Complex flow physics & active plasma flow control in convoluted ducts

Convoluted, s-shaped ducts form an integral part of many subsystems in engineering applications and specifically the aviation industry. They are used, for example, as inlet ducts for fuselage embedded jet engines and as connector pipes between high and low pressure turbine or compressor stages. With a strong curvature and a diffusive nature, the geometry acts on the through-flow making it prone to separate and experience significant cross-stream pressure gradients. The geometry and resulting flow phenomena lead to a non-uniform and highly unsteady flow field in the duct aft the inflection point. Those effects are detrimental to the overall performance of the convoluted duct, reducing the pressure recovery and increasing the distortion parameters. S-shaped ducts have been studied by a large number of researchers for many years. Traditionally, many studies rely on steady state simulations and time averaged experimental data to characterise the flow in convoluted ducts and analyse their performance. However, more recent findings point to the need of transient data to fully understand the dynamic nature of the through-flow and discuss the complex flow physics. This is something that is lacking from many studies reported in the current literature. This is addressed with computational fluid dynamics (CFD) studies of the through-flow in the s-duct using the open source tool OpenFOAM. First low fidelity, steady state simulations are set up before higher fidelity, transient delayed detached eddy simulations (DDES) are conducted. Baseline s-duct through-flow computations are validated against experimental data from literature with very good agreement of pressure recovery values, wall static pressure contours, and flow structures. CFD data is next post processed with statistical and modal decomposition methods. Coherent structures and phase information are obtained from the proper orthogonal decomposition (POD) and the dynamic mode decomposition (DMD) methods. Modal decomposition analysis of DDES data confirms the existance of the horizontal shifing mode. Contrary to previous findings, the presence of a second vertical shifting mode is observed from DDES data. Occurance rates and phase information are determined from the DMD analysis. The recent surge of interest in plasma actuators is clearly illustrated by the high research output that has been reported in literature. Dielectric barrier discharge (DBD) plasma actuators have been studied and successfully applied to control external aerodynamics on aerofoils and bluff bodies. However, successful flow control in convoluted ducts has not been reported with this technology for realistic Reynolds numbers. The DBD plasma characterisation is conducted on two types of actuators: alternating current (ac) and nanosecond (ns) DBD plasma actuators. The Schlieren imaging technique is used with ns-DBD plasmas to record density changes and establish the shock front strength and propagation speed with changing ambient pressure. Higher ambient pressures result in stronger shock waves; this has been observed irrespective of the actuator thickness. This might be explained with fewer air molecules to ionize at lower ambient pressures and hence a lower temperature from the exothermal recombination reactions. For ac-DBD actuators, thinner dielectric materials outperformed thicker ones in terms of ionisation strength with constant voltage input. The smallest dielectric constant of the materials tested resulted in higher induced velocities. Using particle image velocimetry (PIV), a high gradient of velocity reduction with streamwise distance was recorded in the plasma jet. This is significant, as it shows plasma actuators have mostly localised effects. Experimental campaigns are set up such that the DBD experiments are coherent studies in their own right. However, the main purpose of plasma experiments in the context of this thesis is to collect data to validate numerical plasma models. Those phenomenological plasma models are subsequently used for numerical flow control studies on the s-shaped duct. Phenomenological plasma models match the experimental data well when tuned. However, the flow control studies did not show a performance improvement in the convoluted duct

Effect of permittivity and frequency on induced velocity in ac-DBD surface and channel plasma actuators

Plasma actuators have attracted interest for use as active flow control devices due to their many benefits; they have no moving parts and are lightweight, can be flush mounted, and require low power. In this study, the performance of plasma actuators are experimentally characterized with dielectric material, dielectric thickness, and operating frequency for surface and channel actuator geometries. The channel height, changing the effective dielectric constant, is also varied. Induced velocities were measured using a pitot tube and PIV, and power consumption levels were recorded. For the surface plasma actuator, PTFE and GRE dielectric materials show similar performance, with Kapton producing the highest induced velocity jet. Higher plasma ionization tends to occur with operating frequencies of 5 and 10 kHz, with a minimum at 7 kHz – possibly related to a change to streamer discharge from corona discharge. Power consumption was also higher at frequencies of 5 and 10 kHz. Thinner dielectric materials outperformed thicker ones for a given high voltage input. GRE, which has the highest dielectric constant of the tested materials, resulted in higher induced velocities than PTFE and Kapton for the same dielectric thickness. For the channel actuator, the smallest air gap corresponding to the lowest permittivity generated the highest peak induced velocity at 12 kHz. Different air gap heights do not seem to affect the induced velocities for frequencies above 14 kHz. A high gradient of velocity reduction with streamwise distance from the exposed electrode of the channel actuators was also observed

A review of flow control techniques and optimisation in S-shaped ducts

This paper is a review of significant studies in the complex flow physics in diffusive, s-shaped ducts, focusing on flow control methods employed to counteract the onset of separation, swirl formation, and non-uniformity of pressure at the duct exit plane. Passive, active, and hybrid flow control, along with optimisation techniques used to control the dominant flow features are discussed. According to the literature, tapered fin vortex generators and submerged vortex generators improve pressure loss and distortion by double digit percentages, and three-dimensional synthetic jets and pulsed micro-jets show greatest promise amongst active flow control devices. Plasma flow control methods have only sparsely been used in s-ducts with one study performing experiments with alternating-current dielectric-barrier-discharge plasma actuators. The importance of flow unsteadiness has been identified in the literature, with peak values as high as one order of magnitude different from the time-averaged properties. Despite this, very few flow control studies have used time-dependent solution methods to quantify the effect of flow control methods on the unsteadiness of the flow

