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AbstractDetailed information on the flow field structure is often important in numerous industrial applications. Although commercial computational fluid dynamics packages are often capable of providing the required data, they are costly and not universally available. This study was motivated by the operation of an open-pool nuclear research reactor where low radiation levels can be maintained by the installation of a stable purified hot water layer in the upper part of the pool. Maintaining a stable stratification requires a detailed description of the structure of the velocity field. Due to the inherent complications and restrictions of performing accurate measurements in a pool of a real-size operating reactor, either smaller-scale models or oversimplified fluid dynamics computational schemes are routinely used. These methods cannot be validated, and therefore do not necessarily capture the large-scale behavior correctly.We present an alternative approach to evaluate the velocity components in the pool that is based on the potential flow theory. The model results are validated by measurements using particle image velocimetry. The presented potential theory allows for the quick and easy assessment of the global properties of the fluid velocity distribution within the pool, and in particular, close to its surface. The suggested computational models are flexible and allow for easily varying the spatial dimensions of the flow field. The technique thus can be upscaled, and enables the validation of numerical computations in various fluid mechanical installations where the flow field cannot be resolved. [ABSTRACT FROM AUTHOR]

Prosthetic heart valve (PHV) replacement has increased the survival rate and quality of life for heart valve-diseased patients. However, PHV thrombosis remains a critical problem associated with these procedures. To better understand the PHV flow-related thrombosis problem, appropriate experimental models need to be developed. In this study, we present an in vitro fibrin clot model that mimics clot accumulation in PHVs under relevant hydrodynamic conditions while allowing real-time imaging. We created 3D-printed mechanical aortic valve models that were inserted into a transparent glass aorta model and connected to a system that simulates human aortic flow pulse and pressures. Thrombin was gradually injected into a circulating fibrinogen solution to induce fibrin clot formation, and clot accumulation was quantified via image analysis. The results of valves positioned in a normal versus a tilted configuration showed that clot accumulation correlated with the local flow features and was mainly present in areas of low shear and high residence time, where recirculating flows are dominant, as supported by computational fluid dynamic simulations. Overall, our work suggests that the developed method may provide data on flow-related clot accumulation in PHVs and may contribute to exploring new approaches and valve designs to reduce valve thrombosis. [ABSTRACT FROM AUTHOR]

Electro-magnetic brake (EMBr), as a necessary technique to adjust the metal flow within the mould, has been extensively applied in the continuous casting (CC) production. The EMBr technique, if appropriately applied, can take the benefit of braking effect to weaken jet impingement effect, suppress fluid disturbance and stabilize surface fluctuation. As a result, to obtain a reasonable flow pattern within the CC mould, the braking performance of EMBr technique needs to be flexibly adjusted according to the variation of operational parameters. In this article, to obtain a braking performance of a commonly used EMBr-single-ruler technique within an optimal range, the effect of operating parameters on the metal liquid flow and thermal characteristics in a compact strip production (CSP) thin slab mould is described through a 3-D mathematical modeling of multi-physical field coupling. The results illustrate that the braking performance of the EMBr-single-ruler device is weakened with the increase of the casting speed or the mould width. For the effect of the casting speed and mould width on the metal flow with the application of an EMBr device, an optimum braking performance of the EMBr-single-ruler device can be obtained under the matching parameters of a magnetic induction intensity of 0.30 T and a casting speed of 4.5 m ∙ min‒1 together with a mould width of 1500 mm. Relative to the absence of the EMBr-single-ruler device, the maximum surface fluctuation height is controlled at 6.2 mm, and the mean surface temperature is raised to 1803.6 K. Based on these findings, we conclude that the proper utilization of the EMBr-single-ruler device can avoid the entrainment of flux powder and improve the melting performance of flux powder. [ABSTRACT FROM AUTHOR]

In this study, the Large Eddy Simulation (LES) model in OpenFOAM was used to investigate the scale effects in the physical modeling of recirculating shallow flow at low Froude numbers. A laboratory test of turbulent flow through a submerged conical island with a Reynolds number of 6,210 was selected. The lab prototype was scaled with factors of 3 and 10 for both undistorted and distorted models. Our study employed the Froude similarity as the gravitational force is more dominant than the others (viscous, drag, and cohesion forces). Because the fluid (water) used for the prototype and model is the same, it is impossible to match the Reynolds, Weber, and Froude numbers simultaneously, resulting in the scale effects. For a scale of 1:1, the LES model could simulate the experimental data by appropriately capturing the vortices behind the conical island. For the undistorted models with scales of 3 and 10, the numerical model captured weaker magnitudes of vortices than the 1:1 scale, indicated by the discrepancies in velocity. In fact, the magnitudes of vortices became weaker with the distorted models. We also observed a significant increment in energy loss behind the conical island (where recirculating flows exist) as the scale increased. However, no significant discrepancies in velocity were observed between the results of the 1:1 scale and the scaled models in front of the conical island, where vortices were absent. These results indicate that the scale effects due to the Froude similarity are quite significant provided that recirculating turbulent flow occurs. [ABSTRACT FROM AUTHOR]

