Where does wake collide?

The region of disturbed, turbulent flow that trails behind a solid body as it moves through a fluid, or as a fluid streams past it: behind a bluff body such as a cylinder the wake does not sit still but sheds vortices in a repeating pattern, the von Karman vortex street, whose structure sets the drag and the unsteady forces the body feels. Williamson's 1996 Vortex dynamics in the cylinder wake reviewed how these vortices form, interact, and change with the Reynolds number. The referent is a pattern of trailing flow in a fluid, described by its vortices and its velocity deficit, and it has nothing to do with the state of being awake or a vigil kept beside the dead.[1]

Unit

A trailing region of disturbed flow behind a body, by its velocity deficit and shed vortices

Recovery means

The wake widens and its velocity deficit fills in downstream as turbulence mixes momentum back into the surrounding flow

Measured by

Mapped in a wind or water tunnel by hot-wire probes, particle image velocimetry, or flow visualisation, and characterised by the Reynolds number and the vortex-shedding frequency

The state of being awake: the condition of an alert, responsive brain and body, held apart from sleep and switched between the two by interacting neural systems under circadian and homeostatic control. Saper, Chou, and Scammell's 2001 The sleep switch described how wake-promoting and sleep-promoting nuclei in the hypothalamus inhibit one another, so the brain tends to settle into one state or the other rather than drift between them. The referent is a behavioural and physiological state of the whole organism, scored against the sleep-wake cycle, and it is not a trail of turbulence behind a moving body or a gathering kept beside a body before burial.[2]

Unit

The awake state of an organism, scored against sleep across the daily cycle

Recovery means

The wake state is entered from sleep at awakening and held against homeostatic sleep pressure that builds the longer waking lasts

Measured by

Identified by behaviour and by electroencephalography, with low-voltage fast EEG activity and muscle tone marking wake against the stages of sleep in polysomnography