Turbulent coherent structures

The modelling of aerodynamic entrainment is based on the close link between particles take-off and turbulent coherent structures above the surface. In fact, some authors [6,7] have experimentally observed that a particle take-off can be associated to the ejection of fluid from the wall region due to the presence of streamwise counter rotating vortices. If it is assumed that the presence of two streamwise counter rotating vortices produces only one ejection, each pair of streamwise vortices is considered as a possibility that a particle takes-off. Thus, for each of these possibilities, a take-olf criterion is tested. 

Although, these values are established in the case of a smooth wall, they will be considered in the first approximation like the spanwise and streamwise spatial periodicities of the coherent structures over the particle bed. The particle bed is thus subdivided into boxes having spanwise and streamwise sizes given by average spatial periodicities of the turbulent coherent structures. 

Holms, P., Lumley, J. L., and Berkooz, G., Turbulence, Coherent Structures, Dynamical Systems and Symmetry, Cambridge Univ. Press, Cambridge, 1996. 

Holmes, P., J. Lumley, and G. Berkooz. 1996. Turbulence, coherent structures, dynamical systems and symmetry. Cambridge, UK Cambridge University Press. 

Intense mixing of burned and unbumed components within large, coherent, turbulent, eddy structures of a jet may lead to local conditions that may induce the SWACER mechanism and trigger detonation. 

These data indicate that thermal losses during unsteady flame-wall interactions constitute an intense source of combustion noise. This is exemplified in other cases where extinctions result from large coherent structures impacting on solid boundaries, or when a turbulent flame is stabilized close to a wall and impinges on the boundary. However, in many cases, the flame is stabilized away from the boundaries and this mechanism may not be operational. 

DNS results are usually considered as references providing the same level of accuracy as experimental data. The maximum attainable Reynolds number (Re) in a DNS is, however, too low to duplicate most practical turbulent reacting flows, and hence, the use of DNS is neither to replace experiments nor for direct comparisons— not yet at least. However, DNS results can be used to investigate three-dimensional (3D) features of the flow (coherent structures, Reynolds stresses, etc.) that are extremely difficult, and sometimes impossible, to measure. One example of such achievement for nonreacting... 

This part of the turbulent boundary layer is rich in coherent structures, ie the flow exhibits features that are not random. Flow visualization studies [Kline et al (1967), Praturi and Brodkey (1978), Rashidi and Banerjee (1990)] have revealed a fascinating picture. The first observations indicated the occurrence of fluid motions called inrushes and eruptions or bursts in the fluid very close to the wall. During eruptions, fluid... 

Praturi, A.K. and Brodkey, R.S., A stereoscopic visual study of coherent structures in turbulent shear flow, Journal of Fluid Mechanics, 89, pp. 251-72 (1978). 

Grinstein, F.F., and C. R. DeVore. 1996. Dynamics of coherent structures and transition to turbulence in free square jets. J. Physics Fluids 8 1237-51. 

Yu, K. H., and K. C. Schadow. 1997. Role of large coherent structures in turbulent compressible mixing. J. Experimental Thermal Fluid Science 14 75-84. 

In their milestone work, Melander and Hussain found that the method of complex helical wave decomposition was instrumental in modeling both laminar as well as turbulent shear flows associated with coherent vortical structures, and revealed much new important data about this phenomenon than had ever been known before through standard statistical procedures. In particular, this approach plays a crucial role in the description of the resulting intermittent fine-scale structures that accompany the core vortex. Specifically, the large-scale coherent central structure is responsible for organizing nearby fine-scale turbulence into a family of highly polarized vortex threads spun azimuthally around the coherent structure. 

The principle of the model is to scan the bed surface, which is subdivided into boxes whose the width and the length are equal respectively to the spanwise and streamwise statistical periodicities of appearing of the coherent structures. In fact, some authors have shown that the phenomenon of ejection in a turbulent boundary layer could be connected with the particle s take-off. 

Praturi, A. and Brodkey, R. S. (1978). A Stereoscopic Visual Study of Coherent Structures in Turbulent Shear Flow. J. Fluid Mech., 89, 251. 

The orthogonal-plane PIV technique is recently proposed for investigating the 3D characteristics of the coherent structures in a turbulent boundary layer flow (Hambleton et al., 2006 Kim et al., 2006). The hardware components and principle of this technique are the same as polarization-based dual-plane PIV. The only difference is to set up both laser sheets mutually perpendicular to each other instead of parallel to each other in the dual-plane PIV system. This allows for measuring velocity distributions in both streamwise-spanwise and streamwise-wall-normal planes simultaneously, so that the salient features of the coherent structures in a turbulent boundary layer flow as the legs and the head of the hairpin vortices can be detected (Hambleton et al., 2006 Kim et al., 2006). 

There are approaches to analyses of turbulent combustion that, although not deductively based on the Navier-Stokes equations, nevertheless appeal to concepts of coherent structures [68], [69]. We shall not have space here to present these approaches and must refer instead to reviews [18], [27], [40]. These methods share some aspects in common with age theories of stirred reactors [19], theories that we also shall forego discussing for the sake of brevity. Instead, we shall consider a promising approach to the theoretical analysis of turbulent diffusion flames. 

N. Peters and F. A. Williams, Coherent Structures in Turbulent Combustion, in The Role of Coherent Structures in Modelling Turbulence and Mixing, J. Jimenez, ed., Berlin Springer-Verlag, 1981, 364-393... 

---