Etymology and usage: The form fluideja is derived from the root related to flow, with plural or adjectival endings found in several languages. In English-language texts, fluideja are usually translated as fluids; the term is most often encountered in comparative linguistics, world-building materials, or discussions about terminology across languages rather than in standard scientific literature.
Classification: Fluideja are categorized as Newtonian or non-Newtonian, and as incompressible or compressible. They can be single-phase or multi-phase and may be homogeneous or heterogeneous. These classifications influence how their motion and interaction with surfaces are modeled.
Physical properties: Core properties include density (ρ), dynamic viscosity (η), surface tension (σ), and thermal conductivity (k). Fluideja exhibit temperature and pressure dependencies, and their behavior is described using constitutive relations that relate stress to deformation. Key dimensionless numbers, such as the Reynolds number, help predict flow regimes.
Dynamics and flow: The motion of fluideja is governed by balance laws, including conservation of mass and momentum. For many practical problems, the Navier–Stokes equations provide a framework. Laminar flow is typical at low Re, while high Reynolds numbers tend to produce turbulence. Non-Newtonian fluideja require specialized models to describe how viscosity changes with shear rate.
Measurement and modeling: Properties are measured with viscometers, densimeters, rheometers, and related instruments. Modeling often employs computational fluid dynamics (CFD), plus constitutive equations and, for compressible flows, equations of state such as the ideal gas law.