Showing papers on "Shields published in 2020"

Empirical Model for Shields Diagram and Its Applications

Abstract: The Shields diagram is a classic method for quantifying sand incipient motion in sediment transport. It is often expressed by empirical or semiempirical equations. This research applies the...

Initiation of sediment transport in steep channels with coarse bed material

Abstract: Measurements in a flume (0.6 m wide) with a gravel of median size 22 mm, a range of water discharges 0.01−0.1 m3/s and slopes of 3%,5%,7% and 9% are used, along with other flume and field data, to investigate the criteria which determine initiation of sediment transport in steep channels. The most common method for predicting initiation of transport is the Shields diagram which plots the Shields dimensionless shear stress against shear Reynolds number. However, this may not be appropriate to steep channels with small ratios of depth to sediment size because (a) the processes of sediment transport are not adequately described by shear stress considerations; and (b) the Shields diagram is poorly designed mathematically. By contrast the Schoklitsch approach, giving critical flow discharge as a function of slope, has a more suitable, process-related basis. The flume data support this analysis, indicating that the Shields diagram breaks down for steep channels (Fig. 1). The success of the Shields diagram elsewhere probably follows from its past restriction to flows with gentle slopes where variations in the Shields dimensionless shear stress are small (Fig. 2). The Schoklitsch approach explains the major variations in both field and flume data satisfactorily but effects related to bed packing and other field conditions have still to be quantified (Fig. 3). A critical discharge equation is proposed for flows with slopes > 2%.

The Faraday Shields Loss of Transformers

Abstract: Faraday shields are widely used in high-frequency transformers to conduct the common-mode (CM) current to the ground. They are normally located in the high magnetomotive force (MMF) region, where severe eddy currents are induced and contribute considerable magnetic field losses. In this article, an analytical procedure is presented for the magnetically-induced loss in Faraday shields. It is originally derived from shields of foil conductors. Then, it is extended to different shields with round and Litz wire, and different configurations, e.g., multilayer shields, interleaved windings, and coaxial transformers. It is verified by the finite element method (FEM) and experimental results in six case studies. Moreover, the criteria to determine the losses of Faraday shields is given and a design procedure is presented to reduce the shield losses at a certain level while keeping the functionality. Finally, a case study is provided to verify the shield design procedure on a 100-kHz transformer.

Study of the Efficiency of Corrugated Mesh Shields for Spacecraft Protection against Meteoroids and Manmade Space Debris

Abstract: High collision velocities and mass restrictions impose serious requirements on the design of anti-meteoroid spacecraft protection. In this study, the high efficiency of corrugated mesh anti-meteoroid shields in comparison with conventional metal mesh shields is confirmed based on the results of numerical modeling.

Impact of Circular Apertures on Static Shielding Performances of Spherical Magnetic Shields

Abstract: The magnetic field inside a spherical magnetic shield with infinite permeability and a circular aperture in a homogeneous magnetic field of arbitrary direction is analytically derived. The closed-form formulas for the static shielding factors are theoretical upper limits for real shields. We discuss the magnetic field gradient inside the shield caused by a single aperture, implying that using symmetric aperture pairs is more favorable. Based on the theoretical results for ideal shields, an approximate solution to calculate the shielding factor for shields with finite permeability and/or multiple apertures is developed. The calculations for one aperture pair are compared to finite-element analysis. The basic effects could be guidance for the design of apertures in shields of various shapes.

An Experimental Study of the Variability of the Shielding Effectiveness of Circuit Board Shields

Abstract: The paper examines the variability of the Shielding Effectiveness of board level shields measured in a reverberation chamber at frequencies from 200 MHz to 20 GHz. Results show that at any particular frequency the Shielding Effectiveness exhibits a typical variability of +20 dB about the mean value.

Three-Dimensionally Printed On-Skin Radiation Shields Using High-Density Filament

Abstract: Purpose Custom-fabricated lead shields are often used for superficial radiation treatments to reduce radiation doses to adjacent healthy tissue. However, the process for fabricating these lead shields is time consuming, labor intensive, and uncomfortable for patients. Alternatively, patient-specific shields can be 3-dimensionally (3D) printed from a high-density bronze-based filament to address these concerns. This study was performed to assess the shielding characteristics of 3D-printed bronze (3DPB) shields, demonstrate their clinical viability, and report the first ever published case of a patient treated with a 3DPB shield. Methods and Materials The transmission of 6 and 9 MeV electron beams through varying thicknesses of 3DPB was first measured. Percent depth doses and beam profiles were measured with flat 3DPB shields and equivalent lead shields to determine surface dose enhancement, output factors, and field widths. Two 3DPB shields were designed and fabricated for an anthropomorphic phantom, and phantom measurements were performed using optically stimulated luminescence dosimeters and film. Finally, 3DPB shields were used during the treatment of 7 patients' skin lesions. Results Ten and 15 mm of 3DPB were sufficient to shield 6 and 9 MeV electrons, respectively, by 95%. The 3DPB and lead shields had nearly identical beam widths (within 1%). Output factors were on average within 0.8% for bronze shields and 1.2% for lead shields relative to an unshielded field. The skin enhancement for bronze was higher than for lead by an average of 6.3%. Phantom measurements using 3DPB shields generally showed less than 3% transmission of the primary beam under the 3DPB shield. The patients' shields fit as designed and were all deemed clinically acceptable by their physicians. Conclusions The 3DPB shields fit better than lead shields, are easier to design and manufacture, and have similar dosimetric properties. 3DPB shields are a viable clinical option for patient-specific superficial shielding.

