Changes In Velocity Distribution Research Articles

Analysis and design of 2-dimensional acoustic particle velocity horns

A two-dimensional acoustic horn packaged with a 2-D acoustic particle velocity sensor (APVS) is presented. The particle velocity amplification factor of the 2-D acoustic horn is analyzed theoretically and experimentally. The experimental results show a good agreement with the theoretical expectations. Compared with the previous horn model, this model considers the gain attenuation at low frequency and the influence of chip assembly deviation. As the particle velocity distribution changes greatly in the boundary layer, the assembly deviation of the chip has a great influence on the package gain. Furthermore, a microphone and the 2-D particle velocity sensor was packaged in the acoustic horn to detect the acoustic pressure and 2-D particle velocity at one point. The acoustic horn has an omnidirectional response for acoustic pressure. By using the mode domain beamforming, the direction of the incident wave was estimated with this two-dimensional sound intensity probe.

Effects of flue gas recirculation on combustion and heat flux distribution in 660 MW double-reheat tower-type boiler

The ultra-supercritical double-reheat boiler has attracted more attention because of high parameters (steam with high temperature and pressure), low pollution and large capacity. However, there are some difficulties in the development of this technology, such as adjusting the reheat steam temperature. In this study, computational fluid dynamics simulation is used to analyze a 660 MW double-reheat tower-type pulverized coal-fired boiler. The influence of flue gas recirculation (FGR) on heat transfer characteristics and combustion process in the furnace (including heating surfaces in the upper parts of the furnace) are evaluated. The user-defined function approach predicates the CO reduction effect on NOx. The results show that the flow at the horizontal section through the centerline of recirculating flus gas nozzles is rotating. The velocity distribution changes into an elliptical rotating flow when FGR ratio is 20%. At higher FGR ratios, the high-temperature area (1565–1700 K) shrinks and both the NOx concentration at the low-temperature superheater outlet and the O2 concentration in the burner zone descend. The O2 concentration at the low-temperature superheater outlet first increases and then decreases. In the main combustion zone, the heat flux peak of water-cooled wall is about 330 kW m−2. As FGR ratio increases from 0% to 20%, the rate of heat absorption of water-cooled wall to that of total boiler decreases by 3.50%. These rates for reheater and superheater increase by 2.53% and 2.13%, respectively.

Improvement of Velocity Distribution Function Measurement of Hydrogen Atoms in ion Source Discharges

The efficiency of negative hydrogen (H-) ion surface production in an ion source should be highly dependent upon the velocity distribution function of neutral atomic hydrogen (H0) striking the plasm grid (PG). A system composed of forming an atom beam, rotating beam chopper, ionizer and pulse count unit has been constructed to measure H0 velocity distribution functions. The background noise due to stagnation of hydrogen gas in the region between the beam formation skimmer and the ion source PG was improved by inserting a flow reflection structure. The short life time of the secondary electron emission detector (SEM) was prolonged by properly setting the hydrogen ion trajectories from the ionizer to the the first dynode of the SEM. The system is running stably to record the velocity distribution function but statistical noise synchronized to the trigger pulse of the chopper must be removed before the measurment of change in H0 velocity distribution functions dependin upon the type of discharge.

Velocity Distribution and Base Pressure Analysis of Under Expanded Nozzle Flow at Mach 1.0

