A visual study of turbulent shear flow

Abstract

The outer region of a turbulent boundary layer along a flat plate was photographed and analysed; in addition, limited observations of the wall area were also made. The technique involved suspending very small solid particles in water and photographing their motion with a high-speed camera moving with the flow.The single most important event observed in the outer region was fluid motion which in the convected view of the travelling camera appeared as a transverse vortex. This was a large-scale motion transported downstream almost parallel to the wall with an average velocity slightly smaller than the local mean. It appeared to be the result of an instability interaction between accelerated and decelerated fluid, and it is believed to be closely associated with the wall-region ejections. The transverse vortex was part of a deterministic sequence of events; although these events occurred randomly in space and time. The first of these events was a decelerated flow exhibiting velocities considerably smaller than the local mean. It was immediately followed by an accelerated flow. Both these events extended from near the wall to the far outer region. Their interaction resulted in the formation of one or more transverse vortices. While the transverse vortex was transported downstream, small-scale fluid elements, originating in the wall area of the decelerated flow, were ejected outwards (ejection event). After travelling some distance outwards the ejected elements interacted with the oncoming accelerated fluid in the wall region and were subsequently swept downstream (sweep event). The sequence of events closed with two large-scale motions.Estimated positive and negative contributions to the instantaneous Reynolds stress during the events were many times higher than the local mean values.

Conditional Sampling in Turbulence Measurement

“…While there was no indication that the detection was independent of either this level or the filter cutoff, ensemble averages depended only weakly on these settings.2 The detection scheme essentially focuses on the back of the structure where Brown & Thomas observed a "sharp" Blackwelder (1978) and sug gested that this front may be associated with an internal shear layer between accelerated and decelerated fluid regions (e.g. Nychas et al 1973).…”

New Trends in Experimental Turbulence Research

“…Most of the experimental data, visual as well as other types, have been obtained at relatively low Reynolds numbers (in the range 1,000 < Re8 < 5,000 approximately) and any conclusions should be viewed with some caution. Furthermore, most of the observations are spatially limited: Visual observations usually cover a cross-sectional surface (Corino & Brodkey 1969, Grass 1971, Nychas et al 1973, Offen & Kline 1974, while velocity measurements at best give simultaneous data along a linear dimension, possibly including one additional spatial point (Blackwelder & Kaplan 1972). There is, in fact, insufficient information available to form a reliable kinematic picture of the flow.…”

Turbulent characteristics over a rough natural surface part I: Turbulent structures

“…It has been shown that the smooth-wall region is characterized by a randomly recurring 'burst cycle' in which low-speed fluid near the wall is ejected violently into the overlying flow, immediately followed in time by a downsweep of higherspeed fluid into the region close to the wall (e.g., Kim et al, 1971;Nychas et al, 1973); also, intense, intermittent Reynolds-stress contributions and turbulent production rates are associated with these 'ejection' (or 'burst') and 'sweep' events (e.g., Wallace et al, 1972;Lu and Willmarth, 1973). For rough surfaces, there are only a few published studies (e.g.…”