Finite-Element Modeling of Conductive Multilayer Shields by Artificial Material Single-Layer Method

Abstract: The artificial material single-layer (AMSL) method, recently proposed to model solid conductive shields in finite-element solvers without using a fine discretization, is here extended to model multilayer shields. First, the admittance matrix of a multilayer shield is analytically derived by the transmission line (TL) theory. Then, considering that the field through conductive shields propagates normally to the shield surface, the TL admittance matrix is equated to that of a 1-D finite element to extract the physical constants of a homogenized artificial material. These constants are adopted to model the multilayer shield region in the finite-element method (FEM) calculations by using only one layer of finite elements in the direction of the field propagation. By the AMSL-FEM, the field propagation through the multilayer shield is accurately modeled taking into account the skin effect and avoiding the fine discretization of the shield.

Lead-free transparent shields for diagnostic X-rays: Monte Carlo simulation and measurements

Abstract: In recent years, the preference for using lead-free radiation protection shields has increased because of concerns regarding lead poisoning and leakage. In medical and research laboratories, glass shields are preferred because of their transparency. In this study, various glass shields were examined and compared based on the international standards. One commercially available lead-based shield, four recently studied shields, and three new lead-free shields were considered, and their shielding factors were calculated. We presented three glasses based on borate, phosphate, and silicate compounds, which were named Ir1, Ir2, and Ir3, respectively. Based on the International Electrotechnical Commission standard (IEC 61331), the air-kerma ratios (attenuation ratios) and lead equivalent values were derived using Monte Carlo N-Particle eXtended (MCNPX) calculations, and mass attenuation coefficients and effective atomic numbers (Zeff) of all the shields were obtained from XCOM database, in the diagnostic X-ray energy range of 40–120 keV. In addition, some measurements were performed for the reference (lead-based) glass to validate the simulations. The above-mentioned factors for silicate-bismuth-based (Ir3) and borosilicate-barium-based (Tu) glasses were found to be higher than the others and comparable to those of commercially available lead-based glass. In conclusion, Ir3 and Tu glasses were found to be the preferred lead-free transparent shields in the diagnostic X-ray energy range.

Evaluations for Determination of Optimum Shields in Nuclear Medicine.

Abstract: Background 131I source is widely used in the treatment of hyperthyroidism and thyroid cancers 131I emits both beta and gamma-rays Radiation protection is considered for gamma rays emitted by 131I It seems no special shield against 131I source to be designed Objective This research aims to evaluate determination of optimum shields in nuclear medicine against 99Tcm and 131I sources by dosimetric method Additionally, Monte Carlo simulation was used to find the optimum thickness of lead for protection against 131I source Material and methods This is an experimental research in the field of radiation protection A calibrated model of GraetzX5C Plus dosimeter was used to measure exposure rates passing through the shields The efficiency of the shields was evaluated against 99Tcm and 131I Furthermore, Monte Carlo simulation was used to find the optimum thickness of lead for protection against 131I source Results The findings of the dosimetric method show that the minimum and maximum efficiencies obtained by the lead apron with lead equivalent thickness of 025 mm and the syringe holder shields with thickness of 05 mm lead were 5086% and 9950%, respectively The results of the simulations show that the minimum and maximum efficiencies obtained by lead thicknesses of 1 mm and 43 mm were 1936% and 9979%, respectively Conclusion The optimum shields against 99Tcm are the syringe holder shield, the tungsten syringe shield, and the lead partition, respectively Furthermore, based on simulations, the thicknesses of 11-28 mm of lead with efficiencies between 906% to 99% are suggested as the optimum thicknesses to protect against 131I source

Lifetime Prediction for Structural Elements of a Tamman Vacuum High-Temperature Furnace by Means of Mathematical Models

Abstract: A mathematical structural model of a Tamman vacuum high-temperature electric furnace with a heater in the form of a coal or graphite pipe and thermal insulation in the form of graphite cylinder shields is developed and implemented in the Simulink software package. The thermal resistance of the thermal conductivity of graphite shields, which is traditionally neglected in the calculation of widespread thin metal shields, was taken into account in the simulation of thermal processes. Also, the model takes into account the mass flow of the material of the elements of the furnace structure (the speed of the mass flow depends on the temperature and pressure in the working space), causing a change of the size and, correspondingly, the coefficients of the heat transfer caused by the radiation of the elements of the structure. The temperature regime of separate structural elements (heaters, shields, etc.) is analyzed both for a constant supply voltage of heaters and for the case in which a closed temperature control system is used. The influence of the temperature regime of structural elements on the wear of heating elements and shields is shown.

