Reconstructing flow from thermal wall imprint

This thesis develops data-driven flow reconstruction methods to reconstruct the velocity of plane Couette flow from wall temperature. We performed a Direct Numerical Simulation (DNS) for a heated plane Couette flow with imposed flux boundary condition at the bottom wall to create a data-set. Due to the the imposed flux the temperature at bottom wall is free and wall temperature patterns can develop. The focus of this thesis is on the investigations of the strong correlation between the flow velocity and the wall temperature. We analyse their joint probability density function and cross variance spectrum to develop a spectral linear regression model. This model successfully reconstructs wall shear stress from wall temperature except possibly at peaks. To reconstruct flow velocity from wall temperature, we apply flow decomposition modes such as the Proper Orthogonal Decomposition (POD) modes \cite{Holmes2012ProperDecomposition}. We design test problems to develop a framework to reconstruct \emph{gappy} fields with missing information. In this framework, we prescribe suitable regularisation for the under-determined \emph{gappy} fields. We also develop a decomposition method - the subdomain POD method which divides a physical domain into a number of subdomains and then applies the POD method in each subdomain individually. This subdomain POD are locally optimised and inherits properties of the POD modes. In both cases, namely the POD and the subdomain POD method, the reconstructions are found to be in good agreement with the flow velocity obtained form the DNS. To develop data-driven methods with imposed physical constraints, we propose a linear dynamical model based on Orr-Sommerfeld-Squire \cite{Kim2007,Murray2006} system and the scalar transport equation. This model successfully reconstruct some of the key flow structures at $z^+=35$.Open Acces

Inkjet Printed Flexible Patch Antennas for Full Duplex and Circular Polarization Applications

Flexible devices are getting more popular nowadays because of it’s lightweight, small size, portability, less expensive, environment friendly, and disposability. Flexible antenna, part of flexible devices, has become an important component of research for electrical engineers. The flexible antenna can be built using different materials like PET, PEN, liquid crystal polymer, PDMS, and textiles like woolen felt, cotton, cordura, and fleece as substrate. This thesis focuses on designing and fabricating antennas on PET paper by using inkjet printing for full duplex and circular polarization applications. Firstly, a flexible full-duplex antenna is proposed with robust performance and high isolation for 5.8 GHz using foam and PET paper. The patch of the antenna is modified by corner cut and inset feeding, while the defected ground structure is used to improve isolation between transmit and receive ports. Silver nanoparticle ink is used for printing the antenna in an inkjet printer. The fabricated version supports simulated results by showing acceptable performance in desired bandwidth. Bending tests and human body loading experiments are carried out on the fabricated antenna to demonstrate the antenna’s effectiveness for wearable applications. To the best of author’s knowledge, this is the first flexible full duplex antenna designed, achieving a high isolation level of -50 dB. Moreover, wide bandwidth, improved gain, and radiation efficiency, low cost, easy fabrication, and robust performance make it a good option for 5.8 GHz wearable applications. Secondly, a simple and compact CPW-fed circularly polarized antenna is presented. The proposed antenna consists of a modified “S” shaped patch, which has slots in three different places along with a slot in the ground plane. These slots contribute to increasing the bandwidth of the axial ratio. The antenna has a 3 dB axial ratio bandwidth of 10.47% (4.07 GHz- 4.52 GHz) and an impedance bandwidth of 17.53% (3.8 GHz - 4.53 GHz) covering the full region of axial ratio band. Moreover, this antenna is designed using PET paper which makes it flexible in nature and the first flexible antenna in the discussed frequency range to the best of author’s knowledge. Finally, an inkjet printed circularly polarized antenna using CPW feeding on PET substrate is proposed. The antenna is designed and optimized using ANSYS HFSS, which operates at 4.01 GHz - 5.05 GHz and 6.23 GHz - 7.58 GHz with a return loss of < -10 dB. On top of that, the antenna shows an axial ratio of less than 3 dB at 4.23 GHz - 4.62 GHz and 7.11 GHz - 7.36 GHz, whereas left hand circular polarization (LHCP) is observed in the first band and right hand circular polarization (RHCP) is observed in the second band. The dimension of the antenna is 31 mm x 37 mm x 0.135 mm. Measurement of the fabricated version shows good agreement with the simulated version. To the best of author’s knowledge, this antenna is the first flexible CPW-fed circularly polarized antenna with dual band

