Detailed Velocity Profiles Research Articles

Numerical study of hydrodynamics in gas-solid reactors operating within bubbling fluidisation regime

Hydrodynamics of dense gas-solid flows is investigated computationally using Euler-Euler methodology. The method used is primarily based on the kinetic theory of granular flow (KTGF) and additionally by incorporating the features of frictional pressure models (FPM). Frictional stresses are accounted when solid volume fraction reaches the frictional packing limit (FPL). Investigations on the effects of bed pressure drop and other gas-solid flow characteristics have revealed that a value for frictional packing limit around 0.61 yields better results. It is also found that the FPM affects the bed hydrodynamics up to a superficial gas velocity of around 1.5 times the minimum fluidisation velocity. The numerical results of bed pressure drop and bed expansion ratio are validated against the corresponding experimental data available in literature. Detailed velocity and voidage profiles are reported along with the contours of solid volume fraction, and velocity vectors of gas and solid phases.

Wall-Adjacent Velocity Profiles of Nano-scale Gas Flows

Detailed gas velocity profiles in the vicinity of wall surfaces of nano-scale gaps are analysed by molecular dynamics simulations for argon and oxygen gas flows between diamond surfaces. Force driven flows are simulated at a temperature of 330 K and a pressure of 0.23–3.92 MPa. The flow regime is considered as in the slip-transitional flow. It is discussed why the usual binning for statistics may mislead the velocity profiles. To avoid such misleading, the volume based binning is introduced for obtaining correct velocity profiles next to the wall. It is confirmed that the corrected statistical velocity shows a flatter profile next to the wall with an inflection point followed by an increasing curve.

Numerical and Experimental Analysis of Airflow in a Multi-Duct Cleaning System for a Rice Combine Harvester

Abstract. In order to improve the cleaning quality and efficiency of a rice combine harvester, a cleaning system comprising a cen-trifugal fan and four outlet ducts was developed. Three multi-duct centrifugal fans with the same rotor but different up-per outlet positions and volute walls were studied numerically and experimentally to evaluate the impact of the upper outlet on the inner flow field distribution of the fan. Computational fluid dynamics (CFD) was used to analyze the veloc-ity profiles, flow rate, internal velocity, and pressure of the centrifugal fan. Experiments were performed using hot-wire anemometers to measure the velocity at the four outlets of the fan to validate the simulations. CFD simulations were used to perform detailed flow visualization, velocity profile and flow rate estimation, and internal velocity field and pressure field analysis. Special attention was paid to the effect of the upper outlet on the inner flow field distribution and on achieving a balanced airflow division between the four outlets. It was observed that the internal field distribu-tion and the mass flow rate division between the four outlets can be greatly influenced by the upper outlet position. The optimal structure of the multi-duct centrifugal fan was obtained for the rice combine.

A novel instrument to measure differential ablation of meteorite samples and proxies: The Meteoric Ablation Simulator (MASI).

On entering the Earth's atmosphere, micrometeoroids partially or completely ablate, leaving behind layers of metallic atoms and ions. The relative concentration of the various metal layers is not well explained by current models of ablation. Furthermore, estimates of the total flux of cosmic dust and meteoroids entering the Earth's atmosphere vary over two orders of magnitude. To better constrain these estimates and to better model the metal layers in the mesosphere, an experimental Meteoric Ablation Simulator (MASI) has been developed. Interplanetary Dust Particle (IDP) analogs are subjected to temperature profiles simulating realistic entry heating, to ascertain the differential ablation of relevant metal species. MASI is the first ablation experiment capable of simulating detailed mass, velocity, and entry angle-specific temperature profiles whilst simultaneously tracking the resulting gas-phase ablation products in a time resolved manner. This enables the determination of elemental atmospheric entry yields which consider the mass and size distribution of IDPs. The instrument has also enabled the first direct measurements of differential ablation in a laboratory setting.