In this study, the Large Eddy Simulation (LES) model in OpenFOAM was used to investigate the scale effects in the physical modeling of recirculating shallow flow at low Froude numbers. A laboratory test of turbulent flow through a submerged conical island with a Reynolds number of 6,210 was selected. The lab prototype was scaled with factors of 3 and 10 for both undistorted and distorted models. Our study employed the Froude similarity as the gravitational force is more dominant than the others (viscous, drag, and cohesion forces). Because the fluid (water) used for the prototype and model is the same, it is impossible to match the Reynolds, Weber, and Froude numbers simultaneously, resulting in the scale effects. For a scale of 1:1, the LES model could simulate the experimental data by appropriately capturing the vortices behind the conical island. For the undistorted models with scales of 3 and 10, the numerical model captured weaker magnitudes of vortices than the 1:1 scale, indicated by the discrepancies in velocity. In fact, the magnitudes of vortices became weaker with the distorted models. We also observed a significant increment in energy loss behind the conical island (where recirculating flows exist) as the scale increased. However, no significant discrepancies in velocity were observed between the results of the 1:1 scale and the scaled models in front of the conical island, where vortices were absent. These results indicate that the scale effects due to the Froude similarity are quite significant provided that recirculating turbulent flow occurs.

For a combustion-based thermophotovoltaic generator (TPV), the baffle-induced reacting flows are numerically investigated in the micro combustor with a multihole baffle representing a mixed feature of bluff body and multiple jets. The lobed flow feature is realized by the momentum difference between fuel and air streams and the baffle wall, not the geometrical conditions like a lobed nozzle. Compared with the H2–air combustor, the lobed flame and the votical structure are enhanced for the CH4–air combustor. To analyze the lobed structure, the lobe intensity and the equivalent perimeter are defined for the stoichiometric mixture line. The maximum lobe intensity is observed at the beginning zone of flame, whereas the equivalent perimeter is increased near the active reaction zone. As the baffle thickness decreases, the lobe structure is enhanced due to the increased vortices. Also, a close relationship between the maximum vorticities at the baffle exit and the maximized lobe structure is observed. The intensified lobe structure contributes to a shorter flame length and more efficient combustion, resulting in increased radiation available for a micro-TPV system. When the lobe structure is enhanced, the high emitter efficiency for an energy conversion device is obtained as 0.319–0.326 for the H2–air combustor.

Most mixed-flow pumps obtain a saddle-like Q-P curve with a backflow, owing to the increased incidence angle at low flow rates. The backflow was developed near the shroud and followed downstream again at its end to form a recirculating flow. The rotating stall, which could be a part of the recirculating flow, followed the impeller’s rotational direction, and its properties affected the local stability. The reattaching flow became strong when the upstream flow from the blade leading edge deviated from the same circumferential degree as the dominant flow of the rotating stall heading downstream. The fluctuation in the total pressure rise decreased when the average incidence angle was smaller than that of the design flow rate. As a passive control to suppress the saddle and the above flow patterns, the impeller inlet diameter was reduced from the shroud, and the inlet blade angle was further adjusted to maintain the incidence angle. From the reduced inlet diameter, the backflow was mostly suppressed, and the saddle was improved with a wider operating range. Here, the performance near the design flow rate was almost maintained. The stability was evaluated using the fast Fourier transform, and the numerical method was validated through experimental tests.

For a combustion-based thermophotovoltaic generator (TPV), the baffle-induced reacting flows are numerically investigated in the micro combustor with a multihole baffle representing a mixed feature of bluff body and multiple jets. The lobed flow feature is realized by the momentum difference between fuel and air streams and the baffle wall, not the geometrical conditions like a lobed nozzle. Compared with the H2–air combustor, the lobed flame and the votical structure are enhanced for the CH4–air combustor. To analyze the lobed structure, the lobe intensity and the equivalent perimeter are defined for the stoichiometric mixture line. The maximum lobe intensity is observed at the beginning zone of flame, whereas the equivalent perimeter is increased near the active reaction zone. As the baffle thickness decreases, the lobe structure is enhanced due to the increased vortices. Also, a close relationship between the maximum vorticities at the baffle exit and the maximized lobe structure is observed. The intensified lobe structure contributes to a shorter flame length and more efficient combustion, resulting in increased radiation available for a micro-TPV system. When the lobe structure is enhanced, the high emitter efficiency for an energy conversion device is obtained as 0.319–0.326 for the H2–air combustor. [ABSTRACT FROM AUTHOR]

Most mixed-flow pumps obtain a saddle-like Q-P curve with a backflow, owing to the increased incidence angle at low flow rates. The backflow was developed near the shroud and followed downstream again at its end to form a recirculating flow. The rotating stall, which could be a part of the recirculating flow, followed the impeller's rotational direction, and its properties affected the local stability. The reattaching flow became strong when the upstream flow from the blade leading edge deviated from the same circumferential degree as the dominant flow of the rotating stall heading downstream. The fluctuation in the total pressure rise decreased when the average incidence angle was smaller than that of the design flow rate. As a passive control to suppress the saddle and the above flow patterns, the impeller inlet diameter was reduced from the shroud, and the inlet blade angle was further adjusted to maintain the incidence angle. From the reduced inlet diameter, the backflow was mostly suppressed, and the saddle was improved with a wider operating range. Here, the performance near the design flow rate was almost maintained. The stability was evaluated using the fast Fourier transform, and the numerical method was validated through experimental tests. [ABSTRACT FROM AUTHOR]

