Types turbulent motion

Despite the observational identification of a possible site of dust formation, however, the dust-driven wind could not be applied to stars without dust envelope, as noted in Sect.II. Then, a more interesting possibility is a turbulence-driven wind,in which the high turbulent pressure of the transition layer(or cool corona) pushes the gas out of star,just as the high thermal pressure in corona does in solar-type stars. In fact, if the turbulent zone is extended to about 10 stellar radii, the local escape velocity there may already be small enough to be comparable with the observed flow velocities. Thus, the Maxwellian tail of the turbulent motion in the quasi-static molecular formation zone can directly lead to stellar mass-loss in all... 

This stochastic model is in fact one type of turbulent motion model. For the uni-vocity problem, we consider that a gas element can be influenced by any type of elementary process after its insertion into the MWPB at x = 0. The permanent velocity Wg, pushes the gas element outside the bed at x = H and through any of the elementary processes. The presented model can be completed by considering the different frequencies induced by passing from one elementary process to another ... 

The velocity fluctuation represents the flow that varies with periods shorter than the averaging time period. Recall that turbulence is a 3D phenomena. Therefore, we expect that fluctuations in the x-direction might be accompanied by fluctuations in the y- and z- directions. Turbuience, by definition, is a type of motion. Yet motions frequently cause variations in the temperature and concentration fields as well, if there is some mean gradient of that variable across the turbulent domain. Hence, it is common practice to portion each of these variables into mean and turbulent parts in the same manner as for the velocity. 

Chapters 1,4, and 5 emphasized the fact that the rate of mass transfer in multiphase reactors depends on the type and size of the equipment used. The reactors dealt with in this and subsequent chapters are of the type in which the gas phase is dispersed in a continuous liquid phase. The various phases taking part in the overall reaction sequence experience chaotic, turbulent motion in time and space. Under such conditions, mass transfer mainly occurs by a mechanism in which different eddies that come to the interface deliver/receive the solute during their lifetime at the interface and return back to the bulk phase. This unsteady-state mass transfer process has been exhaustively discussed in several texts (Astarita 1967 Danckwerts 1970). In the following, the various approaches to predict mass transfer coefficients in different multiphase reactors are discussed along with the advantages/drawbacks of each approach. 

Mixing can be broadly defined as a process to reduce the non-uniformity of a composition. The basic mechanism of mixing is to induce physical relative motion of the ingredients. The types of motion that can occur are molecular diffusion, turbulent motion, and convective motion. The first two types of motion are essentially limited to gases and low-viscosity liquids. Convective motion is the predominant motion in high-viscosity liquids, such as polymer melts. As discussed in Section 5.3.3, polymer melts are not capable of turbulent motion as a result of their high viscosity motion in polymer melts is always by laminar flow. 

In this type of cell the liquids are in direct contact. They are stirred independently, in turbulent motion and care is taken for not breaking up the interface. So, the interfacial area is well defined. Extraction is followed by sampling the phases. When this type of cell is used the data are sometimes described in the framework of the two-film theory [9, 27], which assiunes that two stagnant layers of fluid exist on either side of the interface. However, the thickness of these layers cannot be calculated theoretically because the liquids are in turbulent motion, with undefined... 

As is widely known, mixing in a bath is governed mainly by large-scale recirculation and turbulent motion. The former is characterized by the mean velocity components in the three directions, while the latter is characterized by the root-mean-square (rms) values of the three turbulence components and the Reynolds shear stresses. Desirable mixing condition would be realized when the two kinds of motions are produced together. Unfortunately, these motions on the mixing time in a bath subjected to surface flow control are poorly understood. This chapter discusses these effects with reference to experiments in which three types of boundary conditions are imposed on the surface of a water bath stirred by bottom gas injection. 

Nq is strongly dependent on the flow regime, Reynolds Number, NRe, and installation geometry of the impeller. The flow from an impeller is only that produced by the impeller and does not include the entrained flow, which can be a major part of the total motion flow from the impeller. The entrained flow refers to fluid set in motion by the turbulence of the impeller output stream [27]. To compare different impellers, it is important to define the type of flows being considered. 

The impeller (Figure 8.20) consists of a series of curved vanes so shaped that the flow within the pump is as smooth as possible. The greater the number of vanes on the impeller, the greater is the control over the direction of motion of the liquid and hence the smaller are the losses due to turbulence and circulation between the vanes. In the open impeller, the vanes are fixed to a central hub, whereas in the closed type the vanes are held between... 

For an incompressible viscous fluid (such as the atmosphere) there are two types of flow behaviour 1) Laminar, in which the flow is uniform and regular, and 2) Turbulent, which is characterized by dynamic mixing with random subflows referred to as turbulent eddies. Which of these two flow types occurs depends on the ratio of the strengths of two types of forces governing the motion lossless inertial forces and dissipative viscous forces. The ratio is characterized by the dimensionless Reynolds number Re. 

Embedded in such models, in which variations were developed [12] are further detailed. The laminar burning velocity is expressed as a function of fuel type, fuel/ air ratio, level of exhaust gas recirculation, pressure, temperature, etc. Furthermore, submodels have been developed to describe the impact of engine speed, port-flow control systems, in-cylinder gross-flow motion (i.e., swirl, tumble, squish), and turbulent fluctuations u. Thus, with a wider knowledge base of the parametric impact of external variables, successful modeling of... 

In real life, the parcels or blobs are also subjected to the turbulent fluctuations not resolved in the simulation. Depending on the type of simulation (DNS, LES, or RANS), the wide range of eddies of the turbulent-fluid-flow field is not necessarily calculated completely. Parcels released in a LES flow field feel both the resolved part of the fluid motion and the unresolved SGS part that, at best, is known in statistical terms only. It is desirable that the forces exerted by the fluid flow on the particles are dominated by the known, resolved part of the flow field. This issue is discussed in greater detail in the next section in the context of tracking real particles. With a RANS simulation, the turbulent velocity fluctuations remaining unresolved completely, the effect of the turbulence on the tracks is to be mimicked by some stochastic model. As a result, particle tracking in a RANS context produces less realistic results than in an LES-based flow field. 

Turbulence and advection can lead to the mixing of adjacent water masses (or types). These water motions create horizontal and vertical gradients in temperature and salinity. As illustrated in Figures 4.17a and 4.17b, vertical mixing at the boundary between two water types produces waters of intermediate temperature and salinity. Since mixing does not alter the ratios of the conservative ions, the water in the mixing zone acquires a salinity intermediate between that of the two water types. The salinity of... 

This will be found almost exclusively in the rough vacuum range. The character of this type of flow is determined by the interaction of the molecules. Consequently Internal friction, the viscosity of the flowing substance. Is a major factor. If vortex motion appears In the streaming process, one speaks of turbulent flow. If various layers of the flowing medium slide one over the other, then the term laminar flow or layer flux may be applied. 

Both large-scale motion (mass flow) and small-scale motion (turbulence) are usually required to bring about effective mixing (R5). Different ratios of mass flow to turbulence can be obtained for a given impeller type for the same power input large ratios for large values of d/T and slow speed, small ratios for small values of d/T and high speed. The requirements peculiar to batch liquid extraction have not been established, but for other services d/T = 0.2 to 0.5 is usually recommended for baffled tanks. 

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