Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session FT: Multiphase Flows IV |
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Chair: Francine Battaglia, Virginia Tech Room: Salt Palace Convention Center Ballroom FH |
Monday, November 19, 2007 8:00AM - 8:13AM |
FT.00001: Cavitation Experiments and Numerical Simulations in Aviation Fuel. Irina Dorofeeva, Michael Davis, Flint Thomas Cavitation is a persistent problem in internal flows due to both performance reduction associated with the introduction of fluid compressibility and component surface damage associated with regions of bubble collapse. The work reported here is focused on understanding cavitation in JP8 aviation fuel and how it differs in character from the more commonly studied cavitation in water. Two flow field geometries are considered. One involves a planar converging-diverging nozzle and the other radial gap flow between circular disks. In both geometries cavitation of water and JP8 occurs but the imposed strain rates are much higher in the disk experiment. We contrast the cavitation of water and JP8 in both geometries and attempt to explain essential differences through numerical modeling. [Preview Abstract] |
Monday, November 19, 2007 8:13AM - 8:26AM |
FT.00002: Investigation of the behavior of a ventilated supercavity Xu Sheng, Ellison Kawakami, William Hambleton, Roger Arndt The topic of supercavitation is of considerable interest to drag reduction and/or speed augmentation in marine vehicles. During the present experimental work, the ventilated supercavity formed behind a sharp-edged disk is investigated. Results regarding cavity shape, cavity closure and ventilation requirements versus cavitation number and Froude number will be presented. Additionally, effects related to flow choking in a water tunnel test section are discussed. Results obtained are similar in character to previously reported results, but differ significantly in measured values. Cavity shape, particularly aft of the maximum cavity diameter, is found to be a strong function of the model support scheme chosen, and results are presented from several configurations. [Preview Abstract] |
Monday, November 19, 2007 8:26AM - 8:39AM |
FT.00003: Jet-Wake Interaction and its Effect on Cavitation Roger Arndt, William Hambleton, Eduard Amromin, Svetlana Kovinskaya Model studies were conducted in support of research on a waterjet propulsion scheme. Of interest are cavitation inception and the formation of a bubbly wake. Most studies of jet cavitation have been for discharge into quiescent flow. However little data exists for jets discharging into a bluff body wake. This study is a preliminary examination of the fundamental physics. A water tunnel model consisting of an axisymmetric Schiebe body, truncated abruptly at the aft end is utilized. A jet issues from a sharp-edged orifice at the aft of the model. Cavitation inception and bubble characteristics are determined as a function of Reynolds number and jet velocity ratio. Additionally, particle image velocimetry is employed to investigate the flow kinematics in the near wake of the model. Complex interactions are found to take place between the jet and wake that seem to have significant implications for cavitation inception. [Preview Abstract] |
Monday, November 19, 2007 8:39AM - 8:52AM |
FT.00004: A dynamic test platform for evaluating control algorithms for a supercavitating vehicle. Arnar Hjartarson, William Hambleton, Gary Balas, Roger Arndt The use of supercavitation to enable marine vehicles to travel at extraordinary speeds is a topic of considerable interest. The control of these vehicles pose new issues not faced with fully wetted vehicles due to a complex interaction between the vehicle and the cavity that it rides in. Existing models make many assumptions that may not be valid for a highly maneuverable vehicle. To this end, several experimental test platforms have been developed to enable test and validation of control algorithms and hydrodynamic models. Previous efforts have revealed the destabilization of marginal super cavities, especially when a cavity is being maintained with ventilation. Our latest water tunnel test platform is a body of revolution with an actuated cavitator on the model forebody, actuated fins which protrude through the cavity surface, and actuated pitch and translation of the model body, all supported by a six-axis balance. Model validation tests and preliminary data regarding the dynamics of the interaction of the cavity with the model will be presented. [Preview Abstract] |
Monday, November 19, 2007 8:52AM - 9:05AM |
FT.00005: Detailed Numerical Simulations of the Primary Atomization of a Turbulent Liquid Jet Marcus Herrmann The atomization process of turbulent liquid jets is as of this day not well understood. Detailed numerical simulations can help study the fundamental mechanisms in regions, where experimental access and analysis is virtually impossible. However, simulating atomization accurately is a huge numerical challenge since time and length scales vary over several orders of magnitude, the phase interface is a material discontinuity, and surface tension forces are singular. The Refined Level Set Grid (RLSG) method is one numerical approach to simulate the primary breakup process of liquid jets and sheets in detail. We will present simulation results of the primary atomization of a Reynolds number 5000 round turbulent liquid jet injected into stagnant high pressure air using the RLSG approach. The physical mechanisms resulting in the initial breakup of the jet will be discussed. Drop size distributions resulting from primary atomization will be presented and their dependence/ independence on the employed numerical grid resolution will be discussed. [Preview Abstract] |
Monday, November 19, 2007 9:05AM - 9:18AM |
FT.00006: An Accurate Conservative Level Set/Ghost Fluid Method for the Simulation of Turbulent Primary Atomization Heinz Pitsch, Olivier Desjardins The atomization of liquid flows plays an important role in many engineering applications, and yet the numerical simulation of the turbulent break-up process remains an outstanding challenge. The large density ratio between the liquid and the gas, as well as the large range of scales involved in the atomization phenomenon, render this problem especially difficult to tackle numerically. We propose a novel strategy that relies on a conservative level set method to ensure adequate mass conservation and on the ghost fluid method for the accuracy of the sharp interface representation. The method shows good robustness in the presence of interfacial forces, as well as large density ratios. A further accuracy improvement is obtained by coupling an additional distance level set to improve the computation of the interface normals. This approach is validated on a range of test cases with realistic water-air conditions and topology changes. This technique is then used to simulate the turbulent atomization of a round liquid diesel jet. The main features of the liquid jet are described, with particular attention given to the turbulence generation process. [Preview Abstract] |
