Nonsteady

Nonsteady is an adjective used to describe processes or systems in which properties change with time and do not remain in a constant or repeating state. It is often synonymous with unsteady, transient, or time-dependent, though terminology varies by field. Nonsteady conditions are contrasted with steady-state conditions, where variables such as temperature, concentration, velocity, or pressure stay constant over time or follow a repeating cycle.

In fluid dynamics, nonsteady or transient flow refers to a velocity field that evolves in time, such as start-up flow in a pipe or flow around a moving body. Such problems are described by time-dependent forms of the governing equations, like the Navier–Stokes equations. In heat transfer, nonsteady (transient) heat conduction describes how temperature changes within a solid after a sudden change at its boundaries, governed by the heat equation with a time derivative. In diffusion, nonsteady or transient diffusion involves time-varying concentration fields, described by the transient diffusion equation.

In chemistry, nonsteady-state reaction kinetics describe systems where intermediate species do not reach a quasi-steady concentration quickly, requiring full time-dependent analysis. In biology and physiology, nonsteady behavior appears in processes such as transient signaling events or changing metabolic states. In electrical engineering and control theory, nonsteady or transient analysis studies circuit and system responses to switches, pulses, or other time-varying inputs until a steady state is attained.

Overall, nonsteady emphasizes temporal dynamics and time-dependent behavior across disciplines, distinguishing with steady-state analyses where variables remain constant or periodic over time.