2D characterization of near-surface V P/V S: surface-wave dispersion inversion versus refraction tomography

International audienceThe joint study of pressure (P-) and shear (S-) wave velocities (Vp and Vs ), as well as their ratio (Vp /Vs), has been used for many years at large scales but remains marginal in near-surface applications. For these applications, and are generally retrieved with seismic refraction tomography combining P and SH (shear-horizontal) waves, thus requiring two separate acquisitions. Surface-wave prospecting methods are proposed here as an alternative to SH-wave tomography in order to retrieve pseudo-2D Vs sections from typical P-wave shot gathers and assess the applicability of combined P-wave refraction tomography and surface-wave dispersion analysis to estimate Vp/Vs ratio. We carried out a simultaneous P- and surface-wave survey on a well-characterized granite-micaschists contact at Ploemeur hydrological observatory (France), supplemented with an SH-wave acquisition along the same line in order to compare Vs results obtained from SH-wave refraction tomography and surface-wave profiling. Travel-time tomography was performed with P- and SH- wave first arrivals observed along the line to retrieve Vtomo p and Vtomo s models. Windowing and stacking techniques were then used to extract evenly spaced dispersion data from P-wave shot gathers along the line. Successive 1D Monte Carlo inversions of these dispersion data were performed using fixed Vp values extracted from Vtomo p the model and no lateral constraints between two adjacent 1D inversions. The resulting 1D Vsw s models were then assembled to create a pseudo-2D Vsw s section, which appears to be correctly matching the general features observed on the section. If the pseudo-section is characterized by strong velocity incertainties in the deepest layers, it provides a more detailed description of the lateral variations in the shallow layers. Theoretical dispersion curves were also computed along the line with both and models. While the dispersion curves computed from models provide results consistent with the coherent maxima observed on dispersion images, dispersion curves computed from models are generally not fitting the observed propagation modes at low frequency. Surface-wave analysis could therefore improve models both in terms of reliability and ability to describe lateral variations. Finally, we were able to compute / sections from both and models. The two sections present similar features, but the section obtained from shows a higher lateral resolution and is consistent with the features observed on electrical resistivity tomography, thus validating our approach for retrieving Vp/Vs ratio from combined P-wave tomography and surface-wave profiling

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Iterative computation of currents in frequency-selective surfaces of finite size

Currents induced across a 20 × 5 dipole frequency selective surface have been computed using the iterative contrast source truncation technique. The results show the same nonuniform current distribution found earlier by the application of Pocklington's equation, particularly at frequencies near the surface reflection band. Edge effects extend to the centre of a 40 × 5 array

Conjugate gradient computation of the current distribution on a tripole FSS array element

Currents on a single tripole set in a domain of 32*64 samples have been computed over a 4:1 frequency range, with one sample across the conductor width. Some results are also given for three samples across the conductors, indicating for the first time the complexity of the current flow around the junction of the three arms of the element

Helicopter performance enhancement by alleviating retreating blade stall using active flow control

In this study, a model was created numerically to study the effect of active flow control on helicopter blade to improve the retreating blade stall. Active flow control is applied to helicopter blades to improve retreating blade stall along with the overall helicopter performance. The novel aspect of this study is using active control in a helicopter with a complex aerodynamic environment and using it in all significant flight modes. This has the advantage of practical application since active control can be used constantly without having to turn it off. The flow control is established by using blowing jets at the leading edge of the airfoil that is used at the tip part of the main rotor blade. 2D and 3D computational fluid dynamics (CFD) Turbulent-unsteady models are developed to investigate the effect of applying blowing jets on different helicopter modes of flight (hover and forward flight). Different excitation parameters and turbulence models are compared to get the maximum possible enhancement of the aerodynamic characteristic of the flow both in 2D and 3D. The results are validated against a NASA helicopter main rotor hub with known geometry and available performance parameters. Both models illustrated a good agreement with the published benchmark problem. The results revealed that the active control blowing jets can enhance the stall characteristics and aerodynamic performance of helicopter blades. The method delays the flow separation and increase the blade lift by about 10% in addition to a 40% increase in the lift to drag ratio. Thus, we can conclude that the present active control technique can enhance the helicopter in forward flight by delaying the retreating blade stall. This research offers insightful information on the usage of blowing jets for rotor flow control

Associations between SNPs in Intestinal Cholesterol Absorption and Endogenous Cholesterol Synthesis Genes with Cholesterol Metabolism

Single nucleotide polymorphisms (SNPs) have been associated with cholesterol metabolism and may partly explain large inter-individual variability in intestinal cholesterol absorption and endogenous cholesterol synthesis rates. This cross-sectional study therefore examined whether SNPs in genes encoding for proteins involved in intestinal cholesterol absorption (ABCG5, ABCG8, and NPC1L1) and endogenous cholesterol synthesis (CYP51A1, DHCR7, DHCR24, HMGCR, HSD17B7, LBR, and MSMO1) were associated with intestinal cholesterol absorption markers (total cholesterol (TC) standardized campesterol and sitosterol levels), an endogenous cholesterol synthesis marker (TC-standardized lathosterol levels), and serum low-density lipoprotein cholesterol (LDL-C) concentrations in a European cohort. ABCG5 (rs4245786) and the tag SNP ABCG8 (rs4245791) were significantly associated with serum campesterol and/or sitosterol levels. In contrast, NPC1L1 (rs217429 and rs217416) were significantly associated with serum lathosterol levels. The tag SNP in HMGCR (rs12916) and a SNP in LBR (rs12141732) were significantly associated with serum LDL-C concentrations. SNPs in the cholesterol absorption genes were not associated with serum LDL-C concentrations. SNPs in CYP51A1, DHCR24, HSD17B7, and MSMO1 were not associated with the serum non-cholesterol sterols and LDL-C concentrations. Given the variable efficiency of cholesterol-lowering interventions, the identification of SNPs associated with cholesterol metabolism could be a step forward towards personalized approaches

Surface and Catalytic Properties of the γ-Irradiated ZnO-Treated CoO/AlO System

Pure and ZnO-doped Co 3 O 4 /Al 2 O 3 catalysts were subjected to γ-irradiation (0.2–1.6 MGy). These catalysts were employed for the conversion of ethanol and isopropanol at 250–400°C using micropulse techniques. The results obtained showed that ZnO treatment increased the activity and selectivity towards dehydrogenation of the alcohols studied. On the other hand, γ-irradiation resulted in a progressive decrease in activity which fell to a minimum value at 0.8 MGy and then increased back to the original (unirradiated) value on increasing the dose further. γ-Irradiation of ZnO-treated catalysts also led to a decrease in their catalytic activity. However, in this case, the extent of such a decrease as a function of the dose applied decreased as the amount of ZnO in the catalysts increased up to 2.63 wt%. This finding was observed when the catalyzed reaction was conducted at 250–350°C, but the reverse trend was exhibited when the reaction was conducted at 350–400°C

Model identification and control analysis for underwater thruster system

992-998This paper conducts an analysis to obtain underwater thruster model using system identification approach. Performance of the developed model is further analysed by simulating it with PID control system. Four realization approaches are investigated whereby methods that carry the highest best fit value are chosen. Open loop and closed-loop control analysis are run for this purpose. From results, model which have best fit value of 98% able to provide good performance for open loop deployment analysis, whereas models with accuracy around 70% only unable to give satisfactory result. However, all the models show good performance for closed-loop analysis, in which, the PID controller model has played a role in improving the system performance