Chamfered circular ring (CCR) innovated from conventional circular ring by introducing chamfered surface on its periphery at 30o attack angle is numerically investigated to study its effect on fluid flow characteristics, heat transfer enhancement, and friction factor in a heat exchanger pipe. In order to study flow blockage effect on overall performance of device, three diameter ratios (DR = Di/Do = 0.6, 0.7 and 0.8) with three spacing ratios (SR = s/D = 1, 2 and 3) are used. Turbulent model k – ε, RNG with enhanced wall treatment is used to solve governing equations. Results showed that CCR chamfered surface produced nozzle jet effect on pipe wall surface through clearance of 1 mm, which further induced turbulent and recirculating flow effect near pipe wall area. This resulted into higher heat transfer rate. All the diameter ratios with SR = 1 showed thermal performance factor value more than unity at most of the lower Reynolds number. The range of TPF shown by DR = 0.6, 0.7 and 0.8 with SR =1 is 1.21–0.93, 1.23–0.97, and 1.19–0.99, respectively. Compromised results of TPF are shown by DR = 0.7 and DR = 0.8 at SR = 1. Chamfered circular ring has shown better results over circular ring, perforated conical ring, short length twisted tape, twin co-twisted tape, wire rod bundles, and perforated twisted tape. Numerical results showed better agreement with predicted results within the range of 10%. Highest TPF of 1.23 is shown by DR = 0.7 and SR = 1 at Re 6000. Graphical Abstract [ABSTRACT FROM AUTHOR]

Microfluidic devices or systems to generate recirculating flow are increasingly utilized in the fields of chemistry, biomedicine, biology, with bottlenecks in integration and miniaturization needed a breakthrough. Here, a concept for a highly integrated valve-less piezoelectric micropump generating recirculating flow (VPMGRF) is proposed, simultaneously realizing innovation in theory and method. Based on fluid inertiaand energy dissipation of vortexes, a novel double-loop tube was designed to achieve fluid flow in different directions along various paths, with the flow characteristics at different cross-sectional areas explored through computational fluid dynamics (CFD). Afterwards, as valve bodies, the double-loop tubes were connected with a piezoelectric-actuated chamber to constitute the VPMGRF, which was followed by establishing a theoretical model of the vibration, fluid dynamics, and net flow rate. Eventually, experimental results showed that VPMGRFs could generate internal recirculating flow and had a pump effect. The maximum net flow rate reached 5.19 mL/min at 8 Hz, corresponding to a vibrator amplitude of 35 μm and an output pressure amplitude of 4.27 kPa. Furthermore, the larger the characteristic length of the cross section, the greater the amplitude of the vibrators, the larger the amplitude of output pressure, and the slower the flow rate.

Microfluidic devices or systems to generate recirculating flow are increasingly utilized in the fields of chemistry, biomedicine, biology, with bottlenecks in integration and miniaturization needed a breakthrough. Here, a concept for a highly integrated valve-less piezoelectric micropump generating recirculating flow (VPMGRF) is proposed, simultaneously realizing innovation in theory and method. Based on fluid inertiaand energy dissipation of vortexes, a novel double-loop tube was designed to achieve fluid flow in different directions along various paths, with the flow characteristics at different cross-sectional areas explored through computational fluid dynamics (CFD). Afterwards, as valve bodies, the double-loop tubes were connected with a piezoelectric-actuated chamber to constitute the VPMGRF, which was followed by establishing a theoretical model of the vibration, fluid dynamics, and net flow rate. Eventually, experimental results showed that VPMGRFs could generate internal recirculating flow and had a pump effect. The maximum net flow rate reached 5.19 mL/min at 8 Hz, corresponding to a vibrator amplitude of 35 μm and an output pressure amplitude of 4.27 kPa. Furthermore, the larger the characteristic length of the cross section, the greater the amplitude of the vibrators, the larger the amplitude of output pressure, and the slower the flow rate. [ABSTRACT FROM AUTHOR]

This paper presents the experiments performed on a high head model Francis turbine during start-up. Synchronized time dependent pressure and velocity measurements were performed to investigate the instabilities in the turbine. A total of four steady state operating points, namely synchronous load, part load, best efficiency point, and high load are considered to perform the turbine start-up. The runner angular speed was observed to increase almost exponentially during the guide vane positions from completely closed to no load condition. The frequency of wave propagation due to the interaction between runner blades and guide vanes was observed to follow the trend of increase of runner angular speed. A vortex rope frequency was captured in the draft tube during synchronous load to part load of the start-up. Two different mechanisms, namely, the development of stagnation point and the available recirculation regions were observed to cause the formation of vortex rope in the draft tube. [ABSTRACT FROM AUTHOR]

