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NASA NTRS2003

Oscillatory Extinction Of Spherical Diffusion Flames

Since extinction has been observed in an oscillatory manner in Le greater than 1 premixed flames, it is not unreasonable to expect that extinction could occur in an unsteady manner for diffusion flames. Indeed, near-limit oscillations have been observed experimentally under microgravity conditions for both candle flames and droplet flames. Furthermore, the analysis of Cheatham and Matalon on the unsteady behavior of diffusion flames with heat loss, identified an oscillatory regime which could be triggered by either a sufficiently large Lewis number (even without heat loss) or an appreciable heat loss (even for Le=1). In light of these recent understanding, the present investigation aims to provide a well-controlled experiment that can unambiguously demonstrate the oscillation of diffusion flames near both the transport- and radiation-induced limits. That is, since candle and jet flames are stabilized through flame segments that are fundamentally premixed in nature, and since premixed flames are prone to oscillate, there is the possibility that the observed oscillation of these bulk diffusion flames could be triggered and sustained by the oscillation of the premixed flame segments. Concerning the observed oscillatory droplet extinction, it is well-known that gas-phase oscillation in heterogeneous burning can be induced by and is thereby coupled with condensed-phase unsteadiness. Consequently, a convincing experiment on diffusion flame oscillation must exclude any ingredients of premixed flames and other sources that may either oscillate themselves or promote the oscillation of the diffusion flame. The present experiment on burner-generated spherical flames with a constant reactant supply endeavored to accomplish this goal. The results are further compared with those from computational simulation for further understanding and quantification of the flame dynamics and extinction.

Law, C. K.↗

NASA NTRS2005

Experimental Investigation of Oscillatory Flow Pressure and Pressure Drop Through Complex Geometries

A series of experiments have been performed to investigate the oscillatory flow pressure and pressure drop through complex geometries. These experiments were conducted at the CSU-SLRE facility which is a horizontally opposed, two-piston, single-acting engine with a split crankshaft driving mechanism. Flow through a rectangular duct, with no insert (obstruction), was studied first. Then four different inserts were examined: Abrupt, Manifold, Diverging Short and Diverging Long. The inserts were mounted in the center of the rectangular duct to represent different type of geometries that could be encountered in Stirling machines. The pressure and pressure drop of the oscillating flow was studied for: 1) different inserts, 2) different phase angle between the two pistons of the engine (zero, 90 lead, 180, and 90 lag), and 3) for different piston frequencies (5, 10, 15, and 20 Hz). It was found that the pressure drop of the oscillatory flow increases with increasing Reynolds number. The pressure drop was shown to be mainly due to the gas inertia for the case of oscillatory flow through a rectangular duct with no insert. On the other hand, for the cases with different inserts into the rectangular duct, the pressure drop has three sources: inertia, friction, and local losses. The friction pressure drop is only a small fraction of the total pressure drop. It was also shown that the dimensionless pressure drop decreases with increasing kinetic Reynolds number.

Ibrahim, Mounir B.↗

NASA NTRS1963

PARTICIPATION OF THE SOLID PHASE IN THE OSCILLATORY BURNING OF SOLID ROCKET PROPELLANTS

An experimental study of the interaction between the solid and gas phases during oscillatory burning of solid propellants has been undertaken with the objective of exploring the validity of the complex theory describing the phenomenon. The apparatus employed was a side-vented, cylindrical vessel with an end-burning propellant grain closely fitted in one end and provision for maintaining the burning zone at a fixed position. Pressure oscillations both at the gas-zone end plate and beneath the grain were recorded. Energy dissipation in the solid propellant, isolated from other losses in the system, was determined from the growth rate constants of the amplitudes of pressure oscillation for grains of varying lengths. The efficiency of acoustical energy or absorption by the grain was found to be maximum when the oscillations are at frequencies close to the natural frequency of the grain. These results confirm the description of oscillatory burning by the acoustical theory in its broad generalizations.

GAS PHASE↗

NASA NTRS1972

Studies of oscillatory combustion and fuel vaporization

Research projects involving oscillatory combustion and fuel vaporization are reported. Comparisons of experimental and theoretical droplet vaporization histories under ambient conditions such that the droplet may approach its thermodynamic critical point are presented. Experimental data on instantaneous heat transfer from a gas to a solid surface under conditions of oscillatory pressure with comparisons to an unsteady one-dimensional model are analyzed. Droplet size and velocity distribution in a spray as obtained by use of a double flash fluorescent method were investigated.

