Efficiency of gas cooling and accretion at the disc-corona interface

In star-forming galaxies, stellar feedback can have a dual effect on the circumgalactic medium both suppressing and stimulating gas accretion. The trigger of gas accretion can be caused by disc material ejected into the halo in the form of fountain clouds and by its interaction with the surrounding hot corona. Indeed, at the disc-corona interface, the mixing between the cold/metal-rich disc gas (T ~ 10^6 K) can dramatically reduce the cooling time of a portion of the corona and produce its condensation and accretion. We studied the interaction between fountain clouds and corona in different galactic environments through parsec-scale hydrodynamical simulations, including the presence of thermal conduction, a key mechanism that influences gas condensation. Our simulations showed that the coronal gas condensation strongly depends on the galactic environment, in particular it is less efficient for increasing virial temperature/mass of the haloes where galaxies reside and it is fully ineffective for objects with virial masses larger than 10^13 Msun. This result implies that the coronal gas cools down quickly in haloes with low-intermediate virial mass (Mvir <~ 3 x 10^12 Msun) but the ability to cool the corona decreases going from late-type to early-type disc galaxies, potentially leading to the switching off of accretion and the quenching of star formation in massive systems.Comment: 14 pages, 8 figures, accepted for publication in MNRA

A static analogy for 2D tolerance analysis

Purpose - This paper aims to present a method for the tolerance analysis of mechanical assemblies that is suitable to nonlinear problems where explicit functional equations are difficult or even impossible to write down. Such cases are usually modelled by linearised tolerance chains, whose coefficients (or sensitivities) are calculated from assembly data. Design/methodology/approach - The method is based on the free-body diagrams of force analysis, which are shown to be related to the sensitivities of linearised functional equations. Such an analogy allows the conversion of a tolerance chain into a corresponding static problem, which can be solved by common algebraic or graphical procedures. Findings - The static analogy leads to a correct treatment of tolerance chains, as the analysis of several examples has confirmed by comparison to alternative methods. Research limitations/implications - Currently, the method has only been tested on two-dimensional chains of linear dimensions for assemblies with nonredundant kinematic constraints among parts. Practical implications - The proposed method lends itself to ready application by using simple operations with minimal software assistance. This could make it complementary to current methods for calculating sensitivities, which are mathematically complex and require software implementation for deployment in industrial practice. Originality/value - Analogy with force analysis, which has not been previously highlighted in the literature, is a potentially interesting concept that could be extended to a wider range of tolerancing problems

Cold flow defects in zinc die casting: prevention criteria using simulation and experimental investigations

High-pressure die casting of zinc alloys is increasingly used in the manufacturing of parts with high aesthetic value. These parts must comply with strict requirements on surface quality, which are generally overlooked in traditional mechanical applications. Cold flow defects, which are a primary concern for surface quality, originate from several different causes that have not yet been fully understood. This report investigates the factors that influence cold flow defects and the choices that can lead to an improvement in surface quality. The research method is based on a case study performed at a die casting company. First, an existing process has been analyzed using simulation to explain the causes of cold flow defects observed in production samples. The temperature at the end of the cavity fill has emerged as a key index for the occurrence of defects, which can be controlled by three primary process parameters: injection velocity, temperature of the cooling medium, and lubricant spraying time. These same factors are then assessed using experimental tests on an existing die, where the number of defects in the selected regions of the casting has been evaluated by image processing. The results suggest that the surface quality can be particularly improved by increasing the flow rate of the molten metal through the gates and avoiding excessive flow turbulence in the wide cavity sections. Consequently, the increase in the gate area has been identified as a specific criterion for the die design. These findings have been validated in the redesign of the die and the selection of the process parameters, which have resulted in a significant reduction in the surface defects

SLM tooling for die casting with conformal cooling channels

The paper reports an experimental study of die-casting dies with conformal cooling fabricated by direct-metal additive techniques. The main objective is to compare the benefits and limitations of the application to what has been widely discussed in literature in the context of plastics injection molding. Selective laser melting was used to fabricate an impression block with conformal cooling channels designed according to part geometry with the aid of process simulation. The tool was used in the manufacture of sample batches of zinc alloy castings after being fitted on an existing die in place of a machined impression block with conventional straight-line cooling channels. Different combinations of process parameters were tested to exploit the improved performance of the cooling system. Test results show that conformal cooling improves the surface finish of castings due to a reduced need of spray cooling, which is allowed by a higher and more uniform cooling rate. Secondary benefits include reduction of cycle time and shrinkage porosity

