LES初始速度场设置和壁面函数设置
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大家好。
最近开始做圆柱绕流，用的LES，pimpleFoam，雷诺数1e5有几个问题想要请教一下大家。-
看了tutorial，和前辈的算例。初始场速度的设置里头，有些设置是直接设置了速度的，比如LES pisoFoam里头motobike算例。
internalField uniform (20 0 0);
有一些则没有，例如同样文件夹下pitzDaily算例
internalField uniform (0 0 0);
不知道这两者对结果和计算速度是否有影响？ 应该如何设置才正确。 -
对于使用LES时的避免函数，在边界层控制在yplus小于1的情况下，是否不需要设置壁面函数？因为看到@random_ran 的贴里有设置，觉得有点困惑。
希望和大家一起探讨，共同进步！
谢谢 -
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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 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。 -
@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：- 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）
- 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！！！
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文件格式不支持上传，我重新上传了压缩文档。settings.zip
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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。 -
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。
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@cccrrryyy 非常感谢您的建议!
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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。 -
目前的时间步库朗数是控制在1.5以下。
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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！
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@random_ran 欢迎吃瓜
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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.
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@random_ran
问得好！- 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？
- 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- 现在这边的例子是光滑圆柱，如果要算看看很高兴分享。（how）
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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）
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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
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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.
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@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。 -
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.
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@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！！！！ -
@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。
