Home
Flow Solver
Contains flow solver related guides, such as the reference and API guides.
Flow solver reference
Insert short description here
Modeling humidity
The flow solver uses the gas mixture scalar equation to model humidity that is defined as a mixture of water vapor in air.
Condensation and evaporation
In transient analysis, the flow solver computes condensation and evaporation at walls when the ambient fluid is air.

Flow Solver
Contains flow solver related guides, such as the reference and API guides.
- Flow solver guides
  Maya HTT's flow solver is included and distributed with Simcenter 3D and Simcenter Femap. This page contains links to flow solver guides, which help you understand how the flow solver works.
- Flow solver reference
  Insert short description here
  - Introduction
    The flow solver computes a solution to the non-linear, partial differential equations for the conservation of mass, momentum, energy, and general scalars in general complex 3D geometries. It uses an element-based finite volume method and iterative Krylov methods to discretize and solve the governing equations.
  - Nomenclature
    The nomenclature table offers a description and sample units or constant values of the variables used in the Flow Solver Reference Manual.
  - Governing equations
    A model can be comprised of multiple flow enclosures. Each enclosure is a separate group of three-dimensional elements that form a separate fluid domain from the remaining geometry. The physical scales of each enclosure, such as the length scale, time scale, pressure, and temperature are independent of the other enclosures. The flow solver solves governing equations separately for each flow enclosure.
  - Source term
    The source term can represent body forces or flow resistance forces to model different physical phenomena.
  - Turbulence models
    In the flow solver, the flow field can be solved as laminar or as turbulent using governing equations.
  - Boundary conditions
    This section outlines different boundary conditions supported by the flow solver and the associated considerations.
  - Modeling a gas mixture
    The flow solver uses the scalar equation to model a gas mixed in any proportion with the main gas. The software supports up to five gases in the mixture. All gases are assumed to behave as ideal gases using the ideal gas law.
  - Modeling humidity
    The flow solver uses the gas mixture scalar equation to model humidity that is defined as a mixture of water vapor in air.
    - Condensation and evaporation
      In transient analysis, the flow solver computes condensation and evaporation at walls when the ambient fluid is air.
    - Fog formation
      The flow solver computes the fog formation in a gas mixture. The gas mixture constitutes the continuum phase, and the airborne condensate that forms the dispersed phase.
    - Semi-permeable flow surface
      The flow solver models semi-permeable flow surfaces for humid air using two methods.
  - Modeling non-Newtonian fluid
  - Modeling particle tracking
    The following section describes the equations and forces that model particle trajectories, including treatment and effects of boundary conditions
  - Modeling fluid structure interaction
    Fluid structure interaction (FSI) computations account for motion or deformation of the solid structure boundaries that are interacting with the adjacent fluid flow. The flow solver handles both transient and static fluid structure interactions.
  - Discretization
    The following section describes the discretization methods used by the flow solver to discretize the governing equations.
  - Solver numerical methods
    The following section describes numerical methods used by the flow solver such as the linear equation solution, which acts as a preconditioning method to the basic iterative algorithm, and other solver methods, such as Krylov methods.
  - Flow solver files
    The following table describes the flow solver files, which are written in the run directory during the solving process.
  - Flow solver bibliography
    The following bibliography lists all sources used to explain how the flow solver works.
- Flow solver flowcharts
  This section consists of flowcharts that illustrate the methodology of the flow solver when you run a simple steady-state or transient simulation, and does not cover all features and situations. It also highlights how the flow solver is coupled to the thermal solver for a simple steady-state or transient coupled thermal-flow simulation.

Condensation and evaporation

In transient analysis, the flow solver computes condensation and evaporation at walls when the ambient fluid is air.

The model assumes film type condensation or evaporation with the following conditions:

The water film on the surface of the walls is very thin.
The temperature of the liquid film is assumed to be the same as that of the wall.
The rate of mass transfer between condensate and the air is small.
The presence of the condensate does not affect the heat transfer coefficient of the surface.

Mass and scalar conservation equations

The flow solver calculates the flux of water vapor from the film to the air as follows [13]:

If the water vapor density is greater than the density at saturation (i.e. 100% relative humidity), then the flow solver evaluates the water vapor flux from:

The mass transfer coefficient, h_m, is evaluated from the Lewis relation:

ρ_B,sat(T_s) is the saturation density of water vapor at the wall temperature.
ρ_Bf is the density of the water vapor in the fluid.
h represents the heat transfer coefficient computed by the thermal solver.
C_pm is the specific heat per unit volume of the air-vapor fluid mixture.
ρ_Af is the density of air in the fluid.
α/D_v is the ratio of the thermal diffusivity of the fluid mixture to the diffusivity of the water vapor, also called the Lewis number.

Evaporation corresponds to a positive flux of water vapor to the fluid, while condensation corresponds to a negative flux.

The flow solver uses the mass flux as a source term in both the mass and momentum equations and the scalar equation.

Energy conservation equation

The addition or retrieval of water vapor to or from the air as a result of evaporation or condensation also affects the overall enthalpy of the fluid mixture. The energy flux to the fluid mixture resulting from evaporation or condensation is evaluated as:

C_p,bf is the specific heat of the water vapor at the fluid temperature, T_f.

This energy flux is a source term in the energy equation.