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.
This package is distributed by:
Energy Science and Technology Software Center
P.O. Box 62
1 Science.Gov Way
Oak Ridge, TN 37831
(865) 576-2606 TEL
(865) 576-6436 FAX
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 timestep or a dynamic timestep selection procedure.
NACHOS2 is rectricted 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).
The program is self-contained with its own mesh generator and plotting facilities. It can also be used with stand-alone mesh generators. The batch plotting package in NACHOS2 permits a full range of plots at the option of the user. Plots of the finite element mesh can be generated, as well as contours, time histories, profiles and vector representations of all the relevant dependent variables in an analysis.
NACHOS2 requires the proprietary CA-DISSPLA Version 10.0 graphics software (see http://www.gaeinc.com). This software is not provided.
Keywords: convection, finite element method, incompressible flow, viscous flow.