Hydrodynamic Flow Field Research Articles

The importance of fluid dynamics for MBR fouling mitigation

The importance of the multiphase fluid dynamics for fouling mitigation in MBR systems has been widely acknowledged with air sparging having been applied commercially for about 20years. However, the effects of air scouring are still not fully understood since the transient orthogonal and parallel flows as well as turbulent eddies created by bubbling generate complex hydrodynamic flow fields in the vicinity of a membrane. There is no generally valid model that describes the relationship between fouling rate and fluid dynamics. So, a reliable and universally applicable model to optimize membrane module and tank geometries, air scouring and filtration cycles is still pending.In addition to providing a discussion on the importance of multiphase fluid dynamics for fouling control, this review aims at developing guidelines to choose appropriate experimental and numerical methods for fluid dynamics investigations in MBR systems.

Mathematical modelling of a hydrocyclone for the down-hole oil–water separation (DOWS)

In this study, a mathematical model is developed to predict the efficiency of a down-hole oil–water separation hydrocyclone. In the proposed model, the separation efficiency is determined based on droplet trajectory of a single oil droplet through the continuous-phase. The droplet trajectory model is developed using a Lagrangian approach in which single droplets are traced in the continuous-phase. The droplet trajectory model uses the swirling flow of the continuous-phase to trace the oil droplets. By applying the droplet trajectory, a trial and error approach is used to determine the size of the oil droplet that reaches the reverse flow region, where they can be separated. The required input for the proposed model is hydrocyclone geometry, fluid properties, inlet droplet size distribution and operational conditions at the down hole. The model is capable of predicting the hydrocyclone hydrodynamic flow field, namely, the axial, tangential and radial velocity distributions of the continuous-phase. The model was then applied for some case studies from the field tested DOWS systems which exist in the literature. The results show that the proposed model can predict well the split ratio and separation efficiency of the hydrocyclone. Moreover, the results of the proposed model can be used as a preliminary evaluation for installing a down-hole oil–water separation hydrocyclone system in a producing well.

Numerical simulation of turbulent flame–wall quenching using a coherent flame model

Flame quenching by fine mesh is one of the oldest known methods for mitigating flame propagation. Sir Humphry Davy pioneered the study of flame–wall quenching while developing the mining safety lamp. Installing a quenching mesh around the equipment is an effective preventive technique against hazardous flame propagation. Quenching of flame at the wall is due to the coupled thermo physical process involving heat transfer, flame stretch and preferential diffusion. While laminar flame–wall quenching has been extensively studied both theoretically and numerically, the reported studies of turbulent flame–wall interactions are limited to some Direct Numerical Simulations (DNS), which have subsequently led to improvement of models for predicting flame characteristics in the vicinity of the wall.Large Eddy Simulation (LES) of turbulent flows is considered as a powerful tool to predict the occurrence of instabilities due to heat release, hydrodynamic flow fields and acoustic waves. It provides a better description of turbulent–combustion interaction than the classical Reynolds Averaged Navier Stokes techniques (RANS). In the present study, the single mesh quenching of turbulent flame deflagration is investigated in stoichiometric methane–air mixture, which is equi-diffusive with unit Lewis number. It is well known that flame stretch and preferential diffusion has negligible influence in flame quenching for equi-diffusive flames. Therefore a unity Lewis number flamelet formulation can be used for simulating the turbulent combustion process within the premixed flamelet regime.In the present study, LES predictions are performed using the OpenFOAM CFD toolbox solver. The Coherent Flame Model (CFM) in the LES context as proposed by Richard et al. (2007) is implemented for modeling flame deflagration. During the flame/wall interactions, enthalpy loss through the wall affects the flamelet speed, flamelet annihilation and flame propagation; and the decrease in turbulence scales near the wall affects turbulent diffusion and flame strain. These flame–wall interactions are accounted for through extending the closures proposed by Bruneaux, Poinsot, and Ferziger (1997) for the CFM–RANS model to the LES context. Preliminary testing has demonstrated good potential of the modified CFM to capture the quenching effect of the wall.

