Fluid Dynamic Mechanisms Research Articles

Hydrodynamic mechanisms of cell and particle trapping in microfluidics.

Focusing and sorting cells and particles utilizing microfluidic phenomena have been flourishing areas of development in recent years. These processes are largely beneficial in biomedical applications and fundamental studies of cell biology as they provide cost-effective and point-of-care miniaturized diagnostic devices and rare cell enrichment techniques. Due to inherent problems of isolation methods based on the biomarkers and antigens, separation approaches exploiting physical characteristics of cells of interest, such as size, deformability, and electric and magnetic properties, have gained currency in many medical assays. Here, we present an overview of the cell/particle sorting techniques by harnessing intrinsic hydrodynamic effects in microchannels. Our emphasis is on the underlying fluid dynamical mechanisms causing cross stream migration of objects in shear and vortical flows. We also highlight the advantages and drawbacks of each method in terms of throughput, separation efficiency, and cell viability. Finally, we discuss the future research areas for extending the scope of hydrodynamic mechanisms and exploring new physical directions for microfluidic applications.

Hydrodynamic Trapping of Particles in an Expansion-Contraction Microfluidic Device

Manipulation and sorting of particles utilizing microfluidic phenomena have been a hot spot in recent years. Here, we present numerical investigations on particle trapping techniques by using intrinsic hydrodynamic effects in an expansion-contraction microfluidic device. One emphasis is on the underlying fluid dynamical mechanisms causing cross-streamlines migration of the particles in shear and vortical flows. The results show us that the expansion-contraction geometric structure is beneficial to particle trapping according to its size. Particle Reynolds number and aspect ratio of the channel will influence the trapping efficiency greatly because the force balance between inertial lift and vortex drag forces is the intrinsic reason. Especially, obvious inline particles contribution presented when the particle Reynolds number being unit. In addition, we selected three particle sizes (2, 7, and 15 μm) to examine the trapping efficiency.

Effect of Orientation of Internal Turbulence Promoting Ribs on Flow Characteristics for Film Cooling

Numerical simulations have been performed to study the film cooling of a flat plate that has an internal cooling passage perpendicular to the main flow. The goal is to understand the effect of the orientation of turbulence promoting ribs installed in the internal cooling passage on the adiabatic film cooling effectiveness in the external main flow downstream of the cooling-air injection. Detached Eddy Simulation is carried out for the flow characteristics for two rib orientations at two blowing ratios. The simulation results have revealed that the rib orientation affects significantly the temperature and flow structures downstream of the cooling hole and therefore affects the film-cooling performance on the surface. The revealed difference is due to the presence/absence of intense spiral motion at the cooling-hole outlet and such a difference originates from an interaction between the flow separation behind the inclined rib and the flow suction at the inlet of inclined cooling hole. The simulation results are exploited to draw a clear picture for the fluid-dynamical mechanisms responsible for the effect of the rib orientation.

Do only the vascular atherosclerosis-related risk factors affect the value of pulsed wave velocity? Blood density or viscosity effect on pulsed wave velocity

Aortic stiffness expressed as aortic pulse wave velocity (PWV) is a strong predictor of future CV events and all-cause mortality. The predictive ability of arterial stiffness is higher in subjects with a higher baseline CV risk. There is a general agreement that arterial stiffness has an important predictive role. Most studies have focused on classic risk factors confined to vascular status. However, there might be some relation between blood flowand vascularwall. So, our hypothesis is that hemorheologic parameters such as blood viscosity, shear rate, and shear stress might affect to PWV as a fluid dynamic mechanism. We have measured and analyzed the PWV, hemorheologic parameters, and other clinical parameters in patients scheduled for coronary angiography. This study showed that an association between known clinical risk factors, such as, age, hypertension, dyslipidemia, smoking, and renal insufficiency were positively associated with the value of PWV. However, the hemorheological abnormalities were diversely associated with PWV. While initial hypothesis was that the viscosity might significantly affect the value of PWV with positive correlation, the results shows that the viscosity and the hematocrit were negatively, and critical shear ratewas positively correlated with PWV. These results suggest that PWV might be affected by shear rate than the viscosity itself. Additionally, flow increasing factors such as shear rate, central blood pressure, and pulse pressuremight affect PWV, hemodynamically. This resultmight provide the useful information on the characteristics of RBC aggregation and hemorheological effect on PWV.

