LES初始速度场设置和壁面函数设置



  • 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。

    1. 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。

    2. 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


  • OpenFOAM讲师

    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



  • @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?


  • OpenFOAM讲师

    @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:

    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!!!


  • 文件格式不支持上传,我重新上传了压缩文档。settings.zip


  • OpenFOAM讲师

    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。



  • @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!

    Screenshot from 2020-02-27 11-33-27.png
    Screenshot from 2020-02-27 11-35-36.png



  • @random_ran 欢迎吃瓜



  • 前排小板凳,:chigua:

    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。
    :chigua:

    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.



  • @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
    
    
    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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