A NEW METHOD FOR MEASURING DIFFUSION COEFFICIENT OF GASES IN LIQUIDS BY PLIF

Gas–liquid mass transfer is a major issue in engineering processes such as wastewater treatment or biogas production since this phenomenon is directly linked to their design and efficiency. In recent years, much research has been done in this area but some gaps still remain in our knowledge of gas–liquid transfer, in particular concerning molecular diffusivity. The determination of molecular diffusivity is commonly based on empirical correlations, such as the widely used Wilke and Chang13 expression, valid under specific conditions and with relatively high uncertainties. In the present work, an innovative and promising technique is proposed to determine diffusion coefficients of gases in liquids. This technique is based on visualizing and quantifying oxygen diffusion across a flat gas–liquid interface, in a Newtonian medium, using planar laser induced fluorescence (PLIF) with inhibition. Particle image velocimetry (PIV) experiments were conducted to confirm the hydrodynamic flow field in the liquid phase. Results included the visualization of oxygen diffusion over time, and the quantification of this visualization. The oxygen diffusivity thus determined is in agreement with values found in the literature.

Hydrodynamic Manipulation of DNA in Nanopost Arrays: Unhooking Dynamics and Size Separation

Micro- or nanofabricated obstacle arrays are widely used as model matrices to perform fast DNA separations by electrophoresis. In this report, a gallery of obstacles of radii spanning from 40 to 250 nm are used to investigate the dynamics of hydrodynamic-field-driven DNA-nanopost collisions at the single-molecule level. The data shows that DNA disengagement dynamics are reasonably well described by conventional electrophoretic models in the limit of a large spacing between obstacles and for moderate migration velocities. It is also demonstrated that the use of hydrodynamic flow fields to convey DNA molecules is associated with changes in the configurational space of hooking events, and to altered relaxation dynamics between consecutive collisions. This study defines experimental conditions for the efficient separation of DNA fragments of tens of base pairs, and provides a complete framework by which to understand the behavior of DNA in the course of hydrodynamic-driven migrations through nanopost arrays.

Computation of Bio-Fluid Sounds

A hybrid method is proposed for prediction of bio-fluid sounds at very low Mach numbers. The unsteady hydrodynamic flow field is computed by the incompressible Navier-Stokes equations (INS), while the sound field is obtained by solving the linearized perturbed compressible equations (LPCE) with sound sources represented by a total derivative of the hydrodyanmic pressure, DP/ Dt. With the present INS/LPCE hybrid method, the vocal sound in human larynx and the buzz sound of bumblebee are computed with more clear understanding on the sound generation processes associated with their characteristic motions such as the self-sustained oscillatory motions of the vocal folds and the figure-eight motion of the flapping wings.

Streaming phenomena in microdroplets induced by Rayleigh surface acoustic wave

This paper reports the numerical simulation and experimental characterization of three-dimensional acoustic streaming behavior of a liquiddroplet subjected to a Rayleigh surface acoustic wave. The streaming behavior of the droplet was studied as a function of radio-frequency (RF) power, aperture of the interdigitated transducer, and size of the liquiddroplet. The hydrodynamic flow field within the droplet was determined by solving the laminar incompressible Navier–Stoke’s equations. The numerical and experimental results are shown to be in good agreement over the range of parameters examined. The ratios of the position of butterfly central line (axis of rotation) to radius of the droplet are demonstrated to be fairly constant for moderate droplet volumes and to vary by less than 12% at large droplet volumes. Besides that, an increase in the RF power and a decrease in the droplet size result in an increased surface acoustic wave(SAW) streaming velocity. The numerical results also suggest that a maximum streaming velocity is achieved when the SAW width is approximately half of the droplet radius.

Numerical modeling of multi micro jet impingement cooling of a three dimensional turbine vane

This paper reports a numerical investigation on the prediction of the thermal and hydrodynamic flow fields of multi micro jet impingement cooling of three dimensional turbine vanes. A three dimensional vane is modeled with an in-line array of impinging jets of diameters 0.5 and 0.25 mm. The numerical model consists of the steady, Reynolds-Averaged Navier–Stokes equations and the Kω SST Turbulence model. The governing equations are solved using a finite volume method. The crossflow mass velocity (Gc) to jet mass velocity (Gj) ratio, and the average and local heat transfer distributions are analyzed with varying mass velocity and jet-to-target spacing. It is found out that a significant decrease in crossflow ratio occurs with the smaller diameters. Due to the lower crossflow and higher exit velocities of the smaller jets, the penetration into the crossflow is much higher. Moreover, at a constant mass flow, the use of micro-jets enhanced the overall average heat transfer coefficient by 63%, while at a fixed pressure drop across the vane instead of the mass flow, the smaller diameters will still yield an enhancement of 34.3% in the overall average heat transfer coefficient.

