Flow Drag Force Research Articles

Improved Nanobubble Immobility Induced by Surface Structures on Hydrophobic Surfaces

In fluid flow on hydrophobic surfaces, boundary slip occurs at the solid-liquid interface and nanobubbles on the surfaces are believed to be the reason for it. Boundary slip is of practical importance in micro/nanofluidics to reduce the drag force in fluid flow. However, nanobubbles tend to move under external disturbance. Therefore, the decreased degree of nanobubble movement (nanobubble immobility) is of interest. In this study, nanobubble immobility is studied on both continuously and partially coated polystyrene films. Experimental results show improved immobility on both surfaces. The nanoindents generated by nanobubbles after immersion in a liquid for a period of time on both films and island-like structures on the partially coated film are thought to be the reasons for improved immobility. A model is developed to reveal the role of nanoindents and island structures in the improvement of nanobubble immobility based on contact angle hysteresis and surface tension. Analysis shows that both structures increase the initial force needed to move nanobubbles. Hence, nanobubble immobility is improved on both surfaces as compared with smooth hydrophobic surfaces.

Magnetically immobilized frits for the preparation of packed columns used in capillary electrochromatography

Fabrication of porous frits to retain stationary phases is a critical issue in column preparation for capillary electrochromatography (CEC). In this work, porous frits were prepared by applying an external magnetic field to magnetically responsive particles placed inside a fused-silica capillary. Three batches of uniform magnetite spheres with particle diameters of 0.3, 0.4, and 0.6 μm and saturation magnetization values of 73.03, 74.41, and 77.83 emu/g, respectively, were used as frit particles and octadecyl- and phenyl-bonded silica gels were packed successfully into frit-containing capillaries. The performance of the resulting magnetically immobilized frits and packed columns was evaluated. The electroosmotic mobilities in capillaries containing outlet frit only were found to be reduced by 2–4% whereas the plate heights of an unretained marker increased by 30–50% as compared to those in open capillaries. These variations are believed to be associated with the inhomogeneities of the packed structure of the frits. The magnetically immobilized frits showed adequate mechanical strength to withstand the flow drag force, allowing separation in capillaries packed with 5-μm stationary phases up to 10–15 cm, thus rendering column efficiency and reproducibility comparable with those obtained with sintered frits. Taken together, retaining frits made of uniform magnetite particles serves as a viable alternative to sintered frits for column preparation, which offers several distinct advantages such as ease of preparation, improved durability as compared to sintered frits where the removal of the polyimide coating makes the packed column susceptible to breakage, and use of large-bore capillaries for semipreparative separations.

Cluster hydrodynamic model of the heat and mass transfer in crystal growth and its significance for space materials science

Two heat-and-mass transfer models are considered as applied to modeling of crystallization from melts in zero gravity, i.e., the cluster hydrodynamic and integral viscosity increase models which take into account the additional melt flow drag force near the phase interface. It was shown that the consideration of this force is required to correctly describe the anomalous impurity distributions in semiconductor single crystals grown under conditions of orbital space flight. The advantages of the first model were indicated. A new efficient method for solving ill-conditioned systems of finite-difference elliptic equations was developed. Calculations of experimental benchmarks with phase transition showed good agreement between calculated and experimental results.

LEUCOCYTE RECRUITMENT UNDER FLUID SHEAR: MECHANICAL AND MOLECULAR REGULATION WITHIN THE INFLAMMATORY SYNAPSE

1. Nature has evolved an exquisite system for regulation of leucocyte recruitment at sites of tissue inflammation. Mechanical energy translated to the red and white blood cells transports them from large arteries down to the microcirculation. 2. Neutrophils overcome the drag forces of blood flow by forming selectin and integrin adhesive bonds with the endothelium that coats the vessel wall. Leucocyte adhesion receptors have evolved unique mechanical and chemical properties that optimize for sequential binding and uptake of traction forces. 3. In the present brief review, we address how dispersive forces acting on a neutrophil in shear flow function to stabilize and synchronize bond formation within a macromolecular membrane complex we denote the inflammatory synapse.

