Tham khảo tài liệu 'kundu fluid mechanics 2 episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 15. Second Lair of ThenniKlptamia Ejilropy Production 109 are small compared to the speed of sound and in which the temperature differences in the flow are small. This is discussed in Section 18. It is shown there that under these restrictions heating due to the viscous dissipation term is negligible in Eq. and that the term p V u can be combined with the left-hand side of Eq. to give for a perfect gas pCp - If the heat flux obeys the Fourier law q -kVT then if k const. Eq. simplifies to DT - kV2T. Dt where K k pCv is the thermal diffusivity stated in m2 s and which is the same as that of the momentum diffusivity V. The viscous heating term Ộ may be negligible in the thermal energy equation but not in the mechanical energy equation . In fact there must be a sink of mechanical energy so that a steady state can be maintained in the presence of the various types of forcing. 15. Second I MU of Thermodynamics Entropy Production The second law of thermodynamics essentially says that real phenomena can only proceed in a direction in which the disorder of an isolated system increases. Disorder of a system is a measure of the degree of uniformity of macroscopic properties in the system which is the same as the degree of randomness in the molecular arrangements that generate these properties. In this connection disorder uniformity and randomness have essentially the same meaning. For analogy a tray containing red balls on one side and white balls on the other has more order than in an arrangement in which the balls arc mixed together. A real phenomenon must therefore proceed in a direction in which such orderly arrangements decrease because of mixing. Consider two possible states of an isolated fluid system one in which there are nonuniformities of temperature and velocity and the other in which these properties are uniform. Both of these states have the same internal energy. Can the system spontaneously go from the state in which its