Dynamics of Red Blood Cells and Vesicles in Microchannels of Oscillating Width

We have studied the dynamics of red blood cells and fluid lipid vesicles in hydrodynamic flow fields created by microchannels with periodically varying channels width. For red blood cells we find a transition from a regime with oscillating tilt angle and fixed shape to a regime with oscillating shape with increasing flow velocity. We have determined the crossover to occur at a critical ratio of channel width L_y and red blood cell velocity v_m approx. 10⊥-3s. These oscillations are superposed by shape transitions from a discocyte to a slipper shape at low velocities and a slipper to parachute transition at high flow velocities.Ref.:H. Noguchi, G. Gompper, L. Schmid, A. Wixforth, T. Franke, EPL (Europhysics Letters) 2010, 89, 28002S. Braunmueller, L. Schmid, T. Franke J.Phys. Condens. Matt. accepted

Perspectives on the Transition from Batch to Continuous Mixing Technologies in the Compounding Industry

Abstract During the last quarter of the 20th century, there was a transition from batch to continuous mixing in the polymer and food industries. We present and summarize experimental studies of flows in both internal and continuous mixers. Simulation of flow in these machines is also described. It is argued that the primary reason for this transition was that continuous mixers produce more uniform and better quality mixes, and that this is inherent in their different fluid mechanics. Batch mixers are always starved and largely unpressurized. Their complex fluid mechanics produce non-uniform mixing in the various portions of the compound material. Continuous mixers set up a steady state hydrodynamic flow field generally consisting of alternating regions of pressurized fully filled flow and starved flow. All of the compounds must pass through these regions to reach the die exit and thus possess uniform mixing histories. This uniform deformation history which includes pressurized mixing regions produces superior uniform products.

Applications of CFD Technique in the Design and Flow Analysis of Implantable Axial Flow Blood Pump

Based on established numerical methods and hydrodynamic performance testing facilities, CFD technique are applied to improve the design of the implantable axial flow blood pump and the flow analysis. (1) Applying brushless machine magnet steel, reducing its thickness while increasing the length, the flow channel's cross-section is increased, with no space connection between the large and the small rotators, and with a cone transition segment from the bearing to the principal axis, the flow is made smoother. The rotating speed is lowered by 1000 rpm – 1200 rpm under the same flow rate and pressure head, and thus the hemolysis can be avoided. (2) Different outlet stator guiding vanes are selected for the same blood pump for analyses of hydrodynamic performances and flow fields. An excellent design not only can regulate the rotating flow field into an axial one, reduce the circulation loss and improve the pump efficiency, but also can avoid backflow, vortex and secondary flow at the pump outlet, and thus the thrombus can be prevented. (3) The calf live tests show that some residual clots exist at the inner wall of the outlet connection bends, which are analyzed and explained by CFD techniques and the corresponding improvements are proposed. All results are verified by hydrodynamic performance tests and PIV flow field tests, and consistent conclusions are obtained.

Kinematics of Specifically Captured Circulating Tumor Cells in Bio-Functionalized Microchannels

The attachment kinematics of cancer cells under hydrodynamic loading in antibody-functionalized microchannels has been studied. Epithelial-cell-adhesion-molecule antibodies are immobilized on the microchannel surface for specific capture of the target cancer cells from homogeneous cell suspensions. The specific interaction between the cancer cell receptors and the immobilized antibodies under static conditions is demonstrated. The capture efficiency of the target cells from homogeneous suspensions under applied hydrodynamic flow field has been investigated, revealing a characteristic shear stress. Applying a lower stress allows the capture of most target cells, while the capture efficiency drops sharply with an increasing shear stress. The captured cells are spatially distributed along the microchannel; both the velocity and the distance travelled by cells prior to capture are measured. The characteristic time and length scales for cell capture are determined, and a log-normal statistical distribution is proposed to describe the observations. Furthermore, a first-order kinetic model for receptor-ligand bond formation provides a rough estimate of the cell adhesion rate constant. Under a low shear stress, the on-rate is much higher than the off-rate, allowing capture of most loaded cells. The off-rate constant increases exponentially with an increasing shear stress, such that above the characteristic stress level, most loaded cells avoid capture.