Borman, G. L.↗

NASA NTRS1974

Oscillatory supersonic kernel function method for interfering surfaces

In the method presented in this paper, a collocation technique is used with the nonplanar supersonic kernel function to solve multiple lifting surface problems with interference in steady or oscillatory flow. The pressure functions used are based on conical flow theory solutions and provide faster solution convergence than is possible with conventional functions. In the application of the nonplanar supersonic kernel function, an improper integral of a 3/2 power singularity along the Mach hyperbola is described and treated. The method is compared with other theories and experiment for two wing-tail configurations in steady and oscillatory flow.

Cunningham, A. M., Jr.↗

NASA NTRS1983

An application of parameter identification to the oscillatory motion of an airplane at high C(L)

This paper presents an application of a stepwise regression incorporating polynomial splines to oscillatory flight data from a light research airplane operating at near stall angles of attack. It is shown that data from several experiments can be combined into a large data set for analysis and that hysteresis phenomena can be observed in this large data set. Finally, it is postulated from the analysis of the flight data and theoretical calculations that the observed oscillatory motion is the result of a combination of wing stall and wing wake position at the tail.

Batterson, J. G.↗

NASA NTRS1986

Uniformly high order accurate essentially non-oscillatory schemes 3

In this paper (a third in a series) the construction and the analysis of essentially non-oscillatory shock capturing methods for the approximation of hyperbolic conservation laws are presented. Also presented is a hierarchy of high order accurate schemes which generalizes Godunov's scheme and its second order accurate MUSCL extension to arbitrary order of accuracy. The design involves an essentially non-oscillatory piecewise polynomial reconstruction of the solution from its cell averages, time evolution through an approximate solution of the resulting initial value problem, and averaging of this approximate solution over each cell. The reconstruction algorithm is derived from a new interpolation technique that when applied to piecewise smooth data gives high-order accuracy whenever the function is smooth but avoids a Gibbs phenomenon at discontinuities. Unlike standard finite difference methods this procedure uses an adaptive stencil of grid points and consequently the resulting schemes are highly nonlinear.

Harten, A.↗

NASA NTRS1986

Lower mass limit of an evolving interstellar cloud and chemistry in an evolving oscillatory cloud

Simultaneous solution of the equation of motion, equation of state and energy equation including heating and cooling processes for interstellar medium gives for a collapsing cloud a lower mass limit which is significantly smaller than the Jeans mass for the same initial density. The clouds with higher mass than this limiting mass collapse whereas clouds with smaller than critical mass pass through a maximum central density giving apparently similar clouds (i.e., same Av, size and central density) at two different phases of its evolution (i.e., with different life time). Preliminary results of chemistry in such an evolving oscillatory cloud show significant difference in abundances of some of the molecules in two physically similar clouds with different life times. The problems of depletion and short life time of evolving clouds appear to be less severe in such an oscillatory cloud.

Tarafdar, S. P.↗

NASA NTRS1987

Extrapolation methods for divergent oscillatory infinite integrals that are defined in the sense of summability

In a recent work by the author an extrapolation method, the W-transformation, was developed, by which a large class of oscillatory infinite integrals can be computed very efficiently. The results of this work are extended to a class of divergent oscillatory infinite integrals in the present paper. It is shown in particular that these divergent integrals exist in the sense of Abel summability and that the W-transformation can be applied to them without any modifications. Convergence results are stated and numerical examples given.

Sidi, Avram↗

NASA NTRS1988

Essentially non-oscillatory shock capturing methods applied to turbulence amplification in shock wave calculations

ENO (essentially non-oscillatory) schemes can provide uniformly high order accuracy right up to discontinuities while keeping sharp, essentially non-oscillatory shock transitions. Recently, an efficient implementation of ENO schemes was obtained based on fluxes and TVD Runge-Kutta time discretizations. The resulting code is very simple to program for multi-dimensions. ENO schemes are especially suitable for computing problems with both discontinuities and fine structures in smooth regions, such as shock interaction with turbulence, for which results for 1-D and 2-D Euler equations are presented. Much better resolution is observed by using third order ENO schemes than by using second order TVD schemes for such problems.

Osher, Stanley↗

NASA NTRS1991

Effects of oscillatory speed and mutual overlap on the tribological behavior of PTFE and selected PTFE-based self-lubricating composites

The wear reducing actions of bronze, graphic powder and discontinuous glass fiber within a PTFE matrix have been demonstrated in oscillatory contact with stainless steel, at two different amplitudes, over a range of speeds. The secondary effects on the friction of the PTFE have also been investigated. The effects of oscillation amplitude on frictional surface morphology have been observed for these composites. The friction and wear of the unfilled PTFE matrix were both found to increase as speed increased, with wear going through a transition from a low- to high-rate regime. The speed at which this transition occurs increases with decreasing oscillatory amplitude.

