Turbulence and ‘coherent structures’

Soon after the first attempts by Lighthill and his successors to predict the sound radiated by turbulent jets a change occurred in the way turbulence is perceived. Turbulent flows were observed to be more ordered than had previously been believed, and a new conceptual flow entity was born, some­times referred to as a ‘coherent structure’, or, alternatively, a ‘wave-packet’. Mollo-Christensen (1967) was one of the first to report such order in the case of the round jet: “…although the velocity signal is random, one should expect to see intermittently a rather regular spatial structure in the shear layer.”. A series of papers followed, confirming these observations and pos­tulating on the nature of this order (Crow and Champagne (1971), Brown

Turbulence and ‘coherent structures’

Figure 4. Flash Schlieren images of jets (Re = 5 x 105; M = 0.83), taken from Moore (1977). Left: random ensemble average; middle: conditional av­erage using axisymmetric nearfield pressure signature as trigger; right: con­ditional average using a single nearfield microphone as trigger (this educes the antisymmetric organisation).

and Roshko (1974) and Moore (1977) to cite just a few). Figure 3(b), taken from Crow and Champagne (1971), provides a visual sense of this under­lying order: by changing visualisation technique, using sheet illumination and carbon dioxide fog, rather than the fine grained patterns revealed by the schlieren technique, an axially-aligned waveform with wavelength of the order of the jet diameter is observed.

A further illustration of the underlying organisation present in high Reynolds number jets is shown in figure 4, which shows the difference be­tween time-averaged and conditionally-averaged images of round jets at high Reynolds and Mach numbers. We will discuss conditional averaging tech­niques later in more detail.

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