Site amplification at the city scale in Basel (Switzerland) from geophysical site characterization and spectral modelling of recorded earthquakes

Hazard assessment at the city scale requires a detailed characterization of the effect of surface geology on ground motion (site effects). Though this analysis is commonly achieved using geophysical site characterization and site response modelling, we propose here a complementary analysis based on amplification functions retrieved from Empirical Spectral Modelling (ESM) of earthquake recordings. We applied this method to the city of Basel (Switzerland) that benefits from a detailed microzonation and a dense Strong Motion Network with 21 modern free-field stations. We first verified the accuracy of ESM amplification functions for this region and used them to determine the bedrock interface at a site with a detailed velocity profile. While the interface between Upper and Lower Tertiary was, until now, considered responsible for the fundamental frequency of resonance in the Rhine Graben, we found that the bedrock interface in fact lies at the Mesozoic limestone. We also investigated the second peak of the H/V ratios that is clustered in a particular area of the basin where amplification is found to be different. We successfully used the ESM amplification functions to verify the microzonation of 2006 and would strongly advise the installation of strong motion stations where such studies are performed in the future. Outside the Rhine Graben, where shallow sediments are found, we propose an amplification functional form based on ESM and the fundamental frequency of resonance. Finally, we combined all our findings and generated amplification maps of the response spectrum at any period of interest for earthquake engineering. This map is proposed for a high resolution real-time implementation in ShakeMap and will be used for seismic loss assessment.

Modelling heat transfer inside an autoclave: Effect of radiation

In this work, computational fluid dynamics simulations are performed to predict the temperature distribution on a part during an autoclave run. Data from an experimental study are used as input to the simulations and also for comparison with the numerical results. A conjugate heat transfer approach was used for the simulations, where best agreement with experiments was obtained from the simulation that included thermal radiation and utilized an experimentally obtained velocity profile as inlet velocity. A yet more detailed inlet velocity profile and more advanced turbulent model could result in an even better agreement.

Numerical Study of Hydrodynamics in Gas-Solid Reactors Operating within Bubbling Fluidization Regime

Hydrodynamics of dense gas-solid flows is investigated computationally using Euler-Euler methodology. The method used is primarily based on the kinetic theory of granular flow (KTGF) and additionally by incorporating the features of frictional pressure models (FPM). Frictional stresses are accounted when solid volume fraction reaches the frictional packing limit (FPL). Investigations on the effects of bed pressure drop and other gas-solid flow characteristics have revealed that a value for frictional packing limit around 0.61 yields better results. It is also found that the FPM affects the bed hydrodynamics up to a superficial gas velocity of around 1.5 times the minimum fluidisation velocity. The numerical results of bed pressure drop and bed expansion ratio are validated against the corresponding experimental data available in literature. Detailed velocity and voidage profiles are reported along with the contours of solid volume fraction, and velocity vectors of gas and solid phases.

Passive probing of the sound fixing and ranging channel with hydro-acoustic observations from ridge earthquakes.

The International Monitoring System includes a hydro-acoustic part to verify the Comprehensive Nuclear-Test-Ban Treaty. Besides explosive signals, monitoring stations also detect acoustic waves from earthquakes that travel through the SOund Fixing And Ranging (SOFAR) channel. The travel times of such detections are listed in the Reviewed Event Bulletin, which is statistically evaluated for the stations located in the Pacific, Indian, and Atlantic Oceans. The celerities of ridge earthquakes are calculated to build up a homogeneous data-set, based on similar source mechanisms. The celerity is defined as the epicentral distance divided by the travel time. The global characteristics of these celerities can be well understood in terms of temperature variations in the SOFAR channel. A detailed velocity profile has been retrieved for the Atlantic Ocean where large differences (14 m/s) are found between the southern and northern parts of the basin. Propagation modeling with normal modes supports these findings, which shows that the celerity gives an estimate of the sound speed in the SOFAR channel. These results compare remarkably well with those from active experiments, showing the ability of passively probing the SOFAR channel with hydro-acoustic waves from earthquake sources.

