Rotation used to be thought as an effective method to prohibit the formation of freckles of a binary alloy unidirectionally solidified from below, which we will show in the present paper is true only ...under restrictive conditions. A linear stability analysis is implemented for the system consisting a dendritic mushy region underlying a bulk fluid region and rotating uniformly about a vertical axis. Emphasis is placed on the rotation effects on the competition between the convection modes, which has a direct influence on the formation of freckles. Results show that rotation generally enhances the stability of the convection modes, but the enhancement is not significant until the rotation speed is very high (such as 10
5 rpm), which in some sense agrees well with the previous numerical and experimental results. In spite of its ineffectiveness as an inhibitory method to the formation of freckles, it is found from the present parametric study that the effects of rotation on convection depend largely on the buoyancy ratio
A
of the alloy. Both the changes of the rotation speed and the buoyancy ratio may lead to a switch of the most critical convection mode, which may in turn influence the characteristics of the subsequent plume convection, and thus the formation of freckles.
Air pollution modelling at a regional scale Tulet, P; Crassier, V; Rosset, R
Environmental modelling & software : with environment data news,
01/2000, Volume:
15, Issue:
6
Journal Article
Peer reviewed
A mesoscale simulation system Meso-NH-C allowing for on line coupling between dynamics and chemistry is presented. Further advantages of this system lie in high vertical resolution, nesting ...capabilities and a full set of parametrization schemes. This system is applied from 9 to 11 August 1997 for a pollution episode over Western Europe. In this episode, both a European and a regional scale analysis have been performed (Greater Paris area). The possibilities of such a modelling system have been emphasized, particularly regarding the evaluation of transboundary fluxes.
The direct gamma dose rates due to a stationary Gaussian plume of radionuclides in the atmosphere have been calculated using different models Lagrangian dose model (LDM), Gaussian plume model (GPM), ...and uniform cloud model (UCM), and the results are compared.
The atmospheric parameters (used in the Lagrangian model) like mean and fluctuating wind components, etc., were obtained from the published field data on a neutral atmosphere. In the LDM, a continuous release of radionuclides into the atmosphere was simulated by liberating a large number of Lagrangian particles, whose trajectories were tracked for various hours in a three-dimensional computational domain. A point isotropic source formula was used for calculating the direct gamma dose contribution from all Lagrangian particles constituting the plume. Each particle represented a point source of radioactivity, whose strength was calculated from the known release rate and was subsequently allowed to decay as a function of time.
The comparison of the LDM results with the GPM indicated that both models predict comparable results in a homogeneous atmosphere. The LDM is, however, more versatile, as it can incorporate variation in meteorological data in space and time (of course when available). The UCM also compared well for ground releases; however, it cannot be used for elevated releases and short downwind distances. The purpose of this work was to test the LDM for simulating the transport, dispersion, and decay of a radionuclide plume. The LDM shall later be used for complex topographic and meteorological conditions, where the GPM is not suitable.
Five atmospheric transport models were evaluated for use in Phase II of the Historical Public Exposures Studies at the Rocky Flats Plant. Models included a simple straight-line Gaussian plume model ...(ISCST2), several integrated puff models (RATCHET, TRIAD, and INPUFF2), and a complex terrain model (TRAC). Evaluations were based on how well model predictions compared with sulfur hexafluoride tracer measurements taken in the vicinity of Rocky Flats in February 1991. Twelve separate tracer experiments were conducted, each lasting 9 hr and measured at 140 samplers in arcs 8 and 16 km from the release point at Rocky Flats. Four modeling objectives were defined based on the endpoints of the overall study: (1) the unpaired maximum hourly average concentration, (2) paired time-averaged concentration, (3) unpaired time-averaged concentration, and (4) arc-integrated concentration. Performance measures were used to evaluate models and focused on the geometric mean and standard deviation of the predicted-to-observed ratio and the correlation coefficient between predicted and observed concentrations. No one model consistently outperformed the others in all modeling objectives and performance measures. About 75% of the maximum hourly concentration predictions were within a factor of 5 of the observations. About 64% of the paired and 80% of the unpaired time-averaged model predictions were within a factor of 5 of the observations. The overall performance of the RATCHET model was somewhat better than the other models. All models appeared to experience difficulty defining plume trajectories, which was attributed to the influence of multilayered flow initiated by terrain complexities and the diurnal flow patterns characteristic of the Colorado Front Range.
