Non ideal compressible fluid dynamics

Non-monotone evolution of the Mach number M in the divergent section of a supersonic nozzle. The fluid is siloxane MM (hexamethyldisiloxane, ) evolving in the non-ideal gasdynamic regime.

Non ideal compressible fluid dynamics (NICFD), or non ideal gas dynamics, is a branch of fluid mechanics studying the dynamic behavior of fluids not obeying ideal-gas thermodynamics. It is for example the case of dense vapors, supercritical flows and compressible two-phase flows. With the term dense vapors, we indicate all fluids in the gaseous state characterized by thermodynamic conditions close to saturation and the critical point.[1] Supercritical fluids feature instead values of pressure and temperature larger than their critical values,[2] whereas two-phase flows are characterized by the simultaneous presence of both liquid and gas phases.[3]

In all these cases, the fluid requires to be modelled as a real gas, since its thermodynamic behavior considerably differs from that of an ideal gas, which by contrast appears for dilute thermodynamic conditions. The ideal-gas law can be employed in general as a reasonable approximation of the fluid thermodynamics for low pressures and high temperatures. Otherwise, intermolecular forces and dimension of fluid particles, which are neglected in the ideal-gas approximation, become relevant and can significantly affect the fluid behavior.[4] This is extremely valid for gases made of complex and heavy molecules, which tend to deviate more from the ideal model.[5]

While the fluid dynamics of compressible flows in ideal conditions is well-established and is characterized by several analytical results,[6] when non-ideal thermodynamic conditions are considered, peculiar phenomena possibly occur. This is particularly valid in supersonic conditions, namely for flow velocities larger than the speed of sound in the fluid considered. All typical features of supersonic flows are affected by non-ideal thermodynamics, resulting in both quantitative and qualitative differences with respect to the ideal gas dynamics.[7]

  1. ^ Callen, Herbert B. (1985). Thermodynamics and an introduction to thermostatistics (2nd ed.). New York: J. Wiley & Sons. pp. 255–261. ISBN 978-0-471-86256-7.
  2. ^ Schlosky, Kevin M. (1989). "Supercritical phase transitions at very high pressure". Journal of Chemical Education. 66 (12): 989. Bibcode:1989JChEd..66..989S. doi:10.1021/ed066p989. ISSN 0021-9584.
  3. ^ Faghri, Amir; Zhang, Yuwen (2006-01-01), Faghri, Amir; Zhang, Yuwen (eds.), "Two-Phase Flow and Heat Transfer", Transport Phenomena in Multiphase Systems, Boston: Academic Press, pp. 853–949, doi:10.1016/b978-0-12-370610-2.50016-7, ISBN 978-0-12-370610-2, S2CID 98384899, retrieved 2023-07-06
  4. ^ Waals, J. D. van der; Rowlinson, John Shipley (1988). On the continuity of the gaseous and liquid states. Studies in statistical mechanics. Amsterdam: North-Holland. ISBN 978-0-444-87077-3.
  5. ^ Colonna, P.; Guardone, A. (2006). "Molecular interpretation of nonclassical gas dynamics of dense vapors under the van der Waals model". Physics of Fluids. 18 (5): 056101–056101–14. Bibcode:2006PhFl...18e6101C. doi:10.1063/1.2196095. ISSN 1070-6631.
  6. ^ Thompson, Philip A. (1972). Compressible-fluid dynamics. Advanced engineering series. New York: McGraw-Hill. pp. 76–99. ISBN 978-0-07-064405-2.
  7. ^ Menikoff, Ralph; Plohr, Bradley J. (1989-01-01). "The Riemann problem for fluid flow of real materials". Reviews of Modern Physics. 61 (1): 75–130. Bibcode:1989RvMP...61...75M. doi:10.1103/revmodphys.61.75. ISSN 0034-6861.