Abrupt Expansion Research Articles

Response surface analysis of nozzle parameters at supersonic flow through microjets

ABSTRACT Base pressure is a crucial component in the measurement of flow parameters in a high-speed aerodynamic flow. In this paper, the microjets impact as a control mechanism is experimentally tested for the nozzles with abrupt expansion at supersonic Mach in an axisymmetric conduit. The flow regulation mechanism is placed at a 90-degree interval in the shape of an orifice of 0.5 mm in radius along the nozzle’s exit diameter, which generates jets at sonic Mach numbers. The flow constraints studied are inertia level (Mach number), expansion level (NPR), and the geometric parameters considered are the pipe’s length (L/D). These three relevant parameters were selected for design of experiments (DOE). In the management of base pressure, this analysis’s primary objective is to evaluate the parameters influencing the flow. The experiments were carried out in two ways: without and with microjets. For the DOE, an L27 orthogonal series, polynomial expression, analysis of variance, and predicted plots were carried out to test the experimental findings. The established prototypes are statistically appropriate and achieved when making precise projections for all the cases. According to the present results, the L/D ratio for a given parameter is the most critical parameter influencing the maximum increase or decrease in the base pressure.

Improvement of the heat transfer enhancement model considering the droplet-wall heat transfer downstream of the flow blockage in the reflood phase

When a flow blockage is formed by fuel swelling in the reflood phase of a large-break loss-of-coolant-accident (LOCA) in a pressurized water reactor (PWR), flow separation and reattachment occur due to the flow area change downstream of the blockage. These phenomena increase turbulence and vorticity in the downstream, resulting in the enhancement of heat transfer by steam and liquid droplets. The existing heat transfer enhancement model has considered the enhancement factor for the single-phase heat transfer only. For the assessment of the existing model, it was implemented in the CUPID code in which a two-fluid three-field model is applied. The existing model was evaluated using the FEBA flow blockage test without flow bypass. As a result, the existing model greatly over-predicted the rod temperature downstream of the blockage. To improve this, a new enhancement model was proposed that considers the wall heat transfer by the droplet deposition. The heat transfer enhancement factor was derived based on the data from the abrupt pipe expansion experiments that measured heat and mass transfer coefficients after flow separation point. For accurate calculation of the amount of droplets approaching the fuel rod, an improved droplet deposition model was developed using the droplet deposition experimental data for dispersed droplet and annular-mist flow. The new model implemented in the CUPID code was evaluated using the FEBA flow blockage tests with different pressure, flooding rate, and blockage ratio. It was shown that the new model gave good predictions for the temperature of the early cooled rod bundle downstream of the blockage.

Stabilization of Unsteady Nonlinear Waves by Phase-Space Manipulation

We introduce a dynamic stabilization scheme universally applicable to unidirectional nonlinear coherent waves. By abruptly changing the waveguiding properties, the breathing of wave packets subject to modulation instability can be stabilized as a result of the abrupt expansion a homoclinic orbit and its fall into an elliptic fixed point (center). We apply this concept to the nonlinear Schrödinger equation framework and show that an Akhmediev breather envelope, which is at the core of Fermi-Pasta-Ulam-Tsingou recurrence and extreme wave events, can be frozen into a steady periodic (dnoidal) wave by a suitable variation of a single external physical parameter. We experimentally demonstrate this general approach in the particular case of surface gravity water waves propagating in a wave flume with an abrupt bathymetry change. Our results highlight the influence of topography and waveguide properties on the lifetime of nonlinear waves and confirm the possibility to control them.

Adherens junction engagement regulates functional patterning of the cardiac pacemaker cell lineage.

Cardiac pacemaker cells (CPCs) rhythmically initiate the electrical impulses that drive heart contraction. CPCs display the highest rate of spontaneous depolarization in the heart despite being subjected to inhibitory electrochemical conditions that should theoretically suppress their activity. While several models have been proposed to explain this apparent paradox, the actual molecular mechanisms that allow CPCs to overcome electrogenic barriers to their function remain poorly understood. Here, we have traced CPC development at single-cell resolution and uncovered a series of cytoarchitectural patterning events that are critical for proper pacemaking. Specifically, our data reveal that CPCs dynamically modulate adherens junction (AJ) engagement to control characteristics including surface area, volume, and gap junctional coupling. This allows CPCs to adopt a structural configuration that supports their overall excitability. Thus, our data have identified a direct role for local cellular mechanics in patterning critical morphological features that are necessary for CPC electrical activity.

