Dissipative large-scale

The convective turbulence tend to dissipate large scale shears (wave length a > pressure scale height H ). The rate of dissipation... 

Properties. Silver difluoride melts at 690°C, bods at 700°C, and has a specific gravity of 4.57. It decomposes in contact with water. Silver difluoride may react violently with organic compounds, quite often after an initial induction period. Provisions must be made to dissipate the heat of the reaction. Small-scale experiments must be mn prior to attempting large-scale reactions. 

With turbulent flow, shear stress also results from the behavior of transient random eddies, including large-scale eddies which decay to small eddies or fluctuations. The scale of the large eddies depends on equipment size. On the other hand, the scale of small eddies, which dissipate energy primarily through viscous shear, is almost independent of agitator and tank size. 

In shear layers, large-scale eddies extract mechanical energy from the mean flow. This energy is continuously transferred to smaller and smaller eddies. Such energy transfer continues until energy is dissipated into heat by viscous effects in the smallest eddies of the spectrum. 

We are essentially assuming that the small scales are in dynamic equilibrium with the large scales. This may also hold in low-Reynolds-number turbulent flows. However, for low-Reynolds-number flows, one may need to account also for dissipation rate anisotropy by modeling all components in the dissipation-rate tensor s j. 

At high Reynolds numbers, we can again expect the small scales of the scalar field to be nearly isotropic. In the classical picture of turbulent mixing, one speaks of scalar eddies produced at large scales, with a distinct directional orientation, that lose their directional preference as they cascade down to small scales where they are dissipated by molecular diffusion. 

Figure 4.9. Sketch of CSTR representation of the SR model for 1 < Sc. Each wavenumber band is assumed to be well mixed in the sense that it can be represented by a single variable

Physically, this closure attempts to model the effect of SGS fluctuations on the filtered reaction rate by assuming that the largest of the SGS are dynamically similar to the smallest of the resolved scales. The influence of the smallest eddies, in particular Kolmogorov eddies, on the filtered reaction rate is assumed negligible because the large viscosity in the flame tends to rapidly dissipate these scales [19]. Further discussion and assessment of this model with comparisons to other SGS combustion closures and DNS results can be found in [17]. 

The large scale molecular motions which take place in the rubber plateau and terminal zones of an uncross-linked linear polymer give rise to stress relaxation and thereby energy dissipation. For narrow molecular weight distribution elastomers non-catastrophic rupture of the material is caused by the disentanglement processes which occur in the terminal zone, e.g., by the reptation process. In practical terms it means that the green strength of the elastomer is poor. 

---