This paper discusses velocity distribution and base pressure control using annular ribs. The annular ribs are located in the enlarged duct. Its impact on the velocity distribution and base pressure changes from a converging nozzle at the sonic Mach number is investigated numerically using ANSYS software. The rib’s height is fixed to 3 mm while the rib’s height is varied from 1 mm to 2 mm. The simulation is performed by placing the rib at various locations in the duct namely 1D, 2D, 3D and 4D and at Nozzle Pressure Ration (NPR) varied from 1.5 to 5. The changes in base pressure (Pb) and velocity variation inside the duct are studied. The presence of rib in the duct causes disturbance in the flow field hence increases the Pb. The tendency of Pb to increase is observed as the height of the rib increases. Varying the NPR from 1.5 to 3 reduced Pb due to shock wave formation. The NPR increases with Pb as the flow undergoes under-expansion due to the expansion fan at the nozzle exit. The changes in Pb are consistent with velocity distribution inside the duct. The increase in turbulent kinetic energy is more significant due to the presence of rib, as compared to duct without rib. For lower NPR, the velocity is found to be highest at the axial direction of the duct while the highest velocity is found just ahead of the nozzle exit at higher NPR.

CFD-DEM study on fluidization characteristics of gas-solid fluidized bed reactor containing ternary mixture

Coupled computational fluid dynamics - discrete element method (CFD-DEM) simulations are carried out to investigate the fluidization characteristics of gas-solid fluidized bed reactor (FBR) containing ternary particles (spheres, ideal and elongated cylinders, the latter two are rod-like particles). The mixing efficiency, velocity distribution, orientation distribution and height change of different particles during the mixing process are fully investigated and discussed. Numerical results show that when the superficial gas velocity is low, the mixing efficiency would decrease with the increase of the volume fraction of elongated cylinders due to the inter-locking. However, the discrepancy of mixing degree induced by particle shape can be weakened by increasing the superficial gas velocity. Elongated cylinders become quite active when the inter-locking is broken with more uncertainty in their motion. Ideal cylinders prefer to be horizontal at all the superficial gas velocities considered but elongated ones only show similar behavior at low superficial gas velocities. When the superficial gas velocity is high, the time-averaged orientation of elongated cylinders becomes quite uniform. The inclined angle of both ideal and elongated cylinders decreases with the increase of height and large inclined angles are found at the two lower corners of the reactor. The ideal cylinders turn to be more vertical when the position moves to the central line of the FBR. In the lower half of the bed material, the elongated cylinders prefer to be horizontal in the near-wall region but more vertical in the center of the upper half. Within the range of currently considered superficial gas velocities, the overall mixing degree of the ternary mixture can be enhanced by increasing the superficial gas velocity both in efficiency and stable mixing degree. The findings can be very useful to guide the improvement of existing reactors and the design of novel ones.Numerical simulation; Fluidization; Ternary granular mixture; Mixing behavior; Rod-like particles.

Study of typical water header flow structure by large eddy simulation and RANS

Water header is the most common structure in the design of flow system for energy and power system. The complex flow structure could result in some problems when CFD simulation is applied in the whole system analysis. The rapid change in velocity distribution of the flow field leads to difficulties to create suitable boundary-layer mesh, and the complex flow structure will also make residuals hard to reach convergence criteria. Large eddy simulation is promising to promote these studies, it is more accurate than RANS method and can capture many non-steady-state characteristics those RANS method cannot obtain. In this study a typical water header flow structure is investigated by RANS and large eddy simulation methods. By comparing the detailed flow structures in the results of two methods, the deficiency of RANS method was found. The results of large eddy simulation can be used to guide the establishment of meshes and the application of time-averaged turbulence models to improve efficiency in engineering. The asymmetric Reynolds stresses may induce asymmetric flow field in symmetric geometry.

Numerical simulation of pulsatile blood flow: a study with normal artery, and arteries with single and multiple stenosis

Numerical simulations of pulsatile transitional blood flow through symmetric stenosed arteries with different area reductions were performed to investigate the behavior of the blood. Simulations were carried out through Reynolds averaged Navier-Stokes equations and well-known k-ω model was used to evaluate the numerical simulations to assess the changes in velocity distribution, pressure drop, and wall shear stress in the stenosed artery, artery with single and double stenosis at different area reduction. This study found a significant difference in stated fluid properties among the three types of arteries. The fluid properties showed a peak in an occurrence at the stenosis for both in the artery with single and double stenosis. The magnitudes of stated fluid properties increase with the increase of the area reduction. Findings may enable risk assessment of patients with cardiovascular diseases and can play a significant role to find a solution to such types of diseases.

