Large-scale turbulent structures

Single-phase flow PIV measurements were made for duee exit velocities 7.62, 3.9 and 0.39m/s. Each resulted in a steady stagnation point flow, and had a uniform velocity profile at the exit, perpendicular to the test plate. The flow field at the exit does not show any large scale turbulent structures that may represent entrainment of outside air. The stagnation point was steady and remained at the center of the hot plate. For the 7.62 m/s exit velocity, the measured velocities ranged from 7.62 m/s at the nozzle exit to 0.1 m/s near the hot plate surface. Similar results were obtained for the addition of water drops. 

The previously mentioned microPIV studies suggest that the turbulence in microchannels is statistically similar to flow in macroscale pipes and channels, but it is also important to consider if the microscale flows are structurally similar to macroscale flows. One way to determine this is to measure the characteristics of the large-scale turbulent structures in turbulent microchannel flow and compare these to turbulent macroscale flow. 

The above discussion holds for dispersion by atmospheric turbulence. In addition, a momentum release of fuel sometimes generates its own turbulence, e.g., when a fuel is released at high pressure in the form of a high-intensity turbulent jet. Fuel mixes rapidly with air within the jet. Large-scale eddy structures near the edges of the jet entrain surrounding air. Compositional homogeneity, in such cases, can be expected only downstream toward the jet s centerline. 

For most medium- and large-scale micromanifold structures, where one passage feeds multiple parallel channels, flow traverses through turbulent and transition flows in the micromanifold region. This fluid in turbulent to transition flow also turns in the micromanifold region as it drops flow into parallel microchannels, which are primarily in the laminar flow regime. 

Pulsation in a spray is generated by hydrodynamic instabilities and waves on liquid surfaces, even for continuous supply of liquid and air to the atomizer. Dense clusters of droplets are projected into spray chamber at frequencies very similar to those of the liquid surface waves. The clusters interact with small-scale turbulent structures of the air in the core of the spray, and with large-scale structures of the air in the shear and entrainment layers of outer regions of the spray. The phenomenon of cluster formation accounts for the observation of many flame surfaces rather than a single flame in spray combustion. Each flame surrounds a cluster of droplets, and ignition and combustion appear to occur in configurations of flames surrounding droplet clusters rather than individual droplets. 

As discussed in Chapter 3, with LES, the smallest scale to be resolved is chosen to lie in the inertial sub-range of the energy spectrum, which means the so-called sub-grid scale (SGS) wave numbers are not resolved. As LES can capture transient large-scale flow structures, it has the potential to accurately predict time-dependent macromixing phenomena in the reactors. However, unlike DNS, a SGS model representing interaction of turbulence and chemical reactions will be required in order to predict the effect of operating parameters on say product yields in chemical reactor simulations. These SGS models attempt to represent an inherent loss of SGS information, such as the rate of molecular diffusion, in an LES framework. Use of such SGS models makes the LES approach much less computationally intensive than the DNS approach. DNS... 

Figure 2 shows an instantaneous velocity field in a 536 pm diameter microtube at Re = 4,500. Large-scale turbulent eddies are clearly visible throughout the velocity field, and a comparison to a velocity field obtained in a macroscale pipe would show many similarities. However, such comparisons are qualitative only, and a true quantitative comparison can only be made by statistically comparing the large-scale structures, as can be done by calculating spatial correlations and turbulent length scales. 

Industrial research. Large scale turbulence and unsteady structures.,... 

The measured values of for each turbulent motion remain almost unchanged in the radial direction, but in a strict sense, those for ejection and sweep changed trend around r/b = 1.0. This implies that the Fle-Wood s metal bubbling jet has two large-scale coherent structures, the boundary being located around r/by = 1.0. This result is consistent with the above-mentioned findings on the appearance frequency and the contributions of each turbulent motion to the turbulence kinetic energies and the Reynolds shear stress. 

A reduced scale of the model requires an increased velocity level in the experiments to obtain the correct Reynolds number if Re < Re for the prob lem considered, but the experiment can be carried out at any velocity if Re > RCj.. The influence of the turbulence level is shown in Fig. 12.40. A velocity u is measured at a location in front of the opening and divided by the exhaust flow rate in order to obtain a normalized velocity. The figure show s that the normalized velocity is constant for Reynolds numbers larger than 10 000, which means that the flow around the measuring point has a fully developed turbulent structure at that velocity level. The flow may be described as a potential flow with a normalized velocity independent of the exhaust flow rate at large distances from the exhaust opening— and far away from surfaces. 

Turbulence is generated by wind shear in the surface layer and in the wake of obstacles and structures present on the earth s surface. Another powerful source of turbulent motion is an unstable temperature stratification in the atmosphere. The earth s surface, heated by sunshine, may generate buoyant motion of very large scale (thermals). 

P. Clavin and F.A. Williams. Effects of molecular diffusion and of thermal expansion on the structure and dynamics of premixed flames in turbulent flows of large scale and low intensity. Journal of Fluid Mechanics, 116 251-282,1982. 

The ability to resolve the dissipation structures allows a more detailed understanding of the interactions between turbulent flows and flame chemistry. This information on spectra, length scales, and the structure of small-scale turbulence in flames is also relevant to computational combustion models. For example, information on the locally measured values of the Batchelor scale and the dissipation-layer thickness can be used to design grids for large-eddy simulation (LES) or evaluate the relative resolution of LES resulfs. There is also the potential to use high-resolution dissipation measurements to evaluate subgrid-scale models for LES. 

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