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Experimental and Numerical Investigation of Flows in Expanding Channels

Description: This is the first year progress report for our grant starting Feb. 1 2004. It describes experimental and theoretical achievements during the first year, lists the articles published during this period, as well as the progress of the graduate students supported by this grant. The timeline for the future is outlined; the current results convince us that the work will be done on time and within the budget.
Date: October 28, 2004
Creator: Vorobieff, Vakhtang Putkaradze Peter
Partner: UNT Libraries Government Documents Department

Simple model for linear and nonlinear mixing at unstable fluid interfaces with variable acceleration

Description: A simple model is described for predicting the time evolution of the half-width h of a planar mixing layer between two immiscible incompressible fluids driven by an arbitrary time-dependent variable acceleration history <i>a(l)</i>a (t): The model is based on a heuristic expression for the kinetic energy per unit area of the mixing layer. This expression is based on that for the kinetic energy of a linearly perturbed interface, but with a dynamically renormalized wavelength which becomes proportional to h in the nonlinear regime. An equation of motion for h is then derived by means of Lagrange�s equations. This model reproduces the known linear growth rates of the Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities, as well as the quadratic RT and power-law RM growth laws in the nonlinear regime. The time exponent in the RM power law depends on the rate of kinetic energy dissipation. In the case of zero dissipation, this exponent reduces to 2/3 in agreement with elementary scaling arguments. A conservative numerical scheme is proposed to solve the model equations, and is used to perform calculations that agree well with published mixing data from linear electric motor experiments. Considerations involved in implementing the model in hydrodynamics codes are briefly discussed.
Date: December 23, 1998
Creator: Ramshaw, J. D. & Rathkopf, J.
Partner: UNT Libraries Government Documents Department

Conditions Contributing to Adverse Loading of Wind Turbines In the Nocturnal Boundary Layer: Final Report, November 15, 2003 -- December 31, 2004

Description: This report summarizes the development of a methodology to describe the characteristics of coherent turbulence in the nocturnal boundary layer that are known to induce excessive structural loads and component vibration in operating wind turbines and suggestions for applying those results in the development of techniques of real-time detection and prediction that can be used for mitigation purposes.
Date: July 1, 2005
Creator: Fritts, D. C.
Partner: UNT Libraries Government Documents Department

The effect of a short wavelength mode on the evolution of a long wavelength perturbation driven by a strong blast wave

Description: Shock-accelerated material interfaces are potentially unstable to both the Richtmyer-Meshkov and Rayleigh-Taylor instabilities. Shear that develops along with these instabilities in turn drives the Kelvin-Helmholtz instability. When driven by strong shocks, the evolution and interaction of these instabilities is further complicated by compressibility effects. In this paper, we present a computational study of the formation of jets at strongly driven hydrodynamically unstable interfaces, and the interaction of these jets with one another and with developing spikes and bubbles. This provides a nonlinear spike-spike and spike-bubble interaction mechanism that can have a significant impact on the large-scale characteristics of the mixing layer. These interactions result in sensitivity to the initial perturbation spectrum, including the relative phases of the various modes, that persists long into the nonlinear phase of instability evolution. We describe implications for instability growth rates, the bubble merger process, and the degree of mix in the layer. Finally, we consider results from relevant deceleration RT experiments, performed on OMEGA, to demonstrate some of these effects.
Date: March 16, 2004
Creator: Miles, A R; Edwards, M; Blue, B; Hansen, J F; Robey, H F; Drake, R P et al.
Partner: UNT Libraries Government Documents Department

Film Boiling on Downward Quenching Hemisphere of Varying Sizes

Description: Film boiling heat transfer coefficients for a downward-facing hemispherical surface are measured from the quenching tests in DELTA (Downward-boiling Experimental Laminar Transition Apparatus). Two test sections are made of copper to maintain low Biot numbers. The outer diameters of the hemispheres are 120 mm and 294 mm, respectively. The thickness of all the test sections is 30 mm. The effect of diameter on film boiling heat transfer is quantified utilizing results obtained from the test sections. The measured data are compared with the numerical predictions from laminar film boiling analysis. The measured heat transfer coefficients are found to be greater than those predicted by the conventional laminar flow theory on account of the interfacial wavy motion incurred by the Helmholtz instability. Incorporation of the wavy motion model considerably improves the agreement between the experimental and numerical results in terms of heat transfer coefficient. In addition, the interfacial wavy motion and the quenching process are visualized through a digital camera.
Date: April 1, 2004
Creator: Kim, Chan S.; Suh, Kune Y.; Rempe, Joy L.; Cheung, Fan-Bill & Kim, Sang B.
Partner: UNT Libraries Government Documents Department

