Low Energy Dissipation Rates Research Articles

Experimental investigation of the effects of contraction on tsunami-induced forces and pressures on a box section bridge

Many bridges that lie within possible tsunami inundation zones are critical links in transport networks. Some efforts have been made to determine the effects of tsunamis on bridges, but only a limited range of published design guidelines are available. Therefore, it is necessary to further investigate the effects of tsunamis on bridges. In the current study, physical modeling experiments were carried out to measure bore impact forces and pressures for various tsunami bore strengths on a bridge deck with different abutment types (wing wall and spill-through) and different opening and submergence ratios. The experiments were conducted in a wave flume with dimensions of 14 × 1.2 × 0.8 m (length × width × height), equipped with an automatic gate designed to generate a tsunami bore. The horizontal and vertical forces showed an increasing trend with increasing submergence ratio for both types of abutment. However, the horizontal force showed a decreasing trend as the opening ratio decreased, while the vertical force initially increased as the opening ratio decreased, until it reached a peak value, and then it started to decrease. The overall shapes of the results for both types of abutment are similar, with higher values for spill-through abutments due to their lower energy dissipation rates. Based on the experimental data, empirical equations are proposed for estimation of tsunami loads as a function of opening and submergence ratios.

Research on Flood Discharge and Energy Dissipation of a Tunnel Group Layout for a Super-High Rockfill Dam in a High-Altitude Region

Under the condition of a large dip angle between the flood discharging structure axis and the downstream cushion pool centerline, the downstream flow connection for the discharging tunnel group is poor, and the lower air pressure in high-altitude areas increases its influence on the trajectory distance of the nappe, further increasing the difficulty of predicting the flood discharge and energy dissipation layout. Based on the RM hydropower project with the world’s highest earth-rockfill dam, this paper studies the problem of a large included angle flip energy dissipation layout of a tunnel group flood discharge using the method of the overall hydraulic physical model test. The test results show that the conventional flip outlet mode has a long nappe falling point, a serious shortage of effective energy dissipation space, a large dynamic hydraulic pressure impact peak value on the bottom slab and side wall of the plunge pool, a poor flow connection between the outlet of the plunge pool and the downstream river channel, and a low energy dissipation rate. Considering the influence of a low-pressure environment, when the “transverse diffusion and downward incidence” outflow is adopted, the nappe falling point shrinks by 11 m, the energy dissipation form of the plunge pool is greatly improved, the effective energy dissipation space is increased by 159%, the RMS of the maximum fluctuating pressure is reduced by 74%, the outflow is smoothly connected with the downstream river, the energy dissipation rate is increased by 0.8%, and the protection range of flood discharge atomization is significantly reduced. This effectively solves the safety problems of large included angle discharge return channels and the energy dissipation and erosion prevention of super-high rockfill dams.

Effects of Density Fluctuations on Alfvén Wave Turbulence in a Coronal Hole

Abstract We present a study of density fluctuations in coronal holes. We used a reduced magnetohydrodynamic (RMHD) model that incorporated observationally constrained density fluctuations to determine whether density fluctuations in coronal holes can enhance Alfvén wave reflection and dissipation, thereby heating coronal holes and driving the fast solar wind. We show results for two models of the background atmosphere. Each model is a solution of the momentum equation and includes the effects of wave pressure on the solar wind outflow. In the first model, the plasma density and Alfvén speed vary smoothly with height. Wave reflection is relatively weak in the smooth model, resulting in a low energy dissipation rate. In the second model, we include additional density fluctuations along the flux tube based on the observations. We find that density ρ fluctuations on the order of δρ/ρ ∼ 0.24 increase the Alfvén wave turbulence to levels sufficient to heat the open field regions in coronal holes.

A novel dual–signal output screwing oscillator based on carbon@MoS2 nanotubes

A novel dual–signal output screwing oscillator built on carbon@MoS2 heterogeneous nanotubes (CNT@MST) is proposed and its oscillation performance is investigated using the molecular dynamics method. The MST acting as a stator is the outer tube of the oscillator, while the CNT acting as a rotor is the inner tube to which an initial rotational excitation and an axial translational excitation are simultaneously applied. Compared with the traditional double-walled carbon nanotube (CNT@CNT) screwing oscillators, CNT@MST screwing oscillators have the following advantages: better stability, lower energy dissipation rates, and wider adjustable intertube spacing.