An in-pond raceway system (IPRS) is an effective intensive aquaculture practice for regions with high water consumption and limited land resources. Water flow and dissolved oxygen (DO) are important for sustainable aquaculture. Several innovations have been made in IPRS design and operation to increase water exchange and DO concentration; one of these is the aeration plug-flow device (APFD). The APFD is commonly used in China as the only power source for water recirculation in aquaculture ponds. Understanding of the hydrodynamics of the system is necessary to improve the design of the IPRS with APFD. To this end, we performed experimental studies on a model system. We measured three-dimensional velocity at various locations using an Acoustic Doppler Velocimeter. Velocity distribution and turbulence characteristics were assessed, and plug-flow characteristics were analysed. Two patterns of velocity and turbulence in horizontal sections were observed: near the APFD, the water flow was intensively pushed downstream and simultaneously recirculated; farther away, the reflux area gradually decreased and the velocity and turbulence distribution trended towards uniform. Secondary flows occurred in different directions, which improved the diffusion of materials and DO retention. The system is effectively self-circulating, and the plug-flow capability may be scaled up for commercial application.

Highlights • Fluid recirculation significantly increases primary particle diameter. • Recirculating and non-recirculating flames both display shell-core nanostructure. • Recirculating soot has both single core and multiple core primary particles. • Proposed secondary soot growth mechanism in flames with complex flow patterns. Abstract While the majority of studies explore soot formation in relatively simple, one-dimensional flames, most real-world flames consist of complex flows defined by large-scale turbulent eddies, recirculating flow patterns, and buoyancy effects. The effects of complex flow on soot physicochemical properties are poorly understood. This work employs an inverted gravity flame reactor (IGFR) to compare differences in soot growth between a one-dimensional laminar diffusion flame and a recirculating flame. Computational fluid dynamics (CFD) and experimental observations show particle oscillations between (i) a rich region with a high concentration of surface growth species, and (ii) a high-temperature oxidation region. Transmission electron microscopy (TEM) shows a significant difference in final primary particle diameter, where the one-dimensional flame produces primary particles 10–25 nm in diameter and the recirculating flame produces primary particles 25–75 nm in diameter. Additionally, larger primary particles from the recirculating flame contain both single and multiple cores. We propose that due to the spheroidal shape of the large primary particles, the secondary surface growth is primarily a result of polyaromatic hydrocarbon (PAH) condensation during re-entrainment of mature soot into the fuel-rich region followed by subsequent liquid layer carbonization in the high-temperature environment of the flame front. The recirculating flow patterns in the IGFR and repeated particle growth/oxidation cycle can serve as a model for soot formation in the large-scale flames with complex flow patterns, such as forest fires, coal fire plants, and other sources. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of a well-established vortex flow receiver and a proposed recirculating flow Solid particle solar receiver (SPSR) capable of achieving high air temperature which is perfectly suitable for the hybrid solar thermal power plant. Solar particle receivers are modeled using discrete particle model (DPM), RNG k- ε flow model and discrete ordinate (DO) radiation model. Numerical analysis is done by considering two cases (i) Recirculating flow solar receiver with non-reacting particle and without non-reacting particle as a medium of transferring heat (ii) Vortex flow solar receiver with non-reacting particle and without non-reacting particle as a medium of transferring heat. The examination is carried out to compare the parametric sensitivity of both solar particle receivers with particle and air and without particle as a medium of transferring heat by considering the solar flux that incident on the aperture of the receiver and varying the air flow rate and particle concentration, recirculation rate. The results depict that use of particle enhance the heat transfer for both cases. It is concluded from the thermal behavior of recirculating flow SPSR that about 30% higher cavity thermal efficiency and exhibit higher and uniform heat transfer than a vortex flow SPSR. [ABSTRACT FROM AUTHOR]

An entropy generation analysis for the Cu-water nanofluid flow through a wavy channel over heat exchanger plat is investigated. Entropy generation is expressed as a function of velocity and temperature. Governing equations, containing mass conservation, momentum and energy equations, are solved by a finite volume technique. All simulations are performed with Ansys-Fluent. The effects of physical parameters such as Reynolds number, dimensionless amplitude, nanoparticles volume fraction and wave number on the total, thermal, and viscous entropy generation rates and Bejan number are examined. The obtained results indicate that the thermal entropy generation is main term in most part of the channel, including near the wavy walls. Moreover, the rise in viscous entropy generation with Reynolds number increases with increasing dimensionless amplitude. [ABSTRACT FROM AUTHOR]

The noise level in the cabin of a car has become one of the primary concerns for automobiles and cooling fans are one of the main noise sources. This paper investigates the source of broadband noise from a shrouded automotive cooling fan using a hybrid approach. Based on the surface pressure data obtained by CFD, the radiated noises are predicted using acoustic analogy. The accuracy of noise prediction and the reliability of the model analysis are verified by comparing both numerical and experimental results. The pressure information on the blade surface is analyzed to clarify the sources of broadband noise induced by the geometry-wake interaction. In future, these results can anticipate reducing the broadband noise in automotive cooling fans. [ABSTRACT FROM AUTHOR]

Recently, small hydropower attracts attention because of its clean, renewable and abundant energy resources to develop. Therefore, a cross-flow hydraulic turbine is proposed for small hydropower in this study because the turbine has relatively simple structure and high possibility of applying to small hydropower. The purpose of this study is to investigate the effect of the turbine's structural configuration on the performance and internal flow characteristics of the cross-flow turbine model using CFD analysis. The results show that nozzle shape, runner blade angle and runner blade number are closely related to the performance and internal flow of the turbine. Moreover, air layer in the turbine runner plays very important roles of improving the turbine performance.