A computational study of leukocyte adhesion and its effect on flow pattern in microvessels

Three-dimensional computational modeling and simulation are presented on the adhesive rolling of deformable leukocytes over a P-selectin coated surface in parabolic shear flow in microchannels. The computational model is based on the immersed boundary method for cell deformation and Monte Carlo simulation for receptor/ligand interaction. The simulations are continued for at least 1 s of leukocyte rolling during which the instantaneous quantities such as cell deformation index, cell/substrate contact area, and fluid drag remain statistically stationary. The characteristic ‘stop-and-go’ motion of rolling leukocytes, and the ‘tear-drop’ shape of adherent leukocytes as observed in experiments are reproduced by the simulations. We first consider the role of cell deformation and cell concentration on rolling characteristics. We observe that compliant cells roll slower and more stably than rigid cells. Our simulations agree with previous in vivo observation that the hydrodynamic interactions between nearby leukocytes affect cell rolling, and that the rolling velocity decreases inversely with the separation distance, irrespective of cell deformability. We also find that cell deformation decreases, and the cells roll more stably with reduced velocity fluctuation, as the cell concentration is increased. However, the effect of nearby cells on the rolling characteristics is found to be more significant for rigid cells than compliant cells. We then address the effect of cell deformability and rolling velocity on the flow resistance due to, and the fluid drag on, adherent leukocytes. While several earlier computational works have addressed this problem, two key features of leukocyte adhesion, such as cell deformation and rolling, were often neglected. Our results suggest that neglecting cell deformability and rolling velocity may significantly overpredict the flow resistance and drag force. Increasing the cell concentration is shown to increase the flow resistance and reduce the fluid drag. The reduced drag then results in slower and more stable rolling of the leukocytes with longer pause time and shorter step distance. But the increase/decrease in the flow resistance/fluid drag due to the increase in the cell concentration is observed to be more significant in case of rigid cells than compliant cells.

Flow structures and drag characteristics of a colony-type emergent roughness model mounted on a flat plate in uniform flow

The characteristics of flow structures around a colony-type emergent roughness model, hereafter called ‘colony model’, mounted on a flat plate in uniform flow and the drag coefficient C dc for the colony model are investigated by flow visualization, spectral analysis, velocity measurement and drag force measurement. Two types of colony models, each comprising seven equally spaced cylinders with grid or staggered arrangement are mounted on a water flume bed. The flow structure around the colony model changes depending on L / D and G / D ( L: space between neighboring cylinders, D: diameter of a cylinder, G: space between cylinders in the cross-stream direction). Two types of flow structures, a large-scale Kármán vortex street (LKV) behind the colony models and a primitive Kármán vortex street (PKV) behind the individual cylinders, are generated. LKV is formed along the shear layer for G / D < 0.4 . When G / D > 1.8 , PKV behind each cylinder is stably formed because of the decrease in difference between the velocity through the colony model and that of the detour flow. The velocity through the colony model, which plays a key role for the phenomenon change, strongly increases with G / D at first and then gradually. The tendency of the velocity curve changes around G / D = 1.8 for each arrangement. It almost coincides with the initiation of the formation of PKV. On the other hand, C dc for the colony model increases in the range of 0.08 < G / D < 0.75 ( 0.25 < L / D < 1 ) and 0 < G / D < 1 ( 1 < L / D < 3 ) for the grid and staggered arrangements, respectively. The changing point of the tendency in the C dc curve is located in the middle of the transition zone for the two vortex structures ( 0.4 < G / D < 1.8 ) . Moreover, the C dc value for the colony model is nearly constant, independent of the two arrangements, when G / D > 1.8 .

Surgical Myectomy Remains the Primary Treatment Option for Severely Symptomatic Patients With Obstructive Hypertrophic Cardiomyopathy

