Direct simulations and modelling of basic three-dimensional bifurcating tube flows

Three-dimensional bifurcating internal flow is studied for a single mother tube branching into two equal but diverging daughter tubes. The mother tube is straight and of circular cross-section, containing a fully developed incident motion, while the diverging daughters are straight and of semi-circular cross-section. This basic configuration is treated first by direct numerical simulation and secondly by slenderflow modelling, for a variety of Reynolds numbers and angles of divergence. The direct simulations and modelling highlight different forms of three-dimensional separation or flow reversal as well as enhanced upstream and downstream influence and pressure loss induced by the bifurcations especially at increased divergence angles. Comparisons between the results from the simulations and those from the slender-flow modelling show relatively close agreement at medium values of Reynolds number. In particular, as the angle of divergence increases for a given Reynolds number, there is generally first an increase in flow attachment on to the inner divider wall(s) and then, at higher angles, an increasing trend to flow reversal at the corners formed by the junctions of the outer wall with the divider; longitudinal vortex motion is also enhanced then. The agreement persists over a surprisingly wide range of divergence angles

Combination of Lighthill Acoustic Analogy and Stochastic Turbulence Modelling for Far-Field Acoustic Prediction

There are many approaches in determining the sound propagated from turbulent flows. Hybrid approaches, in which the turbulent noise source field is computed or modeled separately from the far-field calculation, are frequently used. To have a more feasible approach for basic estimation of sound propagation, cheaper methods can be developed using stochastic modeling of the turbulent fluctuations (turbulent noise source field). In this paper, a simple and easy to use stochastic model for the generation of turbulent velocity fluctuations called continuous filter white noise (CFWN) model is used. This method is based on the use of classical Langevian-equation to model the details of fluctuating field superimposed on averaged computed quantities. The sound propagation due to the generated unsteady field is evaluated using Lighthill's acoustic analogy. Our results are validated by comparing the directivity and the overall sound pressure level (OSPL) magnitudes with the available experimental results. Numerical results show reasonable agreement with the experiments, both in maximum directivity and magnitude of the OSPL

Retinal arteriolar geometry is associated with cerebral white matter hyperintensities on MRI

Background. Cerebral small vessel disease (lacunar stroke and cerebral white matter hyperintensities) is caused by vessel abnormalities of unknown aetiology. Retinal vessels show developmental and pathophysiological similarities to cerebral small vessels and microvessel geometry may influence vascular efficiency. Hypothesis. We hypothesized that retinal arteriolar branching angles or co-efficients (the ratio of the sum of the cross sectional areas of the two daughter vessels to the cross sectional area of the parent vessel at an arteriolar bifurcation) may be associated with cerebral small vessel disease. Methods. We performed a cross-sectional observational study in a tertiary referral hospital, United Kingdom. An experienced stroke physician recruited consecutive patients presenting with lacunar ischaemic stroke with a control group consisting of patients with minor cortical ischaemic stroke. We performed brain magnetic resonance imaging to assess the recent infarct and periventricular and deep white matter hyperintensities. We subtyped stroke with clinical and radiological findings. We took digital retinal photography to assess retinal arteriolar branching co-efficients and branching angles using a semi-automated technique. Results. We recruited 205 patients (104 lacunar stroke, 101 cortical stroke), mean age 68 (Standard Deviation 12) years. With multivariate analysis, increased branching coefficient was associated with periventricular white matter hyperintensities (p=0.006) and ischaemic heart disease (p<0.001); decreased branching co-efficient with deep white matter hyperintensities (p=0.003) but not with lacunar stroke subtype (p=0.96). We found no associations with retinal branching angles. Conclusions. Retinal arteriolar geometry differs between cerebral small vessel phenotypes. More research is needed to ascertain the clinical significance of these findings