Disentangling the photochemical salinity tolerance in Aster tripolium L.: connecting biophysical traits with changes in fatty acid composition.

A profound analysis of A.tripolium photochemical traits under salinity exposure is lacking in the literature, with very few references focusing on its fatty acid profile role in photophysiology. To address this, the deep photochemical processes were evaluated by Pulse Amplitude Modulated (PAM) Fluorometry coupled with a discrimination of its leaf fatty acid profile. Plants exposed to 125-250mm NaCl showed higher photochemical light harvesting efficiencies and lower energy dissipation rates. under higher NaCl exposure, there is evident damage of the oxygen evolving complexes (OECs). On the other hand, Reaction Centre (RC) closure net rate and density increased, improving the energy fluxes entering the PS II, in spite of the high amounts of energy dissipated and the loss of PS II antennae connectivity. Energy dissipation was mainly achieved through the auroxanthin pathway. Total fatty acid content displayed a similar trend, being also higher under 125-250mm NaCl with high levels of omega-3 and omega-6 fatty acids. The increase in oleic acid and palmitic acid allows the maintenance of the good functioning of the PS II. Also relevant was the high concentration of chloroplastic C16:1t in the individuals subjected to 125-250mm NaCl, related with a higher electron transport activity and with the organization of the Light Harvesting Complexes (LHC) and thus reducing the activation of energy dissipation mechanisms. All these new insights shed some light not only on the photophysiology of this potential cash-crop, but also highlight its important saline agriculture applications of this species as forage and potential source of essential fatty acids.

Theoretical model for multiple breakup of fluid particles in turbulent flow field

Generalized phenomenological model, based on the theories of probability and isotropic turbulence, is developed for multiple breakup of fluid particles in turbulent flow field. The approach uses a series of successive binary breakup events occur at a time scale comparable to the colliding eddy turnover time. It was found that the use of energy density, instead of energy, will increase the predicted binary breakup rate which is usually underestimated by the existing models in the literature. Generalization of the binary breakup model for multiple fragmentations is performed by defining a “remaining energy function” for the colliding eddy which means the contribution of original eddy to the later breakup events. For ternary breakage, the model shows a reasonably good agreement with the experimental data. The quaternary fragmentation frequency, however, is of negligible importance at lower energy dissipation rates but its contribution to breakage fraction at higher energy dissipation rates becomes considerable. The results also show that ternary and quaternary breakups have a considerable 90% contribution to the overall fragmentation, while pentenary and further fragmentations are of lower importance at low energy dissipation rates. At higher levels of energy dissipation rate, fragmentations up to six daughter particles contribute to more than 95% of the overall fragmentations. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4508–4525, 2016

HEATING AND ACCELERATION OF THE FAST SOLAR WIND BY ALFVÉN WAVE TURBULENCE

ABSTRACT We present numerical simulations of reduced magnetohydrodynamic (RMHD) turbulence in a magnetic flux tube at the center of a polar coronal hole. The model for the background atmosphere is a solution of the momentum equation and includes the effects of wave pressure on the solar wind outflow. Alfvén waves are launched at the coronal base and reflect at various heights owing to variations in Alfvén speed and outflow velocity. The turbulence is driven by nonlinear interactions between the counterpropagating Alfvén waves. Results are presented for two models of the background atmosphere. In the first model the plasma density and Alfvén speed vary smoothly with height, resulting in minimal wave reflections and low-energy dissipation rates. We find that the dissipation rate is insufficient to maintain the temperature of the background atmosphere. The standard phenomenological formula for the dissipation rate significantly overestimates the rate derived from our RMHD simulations, and a revised formula is proposed. In the second model we introduce additional density variations along the flux tube with a correlation length of 0.04 R ⊙ and with relative amplitude of 10%. These density variations simulate the effects of compressive MHD waves on the Alfvén waves. We find that such variations significantly enhance the wave reflection and thereby the turbulent dissipation rates, producing enough heat to maintain the background atmosphere. We conclude that interactions between Alfvén and compressive waves may play an important role in the turbulent heating of the fast solar wind.

