Contact behavior analysis of elastomeric x-ring under uniform squeeze rate and internal pressure before and after forcing-out using the photoelastic experimental hybrid method

Alunda Ouma Bernard,황재석,신동철,Bai Dong 대한기계학회 2015 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.29 No.5

Many different types of elastomeric rings have been developed to suit various needs in industry. The X-ring was introduced as a resultof the limitations of O-rings that twist, especially during dynamic application. A better understanding of the behavior and the stressdistribution of the X-ring under a uniform squeeze rate and internal pressure is needed. We analyzed the contact stresses and internalstresses developed in an X-ring before and after forcing-out by using the photoelastic experimental hybrid method, ascertained thepacking ability of an X-ring, and studied the failure criterion of an X-ring under uniform squeeze rate and internal pressure. Forcing-outin the X-ring occurred when the internal pressure was 3.92 MPa. After forcing-out, at an internal pressure of 5.88 MPa, the two lobes onthe upper contact surface merged one contact side of the upper side immensely. Even after extrusion of the X-ring, the X-ring can be usedto effectively contain the fluid. This is because the effects of extrusion on the X-ring affected the stress distribution of only two lobesclose to the assembly gap and the two lobes are merge into one lobe. In addition, our experimental results show that the maximum shearfailure criterion is suitable for the prediction of failure in X-ring seals.

A meta-analysis of gender gap in student achievement in African countries

Christine Ouma,남지원 한국행정학회 2015 International Review of Public Administration Vol.20 No.1

We conducted a meta-analysis to examine gender differences in educational assessments in African countries. We analyzed the primary database of the Southern andEastern Africa Consortium for Monitoring Educational Quality (SACMEQ) assessments. This study included a review of the relevant literature on meta-analysis techniques,and an overview of SACMEQ and gender issues in kindergarten (K)–12 in developing countries. The process of meta-analysis employed in this methodological study included searching, coding, calculation of effect sizes and their variances, weighting, confidence interval plots, Q test, funnel plots, and a discussion of results, implications, and future research. We found a small significant gender difference in mathematics in favor of boys and an insignificant gender difference in reading. Moreover, fertility rate was found to be an important predictor of gender gap in reading and math. Finally, we suggest a few implications for theoretical perspectives by connecting the key findings.

Internal stress distribution of X-ring using photoelastic experimental hybrid method

Alunda Ouma Bernard,황재석,신동철,임현석 대한기계학회 2014 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.28 No.5

Sealing elements are essential parts of many machines, and are used to prevent the loss of a fluid or gas. When such fluids are notproperly sealed, catastrophic failures may result. Many different types of rings have been developed to suit various industrial needs. Considerableresearch has been done on the O-ring. We analyze the internal stresses developed in an X-ring under a uniform squeeze rate of20%, which is suitable for static applications, using a photoelastic experimental hybrid method. The internal pressures applied were 0.98,1.96, 2.94, 3.92, 4.90, and 5.88 MPa. We show that sealing rings with X geometry have considerably higher internal stresses than O-ringseals. In addition, we demonstrate that after extrusion, for an internal pressure of 5.88 MPa, the two lobes on the upper contact surfacemerge, thereby increasing the contact length of the upper side significantly. Extrusion in the X-ring occurred when the internal pressurewas 4.90 MPa.

Genetic Diversity in Cultivated Sesame (Sesamum indicum L.) and Related Wild Species in East Africa

Benson Ouma Nyongesa,Beatrice Angiyo Were,Samuel Gudu,Otto George Dangasuk,Augustino Osoro Onkware 한국작물학회 2013 Journal of crop science and biotechnology Vol.16 No.1

Genetic diversity of traditional sesame landraces and related wild species in East Africa remains largely unexplored. Knowing what fraction of the available genetic diversity is actually used by the farmers is of central importance for understanding how cultivation shapes the genetic structure of a crop and for the management of biodiversity preservation. Genetic diversity in cultivated sesame and related wild species in East Africa was determined using inter-simple sequence repeats (ISSR). Six reliable ISSR primers generated 51 amplification fragments of which 36 (70.6%) were polymorphic. The number of amplified fragments ranged from 7 to 12 with a mean of 8.5 fragments per primer. The overall gene diversity and Shannon’s index were 0.28 and 0.34, Jaccard’s similarity coefficient ranged from 0.26 to 0.96, with an average of 0.67. Forty-six accessions of sesame were divided into six clusters, although the clustering did not indicate any clear division among sesame accessions based on their geographical locations. Each wild species was more distant from cultivated sesame than from other wild species, indicating that no cross-pollination with these wild species occurred during sesame domestication. These results showed a relatively high genetic diversity in sesame and related wild species. Indian-1 and Indian-2 accessions showed a good amount of genetic divergence. The genetic diversity data uncovered in this study can be exploited to improve traditional landraces of sesame in East Africa.

