Eulerian grid

Fig. 6. Marker particles in an Eulerian grid. The marker particles are used to track the interface dynamically.

Discussions in Chapter 2 may be referred to for explanations of the various symbols. It is straightforward to apply such conservation equations to single-phase flows. In the case of multiphase flows also, in principle, it is possible to use these equations with appropriate boundary conditions at the interface between different phases. In such cases, however, density, viscosity and all the other relevant properties will have to change abruptly at the location of the interface. These methods, which describe and track the time-dependent behavior of the interface itself, are called front tracking methods. Numerical solution of such a set of equations is extremely difficult and enormously computation intensive. The main difficulty arises from the interaction between the moving interface and the Eulerian grid employed to solve the flow field (more discussion about numerical solutions is given in Chapters 6 and 7). 

Eulerian grid box or Lagrangian particle tracking methods using turbulence models (not suitable for real time emergency response computation), above canopy. Input from obstacle or continuum scale data (for local sources). Upwind data for distant sources. 

The marker points move at the loeal flow veloeity interpolated from the Eulerian grid. The interpolation seheme is important sinee interpolation error may result in a violation of mass eonservation. Ideally, the interpolation scheme must satisfy the mass eonservation at the diserete level in the same way as done in the flow solver [19]. However, the Peskin distribution function or simple bi-linear interpolation is usually used in the fronttracking method [3]. Note that none of these interpolation sehemes satisfies the mass conservation at diserete level. This is in fact an important factor for the change in drop volume espeeially for long simulations. 

The NASA has developed the dispersion code AFGASDM applicable to LH2 and other aviation fuels [22], which is in a sort of interim stage between a Gaussian model and an Eulerian grid model. The model solves the conservation equations following a gas parcel released as a puff until its dilution below the flammability limits. It takes account of air entrainment and phase changes. 

These are time-dependent models which combine the use of an Eulerian grid with marker particles. Each marker represents a fixed mass of pollutants... 

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