# AcuSolve Solver Features (CFD Theory for AcuSolve)

This section on AcuSolve solver features covers the description of various solver features available in AcuSolve such as heat transfer, fluid structure interaction and turbulence modeling.

Commercial solvers for fluid flow analysis come in many forms. They use a collection of numerical methods and approximation approaches to solve the governing continuum equations discussed in this manual.

AcuSolve is a general purpose CFD flow solver that is used in a wide variety of applications and industries. The flow solver is architected for parallel execution on shared and distributed memory computer systems and provides fast and efficient transient and steady state solutions for standard unstructured element topologies.

AcuSolve is based on Galerkin Least Squares (GLS) finite element method. The GLS formulation provides second-order accuracy for spatial discretization of all variables and utilizes tightly controlled numerical diffusion operators to obtain stability and maintain accuracy. In addition to satisfying conservation laws globally, the formulation implemented in AcuSolve ensures local conservation for individual elements. Equal order nodal interpolation is used for all working variables, including pressure and turbulence equations. The semi discrete generalized alpha method is used to integrate the equations in time. This approach has been verified as being second-order accurate in time.

AcuSolve uses a time marching procedure to solve both steady state and transient simulations. In the case of steady state simulations, the inertia (mass) terms of the conservation equations are only included in the Galerkin part of the finite element weighted residual formulation. This inclusion adds stability to the nonlinear iterations. Other parts of the finite element formulation (such as the least-squares operator) do not include the inertia terms. This exclusion accelerates non linear convergence to steady state at the expense of time accuracy. The initial time step size is set to a significantly large value (1.0e+10).

For transient simulations the inertia terms are included in all the operators of the finite element formulation in order to preserve time accuracy.

The resultant system of equations is solved as a fully coupled pressure/velocity matrix system using a preconditioned iterative linear solver. The iterative solver yields robustness and rapid convergence on large unstructured meshes even when high aspect ratio and badly distorted elements are present.

AcuSolve consists of multiple features designed to tackle various flow physics such as temperature flow, solar radiation and rigid body dynamics.