沟槽平板颗粒沉积特性和气膜冷却性能

通过将平板上的气膜冷却孔嵌入横向沟槽中,探究沟槽深度对平板颗粒沉积分布和气膜冷却性能的影响.结果表明:沟槽结构的存在改善了平板表面的颗粒沉积情况和气膜冷却性能,并将一定颗粒阻隔在沟槽内部.在同一吹风比下,沟槽深度的增加使得整体的颗粒碰撞效率增大.在吹风比较小时,3种沟槽深度的结构减小了平板颗粒的沉积和捕获,并改善了冷却...     

电热解法快速致密C/C复合材料研究

该工艺以液烃为前驱体,多孔碳坯浸在液烃中一次完成致密化。沉积温度为800℃～950℃时,坯多经2h～3h致密,其密度达1.62g/cm^3～1.74g/cm^3。以环已烷为前驱体制备的C/C试样具有良好的耐摩擦性能,动摩擦系数为0.2～0.4,静摩擦系数为0.25,每次刹车磨耗的0.89um,满足高性能刹车材料的要求。此外,该工艺所需设备结构简单,工作尺寸放大不受技术性限制,是一种很有开发价值的低成本、快速致密C/C材料的方法。     

热障涂层的制备及热震性能

采用电火花沉积+微弧氧化的方法在GH4169合金表面制备ZrO_2/NiCrAlY涂层,制备过程为:采用电火花沉积技术在GH4169基体表面先沉积厚度为250μm的NiCrAlY涂层,再沉积厚度为150μm的Zr涂层,最后通过微弧氧化的方法将Zr涂层氧化成ZrO_2涂层,从而得到ZrO_2/NiCrAlY涂层。采用Qauta 200F型场发射扫描电镜观察涂层的显微组织和形貌,研究ZrO_2/NiCrAlY涂层在不同温度下的热震性能,结果表明:当热震温度分别为750℃,850℃,950℃时,热震失效次数分别为51次、32次和19次,涂层的热震性能良好。     

Comparison of Ionospheric Total Electron Content (TEC) over Sonmiani (Pakistan) with NeQuick-2 and IRI-2012 during July 2014 – June 2015

In this study, Total Electron Content (TEC) observations acquired by a GNSS receiver installed at Sonmiani (Geog. Coord. 25.19°N, 66.74°E, Geomag. Coord. 17.62°N, 141.5°E) are being reported for the first time. The data utilized is hourly instantaneous TEC values during 10 International Quiet Days (IQDs) per month from Jul-14 to Jun-15, totaling 120 observation days for monitoring nominal TEC. The findings confirm the semi-annual trend of TEC over Sonmiani, which lies at the northern crest of Equatorial Ionization Anomaly (EIA) region. The TEC measurements are then compared with NeQuick-2 and International Reference Ionosphere (IRI-2012) models. It was found that the TEC values derived from NeQuick-2 are in better agreement with GNSS measurements than those from IRI-2012. The TEC measurements also show seasonal variation which is largest during Equinox months. The TEC value in Dec solstice is higher than the Jun solstice, which confirms that the seasonal anomaly is playing a major role in this region during the course of study.     

Investigation on surface integrity of electron beam melted Ti-6Al-4 V by precision grinding and electropolishing

Electron beam melting (EBM), as an excellent Additive Manufacturing (AM) technology, enables the printing of Ti-6Al-4 V alloy for a wide range of applications such as aerospace and biomechanical industries. It improves functionality and integrity of components and negates complexities in assembly processes. However, due to the poor surface and sub-surface integrity represented by the rough surface finish and low dimensional accuracy, achieving a favorable surface condition is quite challenging. Therefore, post processing becomes essential for these electron beam melted (EBM-ed) Ti-6Al-4 V alloys. Being the most common technique to improve such parts, milling of Ti-6Al-4 V alloy is very challenging and resulting tool wear issues, due to its unique material properties. Thus, this paper presents a comprehensive study on the surface integrity of EBM-ed Ti-6Al-4 V parts processed by precision grinding and electropolishing, aiming to qualitatively and quantitatively clarify the interrelation between process parameters and processed surface quality. The surface and subsurface characteristics such as profile accuracy, surface roughness, microstructure, defective layer and residual stress before and after post processing were compared and evaluated. The results show that by precision grinding, the profile accuracy was improved from over 300 µm PV to 7 µm PV, while surface roughness (R_a) was reduced from 30 µm to about 2 µm. The layer with partially melt particles was removed, but introduced a deformed subsurface layer with more residual stress. Then by applying electropolishing, the residual stress was released and the deformed layer was removed. In addition, R_a was further reduced to 0.65 µm. The research can serve as a reference for the integration of post machining processes with AM.     