Spatially resolved instability measurements with a heavy ion beam probe

Description: A heavy ion beam probe was used to make simultaneous measurements of the amplitude and phase of both density and space potential fluctuations in an energetic arc plasma. Detailed comparison with theoretical predictions identifies a 70 KHz coherent oscillation as a Kelvin-Helmholtz instability localized to a region of strong fluid shear. (auth)
Date: January 1, 1975
Creator: Jennings, W. C.; Hickok, R. L. & Glowienka, J. C.
Partner: UNT Libraries Government Documents Department

Accuracy considerations for implementing velocity boundary condiditons in vorticity formulations

Description: A vorticity formulation is described that satisfies the velocity boundary conditions for the incompressible Navier-Stokes equations. Velocity boundary conditions are satisfied by determining the appropriate vortex sheets that must be created on the boundary. Typically, the vortex sheet strengths are determined by solving a set of linear equations that is over-specified. The over-specification arises because an integral constraint on the vortex sheets is imposed. Vortex sheets determined this way do not accurately satisfy both components of the velocity boundary conditions because over-specified systems do not have unique solutions. An integral collocation technique is applied to a generalized Helmholtz decomposition. This formulation implicitly satisfies an integral constraint that is more general that constraints typically used. Improvements in satisfying velocity boundary conditions are shown.
Date: March 1, 1996
Creator: Kempka, S.N.; Glass, M.W.; Peery, J.S.; Strickland, J.H. & Ingber, M.S.
Partner: UNT Libraries Government Documents Department

Numerical analysis of slender vortex motion

Description: Several numerical methods for slender vortex motion (the local induction equation, the Klein-Majda equation, and the Klein-Knio equation) are compared on the specific example of sideband instability of Kelvin waves on a vortex. Numerical experiments on this model problem indicate that all these methods yield qualitatively similar behavior, and this behavior is different from the behavior of a non-slender vortex with variable cross-section. It is found that the boundaries between stable, recurrent, and chaotic regimes in the parameter space of the model problem depend on the method used. The boundaries of these domains in the parameter space for the Klein-Majda equation and for the Klein-Knio equation are closely related to the core size. When the core size is large enough, the Klein-Majda equation always exhibits stable solutions for our model problem. Various conclusions are drawn; in particular, the behavior of turbulent vortices cannot be captured by these local approximations, and probably cannot be captured by any slender vortex model with constant vortex cross-section. Speculations about the differences between classical and superfluid hydrodynamics are also offered.
Date: February 1, 1996
Creator: Zhou, H.
Partner: UNT Libraries Government Documents Department

Computational and experimental studies of hydrodynamic instabilities and turbulent mixing: Review of VNIIEF efforts. Summary report

Description: The report presents the basic results of some calculations, theoretical and experimental efforts in the study of Rayleigh-Taylor, Kelvin-Helmholtz, Richtmyer-Meshkov instabilities and the turbulent mixing which is caused by their evolution. Since the late forties the VNIIEF has been conducting these investigations. This report is based on the data which were published in different times in Russian and foreign journals. The first part of the report deals with calculations an theoretical techniques for the description of hydrodynamic instabilities applied currently, as well as with the results of several individual problems and their comparison with the experiment. These methods can be divided into two types: direct numerical simulation methods and phenomenological methods. The first type includes the regular 2D and 3D gasdynamical techniques as well as the techniques based on small perturbation approximation and on incompressible liquid approximation. The second type comprises the techniques based on various phenomenological turbulence models. The second part of the report describes the experimental methods and cites the experimental results of Rayleigh-Taylor and Richtmyer-Meskov instability studies as well as of turbulent mixing. The applied methods were based on thin-film gaseous models, on jelly models and liquid layer models. The research was done for plane and cylindrical geometries. As drivers, the shock tubes of different designs were used as well as gaseous explosive mixtures, compressed air and electric wire explosions. The experimental results were applied in calculational-theoretical technique calibrations. The authors did not aim at covering all VNIIEF research done in this field of science. To a great extent the choice of the material depended on the personal contribution of the author in these studies.
Date: December 31, 1994
Creator: Andronov, V.A.; Zhidov, I.G.; Meskov, E.E.; Nevmerzhitskii, N.V.; Nikiforov, V.V.; Razin, A.N. et al.
Partner: UNT Libraries Government Documents Department

Kelvin-Helmholtz-like instability of a shear layer subject to free boundary conditions

Description: For free boundary conditions a shear layer with linear velocity profile supports irrotational disturbances, one mode being unstable when its wavelength lambda > 2.619 times the thickness h of the shear layer. For long wavelengths the dispersion relation of the mode approaches Kelvin-Helmholtz form.
Date: January 2, 1979
Creator: Mjolsness, R.C.
Partner: UNT Libraries Government Documents Department

