This Review presents basic facts regarding the long-run evolution of income and wealth inequality in Europe and the United States. Income and wealth inequality was very high a century ago, particularly in Europe, but dropped dramatically in the first half of the 20th century. Income inequality has surged back in the United States since the 1970s so that the United States is much more unequal than Europe today. We discuss possible interpretations and lessons for the future.

Inequality in the long run

DOI: 10.1002/aenm.202000137 different materials that would be electrically charged after being separated. But TE and CE have significant differences. CE occurs just by physical contact of the two materials without rubbing one against the other, but TE is usually inseparably involving friction by rubbing two materials one on the other. Therefore, TE is a “convolution” of two processes between tribology and CE, so that they are inseparable in conventional understanding. We have recently pointed out that CE is a physical effect in science, while TE is an engineering practice that may involve friction and debris.[3] As for the case of solid–solid, CE is defined as a quantum mechanical electron transfer process that occurs for any materials, in any states (solid, liquid, gas), in any application environment, and in a wide range of temperature up to ≈400 °C. Such an effect is universal and is fundamentally unique in nature.

Triboelectric Nanogenerator (TENG)—Sparking an Energy and Sensor Revolution

This paper mainly summarizes the recent progresses for the cavitation study in the hydraulic machinery including turbo-pumps, hydro turbines, etc.. Especially, the newly developed numerical methods for simulating cavitating turbulent flows and the achievements with regard to the complicated flow features revealed by using advanced optical techniques as well as cavitation simulation are introduced so as to make a better understanding of the cavitating flow mechanism for hydraulic machinery. Since cavitation instabilities are also vital issue and rather harmful for the operation safety of hydro machines, we present the 1-D analysis method, which is identified to be very useful for engineering applications regarding the cavitating flows in inducers, turbine draft tubes, etc. Though both cavitation and hydraulic machinery are extensively discussed in literatures, one should be aware that a few problems still remains and are open for solution, such as the comprehensive understanding of cavitating turbulent flows especially inside hydro turbines, the unneglectable discrepancies between the numerical and experimental data, etc.. To further promote the study of cavitation in hydraulic machinery, some advanced topics such as a Density-Based solver suitable for highly compressible cavitating turbulent flows, a virtual cavitation tunnel, etc. are addressed for the future works.

A review of cavitation in hydraulic machinery

Tip clearance on pressure fluctuation intensity and vortex characteristic of a mixed flow pump as turbine at pump mode

Large Eddy Simulation of the Tip-leakage Cavitating flow with an insight on how cavitation influences vorticity and turbulence

Particle image velocimetry (PIV) measurements at varying resolutions focus on the flow structures in the tip region of a water-jet pump rotor, including the tip-clearance flow and the rollup process of a tip leakage vortex (TLV). Unobstructed views of these regions are facilitated by matching the optical refractive index of the transparent pump with that of the fluid. High-magnification data reveal the flow non-uniformities and associated turbulence within the tip gap. Instantaneous data and statistics of spatial distributions and strength of vortices in the rotor passage reveal that the leakage flow emerges as a wall jet with a shear layer containing a train of vortex filaments extending from the tip of the blade. These vortices are entrained into the TLV, but do not have time to merge. TLV breakdown in the aft part of the blade passage further fragments these structures, increasing their number and reducing their size. Analogy is made between the circumferential development of the TLV in the blade passage and that of the starting jet vortex ring rollup. Subject to several assumptions, these flows display similar trends, including conditions for TLV separation from the shear layer feeding vorticity into it.

Measurements of the tip leakage vortex structures and turbulence in the meridional plane of an axial water-jet pump

The complex flow field in the tip region of a turbomachine rotor, including the tip leakage flow and tip leakage vortex (TLV), has been studied for decades. Yet many associated phenomena are still not understood. This paper provides detailed data on the instantaneous and phase averaged inner structure of the tip flow, and evolution of the TLV. Observations are based on series of high resolution planar particle image velocimetry measurements performed in a transparent waterjet pump fitted into an optical refractive index matched test facility. Velocity distributions and turbulence statistics are obtained in several meridional planes inside the rotor. We observe that the instantaneous TLV structure is composed of several unsteady vortex filaments that propagate into the blade passage. These filaments are first embedded into a vortex sheet generated at the suction side of the blade tip, and then they wrap around each other and roll up into the TLV. These vortices do not have sufficient time to merge into a single compact structure within the blade passage. We also find that the leakage vortex induces flow separation at the casing endwall and entrains the casing boundary layer with its counter-rotating vorticity. As it propagates in the rotor passage, the TLV migrates towards the pressure side of the neighboring blade. Unsteadiness associated with observed vortical structures is also investigated. We notice that, at early stages of the TLV evolution, turbulence is elevated in the vortex sheet, in the flow entrained from the endwall, and near the vortex core. Interestingly, the turbulence observed around the core is not consistent with the local distribution of turbulent kinetic energy production rate. This mismatch indicates that, given a TLV section, production likely occurs at preceding stages of the vortex evolution. Then, the turbulence is convected to the core of the TLV, and we suggest that this transport has substantial component along the vortex. Because we observe that the meandering of vortex filaments dominate the flow in the passage, we decompose the unsteadiness surrounding the TLV core to contributions from interlaced vortices and broadband turbulence. Results of this decomposition show that the two contributions are of the same order of magnitude. The TLV is investigated also beyond the trailing edge of the rotor blade. During these late stages of its evolution, the TLV approaches the pressure side of the neighboring blade and vortex breakdown occurs, causing rapid broadening of the phase average core, with little change in overall circulation. Associated turbulence occupies almost half the width of the blade passage and turbulence production there is also broadly distributed. Proximity of the TLV to the pressure side of the neighboring blade also affects entrainment of flow into the incoming tip region.Copyright © 2010 by ASME

