Comment on Viscous Stability of Relativistic Keplerian Accretion Disks

Recently Ghosh (1998) reported a new regime of instability in Keplerian accretion disks which is caused by relativistic effects. This instability appears in the gas pressure dominated region when all relativistic corrections to the disk structure equations are taken into account. We show that he uses the stability criterion in completely wrong way leading to inappropriate conclusions. We perform a standard stability analysis to show that no unstable region can be found when the relativistic disk is gas pressure dominated.Comment: 9 pages, 4 figures, uses aasms4.sty, submitted for ApJ Letter

Performance of wide-area power system stabilizers during major system upsets: investigation and proposal of solutions

© 2021 Springer Nature Switzerland AG. This is a post-peer-review, pre-copyedit version of Benasla, M., Denaï, M., Liang, J. et al. Performance of wide-area power system stabilizers during major system upsets: investigation and proposal of solutions. Electr Eng (2021). The final authenticated version is available online https://doi.org/10.1007/s00202-020-01168-3Wide-area damping controllers (WADCs) are effective means of improving the damping of inter-area oscillations and thereby ensuring a secure operation of modern highly stressed interconnected power systems; however, their implementation costs are high. Therefore, the controller must be well configured and designed to ensure its cost-effectiveness. Several techniques have been proposed in the literature to design effective controllers and good results have been achieved. However, some important practical aspects that could potentially impact the performance of the designed controller have not been addressed or studied in sufficient detail in these previous works. One such aspect is assessing the performance of the designed controllers under major system upsets resulting in large deviations in the frequency and fluctuations in the power. These may lead to controller saturation which could negatively impact its damping performance or even cause instability. In this paper, the impact of such large upsets is investigated on several test systems via extensive small- and large-signal analyses and it is shown that, during severe transients, controller saturation may occur and persist over a long period of time, posing a potential threat to the power system stability. This paper presents a very effective solution to alleviate this problem and help design more robust WADCs. The simulation results show that the proposed solution works well and leads to improved power system stabilisers performance during transient upsets.Peer reviewedFinal Accepted Versio

Wealthy nations called on to boost support efforts. Five-year plan estimated to cost $9.2 billion.

Using the 20-year mark in the history of AIDS as a catalyst, the United Nations and other international organizations have called upon the world's wealthier nations to increase attention, support, and funding to HIV/AIDS prevention and treatment efforts. This call to action included a series of meetings with business and political leaders and culminated in the first United Nations General Assembly Special Session on HIV/AIDS, held in June

In-vitro accuracy and reproducibility evaluation of probing depth measurements of selected periodontal probes.

© 2014, In-vitro accuracy and reproducibility evaluation of probing depth measurements of selected periodontal probes. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/AIM: The purpose of the present in vitro study was to measure the accuracy and reproducibility of three periodontal probes. To eliminate environment- or examiner-related probing errors, two aluminum blocks with predrilled holes of varying depths were examined by participants who had been trained in probing before the study. This methodology improved the likelihood that any probing errors identified were generated by the probes themselves. MATERIALS AND METHODS: Three probes, Williams 14 W (Hu-Friedy Mfg. Co., LLC, UK), Chapple UB-CF-15 (Implantium, Shrewsbury, UK), and Vivacare TPS (Ivoclar Vivadent, Enderby, UK), were randomly distributed to 23 participants (9 males and 14 females; mean age: 31.35 years). Participants measured 30 holes in two aluminum blocks, average 20 days, SD = 341.05. For each hole, the mean measured depth was calculated for each participant and compared to the true depth. Intra- and inter-examiner accuracy and reproducibility for each of the duplicate measurements were calculated. Data were analyzed by paired-samples t-test with the SPSS 18 software package (IBM Portsmouth, UK). A p-value <0.05 indicated statistical significance. Tables were constructed from the data. RESULTS: When used by participants, the Williams 14 W probe was reproducible but not necessarily accurate; Vivacare TPS was neither accurate nor reproducible; and Chapple UBCF-15 was both accurate and reproducible. CONCLUSION: Depth measurements with the Chapple UB-CF-15 probe were more accurate and reproducible compared to measurements with the Vivacare TPS and Williams 14 W probes. This in vitro model may be useful for intra-examiner calibration or clinician training prior to the clinical evaluation of patients or in longitudinal studies involving periodontal evaluation