Calculation of Structural and Operational Parameters of a Facility for Infrared Heating of Base Parts during Restoration with a Polymer Material

Abstract: This paper describes an algorithm and computer program for calculating the structural and operational parameters of infrared radiation facilities for drying of polymer coatings during restoration of end shields of electric motors. On the example of end shields of electric motors, a high convergence of the calculated and experimental data of infrared heating parameters is shown, which indicates the correctness of the developed calculation method.

Pollution control device and method suitable for spacecrafts

Abstract: The invention provides a pollution control device and method suitable for spacecrafts The pollution control device comprises a heating shield, cold shields, a vacuum container, a vacuum pumping system and a liquid nitrogen system; the heating shield and the cold shields are arranged in the vacuum container; the heating shield covers the side face of the vacuum container; the cold shields cover the two ends of the vacuum container; the heating shield and the cold shields form a to-be-cleaned test piece containing space; each cold shield includes an expansion plate and a plurality of elongatedbaffles; each expansion plate consists of two stainless steel plates of which the peripheries are sealed and on which a plurality of bulges communicating with each other are formed about one or more than one laser welding spot through laser welding or supercharging; liquid nitrogen outlets and liquid nitrogen inlets communicate with the bulges; one side, near the heating shield, of each expansionplate is provided with the corresponding multiple elongated baffles; and the multiple elongated baffles are parallel with each other, and an included angle is present between the corresponding multiple elongated baffles and each expansion plate to form a blind window structure The heating shield is employed to bake test pieces, so that the pollution control device has higher universality on the test pieces; high-precision temperature control can also be performed even if a test piece has an irregular shape; and volatile pollutants generated by heating are deposited on the cold shields to avoid the pollution to the space environment

2D BEM Computation of Power Losses in Multiple Thin Conductive Shields

Abstract: This paper proposes a method to calculate Joule and hysteresis losses of thin conductive and magnetic shields in two dimensions. A known BEM formulation enforcing the Thin Layer Impedance Boundary Conditions (TLIBCs) is extended to problems involving multiple closed shields. Analytical formulae for the losses are derived. A typical geometry of shields for a three-phase conductor system is used to show that a good accuracy can be achieved.

Magnetic field normalization in residential building located near overhead line by grid shield

Abstract: This paper deals with the magnetic field mitigation of 110 kV trefoil single-circuit and double-circuit overhead lines by grid shields. The shields under study are made of aluminum conductors connected in parallel. These shields are mounted on the walls of the building. We study the efficiencies of plane and U-shaped grid shields as the dependence from the quantity of metal. As the result, we show that the plane grid shield does not supply the required efficiency of magnetic field mitigation in corner areas of the building. At the same time, the U-shaped grid shield having equivalent quantity of metal allows to mitigate the magnetic field to the reference level 0.5 µT in more than 97 % part of the building.

Ослабление потоков электронов радиационных поясов Земли защитными экранами на основе композита W-Cu

Abstract: For decreasing the radiation effects of the cosmic environment on the electronic components of spacecraft, local protection shields are used. They are manufactured on the basis of materials with high density and large atomic numbers (tungsten, tantalum, the W-Cu composite etc.) and then integrated into the ceramic-and-metal package of electronic components with an insufficient level of radiation resistance. On the basis of the Monte Carlo approach we considered the methods of decreasing the level of the dose absorbed by the crystals of active elements if using the radiation shields based on the W-Cu composite in hybrid metal cases under the action of electrons of a circular orbit with an inclination angle of 30° and an altitude of 8000 km. The electron spectra at the maximum and minimum solar activity were obtained using OMERE 5.3 software. It was established that an increase in the mass thickness of the base and cover of cases with shields up to 1.67 g / cm2 makes it possible to reduce the dose load by 3.5–3.7 times at the minimum and by 3.9–4.1 times at the maximum of solar activity. The optimization of protection by lowering the upper layer of the W-Cu composite to the base to a height of 1.2 mm reduces the absorbed dose by 6.8–9.3 times at the minimum and by 7.6–10.7 times at the maximum solar activity.