Population balance modeling of precipitated calcium carbonate (PCC) flocculation induced by cationic starches

In order to predict and control the flocculation process, it is necessary to develop quantitative model which can describe aggregation, breakage and restructure phenomenon. In this paper, population balance equation was employed to model the PCC flocculation process. Energy dissipation rate was calculated to evaluate the floc strength. It was found, compared to the low charge density starch, the high charge density starch resulted in lower collision efficiency, lower restructure rate and stronger flocs (indicated by higher energy dissipation rate). The higher the temperature, the lower the energy dissipation rate was needed to break the flocs. The addition of NaCl did not show effect on the collision efficiency for both high and low charge density starches, but weaker flocs were formed and more prominent restructure rate was observed at higher NaCl concentration. The collision efficiency decreased with the increase of the shear rate for both starches. At higher starch dosage, lower energy dissipation rate could break the flocs and higher restructure rate was observed.

Turbulence in Continental Stratocumulus, Part II: Eddy Dissipation Rates and Large-Eddy Coherent Structures

This study first illustrates the utility of using the Doppler spectrum width from millimetre wavelength radar to calculate the energy dissipation rate and then to use the energy dissipation rate to study turbulence structure in a continental stratocumulus cloud. It is shown that the turbulence kinetic energy dissipation rate calculated from the radar-measured Doppler spectrum width agrees well with that calculated from the Doppler velocity power spectrum. During the 16-h stratocumulus cloud event, the small-scale turbulence contributes 40 % of the total velocity variance at cloud base, 50 % at normalized cloud depth = 0.8 and 70 % at cloud top, which suggests that small-scale turbulence plays a critical role near the cloud top where the entrainment and cloud-top radiative cooling act. The 16-h mean vertical integral length scale decreases from about 160 m at cloud base to 60 m at cloud top, and this signifies that the larger scale turbulence dominates around cloud base whereas the small-scale turbulence dominates around cloud top. The energy dissipation rate, total variance and squared spectrum width exhibit diurnal variations, but unlike marine stratocumulus they are high during the day and lowest around sunset at all levels; energy dissipation rates increase at night with the intensification of the cloud-top cooling. In the normalized coordinate system, the averaged coherent structure of updrafts is characterized by low energy dissipation rates in the updraft core and higher energy dissipation rates surround the updraft core at the top and along the edges. In contrast, the energy dissipation rate is higher inside the downdraft core indicating that the downdraft core is more turbulent. The turbulence around the updraft is weaker at night and stronger during the day; the opposite is true around the downdraft. This behaviour indicates that the turbulence in the downdraft has a diurnal cycle similar to that observed in marine stratocumulus whereas the turbulence diurnal cycle in the updraft is reversed. For both updraft and downdraft, the maximum energy dissipation rate occurs at a cloud depth = 0.8 where the maximum reflectivity and air acceleration or deceleration are observed. Resolved turbulence dominates near cloud base whereas unresolved turbulence dominates near cloud top. Similar to the unresolved turbulence, the resolved turbulence described by the radial velocity variance is higher in the downdraft than in the updraft. The impact of the surface heating on the resolved turbulence in the updraft decreases with height and diminishes around the cloud top. In both updrafts and downdrafts, the resolved turbulence increases with height and reaches a maximum at cloud depth = 0.4 and then decreases to the cloud top; the resolved turbulence near cloud top, just as the unresolved turbulence, is mostly due to the cloud-top radiative cooling.

Effect of dispersed holdup on drop size distribution in oil–water dispersions: Experimental observations and population balance modeling

The drop diameter distribution (DSD) of dispersed Exxsol D80 oil-in-water in a lab scale stirred tank was predicted numerically using population balance equations (PBEs) modeling. It was assumed that the flow area was split into two zones, around the impeller with high energy dissipation rate and further away from the impeller with low energy dissipation rate. The dispersed phase chord length (CLD) distribution was experimentally measured with a focused beam reflectance method (FBRM) probe, and was then converted to the drop diameter distribution (DSD) numerically using a backward transform. Comparisons between the PBEs model results against experimental data from the current work and from previous literature showed that the PBEs predicted well the data at low dispersed phase fractions but overpredicted them at high ones. The predictions were slightly better when an increased power number was used. In addition, it was found that improved predictions could be obtained when the region of high energy dissipation rate around the impeller was enhanced.

