The objective of this contract is to obtain scientific undertanding and field measurements of spray droplets created by a breaking bow wave.
ONR Program Review, La Jolla CA April 2002.
The formation of steep breaking waves, the generation of spray, and the resulting entrainment of air are three of the most challenging problems in the field of free-surface hydrodynamics. All three of these phenomena play a significant role in the turbulent signature in the near-field flow of a naval combatant. This paper reports on the progress of a Department of Defense Challenge Project aiming at this last frontier of Computational Ship Hydrodynamics. Under the management of Dr. Ki-Han Kim at ONR, the project involves close collaboration between the research group at MIT which is led by Professor Dick K.P. Yue and the research group at SAIC which is led by Dr.'s Douglas Dommermuth and Tricia Waniewski Sur. Unique, scalable parallel-computing capabilities for simulating turbulent, highly mixed flow around a surface ship are developed and implemented on high-performance distributed-memory platforms. At SAIC the NFA (Numerical Flow Analysis) suite of codes use level-set, volume-of-fluid, body-fitted grids, and spray-formation techniques; while at MIT a suite of codes called ShipLES have been developed based on advanced large-eddy simulation of free-surface flows and coupled air-water flows. Through large-scale computations on the IBM SP3 and Cray T3E, the numerical results and their analyses provide us the framework to develop the modeling of wave breaking, spray formation and air entrainment around a surface ship, which heretofore has proven to be an elusive goal. During the first eight months of the project, preliminary steps have been made towards understanding the complex flow field in the aspects of: (i) the flow structures of steep waves; (ii) droplet re-entry dynamics; (iii) the generation of thin spray sheets; and (iv) droplet modeling.
Hendrickson, K., Sur, T.W., Shen, L., Dommermuth, D., and Yue, D.K.P. "Simulation of Steep Breaking Waves and Spray Sheets around a Ship: The Last Frontier in Computational Ship Hydrodynamics." Presented at Department of Defense HPCMP Users Group Conference, June 18-21 2001.
The electronic and other payload power densities in future micro/nano spacecraft are expected to exceed 25 W/cm2 and require advanced thermal control concepts and technologies to keep their payload within allowable temperature limits. A MEMS-based pumped liquid cooling system is being investigated at the Jet Propulsion Laboratory for the thermal control of high power density electronics and other payload on future micro/nano spacecraft. The mechanically pumped cooling system consists of a working fluid circulated through microchannels by a micropump; it provides many advantages over passive thermal control concepts and technologies. First, the working fluid in the cooling loop provides efficient coupling to the hot surface of the electronics. Second, the cooling loop provides flexibility in locating the heat sink inside the spacecraft. Third, the cooling loop potentially provides a simple mating to semiconductor surfaces through bonding techniques. Furthermore, the MEMS cooling system can be easily integrated with the spacecraft thermal control system.
Microchannel heat sinks have been designed and fabricated in silicon at JPL and are currently being tested for hydraulic and thermal performance in simulated microspacecraft heat loads using water as the working fluid. The microchannels are 50 microns deep with widths ranging from 50 to 100 microns. The hydraulic and thermal test data is used for numerical model valication. Optimization studies are being conducted using these numerical models on various microchannel configurations, working fluids, and micropump technologies. This paper presents background on the need for pumped liquid cooling systems for future micro/nano spacecraft and results from this ongoing experimental investigation.
Birur, G.C., Sur, T.W., Paris, A.D., Shakkottai, P., Green, A.A., and Haapanen, S. "Micro/nano spacecraft thermal control using a MEMS-based pumped liquid cooling system." Presented at SPIE Micromaching and Microfabrication, October 21-24, 2001. (pdf version, 397kB)
Sur, T.W. "Microfluidics." Spring 2001 Fluid Mechanics Seminar Series, Mechanical and Aerospace Engineering Department, University of California San Diego.
Birur, G.C., Shakkottai, P., and Sur, T.W. "MEMS-based Pumped Liquid Cooling Systems for Micro/Nano Spacecraft Thermal Control." Presented at NanoSpace 2001: Exploring Interdisciplinary Frontiers, March 13-16 2001.
Experimental studies of air entrainment by breaking waves are essential for advancing the understanding of these flow and creating valid models. The present study used three-dimensional simulations of a bow wave to examine its air entrainment process. The simulated waves were created by a deflecting plate mounted at an angle in a super-critical free sruface flow. Since the air entrainment process is closely coupled with breaking wave dynamics, the present study included both air entrainment and free surface measurements.
Measurements of the free surface were obtained from the simulated bow waves at two scales, and also from the bow wave created by a towed wedge model. Contact line and bow wave profile measurements for the different experiments were compared, demonstrating the similarity of the experimental simulations to the towed model experiments. The plunging wave jet shape was measured in the larger scale stationary model and towed model experiments and used to calculate jet thickness, velocity, and impingement angle. The bow wave profile data from the towed model experiments were used to investigate the scaling of the wave with the flow and geometric parameters. Surface disturbances were observed on the plunging wave face, and their wavelength, frequency, and velocity were measured.
