Fluid Dynamic Forces Research Articles

Application of Lattice-Boltzmann Method for Analysing Detachment of Micron-Sized Particles from Carrier Particles in Turbulent Flows

Numerical calculations based on the Lattice-Boltzmann method were performed for a particle cluster consisting of a large spherical carrier particle covered with hundreds of small spherical drug particles. This cluster, fixed in space within a cubic computational domain, was exposed to turbulent plug airflow with predefined intensity. Such a situation is found in dry powder inhalers where carrier particles blended with fine drug powder are dispersed in a highly turbulent flow with the objective of detaching the drug powder for pulmonary delivery. Turbulence was generated by a digital filtering technique applied to the inflow velocity boundary condition. This technique was first validated by analysing the turbulence intensity at 15 fluid nodes along the stream-wise direction of the computational domain. The size ratio between the drug and carrier particle was 5 μ m/100 μ m, and the coverage degree of the carrier by the small particles was 50%, which is a typical value for carrier particle blending. The range of carrier particle Reynolds numbers considered was between 80 and 200, typical values found in inhaler devices. Exemplarily, at Re = 200 turbulence intensity was varied from 0.3% to 9.0%. The systematic increase of the mean flow (i.e. 80 < Re <200) resulted in varying turbulence intensities from 20 to 9%. These simulations provided the temporal evolution of the fluid dynamic forces on the drug particles in dependence of their angular position on the carrier in order to estimate the possibility of drug particle detachment. For turbulent conditions (i.e. Re = 200 and I = 9.0%) the maximum fluid forces on the drug particles were found to be about 10-times larger than found in laminar flow. The fluctuations in the forces were found to be higher than the flow velocity fluctuations due to the modification of the boundary layer around the cluster and instabilities triggered by the turbulent flow. There are three possibilities for detaching the drug powder, namely, through lift-off and sliding or rolling. Lift-off was found to be of minor importance due to the observed small normal fluid forces even at Re = 200 and I = 9.0%. The probability of sliding and rolling detachment in dependence of the angular position was estimated based on measured adhesion properties, i.e. van der Waals force, adhesion surface energy and friction coefficient. The remarkable rise of detachment probability for both effects due to the action of turbulence is an important finding of this study. In accordance with laminar flow, rolling detachment occurs before sliding, however in turbulent conditions over the entire carrier particle. The present studies improve the understanding of drug particle detachment from carrier particles in an inhaler device. The results will be the basis for developing Lagrangian detachment models that eventually should allow the optimisation of dry powder inhalators through computational fluid dynamics.

A novel Lagrangian agglomerate structure model

Agglomeration of fine particles in fluid flows is important for many processes in powder handling or production. In order to accurately perform numerical calculations of such processes models are needed which realistically describe particle agglomeration. For applications in the frame of an Euler/Lagrange approach such a model was developed which allows predicting the agglomerate structure and its behaviour in the point-particle approximation. The detection of collisions between primary particles or particles and agglomerates is realised by the stochastic collision model. The collision point on the particle or agglomerate surface is generated by accounting for the impact efficiency. The developed structure model is based on storing location vectors from the reference primary particle to all other particles collected in the agglomerate which is still treated as point-mass. Thereby the structure and porosity of the agglomerate is known and an equivalent agglomerate diameter (e.g. gyration diameter) is obtained. With this information the fluid dynamic forces acting on the agglomerates can be predicted correctly as well as the appropriate collision cross-section is used in the collision modelling. For testing the novel agglomerate structure model, fine particle agglomeration in homogeneous isotropic turbulence is calculated considering different properties of the primary particles or droplets. In the case of dry solid particles the van der Waals adhesion force is considered and based on an energy balance the possibility of particle sticking or rebound is calculated. Moreover, high viscous droplets are considered as primary particles, a situation also found in spray dying. For such cases the penetration of two primary particles or a primary particle and a stochastically selected primary particle within an agglomerate is modelled. The structure of the agglomerates produced under these different situations and for mono- and poly-sized primary particles are analysed and compared, using for example gyration diameter, porosity of convex hull and fractal dimension. These results demonstrate that the developed novel agglomerate structure model is applicable in the frame of an Euler/Lagrange approach and provides appropriate results.

