請問關於Thermal LBM的幾個問題

gerok

最近在參考Y.Peng做了個3-D TLBM 代碼

在pr =0.71 低Ra(10^3,10^4) 下可以得到不錯的結果

但在10^5下，程式運算久了之後便會產生下列情形


雖然Nu值算出來是合理的 但看上去似乎非常奇怪

試著修改BC後 有些許改善


但是一樣時間久了之後還是會有不連續的情形發生

想請問這是因為網格數不夠

還是BC還有問題

另外還想請教版上高手們

在 高Ra (1E6) 跟高Pr(7) 條件下的話

Tauf  Taug 兩數值便會差異很大

使得結果很容易發散

這應該要怎麼處理

附上我的算式

XN=121

Bg = Cv**2/(Th-TL)/(XN-1)
NU_P = (Pr*Cv**2*(XN-1)**2/Ra)**0.5
alpha = NU_P/Pr         
TauF = 0.5 + 3.d0*NU_P
TauG = 0.5 + 1.8*alpha  


謝謝!!

luo@odu.edu

A typical phenomenon associated with lattice BGK model, which does not sufficient dissipation to kill spurious density fluctuations. See the following papers:

L.-S. Luo, W. Liao, X. Chen, Y. Peng, and W. Zhang.
Numerics of the lattice Boltzmann method: Effects of collision models on the lattice Boltzmann simulations.
Physical Review E 83(5):056710 (May 2011).

J. Wang, D.H. Wang, P. Lallemand, and L.-S. Luo.
Lattice Boltzmann simulations of thermal convective flows in two dimensions.
Computers and Mathematics with Applications 65(2):262-286 (January 2013).

D. Contrino, P. Lallemand, P. Asinari, and L.-S. Luo.
Lattice-Boltzmann simulations of the thermally driven 2D square cavity at high Rayleigh numbers.
Journal of Computational Physics 275:257-272 (15 October 2014).

which can also be downloaded from my website.

-- LS Luo