Decadal and multidecadal natural variability of African rainfall

Study regionAfrica Study focusAfrican rainfall shows significant year-to-year natural fluctuations that in part are linked to teleconnections associated with modes of variability in the Atlantic, Pacific and Indian oceans. A better understanding of African rainfall variability and potential drivers would help to better prepare African societies for anticipated droughts and floods by taking early precautionary action. Here we are presenting the first continent-wide analysis of African rainfall variability on a month-by-month and country-by-country basis. We have calculated Pearson r values for smoothed monthly rainfall data of 49 African countries over the period 1901–2017 which we compared to six potential climatic drivers of natural variability, namely AMO, NAO, ENSO (El Niño Southern Oscillation), Pacific Decadal Oscillation (PDO), Indian Ocean Dipole (IOD) and solar activity changes. We allowed time lags of up to 11 months for each potential driver (66 months for solar activity). New hydrological insights for the regionThe dynamic temporal-spatial evolution of the seasonal Pearson correlations was mapped out across the continent, tracking the gradual or abrupt expansion, displacement and subsequent waning of the various effects over the course of the year. Relationships are complicated by characteristic time lags, non-stationary correlations and occasional phase shifts. Our empirical results may help to further improve short- to midterm rainfall prognoses in Africa and provide important calibration data for the further improvement of climate models.

Effect of Molar Concentration on the Flow Behavior of Micellar Solutions Passing through Small Slits

The flow behavior of three types of surfactant solution (namely, Lipoquad C-50, Lipothoquad O/12, and cetyltrimethylammonium bromide [CTAB] with an NaSal counterion) passing through a slit was investigated by means of birefringence correlated with the degree of micelle orientation. With Lipoquad C-50, a high birefringence appeared in the slit section and upstream. With Lipothoquad O/12, the birefringence was low. With CTAB, the birefringence varied considerably with elapsed time. Different experimental results were thus obtained depending on the type of surfactant solution. To discuss the experimental results in depth, concentrations of 0.1× and 10× (2×) were used. With 0.1× concentration of all surfactant solutions, no birefringence appeared. With 10× concentration of Lipoquad C-50, there was no sign of the birefringence that was present with 1× concentration. With 10× concentration of Lipothoquad O/12, a birefringence was present that was not present with 1× concentration, and the distribution of the birefringence was almost the same as that with 1× concentration of Lipoquad C-50. The case with 2× concentration of CTAB was similar to that with 1× concentration. Moreover, the aforementioned results depended on the slit width or contraction ratio. To organize the experimental results systematically,the degree of spatial constraint (DSC) was defined, corresponding to the number of micelles occupying a unit volume. The DSC estimations showed that the birefringence appeared only when the DSC was between 5.9 × 10−6 and 1.2 × 10−5 mol (Lipoquad C-50 and Lipothoquad O/12) and between 1.8 × 10−6 and 7.3 × 10−6 mol (CTAB). Thus, the flow-induced structure generated by flow through an abrupt contraction and expansion was found to depend on a certain degree of micelle constraint.

Investigation of Flow Growth in aDuct Flowsfor Higher Area Ratio

Base pressure is a crucial factor in evaluating flow parameters. The effect of flow regulation mechanism is through an experiment examined for the convergent-divergent (C-D) nozzle with abrupt enlargement in an axisymmetric duct. The microjet control is extended along the nozzle's exit diameter at a 90-degree interval having a circular cross-section in the shape of an orifice of 0.5 mm radius. This research's primary aim is to evaluate the flow quality near the wall in the absence and presence of control. The experimental investigation aims to record the data for area ratio 6.25 of the flow field at the separate flow zone's rear end. At ninety degrees apart, four tiny jets are situated at 6.5 mm from the axisof the primary shear layer emanating from the nozzle. The small control jets attain critical conditions. The real Mach values for the main jets were 1.1 and 1.5, respectively, in the present study. The experiments were done at different expansion in the range from 11 to 3. The duct length was in the range from 10 to 1. This research primarily aims to study the quality of the duct's flow and its influence on the pressure and the pressure level along the duct. It is realized that the duct pressure is not harshlyaffected by the control jets in the pipe founded on the current findings.