Detailed Velocity and Concentration Profiles Measurement During Activated Sludge Batch Settling Using an Ultrasonic Transducer

Activated sludge settling is a complex process requiring a thorough experimental study. It is, however, difficult to obtain information on the inner behavior of the sludge blanket without disturbing it. Optical methods are inefficient due to the opacity of sludge suspensions. This paper focuses on the investigation of activated sludge batch settling using a non-invasive method based on an ultrasonic transducer. The treatment of the signal allowed obtaining data on the settling velocity and concentration profiles inside of the suspension. The different settling regimes were clearly shown by the results on the basis of the shape of the settling velocity isolines. The precision of the gathered data is adapted to the calibration and validation of numerical models. The material used in the experimental setup can be installed in actual wastewater treatment plants.

Reprint of: A multiphase, micro-scale PIV measurement technique for liquid film velocity measurements in annular two-phase flow

Prediction methods for two-phase annular flow require accurate knowledge of the velocity profile within the liquid film flowing at its perimeter as the gradients within this film influence to a large extent the overall transport processes within the entire channel. This film, however, is quite thin and variable and traditional velocimetry methods have met with only very limited success in providing velocity data. The present work describes the application of Particle Image Velocimetry (PIV) to the measurement of velocity fields in the annular liquid flow. Because the liquid is constrained to distances on the order of a millimeter or less, the technique employed here borrows strategies from micro-PIV, but micro-PIV studies do not typically encounter the challenges presented by annular flow, including very large velocity gradients, a free surface that varies in position from moment to moment, the presence of droplet impacts and the passage of waves that can be 10 times the average thickness of the base film. This technique combines the seeding and imaging typical to micro-PIV with a unique lighting and image processing approach to deal with the challenges of a continuously varying liquid film thickness and interface. Mean velocity data are presented for air–water in two-phase co-current upward flow in a rectangular duct, which are the first detailed velocity profiles obtained within the liquid film of upward vertical annular flow to the authors’ knowledge. The velocity data presented here do not distinguish between data from waves and data from the base film. The resulting velocity profiles are compared with the classical Law of the Wall turbulent boundary layer model and found to require a decreased turbulent diffusivity for the model to predict well. These results agree with hypotheses previously presented in the literature.

A multiphase, micro-scale PIV measurement technique for liquid film velocity measurements in annular two-phase flow

Prediction methods for two-phase annular flow require accurate knowledge of the velocity profile within the liquid film flowing at its perimeter as the gradients within this film influence to a large extent the overall transport processes within the entire channel. This film, however, is quite thin and variable and traditional velocimetry methods have met with only very limited success in providing velocity data. The present work describes the application of Particle Image Velocimetry (PIV) to the measurement of velocity fields in the annular liquid flow. Because the liquid is constrained to distances on the order of a millimeter or less, the technique employed here borrows strategies from micro-PIV, but micro-PIV studies do not typically encounter the challenges presented by annular flow, including very large velocity gradients, a free surface that varies in position from moment to moment, the presence of droplet impacts and the passage of waves that can be 10 times the average thickness of the base film. This technique combines the seeding and imaging typical to micro-PIV with a unique lighting and image processing approach to deal with the challenges of a continuously varying liquid film thickness and interface. Mean velocity data are presented for air–water in two-phase co-current upward flow in a rectangular duct, which are the first detailed velocity profiles obtained within the liquid film of upward vertical annular flow to the authors’ knowledge. The velocity data presented here do not distinguish between data from waves and data from the base film. The resulting velocity profiles are compared with the classical Law of the Wall turbulent boundary layer model and found to require a decreased turbulent diffusivity for the model to predict well. These results agree with hypotheses previously presented in the literature.

