Abrupt Expansion Research Articles

The dynamics of shock wave propagation far downstream of an abrupt area expansion

Predicting the transient flow fields that develop when a shock wave passes through an area expansion is a fundamental problem in compressible fluid mechanics and significant in many engineering applications. Experiments, large eddy simulations and geometrical shock dynamics are used to study the mechanism by which a normal shock wave that expands across an area expansion evolves into a uniform normal shock far downstream of it. This study analyses shock waves with moderate Mach numbers of 1.1–1.8 that expand at area ratios of up to 5. As the shock wave propagates into the expanded region, it experiences rapid deceleration, forming a non-uniform shock front. Impinging on the walls of the larger cross-section region, the shock wave reflects and generates a complex and highly transient shock pattern near the expansion region. We have found that as the shock front propagates further downstream, a laterally moving shock wave that intersects the shock front at a triple point reverberates laterally between the walls. This process effectively evens out the flow behind the incident shock front, thus reducing the variation of properties behind it. The extended duration of this process leads to significant pressure fluctuation behind the shock front. The results show that the evolution of the shock front can be scaled using the expanded region height and the velocity of the shock wave far downstream of the expansion. The results enabled the formulation of a simple empirical relation, allowing us to predict the shock velocity far downstream of gradual and abrupt area expansions.

The dynamics of capillary flow in an open-channel system featuring trigger valves.

Trigger valves are fundamental features in capillary-driven microfluidic systems that stop fluid at an abrupt geometric expansion and release fluid when there is flow in an orthogonal channel connected to the valve. The concept was originally demonstrated in closed-channel capillary circuits. We show here that trigger valves can be successfully implemented in open channels. We also show that a series of open-channel trigger valves can be placed alongside or opposite a main channel resulting in a layered capillary flow. We developed a closed form model for the dynamics of the flow at trigger valves based on the concept of average friction length and successfully validated the model against experiments. For the main channel, we discuss layered flow behavior in the light of the Taylor-Aris dispersion theory and in the channel turns by considering Dean theory of mixing. This work has potential applications in autonomous microfluidics systems for biosensing, at-home or point-of-care sample preparation devices, hydrogel patterning for 3D cell culture and organ-on-a-chip models.

Influences of stenosis and transplantation on behavior of blood flow in the host and grafted vessels using computational fluid dynamics

In this research, the changes of wall shear stress (WSS) of flow blood affected by stenosis and grafted vessels are studied. For the simulation of non-Newtonian fluid, blood in this paper, the Carreau fluid model is used. The severity of the stenosis is considered to reduce the internal cross-sectional area of the host vessel by 30 %. In this paper, simulations are performed on the human body in sports, which are classified into anemia (LHD), normal (NHD) and high blood pressure (HHD). Results are presented in three sections including after stenosis region, transplantation section, and after transplantation region by calculating wall shear stress (WSS). It is reported that wall shear stress is increased after stenosis location due to immediate variation in the cross-section area when blood flows. To explain, Stenosis reduces the cross-section area of the vessel which causes flow contraction. Consequently, this phenomenon increases flow velocity in the central region of blood flow whereas, after passing from stenosis, blood flow is exposed to an abrupt expansion which causes flow back from the central region to wall vicinity in lateral regions of the vessel. Maximum amounts of wall shear stress are achieved at 280 Pa, 350 Pa, and 550 Pa for anemia, normal, and hypertensive individuals in order. However, the consequence of the mentioned phenomenon in hypertensive individuals is more severe than that of anemic ones. Therefore, the geometry of veins is very important in medical surgeries to prevent vessel failure, especially in stressful points of transplantation.