The performance of flow field channel in direct methanol fuel cell

A 3D computational fluid dynamics (CFD) model is developed to examine the impact of flow field design on the performance of direct methanol fuel cells (DMFCs). Effect of three various type flow fields is investigated in this study: single, double serpentine and honeycomb models. The distribution of velocity and temperature are simulated in 3D models. According to simulation studies, the honeycomb flow field has made uniform flow velocity distribution and minimum temperature change on plate surface. This could result better on DMFC performance. The experimental studies emphasize the performance of a single cell DMFC with different flow field channel designs as well as exhibit maximum power density and open circuit voltage. In subsequent study, electrodeposited Ni-Co alloy on stainless steel mesh surface is utilized to oxidize methanol and the electrode performance has been tested using cyclic voltammetry in alkaline conditions to replace expensive and sensitive platinum and platinum alloy catalysts

Study on viscosity induced contrast in ultrasound color flow imaging of carotid atherosclerosis

Efficient imaging of blood flow disturbances resulted from carotid atherosclerosis plays a vital role clinically to predict brain stroke risk. Carotid atherosclerosis and its development is closely linked with raised blood viscosity. Therefore, study of viscosity changing hemodynamic effect has importance and it might be useful for improved examination of carotid atherosclerosis incorporating the viscosity induced contrast in conventional ultrasound imaging. This work considered the design of realistic models of atherosclerotic carotid artery of different stages and solved to compute the hemodisturbances using computational fluid dynamics (CFD) by finite element method (FEM) to investigate viscosity changes effect. Ultrasound color flow image of velocities of blood have been constructed using phase shift information estimated with autocorrelation of Hilbert transformed simulated backscattered radiofrequency (RF) signals from moving blood particles. The simulated ultrasound images have been compared with CFD simulation images and identified a good match between them. The atherosclerosis stages of the models have been investigated from the estimated velocity data. It has been observed that the blood velocities increase noticeably in carotid atherosclerotic growths and velocity distribution changes with viscosity variations. It is also found importantly that the viscosity induced contrast associated to atherosclerosis is detectable in ultrasound color flow imaging. The findings of this work might be useful for better investigation of carotid atherosclerosis as well as prediction of its progression to reduce the stroke risk.

Control of jet structure utilizing the change in the outlet velocity distribution with time

Flow control using jets is widely implemented. In particular, the clarification of jet technology is indispensable in the control of the blade stall control of blades and for thermal management, such as refrigeration and air conditioning. As part of the fundamental research into the topics mentioned above, various studies of the flow characteristics of steady jets have long been conducted, and the structure of the jets investigated. However, studies on flow control by synthetic jets that essentially exhibit an unsteady flow have begun only recently. Synthetic jets have a flow wherein the jet structure forms downstream of the slot, although the net flow rate is zero, because of repeated ejection and suction. Attempts have been made to control of the jet’s direction of travel by adjusting only the jet’s frequency without any geometric changes. However, many aspects of the effect of oscillation characteristics on the jet’s structure are still unclear. Moreover, since the flow rate is low, it is not suitable for large-scale fluid control, such as that required for high-power fluid machinery. In this study, excited jets are proposed, wherein the velocity distribution change with time at the slot exit and the time-averaged flow rate are considered. An attempt was made to control the jet structure by means of the oscillation characteristics. The influences of the frequency and the phase of the slot outlet velocity distribution on the maximum jet velocity and the jet width were investigated by numerical calculations and experiments. Our research led us to conclude that the staggered vortex street is generated when the phase of the slot exit velocity distribution is π. Further, we concluded that the jet structure could be controlled by the frequency under a certain specific set of conditions.

