virtaus

Virtaus is the Finnish term for flow, describing the movement of a quantity through a region per unit time. In physics and engineering, virtaus commonly refers to the flow of fluids—liquids and gases. The volumetric flow rate, Q, is the volume of fluid passing through a cross-section per second, with Q = A v for a cross-section of area A and average velocity v. The mass flow rate, ṁ, equals ρ Q, where ρ is the fluid density.

Steady, incompressible flow locally satisfies the continuity equation ∇·v = 0; more generally the full continuity equation ∂ρ/∂t + ∇·(ρ v) = 0 applies. Real flows can be laminar, with smooth streamlines, or turbulent, with chaotic fluctuations. A dimensionless Reynolds number, Re = ρ v L / μ, helps predict the transition between regimes.

Virtaus concepts underpin hydraulics, ventilation, aerodynamics, cardiovascular physiology (blood flow), and environmental sciences (air and water currents). In practice, engineers and scientists use Navier–Stokes equations to model virtaus, with simplified forms such as Bernoulli’s equation for certain ideal cases.

In Finnish usage, virtaus appears in phrases such as veren virtaus (blood flow), ilman virtaus (air flow), or virtaama referring to river discharge.