Local Velocity Profiles Research Articles

Effect of products of PLA2 catalyzed hydrolysis of DLPC on motion of rising bubbles

Local velocities of rising bubbles decrease with the increasing concentration in solution of surface-active, water-soluble species. Therefore, it is possible to use this phenomenon to monitor products of enzymatic reactions, which meet such criteria. In this study, hydrolysis of 1,2-dilauroyl-sn-glycero-3-phosphatidylcholine (DLPC) catalyzed by calcium-dependent phospholipase A2 (PLA2) (EC3.1.1.4) from porcine pancreas was used as model reaction. The products of this reaction are lauric acid (LA) and 1-lauroyl-2-hydroxy-sn-glycero-3-phosphatidylcholine (Lyso-PC). DLPC was dispersed in a chloroform/methanol mixture that was spread on a free PLA2 solution surface. Air bubbles were then formed at a capillary orifice and the local velocity of rising bubbles as a function of the distance from the capillary tip was monitored. Local velocity profiles were compared with profiles recorded for solutions of pure enzymatic reaction products and their mixtures. Our experiments showed that the product, which had a dominating effect on bubble motion retardation, was lyso-phosphatidylcholine. This can be explained by differences in the kinetics of lauric acid and lyso-phosphatidylcholine transfer from the spread layer to the solution.

Constraint strength and axial/radial particle velocity profiles for an integrated riser outlet

To study axial/radial profiles of particle velocity in the affected region of an integrated riser outlet, a cold model was developed for the integrated riser reactor combining the gas–solid distributor with the fluidized bed. Constraints, related to the gas–solid distributor and the upper fluidized bed, imposed on the particle flow in the riser outlet region, were investigated experimentally. The experimental results showed that with increasing superficial gas velocity, these constraints have strong influences on particle flow behavior, the particle circulation flux in the riser, and the height of the static bed material of the upper fluidized bed. When the constraints have greater prominence, the axial profile of the cross-sectionally averaged particle velocity in the outlet region initially increases and then decreases, the rate of decrease being proportional to the constraint strength. Along the radial direction of the outlet section, the region where the local particle velocity profile tends to decrease appears near the dimensionless radius r/R=0.30 initially and then, with increasing constraint strength, gradually extends to the whole section from the inner wall. Based on the experimental data, an empirical model describing the constraint strength was established. The average relative error of the model is within 7.69%.

Sand-assisted fluidization of large cylindrical and spherical biomass particles: Experiments and simulation

In this study, bubbling fluidization of a sand fluidized bed with different biomass loadings are investigated by means of the experiments and numerical simulation. The radioactive particle tracking (RPT) technique is employed to explore the impact of the particle shape factor on the biomass distribution and velocity profiles when it is fluidized in a 152mm diameter bed with a 228mm static height. Using a pair of fiber optic sensors, the bubbling characteristics of these mixtures at the upper half of the dense bed are determined at superficial gas velocities ranging from U=0.2m/s to U=1.0m/s. The experimental results show that despite cycling with a similar frequency, spherical biomass particles rise faster and sink slower than the cylindrical biomass particles. Furthermore, bubbles are more prone to break in the presence of biomass particles with lower sphericity. In the separate series of experiments, the reliability of the “frozen bed” technique to quantify the axial distribution of biomass particles is assessed by the RPT results. Using NEPTUNE_CFD software, three-dimensional numerical simulations are carried out via an Eulerian n-fluid approach. Validation of the simulation results with the experiments demonstrates that, in general, simulation satisfactorily reproduces the key fluidization and mixing features of biomass particles such as the global and local time-average distribution and velocity profiles.

