I am doing 1-D numerical modeling of the pulse tube cryocooler. I am referring one paper which says that pressure boundary conditions at left end of regenerator (from where oscillating helium gas flows to the pulse tube system via regenerator) is d2P/dx2 Where P is pressure which is oscillating from 8 bar to 22 bar in a cycle) and x represents space/domain. I want to use this d2P/dx2 information in my code. Please help me. I tried but did not get.