In ANSYS Fluent, you have to use geometry as a cylinder with one inlet. At inlet, use a mass flow inlet or velocity inlet depending upon the conditions of hydrogen available with you from experimental data or physical model. In cell zone conditions you can specify air at atmospheric pressure/ required initial pressure as a fluid . The cylinder walls can be considered adiabatic or iso-thermal or with convective heat loss. Then putting appropriate time step size you can run the simulation. Using species transport model.

Thanks Sanket. Do i need to define separate operating conditions at inlet and inside the tank? In my case compressed hydrogen at high pressure is filling in emptying tank with few mins of time.

Yes while specifying inlet boundary condition you can specify the pressure of the compressed hydrogen and in cell zone conditions you have to specify the ambient pressure. Once compressed gas starts filling you can monitor the pressure on cylinder walls. It must be increasing.