fluent用LES算水翼边界层选择中心差分引起的震荡如何消除

[cccrrryyy]

@心里的小漩涡 好奇FLUENT里面怎么实现这个操作？一直以为格式这一块FLUENT没法儿自定义。

[心里的小漩涡]

@cccrrryyy 直接用TUI命令：
solve/set/numerics/
一直回车到1-st order to higher order blending factor: 1就是纯二阶，0.98就是耦合0.02的一阶迎风
感觉有点像TVD格式，也不是很懂，导师之前教我的操作，具体也不是太懂

[cccrrryyy]

@心里的小漩涡 原来可以这样调！恩，这个很类似TVD/NVD了，有空我也去试试

[gengchen]

感谢老铁的建议，我查了查好像没有这种现成的耦合操作，bounded cd可能是最像的，但是经过计算发现在边界层应该是全部切换成迎风格式了，没有部分耦合这种机制。

在国外cfd论坛上找到一个相似的情况，幸运的是那个人好像不关注边界层就改成迎风就没有震荡了，我看有个人的回答也解释了为什么除了中心差分都算不太准边界层。。
The choice of the scheme for LES should be based on the numerical diffusion it introduces. Ideally you should use linear（中心差分）, or cubic. You need good and prett uniform mesh anyways for LES, since you are assuming the filter operator and the differential operator are commutative, which is not true on non-uniform grids! The filtered schemes are a last resort scheme in my view, if you can't really do better with your mesh. Upwind and linearUpwind, but also limitedLinear and QUICK should be avoided because they are too dissipative.
可惜就是无法消除震荡

[李东岳]

把这个盾体扰流的算例发给我我给你调节一下

[gengchen]

@李东岳 https://www.cfd-online.com/Forums/openfoam-solving/106700-dicretization-schemes-les-pitzdaily.html 李老师，这是钝体扰流那个原贴，里面好像发了几个算例文件，不过我Openfoam水平有限，您可以参考下帖子

[gengchen]

试了下，把水的粘性增大了一百倍，也就是雷诺数降了100倍，震荡就没有了，但是边界层整个就加厚了几十倍。。难道高雷诺数下解析边界层是无解的吗

[李东岳]

我又看了下，你这个是Fluent啊。

不涉密的话可以把网格发给我我用openfoam跑跑

[gengchen]

@李东岳 谢谢李老师 您有时间的话可以试试foam的中心差分格式，我用CFX和fluent都会有震荡。这是约450万的网格文件：https://cowtransfer.com/s/82c878e62de64d
介质为水，边界条件inlet 5m/s, outlet 静压45300 Pa，wall为滑移壁面，foilwall为无滑移壁面，symmetry面设为symmetry，LES模型用的WALE，时间步长用的5e-5

[李东岳]

Sorry that I am using OpenFOAM to set up the test case and my Chinese input is missing. How long does it take to observe the oscillation?

[gengchen]

@李东岳 李老师，我是用的超算3个节点，大概80核，定常SST20分钟，非定常中心差分只要算起来应该就有震荡，算到升力系数稳定的话可能要3个小时左右？（可以先用迎风差分）后面改差分格式是直接用前面算好的结果初始化，算个几十分钟（0.1s）就能看下大概结果。我想知道Openfoam有没有更好的差分方法可以在保证精度的同时抑制震荡

[李东岳]

@gengchen 在 fluent用LES算水翼边界层选择中心差分引起的震荡如何消除 中说：

目前fluent自带的差分格式包括upwind, 2nd upwind, 3ord MUSCL, quick, bound cd 和cd都试了，只有cd能比较精确的算准边界层的速度剖面和脉动强度

Regarding this, do you have any plots?

[gengchen]

李老师，可以先看下0.15弦长处的速度剖面，我是在这个位置插入一根沿z方向4mm线导出平均速度，x=0.0002m, y=-0.059182m, z=0.016964m到0.020964m，根据最大值无量纲化，图表附在文件里。velocity_profile_ldv_test_0.15C.zip

[李东岳]

@gengchen I cannot open opju document. Could you please upload an image?

