The friction of a copper surface, which is exposed to diethyl disulfide (DEDS), dimethyl disulfide (DMDS) or dimethyl trisulfide (DMTS) while rubbing by a tungsten carbide pin, are explored in an ultrahigh vacuum tribometer to investigate the effect of the sulfur-to-carbon stoichiometry on gas-phase lubrication. Surface analyses by Auger spectroscopy of the wear scars after rubbing reveal that the amount of sulfur increases with sulfur content of the gas-phase lubricant, as anticipated. It is found that the friction reduction depends on the normal load, where the friction coefficient tends to decrease more at lower loads, and that the load at which the most friction-reducing tribofilm is formed depends on the stoichiometry of the gas-phase lubricant. DEDS (with a sulfur to carbon ratio of 0.5) only reduces the friction coefficient to a minimum value of ~0.28 at a normal load of 0.25 N, for DMDS (S/C = 1.0) friction is reduced to ~0.28 for loads below ~0.7 N, and DMTS (S/C = 1.5) reduces friction to this value for all loads tested. 相似文献
Turbidity currents in the ocean and lakes are driven by suspended sediment. The vertical profiles of velocity and excess density are shaped by the interaction between the current and the bed as well as between the current and the ambient water above. We present results of a set of 74 experiments that focus on the characteristics of velocity and fractional excess density profiles of saline density and turbidity currents flowing over a mobile bed. The gravity flows include saline density flows, hybrid saline/turbidity currents and a pure turbidity current. The use of dissolved salt is a surrogate for suspended mud that is so fine that it does not settle out readily. Thus, all the currents can be considered to be model turbidity currents. The data cover both Froude-subcritical and Froude-supercritical regimes. Depending on flow conditions, the bed remains flat or bed forms develop over time, which in turn affect vertical profiles. For plane bed experiments, subcritical flow profiles have velocity peaks located higher up in the flow, and display a sharper interface at the top of the current, than their supercritical counterparts. The latter have excess density profiles that decline exponentially upward from the bed, whereas subcritical flows show profiles with a region near the bed where excess density varies little. Wherever bed forms are present, they have a significant effect on the profiles. Especially for Froude-supercritical flow, bed forms push the location of peak velocity upward, and render the near-bed fractional excess density more uniform. In the case of subcritical flow, bed forms do not significantly affect fractional excess density profiles; velocity profiles are pushed farther upward from the bed than in the case of a plane bed, but to a lesser extent than for supercritical bed forms. Overall, the relative position of the velocity peak above the bed shows a dependence upon flow regime, being lowered for increasing Froude number Fd. Gradient Richardson numbers Rig in the near-bed region increase with increasing Fd, but are lower than the critical value of 0.25, indicating that near-bed turbulent structures are not notably suppressed. At the top interface, values of Rig are above the critical value for subcritical and mildly supercritical Fd, effectively damping turbulence. However as Fd increases, Rig goes below the critical value. Shape factors calculated from the profiles for use in the depth-averaged equation of motion are evaluated for different flow and bed conditions. Normalized experimental profiles for supercritical currents scale up well with observations of field-scale turbidity currents in the Monterey Canyon, and the range of average bed slopes and Froude numbers also compare favorably with estimated field-scale flow conditions for the Amazon canyon and fan. This suggests that the experimental results can be used to interpret the kinds of flows that are responsible for the shaping of major submarine canyon-fan systems. 相似文献
During the last ten years 218 dacryocystorhinostomies were done, 8 of them on under 10-years-old children (3.7%). Dacryocystorhinostomy remains the only curative intervention which may be done on children even when the lacrimal ways sounding is inefficient. The surgical technique was the same as the one used for the adults, with some characteristic features imposed by the age of the patient. The intervention was some especially on children with congenital imperforations of the lacrimo-nasal channel, when the repermeability of the lacrimal ways could not be obtained by repeated soundings, but also in traumatic and congenital malformed dacryocystitis. The operation was made with good results even after the extirpation of the lacrimal sac, followed by the formation of a neosac. In the observations we presented the underlimit of the intervention has been 2 years and 4 months. 相似文献