A Refractive Index Matched Experimental Investigation Of The Dynamics Of Inertial Spheres Across Round Pipes And Buoyancy-Driven Flows

A series of refractive index-matched time-resolved tomographic PIV experiments are performed to study the dynamics of motion of solid spheres in flow. The refractive index matching of acrylic spheres with the working fluid facilitates unobstructed optical access around the spheres, enabling the simultaneous estimation of both the 3D flow field and the fluid-structure interactions governing the kinematics of the spheres. Additionally, using normal tracers as well as fluorescent tracers for PIV helps identify the most suitable approach for multiphase index-matched PIV studies. Two different test cases are presented: (1) solid spheres moving in a round channel with an abrupt area expansion and, (2) solid spheres rising in a quiescent flow at terminal velocity. Despite both falling under the category of solid-liquid flows, these cases exhibit entirely different mechanisms and flow characteristics. Nonetheless, refractive index matching provides a valuable tool that can be used to characterize the flow as well as the motion dynamics of the free spherical particles in a liquid. These time-resolved 3D experiments offer a comprehensive insight into the transient evolution of the flow fields and the pressure forces on the spheres, providing a complete picture of the dynamic process. The emphasis of the present paper lies in the experimental methodologies, challenges and data analysis techniques used for refractive index-matched multiphase flow studies.

Minor Loss Coefficient for Abrupt Section Changes in a Cylindrical Pipe Using a Numerical Approach

Abrupt section changes are a classic problem in the study of flow in cylindrical ducts or pipes. For its analysis, there are a wide set of exiting data from previous studies, among which some authors stand out and will be mentioned. Those previous works have been used to obtain reliable results for the resolution of section changes along a pipe, either due to cross area increases or reductions on a 1D basis. It is also known that a numerical 2D axisymmetric simulation (CFD) could find a consistent result compared to experimental data in almost all fluid flow fields. The main novelty of the present study is the development of a simple numerical approach used to solve the minor loss calculation. Firstly, a theoretical analysis is developed, and then the results of the numerical simulations carried out on the behavior that affects the water and air flow rate in an abrupt section change, for both contraction and expansion problems, are presented. In both cases, the results are analyzed with different meshes (discretizations) and turbulence models. Finally, the obtained numerical results are compared with those in the technical literature. Also, a theoretical approach is shown in order to show a whole frame of the discussion. The core results are the loss coefficient evolution as a function of the section change both for the sudden contraction and the expansion of a pipe flow. As the results follow the existing experimental values, it is concluded that the developed model provides a feasible and quick design tool to analyze possible geometrical changes without the need for further experiments.

A mean flow velocity estimation scheme based on flow field characteristics and distribution point optimization for carbon monitoring in coal-fired power plants: A numerical study

CO2 online monitoring provides a method to obtain carbon emissions by directly quantifying the flue gas flow rate and CO2 concentration. However, significant flow rate measurement errors arise due to the complexity of turbulence in large-diameter stacks. To address the issue, this study utilizes turbulence numerical simulation combined with mean velocity estimation error analysis to investigate the flow field of typical stacks in coal-fired power plants. The simulation results demonstrated that the evolution of turbulence exaggerates velocities. The flow field exhibits stable turbulence characteristics in a single-introduced stack, while in the double-introduced stack, the flow field dynamically evolves due to asymmetric collisions and abrupt expansions. Furthermore, the effects of load and temperature on the estimation error of flow velocity are established. The optimum monitoring height, the ideal chord angle and the stepped number of deployment points based on the log-linear method are determined. Within the single-introduced stack, a correction factor is introduced and a corresponding formula is established that links estimation errors with monitoring heights. This resulted in a dramatic reduction of the maximum estimation error from 14% to 0.3%. Finally, the systematic and simplified optimization scheme for estimating mean flow velocity is summarised, which provides a powerful reference for flow rate measurement for online CO2 monitoring in coal-fired power plants.

