Why Fluent does not have 2order RungeKutta scheme?
Sometimes we only need a 2order RungeKutta scheme. A 4order RungeKutta maybe give a strange solution.
Anyone knows? 
Not all numerical schemes are created equal. The 2nd order RungeKutta scheme has very different stability properties with respect to the 4th order one. More specifically, its stability region does not cut the imaginary axis, hence it is not well suited for convection dominated problems.

But whydoes it have Explicit 2order RungeKutta Time Integration Scheme in the commerical sofaware GASPex, produced by AeroSoft. Inc? Please enlighten me. Thanks :)

Well, i didn't say the 2nd order RK scheme is impossible to use, just that it is not well suited for convection dominated flows. If you look at the code page:
http://www.aerosoftinc.com/gasp/features_main.php you can notice that all the convection schemes are dissipative, hence you can't actually use the code in a way such that the eigenvalues are purely imaginary, which should be enough for the 2nd order scheme to work. I'm just guessing... 
ok, many thanks! I will read some textbooks to get better understanding.

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