Numerical investigation into the cavitating jet inside water poppet valves with varied valve seat structures

Owing to the strong three-dimensionality and transient evolution, the flow dynamics of a cavitating jet inside a water poppet valve is poorly understood, leading to insufficient basis for exploring the governing mechanism of cavitation effects. In this paper, a three-dimensional simulation considering the compressibility of each constituent phase is performed, to clarify the governing mechanisms under the cavitating flow inside two water poppet valves. The cavitation structures inside the poppet valves are primarily located at three regions and triggered by different mechanisms. The vortex cavitation, mainly confined within the free shearing layer, is due to vortex dynamics, while the attached cavitation at the poppet trailing edge and within the chamfered groove arises from flow separation. The fast laminar–turbulent transition process contributes to the three-dimensionality within the free shearing layer and the rear part of the chamfered groove. The flow separation due to the chamfered groove leads to increased velocity of the central potential core, contributing to a different flow discharge performance from that of the poppet valves with a sharp seat. In addition, the periodic variation in cavitation reveals the significant interaction between the shed cavitating vortex and attached cavitation at the poppet trailing edge. In conclusion, the change in velocity distribution due to different poppet valve seat structures leads to variation in flow discharge performance, and the vortex dynamics makes sense for all three kinds of cavitation occurring inside poppet valves.

Design and analysis of micro-nano scale nested-grooved surface structure for drag reduction based on ‘Vortex-Driven Design’

Bioinspired grooved surface structure design has been widely used as an efficient passive flow control method in drag reduction. The total drag of plate with grooved surface structure can be decomposed into friction and pressure drag. In this paper, the relationship between them and the distribution of vortex structure in flow field has been analyzed for obtaining the drag reduction mechanism of grooved surface structure. The concept of ‘Vortex-Driven Design’ is proposed to improve the performance of conventional periodic single-level grooved surface structure. A design scheme of micro-nano scale nested-grooved surface structure with better drag reduction performance is given. The authors conduct numerical simulation of rectangular nested-grooved surface structure in laminar flow considering rarefaction based on Lattice Boltzmann Method at high Knudsen number. The results show that the nested-grooved surface structure induces higher complexity in the secondary vortex structure, and that the changes of vorticity distribution and flow characteristics drive the changes of velocity distribution and shear stress in the grooved surface. Compared with conventional grooved surface structure, the average friction drag of the surface is further reduced. Two geometric optimization cases of the conventional grooved surface and nested-grooved surface are realized by Genetic Algorithm, showing that the maximum drag reduction rate of the optimal nested-grooved surface can reach 18.76% while that of the optimal conventional grooved surface only reaches 13.61%. Based on ‘Vortex-Driven Design’, an innovative material improvement method for drag reduction is proposed as a new direction for subsequent research on microstructure in terms of drag reduction and energy conservation.

Effect of Deformation on Droplet Contact Charge Electrophoresis.

The effect of deformation on the droplet contact charge electrophoresis (CCEP) was investigated for consistent droplet movement control. Through systematic experiments and numerical simulations, it has been found that overcharging by deformation is up to about 130% of the sphere and is mainly driven by the concentration of the electric field near the tip of the droplet rather than an increase in the surface area. Dimensional analysis revealed a consistent droplet CCEP motion with the electric capillary number range of 0.01-0.09. We also found that the dimensionless droplet charge follows a universal curve proportional to the electric capillary number, regardless of the droplet size, and the weak dependence on the droplet size shown in the experimental results is due to hydrodynamic effects, not electrostatic ones. Changes in droplet velocity distribution with droplet size and the electric capillary number were also investigated. Using the perfect conductor theory and Stokes law, we derived an analytical relationship between the droplet center velocity and the electric capillary number and analyzed the experimental results based on this relationship. This study implies that if proper hydrodynamic correction is applied, the droplet CCEP and its deformation effect can be explained by a perfect conductor theory.

