# Problem Description

The Problem Description branch under the Global Tree describes the general parameters of the simulation such as title, subtitle, analysis type and flow equation.

- Title
- Provides a description of the problem. The title should not exceed 1,023 characters. Title is an optional description for the problem and has no impact on the solution.
- Sub title
- The sub title of the problem is used to further describe the problem. This parameter is also limited to 1,023 characters and is an optional description of the problem.
- Analysis Type
- This parameter specifies the type of analysis being performed, namely steady state or transient. In either case, a time marching scheme is used to solve the problem. In the case of steady state type, the inertia (mass) terms of the conversation equations are only included in the Galerkin part of the finite element weighted residual formulation. This inclusion adds stability to the non linear 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. For transient type, the inertia terms are included in all the operators of the finite element formulation in order to preserve time accuracy.
- Flow Equation
- This parameter specifies the equations related to the flow physics to be solved in the problem. It has two options of Navier- Stokes and Stokes equations. The Navier-Stokes equation describes the motion of general viscous fluid. The Stokes equation is used to describe the low velocity, high viscosity flows in which the convective inertial forces are negligible compared to viscous forces. The only change for stokes flow is the removal of the (convective) terms from the Navier-Stokes equation.
- Abs. pressure offset
- Usually in an incompressible flow regime the absolute pressures at the boundaries do not affect the solution. Only the pressure difference plays the key role. However, in the compressible flow regime the absolute pressures are important and this parameter Abs. pressure offset specifies the offset to convert to absolute pressure units. For example, if Abs. pressure offset = 101325 Pa and if you define the outlet Pressure as zero, then the absolute pressure at the outlet = 101325+0 = 101325 Pa.
- Viscoelastic equation
- This parameter specifies the viscoelastic material model equation to be used to obtain the viscoelastic stresses in the simulation. AcuSolve supports two types of equations namely, Upper Convected Maxwell and Upper Convected Maxwell Log Model. If the simulation does not involve viscoelastic material effects you can use the option None for this parameter.
- Temperature Equation
- This parameter specifies the type of energy equation to be solved in the problem. AcuSolve supports the Advective-diffusive equation for transport of temperature. If the problem does not involve temperature as a variable then this equation can be set to None.
- Abs. temperature offset
- This parameter specifies the offset to convert to absolute temperature units. If specified, this value is added to all temperatures specified in the simulation.
- Radiation equation
- This parameter specifies the addition of radiation type. AcuSolve supports Enclosure type radiation equation. If the problem does not involve radiation then this equation can be set to None.
- Species equation
- This parameter specifies multi-species transport equation type. AcuSolve supports Advective Diffusive equation for multiple species transport.
- Num. species
- This parameter specifies the number of species present in the problem and is used only with Advective Diffusive species equation.
- Turbulence Equation
- Specifies type of Turbulence equation to be solved. AcuSolve provides a wide range of options in turbulence modeling. The available options for
turbulence modeling are listed below.
- Laminar: Used for Laminar Flow, that is, no turbulence
- Spalart-Allmaras: Reynolds-averaged Navier-Stokes (RANS) based one-equation model
- SST (Shear Stress Transport): RANS based two-equation model
- K Omega: RANS based two-equation model
- Detached Eddy Simulation (DES): Spalart-Allmaras based detached eddy simulation
- SST-DES: Shear stress transport based detached eddy simulation model
- Dynamic LES: Dynamic sub-grid Large Eddy Simulation model
- Classical LES: Classical (Smagorinsky) Large Eddy Simulation model

- Mesh Type
- This parameter specifies the type of mesh motion involved in the simulation. The
following are the available options with the mesh type.
- Fixed: The mesh motion is fixed which means there is no mesh movement of any kind.
- Fully Specified: This option is used when the mesh motion is completely specified using the MESH MOTION command.
- Arbitrary Mesh Movement (ALE): Mesh motion with this option is based on the Arbitrary Lagrangian-Eulerian (ALE) technique involving the benefits of both the Lagrangian based and Eulerian based mesh motions.

- External Code
- This option of on/off specifies whether an external solid/structural code is coupled with AcuSolve to solve a coupled simulation. Forces, moments and heat fluxes are communicated to the solid/structural code while displacements are returned to AcuSolve. The code coupling interface (CCI) library handles the required communication and interpolations. Once a solid/structural code is linked to this library, that code can use CCI functions to run in conjunction with AcuSolve.
- Particle Trace
- This option of on/off specifies whether the AcuTrace code is coupled with AcuSolve to solve a particle-flow problem. Flow quantities like velocity, pressure, temperature, species and so on are communicated to AcuTrace and particle states and source terms are returned to AcuSolve.
- Running average
- This option of on/off specifies whether tocreate running average solution fields. If
running average is set on, then a running average field will be created for every
equation field in the problem. The creation of running average fields does not itself
have any effect on the solution. A running average field is defined by
(1) $$\overline{{f}_{n}}=\frac{1}{N}{f}_{n}+\left(1-\frac{1}{N}\right)\overline{{f}_{n-1}}$$Where ${f}_{n}$ is a solution field (such as velocity and pressure) at step $n$ ;

- $\overline{{f}_{n}}$ is the running average of ${f}_{n}$
- $N$ =min (step, ${N}_{max}$ ) is the number of steps used for averaging
- ${N}_{max}$ = the maximum steps used for averaging, given by Running average steps

- Running average steps
- This option is active only when Running average is set to on and it specifies the maximum number of time steps to average solution fields ( ${N}_{max}$ ).