Chahine G.L.,Dynaflow, Inc.
Fluid Mechanics and its Applications
This chapter presents a numerical model applied to the collapse of a single cavitation bubble near a rigid or deformable plane material boundary. A hybrid incompressible-compressible method is presented, which accounts for the development of a re-entrant jet as well as the emission of shock waves during the collapse or following liquid-liquid or liquid-solid impact. Wall pressures computed on the bubble axis present a first peak due to the impact of the bubble re- entrant jet followed by a second peak due to the shock wave resulting from the residual toroidal bubble collapse. For small standoff distances, both pressure peaks have comparable amplitudes. The re-entrant jet impact pressure is proportional to the liquid impedance and the jet speed and follows the classical water-hammer equation. The effects of the collapse driving pressure, standoff distance, and bubble size are discussed. The permanent plastic deformation (pit) resulting from the bubble collapse is also computed by using a two-way coupled fluid-structure interaction model. The process of pit formation is described and the influence of various parameters (such as bubble size, collapse driving pressure and load duration) on pit geometry is discussed. The analysis shows that the material experiences stresses that are much lower than the fluid generated impulsive loads and that the resulting pit characteristics depend not only on the impulsive load amplitude but also on its duration and spatial extent. © Springer Science+Business Media Dordrecht 2014 Source
Agency: Department of Energy | Branch: | Program: SBIR | Phase: Phase I | Award Amount: 150.00K | Year: 2015
Development of algae as a source of renewable chemicals and fuels is hindered by the difficulty of recovering the algae from the growth solutions. Air flotation can separate solids and liquids by having gas bubbles attach to the solid particles to increase buoyancy and lift them to the surface. The small particle sizes of algae require that small bubble sizes be used. Conventionally very fine bubbles are created using dissolved gas flotation DAF). DAF requires high pressures to supersaturate gases into water and has poor energy efficiency. However, this can be overcome by using bubble generators based on local controlled hydrodynamic cavitation to generate large volumes of very small bubbles with much less energy. These will have long residence time in the water, rise slowly in the gravity field while attaching to the algae cells and lifting them to the free surface. In this proposed Phase I SBIR project, we will investigate the feasibility of using a bubble generator concept based on controlled cavitation to remove water and concentrate the algae from the growth media and recover the intact cells. The objectives of the project will be to design and construct a bench- scale algal growth media dewatering loop with the bubble generator capable of producing large numbers of bubble of diameters less than 50 microns, and to then move to pilot scale systems after addressing R&D issues. A controlled cavitation bubble generator will be tested. The bubble sizes and numbers produced will be diagnosed by high speed video, image analysis, and acoustic techniques. The effects of operating parameters pressure drop, air and water flow rates) on bubble size distributions will be determined. The performance of the solid- liquid separation system with different algae species, algae concentrations, and air-liquid void fractions will be determined. The percent recovery of algae form solution, percentage of solids in the recovered slurry, and algae quality will be measured in these experiments. If necessary to increase the percentage of solids to 20% secondary concentration methods such as vacuum filtration will be tested. An energy efficient method for dewatering algae solutions will reduce the production costs for renewable chemicals and fuels produced from algae, and reduce the barriers to bringing this technology to commercial scale. The separation technology developed in this SBIR would also have applications in other fields such as mining, wastewater treatment, and petroleum process water treatment.
Agency: Department of Defense | Branch: Navy | Program: SBIR | Phase: Phase I | Award Amount: 80.00K | Year: 2016
When a payload exits a submerged launch tube, pressurized gas follows the ejected projectile into the surrounding water and expands between the tube and the upward moving projectile. The gas cavity behind the moving body then deforms and pinches off from the tube and then collapses with the formation of a strong upward reentrant jet. The impact of the high-speed reentrant jet on the projectile presents a threat of damage and imparts an unwanted force to its motion. Previously attempted high speed photography of the reentrant jet has not been successful due to the highly opaque and turbulent bubbly region surrounding the cavity. In order to alleviate this deficiency, we propose in this SBIR to combine high speed video observations of the cavity dynamics with pressure measurements in the water at several locations and impact pressure measurements on the projectile base. The optical method will provide the exterior shape of the evolving cavity, while the acoustic method, combined with a CFD computation of the cavity shape evolution for different input conditions, will enable reconstruction of the reentrant jet shape evolution in space and time using inverse problem optimization techniques that process both the acquired video and the acoustic information. The system will be tested and validated in Phase I using an available small scale uncorking setup in a vacuum tank and then scaled up for the field tests.
Agency: Department of Defense | Branch: Navy | Program: SBIR | Phase: Phase II | Award Amount: 70.00K | Year: 2016
Under a recently completed ONR SBIR Phase II a non-abrasive diver tool for propeller cleaning was developed using DYNAFLOWs cavitating and resonating waterjets. This non-abrasive tool uses the collapse of cavitating microstructures in the submerged jet over the fouling to remove it. It also uses a self-rotating head to achieve high removal rates, a counter-thruster to reduce diver fatigue, and a silencer to reduce noise. The tool was extensively tested on surrogate fouling on NAB plates and on a fouled propeller in the laboratory. Finally field tests were conducted on the USS Leyte Gulf. Recommendation from those tests were to reduce weight and length of the tool for ergonomic considerations. Further R&D since has addressed these recommendations and significantly improved removal ratesIn the proposed effort, the improved tool will be first tested at DYNAFLOW on an APU propeller, then, in coordination with NAVSEA cleaning tests will be conducted on a Navy ship in Norfolk. A well performing pump will be rented and Navy cleared divers from Seaward Marine will conduct tests with the tool and record videos of its performance on the Navy propeller. A report of this effort will be delivered following completion of the tests.
Hsiao C.-T.,Dynaflow, Inc. |
Chahine G.L.,Dynaflow, Inc.
Journal of the Acoustical Society of America
A simplified three-dimensional (3-D) zero-thickness shell model was developed to recover the non-spherical response of thick-shelled encapsulated microbubbles subjected to ultrasound excitation. The model was validated by comparison with previously developed models and was then used to study the mechanism of bubble break-up during non-spherical deformations resulting from the presence of a nearby rigid boundary. The effects of the shell thickness and the bubble standoff distance from the solid wall on the bubble break-up were studied parametrically for a fixed insonification frequency and amplitude. A diagram of bubble shapes versus the normalized shell thickness and wall standoff was derived, and the potential bubble shapes at break-up from reentrant jets were categorized resulting in four distinct zones. © 2013 Acoustical Society of America. Source