Instability and receptivity of boundary layers on concave surfaces and swept wings

This thesis studies the instability and receptivity of boundary layers over a concave wall and a swept Joukowski airfoil. The main interest is in excitation of relevant instability waves by free-stream vortical disturbances and in their subsequent linear development. We first consider excitation of Gortler vortices in a Blasius boundary layer over a concave wall. Attention is focused on disturbances with long streamwise wavelengths, to which the boundary layer is most receptive. The appropriate initial-boundary-value problem describing both the receptivity process and the subsequent development of the induced perturbation is formulated for the generic case where the Gortler number GΛ (based on the spanwise wavelength Λ of the disturbance) is of order one. The impact of free-stream disturbances on the boundary layer is accounted for by the far-field boundary condition and the initial condition near the leading edge, both of which turn out to be the same as those given by Leib, Wundrow and Goldstein (J. Fluid Mech. vol. 380, 1999, p.169) for the flatplate boundary layer. Numerical solutions of the initial-value problem show that for a sufficiently small GΛ, the induced perturbation exhibits essentially the same characteristics as streaks occuring in the flat plate case: the streamwise velocity undergoes considerable amplification and then decays. However, when GΛ exceeds a critical value, the induced perturbation exhibits (quasi-)exponential growth. Comparison with local parallel and non-parallel instability theories reveal that the perturbation acquires the modal shape of Gortler vortices rather quickly, but its growth rate differs appreciably from that predicted by local instability theories before the convergence at large downstream distances. Nevertheless, the overall agreement is close enough to indicate that Gortler vortices have been excited by free-stream disturbances. The amplitude of excited Gortler vortices is found to decrease with the frequency. Steady vortices, generated by steady components of free-stream disturbances, tend to be dominant. Detailed quantitative comparisons with experiments were performed. It is found that the eigenvalue approach predicts the modal shape adequately, but only the initial-value approach can accurately predict the evolution of the amplitude as well as the modal shape. An asymptotic analysis is performed on the assumption of GΛ >>1 to map out distinct regimes through which a disturbance of a fixed spanwise wavelength evolves. The centrifugal force enters the play to influence the generation of the pressure when x* ~ΛRΛG−2/3 Λ , where RΛ denotes the Reynolds number based on Λ. The induced pressure leads to full coupling of the momentum equations when x* ~ ΛRΛG−2/5 Λ . This is the crucial regime linking the pre-modal and modal phases of the perturbation because the governing equations admit a countable set of growing asymptotic eigensolutions, which develop into fully fledged Gortler vortices of inviscid nature when x* ~ ΛRΛ. From this position onwards, local eigenvalue formulations are mathematically justified. The generated Gortler vortices continue to amplify and enter the so-called most unstable regime when x* ~ ΛRΛGΛ, and ultimately approach the right-branch regime when x* ~ ΛRΛG2 Λ. We then extend our study to the receptivity of a three-dimensional boundary layer over a swept wing to free-stream vortical disturbances. The base flow is taken to be the boundary layer over a swept Joukowski airfoil. In contrast to the two-dimensional boundary layer, external disturbances with comparable streamwise and spanwise wavelengths are relevant to receptivity. The appropriate initial-boundary-value problem consists of linearised boundary-layer equations supplemented by the initial condition at the leading edge and the boundary condition in the far field, which are derived by applying the rapid distortion theory, and matching the resultant inviscid solution with the boundary-layer solution. It is found that the linearised boundary-layer equations support spatially growing eigenmodes despite the absence of a pressure gradient. The modes may be first excited by free-stream disturbances, and eventually evolve into fully fledged crossflow vortices

Spatial–temporal QoS assessment of the EV charging network considering power outages

The rapid expansion of electric vehicle charging networks (EVCNs) makes them critical infrastructures bridging power and traffic systems. The EVCN could be vulnerable during power outages at fast charging stations (FCSs), which are induced by planned maintenance or emergency load shedding. This paper proposes an approach to assess the impact of power outages on the Quality-of-service of the EVCN. The Markov decision process is utilized to model the spatial–temporal randomness of EV movement in a graph-based EVCN. The decision of charging by EV drivers is estimated by a fuzzy logic inference system. The spatial–temporal EV charging load at FCSs is formulated by a queuing-based non-linear optimization problem. Yen’s algorithm is adopted to simulate the EV redistribution phenomenon of searching adjacent healthy FCSs in response to the power outage. Quality-of-service (QoS) indices are derived to assess the potential congestions in the adjacent healthy FCSs. The case studies demonstrate that power outages may cause congestion at peripheral FCSs, exacerbating the QoS of the EVCN. Partial charging may alleviate the QoS deterioration in the event of FCS outages