Further study of grain boundary fracture in the breakage of single multiphase particles using X-ray microtomography procedures

The present work aims to verify the 3D quantitative analysis of grain boundary fracture in the breakage of single multiphase particles using X-ray microtomography. The breakage of single multiphase copper ore particles (6mm cubic particles) by slow compression was examined. From XMT reconstructed images using the Marching Cube method, interfacial areas between copper mineral grains and host rock were determined for both parent particles and progeny particles. In this way, the specific interfacial area ratio was calculated as a metric for grain boundary fracture. Preferential grain boundary fracture only occurs at low energy dissipation rates and the current results confirm initial results for 3mm cubes published previously by Garcia et al. (2009).

Flocculation of precipitated calcium carbonate (PCC) by cationic tapioca starch with different charge densities. II: Population balance modeling

A population balance model to describe the flocculation of PCC by two cationic tapioca starches is presented. The model takes into account aggregation, floc breakage and floc restructure. Floc strength as indicated by energy dissipation rate was also evaluated. It was found that the high charge density starch relates to lower collision efficiency, lower restructure rate and higher floc strength (higher energy dissipation rate) compared to the case with the low charge density starch. Lower energy dissipation rate was needed to break the flocs at higher temperature for both starches. On the other hand, the high charge starch was more likely to be negatively affected by the background electrolyte NaCl. The collision efficiency decreased with the increase of the shear rate for both starches. The difference of charge, starch morphology on PCC surface and the charge suppression effect were employed to interpret the results.

Effect of Contaminants on the Gas Holdup and Mixing in Internal Airlift Reactors Equipped with Microbubble Generator

The impact of contaminants on the gas holdup and mixing characteristics encountered in internal airlift reactors was investigated using a 200 L pilot scale unit equipped with a two-phase transonic sparger capable of generating microbubbles. Small dosages of a cationic surfactant (0–50 ppm of sodium dodecyl sulfonate (SDS)) were used to simulate the coalescence-retarding effect encountered in most industrial streams and resulted in the formation of bubbles that varied in size between 280 and 1,900 μm. Gas holdups as high as 0.14 were achieved in the riser under homogeneous flow regime when slowly coalescent systems were aerated at the relatively low superficial velocity of 0.02 ms−1, whereas liquid circulation velocities as high as 1.3 ms−1were achieved in conjunction with rapidly coalescent systems at the same superficial velocity. This excellent hydrodynamic performance represents a 5-fold improvement in the riser gas holdup and up to 8-fold enhancement in the liquid circulation velocity and is expected to yield good mixing and mass transfer performance at low energy dissipation rates.

Physiological responses of CHO cells to repetitive hydrodynamic stress

A majority of the previous investigations on the hydrodynamic sensitivity of mammalian cells have focused on lethal effects as determined by cell death or lysis. In this study, we investigated the effect of hydrodynamic stress on CHO cells in a fed-batch process using a previously reported system which subjects cells to repetitive, high levels of hydrodynamic stress, quantified by energy dissipation rate (EDR). The results indicated that cell growth and monoclonal antibody production of the test cells were very resistant to the hydrodynamic stress. Compared to the control, no significant variation was observed at the highest EDR tested, 6.4 x 10(6) W/m(3). Most product quality attributes were not affected by intense hydrodynamic stress either. The only significant impact was on glycosylation. A shift of glycosylation pattern was observed at EDR levels at or higher than 6.0 x 10(4) W/m(3), which is two orders of magnitude lower than the EDR where physical cell damage, as measured by lactate dehydrogenase release, was observed. While not as extensively investigated, a second monoclonal antibody produced in a different CHO clone exhibited the same glycosylation change at an intensive EDR, 2.9 x 10(5) W/m(3). Conversely, a low EDR of 0.9 x 10(2) W/m(3) had no effect on the glycosylation pattern. As 6.0 x 10(4) W/m(3), the lowest EDR that triggers the glycosylation shift, is about one order of magnitude higher than the estimated, maximum EDR in typically operated, large-scale stirred tank bioreactors, further studies in a lower EDR range of 1 x 10(3)-6.0 x 10(4) W/m(3) are needed to assess the glycosylation shift effect under typical large-scale bioreactor operation conditions. Follow-up studies in stirred tanks are also needed to confirm the glycosylation shift effect and to validate the repetitive hydrodynamic stress model.

