Overview

In free-surface flows the turbulence in the liquid may be sufficient to disturb the surface to the point of entraining air into the flow. This process is important, for example, in water treatment where air is needed to sustain microorganisms for water purification and in rivers and streams for sustaining a healthy fish population. Air entrainment is typically engineered into spillways downstream of hydropower plants to reduce the possibility of cavitation damage at the base of the spillway. Situations where air entrainment is undesirable are in the sprue and runner systems used by metal casters, and in the filling of liquid containers used for consumer products.

The importance of being able to predict the amount and distribution of entrained air at a free liquid surface has led to the development of a unique model in FLOW-3D®. The model has two options. One option, to be used when the volume fraction of entrained air is relatively low, uses a passive scalar variable to record and transport the air volume fraction. This model is passive in that it does not alter the dynamics of the flow.

The second air-entrainment model option is based on a variable density formulation. This model includes the “bulking” of fluid volume by the addition of air and the buoyancy effects associated with entrained air. This dynamically coupled model cannot, however, be used in conjunction with heat transport and natural (thermal) convection.

In addition, when using the variable density formulation, the model can include a relative drifting of air in water, the possible escape of air if it rises to the surface of the water and the removal or addition of air to trapped bubble regions represented as adiabatic bubbles.

The same basic entrainment process is used in both options. It is based on a competition between the stabilizing forces of gravity and surface tension and the destabilizing effects of surface turbulence.

Because turbulence is the main cause of entrainment, a turbulence-transport model must be used in connection with the air-entrainment model. It is recommended that the RNG version of the more traditional k-epsilon turbulence model be employed. All the validation tests reported in this Technical Note were performed using the RNG model.

[다운로드]

FSR_01-12_Air-Entrainment-Report

*FLOW-3D*/MP v6.1 은 FLOW-3D v11.1 솔버에 기초하여 물리 모델, 특징 및 그래픽 사용자 인터페이스가 동일합니다. *FLOW-3D* v11.1의 새로운 기능은 아래 파란색으로 표시되어 있으며 *FLOW-3D*/MP v6.1 에서 사용할 수 있습니다. 새로운 개발 기능에 대한 자세한 설명은 *FLOW-3D* v11.1에서 새로운 기능을 참조하십시오.

- Structured finite difference/control volume meshes for fluid and thermal solutions
- Finite element meshes in Cartesian and cylindrical coordinates for structural analysis
- Multi-Block gridding with nested, linked, partially overlapping and conforming mesh blocks
- Fractional areas/volumes (FAVOR™) for efficient & accurate geometry definition
- Mesh quality checking
- Basic Solids Modeler
- Import CAD data
- Import/export finite element meshes via Exodus-II file format
- Grid & geometry independence
- Cartesian or cylindrical coordinates

Flow Type Options

- Internal, external & free-surface flows
- 3D, 2D & 1D problems
- Transient flows
- Inviscid, viscous laminar & turbulent flows
- Hybrid shallow water/3D flows
- Non-inertial reference frame motion
- Multiple scalar species
- Two-phase flows
- Heat transfer with phase change
- Saturated & unsaturated porous media

Physical Modeling Options

- Fluid structure interaction
- Thermally-induced stresses
- Plastic deformation of solids
- Granular flow
- Moisture drying
- Solid solute dissolution
- Sediment transport and scour
- Cavitation (potential, passive tracking, active tracking)
- Phase change (liquid-vapor, liquid-solid)
- Surface tension
- Thermocapillary effects
- Wall adhesion
- Wall roughness
- Vapor & gas bubbles
- Solidification & melting
- Mass/momentum/energy sources
- Shear, density & temperature-dependent viscosity
- Thixotropic viscosity
- Visco-elastic-plastic fluids
- Elastic membranes & walls
- Evaporation residue
- Electro-mechanical effects
- Dielectric phenomena
- Electro-osmosis
- Electrostatic particles
- Joule heating
- Air entrainment
- Molecular & turbulent diffusion
- Temperature-dependent material properties
- Spray cooling

Flow Definition Options

- General boundary conditions
- Symmetry
- Rigid and flexible walls
- Continuative
- Periodic
- Specified pressure
- Specified velocity
- Outflow
- Grid overlay
- Hydrostatic pressure
- Volume flow rate
- Non-linear periodic and solitary surface waves
- Rating curve and natural hydraulics
- Wave absorbing layer

- Restart from previous simulation
- Continuation of a simulation
- Overlay boundary conditions
- Change mesh and modeling options
- Change model parameters

