1. Give brief explanations (or definitions) of the following fundamental concepts. (4 each subproblem, total 32 ) (1) Irrotional flows; (2) Adiabatic process, and reversible process and isentropic process; (3) Definition of circulation and Kelvin's circulation theorem; (4) Definition of compressibility; (5) Aerodynamic model of the thin airfoil theory (6) Definition of Mach number and Mach angle; (7) Total temperature and Total pressure (8) Down wash