Electro-osmotic non-isothermal flow in rectangular channels with smoothed corners

Microchannel heat sinks are able to provide high cooling capabilities in terms of heat flux rates. This makes them particularly interesting for the thermal management of electronic components such as CPUs, which have high power density and small dimensions. Pressure drop of the coolant across the microchannels may, however, be significant and give rise to viscous heating, thereby preventing the practical use of these devices. When the coolant is a polar fluid and the channel walls possess a net electric charge, an alternative means of moving the fluid is through an applied external electric field. The flow which originates is called electro-osmotic (EOF). EOF does not require moving parts, is free of vibrations and does not need lubrication, but is subject to Joule heating of the fluid and has flow and heat transfer characteristics which differ from those of pressure-drive flows. In spite of several previous investigation on EOF, no attention has been paid to the changes in velocity and temperature distributions caused by modifying the base cross-section of the channels which may be circular, rectangular or polygonal, thanks to the current capabilities of microfabrication. This work investigates numerically the influence of smoothing the corners of the cross-section at fixed hydraulic diameter on the values of the Poiseuille and Nusselt numbers for the laminar, steady and fully developed, electro-osmotic flow in a rectangular channel subject to uniform heat flux and Joule heating. Several aspect ratios are considered, as are different values of the ratio of Joule heating to heat flux through the walls. The results highlight a very slight increase of the Poiseuille number with the radius of curvature, whereas the Nusselt number experiences a significant improvement. Correlations are obtained for both the Poiseuille and Nusselt number as a function of the radius of curvature, aspect ratio and Joule heating-to-heat flux ratio.

Flow field in fouling spiral wound reverse osmosis membrane modules using MRI velocimetry

Magnetic Resonance Imaging (MRI) velocimetry was applied to study non-invasively the water flow field inside a spiral-wound desalination membrane module (diameter: 2.5 in.; length: 18.5 in.), located in a pressure vessel, at typical practice operational conditions as a function of alginate fouling, simulating extracellular polymeric substances (EPS). Cross-sectional velocity images were acquired at an in-plane spatial resolution of 0.137 mm at multiple locations along the length of the reverse osmosis module and were acquired as a function of alginate concentration. At a total system alginate concentration of 3.25 mg/l, significant changes in the cross-sectional velocity map were observed near the module inlet due to alginate fouling, with limited changes observed in the middle and outlet regions of the module. When the total system alginate concentration was increased to 75 mg/l, it caused the module brine seal to fail resulting in significant local water flow by-passing the membrane module. This was clearly discernible in this opaque membrane system using MRI and resulting in dramatic changes in fluid velocity distribution through the membrane module. These observations of significant flow field heterogeneity as fouling develops are consistent with ‘irreversible’ fouling effects noted frequently in practice by the water treatment industry.

Total phosphorus accident pollution and emergency response study based on geographic information system in Three Gorges Reservoir area

In recent years, sudden water pollution accidents in China’s rivers have become more frequent, resulting in considerable effects on environmental safety. Therefore, it is necessary to simulate and predict pollution accidents. Simulation and prediction provide strong support for emergency disposal and disaster reduction. This paper describes a new two-dimensional water quantity and the quality model that incorporates a digital elevation model into the geographic information system. The model is used to simulate sudden water pollution accidents in the main stream of the Yangtze River and Jialing River in the Chongqing section of the Three Gorges Reservoir area. The sectional velocity distribution and concentration change of total phosphorus are then analyzed under four hydrological situations. The results show that the proposed model accurately simulates and predicts the concentration change and migration process of total phosphorus under sudden water pollution accidents. The speed of migration and diffusion of pollutants is found to be greatest in the flood season, followed by the water storage period, drawdown season, and dry season, in that order. The selection of an appropriate water scheduling scheme can reduce the peak concentration of river pollutants. This study enables the impact of pollutants on the ecological environment of river water to be alleviated, and provides a scientific basis for the emergency response to sudden water pollution accidents in the Three Gorges Reservoir area.