Hemodynamics has been recognized as an important factor in the development, growth, and rupture of cerebral aneurysms, and investigated by computational fluid dynamics techniques using a single phase approach. However, flow-dependent cell transport and interactions are usually ignored in single phase models, in which blood is usually treated as a single phase Newtonian fluid. For getting better insight into the underlying pathology of intracranial aneurysm, cell transport and interactions should be covered in hemodynamic studies. In the present study, a multiphase hemodynamic model incorporating cell transport and interactions was developed, in which blood was modeled as multiphase fluid having a continuous phase (plasma) and two particulate phases (erythrocytes and leukocytes). The model showed good agreement with experimental data and observations in the literature, and was applied to four patient-specific aneurysms in a pulsatile manner. Leukocyte accumulations were predicted at locations with flow disturbance and low wall shear stress. The concentrations of leukocyte at accumulation sites were found to exceed 200 to 500% of normal physiological level on three unstable aneurysms, including two ruptured aneurysms and a growing aneurysm where accumulation was observed near a daughter sac and a secondary aneurysm. This suggested that aneurysms with complex secondary flow patterns could be prone to leukocyte accumulation on the wall. As this is the first study to characterize cell transport and interactions in aneurysm hemodynamics, our model can serve as a foundation for future intracranial aneurysm models. [ABSTRACT FROM AUTHOR]

The use of Concentrated Solar Power technology for energy generation relies on the heating of fluids to high temperatures. One of the limitations of this technology is the low working efficiency of current solar receivers which hinders its application and utilization as a renewable clean energy source. This paper presents a novel concept to improve concentrated solar receiver efficiency through the recirculation of solid particles laden flow within the receiver. The recirculating characteristic of the fluid and solid particle mixture is expected to generate higher output fluid temperature than conventional solar receivers. This is expected to lead to significant improvements in the working performance of the concentrated solar receiver. The numerical study investigates this concept, using a commercial CFD package, employing the discrete particle model (DPM), a RNG k- ε flow model and a discrete ordinate (DO) radiation model. It is found that the use of recirculating solid particles improves the energy absorption within the receiver and with it the thermal efficiency of the proposed solar receiver reaches to around 70%. Moreover, comparing this with a conventional solar receiver, this new concept exhibits a more uniform temperature distribution inside the cavity, which is mainly due to the recirculating flow characteristic and the well-controlled mixing of air and particles. The advantages of this novel concept of the solar receiver present its potential benefits in further applications in the process of energy conversion providing on enhanced heat transfer medium. [ABSTRACT FROM AUTHOR]

• The geometric features related to the axis-switching phenomenon are numerically investigated for a multihole baffled micro combustor. • In the combustor of noncircular holes, secondary flows are more developed. • The flow mixing and combustion efficiency are improved by noncircular air holes. • The best thermal field for a combustion-based TPV system is obtained for baffles with 30°-rotated square holes and 15°-rotated triangular holes. • The emitter efficiency for TPV applications is better than the swirling flow combustor. For a thermophotovoltaic (TPV) generator, a combustor design for strengthening the axis-switching phenomenon is explored. Various simulations based on a Reynolds stress model and detailed chemical mechanism of GRI 3.0 are performed for methane (CH 4)–air flames in a multihole baffled combustor. The reacting flow fields are investigated with the baffle plates of rotated square and triangular holes. The spreading and merging of multiple air jets past the baffle plate are significantly affected by rotating noncircular air holes. Considering the crossover location changes of the jet half-width, the air jets have a flow structure that quickly generates the axis-switching phenomenon. Compared with the baffled combustor of circular air holes, the center recirculation zone in the combustor of noncircular holes is enlarged by approximately 200% with a hole rotation. As the hole rotation increases, secondary flows become more complicated and the merging of air jets is quickly finished. Based on these flow structures, the region with the stoichiometric condition is observed in various forms of hexagonal, star, circular, and flower shapes. As a result, the flammable region and combustion efficiency are considerably increased. The flame lengths are roughly evaluated as three times the mixing lengths. From the viewpoint of a heat emitter for the TPV application, the heat transfer on the combustion chamber wall is discussed. Compared with the combustor with circular air holes, the combustor with noncircular air holes shows a 5% increase in flammable region and a 5.9% improvement in combustion efficiency. For a combustion-based TPV system, the best uniformity of wall temperature is observed for baffles with 30°-rotated square holes and 15°-rotated triangular holes and the highest mean wall temperatures are obtained when the flame length is approximately 1/3 of the combustor length. [ABSTRACT FROM AUTHOR]