A New Era: The Growth of Video-Based Visits for Remote Management of Persons with Parkinson's Disease.

The COVID-19 pandemic forced the abrupt and rapid expansion of an alternative care model that embraces the use of video-based visits in the care of persons with Parkinson’s disease. Video-based visits not only eliminate the risk of infection but also reduce geography- and disability-related barriers to accessing specialist care. Research has established that they are feasible, acceptable to persons with Parkinson’s disease and patient-centered. In the Unites States, the relaxation of licensure requirements, adoption of reimbursement parity and investment in telemedicine infrastructure has enabled the rapid growth of video-based visits during the COVID-19 pandemic. Now, we must turn our attention to ensuring that progress made in expanding access to video-based care is not lost and expanded worldwide. More work is needed to identify the optimal video-based care model, establish best practices, and ensure equitable access to care.

3D Numerical simulation of turbulent heat transfer and Fe3O4/nanofluid annular flow in sudden enlargement

In this paper, 3D Simulation of turbulent Fe3O4/Nanofluid annular flow and heat transfer in sudden expansion are presented. k-ε turbulence standard model and FVM are applied with Reynolds number different from 20000 to 50000, enlargement ratio (ER) varied 1.25, 1.67, and 2, , and volume concentration of Fe3O4/Nanofluid ranging from 0 to 2% at constant heat flux of 4000 W/m2. The main significant effect on surface Nusselt number found by increases in volume concentration of Fe3O4/Nanofluid for all cases because of nanoparticles heat transport in normal fluid as produced increases in convection heat transfer. Also the results showed that suddenly increment in Nusselt number happened after the abrupt enlargement and reach to maximum value then reduction to the exit passage flow due to recirculation flow as created. Moreover the size of recirculation region enlarged with the rise in enlargement ratio and Reynolds number. Increase of volume Fe3O4/nanofluid enhances the Nusselt number due to nanoparticles heat transport in base fluid which raises the convection heat transfer. Increase of Reynolds number was observed with increased Nusselt number and maximum thermal performance was found with enlargement ratio of (ER=2) and 2% of volume concentration of Fe3O4/nanofluid. Further increases in Reynolds number and enlargement ratio found lead to reductions in static pressure.

ЧИСЛЕННОЕ ИССЛЕДОВАНИЕ ЗАКРУЧЕННОГО ТУРБУЛЕНТНОГО ТЕЧЕНИЯ В КАНАЛЕ С ВНЕЗАПНЫМ РАСШИРЕНИЕМ

A strongly swirling turbulent flow through an abrupt expansion is studied using the highly resolved DNS, LES, and SAS to shed more light on a stagnation region and spiral vortex destruction, though these methods require high computational expenses. The vortex fracture induced by a sudden expansion resembles the so-called vortex rope that occurs in hydropower draft tubes. It is known that large-scale spiral vortex structures can be captured by regular RANS turbulence models. In this paper, a numerical study of a strongly swirling flow, which abruptly expands, is carried out using the Reynolds stress models SSG / LRR-RSM and EARSM with experimental measurements implemented by Dellenback et al. (1988). Calculations are carried out using the finite volume method. The flow dynamics is studied at the Reynolds number of 3.0 × 104 at almost constant large swirl numbers of 0.6. The time-averaged velocity and pressure fields, as well as the root-mean-square values of the velocity fluctuations are recorded and studied qualitatively. The obtained results are compared with known experimental data. The aim of this work is to test the ability of the models to describe anisotropic turbulence. It is shown that the SSG / LRRRSM model is more appropriate for studying such flows.