A Novel Two-axis Parallel-kinematic High-speed Piezoelectric Scanner for Atomic Force Microscopy

Bernard Ouma Alunda,Yong Joong Lee,Soyeun Park 한국물리학회 2016 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.69 No.5

High-speed atomic force microscopy permits the capture of static, as well as the dynamic, processes present in various physical phenomena. Unlike visualizing static processes, capture of dynamic processes requires high-speed scanning in all three dimensions. Despite the recent increased interest in high-speed atomic force microscopy, relatively few reports concerning piezoelectric actuatordriven scanners for high-speed scanning have been published. In this paper, we propose a novel design for a high-speed two-dimensional piezoelectric scanner unit by combining the positive features developed from works published in the literature. The proposed design ensures high vertical stiffness by utilizing compliant double-hinged flexure that minimizes cross-coupling and parasitic motions. Any high-speed scanner design requires a compromise between the two main competing parameters: maximum scan size and speed. The performance of the proposed scanner was evaluated by using numerical simulations with finite element analyses in terms of the mechanical resonance frequencies and the scan range. Finally, the results from the numerical simulations are compared with the experimental measurements.

Comparative Study of Trans-linear and Trans-impedance Readout Circuits for Optical Beam De ection Sensors in Atomic Force Microscopy

Bernard Ouma Alunda,루크오두어르오티에노,Melody CHEPKOECH,변지수,이영중 한국물리학회 2019 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.74 No.2

The optical beam de ection sensor remains the most popular force detection method used in atomic force microscopy. With the recent development of short cantilevers, a means for measuring small de ections at high frequencies has become a challenge. Minimizing the noise level of the readout electronics without signicantly limiting the detection bandwidth still remains a challenge. In this work, a recently proposed trans-linear readout circuit-based technique, in which necessary analog arithmetics are done in the current domain instead of the voltage domain, is compared to a more traditional trans-impedance readout circuit-based topology. Our developed trans-impedance readout circuit recorded a noise oor of 9:48 1013 V2 Hz1 compared to 1:41 1011 V2 Hz1 for the trans-linear readout circuit. Also, the measured 3 dB bandwidth of 11 MHz for the transimpedance readout circuit was slightly higher than the 10 MHz for the trans-linear readout circuit. Trans-impedance readout circuits, with proper circuit design considerations and careful selection of electronic parts, still remain competitive for use in high-speed operations in atomic force microscopy.

Calcium carbide–induced derangement of hematopoiesis and organ toxicity ameliorated by cyanocobalamin in a mouse model

Pherah A. Ouma,Victoria K. Mwaeni,Peris W. Amwayi,Alfred Orina Isaac,James Nyabuga Nyariki 한국실험동물학회 2022 Laboratory Animal Research Vol.38 No.3

