Large -eddy simulation of ship wakes

The objective of the present study is to improve our understanding of turbulent wake flows. Large-Eddy Simulation (LES) technique is applied for this purpose. A readily available code was used with necessary modifications. Three dimensional incompressible Navier-Stokes equations are solved in non-orthogonal curvilinear coordinates. Finite-volume approach is implemented on a non-staggered grid. The core of the numerical scheme is a fractional step method. The overall accuracy of the method is second order in both space and time.;The LES approach has been validated for four cases: channel flow, flow past a square body, a shear layer flow, and open channel flow. Different subgrid-scale models and numerical schemes have been tested for these benchmarks. Comparisons between the simulations and experiments show the capability of this LES method.;An efficient and accurate Random Flow Generation (RFG) approach has been improved to provide turbulent initial and inflow conditions for developing wake flow calculations. The RFG method can handle anisotropy and inhomogeneity, and it satisfies the instantaneous continuity equations. This approach has been verified by reproducing a turbulent channel flow based on a Direct Numerical Simulation (DNS). Perfect agreements have been obtained.;A typical one equation sub-grid scale model has been selected and modified to include the backscatter of energy by applying the RFG algorithm. Meanwhile, an attempt was made to implement the one equation sub-grid scale model dynamically. Comparisons have been made between the experiments and the simulation results using different one equation sub-grid scale models.;The RFG approach along with the LES technique has been applied to the wake behind a flat plate. Effects of grid resolution and SGS models on the turbulent flow field have been investigated. Good results are obtained as compared to experiments.;Wake flows behind a Naval ship model (5415) have been studied in details by applying the combined LES-RFG method. The calculation domain starts from a plane behind the ship model. Because of the lack of experimental data, Reynolds Averaged Navier-Stokes (RANS) calculations are used to provide the RFG procedure with the information on the inflow boundary. The ship wake flow with an impose surface wave has been studied. The result shows that the wave surface has a significant influence on the turbulent kinetic energy distribution. Finally, part of the ship hull is included in the simulation of the ship wake with a static wave surface (Fr = 0.28) to investigate the difference between RANS and LES results in the near wake of a ship model. The overall quality of the LES calculations is found to be very good. In particular the large coherent structures with significant vorticity concentration and gradients could be captured in the wake of a ship model which is not possible to accomplished by RANS. These structures have significant importance in the transplant of entrained air bubbles

An energy-based coupling degradation propagation model and its application to aviation actuation system

Abstract The degradation of components in complex mechatronic systems involves multiple physical processes which will cause coupling interactions among nodes in the system. The interaction of nodes may be carried out not only by physical connections but also by the environment which cannot be described by single network using the traditional methods. In order to give out a unified model to quantitatively describe the coupling degradation spreading by both physical connections and environment, a novel Energy-Flow-Field Network (EFFN) and a coupling degradation model based on EFFN are proposed in this paper. The EFFN is driven by energy flow and the state transition of spatially related nodes is triggered by the dissipation energy. An application is conducted on aviation actuation system in which the degradation spreading by fluid-thermal-solid interaction is considered. The degradation path and the most probable fault reason can be obtained by combining the state transition and energy output of nodes, which is consistent with the given scenario

Fault Mode Probability Factor Based Fault-Tolerant Control for Dissimilar Redundant Actuation System

This paper presents a Fault Mode Probability Factor (FMPF) based Fault-Tolerant Control (FTC) strategy for multiple faults of Dissimilar Redundant Actuation System (DRAS) composed of Hydraulic Actuator (HA) and Electro-Hydrostatic Actuator (EHA). The long-term service and severe working conditions can result in multiple gradual faults which can ultimately degrade the system performance, resulting in the system model drift into the fault state characterized with parameter uncertainty. The paper proposes to address this problem by using the historical statistics of the multiple gradual faults and the proposed FMPF to amend the system model with parameter uncertainty. To balance the system model precision and computation time, a Moving Window (MW) method is used to determine the applied historical statistics. The FMPF based FTC strategy is developed for the amended system model where the system estimation and Linear Quadratic Regulator (LQR) are updated at the end of system sampling period. The simulations of DRAS system subjected to multiple faults have been performed and the results indicate the effectiveness of the proposed approach

Active Fault Tolerant Control for Vertical Tail Damaged Aircraft with Dissimilar Redundant Actuation System

This paper proposes an active fault-tolerant control strategy for an aircraft with dissimilar redundant actuation system (DRAS) that has suffered from vertical tail damage. A damage degree coefficient based on the effective vertical tail area is introduced to parameterize the damaged flight dynamic model. The nonlinear relationship between the damage degree coefficient and the corresponding stability derivatives is considered. Furthermore, the performance degradation of new input channel with electro-hydrostatic actuator (EHA) is also taken into account in the damaged flight dynamic model. Based on the accurate damaged flight dynamic model, a composite method of linear quadratic regulator (LQR) integrating model reference adaptive control (MRAC) is proposed to reconfigure the fault-tolerant control law. The numerical simulation results validate the effectiveness of the proposed fault-tolerant control strategy with accurate flight dynamic model. The results also indicate that aircraft with DRAS has better fault-tolerant control ability than the traditional ones when the vertical tail suffers from serious damage. © 2016 Chinese Society of Aeronautics and Astronautic