The Helicobacter pylori Genome Project : insights into H. pylori population structure from analysis of a worldwide collection of complete genomes

Helicobacter pylori, a dominant member of the gastric microbiota, shares co-evolutionary history with humans. This has led to the development of genetically distinct H. pylori subpopulations associated with the geographic origin of the host and with differential gastric disease risk. Here, we provide insights into H. pylori population structure as a part of the Helicobacter pylori Genome Project (HpGP), a multi-disciplinary initiative aimed at elucidating H. pylori pathogenesis and identifying new therapeutic targets. We collected 1011 well-characterized clinical strains from 50 countries and generated high-quality genome sequences. We analysed core genome diversity and population structure of the HpGP dataset and 255 worldwide reference genomes to outline the ancestral contribution to Eurasian, African, and American populations. We found evidence of substantial contribution of population hpNorthAsia and subpopulation hspUral in Northern European H. pylori. The genomes of H. pylori isolated from northern and southern Indigenous Americans differed in that bacteria isolated in northern Indigenous communities were more similar to North Asian H. pylori while the southern had higher relatedness to hpEastAsia. Notably, we also found a highly clonal yet geographically dispersed North American subpopulation, which is negative for the cag pathogenicity island, and present in 7% of sequenced US genomes. We expect the HpGP dataset and the corresponding strains to become a major asset for H. pylori genomics

Instability and receptivity of boundary layers on concave surfaces and swept wings

This thesis studies the instability and receptivity of boundary layers over a concave wall and a swept Joukowski airfoil. The main interest is in excitation of relevant instability waves by free-stream vortical disturbances and in their subsequent linear development. We first consider excitation of Gortler vortices in a Blasius boundary layer over a concave wall. Attention is focused on disturbances with long streamwise wavelengths, to which the boundary layer is most receptive. The appropriate initial-boundary-value problem describing both the receptivity process and the subsequent development of the induced perturbation is formulated for the generic case where the Gortler number GΛ (based on the spanwise wavelength Λ of the disturbance) is of order one. The impact of free-stream disturbances on the boundary layer is accounted for by the far-field boundary condition and the initial condition near the leading edge, both of which turn out to be the same as those given by Leib, Wundrow and Goldstein (J. Fluid Mech. vol. 380, 1999, p.169) for the flatplate boundary layer. Numerical solutions of the initial-value problem show that for a sufficiently small GΛ, the induced perturbation exhibits essentially the same characteristics as streaks occuring in the flat plate case: the streamwise velocity undergoes considerable amplification and then decays. However, when GΛ exceeds a critical value, the induced perturbation exhibits (quasi-)exponential growth. Comparison with local parallel and non-parallel instability theories reveal that the perturbation acquires the modal shape of Gortler vortices rather quickly, but its growth rate differs appreciably from that predicted by local instability theories before the convergence at large downstream distances. Nevertheless, the overall agreement is close enough to indicate that Gortler vortices have been excited by free-stream disturbances. The amplitude of excited Gortler vortices is found to decrease with the frequency. Steady vortices, generated by steady components of free-stream disturbances, tend to be dominant. Detailed quantitative comparisons with experiments were performed. It is found that the eigenvalue approach predicts the modal shape adequately, but only the initial-value approach can accurately predict the evolution of the amplitude as well as the modal shape. An asymptotic analysis is performed on the assumption of GΛ >>1 to map out distinct regimes through which a disturbance of a fixed spanwise wavelength evolves. The centrifugal force enters the play to influence the generation of the pressure when x* ~ΛRΛG−2/3 Λ , where RΛ denotes the Reynolds number based on Λ. The induced pressure leads to full coupling of the momentum equations when x* ~ ΛRΛG−2/5 Λ . This is the crucial regime linking the pre-modal and modal phases of the perturbation because the governing equations admit a countable set of growing asymptotic eigensolutions, which develop into fully fledged Gortler vortices of inviscid nature when x* ~ ΛRΛ. From this position onwards, local eigenvalue formulations are mathematically justified. The generated Gortler vortices continue to amplify and enter the so-called most unstable regime when x* ~ ΛRΛGΛ, and ultimately approach the right-branch regime when x* ~ ΛRΛG2 Λ. We then extend our study to the receptivity of a three-dimensional boundary layer over a swept wing to free-stream vortical disturbances. The base flow is taken to be the boundary layer over a swept Joukowski airfoil. In contrast to the two-dimensional boundary layer, external disturbances with comparable streamwise and spanwise wavelengths are relevant to receptivity. The appropriate initial-boundary-value problem consists of linearised boundary-layer equations supplemented by the initial condition at the leading edge and the boundary condition in the far field, which are derived by applying the rapid distortion theory, and matching the resultant inviscid solution with the boundary-layer solution. It is found that the linearised boundary-layer equations support spatially growing eigenmodes despite the absence of a pressure gradient. The modes may be first excited by free-stream disturbances, and eventually evolve into fully fledged crossflow vortices.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Optimization of urban scenic spot planning based on ecological chain and reversed-planning theory—Using Yunlong Lake in Xuzhou city as an example

