LES初始速度场设置和壁面函数设置

[cccrrryyy]

原则上讲，初始条件不应该对最终结果有很大影响。从实际操作来讲，一个是LES本身是瞬态的，需要一定的时间达到稳态，初始条件会影响达到稳态所需要的时间，更接近最终结果的初始场自然会更快的到达稳态；另外一个是LES本身计算起来有时候容易不稳定，一个更接近最终结果的初始值有利于一定程度上抑制这种不稳定。从这个角度讲，初始场的设置最好是参考文献，再加上你自己的物理直觉。目的就是挑选一个尽可能接近最终结果的初始场。从科研兴趣的角度讲嘛，你完全可以两个都试一下，得到一个自己的结论。
yplus小于1不需要设置壁面函数。我自己做过一些壁面函数的粗略分析，现在基本上是用nutUSpaldingWallFunction，但我不做精细的管内流动，所以yplus很少做到1这么小。壁面函数建议参考这个。

[hoversoar]

@cccrrryyy 您好！非常感谢你的回答！
对于yplus小于1而言，我的case表面有很多细小的部分，所以有一些地方的max yplus会达到1.3-1.5这个范围。但是大部分区域和平均值又只有0.5不到。
所以我不知道是不是应该设置壁面函数来解决这个问题。

另外，如果使用LES Y+在2-5的范围内的话，是否可以通过添加壁面函数来使得网格数量减小呢？

[cccrrryyy]

@hoversoar yplus具体应该怎么弄我不清楚，你可以再问问看。印象中，一般的壁面函数都是针对buffer layer而设计的，因为这个是我们不能准确预测速度分布的地方。粘性层（yplus<5）和外面的（yplus>30左右）outer layer是有准确解的。你yplus这么小感觉不需要壁面函数吧？
你是现在的网格预测不准确还是怎么样？像你说的最大yplus也只是到了1.5，平均值都在0.5以下。这种量级如果预测结果和参考结果差别都很大的话，那应该不是壁面函数的问题吧。
我说的都只是大概，因为我不做精细的壁面处理，仅供参考。

[hoversoar]

@cccrrryyy
这两天算了一下初始速度场（0 0 0）和有速度的（1 0 0），发现定量的阻力和升力差别不是太大，确实是有初始速度的稳定起来相对快一些。
我现在遇到的主要有两个问题：

1. 在1w雷诺数下，普通圆柱绕流的阻力系数几乎为1. 据文献来看应该在1.1左右比较正确。3w雷诺数下，阻力系数为1.08左右，相对而言比较可以接受。（两个case的模型一样，用的模型是3w的，也就是说yplus在1w的case下会比较小，大概在0.4-0.5左右）
2. 表面有小凹槽的圆柱模型，3w雷诺数下，阻力系数却相对而言达到了1.2（试验和其他文献来看应该有所降低才对）。
   综上来看，确实不是yplus的原因。不过我也怀疑是不是我的设置有问题，导致结果有偏差。我这里传上来我的几个文件，如果您有空可否帮我看看什么地方的设置有偏差?
   非常感谢！！！

[hoversoar]

文件格式不支持上传，我重新上传了压缩文档。settings.zip

[cccrrryyy]

圆柱扰流之前在论坛看到也有人算，好像不太容易算准。大略看了一下你的设置，有以下几个问题，按我认为的重要性排列。
1、PIMPLE算法一般我会给至少20个nOuterCorrectors，然后2个nCorrectors。你的虽然是不可压的算例，不过nOuterCorrectors是不是也可以多给一些。现在每一个时间步都达到收敛要求了么？我觉得可以尝试pisoFoam，这样就不用管nOuterCorrectors，但相应的时间步可能需要小一点。
2、当前时间步你的CFL数是多少？
3、速度对流项的格式，LES还是尽量避免迎风吧。迎风总会有些数值耗散，会导致细节上有些地方不准确。中心差分最好，当然前提是网格质量要高，不然太容易发散。
你最好参考一下文献里面别人的设置，同样量级雷诺数下别人的入口条件、格式、网格等等，不然修改起来完全没有方向。

[random_ran]

看到有人讨论圆柱绕流就很激动。

可是我已经挖了太多坑还没填。关注中，加油。

[hoversoar]

@cccrrryyy 非常感谢您的建议!

