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The small time behavior of the flow is then found through simplification of the incompressible Navier–Stokes equations using the initial flow to give a step-by-step solution as time progresses. An exact solution in two spatial dimensions is known, and is presented below. Animation of a Taylor-Green Vortex using colour coded Lagrangian tracers
A flow that is not a function of time is called steady flow. Steady-state flow refers to the condition where the fluid properties at a point in the system do not change over time. Time dependent flow is known as unsteady (also called transient [8]). Whether a particular flow is steady or unsteady, can depend on the chosen frame of reference.
In this model the red fluid – initially on top, and afterwards below – represents a more dense fluid and the blue fluid represents one which is less dense. The Rayleigh–Taylor instability is another application of hydrodynamic stability and also occurs between two fluids but this time the densities of the fluids are different. [ 6 ]
Visualisation of the vortex street behind a circular cylinder in air; the flow is made visible through release of glycerol vapour in the air near the cylinder. In fluid dynamics, a Kármán vortex street (or a von Kármán vortex street) is a repeating pattern of swirling vortices, caused by a process known as vortex shedding, which is responsible for the unsteady separation of flow of a fluid ...
In fluid dynamics, a vortex (pl.: vortices or vortexes) [1] [2] is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. [ 3 ] [ 4 ] Vortices form in stirred fluids, and may be observed in smoke rings , whirlpools in the wake of a boat, and the winds surrounding a tropical cyclone , tornado or dust ...
It describes the rate of change of vorticity of the moving fluid particle. This change can be attributed to unsteadiness in the flow ( ∂ω / ∂t , the unsteady term) or due to the motion of the fluid particle as it moves from one point to another ((u ∙ ∇)ω, the convection term).
laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion; turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities. [8] The Reynolds number is defined as: [4]
The Strouhal number gives the vortex shedding frequency resulting from placing an object in a steady flow, so it describes the flow unsteadiness as a result of an instability of the flow downstream of the object. Conversely, the Keulegan–Carpenter number is related to the oscillation frequency of an unsteady flow into which the object is placed.