The evolving alcohol septal ablation versus surgical myectomy controversy represents a crossroad in the management of obstructive hypertrophic cardiomyopathy (HCM). Indeed, in this now polarized debate within the cardiovascular community, between the traditional and established (ie, surgery) and the new and percutaneous (ie, ablation), much is at stake for the HCM patient population. Furthermore, this issue has become increasingly important given the visibility recently afforded the pathophysiological significance and frequency of left ventricular (LV) outflow gradients in this disease.1,2 Response by Fifer p 206 In the course of this discussion, I will vigorously defend surgery as the primary treatment of choice when outflow obstruction (gradient ≥50 mm Hg at rest or with physiological exercise) produces heart failure symptoms refractory to maximal medical management (New York Heart Association functional classes III and IV).3,4 To this purpose, I will rely on the 50-year experience and substantial body of evidence available in HCM, as well as my own personal extensive association with and work in this disease spanning >30 years and several hundred publications—neither as a surgeon or interventional cardiologist nor with any particular allegiance to either discipline. The message expressed herein is prosurgery, but it is by no means antiablation, for this treatment modality has proved useful (although with a selective role) in the management of HCM. ### Historical Context When surgical septal myectomy (Table 1) was initially introduced in the early 1960s at several North American and European centers, it was regarded as revolutionary and has subsequently stood the test of time. The classic myectomy (Morrow operation)5 relieves obstruction by resection of a relatively small amount of muscle (2 to 5g) from the proximal ventricular septum, thereby widening the outflow tract and abolishing flow drag (or Venturi) forces that promote systolic contact between mitral valve and hypertrophied septum, resulting in immediate …

On the Wire Sweep Experiments and Predictions of Gold Wire for Semiconductor Wirebonding Technology

This paper studies the elevated-temperature sweep characteristics of wire bond during transfer molding process for semiconductor package. A set of sweep experiments is also conducted to acquire the sweep stiffness of wire bond for several bond spans and bond heights. The results show the increase of the sweep deflections is more delicate to bond span than bond height. The main objectives of this research are to obtain elevated-temperature material properties of gold wire experimentally and to predict the wire sweep of various bond spans and bond heights subjected to drag force in the compound flow during transfer molding. Based on the analysis results of ANSYS, the effects of bond span and bond height on wire sweep in the elevated-temperature environment can be obtained. Then, the elevated-temperature deflections of wire sweep can be predicted.

수중함의 긴급기동 해석을 위한 유체력계수 모델링에 관한 연구

This paper describes a hydrodynamic modelling study based on the Feldman`s equation to predict the nonlinear and coupled maneuvering characteristics of high speed submarine. The hydrodynamic coefficients set is obtained from the modeling of the cross flow drag force and sail induced vorticity, and the captive model experiments(VPMM and RA test) results used to improved the accuracy. The results contained in this paper will be helpful to predict the behavior of tight turn maneuver and to improve the SOE(Safety Operational Envelope) analysis in case of emergency maneuver

Sediment transport over a flat bed in a unidirectional flow: simulations and validation.

A discrete particle model is described which simulates bedload transport over a flat bed of a unimodal mixed-sized distribution of particles. Simple physical rules are applied to large numbers of discrete sediment grains moving within a unidirectional flow. The modelling assumptions and main algorithms of the bedload transport model are presented and discussed. Sediment particles are represented by smooth spheres, which move under the drag forces of a simulated fluid flow. Bedload mass-transport rates calculated by the model exhibit a low sensitivity to chosen model parameters. Comparisons of the calculated mass-transport rates with well-established empirical relationships are good, strongly suggesting that the discrete particle model has captured the essential elements of the system physics. This performance provides strong justification for future interrogation of the model to investigate details of the small-scale constituent processes which have hitherto been outside the reach of previous experimental and modelling investigations.

Drag force in dense gas-particle two-phase flow

Numerical simulations of flow over a stationary particle in a dense gas-particle two-phase flow have been carried out for small Reynolds numbers (less than 100). In order to study the influence of the particles interaction on the drag force, three particle arrangements have been tested: a single particle, two particles placed in the flow direction and many particles located regularly in the flow field. The Navier-Stokes equations are discretized in the three-dimensional space using finite volume method. For the first and second cases, the numerical results agree reasonably well with the data in literature. For the third case, i.e., the multiparticle case, the influence of the particle volume fraction and Reynolds numbers on the drag force has been investigated. The results show that the computational values of the drag ratio agree approximately with the published results at higher Reynolds numbers (from 34.2 to 68.4), but there is a large difference between them at small Reynolds numbers.

