The effect of na and Sr modification on surface tension and volumetric shrinkage of A356 alloy and their influence on porosity formation

Modification of the eutectic Si in Al-Si foundry alloys by adding strontium or sodium is, unfortunately, accompanied by an increase of porosity in the casting. In an attempt to understand the nature of this problem, this study used a sessile-drop method to investigate the effect of Sr and Na on surface tension and volumetric shrinkage, two probable causes of porosity occur-rence. The addition of 0.01 wt pct Sr and 0.005 wt pct Na to A356 alloy decreases the surface tension of the liquid by about 19 and 10 pct, respectively, and may increase the volume shrink-age by about 12 pct. These changes to surface tension and volumetric shrinkage promote the early formation of the pores during solidification and give the availability of a longer period of growth prior to complete solidification, resulting in a larger pore size. The effect of surface tension on the pores is more significant than volumetric shrinkage. Although the predicted pore diameter increases with lower surface tension or higher volumetric shrinkage, these two effects alone do not seem able to completely account for the observed increase in porosity that is associated with modification.     

Multivariate statistical analysis of a high rate biofilm process treating kraft mill bleach plant effluent.

This study reports on a multivariate analysis of the moving bed biofilm reactor (MBBR) wastewater treatment system at a Canadian pulp mill. The modelling approach involved a data overview by principal component analysis (PCA) followed by partial least squares (PLS) modelling with the objective of explaining and predicting changes in the BOD output of the reactor. Over two years of data with 87 process measurements were used to build the models. Variables were collected from the MBBR control scheme as well as upstream in the bleach plant and in digestion. To account for process dynamics, a variable lagging approach was used for variables with significant temporal correlations. It was found that wood type pulped at the mill was a significant variable governing reactor performance. Other important variables included flow parameters, faults in the temperature or pH control of the reactor, and some potential indirect indicators of biomass activity (residual nitrogen and pH out). The most predictive model was found to have an RMSEP value of 606 kgBOD/d, representing a 14.5% average error. This was a good fit, given the measurement error of the BOD test. Overall, the statistical approach was effective in describing and predicting MBBR treatment performance.     

Machine-learning paradigms for selecting ecologically significant input variables

Harmful algal blooms, which are considered a serious environmental problem nowadays, occur in coastal waters in many parts of the world. They cause acute ecological damage and ensuing economic losses, due to fish kills and shellfish poisoning as well as public health threats posed by toxic blooms. Recently, data-driven models including machine-learning (ML) techniques have been employed to mimic dynamics of algal blooms. One of the most important steps in the application of a ML technique is the selection of significant model input variables. In the present paper, we use two extensively used ML techniques, artificial neural networks (ANN) and genetic programming (GP) for selecting the significant input variables. The efficacy of these techniques is first demonstrated on a test problem with known dependence and then they are applied to a real-world case study of water quality data from Tolo Harbour, Hong Kong. These ML techniques overcome some of the limitations of the currently used techniques for input variable selection, a review of which is also presented. The interpretation of the weights of the trained ANN and the GP evolved equations demonstrate their ability to identify the ecologically significant variables precisely. The significant variables suggested by the ML techniques also indicate chlorophyll-a (Chl-a) itself to be the most significant input in predicting the algal blooms, suggesting an auto-regressive nature or persistence in the algal bloom dynamics, which may be related to the long flushing time in the semi-enclosed coastal waters. The study also confirms the previous understanding that the algal blooms in coastal waters of Hong Kong often occur with a life cycle of the order of 1–2 weeks.     

Statistical analysis of uniform-sampling phase-locked loops based on Volterra functional expansions

With the use of Volterra functional expansions in the domain of the multidimensional z transform, discrete uniform-sampling analogues of digital phase-locked loops of the first, second, and (N + 2)th orders with a delay of N sampling periods are analyzed.     

Authenticating Pollock paintings using fractal geometry

Jackson Pollock’s paintings are currently valued up to US$75 M, triggering discussions that attributation procedures featuring subjective visual assessments should be complimented by quantitative scientific procedures. We present a fractal analysis of Pollock’s patterns and discuss its potential for authenticity research.     

Design techniques for voltage-to-time converters with nonlinearity emphasis

This paper provides an in-depth treatment of voltage-to-time converters (VTCs) for time-based signal processing with a nonlinearity emphasis. The need for VTCs in deployment of time-based techniques for high-speed or high-resolution analog-to-digital converters is investigated. It is followed with the classification of VTCs. A detailed treatment of the principle, topology, operation, and design consideration of variable-slope (VS) and constant-slope (CS) VTCs is provided. The nonlinearity of VS-VTCs and that of CS-VTCs are analyzed in detail analytically. It is shown that VS-VTCs is inherently nonlinear while CS-VTCs is intrinsically linear. Factors contributing to the nonlinearity of these VTCs are investigated. VS-VTCs and CS-VTCs studied are designed in TSMC 130 nm 1.2 V CMOS and analyzed using Spectre from Cadence Design Systems with BSIM3.3 device models. A good agreement between simulation and analytical results is obtained. The average gain of the VS-VTC is 4.4 times that of the CS-VTC. The 2nd and 3rd harmonics of the CS-VTC are significantly smaller as compared with those of the VS-VTC at the price of more power consumption.

     

Supersonic Nozzle Flow Simulations for Particle Coating Applications: Effects of Shockwaves,Nozzle Geometry,Ambient Pressure,and Substrate Location upon Flow Characteristics

Characteristics of supersonic flow are examined with specific regard to nano-particle thin-film coating. Effects of shockwaves, nozzle geometry, chamber pressure, and substrate location were studied computationally. Shockwaves are minimized to reduce fluctuations in flow properties at the discontinuities across diamond shock structures. Nozzle geometry was adjusted to ensure optimal expansion (i.e., P _exit = P _ambient), where shock formation was significantly reduced and flow kinetic energy maximized. When the ambient pressure was reduced from 1 to 0.01316 bar, the nozzle’s diverging angle must be increased to yield the optimum condition of minimized adversed effects. Beyond some critical distance, substrate location did not seem to be a sensitive parameter on flow characteristics when P _amb = 0.01316 bar; however, overly close proximity to the nozzle exit caused flow disturbances inside the nozzle, thereby adversely affecting coating gas flow.     

An innovative method to perform maskless plain waterjet milling for pocket generation: a case study in Ti-based superalloys

This paper presents an innovative methodology/jet path on which plain waterjet (PWJ) can generate pockets of good dimensional/geometrical definition (minimised under/over-erosion) while the proposed method leads to the avoidance of grit embedment on the target workpiece and the elimination of extra cost and time related to the use of mask.The novelty of the paper relies on the proposal of jet-path strategy that minimises the variations in jet dwell time by providing “continuous” relative movement during the jet-part interaction (through minimisation of accelerations/decelerations of the machine head) and by removing a controlled amount of material in a series of layers using special techniques. The proposed method is powerful in its approach from which it ensures (quasi)equal exposure time for each zone of material over which the jet passes, so that the jet path is “totally contained” within the form to be generated; hence, no masking is necessary to define the contour/shape.This approach has been employed for generating pockets on two Ti-based superalloys commonly used in aerospace industries, followed by dimensional, geometrical and surface quality analysis. The results proved that this approach can produce milled surfaces of straightness of the pocket bottom (<200 μm), tolerance on depth of cut per layer (<20 μm), tolerance on the radii at the bottom of the pockets (<100 μm), surface roughness (Ra=4–14 μm) and waviness (Wa=10–13 μm) characteristics in conditions of high surface integrity (no cracks, contaminations, etc.).