Application of Horizontal-to-Vertical Spectral Ratios of Earthquake Ground Motions to Identify Subsurface Structures at and around the K-NET Site in Tohoku, Japan

Abstract We propose an optimal way to use horizontal‐to‐vertical spectral ratios (HVRs) for subsurface structure exploration, based on the diffuse field concept (Kawase et al. , 2011; Sanchez‐Sesma et al. , 2011). This approach is applicable to both earthquake and microtremor ground motions. We show here analyses of the observed ground‐motion data at and around a K‐NET station in Miyagi Prefecture, Japan, where very large peak horizontal ground acceleration was observed during the earthquake of 11 March 2011 off the Pacific coast of Tohoku, Japan. We compare HVRs of the strong motions for the mainshock and the largest peak acceleration aftershock with those averaged over tens of weak motions to observe soil nonlinearity effects on the HVRs. Then, we determine detailed velocity profiles from the HVRs at the K‐NET Tsukidate station and the temporary aftershock observation sites. We find that HVRs can be explained quite well by the identified velocity profiles at all the target sites. The observed peak at 9 Hz for the averaged weak‐motion data originates in the topmost layers lying over the engineering bedrock.

Forced Convection from a Heated Equilateral Triangular Cylinder in Bingham Plastic Fluids

The momentum and forced convection heat transfer characteristics of a heated equilateral triangular cylinder immersed in a Bingham plastic fluid have been studied numerically. The governing equations (continuity, momentum, and thermal energy) are solved for both vertex-upstream and vertex-downstream orientations, over wide ranges of the pertinent parameters, such as Reynolds number: 0.1 ≤ Re ≤30; Prandtl number: 1 ≤ Pr ≤100; and Bingham number: 0 ≤ Bn ≤200. Over the range of conditions, the flow is expected to be steady and symmetric. Detailed analysis of the flow and heat transfer phenomena in the vicinity of the cylinder is performed by a thorough inspection of the streamline and isotherm contours. Furthermore, due to the presence of the yield stress, the flow domain consists of yielded (or fluid-like) and unyielded (or solid-like) zones. The effect of Reynolds number and Bingham number on the shape and size of these zones has been thoroughly examined in terms of the detailed velocity and shear rate profiles. At the next level, the functional dependence of the drag and Nusselt number on the Reynolds number, Bingham number, and Prandtl number is explored and developed. The heat transfer results spanning the above-noted ranges of parameters are consolidated by developing a correlation in terms of the Colburn j h factor as a function of the modified Reynolds number.

Quantifying Water Flow within Aquatic Ecosystems Using Load Cell Sensors: A Profile of Currents Experienced by Coral Reef Organisms around Lizard Island, Great Barrier Reef, Australia

Current velocity in aquatic environments has major implications for the diversity, abundance and ecology of aquatic organisms, but quantifying these currents has proven difficult. This study utilises a simple and inexpensive instrument (<$150) to provide a detailed current velocity profile of the coral-reef system around Lizard Island (Great Barrier Reef, Australia) at a spatial and temporal scale relevant to the ecology of individual benthos and fish. The instrument uses load-cell sensors to provide a correlation between sensor output and ambient current velocity of 99%. Each instrument is able to continuously record current velocities to >500 cms−1 and wave frequency to >100 Hz over several weeks. Sensor data are registered and processed at 16 MHz and 10 bit resolution, with a measuring precision of 0.06±0.04%, and accuracy of 0.51±0.65% (mean ±S.D.). Each instrument is also pressure rated to 120 m and shear stresses ≤20 kNm−2 allowing deployment in harsh environments.The instrument was deployed across 27 coral reef sites covering the crest (3 m), mid-slope (6 m) and deep-slope (9 m depth) of habitats directly exposed, oblique or sheltered from prevailing winds. Measurements demonstrate that currents over the reef slope and crest varies immensely depending on depth and exposure: Currents differ up to 9-fold within habitats only separated by 3 m depth and 15-fold between exposed, oblique and sheltered habitats. Comparisons to ambient weather conditions reveal that currents around Lizard Island are largely wind driven. Zero to 22.5 knot winds correspond directly to currents of 0 to >82 cms−1, while tidal currents rarely exceed 5.5 cms−1. Rather, current velocity increases exponentially as a function of wave height (0 to 1.6 m) and frequency (0.54 to 0.20 Hz), emphasizing the enormous effect of wind and waves on organisms in these shallow coral reef habitats.

