Atwood number

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The Atwood number (A) is a dimensionless number in fluid dynamics used in the study of hydrodynamic instabilities in density stratified flows. It is a dimensionless density ratio defined as

 \mathrm{A} = \frac{\rho_1 - \rho_2} {\rho_1 + \rho_2}


\rho_1 = density of heavier fluid
\rho_2 = density of lighter fluid

Field of application[edit]

Atwood number is an important parameter in the study of Rayleigh–Taylor instability and Richtmyer–Meshkov instability. In Rayleigh–Taylor instability, the penetration distance of heavy fluid bubbles into the light fluid is a function of acceleration time scale, \mathrm{A} g t^2 [1] where g is the gravitational acceleration and t is the time.


  1. ^ Glimm, J.; Grove, J. W.; Li, X.-L.; Oh, W.; Sharp, D. H. (2001). "A critical analysis of Rayleigh–Taylor growth rates". J. Comput. Phys. 169 (2): 652–677. Bibcode:2001JCoPh.169..652G. doi:10.1006/jcph.2000.6590.