Analysis of Flow Past Oscillatory Cylinders Using a Finite Element Fixed Mesh Formulation

In this work, we propose a fixed mesh finite element formulation to solve the fluid dynamic on an Eulerian mesh dealing with immersed bodies in motion. The study is focused on the computation of the fluid dynamic forces acting on immersed bodies which strongly depend on the evolution of the vortex shedding. The frequency of vortex detachment for flow past cylinder problems can be modified when the cylinder moves, promoting the modification of the wake of vortices. Synchronization phenomena appear when the frequencies of the resulting flow pattern coincide with the frequency of the imposed body motion. To study this problem, we propose to describe the immersed body surface by a collection of markers that moves according to the imposed body motion. The markers are updated using a Lagrangian scheme. In this framework, a distinct aspect of the present work is the imposition of the body velocity as an internal immersed boundary condition for the fluid dynamic analysis. To transfer the body velocity to the fluid along the fluid–solid interface, a restriction on the flow velocity is added into the weak form of the Navier–Stokes equations by means of a penalty technique. This work encompasses the study of flows past a crossflow, streamwise, and rotational oscillating cylinders. The results are satisfactorily compared with numerical data reported in the literature, showing a proper behavior for the analysis of long-term vibrating systems at low Reynolds numbers.

石井翼の非定常空力特性

石井翼の非定常空力特性

Dynamic stability analysis of a cylindrical shell subjected to swirling annular flow under thermal loads

Considering thermal loads, a dynamic stability analysis is presented for a flexible cylindrical shell conveying a viscous, incompressible, swirling fluid in the annulus between the inner shell and the outer shell in this paper. The inviscid fluid-dynamic forces associated with shell motions are treated in the frame of the potential flow theory. And the steady viscous forces are derived by using fully developed turbulent theory. The thermal loads are determined by the thermo-elastic theory. Shell motions are described by Flügge's thin shell equations, which are modified to incorporate the prestresses relating to the steady viscous forces. The theoretical analysis is conducted by the zero-level contour method and the Galerkin's method. This study shows that, for annular flow, the effect of viscosity renders the system more unstable. Fluid rotation strongly degrades the stability of the shell. The influences of a combined action of the viscous annular flow having two velocity components and the thermal loads on the stability of shells are discussed in detail. Also, the critical temperature rise is found.

Characterization of Particle Motion and Deposition Behaviour in Electro-Static Fields

As a prerequisite for studying and ultimately improving the powder coating process, particle motion and deposition effects within flow- and electro-static fields need to be thoroughly understood and thus characterized. In this context, a range of dimensionless groups is proposed and new means of characterization are presented. Considering the impact of electro-static, fluid-dynamic and gravity forces on coating particle motion, a triangle chart notation to characterize the state of varying particle size classes, is introduced. Furthermore a derivation of the dimensionless particle momentum equation is shown to lead to a dimensionless chart, representing all possible process states of coating. In combination with a Eulerian-LaGrangian, numerical model, the new means of characterization have led to a far better, over all perspective of occurring phenomena and their causes. Some examples are demonstrated here.

Euler-Lagrangian Model of Particle Motion and Deposition Effects in Electro-Static Fields based on OpenFoam

In order to study the powder coating process of metal substrates, a comprehensive, numerical 3D Eulerian-LaGrangian model, featuring two particle sub-models, has been developed. The model considers the effects of electro-static, fluid-dynamic and gravity forces. The code has been implemented in C++ within the open source CFD platform OpenFoam®, is transient in nature with respect to the applied LaGrangian particle implementation and the electro-static field calculation and is stationary regarding fluid-dynamic phenomena. Qualitative validation of the developed solver has already been achieved by comparison to simple coating experiments and will hereby be presented alongside a thorough description of the model itself. Upon combining knowledge of the relevant dimensionless groups and the numerical model, a dimensionless chart, representing all possible states of coating, was populated with comprehensive, exemplary cases, which are shown here as well.