Quantitative analysis of grain boundary fracture in the breakage of single multiphase particles using X-ray microtomography procedures

Direct determination of intergranular fracture during multiphase particle breakage is a difficult task to achieve. Perhaps the only method of analysis is measurement of interphase area before and after comminution. Conservation of interfacial area after crushing is indicative of transgranular random breakage. On the other hand, if interfacial area is diminished after breakage some degree of preferential grain boundary fracture has occurred. For complete liberation of all grains after crushing the interfacial area goes to zero. Thus the interfacial area criterion is an important metric to assess the significance of preferential grain boundary fracture for different breakage conditions. This study describes the development of procedures for detailed analysis to quantify the extent of preferential grain boundary fracture for different breakage conditions using X-ray microtomography (XMT). The breakage of single multiphase copper ore particles (3 mm cubic particles) by slow compression is examined. Procedures developed for interfacial area measurements are discussed and determination of the extent of grain boundary fracture of multiphase particles for different breakage conditions is presented. For the copper ore studied, it is shown that preferential grain boundary fracture occurs at low energy dissipation rates.

The role of surface vertical mixing in phytoplankton distribution in a stratified reservoir

We investigated convection caused by surface cooling and mixing attributable to wind shear stress and their roles as agents for the transport of phytoplankton cells in the water column by carrying out two daily surveys during the stratified period of the Sau reservoir. Green algae, diatoms, and cryptophyceae were the dominant phytoplankton communities during the surveys carried out in the middle (July) and end (September) of the stratified period. We show that a system with a linear stratification and that is subject to weak surface forcing, with weak winds “4 m s‐1 and low energy dissipation rate values of the order of 10‐8 m2 s‐3 or lower, enables the formation of thin phytoplankton layers. These layers quickly disappear when water parcels mix because there is a medium external forcing (convection) induced by the night surface cooling, which is characterized by energy dissipation rates on the order of ~5 x 10‐8 m2 s‐3. During both surveys the wind generated internal waves during the entire diurnal cycle. During the day, and because of the weak winds, phytoplankton layers rise in the water column up to a depth determined by both solar heating and internal waves. In contrast, during the night phytoplankton mixes down to a depth determined by both convection and internal waves. These internal waves, together with the wind‐driven current generated at the surface, seem to be the agents responsible for the horizontal transport of phytoplankton across the reservoir.

Rock-avalanche dynamics: insights from granular physics experiments

Rock avalanches are known to behave in extraordinary ways unlike other landslides, and their deposits in part reflect their unusual physical behavior. Recent experiments in granular physics suggest that many phenomena and features simply reflect the non-linear nature of granular flows, although some behavior cannot simply be reproduced in lab scale experiments. In a static configuration, grain–grain contact networks dominate the distribution of forces and stresses within a granular mass. During flow, granular collisions damp the system through energy dissipation, while gravitational potential drives the system. The energy distribution between static and collisional stresses within the system can change very rapidly. Threshold events dominate system response, and the challenge is to find which characterization of the static phase helps to predict failure (and thus flow) resistance. Once flowing, many “unusual” rock-avalanche phenomena are entirely consistent with the physics of flow of large granular masses, but the low energy dissipation rate required for long run-out events requires the presence of physical processes that are not involved in experimental flows in the current parameter range or in the assumptions of current models.

