Typical fluid-structure interation (FSI) simulations attempt to couple the full structural analysis FEA code with the CFD solver and compute the structural stresses and deformations after each iteration. The approach taken in the EZ_FSI product takes advantage of a lumped-mass model of the solid part to simplify the analysis.

First, the solid part is meshed and the fluid-structure interaction points (the "wetted" points) are identified. The solid part is then analyzed to determine lumped mass, damping, and stiffness values for the wetted points. The lumped system will behave exactly as the full system for linear systems with small deformations.

Next, the fluid portion of the domain is meshed and the fluid structure interaction points are identified as a special moving boundary. As the transient, CFD solution progresses, the pressure on the wetted points is used as a forcing function on the lumped mass system to determine the new location at the current time. The boundary is moved to the new location, the CFD mesh is updated, and the process repeats. This is all done within the CFD solver -- eliminating the need to communicate with an external structural analysis package every iteration.

Finally, after completing the CFD analysis, the transient pressure data can be imported into the structural analysis package to determine dynamic stresses within the solid.

The tool, dubbed the Urban Dispersion Simulator (UDS), must overcome such modeling obstacles as geometry acquisition, meshing, and attaining a solution on building to city blocks scale. Additionally, prevailing wind and turbulence fields, buoyancy, solar radiation, tree-induced flow losses, HVAC effects, and the dispersal of gas-phase agents as well as solid and liquid dispersants must be considered.

To facilitate rapid modeling of such large-scale structures, CFDRC has hit upon an image based approach to model generation. By analyzing an image representing a height map of the area of interest, a voxel-based solid model is created. The solid model is then used to create a CFD mesh using a 2^N tree-based mesh generator.

The UDS development is being funded under a Small Business Innovation Research contract through the Defense Threat Reduction Agency.

This new level of integration will allow Fluid-Structure interaction between COSMOS/Works and COSMOS/Flow. Fluid-Structure interaction allows transfer of wall forces, computed during CFD analysis, to a structural analysis study. No other analysis software allows this type of coupled analysis for SolidWorks users.

Spherical bubbles tend to rise in pairs, if one bubble is behind, following in the wake of another, it gets speeded up in the upward-moving wake, catches up, hits the top bubble, they tumble, and then both travel in horizontal alignment.

However, the shape of the bubble is dependent on the surface tension of the fluid.

"But as the surface tension drops and the bubbles flatten, this forces the flow to go around in a different way. The ellipsoidal bubbles become little winglets, and that changes the direction of the lift, completely reversing it so that it draws them into the faster moving fluid found in the wakes of passing bubbles. As a result, unlike spherical bubbles, flattened bubbles will sometimes stream together, following each other up in narrow columns,"

In the future, the researchers hope to extend the model to include phase change and mass transfer as well as modeling fundamentally different bubble behavior, for example large single bubbles which can wobble and follow a spiraling pattern as they ascend.

A few of the enhancements in the new version include:

• Interactive Command History Window
• Diagnostic Tools
• Double Precision
• New Elliptic Smoothing Techniques
• Interactive Spotwelds
• More Support for CAD

The Immersive Boundary (IB) Method attempts to model the elastic material as a part of the fluid with additional forces due to elastic stress. The fluid equations are solved in a Lagrangian fashion on a fixed regular lattice mesh. The elastic material is tracked by following a collection of representative points as they move across the mesh. The spacial configuration of these points is used to compute elastic forces which are applied to nearby fluid points in the lattice. In general, the method is applicable to any problem in which a fluid interacts with a flexible material.

The IB method has been applied to a wide variety of problems including a beating three-dimensional heart, valve-less pumping, blood clotting, the swimming motion of fish and eels, and the inner ear.

In such problems, the immersed boundary method opens up the prospect of raw simulation, almost akin to experiment, in which something approaching the full complexity of the system is allowed to operate in the model. -- Charles S. Peskin and David M. McQueen