Spatial Distribution Of Energy Dissipation Research Articles

Energy performance improvement for a mixed flow pump based on advanced inlet guide vanes

The sharp decrease in the efficiency of a mixed flow pump within over-load flow rates presents a challenge for coastal drainage pumping stations. To address this issue, two different structures of advanced inlet guide vanes (AIGV), full-adjustable (FA) and half-adjustable (HA) structures, are designed to approach a better energy performance improvement strategy. Entropy production theory is applied into transient flow field to reveal their influence mechanism on the spatial distribution of energy dissipation. The primary findings are as follows: (1) AIGVs effectively solve the sharp decrease in the energy performance of mixed-flow pumps within the over-load flow rate range, broadening its efficient operation range. (2) The decrease in the axial velocity under the effect of AIGV explains the primary fluid physics of the increased efficiency. (3) The improvement in the match between the impeller inflow angle distribution and the impeller blades structure suppresses the generation and transmission of the flow separation on the pressure side, and reduce the near-wall energy dissipation. The novel HA-AIGV obtains a better flow control effect.

Entropy Production Evaluation within a Prototype Pump-Turbine Operated in Pump Mode for a Wide Range of Flow Conditions

Inside the pump-turbine, energy is irreversibly lost due to turbulent pulsations in the high Reynolds number zone and actions of viscous forces close to the wall. The conventional differential pressure method cannot obtain specific details of the hydraulic loss within the machine’s flow passages; on the other hand, the entropy production method can provide accurate information on the location of irreversible losses and the spatial distribution of energy dissipation. Therefore, based on the entropy production theory, this study investigates the composition and distribution of hydraulic losses under different flow conditions for a prototype pump-turbine in pump mode. Study results indicated that total hydraulic losses significantly decreased, then slowly increased with an increase in flow rate. The entropy production rate caused by turbulence dissipation (EPTD), direct dissipation (EPDD), and wall shear stress (EPWS) displayed the same variation patterns as that of total hydraulic losses, with EPTD and EPDD being the most dominating. The location of hydraulic loss within the pump-turbine’s flow domain strongly depended on flow conditions. High hydraulic losses primarily occurred in the guide vanes (GV) and draft tube under low flow rates. Under high flow conditions, however, high hydraulic losses were mostly concentrated in the stay vanes (SV), spiral casing, and GV. Hydraulic losses at low flow rates were primarily caused by flow separation within the GV flow channels, vortices in the vaneless region, and inlet flow impacts on the runner blade’s leading edge. On the other hand, large vortices within the GV and SV flow channels, GV wake flow, and unsteady flow at the spiral casing were the main contributors to hydraulic loss under high flow conditions. EPDD was mainly caused by strain rate, so it was closer to the main vortex regions, whereas EPTD was affected by turbulence intensity and had a wider distribution range in the unsteady flow.

The effects of driving time scales on coronal heating in a stratified atmosphere

Aims. We investigate the atmospheric response to coronal heating driven by random velocity fields with different characteristic time scales and amplitudes. Methods. We conducted a series of three-dimensional magnetohydrodynamic simulations of random driving imposed on a gravitationally stratified model of the solar atmosphere. In order to understand differences between alternating current (AC) and direct current (DC) heating, we considered the effects of changing the characteristic time scales of the imposed velocities. We also investigated the effects of the magnitude of the velocity driving. Results. In all cases, complex foot point motions lead to a proliferation of current sheets and energy dissipation throughout the coronal volume. For a given driving amplitude, DC driving typically leads to a greater rate of energy injection when compared to AC driving. This ultimately leads to the formation of larger currents, increased heating rates, and higher coronal temperatures in DC simulations. There is no difference in the spatial distribution of energy dissipation across simulations; however, energy release events in AC cases tend to be more frequent and last for less time than in DC cases. This results in more asymmetric temperature profiles for field lines heated by AC drivers. Higher velocity driving is associated with larger currents, higher temperatures, and the corona occupying a larger fraction of the simulation volume. In all cases, the majority of heating is associated with small energy release events, which occur much more frequently than larger events. Conclusions. When combined with observational results that highlight a greater abundance of oscillatory power in lower frequency modes, these findings suggest that energy release in the corona is more likely to be driven by longer time scale motions. In the corona, AC and DC driving occur concurrently and their effects remain difficult to isolate. The distribution of field line temperatures and the asymmetry of temperature profiles may reveal the frequency and longevity of energy release events and therefore the relative importance of AC and DC heating.

