Sea level trend over Malaysian seas from multi-mission satellite altimetry and vertical land motion corrected tidal data

Rise in sea levels is one of the disastrous effects of climate change. A relatively small increase in sea level could affect natural coastal systems. In a study of long-term changes in sea level and measurements of postglacial rebound, monitoring vertical land motion (VLM) is of crucial interest. This study presents an approach to estimate precise sea level trends based on a combination of multi-sensor techniques in the Malaysian region over 19?years. In this study, satellite altimeters (SALT) were used to derive absolute sea levels (ASLs). Tide gauge (TG) stations along the coast of Malaysia were utilised to derive the rate of relative sea levels using sea level changes and VLMs. To obtain ASL at TGs, VLM at these stations were computed using Global Positioning System (GPS), Persistent Scatterer Interferometric Synthetic Aperture Radar (PS InSAR), and SALT minus TG. The computed VLMs mostly show similarities in signs rather than magnitude. The findings from the multi-sensor techniques showed that regional sea level trends ranged from 2.65?±?0.86?mm/yr to 6.03?±?0.79?mm/yr for chosen sub-areas, with an overall mean of 4.47?±?0.71?mm/yr and overall subsidence. This information is expected to be valuable for a wide variety of climatic applications and for studying environmental issues related to flooding and global warming in Malaysia.     

An estimation of tropospheric corrections using GPS and synoptic data: Improving Urmia Lake water level time series from Jason-2 and SARAL/AltiKa satellite altimetry

Tropospheric correction is one of the most important corrections in satellite altimetry measurements. Tropospheric wet and dry path delays have strong dependence on temperature, pressure and humidity. Tropospheric layer has particularly high variability over coastal regions due to humidity, wind and temperature gradients. Depending on the extent of water body and wind conditions over an inland water, Wet Tropospheric Correction (WTC) is within the ranges from a few centimeters to tens of centimeters. Therefore, an extra care is needed to estimate tropospheric corrections on the altimetric measurements over inland waters. This study assesses the role of tropospheric correction on the altimetric measurements over the Urmia Lake in Iran. For this purpose, four types of tropospheric corrections have been used: (i) microwave radiometer (MWR) observations, (ii) tropospheric corrections computed from meteorological models, (iii) GPS observations and (iv) synoptic station data. They have been applied to Jason-2 track no. 133 and SARAL/AltiKa track no. 741 and 356 corresponding to 117–153 and the 23–34 cycles, respectively. In addition, the corresponding measurements of PISTACH and PEACHI, include new retracking method and an innovative wet tropospheric correction, have also been used. Our results show that GPS observation leads to the most accurate tropospheric correction. The results obtained from the PISTACH and PEACHI projects confirm those obtained with the standard SGDR, i.e., the role of GPS in improving the tropospheric corrections. It is inferred that the MWR data from Jason-2 mission is appropriate for the tropospheric corrections, however the SARAL/AltiKa one is not proper because Jason-2 possesses an enhanced WTC near the coast. Furthermore, virtual stations are defined for assessment of the results in terms of time series of Water Level Height (WLH). The results show that GPS tropospheric corrections lead to the most accurate WLH estimation for the selected virtual stations, which improves the accuracy of the obtained WLH time series by about 5%.     

A Rework Reduction Mechanism in Complex Projects Using Design Structure Matrix Clustering Methods

To reduce the uncertainty and reworks in complex projects,a novel mechanism is systematically developed in this paper based on two classical design structure matrix(DSM) clustering methods:Loop searching method(LSM) and function searching method(FSM). Specifically,the optimal working areas for the two clustering methods are first obtained quantitatively in terms of non-zero fraction(NZF) and singular value modularity index(SMI),in which the whole working area is divided into six sub-zones. Then,a judgement procedure is proposed for conveniently choosing the optimal DSM clustering method,which makes it easy to determine which DSM clustering method performs better for a given case. Subsequently,a conceptual model is constructed to assist project managers in effectively analyzing the network of projects and greatly reducing reworks in complex projects by defining preventive actions. Finally,the aircraft design process is presented to show how the proposed judgement mechanism can be utilized to reduce the reworks in actual projects.     

The spatio-spectral localization approach to modeling VTEC over the western part of the USA using GPS observations

The precise ionosphere modeling is crucial and remains a challenge for GPS positioning and navigation, as well as many other Earth observation systems. In this research, two approaches have been proposed to model the vertical total electron content (VTEC) of the ionosphere using spherical slepian function. For the two-dimensional case, VTEC has been modeled in a sun-fixed reference frame and the three-dimensional approach based on the system of the three-dimensional base functions has been defined as the tensor product of the spherical slepian function for the longitude and latitude in an Earth-fixed reference frame, and the polynomial B-spline function for time. Rather than the spherical harmonics, the spherical slepian functions can be employed to produce the locally and globally orthogonal bases to optimally represent the data in any arbitrary region up to a given degree. The spherical slepian functions have been applied to the real data obtained from the ground-based GPS observation across the western part of the USA.     