Optimization of Heat Sinks with Flow Bypass Using Entropy Generation Minimization

Forced air cooling of electronic packages is enhanced through the use of extended surfaces or heat sinks that reduce boundary resistance allowing heat generating devices to operate at lower temperatures, thereby improving reliability. Unfortunately, the clearance zones or bypass regions surrounding the heat sink, channel some of the cooling air mass away from the heat sink, making it difficult to accurately estimate thermal performance. The design of an "optimized" heat sink requires a complete knowledge of all thermal resistances between the heat source and the ambient air, therefore, it is imperative that the boundary resistance is properly characterized, since it is typically the controlling resistance in the path. Existing models are difficult to incorporate into optimization routines because they do not provide a means of predicting flow bypass based on information at hand, such as heat sink geometry or approach velocity. A procedure is presented that allows the simultaneous optimization of heat sink design parameters based on a minimization of the entropy generation associated with thermal resistance and fluid pressure drop. All relevant design parameters such as geometric parameters of a heat sink, source and bypass configurations, heat dissipation, material properties and flow conditions can be simultaneously optimized to characterize a heat sink that minimizes entropy generation and in turn results in a minimum operating temperature of an electronic component. An analytical model for predicting air flow and pressure drop across the heat sink is developed by applying conservation of mass and momentum over the bypass regions and in the flow channels established between the fins of the heat sink. The model is applicable for the entire laminar flow range and any type of bypass (side, top or side and top both) or fully shrouded configurations. During the development of the model, the flow was assumed to be steady, laminar, developing flow. The model is also correlated to a simple equation within 8% confidence level for an easy implementation into the entropy generation minimization procedure. The influence of all the resistances to heat transfer associated with a heat sink are studied, and an order of magnitude analysis is carried out to include only the influential resistances in the thermal resistance model. Spreading and material resistances due to the geometry of the base plate, conduction and convection resistances associated with the fins of the heat sink and convection resistance of the wetted surfaces of the base plate are considered for the development of a thermal resistance model. The thermal resistance and pressure drop model are shown to be in good agreement with the experimental data over a wide range of flow conditions, heat sink geometries, bypass configurations and power levels, typical of many applications found in microelectronics and related fields. Data published in the open literature are also used to show the flexibility of the models to simulate a variety of applications. The proposed thermal resistance and pressure drop model are successfully used in the entropy generation minimization procedure to design a heat sink with bypass for optimum dimensions and performance. A sensitivity analysis is also carried out to check the influence of bypass configurations, power levels, heat sink materials and the coverage ratio on the optimum dimensions and performance of a heat sink and it is found that any change in these parameters results in a change in the optimized heat sink dimensions and flow conditions associated with the application for optimal heat sink performance

Prevalence of depression and anxiety among university students during COVID-19 in Bangladesh: A cross sectional study

Introduction: The COVID-19 outbreak has become a challenging crisis for public health. During the COVID-19 pandemic, the indefinite closure of educational institutions in Bangladesh has a severe impact on the mental health of students. Purpose: The purpose of this study is to investigate factors that might have considerable influence on the mental health of students during quarantine in Bangladesh though they did not explore in previous studies on mental health status during the pandemic. Methodology: A standardized questionnaire was generated using PH9 and GAD7 to measure depression and anxiety levels. A total of 203 responses were collected from university students of Bangladesh through social media. Results: Descriptive statistics found that 37% of the students experienced moderate to severe anxiety while 54% faced moderate to severe depression. Ordinal Logistic Regression analysis found that anxiety is significantly related to gender, students’ current affiliation status in university (e.g., sophomore, masters), and time spent on watching TV while depression was related to family member’s contact with Covid-19, performing multiple activities as hobbies, and spending time in reading and writing. Conclusions: This study adds valuable findings in the existing literature, and it will help Students, university authorities, and government can take productive steps to tackle mental health issue

