FLOW-3D News
Winter 2002Application Note: Descendents
Many of the most interesting fossil finds are located along the shores of bodies of water that existed when the fossilized creatures were still alive. In FLOW-3D many of the program's most interesting aspects are also associated with what happens at its shores, for instance, at boundaries separating liquid and gas regions.
In the earliest version of FLOW-3D gas regions were simply assigned a fixed pressure. Subsequently, an option was added to permit the pressure of an isolated gas region (a bubble) to change adiabatically with its volume. Another, more recent, descendent relaxed the adiabatic assumption by allowing for heat and mass exchanges with liquid and solid surfaces at the boundary of a bubble.
The usefulness of this so called, "homogenous bubble" model is well illustrated by a recent application in the design of a non-mechanical switch for optical circuits. Bubbles are generated in micro channels of a silicon chip by electrical heaters. The change in the index of refraction at a bubble surface redirects light beams that change the configuration of a fiber-optic network. Unfortunately, early experiments with this concept exhibited an unacceptable signal loss. The microscopic size of the devices makes direct observations extremely difficult.
Using FLOW-3D, developers were able to show that
the signal loss problem was caused by a preferential condensation of vapor on
the side of the bubble. The adjacent computational figure illustrates this phenomenon.
Liquid is evaporating at a hot surface at the bottom of the bubble and condensing
on the colder side walls causing bulges in the bubble surface. having bulges
instead of a flat surface causes the observed signal loss. Understanding this
problem allowed the engineers to devise a simple solution. Our thanks go to Agilent
senior scientist, John Uebbing, for this fine example.
The latest descendent in the treatment of bubble regions, which is now available
(Version 8.1) is the inclusion of gas dynamics within bubbles. With this new
species of bubble model, it is possible to account for such things as thermal
stratification effects in a gas region above a liquid pool or the transfer of
heat and mass by a combination of evaporation-condensation through vapor channels.
Evolution continues. Watch for new species of liquid-gas models appearing along the shores of FLOW-3D.