Learn about the numerical capabilities of * FLOW-3D* models, such as surface tension, granular media, air entrainment and bubble and void regions in our Flow Science Reports section. For more information about the physics models in

*, go to the Modeling Capabilities section >*

**FLOW-3D**## Surface Tension Validation – Simple Test Problems

**Flow problems involving surface tension forces arise in many applications such as microfluidics and liquid fuel behavior in spacecraft.** ** FLOW-3D** has an advanced surface tension model that includes complex geometric, wall adhesion and variable surface tension coefficient capabilities. This document describes a variety of simple problems that have been simulated for which there are simple analytic solutions for comparison. These test cases provide a basic level of validation for the computational model.

## Dynamic Droplet Sizes for Drift Fluxes

**There are many two-fluid mixtures of practical interest. For example, mixtures of air and water or oil and water occur frequently in environmental and industrial processes.** The existing modeling capability in ** FLOW-3D** for mixtures of two fluids is the

*DRIFT*routine that computes the relative motion of the two components arising from buoyancy and viscous forces. This model works quite well for many situations because of its simplicity and robustness.

## Continuum Model for Granular Flow

**We present here a continuum model for computationally predicting the dynamics of a concentrated granular material.** In this model, the designation of concentrated granular flow means the volume fraction of the granular material is often 50% or greater. The model is referred to as a “continuum” model because it is based on a continuous fluid representation of the granular solid, making no attempt to treat individual particles. The model utilizes many existing capabilities of the ** FLOW-3D** computer simulation program, which have been modified and enhanced for granular flow.

## Adding Flow Losses to FAVOR™ Methodology

**In practice, the addition of loss terms to momentum advection can be viewed as variations in the finite-difference expressions for advection much like using a donor cell method as opposed to a central difference or higher order approximation.** The losses only occur at localized positions in a grid where there are area changes in the direction of flow. Refinement of a grid means that these loss adjustments shrink with the grid size because they are confined to cells only where FAVOR™ area fractions are changing. These losses also help bring neighboring velocities across an area change together as a grid is refined, a necessity for fluid continuity.

## Core Gas Model

**The making of resin-bonded sand castings has made great strides in quality over its long history.** Even so, there remain some process-related defects that are not fully understood and can cause quality issues. For instance, chemical binders in the sand can produce gas when heated by the molten metal and if not vented adequately, the gas may flow into the metal resulting in a gas porosity defect. This is most likely with cores that form thin interior features of castings that heat up quickly and have long venting paths. The core gas model in ** FLOW-3D** is designed to predict the possibility of such gas defects and is intended to help design core venting that would evacuate safely all the binder product gas from the cores.

## Compliant Mooring Line Model

**A compliant mooring line model has been developed and implemented in FLOW-3D v11.1.** Using a finite segment approach, the model numerically calculates the full 3D dynamics of the mooring lines and the dynamic coupling between the mooring lines and tethered moving objects. Multiple mooring lines with different lengths, diameters, mass densities and other physical properties are allowed to exist in one simulation. A mooring line can have both of its ends connected to moving objects or one end connected to a moving object and the other anchored at a fixed location. Mooring lines can be taut or slack and may fully or partially rest on the sea/river bed. Gravity, buoyancy, fluid drag and tension are considered for calculations of motion and instantaneous shape of the mooring lines. Static equilibrium is assumed for the initial condition of each line.

## Electrically Conducting Fluid/Solid Boundaries

**A weakly conducting fluid containing a positive electric charge is entering a pipe with conducting solid walls.** The pipe is assumed to be grounded on its outside surface (to insure this, a separate solid was wrapped around the outside and defined to be a perfect conductor at zero potential). This problem was originally investigated by a * FLOW-3D* user, Harold Walmsly, who raised questions about simulation results when the solid/fluid boundary is, or is not, coincident with a mesh/grid line.

## Modeling Turbulent Entrainment of Air at a Free Surface

**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.

## Moisture Drying Model

**In the manufacture of paper it is necessary to remove all water from the paper before it is rolled up.** The majority of water is typically removed by squeezing the paper between large rollers. The remaining moisture can be removed by forcing hot air through the paper to accelerate its evaporation. Using heated air can be an expensive process so there is interest in investigating optimum arrangements for achieving the fastest and least expensive means of removing the residual water from paper.

## Sedimentation Scour Model

**Fully-coupled with fluid dynamics, the sediment scour model in FLOW-3D simulates all the sediment transport processes of non-cohesive soil including bedload transport, suspended load transport, entrainment and deposition.** It allows multiple sediment species with different properties including grain size, mass density, critical shear stress, angle of repose, and parameters for entrainment and bedload transport. The model works for both 3D flows and 2D shallow water flows.

## Surface Tension Modeling

**The modeling of surface tension forces is computationally difficult because it requires the evaluation of surface curvatures, i.e., second derivatives of the surface location.** This is particularly true in ** FLOW-3D **since it uses a rectangular grid that does not conform to surface shapes. Although this simple grid structure makes it more difficult to evaluate surface slopes and curvatures, it is this feature that also gives the strength needed to simulate coalescence and breakup of fluids.

## The Sponge Layer Method

**In coastal and ocean engineering applications, a computational domain with a limited size and open boundaries is often used to simulate periodic wave propagation in open water.** When a wave train moving through the domain reaches an open boundary, a proper boundary condition must be used to minimize wave reflection. Otherwise, incorrect wave shape, severe water volume change and unphysical wave-structure interaction may occur. In general, two types of methods exist to reduce wave reflection at open boundaries: the radiation boundary condition and the sponge, or wave-absorbing, layer method.

## Validating the Thermal Stress Evolution Model

**Thermal stresses develop during the solidification of a casting due to non-uniform cooling of the casting part.** These stresses are driven by thermal gradients, expansion and contraction of metal and its interaction with the mold. Engineers are interested in studying thermally induced stresses and the resulting deformations to gain insights in to the integrity of the part. The Thermal Stress Evolution (TSE) model in ** FLOW-3D** is one such tool that allows casting engineers to study this phenomenon. In this paper, the stresses predicted by the TSE model are validated with experimental results.

## Void Regions and Bubble Models

**Void cells represent regions of gas in which spatial variation of pressure and temperature, inertia and friction at the interface with fluid can be neglected.** These assumptions are generally valid if the gas density is much smaller than that of the fluid, the gas speed is comparable with that of the fluid, and the speed of sound in the gas is much greater than the speed of the mean flow. All these conditions are present in many situations such as mold filling with liquid metal, water flow in rivers and ducts, microfluidic devices and so on.