An Etymological Dictionary of Astronomy and Astrophysics
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A continuous, amorphous substance whose molecules move freely past one another and that has the tendency to assume the shape of its container; collective term for liquids and gases.

From L. fluidus "fluid, flowing," from fluere "to flow;" → flux.

Šârré, from šârr, → flux.

The branch of → fluid mechanics that deals with the movement of gases and liquids.

→ fluid; → dynamics.

The branch of physics that extends the ideas developed in → mechanics and → thermodynamics to the study of motion and equilibrium of → fluids.

→ mechanics; → fluid.

The ability of a substance to flow; reciprocal of → viscosity.

→ fluid + → -ity.

A fluid with uniform properties throughout, but meteorologists sometimes designate as homogeneous a fluid with constant density.

→ homogeneous, → fluid.

An fluid which has no internal friction or → viscosity, and no → resistivity as well if the fluid is a plasma.

→ ideal; → fluid.

Any → fluid with a constant → viscosity at a given temperature regardless of the rate of → shear.

→ Newtonian; → fluid.

A → magnetohydrodynamics treatment in which the → plasma consists only of one particle species and moves with the bulk speed. The thermal motion of the particles is neglected and thus there is no motion of particles relative to each other.

→ one; → fluid; → magnetohydrodynamics.

The opposite of an ideal fluid; a fluid which possesses viscosity and therefore exhibits certain frictional phenomena. Viscosity arises due to cohesive forces between molecules and molecular momentum exchange between fluid layers. These effects appear as tangential or shear stresses between moving fluid layers.

→ real; → fluid.

A fluid that is at a temperature and pressure above its thermodynamic critical point. In these conditions the substance acquires unique characteristics of density and mobility. Supercritical fluids exist deep inside some planets; for example, there is supercritical water deep inside the Earth.

→ supercritical; → fluid.

A → fluid that exhibits frictionless flow, very high heat → conductivity, and other unusual physical properties. For example, → liquid helium at the temperature about 2.17 K (→ lambda point) becomes a zero → viscosity fluid which will move rapidly through any pore in the apparatus. See also → helium II.

→ super-; → fluid

The phenomenon occurring in → liquid helium (→ helium I) cooled below the → lambda point temperature of 2.17 K, whereby it flows freely with no measurable → friction and → viscosity.

→ super-; → fluidity.

A fluid whose viscosity is sufficiently large to make the viscous forces a significant part of the total force field in the fluid.

→ viscous; → fluid.