求助，进出口速度不一样

血色蔷薇

最近编了一个计算程序，想模拟矩形通道内的流动（square  duct），入口和出口条件都是压力边界条件，但发现了一个问题，在迭代第一次时，进出口速度基本相同，但迭代50次后进出口速度差了1000倍以上，进口速度稳定在10E-2的量级，出口速度却是10E-6量级...我用的D3Q15模型，1.3的入口密度，1.0的出口密度，相对动力粘度u=0.2，tua值随着密度的变化而计算...网格80X30X30...请问各位大虾...有没有什么好的建议解决这个问题？我菜鸟，勿笑...

luo@odu.edu

Have you checked mass and momentum conservation locally and globally? If not, why?!

血色蔷薇

I think this is a privilege I am lucky to have, thank you very much  Prof. Luo.

At the beginning of the implementation, I do not think mass and momentum should be in conservation, because it is the pressure boundary condition. I initialize the density in square duct with the inlet density , after streaming, the mass cannot be conservated in duct,I think.

If I initialize the density in duct with (inlet density+outlet density)/2.0, can mass and momentum be in conservation?

Moreover,in my code, I have checked every distribution and if it appeare negative density ,the implementation will stop.

My English is poor,sorry...

[ 本帖最后由 血色蔷薇 于 2011-3-16 08:10 编辑 ]

luo@odu.edu

You can respond to my comments in Chinese if that's more convenient to you. But I cannot reply in Chinese because it's too slow for me to type Chinese on my laptop.

You should check the mass influx and outflux, which must be balanced. The mass flux MUST be equal at each and every cross-section of the duct. Also, when the pressure BCs are imposed, only the averaged pressure, but NOT the profile, should be imposed. This is discussed in the following paper:

F. Verhaeghe, L.-S. Luo, and B. Blanpain. Lattice Boltzmann modeling of microchannel flow in slip flow regime. Journal of Computational Physics 228(1):147-157 (January, 2009).

I believe some paper(s) by Prof. Guo ZL et al. also discuss this issue.

Good luck.

luo@odu.edu

PS the mass flux is equal only when the "steady" of final state is attained. You should ALWAYS check mass and momentum conservations no matter what. Why should the pressure BCs allow violation of conservation laws?

血色蔷薇

非常感谢 Prof. Luo，我会学习您所提及的文献的，有问题的话还希望您能解答

血色蔷薇

哦...Prof. Luo，我在学校的数据库中没有找到您提及的文章Lattice Boltzmann modeling of microchannel flow in slip flow regime，能麻烦您发一份给我吗？不胜感激！
wizard_0_0_0_0@163.com

luo@odu.edu

You can find ALL the paper with my name on my website:

http://www.lions.odu.edu/~lluo/journal-pubs.html

I also upload the paper in this message.

血色蔷薇

谢谢，真的很感谢！