Unique erosion features of hafnium cathode in atmospheric pressure arcs of air, nitrogen and oxygen

Experimental investigation of cathode erosion in atmospheric pressure hafnium-electrode plasma torches is reported under different plasma environments along with the results of numerical simulation. Air, nitrogen and oxygen are the plasma gases considered. Distinct differences in the erosion features in different plasmas are brought out. Cathode images exhibiting a degree of erosion and measured erosion rates are presented in detail as a function of time of arc operation and arc current. Physical erosion rates are determined using high precision balance. The changes in the surface microstructures are investigated through scanning electron microscopy (SEM). Evolution of cathode chemistry is determined using energy dispersive x-ray spectroscopy (EDX). Numerical simulation with proper consideration of the plasma effects is performed for all the plasma gases. The important role of electromagnetic body forces in shaping the flow field and the distribution of pressure in the region is explored. It is shown that the mutual interaction between fluid dynamic and electromagnetic body forces may self-consistently evolve a situation of an extremely low cathode erosion rate.

Green production of pristine graphene using fluid dynamic force in supercritical CO2

A green and scalable method was presented for the production of pristine graphene by the exfoliation of graphite using a shear mixer in supercritical CO2. The resulting graphene flakes were characterized by transmission electron microscopy, Raman spectroscopy and atomic force microscopy. The synergistic effects of the fluid dynamic force coupled with the supercritical CO2 actualized the exfoliation of graphite into graphene. The total exfoliation yield reached 63.2%. Up to 79% of the produced graphene was less than five layers, of which monolayer, bilayer and trilayer represented 27%, 25% and 14%, respectively. The average lateral size of the resulting graphene was 5μm. The Raman spectrum reveals that the graphene was defect free. The prepared graphene was utilized to fabricate flexible, transparent and conductive films on a PET substrate through a vacuum filtration and dry transfer method. The transparency of the film reached 83%, the resistance was 0.83kΩ/sq, and there was minimal variation, even after the film was bent more than 500 cycles.

Computational analysis of flow-driven string dynamics in turbomachinery

Abstract We focus on computational analysis of flow-driven string dynamics. The objective is to understand how the strings carried by a fluid interact with the solid surfaces present and get stuck on or around those surfaces. Our target application is turbomachinery, such as understanding how strings get stuck on or around the blades of a fan. The components of the method we developed for this purpose are the Space–Time Variational Multiscale (ST-VMS) and ST Slip Interface (ST-SI) methods for the fluid dynamics, and a one-way-dependence model and the Isogeometric Analysis (IGA) for the string dynamics. The ST-VMS method is the core computational technology and it also has the features of a turbulence model. The ST-SI method allows in a consistent fashion slip at the interface between the mesh covering a spinning solid surface and the mesh covering the rest of the domain, and with this, we maintain high-resolution representation of the boundary layers near spinning solid surfaces such as fan blades. With the one-way-dependence model, we compute the influence of the flow on the string dynamics, while avoiding the formidable task of computing the influence of the string on the flow, which we expect to be small. The IGA for the string dynamics gives us not only a higher-order method and smoothness in the structure shape, but also smoothness in the fluid dynamics forces calculated on the string. To demonstrate how the method can be used in computational analysis of flow-driven string dynamics, we present the pilot computations we carried out, for a duct with cylindrical obstacles and for a ventilating fan.

Modification of a Steam Valve Diffuser for Enhanced Full Load and Part Load Operation Using Numerical Methods

The flow in a steam turbine inlet valve is investigated and improved by numerical methods. From the fluid dynamic point of view two requirements exist: Low pressure losses are desired at the fully opened valve position and dynamic fluid forces acting on the valve plug should be minimized to reduce valve vibration. Usually these undesired dynamic fluid forces occur when the flow is throttled at part load. It is found that these fluid forces are generated by separated jets in the diffuser. The attachment and the separation of the jet are related to the Coanda effect. By understanding the flow physics a way is found to modify the diffuser design in such a way that the flow separations are reduced. Bell-shaped diffusers are able to reduce the flow losses at full load operation. A diffuser contour that fulfils both requirements is developed.