Stoneley wave propagation in transversely isotropic thermoelastic medium with two temperature and rotation

In the present paper we studied Stoneley wave propagation at the interface of two dissimilar homogeneous transversely isotropic thermoelastic media with two temperatures and rotation. The secular equations of Stoneley waves are derived in the form of the determinant by using appropriate boundary conditions. The wave characteristics such as phase velocity and attenuation coefficients are computed and depicted graphically. Effect of rotation and two temperature has been depicted on the phase velocity, attenuation coefficient, displacement component, stress component, and Temperature distribution change. Some particular cases are also deduced from the present investigation.

Flooding Dynamics Within an Amazonian Floodplain: Water Circulation Patterns and Inundation Duration

AbstractFlooding dynamics across a medium‐size (Janauaca Lake, 786 km2) floodplain system along the Amazon/Solimoes River over a 9‐year period (2006–2015) is studied through integration of remote sensing and limited in situ data in hydrologic‐hydrodynamic modelling based on Telemac‐2D model. Model accuracy varies through the hydrological year. We focus on seasonal and interannual spatial variability of water circulation and inundation duration. We highlight strong heterogeneities in water velocity magnitude between the different morphological domains of the floodplain, the highest velocities being encountered in the river‐floodplain channel. In addition to topography, we emphasize the importance of the main channel and the local runoff in controlling the water circulation, at least during part of the hydrological year. From low water to early rising period, local runoff constrains the river incursion across the floodplain, while the rates of main channel rising/receding controls the flood duration. The comparison of several hydrological years highlights the interannual changes of these hydraulic controls and also the influence exerted by prior inundation conditions. While we observed few changes in water velocity distribution among hydrological years, the inundation duration is highly variable. Usually defined by maximum water level, extreme flood events may paradoxically induce positive (up to 40 days) but also negative (up to −20 days) anomalies of inundation duration.

Investigation of the influence of an interceptor on the inlet velocity distribution of a waterjet-propelled ship using SPIV technology and RANS simulation

ABSTRACTTo illustrate the effect of interceptor installation on waterjet-ship wake fields, inlet velocity distribution of a semi-planing waterjet-propelled ship operating under inlet-closed state has been assessed via stereo particle image velocimetry (SPIV) measurements and Reynolds-averaged Navier–Stokes (RANS) simulations. Results reveal that interceptor installation yields substantial drag reduction (6.06% on average) and alts the ship's sailing attitude whilst blocking the water flow in its installation area. Results of SPIV measurements demonstrated good agreement with those obtained via simulations, accurately capturing velocity characteristics in regions affected by fin stabilisers. Interceptor installation was observed to increase boundary-layer thickness near its installation location whilst lowering velocity across the capture area and reducing momentum and energy velocity coefficients by 2.1% and 4.1%, on average, respectively. As observed, changes in the ship's sailing attitude negligibly influenced the velocity distribution; instead, the interceptor's retarding effect was observed to be the main factor driving changes in inlet velocity distribution of waterjet-propelled ships.

Numerical Investigation of the MHD Flow Under the Influence of Heat Transfer in Rectangular Ducts

Liquid metal (LM) breeder blanket is a topic of great interest in the fusion reactor blanket design because of many advantages. In the breeding blanket of the fusion reactor, the dynamic viscosity of LM is impacted by heat transfer, resulting in the change of velocity distribution. The influence of heat transfer on magnetohydrodynamic flow in a rectangular duct at high Hartmann number is investigated by a coupling method. By this method, the velocity field is calculated through a second-order projection method, coupled with the temperature distribution computed by a finite-volume method. The numerical result without temperature influence is validated by Hunt’s and Shercliff’s analytical solutions, which shows very good accuracy. On the basis of this numerical code, the velocity distribution of a Hunt’s case with temperature influence is simulated. The simulation result indicates that the velocity field is different from the benchmark solution as the result of the heat transfer.