The impact study of variable diameter stabilizer on drilling fluid and cuttings transport

Through numerical simulation, this study investigates the flow field characteristics of the variable diameter stabilizer in drilling tools under various conditions. It analyzes the influence of different flow rates and speeds on axial velocity and pressure distribution. The results indicate that more significant flow rates correspond to higher average axial velocities across sections, facilitating the transport of drilling fluid and cuttings. Increasing rotational speed leads to greater pressure differences between adjacent sections, consequently elevating the overall pressure drop of the tool, which, to some extent, aids in transporting drilling fluid with cuttings. During rotation, the vortex zone on the backside of the stabilizer creates a hovering and accumulation of cuttings, causing mud agglomeration, thereby affecting tool performance. During structural optimization of the tool, priority should be given to a transitional design of the outlet area in the functional core zone, aiming to alleviate the impact of abrupt structural expansions on cuttings transport.

Watershed hydrological responses to land use land cover changes at Bilata watershed, Rift Valley Basin, southern Ethiopia

Abstract Most anthropogenic activities have a profound effect on land cover that affects the water cycle and, ultimately, the availability of water in the watershed. The idea of this study was to evaluate the hydrological response to land use and land cover change in the Bilata watershed. To achieve this objective, supervised land use land cover classification was applied for the years 2000, 2010, and 2020 using ERDAS IMAGINE 2015. The watershed underwent significant land use and land cover changes from 2000 to 2020. There was an abrupt expansion of agricultural land and a reduction of forest. The SWAT model was applied to evaluate the impact of land use and land cover change on hydrological processes. The calibration and validation of the SWAT model showed that the model has performed well in simulating the hydrology of the watershed with a coefficient of determination, Nash–Sutcliffe efficiency, and per cent of bias greater than 0.6 for both calibration and validation. Due to intensive agricultural expansion and settlement, the surface runoff increased from 172 to 259.5 and 265.0 mm in 2000, 2010, and 2020, respectively. The average streamflow increased from 132 to 134 and 150 m3/s between 2000, 2010, and 2020.

A new 2D ESPH bedload sediment transport model for rapidly varied flows over mobile beds

A 2D Eulerian meshless bedload sediment transport model is developed using smoothed particle hydrodynamics (SPH) to simulate rapidly varied flows over mobile beds. In the developed model, we adopt a weakly coupled numerical approach to explicitly solve the governing equations, including 2D shallow water equations for fluid flow motion and the Exner equation for bed sediment movement at the same time step. A defined virtual bedload velocity in a weakly coupled approach is involved in the calculation of time step sizes. However, nonphysical virtual bedload velocities, which often occur in cases with nearly flat beds, require extremely small time step sizes. A formulation of the lower and upper bounds of information propagation speeds for an HLLC approximate Riemann solver is thus proposed to remedy the problem. Four case studies involving dam break flows in prismatic and erodible channels, knickpoint migration, dam erosion due to overtopping flow and dam-break flow in an erodible channel with an abrupt expansion are adopted to validate the developed model. In addition, to compare the performances of different particle interaction configurations under Cartesian uniform particle arrangements, four and eight interacting particles are obtained by changing the smoothing length. Against the measured results, the case of eight interacting particles shows more accurate predictions of erosion peaks along the measured cross-sections because the lateral flows are significant. The good agreement between the simulated and measured results shows that the developed 2D Eulerian SPH bedload sediment transport model with eight interacting particles is well suited for simulating complicated flow-induced bed erosion.

Abrupt cooling of cold seasons at the middle-late Holocene transition revealed by alkenone records from North China

In saline lakes, the paleoenvironmental interpretation of long-chain alkenones (LCAs) is difficult due to the inputs of Isochrysidales Group 2i, which are ice-related bloomers. Here, we perform detailed analysis of LCAs and long-chain alkenoates in sediment cores from Lake Daihai in north China. During the late Holocene, alkenone-inferred temperatures and the Group 2i species abundance indicate a long-term warming trend in cold seasons (winter and early spring), possibly driven by regional insolation and by atmospheric CO2 concentrations. Notably, our records indicate cold climate conditions during the period of 4.3–3.0 cal ka BP, possibly linked to the abrupt expansion of Arctic winter sea ice in a given of low insolation and atmospheric CO2 concentration conditions. The abrupt cooling may have led to the collapse of the East Aian Summer Monsoon (EASM) and the deterioration of the ecosystems in northern China during the middle to late Holocene transition, and thus likely accounted for the nearly synchronous Chinese Neolithic cultural collapses.