1. 附上现在1w雷诺数的光滑圆柱绕流的例子的某个时间步的截图，用of里头的monitor监控的residual，我也一直很困惑Uz的曲线不像稳定的样子。
   我现在把2个nOuterCorrectors由2提高到5（从31秒开始），算个十秒左右看看结果。

2. 目前的时间步库朗数是控制在1.5以下。

3. 这个十秒计算过之后，我会把速度对流格式换成中心差分继续跑看看结果是否有改善，到时候再过来更新！

[hoversoar]

@random_ran 欢迎吃瓜

[random_ran]

前排小板凳，

附上现在1w雷诺数的光滑圆柱绕流的例子的某个时间步的截图，用of里头的monitor监控的residual，我也一直很困惑Uz的曲线不像稳定的样子。

为什么关注点再 U_z 不在 残差最大的 p 上投入精力？

我现在把2个nOuterCorrectors由2提高到5（从31秒开始），算个十秒左右看看结果。

这样做的原因是？ 为什么是从2到5？而不是其他的数字，比如7?

目前的时间步库朗数是控制在1.5以下。

这个十秒计算过之后，我会把速度对流格式换成中心差分继续跑看看结果是否有改善，到时候再过来更新！

这个是 case 是机密么？如果能简单运行，我也想试试你的 case.

[hoversoar]

@random_ran
问得好！

1. Uz太抢眼了。我看了下之前算的几个例子，发现p的残差几乎都在0.001这个量级。不知道怎么才能继续降下去？
2. 我一开始尝试增加到10个nOuterCorrectors, 发现第4~5个外循环开始残差下降不明显甚至开始上升，No Iteration也几乎为0。 为了节省时间所以设定到5。