Thermal Modeling Options

- Natural convection
- Forced convection
- Conduction in fluid & solid
- Fluid-solid heat transfer
- Distributed energy sources/sinks in fluids and solids
- Radiation
- Viscous heating
- Orthotropic thermal conductivity
- Thermally-induced stresses

Turbulence Models

- RNG model
- Two-equation k-epsilon model
- Two-equation k-omega model
- Large eddy simulation

Metal Casting Models

- Thermal stress & deformations
- Iron solidification
- Sand core blowing
- Sand core drying
- Permeable molds
- Solidification & melting
- Solidification shrinkage with interdendritic feeding
- Micro & macro porosity
- Binary alloy segregation
- Thermal die cycling
- Surface oxide defects
- Cavitation potential
- Lost-foam casting
- Semi-solid material
- Core gas generation
- Back pressure & vents
- Shot sleeves
- PQ
^{2} diagram - Squeeze pins
- Filters
- Air entrainment
- Temperature-dependent material properties
- Cooling channels
- Fluid/wall contact time

Numerical Modeling Options

**TruVOF** Volume-of-Fluid (VOF) method for fluid interfaces- First and second order advection
- Sharp and diffuse interface tracking
- Implicit & explicit numerical methods
- GMRES, point and line relaxation pressure solvers
- User-defined variables, subroutines & output
- Utilities for runtime interaction during execution

Fluid Modeling Options

- One incompressible fluid – confined or with free surfaces
- Two incompressible fluids – miscible or with sharp interfaces
- Compressible fluid – subsonic, transonic, supersonic
- Stratified fluid
- Acoustic phenomena
- Mass particles with variable density or diameter

Shallow Flow Models

- General topography
- Raster data interface
- Subcomponent-specific surface roughness
- Wind shear
- Ground roughness effects
- Laminar & turbulent flow
- Sediment transport and scour
- Surface tension
- Heat transfer
- Wetting & drying

Advanced Physical Models

- General Moving Object model with 6 DOF–prescribed and fully-coupled motion
- Rotating/spinning objects
- Collision model
- Tethered moving objects (springs, ropes, mooring lines)
- Flexing membranes and walls
- Porosity
- Finite element based elastic-plastic deformation
- Finite element based thermal stress evolution due to thermal changes in a solidifying fluid
- Combusting solid components

Chemistry Models

- Stiff equation solver for chemical rate equations
- Stationary or advected species

Porous Media Models

- Saturated and unsaturated flow
- Variable porosity
- Directional porosity
- General flow losses (linear & quadratic)
- Capillary pressure
- Heat transfer in porous media
- Van Genunchten model for unsaturated flow

Discrete Particle Models

- Massless marker particles
- Mass particles of variable size/mass
- Linear & quadratic fluid-dynamic drag
- Monte-Carlo diffusion
- Particle-Fluid momentum coupling
- Coefficient of restitution or sticky particles
- Point or volumetric particle sources
- Charged particles
- Probe particles

Two-Phase & Two-Component Models

- Liquid/liquid & gas/liquid interfaces
- Variable density mixtures
- Compressible fluid with a dispersed incompressible component
- Drift flux
- Two-component, vapor/non-condensable gases
- Phase transformations for gas-liquid & liquid-solid
- Adiabatic bubbles
- Bubbles with phase change
- Continuum fluid with discrete particles
- Scalar transport
- Homogeneous bubbles
- Super-cooling

Coupling with Other Programs

- Geometry input from Stereolithography (STL) files – binary or ASCII
- Direct interfaces with EnSight
^{®}, FieldView^{®} & Tecplot^{®} visualization software - Finite element solution import/export via Exodus-II file format
- PLOT3D output
- Neutral file output
- Extensive customization possibilities
- Solid Properties Materials Database

Data Processing Options

- State-of-the-art post-processing tool, FlowSight™
- Batch post-processing
- Report generation
- Automatic or custom results analysis
- High-quality OpenGL-based graphics
- Color or B/W vector, contour, 3D surface & particle plots
- Moving and stationary probes
- Measurement baffles
- Arbitrary sampling volumes
- Force & moment output
- Animation output
- PostScript, JPEG & Bitmap output
- Streamlines
- Flow tracers

User Conveniences

- Active simulation control (based on measurement of probes)
- Mesh generators
- Mesh quality checking
- Tabular time-dependent input using external files
- Automatic time-step control for accuracy & stability
- Automatic convergence control
- Mentor help to optimize efficiency
- Change simulation parameters while solver runs
- Launch and manage multiple simulations
- Automatic simulation termination based on user-defined criteria
- Run simulation on remote servers using remote solving

Multi-Processor Computing