The primary mechanisms for air entrainment were the impact of the plunging wave jet and individual droplets in the splash region on the free surface. The air entrainment process was observed in the larger scale stationary model experiments, and the air bubbles were entrained in spatially periodic bubble clouds. Due to the shallow depth in these experiments, measurements of only the larger bubbles in the initial stages of air entrainment were obtained. An impedance based void fraction meter, developed specifically for the purpose, was used to measure void fractions and bubble size distributions beneath the wave. The bubble cloud size and void fraction increased with downstream distance.
There were indications that the surface disturbances control the periodicity of the bubble clouds. Namely, the surface distrubances divide the plunging liquid jet sheet into a series of plunging wave jets, each entraining air into a separate bubble cloud beneath the free surface.
Waniewski, T.A., Brennen, C.E., and F. Raichlen. (2002) "Bow Wave Dynamics." J. Ship Res, vol. 46, pp. 1-15.
Waniewski, T.A., Hunter, C., and Brennen, C.E. (2001) "Bubble Measurements Downstream of Hydraulic Jumps." Int. J. Multiphase Flow, vol. 27, pp. 1271-1284. (pdf version of final manuscript, 155kB)
Waniewski, T.A., Brennen, C.E., and F. Raichlen. (2001) "Measurements of Air Entrainment by Bow Waves." J. Fluids Eng., vol. 123, pp. 57-63. (pdf version of final manuscript, 341kB)
Waniewski, T.A. "Air Entrainment by Bow Waves." Ph.D. thesis. California Institute of Technology, October 1998.
Waniewski, T.A., Brennen, C.E., and F. Raichlen. "Void Fraction Measurment Beneath a Stationary Breaking Wave." Proc. ASME Fluids Engineering Division Meeting Symposium on Aeration Technology, Washington D.C., June 1998.
Waniewski, T.A., Brennen, C.E., and F. Raichlen. "Experimental Simulation of a Bow Wave." Proc. ASME Fluids Engineering Division Summer Meeting, Symposium on Gas-Liquid Two-Phase Flows, June 1997.
Waniewski, T.A., Brennen, C.E., and F. Raichlen. "Air Entrainment in an Experimental Simulation of a Bow Wave." Presented at ONR Bubbly Flow Workshop, February 1998.
Waniewski, T.A., Brennen, C.E., and F. Raichlen. "Experimental Simulation of a Bow Wave." Presented at ONR Bubbly Flow Workshop, February 1997.
Waniewski, T.A., Brennen, C.E., and F. Raichlen. "Breaking Wave Bubble Observations." Presented at ONR Bubbly Flow Workshop, February 1996.
This paper focuses on the different forms that individual cavitating events may take when the cavitation number is below the inception value (but not so low as to produce only attached cavities) and individual nuclei trigger individual cavitation events. It is a sequel to those of Kuhn de Chizelle et al. (1992a, 1992b, 1995) which described a set of cavitation scaling observations on simple Schiebe headforms conducted in the US Navy Large Cavitation Channel (LCC). The most common events observed in those experiments were traveling, hemi-spherical shaped bubbles which grew and collapsed as they were convected through the low pressure region on the headform. Several interesting variations were also observed, including the development of bubble tails and the triggering of patches, or local regions of attached cavitation. In the present paper, the frequency of occurrence of the various types of events is analyzed as well as how those probabilities changed with cavitation number, velocity and headform size. In general, the probabilities of tails and patches increased with decreasing cavitation number, but they also increased with increasing headform size and increasing velocity. A specific parametric dependence on these variables is suggested.
Publication in refereed proceeding:
Waniewski, T.A. and Brennen, C.E. "The Evolution of Cavitation Events with Speed and Scale of the Flow." Proc. 3rd ASME/JSME Fluids Engineering Division Summer Meeting, July 1999. (gzipped postscript version, 1751kB)
In recent years, the dynamic behavior of tilting disc prosthetic heart valves have come under increasing study. Interest in this area arises from several concerns. Recent failures in certain valve designs after long-term implantation had led researchers to believe that the forces acting on the valve struts are not entirely understood. Second, many laboratories have documented cavitation of the fluid downstream of rapidly closing valves (Stinebring et al 1991). Since the occurence of cavitation has been correlated with valve dynamics closure (Graf et al. 1994), a more thorough study of this topic is indicated.
Publication in refereed proceeding:
Gardner, J.F., Waniewski, T.A., and Geselowitz, D.B. "A Newton-Euler Model of Prosthetic Heart Valve Dynamics." Proc. ASME International Mechanical Engineering Congress and Exposition, Bioengineering Division Symposium on Cardiovascular Fluid Mechanics, November 1995.
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