Hydrodynamics of Sperm Cells near Surfaces

Sperm are propelled by an actively beating tail, and display a wide variety of swimming patterns. When confined between two parallel walls, sperm swim either in circles or on curvilinear trajectories close to the walls. We employ mesoscale hydrodynamics simulations in combination with a mechanical sperm model to study the swimming behavior near walls. The simulations show that sperm become captured at the wall due to the hydrodynamic flow fields which are generated by the flagellar beat. The circular trajectories are determined by the chiral asymmetry of the sperm shape. For strong (weak) chirality, sperm swim in tight (wide) circles, with the beating plane of the flagellum oriented perpendicular (parallel) to the wall. For comparison, we also perform simulations based on a local anisotropic friction of the flagellum. In this resistive force approximation, surface adhesion and circular swimming patterns are obtained as well. However, the adhesion mechanism is now due to steric repulsion, and the orientation of the beating plane is different. Our model provides a theoretical framework that explains several distinct swimming behaviors of sperm near and far from a wall. Moreover, the model suggests a mechanism by which sperm navigate in a chemical gradient via a change of their shape.

Experimental Investigation of the Acoustic Power Around Two Tandem Cylinders

Aeroacoustic resonance of bluff bodies exposed to cross flow can be problematic for many different engineering applications and knowledge of the location and interaction of acoustic sources is not well understood. Thus, an empirical investigation of the acoustically coupled flow around two tandem cylinders under two different resonant conditions is presented. It is assumed that the resonant acoustic field could be decoupled from the hydrodynamic flow field, resolved separately, and then recoupled to predict the flow/sound interaction mechanisms using Howe's theory of aerodynamic sound. Particle image velocimetry was employed to resolve the phase-averaged flow field characteristics around the cylinders at various phases in an acoustic wave cycle. It was found that the vortex shedding patterns of the two resonant conditions exhibit substantial differences. For the first condition, which occurred at low flow velocities where the natural vortex shedding frequency was below the acoustic resonance frequency, fully developed vortices formed in both the gap region between the cylinders and in the wake. These vortices were found to be in phase with each other. For the second resonant condition, which occurred at higher flow velocities where the natural vortex shedding frequency was above the acoustic resonant frequency, fully developed vortices only formed in the wake and shedding from the two cylinders were not in phase. These differences in the flow field resulted in substantial variation in the flow-acoustic interaction mechanisms between the two resonant conditions. Corresponding patterns of the net acoustic energy suggest that acoustic resonance at the lower flow velocity is due to a combination of shear layer instability in the gap and vortex shedding in the wake, while acoustic resonance at the higher flow velocity is driven by the vortex shedding in the wake of the two cylinders.

Parametric Design of a Waterjet Pump by Means of Inverse Design, CFD Calculations and Experimental Analyses

The present paper describes the parametric design of a mixed-flow water-jet pump. The pump impeller and diffuser geometries were parameterized by means of an inverse design method, while CFD analyses were performed to assess the hydrodynamic and suction performance of the different design configurations that were investigated. An initial pump design was first generated and used as baseline for the parametric study. The effect of several design parameters was then analyzed in order to determine their effect on the pump performance. The use of a blade parameterization, based on inverse design, led to a major advantage in this study, because the three-dimensional blade shape is described by means of hydrodynamic parameters, such as blade loading, which has a direct impact on the hydrodynamic flow field. On the basis of this study, an optimal configuration was designed with the aim of maximizing the pump suction performance, while at the same time, guaranteeing a high level of hydrodynamic efficiency, together with the required mechanical and vibrational constraints. The final design was experimentally tested, and the good agreement between numerical predictions and experimental results validated the design process. This paper highlights the contrasting requirements in the pump design in order to achieve high hydrodynamic efficiency or good cavitation performance. The parametric study allowed us to determine design guidelines in order to find the optimal compromise in the pump design, in cases where both a high level of efficiency and suction performance must simultaneously be achieved. The design know-how developed in this study is based on flow field analyses and on hydrodynamic design parameters. It has therefore a general validity and can be used for similar design applications.