Interfacial stability with mass and heat transfer

Description: A simplified formulation is presented to deal with interfacial stability problems with mass and heat transfer. For Rayleigh-Taylor stability problems of a liquid-vapor system, it was found that the effect of mass and heat transfer tends to enhance the stability of the system when the vapor is hotter than the liquid, although the classical stability criterion is still valid. For Kelvin-Holmholtz stability problems, however, the classical stability criterion was found to be modified substantially due to the effect of mass and heat transfer.
Date: July 1, 1977
Creator: Hsieh, D. Y.
Partner: UNT Libraries Government Documents Department

Possible mechanisms of macrolayer formation

Description: The high heat flux nucleate boiling region, also called the vapor mushroom region, has been shown to have a thin liquid layer on the heater surface under the large mushroom-shaped vapor bubbles that grow from the heater surface. The name given to this liquid layer is the macrolayer to differentiate it from the microlayer that exists under the discrete bubbles found at lower heat fluxes in the nucleate boiling region. Typical thicknesses of this macrolayer range from 50 to 500 {mu}m for water on a flat horizontal boiling surface and depend upon the heat flux. Thus, the macrolayer is thicker than the wedge-shaped microlayers, found under discrete bubbles, which range in thickness from 1 to 10 {mu}m. Although the mechanism of microlayer formation and its evaporation is conceptually simple that of the macrolayer is still not understood. This paper critically compares the potential mechanisms proposed for macrolayer formation. These mechanisms include the Helmholtz instability applied to the vapor stem above active nucleation sites, liquid trapped by lateral coalescence of discrete bubbles that initially form during the mushroom bubble`s waiting period, and the limitation of liquid resupply at mushroom departure as a result of vapor flow from the active nucleation sites.
Date: May 1, 1992
Creator: Sadasivan, P.; Chappidi, P. R.; Unal, C. & Nelson, R. A.
Partner: UNT Libraries Government Documents Department

Computational and experimental studies of hydrodynamic instabilities and turbulent mixing (Review of NVIIEF efforts)

Description: This report describes an extensive program of investigations conducted at Arzamas-16 in Russia over the past several decades. The focus of the work is on material interface instability and the mixing of two materials. Part 1 of the report discusses analytical and computational studies of hydrodynamic instabilities and turbulent mixing. The EGAK codes are described and results are illustrated for several types of unstable flow. Semiempirical turbulence transport equations are derived for the mixing of two materials, and their capabilities are illustrated for several examples. Part 2 discusses the experimental studies that have been performed to investigate instabilities and turbulent mixing. Shock-tube and jelly techniques are described in considerable detail. Results are presented for many circumstances and configurations.
Date: February 1, 1995
Creator: Andronov, V. A.; Zhidov, I. G.; Meskov, E. E.; Nevmerzhitskii, N. V.; Nikiforov, V. V.; Razin, A. N. et al.
Partner: UNT Libraries Government Documents Department

Comparison of different statistical models of turbulence by similarity methods

Description: The process of implosion by inertial confinement is perturbed by hydrodynamic instabilities such as Rayleigh-Taylor, Richtmyer-Meshkov and Kelvin-Helmholtz instabilities. They may generate turbulent flow, causing the mixing of constituents and the degradation of the symmetry of the implosion. The authors extend Barenblatt`s study (1983) of a one-equation turbulence model to a variety of two-equation models. They consider the problem of the propagation of incompressible turbulence generated by an instantaneous plane source, for which the evolution of the turbulence is determined completely by diffusive and dissipative processes. It is then possible to find for each model a self-similar solution asymptotic to the exact flow. The authors then compare the self-similar temporal and spatial behavior of several two-equation models, including the dependence on model coefficients. They also observe the predicted self-similar behavior and evaluate similarity exponents by numerical solution of the model equations. The combined analytic and numerical approach not only elucidates the analysis but also assists in the validation of the turbulence modeling codes. Some of this analysis has previously been carried out by one of the authors on two turbulence models (Cherfils, 1993), and related work has been reported by Neuvazhaev et al. (1991).
Date: May 1, 1994
Creator: Cherfils, C. & Harrison, A. K.
Partner: UNT Libraries Government Documents Department

Kingfish striations and the Kelvin-Helmholtz instability. Part 1

Description: The role of the Kelvin-Helmholtz instability in initiating the formation of the density striations observed in the Kingfish fireball is examined. Two idealized models are proposed for the velocity shear layer on the sides of the fireball, each of which includes essential characteristics of the Kingfish event insofar as the development of Kelvin-Helmholtz instabilities is concerned. A complete linear analysis is presented for each model.
Date: October 1, 1985
Creator: Hunter, J.H. Jr.
Partner: UNT Libraries Government Documents Department