Aerodynamic Measurements on a Vertical Axis Wind Turbine in a Large Scale Wind Tunnel

This paper presents the first results of a wide experimental investigation on the aerodynamics of a vertical axis wind turbine. Vertical axis wind turbines have recently received particular attention, as interesting alternative for small and micro generation applications. However, the complex fluid dynamic mechanisms occurring in these machines make the aerodynamic optimization of the rotors still an open issue and detailed experimental analyses are now highly recommended to convert improved flow field comprehensions into novel design techniques. The experiments were performed in the large-scale wind tunnel of the Politecnico di Milano (Italy), where real-scale wind turbines for micro generation can be tested in full similarity conditions. Open and closed wind tunnel configurations are considered in such a way to quantify the influence of model blockage for several operational conditions. Integral torque and thrust measurements, as well as detailed aerodynamic measurements were carried out to characterize the 3D flow field downstream of the turbine. The local unsteady flow field and the streamwise turbulent component, both resolved in phase with the rotor position, were derived by hot wire measurements. The paper critically analyses the models and the correlations usually applied to correct the wind tunnel blockage effects. Results highlight that the presently available theoretical correction models do not provide accurate estimates of the blockage effect in the case of vertical axis wind turbines. The tip aerodynamic phenomena, in particular, seem to play a key role for the prediction of the turbine performance; large-scale unsteadiness is observed in that region and a simple flow model is used here to explain the different flow features with respect to horizontal axis wind turbines.

The Tubercles on Humpback Whales' Flippers: Application of Bio-Inspired Technology

The humpback whale (Megaptera novaeangliae) is exceptional among the large baleen whales in its ability to undertake aquabatic maneuvers to catch prey. Humpback whales utilize extremely mobile, wing-like flippers for banking and turning. Large rounded tubercles along the leading edge of the flipper are morphological structures that are unique in nature. The tubercles on the leading edge act as passive-flow control devices that improve performance and maneuverability of the flipper. Experimental analysis of finite wing models has demonstrated that the presence of tubercles produces a delay in the angle of attack until stall, thereby increasing maximum lift and decreasing drag. Possible fluid-dynamic mechanisms for improved performance include delay of stall through generation of a vortex and modification of the boundary layer, and increase in effective span by reduction of both spanwise flow and strength of the tip vortex. The tubercles provide a bio-inspired design that has commercial viability for wing-like structures. Control of passive flow has the advantages of eliminating complex, costly, high-maintenance, and heavy control mechanisms, while improving performance for lifting bodies in air and water. The tubercles on the leading edge can be applied to the design of watercraft, aircraft, ventilation fans, and windmills.

Application of the Wavelet Transform of Pressure Signals for Detecting and Analyzing Rotating Stall Inception in Axial Flow Low Speed Compressor Stage

The paper presents a research program carried out to improve understanding of the fluid dynamics mechanisms that lead to rotating stall in the axial flow low speed compressor stage. The stalling behavior of this compressor stage was studied by measuring unsteady casing pressure by means of a circumferentially and axially spaced array of high frequency pressure transducers. Another probe used was a disc static pressure probe, with the pressure transducer, for in-flow and out-flow measurements along the blade span. It was expected that understanding of the fluid dynamics will facilitate at least two important tasks. The first was to accurately predict of when and how a particular compressor would stall. The second was to control, delay, or eventually suppress the rotating stall and surge. In consequence, one could extend the useful operating range of the axial compressor. Another motivation for the research was to compare the results from the three applied analysis techniques by using a single stall inception event. The first one was a simple visual inspection of the traces, which brought about a very satisfactory effect. The second one was application of spatial Fourier decomposition to the analysis of stall inception data, and the third method of analysis consisted in application of wavelet filtering in order to better understand the physical mechanisms which lead to rotating stall. It was shown that each of these techniques would provide different information about compressor stall behavior, and each method had unique advantages and limitations.

Optimized transformation of the glottal motion into a mechanical model

Abstract During phonation the human vocal folds exhibit a complex self-sustained oscillation which is a result of the transglottic pressure difference, of the characteristics of the tissue of the folds and of the flow in the gap between the vocal folds (Van den Berg J. Myoelastic-aerodynamic theory of voice production. J Speech Hearing Res 1958;1:227–44 [1]). Obviously, extensive experiments cannot be performed in vivo. Therefore, in literature a variety of model experiments that try to replicate the vocal folds kinematics for specific studies within the vocal tract can be found. Here, we present an experimental model to visualize the fluid dynamics which result from the complex motions of real human vocal folds. An existing up-scaled glottal cam model with approximate glottal kinematics is extended to replicate more realistically observed glottal closure types. This extension of the model is a further step in understanding the fluid dynamical mechanisms contributing to the quality of human voice during phonation, in particular the cause (changed glottal kinematics) and its effect (changed aero-acoustic field). For four typical glottal closure types cam geometries of varying profile are generated. Two counter rotating cams covered with a silicone membrane reproduce as well as possible the observed glottal movements.

