Showing papers on "Shields published in 1984"
TL;DR: In this paper, the threshold of movement of loosely packed gravel in a tidal current is described, and the critical friction velocity u*c, corresponding to the initiation of sediment transport, is given by u *c=7.0 D 0.2 ≲D≲ 5.2.
Abstract: Observations of the threshold of movement of loosely packed gravel in a tidal current are described. For gravel with equivalent ‘spherical’ diameters D in the range 0.2 ≲D≲ 5.0cm the critical friction velocity u*c, corresponding to the initiation of sediment transport, is given by u*c=7.0 D0.2. At large values of D within the quoted range, the value u*c is significantly lower than would be obtained by a Shields experiment (u*c∞D0.5). By comparing our values of u*c with those obtained under well-controlled laboratory conditions, the discrepancy with Shields is shown to be due to the open spacing between, and exposure of, individual pebbles on the seabed. By comparing our results with those from upland gravel streams and flume experiments, it is suggested that Shields assumed an excessively large water depth to particle size ratio as a constraint within which the critical sediment entrainment number 0c is valid.
102 citations
TL;DR: In this paper, a method to optimize the location, temperature, and heat dissipation rate of each cooled shield inside an insulation layer was developed, based on the minimization of the entropy production rate which is proportional to the heat leak across the insulation.
Abstract: A method to optimize the location, temperature, and heat dissipation rate of each cooled shield inside an insulation layer was developed. The method is based on the minimization of the entropy production rate which is proportional to the heat leak across the insulation. It is shown that the maximum number of shields to be used in most practical applications is three. However, cooled shields are useful only at low values of the overall, cold wall to hot wall absolute temperature ratio. The performance of the insulation system is relatively insensitive to deviations from the optimum values of the temperature and location of the cooling shields. Design curves for rapid estimates of the locations and temperatures of cooling shields in various types of insulations, and an equation for calculating the cooling loads for the shields are presented.
20 citations
01 Jun 1984
TL;DR: The NASA STARPROBE spacecraft thermal protection system is designed to limit the spacecraft scientific instrument package to a maximum temperature of 55 C during the time of closest approach to the earth's sun when the peak radiative heating rate reaches 393 w/sq cm.
Abstract: This paper describes the preliminary design, development, and initial testing of prototype carbon-carbon components of the thermal protection system of the NASA STARPROBE spacecraft. The thermal protection system is designed to limit the spacecraft scientific instrument package to a maximum temperature of 55 C during the time of closest approach to the earth's sun when the peak radiative heating rate reaches 393 w/sq cm. The thermal protection system is comprised of a carbon-carbon thin shell primary shield and two carbon-carbon sandwich construction secondary shields which block reradiation from the primary shield to the spacecraft. These shields are joined and stiffened by carbon-carbon structural members and attached to the spacecraft with eight thin-walled carbon-carbon struts. Prototype parts of the principal components have been fabricated and will be tested in a solar radiative flux environment simulating peak mission values in late 1984.
1 citations
TL;DR: In this article, a system of four conical radiation shields for a spin stabilized STARPROBE spacecraft has been designed and analyzed with respect to variations of the cone half angles, the intershield spacings, and emissivities.
Abstract: For a system of N diffuse, gray body radiation shields which view only adjacent surfaces and space, the net radiation method for enclosures has been used to formulate a system of linear, nonhomogeneous equations in terms of the temperatures to the fourth power of each surface in the coupled system of enclosures. The coefficients of the unknown temperatures in the system of equations are expressed in terms of configuration factors between adjacent surfaces and the emissivities. As an application, a system of four conical radiation shields for a spin stabilized STARPROBE spacecraft has been designed and analyzed with respect to variations of the cone half angles, the intershield spacings, and emissivities.
1 citations
TL;DR: In this article, the authors presented experimentally-determined benchmark scalar flux spectra for 6.2 MeV photons from a 0.86 m diameter disc source penetrating cylindrical ducts of varying diameters up to 500 mm, machined through concrete shields of thicknesses up to 4.74 m.
TL;DR: In this paper, a computer model was developed to calculate the shield temperature rise with high accuracy for given cable designs, short-circuit currents and sequences, and a simplified shortcircuit test procedure was developed for evaluating new cable designs.
Abstract: Full scale laboratory tests were conducted to determine the maximum temperatures which round wire and flat strap metallic shields can withstand without damage under fault conditions. A computer model was developed to calculate the shield temperature rise with high accuracy for given cable designs, short-circuit currents and sequences. A simplified short-circuit test procedure was developed to facilitate evaluation of new cable designs. This procedure utilizes the computer program and a newly introduced concept of short-circuit equivalence factor which provides a tool for equating laboratory and field conditions. The results of this project, sponsored by EPRI, enable the cable engineer to optimize metallic shield designs with a substantial reduction of initial and operating costs.