PLOT OF THE MONTH

Bubble Behavior in a Propeller's Tip Vortex Flow Field
Jessup, MD

Contributed by:
Jin-Keun Choi, Ph.D.,
Senior Research Scientist
Dynaflow, Inc.

The Engineer

Jin-Keun Choi is a Senior Research Scientist at Dynaflow, Inc. - a private company founded in 1988 in Jessup, M.D. Dynaflow performs advanced research and development in areas such as hydrodynamics, acoustics, and CFD. Their work is used by government agencies (Navy, NOAA, NASA, and the EPA), universities, laboratories, and private companies.

The Simulation

Bubble Behavior in a Propeller's Tip Vortex Flow Field is a bubble nucleus simulated by 3DynaFS©/2DynaFS© - a potential flow-based, free-surface flow solver developed by Dynaflow. 3DynaFS©/2DynaFS© models the interaction between cavitation and explosion bubbles, as well as nearby structures and/or free-surfaces.

Bubbles produce high pressure pulses as they collapse and rebound. Bubble behavior and ensuing reactions vary dramatically depending on whether the structure/free-surface is a submarine, the sea floor, other bubbles, or the ocean surface.

3DynaFS/2DynaFS has many potential uses including:

• Explosions near bodies and the ocean's free surface
• Waves generated by a body's motion
• Breaking waves on a sloped beach
• Bubble deformation in non-uniform flow fields
• Multi-bubble interaction
• Interaction between free surfaces and vortical flow fields
• Fluid structure interaction
• Cavitation inception and noise
• Ship-harbor and ship-ship interactions

The Plot

Bubble Behavior in a Propeller's Tip Vortex Flow Field displays bubble shapes at different times. The initial bubble is 50 micro-meters in radius and elongates due to low pressure along the flow field's vortex axis. In the end, the bubble splits into two sub-bubbles, which develop re-entrant jets along the axis from the split.

This bubble simulation is performed as a part of research funded by the Office of Naval Research to investigate the scaling of propeller tip vortex cavitation inception. Vortex cavitation inception is caused when small bubbles are captured by the core of a strong vortex. This produces a sharp reduction in pressure that can cause bubbles to grow explosively. These bubbles will then collapse violently when they leave the low pressure region, producing sharp pulses of noise.

Cavitation is highly undesirable in mechanical systems since it is a source of damage, noise, and efficiency drop. It is also one of the most important questions in navy ship design. Cavitation inception increases the danger of being detected and hinders the use of sonar equipment. Therefore the speed at which the propeller of a ship or torpedo starts to cavitate is an important design element.

To understand the scaling, Dr. Choi simulates bubbles of different sizes and noises for different sized propellers (i.e., a 12-inch and 10-foot diameter). From this information he determines bubble behavior relationships between cases. Once the scaling is understood, propeller designers can predict cavitation inception by performing small-scale propeller experiments and applying the scaling law.

Cavitation can also be harnessed for useful purposes such as cutting, cleaning, deep-hole drilling and water remediation. In this case, very intense collapses are required. Dynaflow has developed several patents for such applications.

Tecplot

Jin-Keun Choi started using Tecplot in 1995 at the University of Texas in Austin. He typically uses Tecplot to visualize his simulation results with 3-D shade and contour plots, as well as 2-D vector and line plots.

He says, "without visualization, you cannot examine your computational results. Tecplot helps me during the software development stage - I can debug by observing specific parts of the computed geometry, as well as calculated variable values at certain locations."

Jin-Keun Choi believes Tecplot's three greatest strengths as it relates to his work are:

• Plotting capabilities- the ability to plot 3-D geometries (mesh plots) and quickly generate animations with previous layouts.
• Exploration tools - Tecplot's Probe Tool allows him to examine variable values at specific locations with a mouse-click. The ability to interrogate the node/element numbers of the mesh helps him debug problems.
• Data manipulation options - Specify Equations options in Tecplot allow him to create and transform data. This eliminates the need for him to go back and regenerate data.