Effects of Reynolds number, plastic number, and flow intensity on the flow and on the heat transfer of a viscoplastic fluid flowing through a planar expansion followed by a contraction

Non-Newtonian fluids are a constant presence in several industrial applications so it becomes interesting to understand their behavior and to optimize their use. One behavior of commercial interest is the viscoplastic behavior characterized by an abrupt change of the apparent viscosity when a certain amount of stress is achieved. The present work performs a study of the effect of Reynolds number, the plastic number, and the flow intensity over flow parameters of industrial interest, namely, mean Nusselt number, displacement efficiency, and head loss. The mechanical model is approximated by a finite volume method implemented by an OpenFOAM® routine developed by the authors. The SMD model was chosen to model the viscoplastic behavior. The adopted geometry is of a planar channel with an abrupt expansion followed by an abrupt contraction. The channel walls are kept insulated and heat transfer occurs only at the cavity formed between the expansion and contraction planes. For the studied range of parameters, mean Nusselt number has a positive dependence on Reynolds number and flow intensity but a negative dependence on plastic number; displacement efficiency has a positive dependence on Reynolds number and flow intensity but a negative dependence on plastic number. The head loss has a negative dependence on Reynolds number and plastic number and an overall positive dependence on flow intensity.

Influence of the relative momentum flux ratio on the mixing of hydrogen jets in an M=4 crossflow

The paper presents the results of numerical simulation of the 3D flows in a plane channel with the abrupt expansion. Sonic hydrogen jets are supplied from two circle orifices located opposite each other on the channel walls before the backward-facing step. The numerical simulations are carried out under the conditions of experiments performed at the hot-shot wind tunnel IT-302M ITAM SB RAS for the following flow parameters: Mach number at the entrance of the channel M = 3.84, the total temperature T0 = 1715 K, and the total pressure P0 = 6.5 MPa. Calculations were performed with a change in the jet supply pressure from 0.4 to 3.6 MPa, which ensures the variation in the jet-to-crossflow momentum flux ratio in the range of J = 0.7 ÷ 6. Mathematical modeling was performed in ANSYS Fluent based on the Reynolds-Averaged Navier-Stokes equations supplemented by the k-ω SST turbulence model. A comparison of the calculated and experimental data on the flow structure, as well as the static pressure distributions on the walls, indicates a satisfactory agreement. The calculated results made it possible to obtain a clearer understanding of the 3D flow structure in the channel and reveal the influence of the injection pressure on the level of mixing.

Hybrid nanocomposites of tunneled-mesoporous sulfur-doped carbon nanofibers embedded with zinc sulfide nanoparticles for ultrafast lithium storage capability

As effective and simple approach by which to construct hybrid nanocomposite using ZnS nanoparticles (NPs) and conducting carbon materials to realize robust ultrafast lithium storage capabilities, we newly developed tunneled-mesoporous S-doped carbon nanofibers (SCNF) embedded with ZnS NPs through a one-pot carbonization process via the thiourea effect. This hybrid nanocomposite is unique given its tunneled-mesoporous CNF structure with well-dispersed ZnS NPs by ZnO sulfurization, offering available space to accept abrupt structural expansions of ZnS NPs and efficient Li-ion pathways during the electrochemical reaction. Using this material leads to competitive cycling stability and superior rate capabilities. Even at a high current density of 2000 mA g−1, amazing ultrafast electrochemical performance outcomes with a high specific capacity (391.8 mAh g−1) and good long-term cycling stability (97.2%) after 500 cycles were noted. These findings are attributed to the synergistic effects of the accelerated the transportation of Li ions for the ZnS NPs by the internal construction of the tunneled-mesoporous SCNF and facilitation of the electrical conductivity of the electrode via the S doping effect of the CNF matrix. Therefore, the proposed approach for a unique hybrid nanocomposite holds great potential regarding the development of an outstanding anode electrode to enhance the ultrafast lithium storage capabilities.

Flow of yield stress materials through annular abrupt expansion–contractions

We present an experimental study of the flow of yield stress materials through annular abrupt expansions–contractions, to evaluate the flow invasion into the cavity formed in the larger cross section region. Steady inertialess flows of Carbopol® aqueous dispersions were investigated. The flow pattern reveals yielded and unyielded regions, which were visualized using tracer particles, laser sheets, and a digital camera. The yield surfaces were identified in the experiments by choosing large enough exposure times that allow sufficient particle displacement in the yielded region. To estimate the amount of fluid that remains stagnant in the cavity, we defined the invasion ratio, a quantity that was determined through image processing for different combinations of the governing parameters. The influence of the cavity diameter and axial length, eccentricity, and inlet velocity on the invasion ratio was investigated. Fore-aft asymmetric yield surfaces were observed for all tests, probably due to elastic effects.

