Voolukiirus

Voolukiirus is a term from fluid dynamics referring to the magnitude of the velocity of a fluid element as it moves through space or across a cross-section. The word is Estonian for flow speed or flow velocity. It is a key quantity in calculating how much fluid passes a given area, how momentum and energy are transported, and how systems driven by fluids operate.

In general, the instantaneous flow velocity is a vector, whose magnitude is the flow speed. The average velocity across a cross-section with area A relates to the volumetric flow rate Q via Q = ∫A v · n dA; for a uniform velocity across the section, Q = vA, so v = Q/A.

In pipe flow, the velocity profile depends on the flow regime. Laminar flow in a circular pipe has a parabolic profile, with the maximum velocity higher than the average. Turbulent flow yields a flatter profile and greater mixing. The Reynolds number Re = ρvD/μ helps predict the transition between regimes. Fluid properties such as density ρ and dynamic viscosity μ, as well as the driving pressure gradient, influence v.

In open-channel flow, velocity typically varies with depth and along the channel, requiring profiles or measurements at multiple points. Velocity is often measured with instruments such as Pitot tubes, hot-wire anemometers, acoustic Doppler devices, or particle image velocimetry.

Applications of voolukiirus span engineering (pumps, pipelines, HVAC), environmental science (river discharge, rainfall-driven runoff), and meteorology and oceanography, where predicting and controlling flow speed is essential.