Velocity profile variations in granular flows with changing boundary conditions: insights from experiments

We present results of detailed velocity profile measurements in a large series of granular flow experiments in a dam-break setup. The inclination angle, bead size, and roughness of the running surface were varied. In all experiments, the downstream velocity profiles changed continuously from the head to the tail of the avalanches. On rough running surfaces, an inflection point developed in the velocity profiles. These velocity profiles cannot be modeled by the large class of constitutive laws which relate the shear stress to a power law of the strain rate. The velocity profile shape factor increased from the head to the tail of the avalanches. Its maximum value grew with increasing roughness of the running surface. We conclude that flow features such as velocity profiles are strongly influenced by the boundary condition at the running surface, which depends on the ratio of bead size to the typical roughness length of the surface. Furthermore, we show that varying velocity profile shape factors inside gravitationally driven finite-mass flows give rise to an additional term in the depth-averaged momentum equation, which is normally solved in the simulation software of hazardous geophysical flows. We therefore encourage time dependent velocity profile measurements inside hazardous geophysical flows, to learn about the importance of this “new” term in the mathematical modeling of these flows.

Shear velocity estimates in rough‐bed open‐channel flow

ABSTRACTShear velocity u* is an important parameter in geophysical flows, in particular with respect to sediment transport dynamics. In this study, we investigate the feasibility of applying five standard methods [the logarithmic mean velocity profile, the Reynolds stress profile, the turbulent kinetic energy (TKE) profile, the wall similarity and spectral methods] that were initially developed to estimate shear velocity in smooth bed flow to turbulent flow over a loose bed of coarse gravel (D50 = 1·5 cm) under sub‐threshold conditions. The analysis is based on quasi‐instantaneous three‐dimensional (3D) full depth velocity profiles with high spatial and temporal resolution that were measured with an Acoustic Doppler Velocity Profiler (ADVP) in an open channel. The results of the analysis confirm the importance of detailed velocity profile measurements for the determination of shear velocity in rough‐bed flows. Results from all methods fall into a range of ± 20% variability and no systematic trend between methods was observed. Local and temporal variation in the loose bed roughness may contribute to the variability of the logarithmic profile method results. Estimates obtained from the TKE and Reynolds stress methods reasonably agree. Most results from the wall similarity method are within 10% of those obtained by the TKE and Reynolds stress methods. The spectral method was difficult to use since the spectral energy of the vertical velocity component strongly increased with distance from the bed in the inner layer. This made the choice of the reference level problematic. Mean shear stress for all experiments follows a quadratic relationship with the mean velocity in the flow. The wall similarity method appears to be a promising tool for estimating shear velocity under rough‐bed flow conditions and in field studies where other methods may be difficult to apply. This method allows for the determination of u* from a single point measurement at one level in the intermediate range (0·3 &lt; h &lt; 0·6). Copyright © 2013 John Wiley &amp; Sons, Ltd.

Detailed one-dimensional seismic velocity profiles beneath the Himalayan collision zone: evidence for a double Moho?

Seismicity in the Himalaya indicates that relatively deep earthquakes (focal depth 40∼100 km) occur in specific regions beneath the High Himalaya and Nepal. This study focuses on these specific regions to estimate the detailed velocity structure of the lower crust and upper mantle. We selected 202 earthquakes from the Himalaya Nepal Tibet Seismic Experiment (HIMNT) and relocated these earthquakes accurately by applying a genetic algorithm locator, GA-MHYPO (Kim et al., 2006). The detailed onedimensional P-velocity structure is estimated based on travel-time inversion using hypocentral parameters determined by GA-MHYPO. Computational results show two velocity anomalies exhibiting upper-mantle velocities at depth of about 45–50 and 60 km, respectively, beneath the High Himalayan region, whereas a single Moho exists at about 55 km depth beneath Nepal. To validate the stability of these results, a bootstrapping method was used for the inversion.