Heat transfer in turbulent nanofluids: Separation flow studies and development of novel correlations

Convective heat transfer plays a significant role in numerous industrial cooling and heating applications. This method of heat transfer can be passively improved by reconfiguring flow passage, fluid thermophysical properties, or boundary conditions. The broader scope of nanotechnology introduced several studies of thermal engineering and heat transfer. Nano-fluids are one of such technology which can be thought of engineered colloidal fluids with nano-sized particles. In the present study, turbulent forced convection heat transfer to nanofluids in an axisymmetric abrupt expansion heat exchanger was investigated experimentally. During heat transfer investigation, the functionalized multiwalled carbon nanotubes (MWCNT-COOH), polycarboxylate functionalized graphene nanoplatelets (F-GNP), SiO2 and ZnO water-based nanofluids were used. The convective heat transfer coefficient of fully developed turbulent flow of nanofluids flowing through an abrupt enlargement with the expansion ratio (ER) of 2 was experimentally determined at a constant wall heat flux of 12,128.56 W/m2. The experiments were conducted at the Re ranges of 4000–16,000. The observed Nusselt numbers were higher than in the case of fully developed pipe flow indicating the level of the turbulent transport is high even though the recirculating velocities were a few percentages of the bulk mean velocity. The effect of Reynolds number and nanofluid’s volume concentration on heat transfer and friction losses were studied, where all the results reveal that with the increase of weight concentration and Reynolds number, the local Nusselt number enhanced at the increment of axial ratios in all the cases showing greater heat transfer rates than those of the base fluids. Comparison between the examined four types of nanofluids, show that the carbon-based nanofluids have a greater effect on enhancing heat transfer (33.7% and 16.7% heat transfer performance improvement for F-GNP and MWCNT nanofluids respectively at 0.1 wt% concentration) at the downstream of the sudden expansion pipe. There is no reported work dealing with the prediction of the local Nusselt number at the distance equivalent to the axial ratio and flow through sudden expansion. So far, two excellent correlations for the Local Nusselt number are proposed with reasonably good accuracy. Furthermore, a new correlation is developed for the average Nusselt number.

Pressure recovery and acceleration length of gas-solid suspension in an abrupt expansion – An Eulerian-Eulerian approach

The present study based on the Eulerian-Eulerian frame work, reports the role of inter-particle and particle–wall collisions in conjunction with pertinent parameters in an abrupt expansion pipe. The parameters that were investigated are the pressure recovery and the acceleration length. The solid loading was considered between 2 and 10, within which the roles of collisions were found to be prominent. The collisions are identified as the sole mechanism responsible for the lateral dispersion of the relatively coarse particle (400–1000 µm) into the recirculation zone. An increase in the solid loading (from 2 to 10) and the Reynolds number increases the pressure recovery and the acceleration length. On the other side, the lighter (1020 kg/m3) and relatively small sized particles (400 µm) are also responsible for the same. With an increase in the area ratio up to a critical value ≈9.97 the pressure recovery increases and then reduces.

Towards a theory of dynamics of a single cavitation bubble in a rigid micro-confinement

The nonlinear dynamics of a spherical bubble in the confined externally driven liquid cell is considered. It is shown that volume confinement strongly affects the manifestation of the classical cavitation Blake threshold. At relatively large liquid cell exposed to a tension exceeding cavitation Blake threshold, the cavitation bubble abruptly expands to a finite radius in contrast with explosive infinite bubble growth in bulk liquid. For smaller liquid cells, the corresponding finite radius of a cavitation bubble is getting smaller accordingly. At liquid cell size smaller than some critical one, the cavitation is completely suppressed by volumetric confinement. The generalized Rayleigh–Plesset equation for the confined bubble, accounting for mass of gas in the bubble, liquid compressibility, surface tension, and damping due to liquid viscosity in the cell is derived in which the liquid cell size is used as a driving parameter. Three possible regimes of bubble dynamics in confined liquid at different types of driving is studied. First, a simple nonlinear growth initiated by relatively weak liquid cell expansion when the cavitation Blake threshold is not reached yet. Second, an abrupt cavitation expansion with oscillatory transient when cavitation Blake threshold is reached. Third, multiple cavitation inception followed by cavitation vanishing at the periodic liquid cell expansion and contraction. In this case it is also found that for high driving frequency bubble dynamics in the confined liquid resembles the bubble dynamics in an unbounded liquid.