Unsteady stress partitioning and momentum transfer in the wave bottom boundary layer over movable rippled beds

AbstractObservations of the nearbed velocity field over a rippled sediment bed under asymmetric wave forcing conditions were collected using a submersible particle image velocimetry (PIV) system. To examine the role of bed form‐induced dynamics in the total momentum transfer, a double‐averaging technique was implemented on the two‐dimensional time‐dependent velocity field by means of the full momentum equation. This approach allows for direct determination of the bed form‐induced stresses, i.e., stresses that arise due to the presence of bed forms, which are zero in flat bed conditions. This analysis suggests that bed form‐induced stresses are closely related to the presence of coherent motions and may be partitioned from the turbulent stresses. Inferences of stress provided by a bed load transport model suggest that total momentum transfer obtained from the double‐averaging technique is capable of reproducing bed form mobilization. Comparisons between the total momentum transfer and stress estimates obtained from local velocity profiles show significant variability across the ripple and suggest that an array of sensors is necessary to reproduce bed form evolution. The imbalance of momentum obtained by resolving the different terms constituting the near‐bed momentum balance (i.e., acceleration deficit, stress gradient, and bed form‐induced skin friction) provides an estimate of the bed form‐induced pressure that is consistent with flow separation. This analysis reveals three regions in the flow: the free‐stream, where all terms are relatively balanced; the near‐bed, where momentum imbalance is significant during flow weakening; and below ripple crests, where bed form‐induced pressure is the leading order mechanism.

Exploring the phase diagram of fully turbulent Taylor–Couette flow

AbstractDirect numerical simulations of Taylor–Couette flow, i.e. the flow between two coaxial and independently rotating cylinders, were performed. Shear Reynolds numbers of up to $3\times 10^{5}$, corresponding to Taylor numbers of $\mathit{Ta}=4.6\times 10^{10}$, were reached. Effective scaling laws for the torque are presented. The transition to the ultimate regime, in which asymptotic scaling laws (with logarithmic corrections) for the torque are expected to hold up to arbitrarily high driving, is analysed for different radius ratios, different aspect ratios and different rotation ratios. It is shown that the transition is approximately independent of the aspect and rotation ratios, but depends significantly on the radius ratio. We furthermore calculate the local angular velocity profiles and visualize different flow regimes that depend both on the shearing of the flow, and the Coriolis force originating from the outer cylinder rotation. Two main regimes are distinguished, based on the magnitude of the Coriolis force, namely the co-rotating and weakly counter-rotating regime dominated by Rayleigh-unstable regions, and the strongly counter-rotating regime where a mixture of Rayleigh-stable and Rayleigh-unstable regions exist. Furthermore, an analogy between radius ratio and outer-cylinder rotation is revealed, namely that smaller gaps behave like a wider gap with co-rotating cylinders, and that wider gaps behave like smaller gaps with weakly counter-rotating cylinders. Finally, the effect of the aspect ratio on the effective torque versus Taylor number scaling is analysed and it is shown that different branches of the torque-versus-Taylor relationships associated to different aspect ratios are found to cross within 15 % of the Reynolds number associated to the transition to the ultimate regime. The paper culminates in phase diagram in the inner versus outer Reynolds number parameter space and in the Taylor versus inverse Rossby number parameter space, which can be seen as the extension of the Andereck et al. (J. Fluid Mech., vol. 164, 1986, pp. 155–183) phase diagram towards the ultimate regime.

Natural Convection Inside the Open Square Enclosure with Diagonally Placed Twin Square Blocks

A finite difference-based numerical study of steady laminar natural convection inside the ventilated square enclosure having two square cylindrical blocks placed diagonally is presented. The fluid considered is air with a Prandtl number of 0.71. The buoyancy enabled cold air flows from bottom of the left vertical wall to the diagonally opposite top of the right vertical wall by passing the protrusions in the form of two conductive square blocks placed across the through flow. All the exterior circumferential walls are considered as hot. The convective heat transfer inside the enclosure with the different sizes of square blocks is numerically analyzed. The convective heat transfer mechanism inside the enclosure is further modified through the Rayleigh number as 104, 105 and 106. The hydrodynamic block effect due to the square blocks affects the formation and geometry of the vortices and also influences the heat transfer intensities across the enclosure. The hydrodynamic blockage has huge effect on the downstream side of the block than the upstream side. The size of the blocks has great impact on the intensity of heat transfer. The complex physics is illustrated with streamlines, isotherms, local Nusselt number and velocity profiles.