Seismic source mechanism of degassing bursts at Kilauea Volcano, Hawaii: Results from waveform inversion in the 10–50 s band

The current (March 2008 to February 2009) summit eruptive activity at Kilauea Volcano is characterized by explosive degassing bursts accompanied by very long period (VLP) seismic signals. We model the source mechanisms of VLP signals in the 10–50 s band using data recorded for 15 bursts with a 10‐station broadband network deployed in the summit caldera. To determine the source centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous medium that takes topography into account. The VLP signals associated with the bursts originate in a source region ∼1 km below the eastern perimeter of Halemaumau pit crater. The observed waveforms are well explained by the combination of a volumetric component and a vertical single force component. For the volumetric component, several source geometries are obtained which equally explain the observed waveforms. These geometries include (1) a pipe dipping 64° to the northeast; (2) two intersecting cracks including an east striking crack (dike) dipping 80° to the north, intersecting a north striking crack (another dike) dipping 65° to the east; (3) a pipe dipping 58° to the northeast, intersecting a crack dipping 48° to the west–southwest; and (4) a pipe dipping 57° to the northeast, intersecting a pipe dipping 58° to the west–southwest. Using the dual‐crack model as reference, the largest volume change obtained among the 15 bursts is ∼24,400 m3, and the maximum amplitude (peak to peak) of the force is ∼20 GN. Each burst is marked by a similar sequence of deflation and inflation, trailed by decaying oscillations of the volumetric source. The vertical force is initially upward, synchronous with source deflation, then downward, synchronous with source reinflation, followed by oscillations with polarity opposite to the volumetric oscillations. This combination of force and volume change is attributed to pressure and momentum changes induced during a fluid dynamic source mechanism involving the ascent, expansion, and burst of a large slug of gas within the upper ∼150 m of the magma conduit. As the slug expands upon approach to the surface and more liquid becomes wall supported by viscous shear forces, the pressure below the slug decreases, inducing conduit deflation and an upward force on the Earth. The final rapid slug expansion and burst stimulate VLP and LP oscillations of the conduit system, which slowly decay due to viscous dissipation and elastic radiation. Consideration of the fluid dynamic arguments leads us to prefer the dual‐crack VLP source model as it is the only candidate model capable of producing plausible values of length scales and pressure changes. The magnitudes of the vertical forces observed in the 15 bursts appear consistent with slug masses of 104 to 106 kg.

A schlieren optical study of the human cough with and without wearing masks for aerosol infection control

Various infectious agents are known to be transmitted naturally via respiratory aerosols produced by infected patients. Such aerosols may be produced during normal activities by breathing, talking, coughing and sneezing. The schlieren optical method, previously applied mostly in engineering and physics, can be effectively used here to visualize airflows around human subjects in such indoor situations, non-intrusively and without the need for either tracer gas or airborne particles. It accomplishes this by rendering visible the optical phase gradients owing to real-time changes in air temperature. In this study, schlieren video records are obtained of human volunteers coughing with and without wearing standard surgical and N95 masks. The object is to characterize the exhaled airflows and evaluate the effect of these commonly used masks on the fluid-dynamic mechanisms that spread infection by coughing. Further, a high-speed schlieren video of a single cough is analysed by a computerized method of tracking individual turbulent eddies, demonstrating the non-intrusive velocimetry of the expelled airflow. Results show that human coughing projects a rapid turbulent jet into the surrounding air, but that wearing a surgical or N95 mask thwarts this natural mechanism of transmitting airborne infection, either by blocking the formation of the jet (N95 mask), or by redirecting it in a less harmful direction (surgical mask).