Blanchet, Thierry A.↗

NASA NTRS1992

Intermodulation in the oscillatory magnetoresistance of a two-dimensional electron gas

The oscillatory magnetoresistance wave form of a 2-D electron gas shows multiple structures when two subbands are populated. In addition to high-field oscillations at a frequency equal to the sum of the two frequencies corresponding to the concentrations of the subbands, and to a superposition at intermediate fields, we observed oscillations at the difference frequency at low fields and higher temperatures. The field range at which the frequency difference is observed increases with increasing temperature. The crossover from superposition to frequency difference is accompanied by a beat. Similar beats, whose field location shows identical temperature dependence, can be observed in data obtained by other groups on different structures. The various components of the wave form can be attributed to different phase relations between the diagonal and off-diagonal elements of the conductivity tensor. It is shown how the intermodulation term, when inserted into the extended oscillatory equation, can give rise to all three structures.

Schacham, S. E.↗

NASA NTRS1992

An experimental study of oscillatory thermocapillary convection in cylindrical containers

An experimental study of oscillatory thermocapillary in small cylindrical containers with a heating wire placed along the center axis is performed by investigating the flow structures and temperature distributions under various conditions. To supplement the flow visualization the surface is scanned using an infrared imager. Here, 2 cS viscosity (Pr = 27) silicone oil is used as the test fluid. It is observed that beyond a certain temperature difference between the container wall and the heating wire, a distinctive unsteady flow pattern appears. This unsteady phenomenon is identified as oscillatory thermocapillary. After the onset of oscillations the flow structure becomes nonaxisymmetric and wave motion is observed at the free surface. It is shown that the critical temperature difference is independent of container dimensions if the aspect ratio is fixed.

Kamotani, Y.↗

NASA NTRS1992

On essentially non-oscillatory schemes on unstructured meshes: Analysis and implementation

A few years ago, the class of Essentially Non-Oscillatory Schemes for the numerical simulation of hyperbolic equations and systems was constructed. Since then, some extensions have been made to multidimensional simulations of compressible flows, mainly in the context of very regular structured meshes. In this paper, we first recall and improve the results of an earlier paper about non-oscillatory reconstruction on unstructured meshes, emphasizing the effective calculation of the reconstruction. Then we describe a class of numerical schemes on unstructured meshes and give some applications for its third order version. This demonstrates that a higher order of accuracy is indeed obtained, even on very irregular meshes.

Abgrall, R.↗

NASA NTRS1992

Intermodulation in the oscillatory magnetoresistance of a two-dimensional electron gas

Schacham, S. E.↗

NASA NTRS1992

Transition of oscillatory flow in tubes - An empirical model for application to Stirling engines

Attention is given to an empirical model for transition to turbulence in oscillatory flows in straight tubes. Designed after a correlation for transition of a boundary layer on a flat plate, the model yields the laminar flow momentum thickness Reynolds number that must be met before transition to turbulence will occur. The transition point is located by comparing this to the actual momentum thickness Reynolds number. A scheme is proposed for estimating the momentum thickness Reynolds number in terms of the position within the cycle, the maximum value of the diameter Reynolds within the cycle, Re(max), and the dimensionless frequency, Valensi number. Results from an experimental study of oscillatory flow in a tube are employed to develop the model. When the flow is determined to be turbulent, it is proposed that a fully-developed, steady flow friction coefficient be applied. When the flow is laminar, the assumption of fully developed flow cannot be made; thus, a method is suggested for estimating the friction factor.

Simon, T. W.↗

NASA NTRS1994

Control of oscillatory thermocapillary convection in microgravity

Laboratory and numerical experiments are underway to generate, and subsequently suppress, oscillatory thermocapillary convection in thin layer of silicone oil. The laboratory experiments have succeeded in characterizing the flow state in a limited range of Bond number-Marangoni number space of interest, identifying states of: (1) steady, unicellular, thermocapillary convection; (2) steady, multicellular, thermocapillary convection; and (3) oscillatory thermocapillary convection. Comparisons between experimental results and stability computations for a related basic state will be made.

Neitzel, G. Paul↗

NASA NTRS1994

Oscillatory thermocapillary convection

We study thermocapillary and buoyant thermocapillary convection in rectangular cavities with aspect ratio A = 4 and Pr = 0.015. Two separate problems are considered. The first is combined buoyant thermocapillary convection with a nondeforming interface. We establish neutral curves for transition to oscillatory convection in the Re-Gr plane. It is shown that while pure buoyant convection exhibits oscillatory behavior for Gr is greater than Gr(sub cr) (where Gr(sub cr) is defined for the pure buoyant problem), pure thermocapillary convection is steady within the range of parameters tested. In the second problem, we consider the influence of surface deformation on the pure thermocapillary problem. For the range of parameters considered, thermocapillary convection remained steady.

Mundrane, Michael R.↗