Micro-PIV analysis of gas–liquid Taylor flow in a vertical oriented square shaped fluidic channel

Taylor bubbles correspond to a sequence of gas bubbles surrounded by liquid, which are encountered in many practical applications. Besides their dimensions (e.g. bubble length) and the distance between consecutive Taylor bubbles (i.e. slug length) the main characteristics, rather advantageous, for investigating Taylor bubbles is their reproducibility. So far, most experimental investigations are based on circular cross sections. In addition, most investigations deal with the flow field in the liquid slug only. Little is known about the velocity profile in the liquid film between the bubble and the channel wall. The observations presented here are based on square shaped channels. Moreover, this study is based on a combination of experimental observations and numerical simulations. The experimental setup is vertically oriented and a special injection valve with an extraordinary compensation pipe is tailored to generate steady and reproducible Taylor bubbles. Furthermore, refractive index matching (RIM) is used to avoid refraction caused by curved surfaces. The use of RIM enables very accurate micro Particle Image Velocimetry (μPIV) observations of the flow fields in the liquid slug and in the lateral liquid film. The μPIV observations and the numerical simulations both indicate a recirculation flow pattern in the liquid slug. Additionally, both observations indicate a Couette shaped local velocity profile within the liquid film. The no-slip boundary condition is assumed at the gas–liquid interface in the liquid film. This condition leads to the existence of a toroidal circulation within the Taylor bubble, which is similar to the Rybczynski–Hadamard circulation. Both, the experimental observations and the numerical simulations show good agreement for the flow fields in the liquid slug and the liquid film.

Experimental investigation of viscoplastic free-surface flows in a steady uniform regime

AbstractWe present experimental results focused on the hydraulic properties of free-surface flows of viscoplastic fluids. The objective is to investigate the possibility of predicting macroscopic flow properties on the base of conventional rheometrical characterization of the fluids. The experiments are performed in an inclined conveyor-belt channel allowing us to generate gravity-driven surges which remain stationary in the laboratory frame. Two different types of materials are studied: Kaolin slurries and Carbopol microgels. Global height–velocity relationships and local velocity profiles are measured in the uniform zone for different experimental conditions (slope angle, rheological parameters). These data are then compared to theoretical predictions based on the Herschel–Bulkley constitutive law and independent measurements of the rheological parameters. Great care has been devoted to the determination of experimental uncertainties, including those associated with the rheometrical characterization. For Kaolin, the experimental results show excellent agreement with theoretical predictions. With Carbopol, on the contrary, a systematic discrepancy between measured and theoretical flow heights is observed. The velocity profiles do nevertheless remain consistent with a Herschel–Bulkley rheology, and we show that all experimental data can be explained by increasing the rheological parameters (yield stress and consistency) by 10–20 % compared to the values measured in the rheometer. Potential interpretations for this discrepancy are discussed.

Buoyancy enhanced natural convection inside the ventilated square enclosure with a partition and an overhanging transverse baffle

Finite difference based numerical studies of steady laminar natural convection inside the open square enclosure with protrusions are presented. The fluid considered is air with a Prandtl number of 0.71. The buoyancy enabled cold air flows from bottom-left vertical wall to the diagonally opposite top-right vertical wall crossing the obstacles in the form of a partition and transverse baffles. All the external peripheral walls are considered as hot and the interior cross walls are assumed as cold. The effect of obstacles on the heat transfer is numerically investigated. The convective heat transfer mechanism is enhanced further through the Rayleigh number by varying Ra as 103, 104 and 105. The subtle variation in buoyancy, momentum, hydrodynamic blockage and protrusions influences the formation, size and geometry of the vortices and also influences the heat transfer intensities inside the enclosure. The intriguing physics is elucidated with streamlines, isotherms, Local Nusselt number and velocity profiles. The study reveals that the introduction of cold baffle intensifies the heat transfer inside the enclosure irrespective of its position and length. The hydrodynamic blockage effect is dominant when the transverse baffle is positioned on the right side of the partition.