すべり軸受を有する鉛直ロータ系の自励振動の特性

In this paper, vibration characteristics of a vertical rotor system were investigated. Both the additional degree of freedom of the journal bearing support portion and rotor dynamic(RD) nonlinear fluid force by the journal bearing were considered, and their effects on the vibration characteristics were investigated, numerically. As a result, it was observed that an existence region of self-excited vibration was reduced by supporting the journal bearing flexibly. Moreover, influence of parameters; stiffness for the support part, a damping for the support part, and radial clearance, on the vibration phenomenon was clarified.

Microfluidic immunomagnetic cell separation from whole blood.

Immunomagnetic-based separation has become a viable technique for the separation of cells and biomolecules. Here we report on the design and analysis of a simple and efficient microfluidic device for high throughput and high efficiency capture of cells tagged with magnetic particles. This is made possible by using a microfluidic chip integrated with customized arrays of permanent magnets capable of creating large magnetic field gradients, which determine the effective capturing of the tagged cells. This method is based on manipulating the cells which are under the influence of a combination of magnetic and fluid dynamic forces in a fluid under laminar flow through a microfluidic chip. A finite element analysis (FEA) model is developed to analyze the cell separation process and predict its behavior, which is validated subsequently by the experimental results. The magnetic field gradients created by various arrangements of magnetic arrays have been simulated using FEA and the influence of these field gradients on cell separation has been studied with the design of our microfluidic chip. The proof-of-concept for the proposed technique is demonstrated by capturing white blood cells (WBCs) from whole human blood. CD45-conjugated magnetic particles were added into whole blood samples to label WBCs and the mixture was flown through our microfluidic device to separate the labeled cells. After the separation process, the remaining WBCs in the elute were counted to determine the capture efficiency, and it was found that more than 99.9% WBCs have been successfully separated from whole blood. The proposed design can be used for positive selection as well as for negative enrichment of rare cells.

Experimental study on flow past a circular cylinder with rough surface

Flow past a circular cylinder with smooth, grooved and dimpled surface (roughness coefficient k/D=0.05, where k is the depth of the grooves/dimples and D is the outer diameter of the cylinder) was investigated in an open channel and a towing tank. Two different sizes of cylinder (D=40 and 100mm) were considered in the present study. The Reynolds numbers based on the cylinder diameter ranged from 7.4×103 to 8×104. The fluid dynamic forces (drag and lift) on the cylinder were directly measured using a peizoelectric load cell, in conjunction with measurements of the wake flow pattern using digital Particle Image Velocimetry (PIV) for some cases. The results reveal that the grooved and dimpled cylinders produce a lower mean drag than that of a smooth cylinder. The strength of vertex shedding from the cylinder becomes weaker due to the effect of grooved or dimpled surfaces, as indicated by a smaller and narrower wake than that of the smooth cylinder.

Bacterial Hydrodynamics

Bacteria predate plants and animals by billions of years. Today, they are the world's smallest cells, yet they represent the bulk of the world's biomass and the main reservoir of nutrients for higher organisms. Most bacteria can move on their own, and the majority of motile bacteria are able to swim in viscous fluids using slender helical appendages called flagella. Low–Reynolds number hydrodynamics is at the heart of the ability of flagella to generate propulsion at the micrometer scale. In fact, fluid dynamic forces impact many aspects of bacteriology, ranging from the ability of cells to reorient and search their surroundings to their interactions within mechanically and chemically complex environments. Using hydrodynamics as an organizing framework, I review the biomechanics of bacterial motility and look ahead to future challenges.