PIMPLE: iteration 1
smoothSolver:  Solving for Ux, Initial residual = 0.000154843, Final residual = 2.69652e-07, No Iterations 1
smoothSolver:  Solving for Uy, Initial residual = 0.00295664, Final residual = 4.75419e-06, No Iterations 1
smoothSolver:  Solving for Uz, Initial residual = 0.00438083, Final residual = 8.2538e-06, No Iterations 1
GAMG:  Solving for p, Initial residual = 0.00648663, Final residual = 8.19988e-05, No Iterations 4
GAMG:  Solving for p, Initial residual = 0.000125942, Final residual = 4.5224e-05, No Iterations 1
time step continuity errors : sum local = 3.53308e-10, global = -1.25279e-13, cumulative = -2.33184e-10
GAMG:  Solving for p, Initial residual = 0.000354817, Final residual = 8.87088e-05, No Iterations 1
GAMG:  Solving for p, Initial residual = 0.000110154, Final residual = 9.47584e-07, No Iterations 8
time step continuity errors : sum local = 7.40376e-12, global = -2.0947e-13, cumulative = -2.33394e-10
PIMPLE: iteration 2
smoothSolver:  Solving for Ux, Initial residual = 1.15813e-06, Final residual = 1.15813e-06, No Iterations 0
smoothSolver:  Solving for Uy, Initial residual = 1.59673e-05, Final residual = 4.28047e-08, No Iterations 1
smoothSolver:  Solving for Uz, Initial residual = 3.989e-05, Final residual = 1.15289e-07, No Iterations 1
GAMG:  Solving for p, Initial residual = 0.00371771, Final residual = 8.15447e-05, No Iterations 3
GAMG:  Solving for p, Initial residual = 0.000113078, Final residual = 4.60568e-05, No Iterations 1
time step continuity errors : sum local = 3.60026e-10, global = 4.84724e-15, cumulative = -2.33389e-10
GAMG:  Solving for p, Initial residual = 0.000230915, Final residual = 6.19172e-05, No Iterations 1
GAMG:  Solving for p, Initial residual = 8.10529e-05, Final residual = 8.7777e-07, No Iterations 8
time step continuity errors : sum local = 6.86178e-12, global = 1.42834e-14, cumulative = -2.33375e-10
PIMPLE: iteration 3
smoothSolver:  Solving for Ux, Initial residual = 2.91121e-08, Final residual = 2.91121e-08, No Iterations 0
smoothSolver:  Solving for Uy, Initial residual = 3.97728e-07, Final residual = 3.97728e-07, No Iterations 0
smoothSolver:  Solving for Uz, Initial residual = 8.90301e-07, Final residual = 8.90301e-07, No Iterations 0
GAMG:  Solving for p, Initial residual = 0.000168175, Final residual = 1.98109e-05, No Iterations 1
GAMG:  Solving for p, Initial residual = 3.82521e-05, Final residual = 3.82521e-05, No Iterations 0
time step continuity errors : sum local = 2.99032e-10, global = -2.47605e-14, cumulative = -2.33399e-10
GAMG:  Solving for p, Initial residual = 4.55083e-05, Final residual = 4.55083e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 4.55083e-05, Final residual = 9.29993e-07, No Iterations 3
time step continuity errors : sum local = 7.27013e-12, global = -1.36329e-15, cumulative = -2.33401e-10
PIMPLE: iteration 4
smoothSolver:  Solving for Ux, Initial residual = 1.71081e-09, Final residual = 1.71081e-09, No Iterations 0
smoothSolver:  Solving for Uy, Initial residual = 2.12255e-08, Final residual = 2.12255e-08, No Iterations 0
smoothSolver:  Solving for Uz, Initial residual = 4.28694e-08, Final residual = 4.28694e-08, No Iterations 0
GAMG:  Solving for p, Initial residual = 3.37196e-05, Final residual = 3.37196e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 3.37196e-05, Final residual = 3.37196e-05, No Iterations 0
time step continuity errors : sum local = 2.63599e-10, global = -1.96671e-15, cumulative = -2.33403e-10
GAMG:  Solving for p, Initial residual = 3.64374e-05, Final residual = 3.64374e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 3.64374e-05, Final residual = 6.46133e-07, No Iterations 2
time step continuity errors : sum local = 5.05107e-12, global = 3.57324e-16, cumulative = -2.33402e-10
PIMPLE: iteration 5
smoothSolver:  Solving for Ux, Initial residual = 3.81718e-10, Final residual = 3.81718e-10, No Iterations 0
smoothSolver:  Solving for Uy, Initial residual = 3.41675e-09, Final residual = 3.41675e-09, No Iterations 0
smoothSolver:  Solving for Uz, Initial residual = 5.30364e-09, Final residual = 5.30364e-09, No Iterations 0
GAMG:  Solving for p, Initial residual = 2.63333e-05, Final residual = 2.63333e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 2.63333e-05, Final residual = 2.63333e-05, No Iterations 0
time step continuity errors : sum local = 2.05857e-10, global = 4.08418e-16, cumulative = -2.33402e-10
GAMG:  Solving for p, Initial residual = 2.98151e-05, Final residual = 2.98151e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 2.98151e-05, Final residual = 9.1923e-07, No Iterations 1
time step continuity errors : sum local = 7.18597e-12, global = 1.67701e-15, cumulative = -2.334e-10
PIMPLE: iteration 6
