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A theoretical framework for studying supercooling and ice formation in turbulent waters is developed. The basic idea is that the problem can be described by a boundary layer theory in which buoyancy effects become important because of suspended ice crystals. A mathematical model based upon transient Ekman dynamics is formulated and explored. The model is based on the conservation equations for mean momentum, heat energy, salinity, and suspended ice concentration in their one‐dimensional form. The turbulent exchange coefficients are calculated with a two‐equation model of turbulence. The ice nucleation is assumed to be secondary, which means that ice crystals are assumed to be always present in the water as a result of mass exchange with the atmosphere. In the model the mass exchange is treated as a surface boundary condition for the ice concentration equation. Calculated time histories of temperature and ice concentration for different meteorological conditions and different rise velocities are presented and discussed. The results are in good qualitative agreement with field and laboratory measurements. The importance of the strong interaction between the ice formation process and the hydrodynamics of the boundary layer is emphasized.

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A computer code for simulation of groundwater flow and transport is described. Both porous and fractured media are handled by the code. The main intended application is the analysis of a deep repository for nuclear waste and for this reason flow and transport in a sparsely fractured rock is in focus. The mathematical and numerical models are described in some detail. In short, one may say that the code is based on the traditional conservation and state laws, but also embodies a number of submodels (subgrid processes, permafrost, etc). An unstructured Cartesian grid and a finite volume approach are the key elements in the discretization of the basic equations. A multigrid solver is part of the code as well as a parallelization option based on the SPMD (Single-Program Multiple-Data) method. The main application areas are summarized and an application to a deep repository is discussed in some more detail.

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