Validation of fluid–particle interaction force relationships in binary-solid suspensions

In this work several relationships governing solid–fluid dynamic interaction forces were validated against experimental data for a single particle settling in a suspension of other smaller particles. It was observed that force relationships based on Lattice-Boltzmann simulations did not perform as well as other interaction types tested. Nonetheless, it is apparent that, in the case of a suspension of different particle types, it is important that the correct choice is made as to how the contribution to the overall fluid–particle interaction force is split between buoyancy and drag. Experimental evidence clearly suggests that the “generalized” Archimedes’ principle (where the foreign particle is considered to displace the whole suspension and not just the fluid) provides the best result.

Impacts of top-end vessel sway on vortex-induced vibration of the submarine riser for a floating platform in deep water

The dynamic coupling between moving top-end vessel and submarine riser becomes more remarkable for floating platform in deep water due to its larger amplitude of top-end motion, compared to fixed platform in shallow water. The impacts of top-end motion on the riser undergoing vortex-induced vibration (VIV) are explored in this study. A coupled hydrodynamic force approach, involving the vortex-induced lift force along with the fluid drag force, is developed, which takes into account the interaction between fluid dynamic force and instantaneous riser motion. Then the dynamic behaviors of the riser suffering both top-end motion and VIV are examined by means of finite element simulations. The effects of the amplitude and frequency of top-end vessel sway on riser׳s VIV are studied. During the riser׳s dynamic responses, an interesting phenomenon, called nonlinear response amplification, is observed, which demonstrates that top-end motion may be amplified as the motion propagates along riser length. Our numerical results show that the riser׳s displacement becomes several times larger than that of the case without top-end motion. Moreover, the nonlinear amplification gets more pronounced as the number of mode order drops, but the amplification factor just slightly changes with the increase of sway amplitude.

Forces on micron-sized particles randomly distributed on the surface of larger particles and possibility of detachment

Numerical calculations based on the Lattice-Boltzmann method were performed for a particle cluster consisting of a large carrier particle covered with small spherical drug particles which were exposed to a constant velocity air flow. Prior to these studies, the simulations were validated based on a test case where a single particle is situated on a plane wall and exposed to a linear shear flow. The present simulations were compared with analytical results and other simulations. Moreover, the required small particle resolution and the domain size were properly selected based on an extensive numerical study. The diameter of the carrier particles was 100μm, while the fine particles had diameters of 3μm and 5μm, respectively. The range of Reynolds numbers considered was between 1 and 200. Moreover, the coverage degree of the carrier by the small particles was varied in the simulations between 10% and 50%, and this influence on the detachment was determined. From these simulations the fluid dynamic forces on the drug particles were extracted in dependence of the angular position in order to estimate the possibility of drug particle detachment. Detachment might occur through lift-off, sliding or rolling. Lift-off was found to be not possible due to the acting small normal forces even at Re=200. The probability of sliding and rolling detachment in dependence of the angular position was estimated based on measured adhesion properties, i.e. van der Waals force, adhesion surface energy and friction coefficient. With these studies it is aimed to understand drug particle detachment from carrier particles in an inhaler device as a basis of modelling and optimising dry powder inhalation.

Calculation and analysis of fluid dynamic thrust force in main piping double-ended break of AP1000 secondary loop

Calculation and analysis of fluid dynamic thrust force in main piping double-ended break of AP1000 secondary loop

J2010203 放物運動の最適角 : 初等力学と最適化問題

The derivation of the maximum projection angle in projectile motion of a particle gives one of the most popular examples in elementary mechanics. This procedure is also a typical example of optimization problem although it has not been emphasized in many textbooks of mechanics. We formulate the problem on the basis of the optimization approach, and obtain the perturbed solution in terms of aerodynamic parameters. This solution yields the optimum angles in the following three examples: simple projection with vertical displacement between launch and landing positions, projection with initial speed depending on initial angle and also projection under the action of fluid dynamic forces such as drag and lift proportional to flight speed. These results show realistic angles in several projection phenomena appearing in our usual life, and successfully explain the mechanism. This fact means the significance of optimization approach in elementary mechanics, and it will be effective to introduce this concept to early education in mechanics.