Delayed extensive brain edema caused by the growth of a giant basilar apex aneurysm treated with basilar artery obliteration: a case report

BackgroundPartially thrombosed giant aneurysms at the basilar apex (BA) artery are challenging lesions with a poor prognosis if left untreated. Here we describe a rare case of extensive brain edema after growth of a surgically treated and thrombosed giant basilar apex aneurysm.Case presentationWe performed a proximal surgical basilar artery occlusion on a 64-year-old female with a partially thrombosed giant BA aneurysm. MRI showed no ischemic lesions but showed marked edema adjacent to the aneurysm. She had a good recovery, but 3 months after surgical occlusion, her gait deteriorated together with urinary incontinence and worsening right hemiparesis. MRI showed that the aneurysm had grown and developed intramural hemorrhage, which caused extensive brain edema and obstructive hydrocephalus. She was treated by a ventriculoperitoneal shunt placement. Follow-up MRI showed progressive brain edema resolution, complete thrombosis of the lumen and shrinkage of the aneurysm. At 5 years follow-up the patient had an excellent functional outcome.ConclusionsDelayed growth of a surgically treated and thrombosed giant aneurysm from wall dissection demonstrates that discontinuity with the initial parent artery does not always prevent progressive enlargement. The development of transmural vascular connections between the intraluminal thrombus and adventitial neovascularization by the vasa vasorum on the apex of the BA seems to be a key event in delayed aneurysm growth. Extensive brain edema might translate an inflammatory edematous reaction to an abrupt enlargement of the aneurysm.

Electro-elastic flow instabilities of viscoelastic fluids in contraction/expansion micro-geometries

Electro-elastic instabilities in electro-osmotic contraction/expansion flows (EOF) of viscoelastic fluids are experimentally investigated in this work. Several microchannel configurations are used, including the cases with hyperbolic-shaped contractions followed by an abrupt expansion, or abrupt contractions followed by a hyperbolic-shaped expansion. Such cases were selected to assist in the understanding of the EOF instability mechanisms and at which conditions they occur. A reference Newtonian fluid was also used in the experiments, and a wide range of electric fields were imposed to drive the flow. Flow visualizations for a Newtonian fluid allowed to assess the impact of dielectrophoresis on the velocity field of the seeding particles. For each geometry, depending on the polymer concentration, the flow was found to be quasi-Newtonian below a critical electric potential difference, characterized by smooth parallel steady flow patterns in the upstream and downstream channels. Above the critical voltage difference two types of electro-elastic instabilities were found to occur, leading to an unsteady symmetric flow, and to time-dependent irregular flow.

A 2-D CFD model for the decompression of carbon dioxide pipelines using the Peng-Robinson and the Span-Wagner equation of state

Pressurized liquefied gases such as carbon dioxide are transported at a pressure above their saturation pressure. Therefore, if a pipeline transporting this substance ruptures, an abrupt expansion occurs, causing the flashing of the fluid. Computational tools that predict how fast a depressurization develops, help to assess the consequences of potential pipeline rupture scenarios. This paper describes the development of a 2-D full-bore rupture decompression model to simulate the transient depressurization of a pipeline transporting pure liquefied CO2, using ANSYS Fluent as CFD software. The scope of work focuses on incorporating non-equilibrium phase transition, while addressing the calculation of properties for metastable liquid. Additionally, it includes the comparison of model predictions when implementing two thermodynamic approaches: the Peng-Robinson Equation of State (EoS), and correlations developed in this work based on the Span-Wagner EoS. It was found that the thermodynamic approach is deemed to have a predominant effect on the arrival time of the decompression wave front at different locations along the computational domain, while the mass transfer coefficient in the source terms (C) governs the phase transition and the pressure plateau representing this phenomenon.