The quiet nighttime low-latitude ionosphere as observed by TIMED/GUVI

In this paper, we examine the nighttime ionosphere climatology structure in the low latitude region and discrepancies between Global Ultraviolet Imager (GUVI) observations and the IRI model predictions using (1) the magnetic zonal mean of electron number density as a function of altitude and magnetic latitude, (2) vertical electron density profiles at various levels of F10.7 index, (3) nighttime descent and magnitude decrease of the ionosphere, (4) point-to-point comparisons of F-peak height (hmF2) and density (NmF2), and (5) the magnetic longitudinal variations of hmF2 and NmF2. The data collected from the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) mission since its launch in December 2001 have provided great opportunities for many scientific investigations of the ionosphere. In this analysis, we investigate the climatology of the nighttime low-latitude ionosphere under low geomagnetic activity (kp ? 4) using the electron density profiles inferred from the airglow measurements obtained by the GUVI aboard the TIMED spacecraft and compared with the results obtained from IRI (International Reference Ionosphere) model-2001. The observed climatology is an essential tool for further understanding the electrodynamics in the low-latitude region and improving the model’s prediction capability. The time range of the GUVI data used in this study is from 2002 (day 053) to 2006 (day 304), and the IRI model predictions were produced at every GUVI location. The ionosphere observed is generally of greater density than what IRI predicts throughout the night for all four seasons for low and moderate solar activity while the model over-predicts the electron density near the F-region peak at high solar activity before midnight. Observations show that the height of the F-region peak has a steep descent from dusk to midnight and near midnight the height of layer is insensitive to solar conditions, significantly different than what is predicted by IRI. Longitudinal features shown in GUVI data are present in the low-latitude ionosphere after sunset and continue through to midnight after which the low-latitude ionosphere is largely zonally symmetric.     

Improvements in bottomside electron density definition in the Autoscala program

Some improvements introduced in the Autoscala program are presented. They include improvements in E valley modeling of the electron density profile N_e(h), and in the link between the E valley and bottom-side F regions. An abrupt variation in N_e(h) generated by the previous version of Autoscala under night conditions has been eliminated.A series of ionograms recorded by the Millstone Hill digisonde (42.6°, 288.5°) were automatically interpreted by the previous version of Autoscala and by the new one. Data from Incoherent Scatter Radar (ISR) were used to comparatively assess the performance of the two versions. For this purpose, the root mean square errors (RMSEs) of the N_e(h) provided by Autoscala were calculated relative to the corresponding values provided by ISR.A more accurate overall modeling of N_e(h) was achieved by the new Autoscala version (RMSE = 0.51 MHz for the new version against RMSE = 0.67 MHz for the previous one).     

On the mid-latitude ionospheric storm association with intense geomagnetic storms

The bulk association between ionospheric storms and geomagnetic storms has been studied. Hemispheric features of seasonal variation of ionospheric storms in the mid-latitude were also investigated. 188 intense geomagnetic storms (Dst ≤ 100 nT) that occurred during solar cycles 22 and 23 were considered, of which 143 were observed to be identified with an ionospheric storm. Individual ionospheric storms were identified as maximum deviations of the F2 layer peak electron density from quiet time values. Only ionospheric storms that could clearly be associated with the peak of a geomagnetic storm were considered. Data from two mid-latitude ionosonde stations; one in the northern hemisphere (i.e. Moscow) and the other in the southern hemisphere (Grahamstown) were used to study ionospheric conditions at the time of the individual geomagnetic storms. Results show hemispheric and latitudinal differences in the intensity and nature of ionospheric storms association with different types of geomagnetic storms. These results are significant for our present understanding of the mechanisms which drive the changes in electron density during different types of ionospheric storms.     

Low solar activity variability and IRI 2007 predictability of equatorial Africa GPS TEC

Diurnal, seasonal and latitudinal variations of Vertical Total Electron Content (VTEC) over the equatorial region of the African continent and a comparison with IRI-2007 derived TEC (IRI-TEC), using all three options (namely; NeQuick, IRI01-corr and IRI-2001), are presented in this paper. The variability and comparison are presented for 2009, a year of low solar activity, using data from thirteen Global Positioning System (GPS) receivers. VTEC values were grouped into four seasons namely March Equinox (February, March, April), June Solstice (May, June, July), September Equinox (August, September, October), and December Solstice (November, December, January). VTEC generally increases from 06h00 LT and reaches its maximum value at approximately 15h00–17h00 LT during all seasons and at all locations. The NeQuick and IRI01-corr options of the IRI model predict reasonably well the observed diurnal and seasonal variation patterns of VTEC values. However, the IRI-2001 option gave a relatively poor prediction when compared with the other options. The post-midnight and post-sunset deviations between modeled and observed VTEC could arise because NmF2 or the shape of the electron density profile, or both, are not well predicted by the model; hence some improvements are still required in order to obtain improved predictions of TEC over the equatorial region of the Africa sector.