Turbulence transport mechanism

The use of local theories, incorporating parameters such as the eddy viscosity Km and eddy thermal conductivity Ke, has given reasonable descriptions of numerous important flow phenomena, notably large scale atmospheric circulations with small variations in topography and slowly varying surface temperatures. The main reason for this success is that the system dynamics are dominated primarily by inertial effects. In these circumstances it is not necessary that the model precisely describe the role of turbulent momentum and heat transport. By comparison, problems concerned with urban meso-meteorology will be much more sensitive to the assumed mode of the turbulent transport mechanism. The main features of interest for mesoscale calculations involve abrupt... 

The micro-scale temperature fluctuations at the water surface give a direct insight into the turbulent transport mechanism and the influence of surface chemical enrichments on near surface turbulence. Figure 9 shows two sequences of three consecutive infrared images of the water surface recorded at a wind speed of 2.2 m s 1 one sequence was taken while the interface was clean, and the other while a surfactant was present. The formation of stationary temperature streaks at the water surface which are oriented parallel to the wind direction occur in both cases and result from organized turbulent structures close to the interface. In presence of the surfactant the temperature streaks seem to be more regularly oriented in the... 

Using heat as a proxy tracer for gases not only delivers the transfer coefficient at high spatial and temporal resolution but also gives direct insight into the spatial structure of near surface turbulence enabling detailed investigations of the influence of surface chemical enrichments on the gas transfer velocity and the turbulent transport mechanisms. 

Uniform Fluid Properties. Analyses of turbulent boundary layers experiencing surface transpiration employ a hierarchy of increasingly complex models of the turbulent transport mechanisms. Most of the analyses, supported by complementary experiments, have emphasized the transpiration of air into low-speed airstreams [110-112], Under these conditions, the fluid properties in the boundary layer are essentially constant, and the turbulent boundary layer can be described mathematically with Eqs. 6.170 and 6.179. In addition, when small quantities of a foreign species are introduced into the boundary layer for diagnostic purposes or by evaporation, the local foreign species concentration in the absence of thermal diffusion is given by... 

Creeping flow of an incompressible Newtonian fluid around a solid sphere corresponds to g (0) = I sin0. For any flow regime that does not include turbulent transport mechanisms, the dimensionless boundary layer thickness is... 

Eddy diffusion as a transport mechanism dominates turbulent flow at a planar electrode ia a duct. Close to the electrode, however, transport is by diffusion across a laminar sublayer. Because this sublayer is much thinner than the layer under laminar flow, higher mass-transfer rates under turbulent conditions result. Assuming an essentially constant reactant concentration, the limiting current under turbulent flow is expected to be iadependent of distance ia the direction of electrolyte flow. 

Linear-eddy modelling of turbulent transport. Part 3. Mixing and differential diffusion in round jets. Journal of Fluid Mechanics 216, 411 —4-35. 

Chapters Turbulent Diffusion. Turbulent diffusion is an important transport mechanism in the atmosphere, oceans, lakes, estuaries, and rivers. In fact, most of the atmosphere and surface waters of the Earth are turbulent. If you are going to work in any of these systems, it will be important to have at least a working knowledge of turbulent diffusion. 

Table 5.1 shows that, with the boundary conditions present in most environmental flows (i.e., the Earth s surface, ocean top and bottom, river or lake bottom), turbulent flow would be the predominant condition. One exception that is important for interfacial mass transfer would be very close to an interface, such as air-solid, solid-liquid, or air-water interfaces, where the distance from the interface is too small for turbulence to occur. Because turbulence is an important source of mass transfer, the lack of turbulence very near the interface is also significant for mass transfer, where diffusion once again becomes the predominant transport mechanism. This will be discussed further in Chapter 8. 

When a solute is transferred from a solid into a high-pressure gas, it is then taken downstream in the bulk fluid by convective transport. Depending on turbulence, the solute may travel further by other mass-transport mechanisms such as dispersion. Dispersion spreads the solute axially and radially in a cylindrical stet. Eaton and Akgerman [30] considered both axial and radial effects in a model for the desorption of heavy organics, from carbon, by a dense gas. 