Functionalized nanoparticles (NPs) offer diverse biomedical applications, but their synthesis is complex, costly, and labour-intensive, particularly when providing for additional functionalization requirements which are a key feature of biomedical applications. Pulsed laser ablation in liquids (PLAL) has previously allowed for the synthesis of metal and metal oxide nanoparticles using metal targets, which can then be surface functionalized during synthesis by the surrounding liquid species. Therefore, it should be possible to achieve biomolecule functionalization by ablating in biomolecular solutions. We explore a novel controlled recirculation PLAL scheme which should increase functionalization and productivity of functionalized nanoparticles. Traditional PLAL was performed by ablating a silicon target in a DNA solution. We have extended beyond traditional approaches by ablating the silicon target under novel flow conditions in a controlled recirculating loop of DNA solution. Ablating in a DNA solution allows for high efficiency binding of DNA to silicon nanoparticles (SiNPs) in a single step process. By using SiNPs we are significantly reducing the overall cost of the process, when compared with the more traditional use of gold or silver; as well as, using a biocompatible material with an affinity for protein and nucleic acid binding. Reducing the laser shielding effects of particles and debris by removing them from the ablation site produces higher volumes and concentrations of functionalized colloid. Recirculating this liquid over the target has resulted in a 50% relative increase in binding efficiency, compared with static and single-pass flow conditions processing, achieving an average maximum binding efficiency of 78% in flow compared to 52% under static conditions. Furthermore, by reducing the initial DNA concentration, we were able to achieve 100% binding efficiency, which we believe to be the highest reported in literature to date. [Display omitted] • A recirculatory flow configuration of pulsed laser ablation in liquids was demonstrated. • Higher functionalization efficiencies and nanoparticle concentrations were obtained compared to previously established flow set-ups. • 100% functionalization efficiency is achievable for nucleic acids with low-cost, biocompatible silicon nanoparticles. [ABSTRACT FROM AUTHOR]

Recirculating fluid regions occur in the human body both naturally and pathologically. Diffusion is commonly considered the predominant mechanism for mass transport into a recirculating flow region. While this may be true for steady flows, one must also consider the possibility of convective fluid exchange when the outer (free stream) flow is transient. In the case of an open cavity, convective exchange occurs via the formation of lobes at the downstream attachment point of the separating streamline. Previous studies revealed the effect of forcing amplitude and frequency on material transport rates into a square cavity (Horner in J Fluid Mech 452:199-229, ). This paper summarizes the effect of cavity aspect ratio on exchange rates. The transport process is characterized using both computational fluid dynamics modeling and dye-advection experiments. Lagrangian analysis of the computed flow field reveals the existence of turnstile lobe transport for this class of flows. Experiments show that material exchange rates do not vary linearly as a function of the cavity aspect ratio ( A = W/H). Rather, optima are predicted for A ≈ 2 and A ≈ 2.73, with a minimum occurring at A ≈ 2.5. The minimum occurs at the point where the cavity flow structure bifurcates from a single recirculating flow cell into two corner eddies. These results have significant implications for mass transport environments where the geometry of the flow domain evolves with time, such as coronary stents and growing aneurysms. Indeed, device designers may be able to take advantage of the turnstile-lobe transport mechanism to tailor deposition rates near newly implanted medical devices. [ABSTRACT FROM AUTHOR]

Turbulent flow characteristics and heat transfer performances in square channels with different cylindrical-shaped grooves are analyzed and compared numerically in this research. The novel groove geometries are conventional cylindrical grooves with rounded transitions to the adjacent flat surfaces and with modifications to their bases. The objective of this work is to determine optimal configuration for augmenting heat transfer rates with minimal pressure drop penalties. The paper documents also provide the flow details near the groove surface. All turbulent fluid flow and heat transfer results are obtained using computation fluid dynamics with a verified v 2 f turbulence closure model. Five rectangular channels with different cylindrical groove shapes are computed. Heat transfer enhancement and flow details are analyzed and compared with results for conventional cylindrical groove geometry and for conventional square ribs. This investigation shows that the conventional cylindrical grooves have similar overall heat transfer enhancement with conventional square ribs, but the pressure loss penalty is much decreased from square rib values. The rounded transition of the grooves has a large advantage over conventional cylindrical grooved surfaces in both enhancing heat transfer and reducing pressure loss penalty. For the rounded-transition grooves, recirculating flows inside the groove are reduced and reattachment develops more smoothly and the separating zone is reduced compared with the flow over the conventional cylindrical groove. The velocity magnitude in the near-wall region of grooved surface is much larger compared with that over a ribbed surface, which essentially causes the larger heat transfer enhancement for the groove surfaces. The design for the rounded transitions is shown to improve the overall thermal performance for channel internal cooling. [ABSTRACT FROM AUTHOR]

