Three-dimensional numerical simulations are carried out by commercial software STAR-CCM+ to investigate the hydraulic transient and flow behaviours of a prototype pump-turbine during the normal shutdown process. The global characteristics and pressure fluctuations are analysed as the pump-turbine runs through the turbine mode, into the turbine braking mode, and finally into the reversed pump mode. As the guide vanes close, flow rate and runner torque both decrease while the rotational speed drops in response to the negative torque on the runner. The proposed approach accurately predicts the transient characteristics during the shutdown process and provides insight into the inner flow behaviour of a pump-turbine. Thus, this study has the potential to facilitate both exploration and diagnostics of faults of turbo machines during transient operation.

Three-dimensional transient simulation of a prototype pump-turbine during normal turbine shutdown

Three-dimensional numerical simulation for a prototype pump-turbine has been performed to investigate the transient characteristics and flow behaviors in the startup process. The rotational speed, ...

Three-dimensional simulation of unsteady flows in a pump-turbine during start-up transient up to speed no-load condition in generating mode

In this study, an attempt has been made to study the cavitating flows of a pumpjet propulsor. A numerical study based on the unsteady Reynolds-averaged Navier–Stokes computational fluid dynamics method of homogeneous multiphase using structured grids and the shear stress transport (SST) k–ω turbulence model has been carried out. Considering the effects of non-condensable gas (NCG) on the cavitation performance, the Schnerr–Sauer cavitation model has been applied and improved by introducing the NCG. The numerical calculations of non-cavitating and cavitating flows for the marine propeller E779A are carried out with different advance ratios at several cavitation numbers to verify the numerical simulation method. Results show that the thrust, the torque, and the cavitation performance, such as the cavitation location and shape, are in good agreement with experimental data. Furthermore, numerical predictions for a pumpjet propulsor on an unmanned underwater vehicle are made with different non-cavitating and c...

Numerical simulation of unsteady cavitating flows of pumpjet propulsor

Domain decomposition (DD) is a powerful approach to numerically solve partial differential equations, and it has become popular and indispensable in simulations of fluid flows, especially those that arise from models of the real world that require large-scale, parallel computation. Domain decomposition methods (DDMs) were originally introduced for problems on “complex” geometries, and they are now widely used as parallel algebraic solvers and preconditioners for solutions of various problems. Research in theory and practical application of DDMs for fluid flows has seen considerable progress with a substantial amount of work published in the past forty years. This paper makes a comprehensive review of the development and current status of the DDMs, and, for both beginners and experts in this area, it surveys the concepts, algorithms, and the utilization of DD to simulate compressible flows, incompressible flows, and flows in various applications. Along with numerical examples, difficulties are highlighted with potential methods to overcome them, and topics for future study are identified.

A Review of Domain Decomposition Methods for Simulation of Fluid Flows: Concepts, Algorithms, and Applications

Numerical study on characteristic correlation between cavitating flow and skew of ship propellers

Numerical investigation of cavitating flows for marine propulsors using an unstructured mesh technique

An ultra-low voltage microelectromechanical system (MEMS) switch for low-power integrated circuit (IC) applications is proposed, fabricated and demonstrated. The folded hinge structure allows a large beam structure to be suspended with a designed air gap, effectively suppressing unwanted deflection. The actuation voltage of the switch was measured to be 1.7 V, the lowest among electrostatic switches. There was no variation in the actuation voltage until 106 cyclic actuations, showing the stability of a low actuation voltage in electrostatic actuation for the first time. The contact resistance was around 12 Ω, caused by a low contact force below 1 μN despite an Au–Au contact.

/pdf/ultra-low-voltage-mems-switch-using-a-folded-hinge-structure-95rmehlaxz.pdf

Ultra-low voltage MEMS switch using a folded hinge structure

Reliability in hot switching operations of MEMS DC switches is enhanced by introducing a drain voltage-sustaining capacitor, which leads to electric field-less or arc-less switching operations. The role of the capacitor is to eliminate the electric potential difference between the suspended beam and the drain electrode at the switch “opening” instant. In the case of a pure Au-Au contact, we have experimentally demonstrated that the lifetime of conventional MEMS switches with the suggested method was improved dramatically, by an order of magnitude, in the relatively high electric-field hot switching condition. We clearly observed material transfer at failure in the device without the capacitor, whereas stiction was the main failure mechanism in the device with the capacitor. Owing to the simplicity of the proposed method, it can be adopted in any type of MEMS DC switches in order to greatly enhance the switch lifetime.

/pdf/improvement-of-hot-switching-lifetime-in-mems-dc-switches-4kklmrgeu3.pdf

Improvement of hot switching lifetime in MEMS DC switches using a drain voltage-sustaining capacitor

Objective: Brain abscess is relatively uncommon because the brain has an abundant blood supply and protection from the blood-brain barrier. The objective of this study was to analyze the outcomes of brain abscesses and to identify factors associated with patients’ outcomes. Methods: The characteristics (age, sex, medical history, initial neurologic status, laboratory data, abscess morphology) and outcomes of 54 patients with brain abscesses who underwent surgical treatment at our hospital from January 2011 to December 2020 were retrospectively analyzed.Results: There were more male patients than female patients (44:10). Sixteen patients died or became disabled, whereas 38 patients recovered. Statistical analyses indicated that those with a penetration cause, Glasgow coma scale≤12, ventriculitis, or Gram-negative bacilli grown in abscess culture had significantly poorer outcomes, whereas those with a contiguous cause and isolated brain abscess had significantly better outcomes (p<0.05). Conclusion: This study identified poor prognostic factors in brain abscess patients. Despite advances in diagnosis and treatment techniques, brain abscess still has a high mortality rate. The results of this study are expected to help with exact diagnoses and more accurate prognostication of brain abscess patients.

Sang Joon Ahn

Papers

Numerical investigation of cavitating flows for marine propulsors using an unstructured mesh technique

Ultra-low voltage MEMS switch using a folded hinge structure

Improvement of hot switching lifetime in MEMS DC switches using a drain voltage-sustaining capacitor

Factors influencing the outcomes and mortality of brain abscess: a retrospective analysis of 54 cases