The Internal Structure of the Tip Leakage Vortex Within the Rotor of an Axial Waterjet Pump

Stereo particle image velocimetry measurements focus on the flow structure and turbulence within the tip leakage vortex (TLV) of an axial waterjet pump rotor. Unobstructed optical access to the sample area is achieved by matching the optical refractive index of the transparent pump with that of the fluid. Data obtained in closely spaced planes enable us to reconstruct the 3D TLV structure, including all components of the mean vorticity and strain-rate tensor along with the Reynolds stresses and associated turbulence production rates. The flow in the tip region is highly three-dimensional, and the characteristics of the TLV and leakage flow vary significantly along the blade tip chordwise direction. The TLV starts to roll up along the suction side tip corner of the blade, and it propagates within the passage toward the pressure side of the neighboring blade. A shear layer with increasing length connects the TLV to the blade tip and initially feeds vorticity into it. During initial rollup, the TLV involves entrainment of a few vortex filaments with predominantly circumferential vorticity from the blade tip. Being shed from the blade, these filaments also have high circumferential velocity and appear as swirling jets. The circumferential velocity in the TLV core is also substantially higher than that in the surrounding passage flow, but the velocity peak does not coincide with the point of maximum vorticity. When entrainment of filaments stops in the aft part of the passage, newly forming filaments wrap around the core in helical trajectories. In ensemble-averaged data, these filaments generate a vortical region that surrounds the TLV with vorticity that is perpendicular to that in the vortex core. Turbulence within the TLV is highly anisotropic and spatially non-uniform. Trends can be traced to high turbulent kinetic energy and turbulent shear stresses, e.g., in the shear layer containing the vortex filaments and the contraction region situated along the line where the leakage backflow meets the throughflow, causing separation of the boundary layer at the pump casing. Upon exposure to adverse pressure gradients in the aft part of the passage, at 0.65–0.7 chord fraction in the present conditions, the TLV bursts into a broad turbulent array of widely distributed vortex filaments.

Three-dimensional flow structures and associated turbulence in the tip region of a waterjet pump rotor blade

Inventors at Georgia Tech have developed an electrohydrodynamic jet that utilizes a rotary freestanding triboelectric nanogenerator (TENG) connected to a simple boost circuit that could supply an open circuit DC voltage above 1 kV. This is adequate to induce continuous droplet formation and ejection from the printing nozzle. A rotary freestanding TENG used in this technology acts as the high voltage power source for generating stable ink droplet ejection. Results reveal that the TENG's operation frequency allows for higher resolution printing with feature sizes smaller than nozzle size.

Electrohydrodynamic Jet Printing Driven by a Triboelectric Nanogenerator

Stereoscopic particle image velocimetry measurements are performed in an optical refractive index matched facility to investigate the evolution of turbulence in the tip region of an axial waterjet pump rotor. Presented analysis of mean flow velocity, vorticity, Reynolds stresses, and turbulence production/transport within the rotor passage focus on the tip-leakage vortex and associated flows. Turbulence production peaks in the shear layer that connects the blade-tip suction side with the vortex as well as in a region of flow contraction situated at the casing wall. Flow separation occurring there, as the leakage backflow meets the throughflow, detaches the boundary-layer vorticity, which is entrained into the tip-vortex perimeter. Upon the inclusion of turbulence transport in the analysis, a discrepancybetweendistributions of turbulent stresses andassociated production vanishes, except at the vortex core. There, the elevated turbulent energy (but relatively low production of Reynolds stresses) is presumably due to low dissipation. Within the aft part of the rotor passage, shortly after vortex bursting, the tip-leakage backflow reaches the neighboring blade. There, radial motion induced by the tip-vortex residual swirl detaches the pressure-side boundary-layer vorticity and injects it into the rotor passage.

Huixuan Wu

Papers

Measurements of the tip leakage vortex structures and turbulence in the meridional plane of an axial water-jet pump

The Internal Structure of the Tip Leakage Vortex Within the Rotor of an Axial Waterjet Pump

Three-dimensional flow structures and associated turbulence in the tip region of a waterjet pump rotor blade

Electrohydrodynamic Jet Printing Driven by a Triboelectric Nanogenerator

Turbulence Within the Tip-Leakage Vortex of an Axial Waterjet Pump