Unsteady

Unsteady is an adjective describing something that is not in a steady state; its properties vary with time. In scientific and engineering contexts, unsteady implies a time-dependent state, as opposed to steady or steady-state conditions where variables remain constant or evolve to a time-invariant regime.

In fluid dynamics, unsteady flow refers to a fluid motion whose velocity field depends on time. Unsteadiness arises from changing boundary conditions, transient forces, or pulsatile inputs. Such flows can be periodic, quasi-periodic, or transient, and they often require time-resolved measurements or numerical simulations. By contrast, steady flow assumes time-invariant conditions and neglects accelerations.

Mathematically, unsteady problems involve solving time-dependent equations, such as partial differential equations or ordinary differential equations, with explicit initial conditions. The solution typically evolves from an initial state toward a future behavior, which may approach a steady state or remain inherently time-varying.

Unsteadiness is common across disciplines. In mechanical and aerospace engineering, unsteady aerodynamics, transient combustion, and valve or pump start-up are practical concerns. In physiology, blood flow can be unsteady due to the heartbeat, while in meteorology and oceanography, winds, waves, and currents exhibit time-dependent changes. In signal processing and statistics, non-stationary or time-varying processes are described as unsteady.

Note that unsteady does not inherently imply instability; a system can be unsteady yet stable, with bounded fluctuations. Related terms include transient, time-dependent, non-steady, and, in contrast, steady-state.