Generating vegetation leaf area index earth system data record from multiple sensors. Part 1: Theory

The generation of multi-decade long Earth System Data Records (ESDRs) of Leaf Area Index (LAI) and Fraction of Photosynthetically Active Radiation absorbed by vegetation (FPAR) from remote sensing measurements of multiple sensors is key to monitoring long-term changes in vegetation due to natural and anthropogenic influences. Challenges in developing such ESDRs include problems in remote sensing science (modeling of variability in global vegetation, scaling, atmospheric correction) and sensor hardware (differences in spatial resolution, spectral bands, calibration, and information content). In this paper, we develop a physically based approach for deriving LAI and FPAR products from the Advanced Very High Resolution Radiometer (AVHRR) data that are of comparable quality to the Moderate resolution Imaging Spectroradiometer (MODIS) LAI and FPAR products, thus realizing the objective of producing a long (multi-decadal) time series of these products. The approach is based on the radiative transfer theory of canopy spectral invariants which facilitates parameterization of the canopy spectral bidirectional reflectance factor (BRF). The methodology permits decoupling of the structural and radiometric components and obeys the energy conservation law. The approach is applicable to any optical sensor, however, it requires selection of sensor-specific values of configurable parameters, namely, the single scattering albedo and data uncertainty. According to the theory of spectral invariants, the single scattering albedo is a function of the spatial scale, and thus, accounts for the variation in BRF with sensor spatial resolution. Likewise, the single scattering albedo accounts for the variation in spectral BRF with sensor bandwidths. The second adjustable parameter is data uncertainty, which accounts for varying information content of the remote sensing measurements, i.e., Normalized Difference Vegetation Index (NDVI, low information content), vs. spectral BRF (higher information content). Implementation of this approach indicates good consistency in LAI values retrieved from NDVI (AVHRR-mode) and spectral BRF (MODIS-mode). Specific details of the implementation and evaluation of the derived products are detailed in the second part of this two-paper series.     

Design-Space Exploration for Block-Processing Based Temporal Partitioning of Run-Time Reconfigurable Systems

The reconfiguration capability of modern FPGA devices can be utilized to execute an application by partitioning it into multiple segments such that each segment is executed one after the other on the device. This division of an application into multiple reconfigurable segments is called temporal partitioning. We present an automated temporal partitioning technique for acyclic behavior level task graphs. To be effective, any behavior-level partitioning method should ensure that each temporal partition meets the underlying resource constraints. For this, a knowledge of the implementation cost of each task on the hardware should be known. Since multiple implementations of a task that differ in area and delay are possible, we perform design-space exploration to choose the best implementation of a task from among the available implementations.To overcome the high reconfiguration overhead of the current day FPGA devices, we propose integration of the temporal partitioning and design space exploration methodology with block-processing. Block-processing is used to process multiple blocks of data on each temporal partition so as to amortize the reconfiguration time. We focus on applications that can be represented as task graphs that have to be executed many times over a large set of input data. We have integrated block-processing in the temporal partitioning framework so that it also influences the design point selection for each task. However, this does not exclude usage of our system for designs for which block-processing is not possible. For both block-processing and non block-processing designs our algorithm selects the best possible design point to minimize the execution time of the design.We present an ILP-based methodology for the integrated temporal partitioning, design space exploration and block-processing technique that is solved to optimality for small sized design problems and in an iterative constraint satisfaction approach for large sized design problems. We demonstrate with extensive experimental results for the Discrete Cosine Transform (DCT) and random graphs the validity of our approach.     

Some experiments on the scratching of silicon:: In situ scratching inside an SEM and scratching under high external hydrostatic pressures

To elucidate the mechanisms of material removal in ultra precision machining of silicon involving deformation and fracture, in situ scratching of silicon with a diamond stylus inside a scanning electron microscope (SEM) using a specially designed tribometer and scratching under zero and high (400 MPa) external hydrostatic pressures using a specially designed high-pressure machining apparatus were conducted. The resulting scratches were examined in an SEM to evaluate the influence of depth of cut and hydrostatic pressure on the nature of scratches produced (smooth versus fractured surfaces) and the possible brittle–ductile transition. Experimental results indicate that hydrostatic pressure plays an important role in minimizing fracture and producing smooth surfaces. Reasonable agreement of the experimental results with the meso-plasticity FEM simulation of indentation was obtained.     

Molecular dynamics (MD) simulation of uniaxial tension of some single-crystal cubic metals at nanolevel

Molecular Dynamics (MD) simulations of uniaxial tension at nanolevel have been carried out at a constant rate of loading (500 ms⁻¹) on some single-crystal cubic metals, both FCC (Al, Cu, and Ni) and BCC (Fe, Cr, and W) to investigate the nature of deformation and fracture. Failure of the workmaterials due to void formation, their coalescence into nanocracks, and subsequent fracture or separation were observed similar to their behavior at macroscale. The engineering stress–strain diagrams obtained by the MD simulations of the tensile specimens of various materials show a rapid increase in stress up to a maximum followed by a gradual drop to zero when the specimen fails by ductile fracture. The radius of the neck is found to increase with an increase in the deformation of the specimen and to decrease as the ductility of the material decreases. In this investigation, the strain to fracture is observed to be lower with the BCC materials than FCC materials. In the case of BCC crystals, no distinct linear trend in the engineering stress–strain characteristics is observed. Instead, rapid fluctuations in the force values were observed. If the drop in the force curves can be attributed to the rearrangement of atoms to a new or modified crystalline structure, it appears that BCC materials undergo a significant change in their structure and subsequent realignment relative to the FCC materials, as previously reported in the literature. While good correlation is found between the D- and α-parameters of the Morse potential with the ultimate strength and the strain to failure for the FCC metals, no such correlation is found for the BCC metals. From this, it appears that Morse potentials may not represent the deformation behavior of BCC metals as accurately as FCC metals and alternate potentials may need to be considered.     