The enhancement of heat transfer attributed from rib turbulators relative to the increased pressure drop in the channel is a crucial design parameter. Thus, the design of the truncated ribs (whose length is less than the passage width) provides options to address such cooling requirements when the pressure loss is a critical factor. Considering different types of truncated ribs, mid-truncated ribs (which are truncated in the central part of the rib) have been proved to show better thermal performance than other types of truncated ribs. A numerical study of mid-truncated ribs with different offset placements or angles on turbulent heat transfer inside a non-rotating cooling passage of a gas turbine blade is performed for inlet Reynolds number ranging from 10,000 to 50,000. The overall performance characteristics of six different types of mid-truncated ribbed arrangements are also compared: Cases A1, A2, A3 utilize mid-truncated spanwise ribs with different relative rib stagger positions at different spanwise locations; Cases B, C, D utilize mid-truncated angled ribs with the ribs oriented at ±45° (Case B), +45°/+45° (Case C), and ±135° (Case D). In all cases, a gap is present between the ribs placed on opposite spanwise parts of the channel, to provide the mid-truncation. It is found that the 135° mid-truncated rib (Case D) has the highest heat transfer coefficient, while the 90° mid-truncated ribs with no offset placement (Case A1) behave best in reducing pressure loss penalty. Although Case A shows larger heat transfer augmentation, Case D is advantageous for augmenting side-wall heat transfer when the pressure loss is considered and the Reynolds number is comparatively large. Case C exhibits the best overall thermal performance over the largest range of experimental conditions when the pressure drop is also considered. Staggered arrangement for 90° mid-truncated ribs can enhance heat transfer efficiently and makes a good overall performance at low Reynolds numbers. Case A1 can be used in practical operation because of reduced weight and good thermal performance at high Reynolds numbers. This is the first study on various offset mid-truncated ribs aiming to improve the heat transfer of turbine blade internal cooling passages with reduced pressure loss penalty. [ABSTRACT FROM AUTHOR]

Assessing the amount of energy dissipation that can be achieved by simple jet diffusion in a confined environment has considerable practical importance. Previous works on jets in confined enclosures have indicated that the entrainment of a recirculating fluid greatly enhances the dissipation of the inflow kinetic energy and reduces the length of the potential core. In the present experimental work, a vertical jet issuing into a rectangular chamber is investigated under two types of confinement and three locations of entry. Detailed velocity measurements for an eccentric jet are obtained to assess the evolution of the axial jet centreline velocity and the turbulent characteristics. These results are compared with classical jet behaviour and jets under other confined conditions. A method to predict the energy dissipation attained by jet diffusion in a confined chamber is also proposed. [ABSTRACT FROM AUTHOR]

Abstract: In the reviewed literature about swirling flows through ducts of circular cross section, there are contradictions regarding the detection and measurement of the inverse axial flow generated in the center of a swirling flow. The main motivation for this investigation was to determine the existence of the inverse flow and the feasibility of measuring it by the flying hot wire technique using unidimensional and bidimensional hot wires driven with an intermittent linear servomotor with a maximum velocity of 2m/s and with a distance trip of 0.595m. The inverse flow was detected with both sensors but it was not constant during the maximum speed trip. The maximum speed and the duration of the inverse flow was obtained with the unidimensional sensor up to a radial position of 3cm, although the axial position in which the inverse flow is detected, it is not the same in every radial position. Therefore, it is not possible to make a radial distribution of the axial velocity using only one sensor. Finally, in the center of the swirling flow there is an intermittent inverse flow similar to the recirculating flow that is generated in the wake of a body in relative motion to the fluid mass. [Copyright &y& Elsevier]

Abstract: Brian Spalding did not invent CFD. He did not even coin the name. But more than anyone else, he created the practice of CFD – its application to problems of interest to engineers. The author was associated with, and was an integral part of the team led by, Prof. Spalding that developed the basic engineering practice that came to be known as the Imperial College (IC) approach to “CFD”. Most of today’s commercially available CFD software tools trace their origin to the work done by the IC group in the decade spanning the mid-60s and mid-70s. This paper is a personal recollection of the key moments of the CFD developments at Imperial College and the role played by Brian Spalding as a leader of, and as an active contributor to, the IC Group. His key insights during this decade often made breakthroughs possible and re-directed the focus at critical moments. The paper also explores the opportunities missed by the IC Group during this decade of breakneck progress in CFD. [Copyright &y& Elsevier]

The flow over a stepped spillway is free surface, multiphase and turbulent. Moreover, a recirculation region occurs at each step of the spillway. The flow characteristic is hence complicated and difficult to predict. In previous work on stepped spillways (Chen et al. Journal of Hydraulic Engineering, ASCE, 128 (7), 683-688 and Xiangju et al. Science in China Series E: Technological Sciences, 49 (3), 674-684), only the linear turbulence model was applied which gave low accuracy. Furthermore, there are still no optimised model constants for this kind of flow. The present work therefore aims to apply the large eddy simulation (LES), the non-linear and the modified non-linear turbulence models to simulate the flow over the spillways with various slopes and step heights. It is found that the modified model is able to predict the flow close to the LES but requires less computational time by a factor of up to 4. Moreover, the modified model gives higher accuracy than the LES for the cases of the spillways with higher slopes and step heights. [ABSTRACT FROM AUTHOR]