Three different scenarios are studied to estimate how different structures at the side of a road, influence concentration levels. A solid fence gives the strongest effect on concentration. It forces ...the mean flow to move vertically which promotes mixing in the wake behind the fence. A hedge reduces the level of turbulence and the level of concentration becomes higher due to less turbulent dispersion. A road-valley generates turbulence in an intermediate regime. It captures a part of the plume in the valley which gives high concentrations locally. Atmospheric stability increases the concentration levels at a distance from the topographical disturbance. A heavy gasplume encountering a solid fence is broadened in front of the fence depending on the height of the fence and internal stability in the plume. The relation between these parameters, wind speed and the source rate will also affect the level of concentration downwind the fence.
Thermal plumes in wall free space were numerically simulated with the standard k- e model and the results were compared with the basic experiment which had been conducted by Yokoi. The purpose of ...this study is to confirm the accuracy and reliability of the numerical simulation of thermal plumes from the view point of practical use of simulation for fire safety planning. The flow fields associated with various conditions concerning the computational domain, mesh dividing system and so on were simulated and compared with the Yokoi's formula in order to evaluate the accuracy and reliability of the CFD performance based on the standard k-ε model. The distributions of Cell Reynolds numbers gave us a good criteria for practically reliable simulations.
The main objective of this study is to understand the flow structure close to the roughness elements over which a thick turbulent boundary-layer is developed and the way it interacts with a surface ...mounted hot cube having in mind possible applications to the thermal interaction of a building located in a urban area under light wind or calm atmospheric conditions. The whole structure of the turbulent boundary-layer is studied in what concerns mean flow field, turbulent intensity and shear stress profiles. Boundary-layer integral parameters are calculated and their along wind development is studied, in a relative wide range of different roughnesses. Different flow conditions are used, ranging from natural to forced convection. Particular attention is given to mixed convection situations typical of a wide variety of practical problems such as building environment and industrial equipment. The influence of wind incidence is also studied. The study comprises also a numerical simulation, using a three-dimensional computer code based on the
k–
ε two equation turbulence model. Numerical results are compared with experimental ones in what concerns the development of the turbulent boundary layer itself and the way it interacts with the heated bluff body.
This study concerns the application of a class of `time-iterative' methods to several different classes of problems in fluid mechanics. These new iterative methods combine ideas from multi-stage ...Runge–Kutta (RK) integration together with a selection of Chebyshev iteration parameters. The underlying performance of these Chebyshev parameterized Runge–Kutta (CPRK) solvers is studied for steady-state solution to a representative sampling from: (i) transport involving convection and diffusion; (ii) incompressible viscous flow governed by the Navier–Stokes equations; (iii) supercritical flume flow in shallow-water theory and (iv) compressible, viscous hypersonic gas flow with multiple reacting species. We provide numerical results to demonstrate convergence to the steady-state flow in each case and give graphs showing residual decay for the CPRK and standard RK schemes.
We present experimental results of heat transfer processes in mixed-convective flow from a ducted vertical hot-plate thermal flow sensor for aiding (upward) and opposing (downward) flows. The results ...are obtained for three different Grashof numbers, Gr = 289, 411, and 456, using air, in the Reynolds number range from 0 to 120. The Nusselt number for aiding flows can be adequately described by Nu tot - Nu 0 = (Nu n forced + Nu n free ) 1/n , with Nu 0 = 0.5, as originally proposed by Churchill for a free flat plate. For n, a value of 1.5-1.7 is found. For opposing flows in the mixed-flow region (0.1 h Gr/Re 2 S 10), flow visualization shows an oscillating buoyant plume around the flat plate. In the transition from free to mixed and to essentially forced convection, distinct sequences of instabilities of this plume are observed, leading to several local minima and maxima in the heat transfer from the plate. The results are summarized in a bifurcation diagram. Here, several windows with instabilities are found, both nonperiodic, with strong indications of chaotic behavior, and (quasi-) periodic. Typical fundamental frequencies of the instabilities range from 0.15 to 1 Hz.
PDF
