Reliability, mesoscale models

In this chapter we focus on atomistic predictions of thermophysical and mechanical properties of HMX crystals and liquid important to the development of reliable mesoscale equations of state. The outline of the remainder of the chapter is as follows In section 2 we describe briefly the philosophy and overall approach we have taken to force field development, including the results of quantum chemistry calculations for HMX and smaller model compounds that were used in the force field parameterization. The focus of section 3 is on the properties of liquid HMX, for which experimental data are completely lacking. Structural, thermal, and mechanical properties of the three pure crystal polymorphs of HMX are presented in section 4, where the results are compared to the available experimental data. At the ends of sections 3 and 4 we discuss briefly the importance of the various properties with mesoscale models of high explosives, with an emphasis on conditions relevant to weak shock initiation. We conclude in section 5, and provide our opinions (and justifications, based on our interactions with mesoscale modelers) regarding which HMX properties and phenomena should comprise the next targets for study via atomistic simulation. 

As explained in Table 2.4, these mesoscale models take many hours to compute a single meteorological situation. Nevertheless, they are sufficiently reliable that they are used operationally in the USA for short term forecasts and for computing the future scenarios of air quality in large urban areas. For complex dispersion e.g. from a localised source, stochastic simulations are sometimes used, (e.g. NAME model of UK Met Office, Maryon and Buckland [396] (1995)). Even then their predictions have to be supplemented with more detailed input on the properties of the turbulence. 

It is meaningful to examine the relation between microscale model, mesoscale model, and micromodel. For reaction kinetics, microscale and mesoscale models adopt the same kinetics that based on element reaction system. For diffusion, mesoscale model embodies two diffusion mechanisms (one for micropores and another for mesopores and macropores), and microscale model considers one diffusion mechanism since it only has micropores. No diffusion was considered within the macropores. It is obvious that the mesoscale model possesses the same theoretical foundation as the microscale model, but its application scope has been enlarged compared to the microscale model. Therefore, it could be reliably used as a tool to derive some parameters, such as effective chemical kinetics and effective diffusion parameters, for macroscale model. In the section following, we discuss the method on how to link the microscale kinetics to the lumped macroscale kinetics via the mesoscale modeling approach. 

The consistency and reliability of well designed model libraries is inevitably getting lost over time. Now, even though the market for these simulators is in full evolution, spectacular progress is not expected because the basic models of the units stay at mesoscale or macroscale. 

In summary, while most studies of atmospheric boundary layer flows have used local theories involving eddy transport coefficients, it is now recognized that turbulent transport coefficients are not strictly a local property of the mean motion but actually depend on the whole flow field and its time history. The importance of this realization in simulating mean properties of atmospheric flows depends on the particular situation. However, for mesoscale phenomena that display abrupt changes in boundary properties, as is often the case in an urban area, local models are not expected to be reliable. 

A problem area that is not so amenable to mesoscale methods is polymer crystallization. This has proven to be one of the most difficult computational challenges in all of polymer science because the pertinent phenomena operate simultaneously over a wide range of length scales. The pol5uner crystallizes into a particular space group because of atomic detail, and the mechanical properties of the crystallites are determined by, and can only be calculated reliably with, atomic force fields with all atoms represented (126,127). Yet the size of the crystallites or spherulites is so large as to require mesoscopic methods for comprehension. But a crystalline polymer is almost never 100% crystalline. The interphases between crystalline and amorphous domains, with the possibilities for adjacent or nonadjacent reentry and tie-chain distributions, are critical to the properties of semicrystalline polymers. Only recently have models been developed (203) to rigorously address this problem area. 

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