Monday, November 19, 2007 9:18AM - 9:31AM |
FT.00007: Heat-transport measurement in a turbulent fluid above the critical point Guenter Ahlers, Jin-Qiang Zhong Below the critical point (CP) at $P_c,T_c$ liquid and vapor co-exist along a line $T_\phi(P)$ in the temperature-pressure plane. When a fluid at $P < P_c$ and in the presence of gravity is heated from below and the resulting temperature difference $\Delta T = T_b - T_t$ ($T_b$ and $T_t$ are the temperaturs at the bottom and top of the sample respectively) straddles $T_\phi$, then liquid can condense at the top and drop to the bottom. By virtue of the latent heat of vaporization this process will contribute strongly to the effective conductivity $\lambda_{eff}$ of the sample. Since the latent heat vanishes at the CP, one would expect this enhancement to vanish as $P \rightarrow P_c$ from below. We measured $\lambda_{eff}$ using ethane close to but {\bf above} the CP along various isobars using a constant $\Delta T$ and varying $T_m = (T_t + T_b)/2$. Contrary to our expectations, even for $P > P_c$ we found that $\lambda_{eff}$ had a maximum for $T_m$ close to the temperature corresponding to the critical isochore and reached values well above those expected for a single-phase Boussinesq fluid at the same Rayleigh numbers. [Preview Abstract] |
Monday, November 19, 2007 9:31AM - 9:44AM |
FT.00008: Evaluation of Quasi-Static and Computational Solutions of Phase Change with Internal Heat Generation John Crepeau, Ali Siahpush, Blaine Spotten During a solid-liquid phase change, the internal heat generation of the material enhances the melting process and impedes solidification. Previously presented results have given an approximate, closed form solution of this problem, using a quasi-static method, valid for Stefan numbers less than one. It was shown that the steady-state location of the front was inversely proportional to the square root of the internal heat generation. To test the validity of the quasi-static solution, a computational fluid dynamics model was developed, and the results were compared to the closed-form solution. A cylindrical geometry was chosen, with both constant temperature and constant heat flux boundary conditions. For the constant temperature boundary condition, there was excellent agreement between the quasi-static and computational solutions for Stefan numbers less than one, validating the approximate solution technique. The agreement between the two methods diverged for Stefan numbers of one and greater, with the approximate solution reaching steady-state faster than the computational solution. For the constant heat flux boundary condition, there was excellent agreement between the approximate and computational solutions when the ratio between the volumetric internal heat generation and surface heat flux was close to one. [Preview Abstract] |
Monday, November 19, 2007 9:44AM - 9:57AM |
FT.00009: Heat-transport measurements for ethane in a turbulent liquid-vapor two-phase state. Jin-Qiang Zhong, Guenter Ahlers Below the critical point (CP) at $P_c,T_c$ liquid and vapor co-exist along a line $T_\phi(P)$ in the temperature-pressure plane. When a fluid at $P < P_c$ is heated from below and $\Delta T = T_b - T_t$ ($T_b$ and $T_t$ are the temperatures at the bottom and top of the sample respectively) straddles $T_\phi$, then liquid can condense at the top and drop to the bottom. This process will contribute strongly to the effective conductivity $\lambda_{eff}$ of the sample. We measured $\lambda_{eff}$ using ethane close to but {\bf below} the CP along various isobars using a constant $\Delta T$ and varying $T_m$ = $(T_t+T_b)/2$. For $T_t > T_\phi$ the sample was in the single-phase vapor region and $\lambda_{eff}$ exceeded the pure conduction value because the sample underwent turbulent convection. As $T_m$ was decreased so that $T_t$ entered the two-phase region, we found that the heat transport was enhanced further, but that the enhancement did not start until $T_t$ reached a critical value $T_t^c < T_\phi$. At that point a meta-stable boundary layer at the sample top was assumed to have reached a sufficient thickness for nucleation of liquid to occur. For $T_t < T_t^c$ the heat transport increased continuously and linearly with decreasing $T_t$. When $T_t$ decreased sufficiently, $\lambda_{eff}$ reached a maximum where it was an order of magnitude larger than in the single-phase state. [Preview Abstract] |
Monday, November 19, 2007 9:57AM - 10:10AM |
FT.00010: Experimental Study and CFD modeling of high speed water jets in air Anirban Guha, R.M. Barron, R. Balachandar High speed turbulent water jets are extensively used in industrial cleaning applications. They interact vigorously with the surrounding air and loose mass in the form of water droplets which moves along with the entrained air stream. The transfer of momentum to the surroundings reduces the jet velocity and thus the pressure at the impinging surface is significantly lower than the supply pressure. Laser Doppler Anemometer (LDA) measurements of velocity field and pressure measurements at different axial and radial locations were performed. The potential core of the jet was found to extend to around 100 nozzle diameters. The dynamic pressure along the centerline was found to decay linearly, which can be used to estimate the decay of water volume fraction along the centerline. An empirical formulation of mass transfer (in the form of droplets) from water phase to the surroundings has been developed and incorporated into the commercial CFD code FLUENT. The flow was simulated using the RNG k-$\varepsilon $ turbulence model and Eulerian-Eulerian multiphase model. The predicted pressure distribution at the impinging surface was found to match closely with the experimental findings. [Preview Abstract] |
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