Energy analysis for damage and catastrophic failure of rocks

The development history and current state of studies on the characteristics and mechanisms of deformation and failure of rock materials were briefly reviewed from the viewpoint of energy. The main scope and the achievable objectives of the energy-based research system were expatiated. It was validated by experiments that the damage process of rocks can be well described by the rock damage evolution equation established based on energy dissipation. It was found from the uniaxial compression and biaxial compression tests that only a small proportion of the total input energy in hard rocks is dissipated before peak load and a large proportion in soft rocks is dissipated before peak load. For both hard and soft rocks, the energy dissipated after peak load accounts for a greater proportion. More energy would be required for rock failure under equal biaxial compression than under unequal biaxial compression. The total absorbed energy is different for rock failure under high-rate loading and low-rate loading. More fragmented failure pattern usually corresponds to higher energy absorption. The mesoscopic analysis on the damage and failure of bedded salt rocks showed that the energy dissipation is prominent and the total absorbed energy for rock failure is low when cracks propagate in the weak mud interlayer while it is contrary when cracks propagate in the salt rock. The energy accumulation, transfer, dissipation and release during the failure process of tunnel with impending failure under disturbance were analyzed theoretically based on the elastoplastic mechanics theory. Furthermore, the spatial distribution of energy dissipation and energy release of fractured rocks under unloading was simulated numerically. It was demonstrated that energy is likely to be released from the weakest surface under compression, which triggers the global failure of rocks.

Resonance and sea level variability in Chesapeake Bay

A numerical model is used to determine the resonant period and quality factor Q of Chesapeake Bay and explore physical mechanisms controlling the resonance response in semi-enclosed seas. At the resonant period of 2 days, the mouth-to-head amplitude gain is 1.42 and Q is 0.9, indicating that Chesapeake Bay is a highly dissipative system. The modest amplitude gain results from strong frictional dissipation in shallow water. It is found that the spatial distribution of energy dissipation varies with forcing frequency. While energy at tidal frequencies is dissipated around topographic hotspots distributed throughout the Bay, energy dissipation at subtidal frequencies is mainly concentrated in the shallow-water lower Bay. An analytic calculation shows that the bottom friction parameter is much larger in Chesapeake Bay than in other coastal systems with strong resonance response. The model-predicted amplitude gains and phase changes agree well with the observations at semidiurnal and diurnal tidal frequencies. However, the predicted amplitude gain in the resonant frequency band (34–54 h period) falls below that inferred from band-passed sea level observations. This discrepancy can be attributed to the local wind forcing which amplifies the sea level response in the upper Bay. The model is also used to show that rising sea levels associated with global warming will shift the resonance period of Chesapeake Bay closer to the diurnal tides and thus exacerbate flooding problems by causing an increase in tidal ranges.

Spatial resolution limits in electron beam nanolithography

Computer simulation of high energy primary electron scattering and subsequent generation of ‘‘fast’’ secondary electrons in thin film targets is demonstrated with Monte Carlo techniques. A hybrid Monte Carlo model is utilized to calculate the generation and subsequent spatial trajectory of each secondary electron in the target. The three-dimensional spatial distribution of energy dissipation by such fast secondary electrons is shown to be the fundamental resolution limit for electron beam lithography with high voltage beams (100 keV) and thin film polymer targets. The dependence of resolution on beam voltage and film thickness is presented, and quantitative comparison is made between these new Monte Carlo calculations and the limited amount of experimental data available in the scientific literature.

The tidal regime of the Bristol Channel: a numerical modelling approach

Summary. Two-dimensional numerical models are used to examine the tidal regime of the Bristol Channel. The effect of including S2 in addition to M2 in the models on both the tidal elevations and the residual Eulerian circulation is investigated. The spatial distribution of the maximum spring currents and the time of their occurrence are also computed. The Eulerian and Lagrangian residual circulations are compared and the effect of model grid size on the former is examined. Finally the spatial distribution of energy dissipation within, and the total flux of energy into, the Bristol Channel are given. htroduction h the course of investigating the effect on the tides of tidal power barrages in the Bristol Channel a series of numerical models have been developed (Owen & Heaps 1979; Owen 1979). These models have yielded general information on the tidal dynamics of the Bristol Channel and some interesting points concerning the numerical modelling of shallow coastal qions in general. It is the purpose of this paper to present these results. On the basis of an M2 + S2 tidal simulation from springs to neaps the structure of the €ulerian tidal streams in the Bristol Channel over a cycle at mean tide are examined, and contours of maximum spring velocities are given. In addition, the tidally induced residual Eulerian and Lagrangian circulations are plotted and compared. The effect of different &el grid sizes on the residual circulation is also considered. It is usual practice to deduce &e residual circulation from numerical models driven with the M2 tide only, i.e. no account B taken of the spring neap cycle. Here, the residual circulation obtained from the M2 tide is ampared with that from the M2 + S2 tide. Finally the spatial distribution of energy dissipation due to the M2 tide is plotted and the mtal M2 energy flux into the Bristol Channel is computed and compared to previous admates.

Spatial Distribution of Energy Dissipation by High-Energy X-Rays

The space distribution of energy dissipated by betatron x-rays results from the combined propagation of the x-rays and of their secondary electrons. This distribution has been calculated for a 40-Mev bremsstrahlung spectrum incident on a mass of water. The calculation is described and the results are compared with related experiments.