Model shortcomings

Model shortcomings. The major shortcoming of the single reservoir degassing models is that mantle heterogeneities, especially for He/ He ratios, cannot be explained. Therefore, these models have been superceded by those involving a greater number of reservoirs. 

The simulation methods presented in this section demonstrate the range of approaches developed to tackle the computational challenges associated with simulations of folding processes. Despite limitations intrinsic to each method, along with force field and water model shortcomings, these simulations have been able to provide insight into folding at a level of detail not accessible to experiments. 

If there is sufficient flexibility in the choice of model and if the number of parameters is large, it is possible to fit data to within the experimental uncertainties of the measurements. If such a fit is not obtained, there is either a shortcoming of the model, greater random measurement errors than expected, or some systematic error in the measurements. 

One flux model for a porous medium—the dusty gas model- has already been described in Chapter 3. Although it is perhaps the most important and generally useful model currently available, it has certain shortcomings, and other models have been devised in attempts to rectify these. However, before describing these, we will review certain general principles to which all reasonable flux models must conform. 

Of course, these shortcomings of the Wakao-Smith flux relations induced by the use of equations (8.7) and (8.8) can be removed by replacing these with the corresponding dusty gas model equations, whose validity is not restricted to isobaric systems. However, since the influence of a strongly bidisperse pore size distribution can now be accounted for more simply within the class of smooth field models proposed by Feng and Stewart [49], it is hardly worthwhile pursuing this."... 

To move up the scale of complexity one now needs to consider the energetics o rotation about each bond. The simplest approach is to assume that each bond can be treatec independently 2md that the total energy of the chain is the sum of the individual torsiona energies for each bond. However, this particular model has some serious shortcoming arising from the assumption of independence. 

Much of what we currently understand about the micromechanics of shock-induced plastic flow comes from macroscale measurement of wave profiles (sometimes) combined with pre- and post-shock microscopic investigation. This combination obviously results in nonuniqueness of interpretation. By this we mean that more than one micromechanical model can be consistent with all observations. In spite of these shortcomings, wave profile measurements can tell us much about the underlying micromechanics, and we describe here the relationship between the mesoscale and macroscale. 

S. R, Wilson, G. A. Al-Jumaily, and J. R. McNeil. Nonlinear Characteristics of a Stylus Profilometer. Opt. Eng. 26,953, 1987. This describes modeling stylus profilometer response characteristics and explains their shortcomings. 

The variations of i, 2 and tani with co are shown in Fig. 2.56. It may be seen that the variations of and 2 are of the general shape that would be expected. The tani curve does not go through a maximum and so this is a further shortcoming of this simple model. 

The model is intrinsically irreversible. It is assumed that both dissociation of the dimer and reaction between a pair of adjacent species of different type are instantaneous. The ZGB model basically retains the adsorption-desorption selectivity rules of the Langmuir-Hinshelwood mechanism, it has no energy parameters, and the only independent parameter is Fa. Obviously, these crude assumptions imply that, for example, diffusion of adsorbed species is neglected, desorption of the reactants is not considered, lateral interactions are ignored, adsorbate-induced reconstructions of the surface are not considered, etc. Efforts to overcome these shortcomings will be briefly discussed below. 

Don t be overly alarmed by the large maximum errors for many model chemistr and the rather poor results in general. It is not surprising that these methods are ve inaccurate at predicting thermochemical data. It is because of the knov shortcomings of existing methods that the compound methods to be discussed in t next section were developed. 

We have attempted to cover all aspects of the field, trying to display its richness of thoughts and models, as well as its still considerable shortcomings. However, it is not possible to detail all developments and we apologize for any omissions which others may have included. 

We then turn to a more recent approach to the determination of i by Point [51, 52] in Sect. 3.7 explaining how it differs from that of Lauritzen and Hoffman (LH). Section 3.8 covers other proposed nucleation models and we conclude with an overview of nucleation theories and their successes and most notable shortcomings. 

In order to overcome the shortcomings of the power-law model, several alternative forms of equation between shear rate and shear stress have been proposed. These are all more complex involving three or more parameters. Reference should be made to specialist works on non-Newtonian flow 14-171 for details of these Constitutive Equations. 

The behavior of the Hg/pc interface is very similar to the Hg/DMF interface 294,301,310 It can be qualitatively described by a multistate model.291 However, although the model can reproduce the electric field and temperature dependencies of the inner-layer properties, the shortcomings of the approach should not be overlooked. 1... 

The optimization of empirical correlations developed from the ASPEN-PLUS model yielded operating conditions which reduced the steam-to-slurry ratio by 33%, increased throughput by 20% while maintaining the solvent residual at the desired level. While very successful in this industrial application the approach is not without shortcomings. The main disadvantage is the inherent assumption that the data are normally distributed, which may or may not be valid. However, previous experience had shown the efficacy of the assumption in other similar situations. 

To address the shortcoming of the simple MPI model, Medvedev and coworkers [40] developed a hybrid OpenMP/MPI method that takes advantage of both distributed and shared memory features of these clusters of multiprocessor nodes. The features of this model are ... 

Equation (55) illustrates the major shortcoming of the original electroelastic model it predicts membrane thirming of almost 40%, a result vastly different from the experimental... 

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