[gengchen]

抱歉，李老师，那我直接把LDV的数据贴上吧，谢谢老师！

0.51874	0.02
0.54523	0.04
0.57821	0.06
0.60027	0.08
0.61619	0.1
0.63168	0.12
0.65259	0.16
0.67519	0.2
0.69035	0.24
0.71312	0.28
0.72578	0.32
0.75144	0.38
0.76757	0.42
0.78385	0.46
0.79825	0.5
0.81624	0.54
0.83174	0.58
0.86305	0.66
0.89698	0.74
0.9205	0.82
0.94188	0.9
0.96355	0.98
0.98842	1.14
0.99863	1.3
1.00000 1.46
0.99797	1.62
0.99706	1.78

[李东岳]

1. is it time-averaged U predicted by LES?
2. What is the maxima? the Umax in this line or in the computational domain?

Your results look quite good. Is it predicted by linear scheme? How about the other schemes, e.g., limited schemes?

I am runing LES simulation, but it is very slow. I try to limit Co under 0.6. Since the global Umax is around 10 m/s, it indicates $\mathrm{\Delta }t=2.6e-6$. What value is your global Umax?

Courant Number mean: 0.00518159 max: 0.59921
deltaT = 2.67779e-06
Time = 0.0168822

GAMG:  Solving for p, Initial residual = 0.0474207, Final residual = 0.00156874, No Iterations 2
time step continuity errors : sum local = 3.90116e-10, global = -6.12223e-14, cumulative = 5.73243e-11
DICPCG:  Solving for p, Initial residual = 0.0105353, Final residual = 9.99036e-07, No Iterations 608
time step continuity errors : sum local = 2.48515e-13, global = -3.59741e-15, cumulative = 5.73207e-11
ExecutionTime = 401.48 s  ClockTime = 421 s

fieldMinMax minMaxp write:
    min/max(mag(U)) = 0 10.5464

Courant Number mean: 0.0051816 max: 0.601813
deltaT = 2.61829e-06
Time = 0.0168848

GAMG:  Solving for p, Initial residual = 0.0479045, Final residual = 0.00156265, No Iterations 2
time step continuity errors : sum local = 3.72593e-10, global = 2.37179e-14, cumulative = 5.73444e-11
DICPCG:  Solving for p, Initial residual = 0.0102726, Final residual = 9.97099e-07, No Iterations 152
time step continuity errors : sum local = 2.38352e-13, global = 2.12724e-14, cumulative = 5.73657e-11
ExecutionTime = 402.54 s  ClockTime = 423 s

[gengchen]

@李东岳 辛苦李老师！
1、图中是fluent统计的时间平均速度 2. 最大值为这条线上的最大值
我的计算是这样，图中是中心差分的结果，但是会有震荡。其他结果，比如有界中心差分，会使边界层剖面有整体的改变，二阶迎风能稍微好一点，但是在边界层的粘性底层和过渡层偏差会比较大
Umax差不多11m/s，时间步长的话，因为隐式格式为了快速看结果，我用的5e-5，试了从1e-5到5e-5，结果基本不变。

[李东岳]

I did a rough calculation, my time step is 3.3e-6, Co = 0.6, my server takes 2 s to calculate one time step. If I want to calculate 0.5 s, it takes me $0.5/(3.3e-6)\ast 2/3600/24=2.5$ days to run, which looks too long for me.

Another point we need to discuss is that, if Umax is 11 m/s, to ensure Co < 1, you should use similar time step like mine.

试了从1e-5到5e-5

No matter 1e-5 or 5e-5, the Co is larger than 1. Unless time step is smaller than 5e-6, Co is smaller than 1.

Meanwhile, I heard that commercial software can employ much larger Co number. Since the code is not public to the users, I dont understand why.

[gengchen]

@李东岳 谢谢李老师的说明，我这边只试了中心差分Co<1的情况，确定是有震荡的，其他格式结果我还可以再减小时间步长测试下变化，您可以在确保稳定计算的情况增大下时间步长，简单看下foam的中心差分格式结果，有没有类似的不光滑速度等值面，或者破碎的震荡涡，不需要算那么长时间统计平均速度。有改善的话，我就全力研究争取用Openfoam计算。如果太麻烦的话就算啦，还是十分感谢李老师！

关于库朗数可以大于1的原因，我也不甚清楚，看有篇文献说测试平均库朗数到5还能保证计算结果，可能让我没有重视用小时间步计算。

[李东岳]

Yes. My OpenFOAM results show that oscillation occurs if linear scheme is used. Although this can be explained by theory, you know that. You can try other schemes like superbee which is quite sharp to verify if it predicts better results.

Now, I terminate my calculation.