Solar Radiation Assisted Mixed Convection Flow Along a Vertical Plate

ABSTRACTSolar radiation assisted mixed convection boundary layer flow of Newtonian fluid along a non- reflecting, non-absorbing and ideally transparent semi-infinite vertical plate is studied here. Beer's law is used to express the solar radiation term. It is convenient to transform the non-linear dimensionless parabolic partial differential equations into (i) primitive variable formulation (PVF) and (ii) stream function formulation (SFF) before applying the numerical schemes. Coupled equations thus obtained from PVF are integrated numerically through implicit finite difference method together with the Gaussian elimination technique whereas block tridiagonal Keller-box technique is adopted to simulate the system of equations obtained from SFF. Numerical results from these two methods are also compared graphically in order to test the validation of the two schemes. However, due to less computation time and accuracy numeric results of shear stress, local Nusselt number coefficient, velocity and temperature profiles are obtained via SFF. It is found that velocity as well as temperature of the non-absorbing fluid enhances owing to the increase in solar radiation parameter.

Scale effects in gas nano flows

Most previous studies on gas transport in nano-scale confinements assume dynamic similarity with rarefied gas flows, and employ kinetic theory based models. This approach is incomplete, since it neglects the van der Waals forces imposed on gas molecules by the surfaces. Using three-dimensional molecular dynamics (MD) simulations of force driven gas flows, we show the significance of wall force field in nano-scale confinements by defining a new dimensionless parameter (B) as the ratio of the wall force-penetration length to the channel height. Investigation of gas transport in different nano-channels at various Knudsen numbers show the importance of wall force field for finite B values, where the dynamic similarity between the rarefied and nano-scale gas flows break down. Comparison of MD results employing molecularly structured three-dimensional walls versus reflection of gas molecules from a two-dimensional planar surface with Maxwell distribution show that the nano-confinement effects cannot be resolved by the latter approach, frequently used in kinetic theory calculations. Molecularly structured walls determine the bulk flow physics by setting a proper tangential momentum accommodation coefficient, and they also determine the transport in the near wall region. Gas nano-flows with finite B exhibit significant differences in the local density and velocity profiles, affecting the mass flow rate and the formation of Knudsen's minimum in nano-channels.

Optimal Taylor–Couette flow: radius ratio dependence

AbstractTaylor–Couette flow with independently rotating inner ($i$) and outer ($o$) cylinders is explored numerically and experimentally to determine the effects of the radius ratio $\eta $ on the system response. Numerical simulations reach Reynolds numbers of up to $\mathit{Re}_i=9.5\times 10^3$ and $\mathit{Re}_o=5\times 10^3$, corresponding to Taylor numbers of up to $\mathit{Ta}=10^8$ for four different radius ratios $\eta =r_i/r_o$ between 0.5 and 0.909. The experiments, performed in the Twente Turbulent Taylor–Couette ($\mathrm{T^3C}$) set-up, reach Reynolds numbers of up to $\mathit{Re}_i=2\times 10^6$ and $\mathit{Re}_o=1.5\times 10^6$, corresponding to $\mathit{Ta}=5\times 10^{12}$ for $\eta =0.714\mbox{--}0.909$. Effective scaling laws for the torque $J^{\omega }(\mathit{Ta})$ are found, which for sufficiently large driving $\mathit{Ta}$ are independent of the radius ratio $\eta $. As previously reported for $\eta =0.714$, optimum transport at a non-zero Rossby number $\mathit{Ro}=r_i |\omega _i-\omega _o |/[2(r_o-r_i)\omega _o]$ is found in both experiments and numerics. Here $\mathit{Ro}_{opt}$ is found to depend on the radius ratio and the driving of the system. At a driving in the range between $\mathit{Ta}\sim 3\times 10^{8}$ and $\mathit{Ta}\sim 10^{10}$, $\mathit{Ro}_{opt}$ saturates to an asymptotic $\eta $-dependent value. Theoretical predictions for the asymptotic value of $\mathit{Ro}_{opt}$ are compared to the experimental results, and found to differ notably. Furthermore, the local angular velocity profiles from experiments and numerics are compared, and a link between a flat bulk profile and optimum transport for all radius ratios is reported.