smoothSolver:  Solving for Ux, Initial residual = 1.86276e-10, Final residual = 1.86276e-10, No Iterations 0
smoothSolver:  Solving for Uy, Initial residual = 2.12072e-09, Final residual = 2.12072e-09, No Iterations 0
smoothSolver:  Solving for Uz, Initial residual = 1.8065e-09, Final residual = 1.8065e-09, No Iterations 0
GAMG:  Solving for p, Initial residual = 2.79915e-05, Final residual = 2.79915e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 2.79915e-05, Final residual = 2.79915e-05, No Iterations 0
time step continuity errors : sum local = 2.1882e-10, global = 1.66412e-15, cumulative = -2.33398e-10
GAMG:  Solving for p, Initial residual = 3.20665e-05, Final residual = 3.20665e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 3.20665e-05, Final residual = 7.84955e-07, No Iterations 1
time step continuity errors : sum local = 6.13629e-12, global = 2.44261e-15, cumulative = -2.33396e-10
PIMPLE: iteration 7
smoothSolver:  Solving for Ux, Initial residual = 1.86182e-10, Final residual = 1.86182e-10, No Iterations 0
smoothSolver:  Solving for Uy, Initial residual = 1.67569e-09, Final residual = 1.67569e-09, No Iterations 0
smoothSolver:  Solving for Uz, Initial residual = 1.11517e-09, Final residual = 1.11517e-09, No Iterations 0
GAMG:  Solving for p, Initial residual = 3.13138e-05, Final residual = 3.13138e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 3.13138e-05, Final residual = 3.13138e-05, No Iterations 0
time step continuity errors : sum local = 2.44792e-10, global = 2.48318e-15, cumulative = -2.33394e-10
GAMG:  Solving for p, Initial residual = 3.63778e-05, Final residual = 3.63778e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 3.63778e-05, Final residual = 7.91691e-07, No Iterations 1
time step continuity errors : sum local = 6.18895e-12, global = 3.12078e-15, cumulative = -2.3339e-10
PIMPLE: iteration 8
smoothSolver:  Solving for Ux, Initial residual = 1.92788e-10, Final residual = 1.92788e-10, No Iterations 0
smoothSolver:  Solving for Uy, Initial residual = 2.05456e-09, Final residual = 2.05456e-09, No Iterations 0
smoothSolver:  Solving for Uz, Initial residual = 9.56459e-10, Final residual = 9.56459e-10, No Iterations 0
GAMG:  Solving for p, Initial residual = 3.49563e-05, Final residual = 3.49563e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 3.49563e-05, Final residual = 3.49563e-05, No Iterations 0
time step continuity errors : sum local = 2.73267e-10, global = 3.06099e-15, cumulative = -2.33387e-10
GAMG:  Solving for p, Initial residual = 4.11002e-05, Final residual = 4.11002e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 4.11002e-05, Final residual = 7.86332e-07, No Iterations 1
time step continuity errors : sum local = 6.14706e-12, global = 3.60325e-15, cumulative = -2.33384e-10
PIMPLE: iteration 9
smoothSolver:  Solving for Ux, Initial residual = 2.26526e-10, Final residual = 2.26526e-10, No Iterations 0
smoothSolver:  Solving for Uy, Initial residual = 2.22568e-09, Final residual = 2.22568e-09, No Iterations 0
smoothSolver:  Solving for Uz, Initial residual = 8.66044e-10, Final residual = 8.66044e-10, No Iterations 0
GAMG:  Solving for p, Initial residual = 3.87527e-05, Final residual = 3.87527e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 3.87527e-05, Final residual = 3.87527e-05, No Iterations 0
time step continuity errors : sum local = 3.02945e-10, global = 3.67155e-15, cumulative = -2.3338e-10
GAMG:  Solving for p, Initial residual = 4.62632e-05, Final residual = 4.62632e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 4.62632e-05, Final residual = 8.09623e-07, No Iterations 1
time step continuity errors : sum local = 6.32914e-12, global = 4.16309e-15, cumulative = -2.33376e-10
PIMPLE: iteration 10
smoothSolver:  Solving for Ux, Initial residual = 2.52859e-10, Final residual = 2.52859e-10, No Iterations 0
smoothSolver:  Solving for Uy, Initial residual = 2.63933e-09, Final residual = 2.63933e-09, No Iterations 0
smoothSolver:  Solving for Uz, Initial residual = 8.84599e-10, Final residual = 8.84599e-10, No Iterations 0
GAMG:  Solving for p, Initial residual = 4.41963e-05, Final residual = 4.41963e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 4.41963e-05, Final residual = 4.41963e-05, No Iterations 0
time step continuity errors : sum local = 3.455e-10, global = 4.12535e-15, cumulative = -2.33372e-10
GAMG:  Solving for p, Initial residual = 5.2893e-05, Final residual = 5.2893e-05, No Iterations 0
GAMG:  Solving for p, Initial residual = 5.2893e-05, Final residual = 8.16536e-07, No Iterations 1
time step continuity errors : sum local = 6.38319e-12, global = 4.56977e-15, cumulative = -2.33367e-10
smoothSolver:  Solving for k, Initial residual = 0.000235852, Final residual = 4.67513e-07, No Iterations 1