Assessing the criticality of germanium as a by-product

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 37-38).Although germanium production is currently nowhere near its supply potential, many sources cite germanium, a by-product material produced primarily from zinc and coal, as a critical metal. Current methods for assessing criticality include frameworks that rely on geopolitical risk metrics, geological reserves, substitutability, and processing limitations during extraction among others but there is a gap in understanding the complex supply and demand dynamics that are involved in the market for by-products. This thesis addressed this gap by assessing the supply risk of germanium using an econometric framework to generate estimates of price elasticities. Annual world production and price data of years 1967 - 2014 for germanium was used to construct supply and demand models in order to obtain estimates for the price elasticities of supply and demand. Ordinary least squares (OLS) regression was used on an autoregressive distributed lag (ARDL) model for both supply and demand. The supply model was constructed with price, zinc production, and 5-year interest rate as shifters along with lag terms for germanium production, germanium price, 5-year interest rate, and zinc production. The adjusted R2 was 0.761 and the long term supply price elasticity was found to be 0.05 with an upper bound of 0.7 and a lower bound of -0.6 indicating that germanium supply is price inelastic. In a similar fashion, a demand model was constructed with two structural breaks accounting for fundamental changes in the market structure in 1991 and 2003, along with lag terms for germanium production, germanium price and antimony price. The adjusted R2 value for the demand model was 0.683 and the price elasticity was 0.05 with an upper bound of 1 and a lower bound of -1 indicating that demand, too, is price inelastic. This creates an added risk for supply shortages, adding to the criticality of germanium. However, the stabilizing behavior of its carriers, coal and zinc, reduce the likelihood of an actual shortage. This type of analysis improves upon existing methods and can lead to more accurate quantified estimates for long-term criticality.by Nagisa Tadjfar.S.B

Numerical study of aerodynamic forces of two airfoils in tandem configuration at low reynolds number

For a tandem airfoil configuration, an airfoil is placed in the wake of an upstream airfoil. This interaction affects the aerodynamic forces of the airfoils, especially the downstream one. In the present study a tandem configuration consists of an upstream pitching airfoil and a downstream stationary airfoil is investigated. This study aims to investigate the role of reduced frequency and pitch amplitude of the upstream airfoil s motion on lift and drag coefficients of two airfoils. These two parameters play an important role in the formation of vortices. The investigation is done for Selig-Donovan 7003 (SD7003) airfoils at low Reynolds number of 30,000 using a computational fluid dynamics. Incompressible URANS equations were employed for solving the flow field. It was found that for a fixed reduced frequency of 0.5 thrust is produced on the hindfoil for a part of cycle for different pitch amplitudes from light to deep stall while for a fixed pitch amplitude at different reduced frequencies high level of thrust or drag can be produced. The reason is related to the type and intensity of vortex-blade interaction

Configuration Optimization of Two Tandem Airfoils at Low Reynolds Numbers

Optimization was used to find the best configuration of two airfoils in tandem placed into an incoming flow. Upstream airfoil (forefoil) was pitching at a fixed frequency, while the downstream airfoil (hindfoil) was kept at a fixed angle of attack. Study was performed at a low Reynolds number of 30,000 based on chord length. Selig-Donovan 7003 (SD7003) was used for both airfoils, which is a high-performance airfoil specially designed for low Reynolds number flows. The optimization studies were conducted using a genetic algorithm (GA) to maximize aerodynamic performance. The design variables in this study were: horizontal and vertical spacing between the airfoils and hindfoil's angle of attack. Since the optimization process is time-consuming, machine learning was used to train four artificial neural networks (ANNs) to be coupled with genetic algorithm to reduce the computational cost. Two separate optimization cases were considered at two different orders of magnitude in pitching amplitudes of the forefoil, while the pitching frequency was kept at constant value. We found that in both cases, optimum tandem configurations had a smaller combined drag coefficient in comparison with the addition of two separate airfoils. The case with high pitching amplitude produced higher magnitude of lift, while the low amplitude case resulted in a significant improvement in aerodynamic performance