Simulation of windblown dust transport from a mine tailings impoundment using a computational fluid dynamics model.

Mining operations are potential sources of airborne particulate metal and metalloid contaminants through both direct smelter emissions and wind erosion of mine tailings. The warmer, drier conditions predicted for the Southwestern US by climate models may make contaminated atmospheric dust and aerosols increasingly important, due to potential deleterious effects on human health and ecology. Dust emissions and dispersion of dust and aerosol from the Iron King Mine tailings in Dewey-Humboldt, Arizona, a Superfund site, are currently being investigated through in situ field measurements and computational fluid dynamics modeling. These tailings are heavily contaminated with lead and arsenic. Using a computational fluid dynamics model, we model dust transport from the mine tailings to the surrounding region. The model includes gaseous plume dispersion to simulate the transport of the fine aerosols, while individual particle transport is used to track the trajectories of larger particles and to monitor their deposition locations. In order to improve the accuracy of the dust transport simulations, both regional topographical features and local weather patterns have been incorporated into the model simulations. Results show that local topography and wind velocity profiles are the major factors that control deposition.

CFD application to advanced design for high efficiency spacer grid

Pressurized water reactor (PWR) fuels have been developed to meet the needs of the market. A spacer grid is a key component to improve thermal hydraulic performance of a PWR fuel assembly. Mixing structures (vanes) of a spacer grid promote coolant mixing and enhance heat removal from fuel rods. A larger mixing vane would improve mixing effect, which would increase the departure from nucleate boiling (DNB) benefit for fuel. However, the increased pressure loss at large mixing vanes would reduce the coolant flow at the mixed fuel core, which would reduce the DNB margin. The solution is to develop a spacer grid whose pressure loss is equal to or less than the current spacer grid and that has higher critical heat flux (CHF) performance. For this reason, a requirement of design tool for predicting the pressure loss and CHF performance of spacer grids has been increased.The author and co-workers have been worked for development of high efficiency spacer grid using Computational Fluid Dynamics (CFD) for nearly 20 years. A new laser Doppler velocimetry (LDV), which is miniaturized with fiber optics embedded in a fuel cladding, was developed to investigate the local velocity profile in a rod bundle and inside a spacer grid. The rod-embedded fiber LDV (rod LDV) can be inserted in an arbitrary grid cell instead of a fuel rod, and has the advantage of not disturbing the flow field since it is the same shape as a fuel rod. The probe volume of the rod LDV is small enough to measure spatial velocity profile in a rod gap and inside a spacer grid. According to benchmark experiments such as flow velocity measurement using the rod LDV, CFD methodology that predicts flow field in a PWR fuel precisely has been developed and design information that utilizes for high efficiency spacer grid has been obtained.The new spacer grid that designed using the CFD methodology has almost equal pressure loss and increased CHF performance compared to the current grid. The result of the study indicates that the CFD simulation can play a significant role in designing high efficiency PWR spacer grids.

Numerical study of turbulent flow in heated circular tube using transitional shear stress transport turbulence model

Low Reynolds number turbulent air flow moving upward in a vertical circular tube subjected to high heating rates was simulated using the transitional shear stress transport (SST) model for an inlet Reynolds number range of 4000 ≤ Rein ≤ 10,000 and a non-dimensional heating rate range of 0.001≤qw+≤0.005. The transitional SST model was validated by comparing pressure drop, Stanton number, and local velocity and temperature profiles with available experimental data and DNS results. The good agreement obtained between the transitional SST turbulence model predictions and the experimental data and DNS results shows that the transitional SST turbulence model is able to predict laminarization accurately. Parametric calculations using the validated transitional SST model were performed to obtain Nusselt number and pressure drop correlations. In addition, a laminarization criterion that is based on the attenuation of Reynolds shear stress due to flow laminarization is proposed. The laminarization criterion predicts a reduction in the Nusselt number as compared to the constant property results, ranging from as much as 62% reduction for Rein = 4000–23% reduction for Rein = 10,000.