Background: Calcium carbide (CaC2) is a chemical primarily used in the production of acetylene gas. The misuse of CaC2 to induce fruit ripening is a global challenge with a potential adverse effects to human health. Additionally, CaC2 is known to contain some reasonable amount of arsenic and phosphorous compounds that are toxic and pose a danger to human health when ingested. The current study sought to characterize CaC2 toxicity and elucidate any protective effects by cyanocobalamin (vitamin B12), a well-established antioxidant and anti-inflammatory bio-molecule. Female Swiss white mice were randomly assigned into three groups; the first group was the control, while the second group was administered with CaC2. The third group received CaC2 followed by administration of vitamin B12. The mice were sacrificed at 60 days post treatment, hematological, biochemical, glutathione assay, cytokine ELISA and standard histopathology was performed. Results: CaC2 administration did not significantly alter the mice body weight. CaC2 administration resulted in a significant decrease in packed cell volume (PCV), hemoglobin (Hb), red blood cells (RBCs) and RBC indices; indicative of CaC2-driven normochromic microcytic anaemia. Further analysis showed CaC2-driven leukopenia. Evidently, vitamin B12 blocked CaC2-driven suppression of PCV, Hb, RBCs and WBCs. Monocytes and neutrophils were significantly upregulated by CaC2. CaC2-induced elevation of aspartate aminotransferase (AST), alanine aminotransferase (ALT) and bilirubin signaled significant liver damage. Notably, vitamin B12 stabilized AST, ALT and bilirubin in the presence of CaC2, an indication of a protective effect. Histopathological analysis depicted that vitamin B12 ameliorated CaC2-driven liver and kidney injury. CaC2 resulted in the depletion of glutathione (GSH) levels in the liver; while in the brain, kidney and lungs, the GSH levels were elevated. CaC2 administration resulted in elevation of pro-inflammatory cytokines TNF-α and IFN-γ. Vitamin B12 assuaged the CaC2-induced elevation of these pro-inflammatory cytokines. Conclusions: These findings demonstrate for the first time that oral supplementation with vitamin B12 can protect mice against CaC2-mediated toxicity, inflammation and oxidative stress. The findings provide vital tools for forensic and diagnostic indicators for harmful CaC2 exposure; while providing useful insights into how vitamin B12 can be explored further as an adjunct therapy for CaC2 toxicity.

A simple way to higher speed atomic force microscopy by retrofitting with a novel high-speed flexure-guided scanner

Alunda, Bernard Ouma,Lee, Yong Joong,Park, Soyeun Institute of Pure and Applied Physics 2018 Japanese Journal of Applied Physics Vol. No.

Comparative study of two types of parallel kinematic flexure scanners for atomic force microscopy

Alunda, Bernard Ouma,Lee, Yong Joong,Park, Soyeun Informa UK (TaylorFrancis) 2018 Instrumentation science & technology Vol.46 No.1

<P>A little consideration will show that a scanner is one of the most critical components of any atomic force microscope (AFM), and properly designing a scanner remains a challenging aspect in the minds of developers. We closely examined two types of flexure-based parallel kinematic scanners (push-pull and push-only configurations) as they have been applied to AFM. The custom-fabricated scanners have been installed on a commercial AFM while keeping other parameters identical except for the scanners. The results show that intrinsically there is no significant difference in performance of both scanner designs. However, it was found that preloading conditions more critically affect the performance of the push-pull scanner than the push-only scanner. In addition, the Prandtl-Ishlinskii model has been applied to model the obtained hysteresis curves for both scanners. The application of the inverse of the Prandtl-Ishlinskii model improved the linearity of the measured hysteresis. Although both scanners possess similar characteristics and can operate at higher speeds than commercial scanners in reduced scan areas, simpler operating requirements and the monolithic construction make the push-only scanner a preferred choice for AFM.</P>

Genetic Diversity in Cultivated Sesame (Sesamum indicum L.) and Related Wild Species in East Africa

Nyongesa, Benson Ouma,Were, Beatrice Ang'iyo,Gudu, Samuel,Dangasuk, Otto George,Onkware, Augustino Osoro 한국작물학회 2013 Journal of crop science and biotechnology Vol.16 No.1

Genetic diversity of traditional sesame landraces and related wild species in East Africa remains largely unexplored. Knowing what fraction of the available genetic diversity is actually used by the farmers is of central importance for understanding how cultivation shapes the genetic structure of a crop and for the management of biodiversity preservation. Genetic diversity in cultivated sesame and related wild species in East Africa was determined using inter-simple sequence repeats (ISSR). Six reliable ISSR primers generated 51 amplification fragments of which 36 (70.6%) were polymorphic. The number of amplified fragments ranged from 7 to 12 with a mean of 8.5 fragments per primer. The overall gene diversity and Shannon's index were 0.28 and 0.34, Jaccard's similarity coefficient ranged from 0.26 to 0.96, with an average of 0.67. Forty-six accessions of sesame were divided into six clusters, although the clustering did not indicate any clear division among sesame accessions based on their geographical locations. Each wild species was more distant from cultivated sesame than from other wild species, indicating that no cross-pollination with these wild species occurred during sesame domestication. These results showed a relatively high genetic diversity in sesame and related wild species. Indian-1 and Indian-2 accessions showed a good amount of genetic divergence. The genetic diversity data uncovered in this study can be exploited to improve traditional landraces of sesame in East Africa.