Engineering topics

Shankar, N.

Publications and source records attributed to Shankar, N..

7 records

NASA NTRS2000

An Overview of SAL

To become practical for assurance, automated formal methods must be made more scalable, automatic, and cost-effective. Such an increase in scope, scale, automation, and utility can be derived from an emphasis on a systematic separation of concerns during verification. SAL (Symbolic Analysis Laboratory) attempts to address these issues. It is a framework for combining different tools to calculate properties of concurrent systems. The heart of SAL is a language, developed in collaboration with Stanford, Berkeley, and Verimag for specifying concurrent systems in a compositional way. Our instantiation of the SAL framework augments PVS with tools for abstraction, invariant generation, program analysis (such as slicing), theorem proving, and model checking to separate concerns as well as calculate properties (i.e., perform, symbolic analysis) of concurrent systems. We. describe the motivation, the language, the tools, their integration in SAL/PAS, and some preliminary experience of their use.

Bensalem, Saddek↗

NASA NTRS1990

Mechanical proofs of fault-tolerant clock synchronization

The following topics are covered in viewgraph form: (1) introduction to clock synchronization protocols; (2) a schematic formulation of clock synchronization; (3) the interactive convergence algorithm; (4) verification of Schneider's formulation; (5) verification of interactive convergence; (6) a hardware-oriented clock synchronization protocol; (7) verification of Infis/Moore's protocol; and (8) the EHDM specification/verification environment.

Shankar, N.↗

NASA NTRS1984

The dissolution or growth of a sphere

The problem of the dissolution or growth of an isolated stationary sphere in a large fluid body is analyzed. The motion of the boundary as well as the the resulting motion in the liquid are properly taken into account. The governing equations are solved using a recently developed technique (Subramanian and Weinberg, 1981) which employs an asymptotic expansion in time. Results for the radius of the sphere as a function of time are calculated. The range of utility of the present solution is established by comparison with a numerical solution of the governing equations obtained by the method of finite differences.

Shankar, N.↗

NASA NTRS1982

The motion of bubbles inside drops in containerless processing

A theoretical model of thermocapillary bubble motion inside a drop, located in a space laboratory, due to an arbitrary axisymmetric temperature distribution on the drop surface was constructed. Typical results for the stream function and temperature fields as well as the migration velocity of the bubble were obtained in the quasistatic limit. The motion of bubbles in a rotating body of liquid was studied experimentally, and an approximate theoretical model was developed. Comparison of the experimental observations of the bubble trajectories and centering times with theoretical predictions lends qualified support to the theory.

Shankar, N.↗

NASA NTRS1982

Physical phenomena in containerless glass processing

Experiments were conducted on bubble migration in rotating liquid bodies contained in a sphere. Experiments were initiated on the migration of a drop in a slightly less dense continuous phase contained in a rotating sphere. A refined apparatus for the study of thermocapillar flow in a glass melt was built, and data were acquired on surface velocities in the melt. Similar data also were obtained from an ambient temperature fluid model. The data were analyzed and correlated with the aid of theory. Data were obtained on flow velocities in a pendant drop heated from above. The motion in this system was driven principally by thermocapillarity. An apparatus was designed for the study of volatilization from a glass melt.

Subramanian, R. S.↗

NASA NTRS1982

Bubble migration inside a liquid drop in a space laboratory

The design of experiments in materials processing for trials on board the Shuttle are described. Thermocapillary flows will be examined as an aid to mixing in the formation of glasses. Acoustically levitated molten glass spheres will be spot heated to induce surface flow away from the hot spot to induce mixing. The surface flows are also expected to cause internal convective motion which will drive entrained gas bubbles toward the hot spot, a process also enhanced by the presence of thermal gradients. The method is called fining, and will be augmented by rotation of the sphere to cause bubble migration toward the axes of rotation to form one large bubble which is more easily removed. Centering techniques to fix the maximum centering accuracy will also be tried. Ground-based studies of bubble migration in a rotating liquid and in a temperature gradient in a liquid drop are reviewed.

Annamalai, P.↗

NASA NTRS1982

Thermocapillary motion of bubbles inside drops

A quasi-static analysis is performed for the thermocapillary motion of a bubble located inside a drop in free fall, with arbitrary axisymmetric temperature fields prescribed on the drop surface. It is shown that in the case of an axially symmetric temperature field, the bubble moves along the axis of symmetry toward the nearest warm pole. The bubble velocity as well as the velocity and temperature fields in the drop can be predicted on the basis of the quasi-static assumptions. An approximation is presented which adequately describes bubble migration velocities in the case where the ratio of the bubble radius to the drop radius is relatively small.

Shankar, N.↗