The Atwood number (A) is a
dimensionless number
A dimensionless quantity (also known as a bare quantity, pure quantity, or scalar quantity as well as quantity of dimension one) is a quantity to which no physical dimension is assigned, with a corresponding SI unit of measurement of one (or 1) ...
in
fluid dynamics
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) a ...
used in the study of
hydrodynamic instabilities in density stratified flows.
It is a dimensionless density ratio defined as
:
where
:
= density of heavier fluid
:
= density of lighter fluid
Field of application
Atwood number is an important parameter in the study of
Rayleigh–Taylor instability
The Rayleigh–Taylor instability, or RT instability (after Lord Rayleigh and G. I. Taylor), is an instability of an interface between two fluids of different densities which occurs when the lighter fluid is pushing the heavier fluid.
Drazin ( ...
and
Richtmyer–Meshkov instability The Richtmyer–Meshkov instability (RMI) occurs when two fluids of different density are impulsively accelerated. Normally this is by the passage of a shock wave. The development of the instability begins with small amplitude perturbations which ...
. In Rayleigh–Taylor instability,
the penetration distance of heavy fluid bubbles into the light fluid is a function of acceleration time scale,
where ''g'' is the gravitational acceleration and ''t'' is the time.
References
Dimensionless numbers of fluid mechanics
Fluid dynamics
{{NonDimFluMech