Developments in turbulence research: a review based on the 1999 Programme of the Isaac Newton Institute, Cambridge

Abstract

Recent research is making progress in framing more precisely the basic dynamical and statistical questions about turbulence and in answering them. It is helping both to define the likely limits to current methods for modelling industrial and environmental turbulent flows, and to suggest new approaches to overcome these limitations. Our selective review is based on the themes and new results that emerged from more than 300 presentations during the Programme held in 1999 at the Isaac Newton Institute, Cambridge, UK, and on research reported elsewhere. A general conclusion is that, although turbulence is not a universal state of nature, there are certain statistical measures and kinematic features of the small-scale flow field that occur in most turbulent flows, while the large-scale eddy motions have qualitative similarities within particular types of turbulence defined by the mean flow, initial or boundary conditions, and in some cases, the range of Reynolds numbers involved. The forced transition to turbulence of laminar flows caused by strong external disturbances was shown to be highly dependent on their amplitude, location, and the type of flow. Global and elliptical instabilities explain much of the three-dimensional and sudden nature of the transition phenomena. A review of experimental results shows how the structure of turbulence, especially in shear flows, continues to change as the Reynolds number of the turbulence increases well above about 10 4 in ways that current numerical simulations cannot reproduce. Studies of the dynamics of small eddy structures and their mutual interactions indicate that there is a set of characteristic mechanisms in which vortices develop (vortex stretching, roll-up of instability sheets, formation of vortex tubes) and another set in which they break up (through instabilities and selfdestructive interactions). Numerical simulations and theoretical arguments suggest that these often occur sequentially in randomly occurring cycles. The factors that † Now at

Mechanics of the Turbulent-Nonturbulent Interface of a Jet

“…0 and consequently that E b 0. To overcome this unphysical condition, turbulence models usually make some assumption about a finite ''background'' value for T [3,20]. However, for our conditional data the eddy viscosity vanishes in the irrotational (i.e., nonturbulent) flow domain, yet at the same time predicts a nonzero constant eddy viscosity defined in laboratory coordinates.…”

“…The sharp interface between the turbulent and nonturbulent flow regions is strongly contorted over length scales proportional to the integral length scale and propagates into the irrotational flow region while irrotational fluid is entrained into the turbulent flow region. A long-standing problem about these unconfined, but localized, turbulent flows is to describe and quantify the characteristic features of the inhomogeneous interface [1][2][3][4], and to identify the nature of the entrainment process by which irrotational fluid becomes turbulent. It has been unclear whether this occurs as the result of outward spreading of small-scale vortices (''nibbling'') or large-scale engulfment by the inviscid action of the dominant eddies in the turbulent flow region.…”

“…Conclusions drawn from early attempts to measure U from conditionally sampled hot-wire data [15] were not substantiated by the data. Ever since, the existence of this jump has been debated [3,14].…”

Detection of Passive Scalar Interface Directly from PIV Particle Images in Inhomogeneous Turbulent Flows

“…Another application of scalar interface in the field of experimental non-reacting fluid mechanics is that it has been used before as one of the ways to mark out turbulence non-turbulence interface (TNTI). TNTI is usually defined by the transitional zone between the outside irrotational region and the inside highly concentrated, fully developed turbulent region (see 11,12,25 for example.…”

Characteristics of the turbulent/non-turbulent interface of a non-isothermal jet

“…This gradual change is the result of intermittency, in which turbulent and non-turbulent-flow regions pass along a fixed point. A long-standing problem about these unconfined, but localized, turbulent flows is to describe and quantify the characteristic features of the inhomogeneous interface [1][2][3][4], and to identify the nature of the entrainment process by which irrotational fluid becomes turbulent. Until recently it had been unclear whether this occurs as the result of outward spreading of small-scale vortices ('nibbling') or large-scale engulfment by the inviscid action of the dominant eddies in the turbulent-flow region.…”