sod激波管问题，x<0:(u,ρ,p)=(0,1,1);x≥0:(u,ρ,p)=(0,0.125,0.1),画出t=0.14时刻的u,ρ,p曲线.(The Sod shock tube problem, named after Gary A. Sod, is a common test for the accuracy of computational fluid codes, like Riemann solvers, and was heavily investigated by Sod in 1978.)