On the mechanics of the grinding process – Part I. Stochastic nature of the grinding process

Grinding of metals is a complex material removal operation involving cutting, ploughing, and rubbing depending on the extent of interaction between the abrasive grains and the workmaterial under the conditions of grinding. It is also a stochastic process in that a large number of abrasive grains of unknown geometry, whose geometry varies with time, participate in the process and remove material from the workpiece. Also, the number of grains passing through the grinding zone per unit time is extremely large. To address such a complex problem, it is necessary to analyze the mechanics of the grinding process using probability statistics, which is the subject of this investigation. Such an analysis is applicable to both form and finish grinding (FFG), such as surface grinding and stock removal grinding (SRG), such as cut-off operation. In this investigation, various parameters of the process including the number of abrasive grains in actual contact, the number of actual cutting grains per unit area for a given depth of wheel indentation, the minimum diameter of the contacting and cutting grains, and the volume of the chip removed per unit time were determined analytically and compared with the experimental results reported in the literature. Such an analysis enables the use of actual number of contacting and cutting grains in the grinding wheel for thermal and wheel wear analyses. It can also enable comparison of analytical work with the experimental results and contribute towards a better understanding of the grinding process. The analysis is applied to some typical cases of fine grinding and cut-off operations reported in the literature. It is found that out of a large number of grains on the surface of the wheel passing over the workpiece per second (˜million or more per second), only a very small fraction of the grains merely rub or plough into the workmaterial (3.8% for FFG and 18% for SRG) and even a smaller fraction (0.14% for FFG and 1.8% for SRG) of that participate in actual cutting, thus validating Hahn’s rubbing grain hypothesis.     

The significance of normal rake in oblique machining

3D molecular dynamics (MD) simulations of oblique machining of an aluminum workmaterial with a single straight cutting edge were conducted over a wide range of normal rake angles (−45° to +45°) and inclination angles (0° to 45°). Three distinct rake angles, namely, the normal rake angle, α_n, the velocity rake angle, α_v, and the so-called effective rake angle, α_e, associated with oblique machining were considered. Variation of the three components of force (cutting, thrust, and oblique), force ratio (thrust force/cutting force), and specific energy (energy required for unit volume of material removed) with rake angle and inclination angle were determined. Based on the analysis of the simulation results, it is shown that normal rake angle is the angle of significance influencing the mechanics of oblique machining, especially from the point of view of cutting force and specific energy in machining, as reported at the macro scale by many in the literature.     

On thermoplastic shear instability in the machining of a titanium alloy (Ti-6Al-4V)

A thermal model for the thermoplastic shear instability in the machining of a titanium alloy (Ti-6Al-4V) is developed. It is based on the analysis of the shear-localized chip formation process and the temperature generated in the shear band due to various heat sources (primary, preheating, and image) in machining. The temperature in the shear band was determined analytically using the Jeager’s classical stationary- and moving-heat-source methods. Using Recht’s classical model of catastrophic shear instability (thermal softening vs strain hardening), the onset of shear localization was determined. The shear stress in the shear band is calculated at the shear-band temperature and compared with the value of the shear strength of the bulk material at the preheating temperature. If the shear stress in the shear band is less than or equal to the shear strength of the bulk material, then shear localization is imminent. The cutting speed at which this occurs is taken as the critical speed for the onset of shear localization, which continues at all speeds above this value. In the case of titanium alloys, this speed is rather low, indicating shear localization practically at all conventional cutting speeds. The effect of the depth of the cut on the onset of shear localization was also considered, as it may affect the heat transfer from the shear-localized region, i.e., between the segments in the chip, to the rest of the chip and preheating of the segment. For example, there can be a significant difference in the thermal aspects of shear localization in ultraprecision machining (where the depths of cuts are a few micrometers or less) compared to conventional machining (where the depths of cuts are several hundred micrometers). This is because of the differences in the distances between the segments as well as the energy inputs in each case.     

A mechanism for the accelerated wear of high speed sliding surfaces in the presence of spherical particles

A mechanism for the accelerated wear of bearing surfaces due to the cutting action of the spherical particles between bearing surfaces in sliding contact is proposed and verified experimentally.     

Converging spherical and cylindrical shock waves

The self-similar solutions for converging spherical and cylindrical strong shock waves in a non-ideal gas satisfying the equation of state of the Mie-Gruneisen type are investigated. The equations governing the flow, which are highly non-linear hyperbolic partial differential equations, are first reduced to a Poincaré-type ordinary differential equation with suitable approximation. Such an approximation helps in obtaining the self-similar solutions and the similarity exponent numerically by phase-plane analysis.     

Self-similar flows with increasing energy-3. Radiation-driven shock wave

Self-similar flows behind a radiation-driven shock wave have been investigated. Approximate analytical solutions are presented when the flow is adiabatic. These solutions are agreeing well with the exact numerical solutions. A simple method to obtain the shock propagation law has been presented. Also explicit approximate analytical solutions are obtained when the flow behind the shock is taken to be isothermal.