Mixed convection stagnation-point flow on vertical stretching sheet with external magnetic field

The problem of steady laminar magnetohydrodynamic (MHD) mixed convection stagnation-point flow of an incompressible viscous fluid over a vertical stretching sheet is studied. The effect of an externally magnetic field is taken into account. The transformed boundary layer equations are solved numerically by using an implicit finite-difference scheme. Numerical results are obtained for various values of the mixed convection parameter, Hartmann number, and Prandtl number. The effects of an externally magnetic field on the skin friction coefficient, local Nusselt number, velocity, and temperature profiles for both A > 1 and A < 1, where A is the velocity ratio parameter, are presented graphically and discussed in detail. Both assisting and opposing flows are considered, and it is found that dual solutions exist for the opposing flow.

Effects of Temperature Dependent Thermal Conductivity on Natural Convection Flow Along a Vertical Wavy Cone with Heat Flux

A steady natural convection along a vertical cone with uniform surface heat flux for temperature dependent thermal conductivity k (T) has been investigated numerically. Using the appropriate variables the basic equations are transformed to non-dimensional boundary layer equations and then solved employing marching order implicit finite difference method. In this paper, attention is mainly focused to the evolution of the surface shear stress in terms of local skin-friction and the rate of heat transfer in terms of local Nusselt number, velocity and temperature profiles, velocity vector field, streamlines and isotherms for a wide range of values of thermal conductivity variation parameter. The present numerical results are compared with the available published results which show a good agreement indeed.

Experimental Investigation of Ion Mass Dependency of Magnetic Reconnection Outflow by using High Precision Ion Doppler Spectroscopy

Doppler spectroscopy diagnostics with a new reconstruction method has been developed to evaluate local ion velocity profile of magnetic reconnection outflow. The multi-chord ion Doppler diagnostic can simultaneously measure spectral profiles of line-integrated emission from 90 chords across the plasma using three monochromators and three intensified CCD cameras. Under an assumption of axisymmetry, 2-dimensional profiles of reconnection outflow ion velocity are achieved with satisfactory accuracy by a vector-based tomography technic for the first time.

Validation of numerical flow simulations against in vitro phantom measurements in different type B aortic dissection scenarios

An aortic dissection (AD) is a serious condition defined by the splitting of the arterial wall, thus generating a secondary lumen [the false lumen (FL)]. Its management, treatment and follow-up are clinical challenges due to the progressive aortic dilatation and potentially severe complications during follow-up. It is well known that the direction and rate of dilatation of the artery wall depend on haemodynamic parameters such as the local velocity profiles, intra-luminal pressures and resultant wall stresses. These factors act on the FL and true lumen, triggering remodelling and clinical worsening. In this study, we aimed to validate a computational fluid dynamic (CFD) tool for the haemodynamic characterisation of chronic (type B) ADs. We validated the numerical results, for several dissection geometries, with experimental data obtained from a previous in vitro study performed on idealised dissected physical models. We found a good correlation between CFD simulations and experimental measurements as long as the tear size was large enough so that the effect of the wall compliance was negligible.

Multiscale image‐based modeling and simulation of gas flow and particle transport in the human lungs

Improved understanding of structure and function relationships in the human lungs in individuals and subpopulations is fundamentally important to the future of pulmonary medicine. Image-based measures of the lungs can provide sensitive indicators of localized features, however to provide a better prediction of lung response to disease, treatment, and environment, it is desirable to integrate quantifiable regional features from imaging with associated value-added high-level modeling. With this objective in mind, recent advances in computational fluid dynamics (CFD) of the bronchial airways-from a single bifurcation symmetric model to a multiscale image-based subject-specific lung model-will be reviewed. The interaction of CFD models with local parenchymal tissue expansion-assessed by image registration-allows new understanding of the interplay between environment, hot spots where inhaled aerosols could accumulate, and inflammation. To bridge ventilation function with image-derived central airway structure in CFD, an airway geometrical modeling method that spans from the model 'entrance' to the terminal bronchioles will be introduced. Finally, the effects of turbulent flows and CFD turbulence models on aerosol transport and deposition will be discussed.