The austenite formation kinetics in unalloyed cast ductile iron was studied on the basis of dilatometry measurements, and Avrami’s equation was used to estimate the material’s kinetic parameters. A con... 相似文献
In this paper, a multiple UAVs control scheme is developed considering the full nonlinear position/orientation model of a j-Quadrotor system. A novel second order sliding mode controller is presented which guarantees exponential and robust tracking of admissible time-varying pose. The harmful chattering is not involved and no dynamic model is required to implement the controller to yield fast and precise tracking. Additionally, well-posed terminal and controlled time convergence allows an enforced contact at given pre-defined stable contact points at the same time. A stiffness control is proposed for grasping objects considering virtual linkages approach. Our approach yields high performance from the control system, in contrast to other simple controllers proposed for load carrying. In this sense, our advanced nonlinear control solves the apparent limitations imposed by the available technology from the viewpoint of the precise tracking control, and control of the inherent unstable underactuated dynamics, for frictionless contact points (neither rolling nor sliding are considered). A numerical simulation study, under various conditions, shows the numerical feasibility of the proposed approach. 相似文献
An assessment of the standard fabrication Micro-Electro-Mechanical Systems (MEMS) process Multi-User MEMS Processes (MUMPs) for complex air-coupled capacitive Micromachined Ultrasonic Transducer array aperture manufacture is reported. A 1-D linear array and a 2-D sparse symmetric binned-array have been designed and manufactured, and then characterised experimentally using electrical impedance measurements, laser vibrometry and air-coupled field measurement; the experimental data are supported by simulated data using Finite Element technique and field simulation based on Huygens’ principle. A methodology for the manufacture of the array structures using the MUMPs process is described. Electrical characterisation shows the devices operation at 770 kHz and the existence of large parasitic capacitances and electrical losses. Mechanical crosstalk of array substrate has been measured at −40 dB using laser vibrometry. Moreover, the laser vibrometry measurement and the field characteristics of one element reveal that each element operates as a piston radiator.
Nutrient Cycling in Agroecosystems - Increasing cropping intensity may affect phosphorus (P) recycling through crop residue inputs, which may differ in soils with different productivity, P... 相似文献
Despite the high prevalence of lactic acid bacteria in dark fermentation (DF) processes, their ecological role is not yet completely elucidated, preventing their systematic use as “helpers” for hydrogen production. The aim of this study was to investigate the microbial community structure of a lactate-driven DF process that successfully produced hydrogen under carbohydrate-limiting conditions using tequila vinasse as a substrate. Microbial responses to stepwise decreases in hydraulic retention time (HRT) from 24 to 4 h were assessed by using Illumina MiSeq sequencing. HRTs above 12 h and below 6 h led to a lower hydrogen production rate (HPR; 0.2–3.3 L/L-d) and process stability (HPR variations within 25–65%), which were associated with the presence of Acetobacter lovaniensis, Clostridium luticellari, Blautia coccoides, and the high abundance of propionate and lactate. Interestingly, transient conditions from unsteady-to-steady state occurred at an HRT of 12 h, where species richness and evenness decreased remarkably. Accordingly, HRTs between 12 and 6 h resulted in higher HPRs of up to 11.7 ± 0.7 L/L-d with HPR variations of less than 10%, which closely matched with the dominance of Clostridium sp., and butyrate and acetate as the main aqueous products. Overall, the results indicate that the successfulness of exploiting the ‘unwanted’ LAB proliferation through lactate-driven DF processes requires the enrichment of lactate-consuming and hydrogen-producing bacteria, which entails the selection of proper biocatalysts and operating conditions/strategies such as the operation of DF reactors under carbohydrate-limiting conditions and low HRTs. 相似文献