Effect of blade tip geometry on the tip leakage flow in axial turbine cascades

The phenomenon of tip leakage has been studied in two linear cascades of turbine blades. The investigation includes an examination of the performance of the cascades with a variety of tip geometries. The effects of using plain tips, suction side squealers, and pressure side squealers are reported. Traverses of the exit flow field were made in order to determine the overall performance. A method of calculating the tip discharge coefficients for squealer geometries is put forward. In linking the tip discharge coefficient and cascade losses, a procedure for predicting the relative performance of tip geometries is developed. The model is used to examine the results obtained using the different tip treatments and to highlight the important aspects of the loss generation process.

Building damage and repair due to leakage in a deep excavation

The north-south line in Amsterdam is being built underneath the historic centre of the city. Three deep stations are being constructed in deep excavations supported by diaphragm walls. During the excavation for Vijzelgracht station, leakage through the wall resulted in large settlements and damage to historic buildings, which threatened continuation of the project. The authors analysed the cause of the leakage and the damage to the buildings. With the application of robust preventative measures at two of the deep excavations it was possible to continue the project. This paper reports on the cause of the events, the damage to the buildings and the counter-measures taken. It includes lessons learned for the project and for the foundations industry.

CHANGES IN PREDICTED MOS DEVICE CHARACTERISTICS DUE TO THE USE OF TWO DIMENSIONAL SOURCE/DRAIN PROFILES.

Two-dimensional MOS device simulation programs such as MINIMOS left bracket 1 right bracket are limited in their validity due to assumptions made in defining the initial two-dimensional source/drain profiles. The two options available to define source/drain regions both construct a two-dimensional profile from one-dimensional profiles normal to the surface. Inaccuracies in forming these source/drain profiles can be expected to effect predicted device characteristics as channel dimensions of the device are reduced. This paper examines these changes by interfacing numerically similated two dimensional source/drain profiles to MINIMOS and comparing predicted I//D-V//D characteristics with 2-D interfacing, 2-D profiles constructed from interfaced 1-D profiles and MINIMOS self generated profiles. Data obtained for simulations of 3 mu m N and P channel devices are presented.

Drain groups in liquefiable soil

One of the major concerns for engineers in seismically active regions is the prevention of damage caused by earthquake-induced soil liquefaction. Vertical drains can aid dissipation of excess pore pressures both during and after earthquakes. Drain systems are designed using standard design charts based around the concept of a unit cell, assuming each drain is surrounded by more drains. It is unclear how predictable drain performance is outside that unit cell concept, for example, drains at the edge of a group. Centrifuge testing is a logical method of performing controlled experiments to establish the efficacy of vertical drains. Centrifuge testing is used to identify the effect of drains dealing with very different catchment areas. The importance of this is further highlighted by the results of a test where the same drains have been modified so that each should behave as a unit cell. It is shown that drains with large catchment areas perform more poorly than unit cells, and also have a knock-on detrimental effect on other drains. Copyright © 2011, IGI Global.

Effects of Arsenate on Microcystin Content and Leakage of Microcystis Strain PCC7806 Under Various Phosphate Regimes

Both arsenic pollution and eutrophication are prominent environmental issues when considering the problem of global water pollution. It is important to reveal the effects of arsenic species on cyanobacterial growth and toxin yields to assess ecological risk of arsenic pollution or at least understand naturally occurring blooms. The sensitivity of cyanobacteria to arsenate has often been linked to the structural similarities of arsenate and phosphate. Thus, we approached the effect of arsenate with concentrations from 10(-8) to 10(-4) M on Microcystis strain PCC7806 under various phosphate regimes. The present study showed that Microcystis strain PCC7806 was arsenate tolerant up to 10(-4) M. And such tolerance was without reference to both content of intra- and extra-cellular phosphate. It seems that arsenate involved the regulation of microcystin synthesis and cellular polyphosphate contributed to microcystin production of Microcystis responding to arsenate, since there was a positive linear correlation of the cellular microcystin quota with the exposure concentration of arsenate when the cells were not preconditioned to phosphate starvation. It is presumed that arsenate could help to actively export microcystins from living Microcystis cells when preconditioned to phosphate starvation and incubated with the medium containing 1 mu M phosphate. This study firstly provided evidence that microcystin content and/or release of Microcystis might be impacted by arsenate if it exists in harmful algal blooms. (C) 2008 Wiley Periodicals, Inc. Environ Toxicol 24:97 94, 2009.

High performance non-volatile ferroelectric copolymer memory based on a ZnO nanowire transistor fabricated on a transparent substrate

A high performance ferroelectric non-volatile memory device based on a top-gate ZnO nanowire (NW) transistor fabricated on a glass substrate is demonstrated. The ZnO NW channel was spin-coated with a poly (vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) layer acting as a top-gate dielectric without buffer layer. Electrical conductance modulation and memory hysteresis are achieved by a gate electric field induced reversible electrical polarization switching of the P(VDF-TrFE) thin film. Furthermore, the fabricated device exhibits a memory window of ∼16.5 V, a high drain current on/off ratio of ∼105, a gate leakage current below ∼300 pA, and excellent retention characteristics for over 104 s. © 2014 AIP Publishing LLC.

The influence of boundary conditions on tip leakage flow

Most of the current understanding of tip leakage flows has been derived from detailed cascade experiments. However, the cascade model is inherently approximate since it is difficult to simulate the boundary conditions present in a real machine, particularly the secondary flows convecting from the upstream stator row and the relative motion of the casing and blade. This problem is further complicated when considering the high pressure turbine rotors of aero engines, where the high Mach numbers must also be matched in order to correctly model the aerodynamics and heat transfer. More realistic tests can be performed on high-speed turbines, but the experimental fidelity and resolution achievable in such set-ups is limited. In order to examine the differences between cascade models and real-engine behavior, the influence of boundary conditions on the tip leakage flow in an unshrouded high pressure turbine rotor is investigated using RANS calculations. This study examines the influence of the rotor inlet condition and relative casing motion. A baseline calculation with a simplified inlet condition and no relative endwall motion exhibits similar behavior to cascade studies. Only minor changes to the leakage flow are induced by introducing either a more realistic inlet condition or relative casing motion. However when both of these conditions are applied simultaneously the pattern of leakage flow is very different, with ingestion of flow over much of the early suction surface. The paper explores the physical processes driving this change and the impact on leakage losses and modeling requirements. Copyright © 2013 by ASME.