NACHOS2, Incompressible Viscous Fluid Dynamic

1 - Description of program or function: NACHOS2 is a finite element program designed for the analysis of two-dimensional, incompressible viscous fluid flow problems. The basic flows considered may be isothermal, nonisothermal, or may involve other physical processes, such as mass transport. Both steady and transient flows may be analyzed. The class of problems treated are those described by the two-dimensional (plane or axisymmetric) incompressible form of the Navier-Stokes equations. An energy transport equation is included in the formulation for problems in which heat transfer effects are important. Two auxiliary transport equations can be added to describe other physical processes, e.g. mass transfer, chemical reactions. Among the specific types of flow problems treated are: isothermal flow: forced, free, or mixed convection; conjugate heat transfer; flow in saturated porous media with or without heat transfer; and inelastic, non-Newtonian flows with or without heat transfer. Other problem classes are possible depending on the specific definitions applied to the auxiliary transport equations. 2 - Method of solution: NACHOS2 is based on the Galerkin form of the finite element method. It utilizes an element library consisting of six-node triangular elements, and eight-, and nine-node quadrilateral elements. The fluid velocity and temperature are approximated using quadratic interpolation. The pressure is represented using either linear or bilinear functions. The frontal solution method is used for direct solution of the matrix problem. All equations in a problem are solved in a fully coupled manner. For steady-state simulations the standard Picard method is augmented with a full Newton method and a quasi-Newton procedure. Transient analyses are performed using either a backward Euler or a trapezoidal rule integration procedure. Either method can be run with a fixed time step or a dynamic time step selection procedure. 3 - Restrictions on the complexity of the problem: NACHOS2 is restricted to problems for which the fluid of interest may be assumed incompressible within the context of the Boussinesq approximation. Isotropic behavior is assumed for all fluids, solids, and porous materials; all fluids are assumed to obey a Newtonian or generalized Newtonian (inelastic) constitutive law. No provision is made for the explicit computation of fluid/fluid interfaces (i.e. free surfaces)