The purpose of this technical note is to describe to the reader general details of the treatment of void regions and their interaction with fluid in FLOW-3D1 and to help better understand the program’s capabilities and results, as well as its limitations.

In one-fluid problems in FLOW-3D the volume-of-fluid (VOF) function F defines the location of fluid #1 in the mesh. For example, F(ijk)=1.0 in a cell full of fluid. An open cell with no fluid is a void cell; F(ijk)=0.0 in such a cell.

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,
– 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 (with the exception of high winds, which could be accounted for with a special type of a free surface boundary condition in FLOW-3D), micro-fluidic devices and so on.

The one-fluid/void approach to modeling free surface is a powerful method that provides efficient and accurate solutions to general free surface flows [1]. Including the details of gas flow in such cases is usually computationally expensive and unnecessary.

The terms ‘void’ and ‘bubble’ in this article largely refer to the same computational objects. A slight distinction is that a void region may be called a ‘bubble’ when it is surrounded by fluid and its pressure and temperature vary dynamically – as in an actual bubble moving through liquid. In the context of the solver all such objects are referred to as ‘void regions’.

Bubble models in FLOW-3D have many uses in complex flow situations involving non-equilibrium thermal and dynamic processes. Bubbles containing vapor can arise or disappear in a simulation because of phase change associated with cavitation or boiling. Gas bubbles can be affected or created by gas mass sources as well as through vents and valves connected to external
pressure reservoirs. Turbulent liquid surfaces may entrain gas from bubbles and be bulked up by the added gas volume. All these possibilities as well as several basic techniques dealing with the
identification and labeling of bubble regions are described in the remainder of this document.