A review of propulsive mechanisms in rowing

This paper reviews the fluid dynamic mechanisms that are fundamental to the rowing stroke. Complex interactions occur between the oar blade and water, and over the last 30 years our understanding of the mechanisms that govern rowing propulsion has developed significantly. The oar blade was once believed to rip through the water, generating a drag force acting normal to the blade. Current research indicates that the oar blade acts as an aerofoil making use of lift forces to propel the boat through the water early and late in the stroke, with drag being the dominant propulsive force when the oar is perpendicular to the boat. Early on-water research showed variations in the fluid dynamic behaviour of different oar blades. Recently, more controlled laboratory tests have isolated the oar blade from the rower—boat—water system to obtain blade characteristics. The isolated nature of recent oar blade studies and the complex nature of the oar blade—water interaction have led to suggestions that computational fluid dynamics (CFD) may be used to advance understanding of oar blade behaviour as a precursor to a more informed design process. By integrating a dynamic CFD model with a mathematical model of rowing mechanics, a full optimization of rower technique, boat rigging, and equipment design could be performed.

Wake structure of circular cylinder in microbubble mixture

Injecting microbubbles near wall can reduce the frictional drag. While it has been confirmed by a variety of experiments to date, the fluid dynamics mechanism of drag reduction has not been comprehensively understood so far. We measure the wake structure of a circular cylinder in microbubble mixture to figure out the characteristic of the interaction between microbubbles and liquid flow accompanying high turbulence. The flow field is restricted to be two-dimensional by confining the bubbly two-phase flow into a thin horizontal channel of 2 mm in height. Microbubbles around 100 μm in diameter are generated with water electrolysis at far upstream the measurement section. Experiment is conducted at Reynolds number higher than 104 to clarify the role of microbubbles in highly turbulent situation. We use solid particles as the PTV-tracer in the single-phase flow and assume that microbubbles trace the liquid flow in the microbubble mixture. We measure the turbulent intensity and Reynolds shear stress distribution from the PTV data and demonstrate that microbubbles significantly suppress the turbulence. In addition, the Karman vortex shedding frequency, which is measured from the unsteady stream function, increases when microbubbles are mixed.

Fluid Dynamic Mechanism Responsible for Breaking the Left-Right Symmetry of the Human Body: The Nodal Flow

A left-right (LR) asymmetric body plan is crucial for the development of the human body. The past decade has seen rapid progress in our understanding of the LR symmetry-breaking process in vertebrate development. A series of experimental studies has demonstrated that leftward movement of fluid at the ventral node, designated nodal flow, is the central process in symmetry breaking. Nodal flow is autonomously generated by the posteriorly tilted rotation of cilia. The underlying fluid dynamic mechanism, especially the importance of viscous interactions between the cilia and the cell surface, has been clarified by theoretical analyses. Recent experiments have suggested how this leftward nodal flow can be interpreted to create LR asymmetry. Specifically, leftward transport of lipoprotein particles, called nodal vesicular parcels (NVPs), was uncovered. Nodal flow thus triggers left-specific signaling pathways by transporting signaling molecules to the left side using NVPs.

Fluid Dynamics of Gas Exchange in High-Frequency Oscillatory Ventilation: In Vitro Investigations in Idealized and Anatomically Realistic Airway Bifurcation Models

The objective of this work is to develop understanding of the local fluid dynamic mechanisms that underpin gas exchange in high-frequency oscillatory ventilation (HFOV). The flow field during HFOV was investigated experimentally using particle image velocimetry in idealized and realistic models of a single bifurcation. Results show that inspiratory and expiratory fluid streams coexist in the airway at flow reversal, and mixing between them is enhanced by secondary flow and by vortices associated with shear layers. Unsteady flow separation and recirculation occurs in both geometries. The magnitude of secondary flow is greater in the realistic model than in the idealized model, and the structure of secondary flow is quite different. However, other flow structures are qualitatively similar.

Near-Surface Intensification of Tornado Vortices

Abstract An idealized analytical model and numerical large-eddy simulations are used to explore fluid-dynamic mechanisms by which tornadoes may be intensified near the surface relative to conditions aloft. The analytical model generalizes a simple model of Barcilon and Fiedler and Rotunno for a steady supercritical end-wall vortex to more general vortex corner flows, angular momentum distributions, and time dependence. The model illustrates the role played by the corner flow swirl ratio in determining corner flow structure and intensification; predicts an intensification of near-surface swirl velocities relative to conditions aloft of Iυ ∼ 2 for supercritical end-wall vortices in agreement with earlier analytical, numerical, and laboratory results; and suggests how larger intensification factors might be achieved in some more general corner flows. Examples of the latter are presented using large-eddy simulations. By tuning the lateral inflow boundary conditions near the surface, quasi-steady vortices exhibiting nested inner and outer corner flows and Iυ ∼ 4 are produced. More significantly, these features can be produced without fine tuning, along with an additional doubling (or more) of the intensification, in a broad class of unsteady evolutions producing a dynamic corner flow collapse. These scenarios, triggered purely by changes in the far-field near-surface flow, provide an attractive mechanism for naturally achieving an intense near-surface vortex from a much larger-scale less-intense swirling flow. It is argued that, applied on different scales, this may sometimes play a role in tornadogenesis and/or tornado variability. This phenomenon of corner flow collapse is considered further in a companion paper.