RESPONSES TO COVID-19 PANDEMIC IN A DEVELOPING COUNTRY: A SCENARIO OF BANGLADESH

Emerging at the end of 2019 in China, novel coronavirus (SARS-CoV-2) infected millions and killed thousands of people across the globe. Like other South Asian countries such as India and Pakistan, Bangladesh is also struggling to control the COVID-19 pandemic. Since the first detection on March 2020, terrible touch of the corona had been inundated Bangladesh with a total of 5,661,926 suspected patients and reported 776,257 (13.7%) cases, 12,005 deaths (1.5% CFR), and an overall recovery of 715,321 (92%) cases as of May 11, 2020. In attempts to reduce the burden of corona cases in one of the most densely populated country of the world, the government has taken several initiatives. Bangladesh has been stepped up a large number of medical &amp; testing facilities, manpower, social awareness campaigns, and has taken many initial steps to fight against COVID-19. This review article is focused on the overall outbreak situation, the preparedness of the government of Bangladesh as well as challenges and limitations faced by the country so far to tackle the novel coronavirus pandemic 2019

Requirement of ESWL after ureterorenoscopy and lithotripsy in the management of upper ureteric stone: comparison between Holmium Yag laser and pneumatic lithotripsy in a referral hospital, Bangladesh

Various methods have been employed for the removal of ureteric calculi all over the world. Both Ureteroscopic Pneumatic and Laser lithotripsy are popular procedures for this purpose. However proximal migration of stone fragments are not uncommon in these procedures while treating upper ureteric stones. Extra corporeal shock wave lithotripsy (ESWL) may require to fragment those proximally migrated stones after the ureteroscopic Pneumatic or Laser lithotripsy. To compare the requirement of ESWL in the management of ureteric stone between Holmium Yag Laser and Pneumatic Lithotripsy. This study included 100 patients with upper ureteric stones who underwent ureteroscopic lithotripsy at the Department of Urology, CMH, Dhaka, between October 2010 and September 2012. Laser lithotripsy was used in 50 patients (Group A), and pneumatic lithotripsy was used in the remaining 50 patients (Group B). In each case, the same ureteroscope, video monitor, baskets, or irrigation devices were used. A kidney ureter and bladder radiograph and ultrasonograph were performed on patients one month and three months after lithotripsy. Patients with migrated fragments or insufficient clearance underwent a supplementary procedure such as ESWL. Mean age was 41.9}10.9 years and 41.3}12.3 years in Group A and Group B respectively. Males were predominant in both groups. Mean stone size was 1.36 } 0.36 cm in Group A and 1.37} 0.36 cm in Group B. Complete stone clearance was 94.0% in Group A and 76.0% in Group B. EWSL requirement rate was significantly higher in Group B (24.0%) than Group A (6.0%). Peri procedural complications like hemorrhage was significantly higher in Group B and mucosal disruption/perforation was almost same in both the groups. EWSL requirement rate was comparatively higher in pneumatic lithotripsy than laser lithotripsy. BSMMU J 2022; 15(2): 111-11

Evaluation of The Current Level of Knowledge of The Residents of Dhaka City Regarding Earthquake Hazard

An earthquake is a sudden disaster that is not possible to predict. This impulsive behavior makes it very dangerous for humankind. Precautionary measures are immense for reducing damage. The first step of preventive measures for an earthquake is raising awareness. Dhaka City has a high earthquake risk due to its large population and urbanization. Researchers have said that an earthquake in this zone can be fatal, resulting in heavy casualties with structural damage. For this reason, proper awareness is essential for the residents of this area. This study shows an insight into the knowledge level of the residents of Dhaka City regarding earthquake incidents. Survey data indicates that many people lack Knowledge of what to do before and during an earthquake. Especially school and college-going students are unaware of the essential things to do during an earthquake incident. Many people still don’t know the importance of a seismic-resistant building system and are unprepared for a seismic event. This study brings these aspects together to learn about the knowledge level, which can help policymakers raise awareness among this city's residents

Reserve Allocation in Active Distribution Systems for Tertiary Frequency Regulation: A Coalitional Game Theory-based Approach

This paper proposes a coalitional game theory-based approach for reserve optimization to enable DERs participate in tertiary frequency regulation. A two-stage approach is proposed to effectively and precisely allocate spinning reserve requirements from each DER in distribution systems. In the first stage, two types of characteristic functions: worthiness index (WI) and power loss reduction (PLR) of each coalition are computed. In the second stage, the equivalent Shapley values are computed based on the characteristic functions, which are used to determine distribution factors for reserve allocation among DERs