Numerical Simulation of MR Fluid Damping Characteristics Using a Modified Bingham Model

A numerical simulation is conducted to clarify the unsteady flow behavior and drag force around the oscillating flat plate immersed in MR fluids under the applied magnetic field. A modified Bingham model is adopted to take continuous viscous flow behavior at the very small shear rate and also magnetically variable yield stress into account. The measured yield stress is given to the model as a function of magnetic field intensity. The governing equations for the oscillating flat plate and unsteady MR fluids flow are derived by a modified Bingham model taking magnetic body force, yield stress and viscous force into account. The effects of magnetic field intensity, oscillation frequency and amplitude and further plate thickness on the fluid flow and damping characteristics around the oscillating flat plate are clarified by the numerical simulation.

Single-molecule DNA digestion by lambda-exonuclease.

We used a bead displacement sensor to determine the enzymatic shortening of individual molecules of unstained lambda-DNA attached to optically trapped beads. The setup has been described previously (Dapprich and Nicklaus: Bioimaging 6:25-32, 1998) and works by observing the change in position of a trapped bead depending on its viscous drag force during motion. The drag force of a naked bead increases with each attached DNA molecule to a characteristic level that depends on the length and the number of DNAs per bead. A single undigested DNA molecule on a bead will remain stable for extended periods and exhibit a constant drag force in flow. If lambda-exonuclease is added, the drag force decreases from the level for one strand of DNA on a bead to that of a naked bead in about 45 min. This result indicates that the digestion of native lambda-DNA by lambda-exonuclease occurs at an average rate of approximately 15-20 Hz.

Force-mediated kinetics of single P-selectin/ligand complexes observed by atomic force microscopy.

Leukocytes roll along the endothelium of postcapillary venules in response to inflammatory signals. Rolling under the hydrodynamic drag forces of blood flow is mediated by the interaction between selectins and their ligands across the leukocyte and endothelial cell surfaces. Here we present force-spectroscopy experiments on single complexes of P-selectin and P-selectin glycoprotein ligand-1 by atomic force microscopy to determine the intrinsic molecular properties of this dynamic adhesion process. By modeling intermolecular and intramolecular forces as well as the adhesion probability in atomic force microscopy experiments we gain information on rupture forces, elasticity, and kinetics of the P-selectin/P-selectin glycoprotein ligand-1 interaction. The complexes are able to withstand forces up to 165 pN and show a chain-like elasticity with a molecular spring constant of 5.3 pN nm-1 and a persistence length of 0.35 nm. The dissociation constant (off-rate) varies over three orders of magnitude from 0.02 s-1 under zero force up to 15 s-1 under external applied forces. Rupture force and lifetime of the complexes are not constant, but directly depend on the applied force per unit time, which is a product of the intrinsic molecular elasticity and the external pulling velocity. The high strength of binding combined with force-dependent rate constants and high molecular elasticity are tailored to support physiological leukocyte rolling.

Flow Resistance and Drag Forces Due to Multiple Adherent Leukocytes in Postcapillary Vessels

Computational fluid dynamics was used to model flow past multiple adherent leukocytes in postcapillary size vessels. A finite-element package was used to solve the Navier-Stokes equations for low Reynolds number flow of a Newtonian fluid past spheres adhering to the wall of a cylindrical vessel. We determined the effects of sphere number, relative geometry, and spacing on the flow resistance in the vessel and the fluid flow drag force acting to sweep the sphere off the vessel wall. The computations show that when adherent leukocytes are aligned on the same side of the vessel, the drag force on each of the interacting leukocytes is less than the drag force on an isolated adherent leukocyte and can decrease by up to 50%. The magnitude of the reduction depends on the ratio of leukocyte to blood vessel diameter and distance between adherent leukocytes. However, there is an increase in the drag force when leukocytes adhere to opposite sides of the vessel wall. The increase in resistance generated by adherent leukocytes in vessels of various sizes is calculated from the computational results. The resistance increases with decreasing vessel size and is most pronounced when leukocytes adhere to opposite sides of the vessel.

