，再逐層返回到細網(wǎng)格上。
。FLUENT 中有四種多重網(wǎng)格循環(huán)：V，W，F(xiàn) 以及靈活("flex")循環(huán)。V 和W 循環(huán)可以用在AMG 和FAS 中，F(xiàn) 和靈活循環(huán)只限用于AMG 方法。（W 和靈活AMG 循環(huán)由于要花費大量的計算而不可用于解耦合方程組。），F(xiàn) 循環(huán)比V 循環(huán)需要更多的計算，但是比W 循環(huán)花費要少一些。但是它的收斂性比V 循環(huán)要好，大致和W 循環(huán)的收斂性差不多。對于耦合求解器設置來說，F(xiàn) 循環(huán)是默認的AMG 循環(huán)類型。
靈活循環(huán)和V，W 循環(huán)之間的主要區(qū)別是：靈活循環(huán)會通過殘差減小的公差和終止判據(jù)的滿足情況來確定什么時候，按什么樣的頻率來處理每一層網(wǎng)格，而V 和W 循環(huán)則明確定義了各個層面之間的轉換模式。
靈活循環(huán)：當當前層面的誤差減小速度不夠快時，多重網(wǎng)格程序就會調用下一個網(wǎng)格層面的計算（restriction），。B 的值控制了處理的粗化
網(wǎng)格層面的頻率。默認值是0.7。如果b 的值較大，就會處理較小的頻率，反之亦然。當校正解的誤差減小到該網(wǎng)格層初始誤差的某一分數(shù)a（在0 和1 之間）時，當前網(wǎng)格層上的校正方程就可以被認為是充分收斂了。參數(shù)a 被稱為終止判據(jù)（termination）。默認值是0.1。
FAS 優(yōu)于AMG 方法的地方在于，對于非線性問題前者可以做得更好，這是因為系統(tǒng)的非線性可以通過重新離散傳到粗糙層面；當使用AMG 時，一旦系統(tǒng)被線化，直到細化層面算子被更新，求解器才會“感覺到”非線性。
Turning On FAS Multigrid
FAS 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 Solution
you can initialize the entire flow domain, also you can Patching Values in Selected Cells.
Special Treatment for Strong Body Forces in Multiphase Flows：1）The Frozen Flux Formulation－－－This option is only available for single-phase transient problems that use the segregated iterative solver and do not use a moving/deforming mesh model.2）Time-Advancement Schemes contain two types：Iterative Time-Advancement Scheme－－－The iterative scheme is the default in FLUENT and Non-Iterative Time-Advancement Scheme－－－ FLUENT offers two versions of NITA schemes; the non-iterative fractional step method and the non-iterative PISO method .
用殘差光順的方法增加庫朗數(shù)
在Solution Controls（求解過程控制）面板中，殘差光順的迭代值在缺省設置中被設定為0，即在缺省設置中沒有使用殘差光順技術。如果將Iterations（迭代計數(shù)器）增加為1或更大的數(shù)，則可以進一步設置Smoothing Factor（光順因子）。將光順因子設定為0.5 可以將庫朗數(shù)增加為原數(shù)值的兩倍。
改變多步格式
首先啟動Multi-Stage Parameters（多步格式參數(shù)）面板：Solve->Controls->Multi-Stage...
在缺省設置中，F(xiàn)LUENT 使用5 步格式，每步的系數(shù)分別為0.25、0.166666、0.375、0.5 和1.0。在對多步格式非常熟悉的情況下可以增加多步格式的步數(shù)，同時修改每步的系數(shù)。修改系數(shù)的一般要求是：
（1）系數(shù)為介于0 和1 之間的實數(shù)。
（2）最后一步的系數(shù)必須為1。
使用無反射邊界條件
如果要采用無反射邊界條件，最好先在不使用這類邊界條件時將問題計算一遍，在獲得收斂解之后，再加入無反射邊界條件，繼續(xù)進行計算并再度獲得收斂解。使用無反射邊界條件的步驟如下：
（1）加入無反射邊界條件的文本命令如下：define->boundary-conditions->non-reflecting->enable? 如果不知道無反射邊界條件是否已經(jīng)加入計算可以用文本命令show-status 進行查看。
（2）無反射邊界條件初始化的文本命令如下：define->boundary-conditions->non-reflecting->initialize 初始化成功后，系統(tǒng)會顯示相應的系統(tǒng)信息。
（3）如果有必要，可以修改相關參數(shù)，相關的文本命令如下：define->boundary-conditions->non-reflecting->set 相關參數(shù)的含義為：
under-relaxation：設定亞松弛因子，缺省值為0.75。
discretization：設定離散格式，缺省為高階格式。
verbosity：設定信息長度，0 為不顯示，1 為顯示基本信息，2 為顯示詳細信息。
1. 在混合平面模型中使用無反射邊界條件
如果計劃在計算中同時采用無反射邊界條件和混合平面模型，則首先要將混合平面定義為壓強
2. 在并行版FLUENT 中使用無反射邊界條件
在無反射邊界條件與并行求解器共同使用時，采用無反射邊界條件的網(wǎng)格單元必須處于同一個分區(qū)中。為保證所有單元在同一個分區(qū)中，可以用人工方式進行網(wǎng)格分區(qū)。
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