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    k-omega 模型的网格无关性验证

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    • zousiyu
      zousiyu last edited by zousiyu

      各位老师,大家好!我用标准的 k-omega 模型计算了一个平面自由射流(plane free jet)的案例,分别采用了 4 种不同的网格计算。
      网格信息如下,
      7d88ca2f-e7af-4a3d-9d59-5986e40bbefd-image.png
      19fbfe3c-a53d-4b64-8527-32192c3d0ba2-image.png
      边界条件使用 fluent 常用的设置方式。
      43cd035b-2108-4608-b2b2-13026b8d5288-image.png
      离散格式如图,
      8f006db4-0302-42a5-adbb-be5c0eb542dc-image.png
      计算的残差,mesh 1 和 mesh 2 的 continuity 在 3e-3 左右,其他物理量的残差各异,且无法小于 1e-6。mesh 3 和 mesh 4 的 continuity 在 3e-6 到 6e-6 左右,其他物理量的残差小于 1e-6。
      但是这 4 种网格的计算结果并没有显示出网格无关性。结果如图所示,
      637579d2-911c-4918-90a6-c79a6e87e03a-image.png
      继续加密的 mesh 5 也没有呈现网格无关性。而 k-epsilon 模型非常容易达到网格无关性,mesh 1 到 mesh 4 的结果几乎没有区别。
      请教各位熟悉湍流模型的老师,k-omega 模型如何达到网格无关性。

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      • 李东岳
        李东岳 管理员 last edited by

        @zousiyu 你尝试更改过omega的进口条件么

        CFD课程 改成线上了 http://dyfluid.com/class.html
        CFD高性能服务器 http://dyfluid.com/servers.html

        zousiyu 1 Reply Last reply Reply Quote
        • zousiyu
          zousiyu @李东岳 last edited by

          @东岳 改成什么呀?

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          • 李东岳
            李东岳 管理员 last edited by

            omega公式中的$l$怎么选取de?

            CFD课程 改成线上了 http://dyfluid.com/class.html
            CFD高性能服务器 http://dyfluid.com/servers.html

            zousiyu 1 Reply Last reply Reply Quote
            • zousiyu
              zousiyu @李东岳 last edited by

              @东岳 $l=0.07L/C_{\mu}^{3/4}$。$L$是射流喷嘴的高度。

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              • zousiyu
                zousiyu last edited by

                @random_ran 老师您可以给一些意见吗?

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                • R
                  random_ran 副教授 last edited by

                  Sorry I had hard time to type Chinese.

                  Thanks for sharing this. Your finding is consistent with what I've
                  read this in Wilcox's book (P98).

                  In free shear flow, k-epsilon model is unaffected by the numerical
                  value of $k$ and $\epsilon$, but k-omega is very sensitive to the
                  choice of $\omega$.

                  But the book does not provide any suggestions on how to achieve mesh
                  independence on k-omega model.

                  Speaking to mesh independence study. The idea is simple: refining the
                  mesh at the region with high gradient. In reality, it depends on
                  researchers to define where are these regions. Even in the simple
                  context like flow over a circular cylinder. Different people choose
                  different types of the computational domain, different strategies to
                  generate the mesh, different parameters to build the block.

                  There are something I would investigate:

                  1. continue refine the mesh in the "slot exit"
                  2. expand the computational domain
                  3. expand the "slot exit" region, especially where the shear layers
                    are.
                  4. use unsteady solver
                  5. try LES if you have computational resource

                  Wilcox, D. C., & others, (1998). Turbulence modeling for cfd. : DCW
                  industries La Canada, CA.

                  Yours in CFD,

                  Ran

                  zousiyu 2 Replies Last reply Reply Quote
                  • zousiyu
                    zousiyu @random_ran last edited by zousiyu

                    @random_ran 我还有一个想法。书上都说 standard k-omega 模型是一个高雷诺数模型。我图上测试的是 Re=3000 的结果,无法达到网格无关解。我另外测试了一个 Re=16500 的结果,相同的网格,居然可以达到网格无关解。
                    ec39331b-e09d-4073-b095-4d1aaf904baa-image.png

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                    • zousiyu
                      zousiyu @random_ran last edited by

                      @random_ran 感谢老师的意见!

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                      • J
                        Juanyong last edited by

                        图很好看啊,是什么软件做的?

                        西湖冷月 zousiyu 2 Replies Last reply Reply Quote
                        • 西湖冷月
                          西湖冷月 @Juanyong last edited by

                          @Juanyong Origin出这种图应该很容易吧,哈哈

                          努力守恒。

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                          • zousiyu
                            zousiyu @Juanyong last edited by

                            @Juanyong 图是 MATLAB 绘制的,因为可编程才是最灵活的。参见我的文章,https://zhuanlan.zhihu.com/p/85068748

                            李东岳 1 Reply Last reply Reply Quote
                            • 李东岳
                              李东岳 管理员 @zousiyu last edited by

                              @zousiyu 图片风格确实不错

                              CFD课程 改成线上了 http://dyfluid.com/class.html
                              CFD高性能服务器 http://dyfluid.com/servers.html

                              zousiyu 1 Reply Last reply Reply Quote
                              • zousiyu
                                zousiyu @李东岳 last edited by

                                @东岳 那老师对这个问题有什么看法呢?standard k omega 在 Re=3000 时并不能表现出网格无关性,但相同的条件,在 Re=10000 下可以表现出网格无关性。

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                                • walten
                                  walten last edited by

                                  绘图matlab代码好评,已收藏,感谢楼主的干货分享。这种类型的图,excel是不是也能做出来,万年excel选手路过。。。。。。

                                  zousiyu 1 Reply Last reply Reply Quote
                                  • zousiyu
                                    zousiyu @walten last edited by

                                    @walten excel当然也可以,但是如果你有很多数据,一般都是编程处理比较方便。

                                    walten 1 Reply Last reply Reply Quote
                                    • walten
                                      walten @zousiyu last edited by

                                      @zousiyu
                                      一段程序可以处理所有的数据对吧,微调就可以了,改明儿试试

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