Possible mechanisms of macrolayer formation

Description: The high heat flux nucleate boiling region, also called the vapor mushroom region, has been shown to have a thin liquid layer on the heater surface under the large mushroom-shaped vapor bubbles that grow from the heater surface. The name given to this liquid layer is the macrolayer to differentiate it from the microlayer that exists under the discrete bubbles found at lower heat fluxes in the nucleate boiling region. Typical thicknesses of this macrolayer range from 50 to 500 {mu}m for water on a flat horizontal boiling surface and depend upon the heat flux. Thus, the macrolayer is thicker than the wedge-shaped microlayers, found under discrete bubbles, which range in thickness from 1 to 10 {mu}m. Although the mechanism of microlayer formation and its evaporation is conceptually simple that of the macrolayer is still not understood. This paper critically compares the potential mechanisms proposed for macrolayer formation. These mechanisms include the Helmholtz instability applied to the vapor stem above active nucleation sites, liquid trapped by lateral coalescence of discrete bubbles that initially form during the mushroom bubble's waiting period, and the limitation of liquid resupply at mushroom departure as a result of vapor flow from the active nucleation sites.
Date: January 1, 1992
Creator: Sadasivan, P.; Chappidi, P.R.; Unal, C. & Nelson, R.A.
Partner: UNT Libraries Government Documents Department

Lattice gas simulations of one and two-phase fluid flows using the Connection Machine-2

Description: In this paper, we report recent lattice gas simulations for single-phase and two-phase flows for two dimensional problems using the Connection Machine-2. For the single-phase fluid problem, we use the standard 7-bit lattice gas model with the maximum collision rules. The velocity and vorticity field of the Kelvin-Helmholtz instability is studied. It is shown that the lattice gas method preserves the main properties of the flow patterns observed in other numerical simulations. Using colored particles and holes, the lattice gas method is extended to simulate immiscible fluids with adjustable surface tension, using a purely local collision scheme. The locality of this model allows us to implement a very fast and parallel algorithm on the Connection Machine-2. Because this new model correctly describes short-range particle-particle interactions between liquids and also particle-solid interactions between the fluid and the wall, cohesion and wettability can be simulated. Applications of the current model to several physical systems including spinodal decomposition, Rayleigh-Taylor flows and wettability in two-phase flows through porous media are discussed. 15 refs., 10 figs.
Date: 1990-09~
Creator: Chen, S.; Doolen, G. D.; Eggert, K.; Grunau, D. & Loh, E. Y., Jr.
Partner: UNT Libraries Government Documents Department

Pattern formation by shock processes

Description: Shock waves in condensed media often produce and leave behind periodic patterns and textures. These patterns have been observed both in real time and in postmortem examination. In many cases the patterns can be related to analogous Pattern-forming mechanisms in classical fluid dynamics, such as the Rayleigh-Taylor and Helmholtz instabilities. In other cases, the textures arise from peculiarities in the dynamic stress state immediately behind the leading edge of the shock wave. Periodic waves in the interface between two shock welded metals have a close resemblance to the classical Helmholtz instability. From a practical point of view, these waves are crucial to the formation of a good bond. Impulsive acceleration of an interface can result in the Meshkov instability, which forms patterns qualitatively similar to the Rayleigh-Taylor instability driven by continuous acceleration. However, the patterned stress state left behind after a shock crosses a perturbed interface can result in perturbation growth for shock propagation in either direction across the interface. Even in homogeneous media, the non-hydrostatic component of the stress behind a shock can drive a pattern forming instability. Adiabatic shear banding has been proposed as a mechanism to explain both the patterns observed in shock-compressed and recovered metal samples and the apparent loss of macroscopic shear strength of shocked ceramics. New optical photographs of shocked quartz support this mechanism. 27 references.
Date: January 1, 1983
Creator: Shaner, J.W.
Partner: UNT Libraries Government Documents Department

Nonlinear aspects of the Rayleigh-Taylor instability in laser ablation

Description: We report on our investigation of the Rayleigh-Taylor (R-T) and Kelvin-Helmholtz (K-H) instabilities in laser ablatively accelerated targets for single mode perturbations for a series of wavelengths in the parameter regime 1/2 less than or equal to lambda/..delta..R less than or equal to 10, where lambda is the wavelength of the perturbation and ..delta..R is the cold foil thickness. We find linear growth rates well below classical values (by a factor on the order of 3 to 4). We also find a cutoff in the growth rates for wavelengths less than the foil thickness. The striking result is the dominance of nonlinear effects; i.e., the K-H instability, for short wavelength perturbations. Although the linear growth rates increase as k/sup 1/2/ up to the cutoff, the K-H rollup dominates at large k, drastically reducing the penetration rate of the dense spike below its free fall value and effectively doubling the aspect ratio of the foil. In other words, it is the long wavelength perturbations that are most effective in destroying the symmetric implosion of the shell.
Date: September 3, 1982
Creator: Emery, M. H.; Gardner, J. H. & Boris, J. P.
Partner: UNT Libraries Government Documents Department