Modernized urban organization relies more on the foundation of ecological infrastructure (EI) with the planning objectives based more on synthetical benefits of economy, society and ecological environment. The effect of multiple factors ought to be taken into consideration in the urban scenic spot planning and and the process of formulating the related plan faces a challenge of how to organically bound environmental factors to tour function of urban scenic spot. This thesis, taking YunLong Lake, a scenic spot in XuZhou, JiangSu province as an example, studies how to ameliorate the planning of urban scenic spot combined with the ecological chain and reversed-planning theory. The result shows that utilizing the theory flexibly when analyzing the urban scenic spot planning design can ensure the existing ecological connection between each system and maintain the ecological stability of urban scenic spot. Thus, with the basis of ecological chain and reversed-planning theory, this paper puts forward a new planning concept and work flow, in order to provide new ideas for modernized urban scenic spot planning

Load Transfer Efficiency Assessment of Concrete Pavement Joints Using Distributed Optical Vibration Sensor

This paper presents a method to assess the load transfer efficiency (LTE) of concrete pavement joints using distributed optical vibration sensors. First, a theoretical analysis of concrete pavement vibration was conducted to investigate how to reflect LTE by spectral amplitude. Second, distributed optical vibration sensor (DOVS) was applied to measure vibration around joints distributedly. Third, the corresponding processing method for DOVS data was proposed to calculate the ratio of spectral amplitude from different slabs through power spectral density (PSD) analysis. Then, field tests were conducted on nine concrete pavement slabs with three different types of joints (dummy joint, rabbet joint, and dowel bars). The deflection-based method as well as the proposed vibration-based method were employed to assess the LTE of eleven joints on two different dates. The comparative analysis results indicate the deflection-based LTE (DLTE) and the ratio of PSD (RPSD) have a strong correlation (0.871) and a slight difference (<±0.03) overall. The correlation is robust in different dates and types of joints (0.844~0.88). These findings prove the accuracy and effectiveness of the proposed vibration-based method

Optimization Approaches of Multi-Dimensional Environments in Rural Space Reproduction Driven by Tourism

Rural tourism is an important driving force for the transformation of rural society in the process of urbanization. However, besides promoting rural economy, it also interrupts the development of the rural living, human, manage and economic multi-dimensional environments. Space reconstruction in rural settlements is an important means to optimize multi-dimensional environments and revitalize rural tourism. Focusing on two types of rural settlements, namely the “relocation village” and the “sightseeing and settlement symbiosis village”, this study has revealed the affecting mechanism of space reconstruction in rural tourist locations on the local multi-dimensional environment by establishing a multi-dimensional environment evaluation system through an analytic, hierarchical process. Subsequently, based on the theoretical perspective of the reproduction of the space and the paradigm of the three-fold model, a comprehensive optimization path of rural multi-dimensional environments has been proposed. The results show that in the space reconstruction process driven by tourism: (1) the multi-dimensional environments of the “sightseeing and settlement symbiosis village” generally develop better than the “relocation village”; (2) the ecological environment is the direct benefit while the management environment and economic environment are the bottlenecks; (3) the “sightseeing and settlement symbiosis village” is more appealing to tourists who are interested in rural tourism since it keeps more rustic characteristics than the “relocation village”; (4) in order to optimize the multi-dimensional environments, “relocation village” should pay more attention to the space needs of residents, improve space utilization rates, and increase the residents’ sense of belonging; (5) “sightseeing and settlement symbiosis village” needs to resolve the contradiction between the tourism and living demands, improve the reproduction process driven by diversified entities, and reconstruct the neighborhood network under rural tourism