Modeling of Reliability and Performance Assessment of a Dissimilar Redundancy Actuation System With Failure Monitoring

Actuation system is a vital system in an aircraft, providing the force necessary to move flight control surfaces. The system has a significant influence on the overall aircraft performance and its safety. In order to further increase already high reliability and safety, Airbus has implemented a dissimilar redundancy actuation system (DRAS) in its aircraft. The DRAS consists of a hydraulic actuation system (HAS) and an electro-hydrostatic actuation system (EHAS), in which the HAS utilizes a hydraulic source (HS) to move the control surface and the EHAS utilizes an electrical supply (ES) to provide the motion force. This paper focuses on the performance degradation processes and fault monitoring strategies of the DRAS, establishes its reliability model based on the generalized stochastic Petri nets (GSPN), and carries out a reliability assessment considering the fault monitoring coverage rate and the false alarm rate. The results indicate that the proposed reliability model of the DRAS, considering the fault monitoring, can express its fault logical relation and redundancy degradation process and identify potential safety hazards

Modeling of reliability and performance assessment of a dissimilar redundancy actuation system with failure monitoring

AbstractActuation system is a vital system in an aircraft, providing the force necessary to move flight control surfaces. The system has a significant influence on the overall aircraft performance and its safety. In order to further increase already high reliability and safety, Airbus has implemented a dissimilar redundancy actuation system (DRAS) in its aircraft. The DRAS consists of a hydraulic actuation system (HAS) and an electro-hydrostatic actuation system (EHAS), in which the HAS utilizes a hydraulic source (HS) to move the control surface and the EHAS utilizes an electrical supply (ES) to provide the motion force. This paper focuses on the performance degradation processes and fault monitoring strategies of the DRAS, establishes its reliability model based on the generalized stochastic Petri nets (GSPN), and carries out a reliability assessment considering the fault monitoring coverage rate and the false alarm rate. The results indicate that the proposed reliability model of the DRAS, considering the fault monitoring, can express its fault logical relation and redundancy degradation process and identify potential safety hazards

Anomalous optical and electronic properties of dense sodium

Based on ab initio density-functional-theory using generalized gradient approximation, we systematically study the optical and electronic properties of the insulating dense sodium phase (Na-hp4) reported recently [Ma \textit{et al.}, Nature \textbf{458}, 182 (2009)]. The structure is found optically anisotropic and transparent to visible light, which can be well interpreted using its electronic band structure and angular moment decomposed density of states. Through the bader analysis of Na-hp4 at different pressures, we conclude that ionicity exists in the structure and becomes stronger with increasing pressure. In addition, the absorption spectra in the energy range from 1.4 to 2.4 eV are compared with recent experimental results and found good agreement. It is found that the deep-lying valence electrons participate in the interband transition.Comment: 7 pages, 7 figure

Changes in sleep quality of children with epilepsy and anxiety of their caregivers after COVID-19 infection: a case-series report

ObjectiveTo study the changes in epileptic seizures and sleep quality in children with epilepsy (CWE) and the changes in anxiety of their caregivers after infection with COVID-19.MethodsOutpatients and inpatients of CWEs were selected as subjects and a questionnaire survey was used to carry out this case-series study. The demographic information of the CWEs and their caregivers, information about epilepsy, and information about the vaccination, infection, and treatment of COVID-19 were collected. The changes in sleep quality of CWEs and the changes in anxiety of their caregivers were assessed by the Child Sleep Habits Questionnaire (CSHQ) and Caregiver Anxiety Scale (CAS). Risk factors affecting sleep habits in CWEs and caregiver anxiety were further analyzed by one-way analysis of variance.ResultsA total of 312 children were included in the study. Among them, 134 patients (42.9%) were female. The average age of the children was 9.30 ± 3.88 years, and the duration of epilepsy was 4.59 ± 3.36 years. A total of 221 of the 312 children were infected with COVID-19, and all the infected children developed fever, which lasted for 1.71 ± 1.13 days. 10 children were satisfied with controlled seizures for more than 1 year and relapsed after COVID-19 infection (4.2%), 4 cases (3.6%) with increased seizures, and 8 children with reduced seizures (7.7%), 17 children (7.7%) had no change in seizures, and 182 children (82.3%) remained seizure-free after the COVID-19 infection. The average sleep time of the CWEs was 9.25 ± 1.04 h and the average total score of the CSHQ was 37.25 ± 5.19, among which 44 cases (14.1%) had more than 41 points. As the result of the CAS, 16 of them (5.13%) scored above 50 and the average total score was 31.49 ± 8.09. The control of seizures, age of onset, types of anti-seizure medicines (ASMs), and seizure duration were risk factors affecting sleep quality. Accordingly, the score of CAS was significantly lower when there was more than one caregiver who cared for the CWE.ConclusionsCOVID-19 infection did not cause an increase in seizures in CWEs, nor did it worsen their sleep quality of them or aggravate the anxiety of their caregivers