1543 サッカーボールの低回転魔球飛翔のメカニズム(S31 スポーツ流体工学,S31 スポーツ流体工学)

1543 サッカーボールの低回転魔球飛翔のメカニズム(S31 スポーツ流体工学,S31 スポーツ流体工学)

An Experimental Study on Lift Force Generation Resulting from Spanwise Flow in Flapping Wings

Using a combination of force transducer measurement to quantify net lift force, high frame rate camera to quantify and subtract inertial contributions, and Digital Particle Image Velocimetry (DPIV) to calculate aerodynamic contributions in the spanwise plane, the contribution of spanwise flow to the generation of lift force in wings undergoing a pure flapping motion in hover is shown as a function of flapping angle throughout the flapping cycle. These experiments were repeated at various flapping frequencies and for various wing planform sizes for flat plate and span wise cambered wings. Despite the previous identification of the importance of span wise fluid structures in the generation of lift force in flapping wings throughout the existing body of literature, the direct contribution of spanwise flow to lift force generated has not previously been quantified. Therefore, in the same manner as commonly applied to investigate the chordwise lift distribution across an airfoil in flapping wings, spanwise flow due to bulk flow and rotational fluid dynamic mechanisms will be investigated to validate the existence of a direct component of the lift force originating from the flapping motion in the spanwise plane instead.

Microwave generation of stable atmospheric-pressure fireballs in air

The generation of stable buoyant fireballs in a microwave cavity in air at atmospheric pressure without the use of vaporized solids is described. These fireballs have some of the characteristics of ball lightning and resemble those reported by Dikhtyar and Jerby [Phys. Rev. Lett. 96, 045002 (2006)], although of a different color, and do not require the presence of molten or vaporized material. Mechanisms of microwave plasma formation and fluid dynamics can account for the observed behavior of the fireballs, which do not appear to meet the accepted definition of dusty plasmas in this case. Relevance to models of ball lightning and industrial applications are discussed.

Observations of “granular jump” in the pneumatic conveying system

This paper presents a preliminary study of a previously unreported phenomenon of the “gas driven granular jump”, observed in the gas–solids flow within the pneumatic conveying system. From the phenomenological point of view, it resembles the already known processes such as hydraulic jumps in shallow water or granular jumps in granular flows in chutes or avalanches (although it seems most appropriate to explain it by analogy to a propagating granular bore). Clearly, unlike in classical phenomena of this type, the flow itself is driven by the aerodynamic forces related to the gas flow and the behaviour of the front of the “jump” is modified significantly by their presence. A series of high-speed camera visualisations are presented, which focus on this unusual behaviour of the flow on the border-line between cluster and stratified flow regimes in a horizontal pipe. Some similarities are drawn between the observed phenomenon and the broader class of problems exhibiting transition between super- and sub-critical flows. The fluid dynamical aspects and possible mechanisms behind the new phenomenon are discussed and the results obtained are compared quantitatively with simple theoretical models.

Investigation of Zigzag Rising Bubble Using Spinning Sphere Model

A zigzag rising motion is a typical motion of bubbles in bubbly flows. Vortices shedding oscillatory behind a bubble and the wake of the bubble turning periodically similar to a sign of inequality are regarded as the most fundamental causes of the turbulence excited in bubbly flows, because these phenomena produce an alternating lift force on the basis of both conservation laws of circulation and momentum, respectively. In this paper, we discuss the lift production on a zigzag rising bubble trailing a hairpin vortex using a spinning sphere model, which is constructed by us to hold the aerodynamic universality independent of Reynolds number. We fluid-dynamically explicate the alternating lift production on a rising originally nonspin bubble and the zigzag turning of its path, and propose a chain of processes considered in this paper as a fluid-dynamic mechanism that produces the rising bubble zigzag motion.