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The code exact_riemann.f solves the riemann problem of a discontinuity across an interface, and in its original form is from Bruce Fryxell. While the Sod problem has become a standard hydrodynamic test case, it isn't a very discriminating test for modern software instruments. The strong shock tube problem below is more demanding because of the stronger discontinuities across the shock interface and the narrow density peak that forms behind the shock. The γ=1.4 test case shown below sets the time=0 density to ρ(x < 2)=10 and ρ(x > 2)=1, and the time=0 pressure to P(x < 2)=100 and P(x >2)=1. The exact Riemann solution at 0.4 s is (click image to see a pdf version)
If you want a simple 1D hydrodynamics code, check out this PPM code. If you want a reasonably modern hydrodynamics code, check out The FLASH code. Mike Zingale's software page offers several opensource hydro codes for research and teaching. 


Please cite the relevant references if you publish a piece of work that use these codes, pieces of these codes, or modified versions of them. Offer coauthorship as appropriate. 