Direct digital manufacturing of shoe heels through Fused Deposition Modeling

Additive manufacturing process chains for direct part production in the footwear sector are being investigated almost exclusively for athlete shoe soles and orthotics. This paper focuses on the direct digital manufacturing of high heels for woman shoes, which could provide solutions for personalized production and complex design for high added-value shoes. The study aims at testing the introduction of additively manufactured components into conventional shoe production lines. Sample heels of two common designs have been fabricated by the Fused Deposition Modeling technique, assembled with conventional mounting machines, and tested for wearability according to standard procedures. The experimentation has shown that fully process compatible and functional heels can be produced for a basic design under some restrictions, and has allowed to identify critical issues to be dealt with for future applications on more critical heel shapes. These results will help to develop methods for a robust heel design exploiting the flexibility and creative freedom allowed by additive processes

The survival of gas clouds in the Circumgalactic Medium of Milky Way-like galaxies

Observational evidence shows that low-redshift galaxies are surrounded by extended haloes of multiphase gas, the so-called 'circumgalactic medium' (CGM). To study the survival of relatively cool gas (T < 10^5 K) in the CGM, we performed a set of hydrodynamical simulations of cold (T = 10^4 K) neutral gas clouds travelling through a hot (T = 2x10^6 K) and low-density (n = 10^-4 cm^-3) coronal medium, typical of Milky Way-like galaxies at large galactocentric distances (~ 50-150 kpc). We explored the effects of different initial values of relative velocity and radius of the clouds. Our simulations were performed on a two-dimensional grid with constant mesh size (2 pc) and they include radiative cooling, photoionization heating and thermal conduction. We found that for large clouds (radii larger than 250 pc) the cool gas survives for very long time (larger than 250 Myr): despite that they are partially destroyed and fragmented into smaller cloudlets during their trajectory, the total mass of cool gas decreases at very low rates. We found that thermal conduction plays a significant role: its effect is to hinder formation of hydrodynamical instabilities at the cloud-corona interface, keeping the cloud compact and therefore more difficult to destroy. The distribution of column densities extracted from our simulations are compatible with those observed for low-temperature ions (e.g. SiII and SiIII) and for high-temperature ions (OVI) once we take into account that OVI covers much more extended regions than the cool gas and, therefore, it is more likely to be detected along a generic line of sight.Comment: 12 pages, 10 figures. Accepted for publication in MNRA

Application of Low Potential Electric Fields for Improving Slope Stability

AbstractThe aim of this research is the application of low potential direct currents in order to improve slope stability by inducing the reduction of potential swelling and water content, and the precipitation of carbonates in cohesive soil pertaining to a possible sliding surface. Two different types of tests were performed: the first one on small samples and the other one on a physical model reproducing a slope. Main results showing the effectiveness of this application are described

Allocation of geometric tolerances in one-dimensional stackup problems

Many tolerancing problems on mechanical assemblies involve a functional requirement depending on a chain of parallel dimensions on individual parts. In these one-dimensional cases, simple methods are available for the analysis and the allocation of dimensional tolerances. However, they are difficult to extend to geometric tolerances, which must be translated into equivalent dimensional tolerances; this allows the analysis but makes the allocation generally impossible without Monte Carlo simulation and complex search strategies. To overcome this difficulty, the paper proposes a way of dealing directly with geometric tolerances in the allocation problem. This consists in expressing the functional requirement as a linear model of geometric tolerances rather than equivalent dimensional tolerances; the coefficients of the model (sensitivities) are calculated considering both the dimension chain and the standard definition of the geometric tolerances. The approach can be combined with any constrained optimization method based on sensitivities. The optimal scaling method, previously proposed for dimensional tolerances, is extended to geometric tolerances and used in two examples to demonstrate the simplicity of the overall workflow and the quality of the optimal solution