Numerical simulation and experimental study of the hydrodynamics of a modeled reef located within a current

The hydrodynamic forces and flow field of artificial reef models in steady flow were numerically investigated using the RNG k-ɛ turbulent model. The numerical simulation results are consistent with results observed by experimental means. A comparative study indicates that the corresponding errors of forces between calculated values and values observed in the experiment vary in the range of 2.3%–11.2% and that the corresponding errors of velocities vary in the range of 1.3%–15.8%. The flow field numerical results show that upstream and vortices exist when the current passes over and through the surface of the reef model. This study suggests that the numerical simulation method can be applied to predict the forces and flow field associated with artificial reefs.

Swimmer-tracer scattering at low Reynolds number

Understanding the stochastic dynamics of tracer particles in active fluids is important for identifying the physical properties of flow generating objects such as colloids, bacteria or algae. Here, we study both analytically and numerically the scattering of a tracer particle in different types of time-dependent, hydrodynamic flow fields. Specifically, we compare the tracer motion induced by an externally driven colloid with the one generated by various self-motile, multi-sphere swimmers. Our results suggest that force-free swimmers generically induce loop-shaped tracer trajectories. The specific topological structure of these loops is determined by the hydrodynamic properties of the microswimmer. Quantitative estimates for typical experimental conditions imply that the loops survive on average even if Brownian motion effects are taken into account.

Induction of mammalian cell death by simple shear and extensional flows

In this work we investigated whether the type of shear flow, to which cells are exposed, influences the initiation of cell death. It is shown that mammalian cells, indeed, distinguish between discrete types of flow and respond differently. Two flow devices were employed to impose accurate hydrodynamic flow fields: uniform steady simple shear flow and oscillating extensional flow. To distinguish between necrotic and apoptotic cell death, fluorescence activated cell sorting and the release of DNA in the culture supernatant was used. Results show that Chinese Hamster Ovaries and Human Embryonic Kidney cells will enter the apoptotic pathway when subjected to low levels of hydrodynamic stress (around 2.0 Pa) in oscillating, extensional flow. In contrast, necrotic death prevails when the cells are exposed to hydrodynamic stresses around 1.0 Pa in simple shear flow or around 500 Pa in extensional flow. These threshold values at which cells enter the respective death pathway should be avoided when culturing cells for recombinant protein production to enhance culture longevity and productivity.

Generic theory of colloidal transport

We discuss the motion of colloidal particles relative to a two-component fluid consisting of solvent and solute. Particle motion can result from i) net body forces on the particle due to external fields such as gravity; ii) slip velocities on the particle surface due to surface dissipative phenomena. The perturbations of the hydrodynamic flow field exhibit characteristic differences in cases i) and ii) which reflect different patterns of momentum flux corresponding to the existence of net forces, force dipoles or force quadrupoles. In the absence of external fields, gradients of concentration or pressure do not generate net forces on a colloidal particle. Such gradients can nevertheless induce relative motion between particle and fluid. We present a generic description of surface dissipative phenomena based on the linear response of surface fluxes driven by conjugate surface forces. In this framework we discuss different transport scenarios including self-propulsion via surface slip that is induced by active processes on the particle surface. We clarify the nature of force balances in such situations.

Sound production and hearing in coral reef butterflyfishes.

Butterflyfishes (family Chaetodontidae) exhibit a diversity of close affiliative social behaviors. All 87 species of Chaetodon have a laterophysic connection and extensions of the anterior horns of the swim bladder near the otic capsule, which are proposed to impart sound pressure sensitivity to the normally mechanosensitive lateral line and acceleration‐sensitive inner ear. Video kinematic analyses reveal that the motor action patterns used to produce pulsed sounds vary among species. The sound production mechanisms in Chaetodon and Forcipiger do not appear to be conserved, thus sound production may have evolved independently within the family. Species with long swim bladder horns have lower AEP hearing thresholds than those without, and deflation of the horns and bladder decreases hearing sensitivity in the former group. When the laterophysic connection and swim bladder horns are physically disrupted in wild fish, movement within their territories decreases and fish pairs swim closer together. Thus, the low intensity sounds produced by butterflyfishes may favor close social behaviors that increase communication efficiency among individuals on a noisy coral reef. We are currently investigating how hydrodynamic flow fields produced during social interactions may be used for intraspecific communication during these natural behaviors.