Deployable noise shield panels

Abstract: Technologies for providing noise shielding are described herein. In some examples, noise shields are installed proximate to one or more of the main engines of the aircraft. The noise shields can be extended during terminal operations and retracted during flight operations.

Modeling the Interaction of Solit-Like Pulse Signals with Electromagnetic Shields in the Form of Heterogeneous Media

Abstract: Modeling of electromagnetic radiation interaction with electromagnetic shields is an important problem that is solved during their development. By solving this problem, as a rule, it is possible to reduce the time and financial costs necessary to obtain electromagnetic shields, characterized by the required electromagnetic radiation attenuation and reflection coefficient. Currently, electromagnetic shields are usually developed in the form of heterogeneous media, which is due to lower the values of electromagnetic radiation reflection coefficient of such shields in comparison with the shields in the form of homogeneous media made in the form of continuous materials sheets. The authors of the article have proposed a new approach to modeling of electromagnetic radiation interaction with electromagnetic shields in the form of heterogeneous media. This approach is based on the use of difference schemes of end-to-end counting without explicitly distinguishing the interface between adjacent media, fulfilling the conditions of equality of full currents and charge flows on this boundary, and also on describing electromagnetic waves in the form of soliton-like signals, characterized by a greater penetration depth compared to other waves used in currently in the process of modeling (rectangular, sawtooth, etc). When using soliton-like signals that take into account broadening of spectral lines, the matching conditions for the first initial-boundary-value problem are satisfied. The existing software packages for electrodynamic tasks solving don’t take into account the matching conditions. On the base of the proposed approach, using the COMSOL Multiphysics software package, the authors first simulated the electromagnetic radiation interaction with a silver-based shield, the surface of which is rough and characterized by roughness sizes significantly smaller than the length of electromagnetic waves interacting with them, and with a shield in the form of a copper plate, the surface of which has slots, diameters and the depth of which is much less than the length of the electromagnetic waves interacting with them. The selection of these objects of study is due to the wide use of copper and silver for the electromagnetic shields manufacture, as well as the prospects for the development of shields formation technology, which consists in the heterogenization of the solid sheet metal materials surface.

Reduction of Dust Extraction from an Aspiration Hood via Mechanical Shielding

Abstract: The article explores the dust dynamics in an aspiration hood equipped with various thin mechanical shields. A differential equation of motion of dust particles is used to calculate their trajectories, and the velocity fields of the air flow are calculated by the method of boundary integral equations. The influence of different mechanical shields on the maximum diameter of dust particles is determined. The results demonstrate that by using curvilinear mechanical shields, dust extraction into the aspiration network can be significantly reduced.

Methods of applying thermal interface materials to board level shields

Abstract: Disclosed are exemplary embodiments of board level shields including thermal interface materials Also disclosed are methods of applying thermal interface materials to board level shields

Shielded Wires and Cables

Abstract: Shielded wires and cables are used within electrical and electronic system to reduce electromagnetic interference (EMI) within a system. The interference of concern is capacitive and inductive coupling from wire to wire. The termination of the shields for both capacitive and inductive coupling is critical and fully discussed. Shielded cables are often used to tie subsystems together to form a large system. The level of shielding by these cables can vary significantly, where various methods used to design the shields are discussed. The methods used to terminate the shields to ground are presented. These methods include referencing the shields to ground with wires brought through the interface connector as well as the use of back shells and connectors. The advantage and disadvantage of referencing the back shells to the outside and inside of an electronic system are thoroughly discussed. Coaxial cables and their use are thoroughly presented. This includes the shielding to the E and H fields.

Span with shields of covering of inter-wheel space

Abstract: FIELD: building of bridges.SUBSTANCE: invention relates to mechanized bridges. In mechanized bridge including wheeled self-propelled chassis, mechanism for installation of obstacle bridge and arranged on it bridge unit consisting of span, intermediate support and auxiliary equipment, in span, which is a track-type structure allowing folding of the pins in two and shifting them to provide for convenience of transportation, consisting of two ruts, on the inner edge of the roadway of each of the ruts, the shields of overlapping of the interstore space are installed and fixed, made in the form of plates of rectangular shape, curved out of metal sheet, reinforced with stiffeners, with the possibility of movement of these shields relative to span of span during transfer of stakes from transport to working position and back. Shields of covering one of sides through two holes are fixed to the edge of the roadway of one of the span using brackets, spring retainers locked by needle spring pins, without possibility of considerable movement relative to the carriageway. Other side of the flooring shield is secured through the gaps between the angles to the edge of the driveway. Shields of overlap on folding halves of track span are fixed mirror-like to balance center of gravity relative to axis of machine.EFFECT: disclosed is superstructure with shields of overlapping of inter-wheel space.1 cl, 4 dwg