6,233 research outputs found
Properties of Concurrent Programs
A program property is a predicate on programs. In this paper we explore program properties of the form U -> V where U and V are either predicates on states of a program or program properties, and -> satisfies three rules that are also used in reasoning about sequential programs and safety properties of parallel programs. We show how such properties can be used to reason about concurrent programs
Dynamics and symmetries of flow reversals in turbulent convection
Based on direct numerical simulations and symmetry arguments, we show that
the large-scale Fourier modes are useful tools to describe the flow structures
and dynamics of flow reversals in Rayleigh-B\'enard convection (RBC). We
observe that during the reversals, the amplitude of one of the large-scale
modes vanishes, while another mode rises sharply, very similar to the
"cessation-led" reversals observed earlier in experiments and numerical
simulations. We find anomalous fluctuations in the Nusselt number during the
reversals. Using the structures of the RBC equations in the Fourier space, we
deduce two symmetry transformations that leave the equations invariant. These
symmetry transformations help us in identifying the reversing and non-reversing
Fourier modes.Comment: 4 pages, 3 figure
Systematic composition of distributed objects: Processes and sessions
We consider a system with the infrastructure for the creation and interconnection of large numbers of distributed persistent objects. This system is exemplified by the Internet: potentially, every appliance and document on the Internet has both persistent state and the ability to interact with large numbers of other appliances and documents on the Internet. This paper elucidates the characteristics of such a system, and proposes the compositional requirements of its corresponding infrastructure. We explore the problems of specifying, composing, reasoning about and implementing applications in such a system. A specific concern of our research is developing the infrastructure to support structuring distributed applications by using sequential, choice and parallel composition, in the anarchic environment where application compositions may be unforeseeable and interactions may be unknown prior to actually occurring. The structuring concepts discussed are relevant to a wide range of distributed applications; our implementation is illustrated with collaborative Java processes interacting over the Internet, but the methodology provided can be applied independent of specific platforms
A Primer for Program Composition Notation
This primer describes a notation for program composition. Program composition is putting programs together to get larger ones. PCN (Program Composition Notation) is a programming language that allows programmers to compose programs so that composed programs execute efficiently on uniprocessors, distributed-memory multicomputers or shared-memory multiprocessors. (Revised December 12, 1990
Experimental and theoretical studies of subsonic fan noise
The noise generated by inlet turbulence impinging on a subsonic axial flow fan was studied as a function of tip speed, flow coefficient, and intensity and scale of turbulence was carried out. Both turbulence and far field acoustic measurements were made. The new elements introduced in the theoretical analysis were accounting for blade loading dependent noise mechanisms and consideration of anisotropic turbulence impinging on the rotor because of inlet flow contraction effects. Experimentally an unexplained increase of noise at about 1/2 and 1 1/2 times blade passsing frequency was observed at low flow coefficients even though there was no evidence of compressor surge. In the final version the theory does a fair job of predicting variations of noise with blade loading and tip speed. Alteration of inlet turbulence length scales produced some but not very pronounced changes in the far field PWL spectra. Some degree of eddy contraction and resulting anisotropy were essential to explain the concentration of energy around blade passing frequencies
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