Calibration of FAGE1 from a static reactor (a Teflon film bag that collapses as sample is withdrawn) has been reported (78). In static decay, HO reacts with a tracer T that has a loss that can be measured by an independent technique T necessarily has no sinks other than HO reaction (see Table I) and no sources within the reactor. From equation 17, the instantaneous HO concentration is calculated from the instantaneous slope of a plot of ln[T] versus time. The presence of other reagents may be necessary to ensure sufficient HO however, the mechanisms by which HO is generated and lost are of no concern, because the loss of the tracer by reaction with whatever HO is present is what is observed. Turbulent transport must keep the reactor s contents well mixed so that the analytically measured HO concentration is representative of the volume-averaged HO concentration reflected by the tracer consumption. If the HO concentration is constant, the random error in [HO] calculated from the tracer decay slope can be obtained from the slope uncertainty of a least squares fit. Systematic error would arise from uncertainties in the rate constant for the T + HO reaction, but several tracers may be employed concurrently. In general, HO may be nonconstant in the reactor, so its concentration variation must be separated from noise associated with the [T] measurement, which must therefore be determined separately. 

Airborne particles may be delivered to surfaces by wet and dry deposition. Several transport mechanisms, such as turbulent diffusion, precipitation, sedimentation, Brownian diffusion, interception, and inertial migration, influence the dry deposition process of airborne particles. Large particles (dNIOAm) are transported mainly by sedimentation hence, large particulate PAHs tend to be deposited nearer the sources of emission Small particles (dblAm), which behave like gases, are often transported and deposited far from where they originated (Baek et al., 1991 Wu et al., 2005). 

Obtain a dimensionless relation for the velocity profile in the neighbourhood of a surface for the turbulent flow of a liquid, using Prandtl s concept of a Mixing Length (Universal Velocity Profile). Neglect the existence of the buffer layer and assume that, outside the laminar sub-layer, eddy transport mechanisms dominate. Assume that in the turbulent fluid the mixing length Xe is equal to 0.4 times the distance y from the surface and that the dimensionless velocity u1 is equal to 5.5 when the dimensionless distance y+ is unity. 

The mass transfer coefficient, K, is defined as the ratio of the mass transport controlled reaction rate to the concentration driving force. The concentration driving force will depend on both turbulent and bulk convection. Bulk convection depends on molecular diffusivity, while the turbulent component depends on eddy diffusivity (4). The mass transfer coefficient considers the combination of the two transport mechanisms, empirically. 

Generally, this vector contains three components, which correspond to the mechanisms characterizing the behavior of the property carriers during their movement. The molecular, convective and turbulent moving mechanisms can together contribute to the vector flux formation [3.6]. In the relation below (3.12), Dj- is the ordinary diffusion coefficient of the property. Dj-, represents the diffusion coefficient of the turbulences and w is the velocity flow vector, then the general relation of the transport flux of the property is ... 

Laminar and turbulent flows, which occur in large pores where the mean pore diameter (say, larger than 1 )jun) is larger than the mean free paths of the fluid molecules involved, do not discriminate components in fluid mixtures. The same applies to molecular diffusion which occurs at relatively high pressures in large pores, llierefore, no separation is effected when only laminar or turbulent flow or bulk diffusion occurs in the pores. The permselectivity of a porous membrane will then have to rely on other transport mechanisms. 

Angular Momentum Transport Mechanisms. 04.4.1.1 Hydrodynamic turbulence. 04.4.1.2 Magnetorotational-driven turbulence... 

The dominant transport mechanism for both aerosol and gaseous agents in the atmosphere is advection associated with the bulk motion of the atmosphere. Since airflows in the planetary boundary layer exhibit signihcant turbulence under most conditions (though turbulence may be suppressed under conditions of temperature inversion), this will cause aerosol releases to disperse into a plume or puff that expands... 

The basic similarity hypothesis states simply that the turbulent transport processes of momentum, heat and mass are caused by the same mechanisms, hence the functional properties of the transfer coefficients are simiiar. The different transport coefficients can thus be related through certain dimensionless groups. The closure problem is thus shifted and henceforth consist in formulating sufficient parameterizations for the turbulent Prandtl Pr )- and Schmidt (Sct) numbers. 

Imboden and Schwarzenbach (1985) have illustrated how the mass-balance equation is a means of accounting for chemical and biological reactions that produce or consume a chemical within a test volume, and for transport processes dial import or export the chemical across the boundaries. Each process acting on a chemical can be characterized by an environmental first-order rate constant, expressed in units of time-1. Transport mechanisms include water renewal by nvers, horizontal and vertical turbulent diffusion, advection by lake particles, and settling of particles (Imboden and Schwarzenbach, 1985). Chemical reaction i ales and reaction half-lives for a wide variety of reactions have been summarized by I loffmann (1981), Pankow and Morgan(1981), Morgan and Stone(1985),and Santsehi (1988). 

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