Field Measurement of Boulder Flow Drag

A field method for measuring the flow drag force on individual boulders on a riverbed is identified and tested. Drag is determined from the momentum deficit in the boulder wake, obtained by measuring the flow velocity in a traverse across the wake. A field test yielded a mean drag coefficient of 0.36, which is low compared with flume measurements of 1.1–1.5 for cubes. This may be the result of higher velocity water near the riverbed being sucked into and reenergizing the wake. Further tests are needed to define the conditions in which the method can be used and to indicate how well the assumptions behind the method can be accommodated in a natural flow environment.

Model studies of leukocyte-endothelium-blood interactions: I. The fluid flow drag force on the adherent leukocyte

Using a large scale model and the concept of dimensional similarity, we have developed a dimensionless correlation which permits the estimation of the drag force of blood flow in an in vivo vessel on a leukocyte adherent to the vessel wall. This relationship is Re ⋅ C d = ( 8.15 − 7.52 ln [ d / D] ) 2 where Re is a Reynolds number, C d is a drag coefficient, and d/D is the leukocyte-to-vessel diameter ratio. This function increases rapidly with increasing d/D when d/D is greater than 0.5. A relationship between the drag force on the leukocyte, F f, and the commonly used wall shear stress in the absence of the adherent cell, τ w, has also been developed, and is given by F f τ w D 2 = ( 0.598 + 1.847 ln [d/D] ) 2 This study shows that the discrepancy between the in vivo and in vitro critical wall stresses (above which leukocytes do not adhere to endothelial cell layers) is not due to misrepresentation of the corresponding drag forces on the leukocytes. The discrepancy must be due to real differences in the flow and/or cellular conditions between the in vivo and in vitro experiments.

Characteristics of Debris Flow Caused by Denser Fluid

Sediment discharge and concentration are experimentally investigated for debris flows caused by denser fluid and by plain water, respectively. Denser fluid is represented by the mixture flow of sand and water. The former debris flow is found to carry more gravels than the latter. Gravel discharge and concentration increase with the decrease in the sand grain diameter. This can be explained by critical fluid velocity for gravel movement in the mixture flow. For the estimate of the critical velocity the drag force is also investigated. The value of the drag force in the mixture flow of very fine sand is close to that in plain water. Therefore, such mixture flow is found to behave like a single fluid.

Use of the falling ball viscometer to obtain flow curves for inelastic, non-newtonian fluids

This paper reports on a numerical study of the drag force on a sphere moving at constant velocity in inelastic non-Newtonian fluids. The intended application of this work is in the use of the falling ball viscometer to determine flow curves for these fluids. Numerical solutions were obtained for the case of the sphere moving along the axis of a cylinder and ratios of cylinder to sphere diameter from 10 to 40 were examined. In obtaining the solutions inertia was neglected. To represent the fluid's flow curve the modified power law model was used. This model produces Newtonian behavior as the shear rate approaches zero and power law behavior as the shear rate becomes large. A control volume finite difference method was used to calculate the flow field and drag force. Solutions were obtained for values of the power law index from 1.0 to 0.35. Results are compared with others' calculated results in the power law limit and with published experimental data in the transition region of the flow curve. Two methods of using the falling ball viscometer to determine the flow curves of non-Newtonian fluids are presented. The first assumes that sphere drag data cover the Newtonian and power law limits. The second uses an apparent flow index, and an average shear rate and the corresponding shear stress. This is an extension of an approach used by others.

Finite element solution of multi-dimensional two-phase flow through porous media with arbitrary heating conditions

Mechanistic models for flow regime transitions and drag forces proposed in an earlier work are employed to predict two-phase flow characteristics in multi-dimensional porous layers. The numerical scheme calls for elimination of velocities in favor of pressure and void fraction. The momentum equations for vapor and liquid then can be reduced to a system of two partial differential equations (PDEs) which must be solved simultaneously for pressure and void fraction. Solutions are obtained both in two-dimensional cartesian and in axi-symmetric coordinate systems. The porous layers in both cases are composed of regions with different permeabilities. The finite element method is employed by casting the PDEs in their equivalent variational forms. Two classes of boundary conditions (specified pressure and specified fluid fluxes) can be incorporated in the solution. Volumetric heating can be included as a source term. The numerical procedure is thus suitable for a wide variety of geometry and heating conditions. Numerical solutions are also compared with available experimental data.