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Flow Solver
Contains flow solver related guides, such as the reference and API guides.
Flow solver reference
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Modeling particle tracking
The following section describes the equations and forces that model particle trajectories, including treatment and effects of boundary conditions
Brownian and turbulent diffusion
The phenomenon of small scale chaotic motion of particles through a fluid, due to the sub‑continuum interactions with individual fluid particles, is known as Brownian motion. In the case of a turbulent flow, chaotic motion of particles is also observed. It is due to the instantaneous forces arising from the turbulent, continuum-scale fluctuations of the velocity and pressure fields. This chaotic motion is estimated within the boundaries of an element during a particle time step.

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
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  - 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.
  - 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 of the particle motion
      The flow solver uses a one-way coupling between the flow field and the injected particles. Thus, the flow field is not affected by the particle transport problem.
    - Drag force
      The drag force on a particle is defined using the local Reynolds number, which considers the local fluid velocity field unaffected by the particle's presence or motion. By default, the flow solver uses a specific drag coefficient correlation, but you can also specify a fixed drag coefficient for the calculations.
    - Buoyancy
      The flow solver models buoyancy force acting on a spherical particle as a function of the particle's volume and density, as well as the fluid density.
    - Non-drag forces
      The non-drag force on a particle consists of the force due to the pressure gradient in the fluid surrounding the particle, and the added mass force to accelerate the virtual mass of the fluid in the volume occupied by the particle.
    - Brownian and turbulent diffusion
      The phenomenon of small scale chaotic motion of particles through a fluid, due to the sub‑continuum interactions with individual fluid particles, is known as Brownian motion. In the case of a turbulent flow, chaotic motion of particles is also observed. It is due to the instantaneous forces arising from the turbulent, continuum-scale fluctuations of the velocity and pressure fields. This chaotic motion is estimated within the boundaries of an element during a particle time step.
    - Particle motion equations
      The flow solver uses many equations to modelize particle trajectories. This page groups them for easy access.
    - Particle slip correction factor
      Understand how the flow solver corrects the particle traction force when the no-slip assumption cannot be used.
    - Initial conditions for injected particles
      Understand how the flow solver modelizes initial conditions for injected particles for transient and steady state solutions.
    - Boundary treatment
      Boundaries that represent a discontinuity in the equations of particle motion and particle position require a careful treatment. Learn how the flow solver represents these boundaries.
    - Wall boundaries
      The flow solver treats both slip or no-slip wall boundaries identically in the particle transport problem. However it applies different boundary treatments depending on the particle impact type.
    - Inlet and symmetry boundaries
      Learn how the flow solver modelizes the motion of particles for inlets or symmetry boundaries.
    - Periodic boundaries for particle tracking
      Understand how the flow solver relates the aproach and departure velocities for rotational and translational periodic boundaries.
  - 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.

Brownian and turbulent diffusion

The phenomenon of small scale chaotic motion of particles through a fluid, due to the sub‑continuum interactions with individual fluid particles, is known as Brownian motion. In the case of a turbulent flow, chaotic motion of particles is also observed. It is due to the instantaneous forces arising from the turbulent, continuum-scale fluctuations of the velocity and pressure fields. This chaotic motion is estimated within the boundaries of an element during a particle time step.

A particle moves an average distance | r² | in Δt according to , where D_p is the particle diffusivity:

D_T is the turbulent diffusivity associated with the unresolved turbulent motions.
D_B is the Brownian diffusivity associated with the sub-continuum effects.

The turbulent diffusivity is computed based on the turbulent eddy diffusivity, as follows:

The Brownian diffusivity is given by:

In these equations:

μ_t is the eddy viscosity based on the turbulent model.
ρ_f is the fluid density.
μ_f is the fluid viscosity.
k_B is the Boltzmann constant.
T is the fluid temperature.
d is the diameter of the particle.

Therefore the perturbations of velocity field due to Brownian and turbulent forces is estimated as:

The presence of the particle inside the element is estimated as:

Substituting expressions for Brownian and turbulent diffusivities along with the expression for the perturbation of flow field, into the expression for the drag force, becomes:

where the unit vector that represents the direction of the velocity perturbation is generated randomly, and V_e is the element volume of the mesh.

By default, the flow solver computes the Brownian and turbulent diffusivity term . When you deactivated this term, the flow solver uses . Usually, you neglect this term when you apply the particle slip correction. For more information, see Particle slip correction factor.