3. 现在这边的例子是光滑圆柱，如果要算看看很高兴分享。（how）

[random_ran]

Uz太抢眼了。

不知道坐标系是如何定义？

我看了下之前算的几个例子，发现p的残差几乎都在0.001这个量级。

非常有意思的问题，我之前没怎么考虑过 压力 残差的取值问题。我也好好思考一下。

不知道怎么才能继续降下去？

减小时间步长？提高收敛准则？或者优化网格形态？...

我一开始尝试增加到10个nOuterCorrectors, 发现第4~5个外循环开始残差下降不明显甚至开始上升，No Iteration也几乎为0。 为了节省时间所以设定到5。

我之前也一直在用 piso， 不过是默认值，没有动，我得好好看看 piso 算法再来回答你的问题。

现在这边的例子是光滑圆柱，如果要算看看很高兴分享。（how）

• 我记得论坛里有人分享过 *.zip 包含算例和运行脚本，简单点的办法就是做相同的事情

• 稍微难一点的是可以把楼主的 case 文件，在层主的 compute 仓库下 caseIdoNotUnderstand

然后我们一起来 hack.

[hoversoar]

@random_ran
我上传了case在google drive , 点击链接！（github 文件太大，我放弃了...）
晚些我会更新一下最近算的这个case的结果。

[hoversoar]

更新一下。按照@cccrrryyy 的建议算了几天，虽然结果还是不尽如人意，还是贴上来和大家讨论下。

Step1，31-38s，把nOutercorrectors 从2改成5.

几点观察：

• 阻力系数，升力系数主要与Uy残差曲线有关
• 在34.5s处震荡减小，CdCl均明显下降。
• Uz曲线不稳定

[hoversoar]

Step2. 38.5-51s, fvscheme中的速度对流项 从upwind 改为 Gauss linear

• 残差相比迎风格式，有明显的量级上升。Uz 呈现出收敛的趋势，稳定了。
• Uy同样的在47s处出现振幅下降，CdCl下降。
• 把40s处当成稳定，分为两个区段来考虑。CdClSt系数如下表。

看趋势50.5s处出现第二次Uy残差的振幅下降，51s之后会怎样？我打算继续算下去10s左右看看是不是周期性的。

[random_ran]

楼主 mesh 太大了,我把设置文件拷贝下来,打算用很小的脚本来生成圆柱网格.

这样的话,别人可以更方便地帮助你.

我们就可以一起讨论了.

仓库地址: https://github.com/randomwangran/chigua

[hoversoar]

@random_ran这个mesh是用ICEM生成的，我已经转化到openfoam了。
你真的太nice了！！！！

[random_ran]

@hoversoar 台兄过奖了。我只是好奇罢了。自己玩的很杂，可是遇到能一起玩的，实在不易，所以慢慢来。我在找 of 的脚本，自动生成。最好简单可控，小 mesh 大家可以互相交流，大 mesh 大家自己留着做研究就好。

[李东岳]

科研整的挺好 老铁们 @random_ran 是这个领域的专家 hoversoar命中遇到天子

[hoversoar]

@东岳 对，抱紧大腿！怎么加表情 - -