Scale-up criteria for an injector with a confined mixing chamber during precipitation with a compressed-fluid antisolvent

Four scaling criteria were used to scale-up the precipitation with a compressed-fluid antisolvent (PCA) process from the laboratory to the pilot plant scale using an injector with a confined mixing chamber. The scale-up criteria consisted of maintaining either: (1) constant Reynolds numbers, (2) constant axial velocity, (3) constant residence time and suspension density, or (4) a constant energy dissipation rate, as process scale was changed. Experiments were conducted in a semi-continuous PCA flow apparatus and the biodegradable polymer poly ( l-lactic acid) (PLLA) was used as the model solute. Successful scale-up was based on maintaining measured particle size distributions for PLLA microparticles as process scale was increased. The scale-up criteria of maintaining constant Reynolds numbers or constant axial velocities were not sufficient to ensure equivalent process performance during scale-up. Scale-up at constant Reynolds number resulted in a 40% increase in the average PLLA diameter with a feed concentration of 0.125 wt% PLLA, and a 36% increase in the average diameter was calculated when the feed concentration was increased to 0.25 wt% PLLA. Scale-up maintaining a constant axial velocity resulted in a 32% increase in the average PLLA diameter with a feed concentration of 0.125 wt% PLLA. The shift in the PLLA particle size distributions towards larger size fractions with this criteria was attributed to a low energy dissipation rate in the confined mixing chamber, which directly affected mixing quality and secondary nucleation mechanism(s) in the confined mixing chamber. Scaling the injector with a constant residence time and suspension density, or a constant energy dissipation rate resulted in similar PLLA particle size distributions at each scale of operation, and was attributed to maintaining comparable mixing quality and similar precipitation kinetics with a change in process scale.

Model for drop coalescence in a locally isotropic turbulent flow field

The proposed model views drop coalescence in a turbulent flow field as a two-step process consisting of formation of a doublet due to drop collisions followed by coalescence of the individual droplets in a doublet due to the drainage of the intervening film of continuous phase under the action of colloidal (van der Waals and electrostatic) and random turbulent forces. The turbulent flow field was assumed to be locally isotropic. A first-passage-time analysis was employed for the random process of intervening continuous-phase film thickness between the two drops of a doublet in order to evaluate the first two moments of coalescence-time distribution of the doublet. The average drop coalescence time of the doublet was dependent on the barrier for coalescence due to the net repulsive force (net effect of colloidal repulsive and turbulent attractive forces). The predicted average drop coalescence time was found to be smaller for larger turbulent energy dissipation rates, smaller surface potentials, larger drop sizes, larger ionic strengths, and larger drop size ratios of unequal-sized drop pairs. The predicted average drop coalescence time was found to decrease whenever the ratio of average turbulent force to repulsive force barrier became larger. The calculated coalescence-time distribution was broader, with a higher standard deviation, at lower energy dissipation rates, higher surface potentials, smaller drop sizes, and smaller size ratios of unequal drop pairs. The model predictions of average coalescence-rate constants for tetradecane-in-water emulsions stabilized by sodium dodecyl sulfate (SDS) in a high-pressure homogenizer agreed fairly well with the inferred experimental values as reported by Narsimhan and Goel (J. Colloid Interface Sci. 238 (2001) 420–432) at different homogenizer pressures and SDS concentrations.

Phosphate removal in a fluidized bed—II. Process optimization

The aggregation of fine primarily formed calcium phosphate particles with sand grains in a fluidized bed for phosphate removal was studied experimentally by means of a set-up which isolated aggregation from other processes during calcium phosphate precipitation, as well as through experiments under normal operation of the fluidized bed. The net aggregation process was described by means of a mathematical model which takes into account two competing mechanisms: orthokinetic aggregation and breakage. The net aggregation process was found to account for ∼ 60% of the phosphate removed by the fluidized bed. It was found that the orthokinetic aggregation can be improved by spreading the supersaturation more evenly throughout the reactor, and breakage can be diminished by a low energy dissipation rate in the bed. Optimization of the phosphate removal efficiency was therefore achieved by selecting sand grains of small sizes (0.1–0.3 mm) and a low superficial velocity (7·10 −3 m/s), and by spreading the addition of the NaOH solution (reactant) over two dosage points. Under these conditions the phosphate removal efficiency was ∼ 80%.