The next generation of thermodynamic power cycles offers great potential as the conceptual basis for sustainable energy converters. Examples are the supercritical and superheated Organic Rankine cycle, the transcritical condensation cycle, the supercritical Brayton cycle, the Organic Stirling cycle and the transcritical vapor compression cycle. They can be considered the next generation of well-known thermodynamic cycles, since they work with different working fluids and operate at different thermodynamic conditions, namely, close to or above the critical point of the working fluid or, at slightly lower pressures, in the dense-gas region. The anomalous thermodynamic behavior at these conditions and for these fluids offers various advantages, such as expansion across a reduced number of turbine stages, reduced compression work and a better match between the heating trajectory of the working fluid and the cooling trajectory of the heat source in the heat exchanger. This new generation of energy conversion systems could therefore attain higher conversion efficiencies, be more compact and have lower investment costs than their conventional counterparts at relatively low temperatures and small power levels, making them very attractive for the utilization of sustainable energy sources such as solar radiaton, geothermal heat and biomass. A crucial barrier to their development is the lack of understanding of the anomalous phenomena in flows involving supercritical and transcritical states. The aim of the work documented in this thesis is to gain a better understanding of the fluid dynamics of expansions at these conditions for various fluids. To this purpose, this dissertation presents theoretical and numerical research conducted into various aspects of the fluid dynamics of supercritical, transcritical, and dense-gas expansions, showing how the different thermodynamic states and the molecular characteristics of the working fluid affect fluid dynamic behavior upon expansion, and demonstrates the effects in the context of realistic turbomachinery flows of some next-generation thermodynamic cycles.