Abstract: The development of light and efficient Compact Heat Exchangers (CHE) for automotive or aerospace applications has motivated recent research for the energetic optimisation of these devices. Owing to their geometrical complexity, such a goal requires valuable knowledges on the more simple case of a single plate in a fluid flow. The purpose of this study is to present an experimental and numerical study of the heat transfer between an incompressible fluid and a blunt flat plate with constant wall temperature. An experimental test rig is built in which air flows around a constant temperature copper plate. The Reynolds number defined on the plate thickness is varied from 120 to 500. Flow measurements are achieved with laser anemometry and the temperature field is determined using the cold wire technique. For each experiment, the experimental data are compared to Direct Numerical Simulations. Classical features of the flow are observed, i.e. a recirculating vortex near the leading edge of the plate, which is followed by a boundary layer from which vortices are alternately shed from both faces of the plate. Unlike previous studies, a particular attention is paid to the longitudinal distribution of the heat transfer coefficient for low to moderate Reynolds numbers. It is shown that a Direct Numerical Simulation of the flow and heat transfer around a thick plate is an efficient tool of optimisation which could be used for the design of industrial CHE. [Copyright &y& Elsevier]

A transonic/supersonic axisymmetric backward facing step nozzle flow in an air-jet loom has been analyzed numerically by using a time accurate characteristic based upwind flux difference splitting compressible Navier-Stokes method. The unsteady pressure and Mach number behavior along the center line of the main nozzle were analyzed by periodic inlet condition changes to simulate the intermittent flow inside main nozzle of an air-jet loom. [ABSTRACT FROM AUTHOR]

Hot-wire measurements of the full mapping of the velocity and Reynolds stress components are reported for developing turbulent flow in a strongly curved 180 deg pipe and its tangents. A slanted wire is rotated into 6 orientations and the voltage outputs from wires are combined to obtain the mean velocity and Reynolds stress components. The strength of secondary flow reaches up to the 28% of bulk mean velocity. The strong counter-rotating vortex pair induced by the transverse pressure gradient and centrifugal force imbalance grows up to Θ = 67.5° into the bend. But the vortex pair breaks down into two cell pattern after Θ=90° Core vortex formation and reversal of secondary flow direction along the bend symmetry plane is cleanly found in the secondary vector plot. At Θ=67.5° and Θ = 90° into bend a large “trough” develops in the longitudinal velocity toward the inside of the bend due to the breakdown of secondary flow. In the bend, the mean longitudinal velocity component changes little after Θ=90°, but secondary flow never achieves fully-developed state. Similar behaviors are observed in the radial and circumferential stresses. [ABSTRACT FROM AUTHOR]

Abstract: A charged spherical particle is concentrically positioned in a converging–diverging nanotube filled with an electrolyte solution, resulting in an electric double layer (EDL) forming around the particle''s surface. In the presence of an axially applied electric field, the particle electrophoretically migrates along the axis of the nanotube due to the electrostatic and hydrodynamic forces acting on the particle. In contrast to a cylindrical nanotube with a constant cross-sectional area in which the electric field is almost uniform, the presence of a converging–diverging section in a nanotube alters the electric field, perturbs the charge distribution, and induces a pressure gradient and a recirculating flow that affect the electrostatic and hydrodynamic forces acting on both the particle and the fluid. Depending on the magnitude of the surface charge density along the nanotube''s wall, the particle''s electrophoretic motion may be significantly accelerated as the particle transverses the converging–diverging section. A continuum model consisting of the Nernst–Planck, Poisson, and Navier–Stokes equations for the ionic concentrations, electric potential, and flow field is implemented to compute the particle''s velocity as a function of the particle''s size, the nanotube''s geometry, surface charges, electric field intensity, bulk electrolyte concentration, and the particle''s location. When the particle is negatively charged and the wall of the nanotube is uncharged, the particle migrates in the direction opposite to that of the applied electric field and the presence of the converging–diverging section significantly accelerates the particle''s motion. This, however, is not always true when the nanotube''s wall is charged with the same sign as that of the particle. Once the ratio of the surface charge density of the nanotube''s wall to that of the particle exceeds a certain value, the negatively charged particle will not translocate through the tube toward the anode and does not attain the maximum velocity at the throat of the converging–diverging section. One can envision such a device to be a nanofilter that allows molecules with surface charge densities much higher than that of the wall to go through the nanofilter, while preventing molecules with surface charge densities lower than that of the wall from passing through the nanofilter. The induced recirculating flow may be used to enhance mixing and to stretch, fold, and trap molecules in nanofluidic detectors and reactors. [Copyright &y& Elsevier]

Abstract: Using an applied electrical field to drive fluid flows becomes desirable as channels become smaller. Although most discussions of electroosmosis treat the case of thin Debye layers, here electroosmotic flow (EOF) through a constricted cylinder is presented for arbitrary Debye lengths () using a long wavelength perturbation of the cylinder radius. The analysis uses the approximation of small potentials. The varying diameter of the cylinder produces radially and axially varying effective electric fields, as well as an induced pressure gradient. We predict the existence of eddies for certain constricted geometries and propose the possibility of electrokinetic trapping in these regions. We also present a leading-order criterion which predicts central eddies in very narrow constrictions at the scale of the Debye length. Eddies can be found both in the center of the channel and along the perimeter, and the presence of the eddies is a consequence of the induced pressure gradient that accompanies electrically driven flow into a narrow constriction. [Copyright &y& Elsevier]