Pressure dependency on a nanosecond pulsed dielectric barrier discharge plasma actuator

The behavior of a nanosecond pulsed dielectric barrier discharge (ns-DBD) plasma actuator with the ambient pressure from 30 to 100 kPa was characterized with Schlieren images. Shock wave propagation speed and strength were recorded, showing clear trends with decreasing ambient pressure. Higher ambient pressures result in stronger shock waves; this has been observed irrespective of the actuator thickness. This might be explained with fewer air molecules to ionize at lower ambient pressures and hence a lower temperature from the exothermal recombination reactions. The thickness of the dielectric barrier also influences the shock strength. In accordance with previous findings, it was confirmed that a thinner dielectric barrier results in a greater shock strength. NS-DBD shock waves were modeled numerically using OpenFOAM through a source term added to the energy equation, which controls the amount of thermal energy added to the near-wall deposition region. The compressible, unsteady sonicFoam solver was used with second order schemes. A mesh sensitivity study gives confidence that the solution is grid independent. The overall shock wave structure and propagation speed match well with experimental data. The heat addition required to reproduce experimental results varied with ambient pressure. Less heating of the near-wall region was needed with lower ambient pressures

Numerical study of complex flow physics and coherent structures of the flow through a convoluted duct

No abstract available

Effect of Vitamin D Supplementation on Outcomes in People with Early Psychosis: The DFEND Randomized Clinical Trial

IMPORTANCE: People with psychotic disorders have an increased risk of vitamin D deficiency, which is evident during first-episode psychosis (FEP) and associated with unfavorable mental and physical health outcomes. OBJECTIVE: To examine whether vitamin D supplementation contributes to improved clinical outcomes in FEP. DESIGN, SETTING, AND PARTICIPANTS: This multisite, double-blind, placebo-controlled, parallel-group randomized clinical trial from the UK examined adults 18 to 65 years of age within 3 years of a first presentation with a functional psychotic disorder who had no contraindication to vitamin D supplementation. A total of 2136 patients were assessed for eligibility, 835 were approached, 686 declined participation or were excluded, 149 were randomized, and 104 were followed up at 6 months. The study recruited participants from January 19, 2016, to June 14, 2019, with the final follow-up (after the last dose) completed on December 20, 2019. INTERVENTIONS: Monthly augmentation with 120 000 IU of cholecalciferol or placebo. MAIN OUTCOMES AND MEASURES: The primary outcome measure was total Positive and Negative Syndrome Scale (PANSS) score at 6 months. Secondary outcomes included total PANSS score at 3 months; PANSS positive, negative, and general psychopathology subscale scores at 3 and 6 months; Global Assessment of Function scores (for symptoms and disability); Calgary Depression Scale score, waist circumference, body mass index, and glycated hemoglobin, total cholesterol, C-reactive protein, and vitamin D concentrations at 6 months; and a planned sensitivity analysis in those with insufficient vitamin D levels at baseline. RESULTS: A total of 149 participants (mean [SD] age, 28.1 (8.5) years; 89 [59.7%] male; 65 [43.6%] Black or of other minoritized racial and ethnic group; 84 [56.4%] White [British, Irish, or of other White ethnicity]) were randomized. No differences were observed in the intention-to-treat analysis in the primary outcome, total PANSS score at 6 months (mean difference, 3.57; 95% CI, −1.11 to 8.25; P = .13), or the secondary outcomes at 3 and 6 months (PANSS positive subscore: mean difference, −0.98; 95% CI, −2.23 to 0.27 at 3 months; mean difference, 0.68; 95% CI, −0.69 to 1.99 at 6 months; PANSS negative subscore: mean difference, 0.68; 95% CI, −1.39 to 2.76 at 3 months; mean difference, 1.56; 95% CI, −0.31 to 3.44 at 6 months; and general psychopathology subscore: mean difference, −2.09; 95% CI, −4.36 to 0.18 at 3 months; mean difference, 1.31; 95% CI, −1.42 to 4.05 at 6 months). There also were no significant differences in the Global Assessment of Function symptom score (mean difference, 0.02; 95% CI, −4.60 to 4.94); Global Assessment of Function disability score (mean difference, −0.01; 95% CI, −5.25 to 5.23), or Calgary Depression Scale score (mean difference, −0.39; 95% CI, −2.05 to 1.26) at 6 months. Vitamin D levels were very low in the study group, especially in Black participants and those who identified as another minoritized racial and ethnic group, 57 of 61 (93.4%) of whom had insufficient vitamin D. The treatment was safe and led to a significant increase in 25-hydroxyvitamin D concentrations. CONCLUSIONS AND RELEVANCE: In this randomized clinical trial, no association was found between vitamin D supplementation and mental health or metabolic outcomes at 6 months. Because so few patients with FEP were vitamin D replete, the results of this study suggest that this group would benefit from active consideration in future population health strategies. TRIAL REGISTRATION: isrctn.org Identifier: ISRCTN1242484