Near-Wake Turbulent Flow Structure and Mixing Length Downstream of a Fractal Tree

In order to study the turbulence structure behind a multiscale tree-like element in a boundary layer, detailed particle image velocimetry measurements are carried out in the near-wake of a fractal-like tree. The tree is a pre-fractal with five generations, each consisting of three branches and a scale-reduction factor of 1/2 between consecutive generations. Detailed mean velocity and turbulence stress profiles are documented, as well as their downstream development. Scatter plots of mean velocity gradient (transverse shear in the wake) and Reynolds shear stress exhibit a good linear relation at all locations in the flow. Therefore, in the transverse direction of the wake evolution, the data support the Boussinesq eddy-viscosity concept. The measured mixing length increases with streamwise distance, in agreement with classic wake expansion rates. Conversely, the measured eddy viscosity and mixing length in the transverse direction decrease with increasing elevation, which differs from the behaviours measured in the vertical direction in traditional boundary layers or in canopy flows studied before. In order to find an appropriate single length scale to describe the wake evolution behind a multiscale object, two models are proposed, based on the notion of superposition of scales. One approach is based on the radial spectrum of the object while the second is based on its length-scale distribution evaluated using fractal geometry tools. Both proposed models agree well with the measured mixing length. The results suggest that information about multiscale clustering of branches must be incorporated into models of the mixing length for flows through single or sparse canopies of multiscale trees.

IMPULSIVE ACCELERATION OF CORONAL MASS EJECTIONS. I. STATISTICS AND CORONAL MASS EJECTION SOURCE REGION CHARACTERISTICS

We use high time cadence images acquired by the STEREO EUVI and COR instruments to study the evolution of coronal mass ejections (CMEs) from their initiation through impulsive acceleration to the propagation phase. For a set of 95 CMEs we derived detailed height, velocity, and acceleration profiles and statistically analyzed characteristic CME parameters: peak acceleration, peak velocity, acceleration duration, initiation height, height at peak velocity, height at peak acceleration, and size of the CME source region. The CME peak accelerations we derived range from 20 to 6800 m s−2 and are inversely correlated with the acceleration duration and the height at peak acceleration. Seventy-four percent of the events reach their peak acceleration at heights below 0.5 R☉. CMEs that originate from compact sources low in the corona are more impulsive and reach higher peak accelerations at smaller heights. These findings can be explained by the Lorentz force, which drives the CME accelerations and decreases with height and CME size.

Crust and upper mantle structures of the region between Korea and Taiwan by surface wave dispersion study

We have investigated the crust and upper-mantle shear velocity structures of the region between Korea and Taiwan by analyzing the path-averaged group-velocity dispersion characteristic curves derived from surface waves. The depth of the East China Sea between Korea and Taiwan is mostly less than 100 m. We selected data from earthquakes of magnitude greater than 6.0 that occurred in Taiwan from 1999 to 2007. We used data from 219 seismograms recorded at three-component broadband seismic stations in South Korea. We also used data from 19 events from South Taiwan for our study of crust and upper-mantle shear-wave velocity structures. By examining the wave period between 5 s and 100 s, the group velocities of Rayleigh waves and Love waves were inverted jointly. These waves were computed using the multiple filter technique (MFT) and using the single-station method. The inversion results from the dispersion curves provide a detailed one-dimensional velocity profile of the East China Sea. The main features of the derived velocity profile are: 1) the unconsolidated sediments surface layer shows a fairly low shear-wave velocity value; 2) the crustal shear-wave velocity increases from 2.92 km/s to 3.90 km/s, as the depth increases to 30 km; 3) shear-wave velocities in the uppermost mantle exhibit a clear low-velocity zone between 50 km and 90 km, with the velocity varying between 4.33 km/s and 3.99 km/s; 4) the variation in shear-wave velocities in the upper mantle increases with increasing depth. We can infer that the crustal structure of the study area is continental crust. The average Moho depth is 30 km beneath the East China Sea.