线切割放电加工助航空客户飞得更高

过去20年里线切割的切割速度几乎增加了5倍,粗加工的直线切割速度可达600mm2/min;对大多数复杂型腔的加工速度较慢,但也可达到约350mm2/min。通过优化粗加工和精加工的策略,变质层几乎为零,同时没有材料或微观结构的变化,放电脉冲的控制特别是脉冲的持续时间和频率是达到最小损伤的关键因素。     

Recovery of Moho’s undulations based on the Vening Meinesz–Moritz theory from satellite gravity gradiometry data: A simulation study

In the gravimetric approach to determine the Moho depth an isostatic hypothesis can be used. The Vening Meinesz–Moritz isostatic hypothesis is the recent theory for such a purpose. Here, this theory is further developed so that the satellite gravity gradiometry (SGG) data are used for recovering the Moho depth through a nonlinear integral inversion procedure. The kernels of its forward and inverse problems show that the inversion should be done in a larger area by 5° than the desired one to reduce the effect of the spatial truncation error of the integral formula. Our numerical study shows that the effect of this error on the recovered Moho depths can reach 6 km in Persia and it is very significant. The iterative Tikhonov regularization in a combination with either generalized cross validation or quasi-optimal criterion of estimating the regularization parameter seems to be suitable and the solution is semi-convergent up to the third iteration. Also the Moho depth recovered from the simulated SGG data will be more or less the same as that obtained from the terrestrial gravimetric data with a root mean square error of 2 km and they are statistically consistent.     

Variable-fidelity optimization with design space reduction

Advanced engineering systems, like aircraft, are defined by tens or even hundreds of design variables. Building an accurate surrogate model for use in such high-dimensional optimization problems is a difficult task owing to the curse of dimensionality. This paper presents a new algorithm to reduce the size of a design space to a smaller region of interest allowing a more accurate surrogate model to be generated. The framework requires a set of models of different physical or numerical fidelities. The low-fidelity (LF) model provides physics-based approximation of the high-fidelity (HF) model at a fraction of the computational cost. It is also instrumental in identifying the small region of interest in the design space that encloses the high-fidelity optimum. A surrogate model is then constructed to match the low-fidelity model to the high-fidelity model in the identified region of interest. The optimization process is managed by an update strategy to prevent convergence to false optima. The algorithm is applied on mathematical problems and a two-dimen-sional aerodynamic shape optimization problem in a variable-fidelity context. Results obtained are in excellent agreement with high-fidelity results, even with lower-fidelity flow solvers, while showing up to 39% time savings.     

Effect of an end plate on surface pressure distributions of two swept wings

A series of wind tunnel tests was conducted to examine how an end plate affects the pressure distributions of two wings with leading edge(LE) sweep angles of 23° and 40°. All the experiments were carried out at a midchord Reynolds number of 8×10~5, covering an angle of attack(AOA) range from -2° to 14°. Static pressure distribution measurements were acquired over the upper surfaces of the wings along three chordwise rows and one spanwise direction at the wing quarter-chord line. The results of the tests confirm that at a particular AOA, increasing the sweep angle causes a noticeable decrease in the upper-surface suction pressure. Furthermore, as the sweep angle increases, the development of a laminar separation bubble near the LEs of the wings takes place at higher AOAs. On the other hand, spanwise pressure measurements show that increasing the wing sweep angle results in forming a stronger vortex on the quarter-chord line which has lower sensitivity to AOA variation and remains substantially attached to the wing surface for higher AOAs than that can be achieved in the case of a lower sweep angle. In addition, data obtained indicate that installing an end plate further reinforces the spanwise flow over the wing surface, thus affecting the pressure distribution.     

Harmonic analysis of the ionospheric electron densities retrieved from FORMOSAT-3/COSMIC radio occultation measurements

In order to investigate the regular variations of the ionosphere, the least-squares harmonic estimation is applied to the time series of ionospheric electron densities derived from about five years of Global Positioning System radio occultation observations by FORMOSAT-3/COSMIC satellites. The analysis is done for different latitudes and altitudes in the region of Iran. The least-squares harmonic estimation is found to be a powerful tool for the frequency analysis of the completely unevenly spaced time series of radio occultation measurements. Although the obtained results are slightly different from the exact expected cycles (i.e. annual and diurnal components with their Fourier decompositions, and the 27-day period) due to the low horizontal resolution of radio occultation measurements, high vertical resolution of the observations enables us to detect not only the total electron content variations but also periodic patterns of electron densities at different altitudes of the ionosphere. The dominant diurnal and annual signals together with their Fourier series decompositions are obtained, which are consistent with the previous analyses on the total electron content. In the equatorial anomaly band, the annual component is weaker than its Fourier decomposition periods. In particular, the semiannual period dominates the annual component, indicating the relationship between the semiannual variation of the electron densities and the ionospheric equatorial anomaly. From detection of the phases of the components, it is revealed that the annual signal generally has its maximum value in summers at high altitudes, and in the winters at low altitudes. This is probably due to the higher [O/N₂] ratios in winter than in the summer in the lower ionosphere. Furthermore, the semiannual component mostly peaks around solstices or about a month before/after them.