Alfvén number

(physics) The ratio of the speed of the Alfvén wave to the speed of the fluid at a point in the fluid.

[Etymology: H. O. G. Alfvén; Sweden 1908-95] rheology. Symbol Al. A dimensionless quantity characterizing the steady flow past a fixed object of a conducting fluid (such as a physical plasma) subject to a magnetic field parallel to the direction of flow. The number is the ratio of the speed of flow to the Alfvén speed, which is the ratio of the magnetic flux density to the square root of magnetic permeability times volumic mass.International Standards Association
[ ISO 31-12:1992 Quantities and Units: Characteristic Numbers]
[Mills I., Cvitas T., Homan K., Kuchitsu K. Quantities, Units and Symbols in Physical Chemistry, 2nd edn (Oxford: Blackwell, 1993)]