How hydraulic jumps form downstream of a negative step with channel expansion: experimental and numerical investigations of the transitions between wave jumps and submerged jets

AbstractWeirs and sills, particularly negative steps, play a pivotal role in modulating water flow, inducing hydraulic jumps that efficiently dissipate downstream energy. Beyond their aesthetic appeal, these features hold crucial engineering significance. This study combines physical experiments and numerical simulations downstream of a negative step featuring an abrupt width expansion. The spontaneous alteration of water flow conditions upstream and downstream of the step results in distinct flow regimes. By considering the critical Froude number to sustain an undular jump without wave breaking on a flatbed, we establish a framework for evaluating energy loss. Our analysis successfully delineates the transition limit between wave jumps and submerged jets downstream of a negative step. The co-existence regime of both jumps is explained by the analysis showing that the additional energy loss induced by the negative step is larger for the wave jump compared to the submerged jet. The abrupt width expansion at the negative step significantly reduces the transition depth between the submerged jet and wave jump, attributed to energy loss with intricate three-dimensional vortex motions—exceeding losses incurred by the negative step alone. We delve into the detailed mechanisms of these transitions through a three-dimensional numerical simulation of the energy-loss process and water surface profiles downstream of the step with expansion. The maximum energy loss by the undular jump and the minimum energy loss by the submerged jet are defined by the wave steepness at the limit of maintaining the undular jump and the jet plunging angle capable of sustaining the submerged jet, respectively.

Collision Behaviors of Two Successive Compound Droplets in an Abrupt Expansion Microchannel

Collision Behaviors of Two Successive Compound Droplets in an Abrupt Expansion Microchannel

Computational Modeling of High-Speed Flow of Two-Phase Hydrogen through a Tube with Abrupt Expansion

Hydrogen can become a prevalent renewable fuel in the future green economy, but technical and economic hurdles associated with handling hydrogen must be overcome. To store and transport hydrogen in an energy-dense liquid form, very cold temperatures, around 20 K, are required. Evaporation affects the achievable mass flow rate during the high-speed transfer of hydrogen at large pressure differentials, and accurate prediction of this process is important for the practical design of hydrogen transfer systems. Computational fluid dynamics modeling of two-phase hydrogen flow is carried out in the present study using the volume-of-fluid method and the Lee relaxation model for the phase change. Suitable values of the relaxation time parameter are determined by comparing numerical results with test data for high-speed two-phase hydrogen flows in a configuration involving a tube with sudden expansion, which is common in practical systems. Simulations using a variable outlet pressure are conducted to demonstrate the dependence of flow rates on the driving pressure differential, including the attainment of the critical flow regime. Also shown are computational results for flows with various inlet conditions and a fixed outlet state. Field distributions of the pressure, velocity, and vapor fractions are presented for several flow regimes.

EFFECT OF THE NANOFLUID FLOW AND EXTENDED SURFACES ON AN ABRUPT EXPANSION TUBE REGARDING THERMODYNAMIC IRREVERSIBILITY

Numerous scientists have examined circular dimpled surfaces, tubes, and other approaches for enhancing heat transfer. Moreover, the dimples' angle of attack has a substantial effect on the tube's flow and thermal features. This numerical study presents a novel approach to enhancing heat transfer rate in a tube subjected to constant heat flux by incorporating a surface dimpling strategy and evaluates three different tube layouts with elliptical dimpled fins for different working fluids such as DW and Al<sub>2</sub>O<sub>3</sub>/DW nanofluid (φ = 0.5-1.0%) by using ANSYS Fluent v2020R2 under laminar flow conditions. Under identical circumstances, the thermal performances of proposed designs are compared to those of a smooth tube, and the influence of the elliptical dimpled fin angle of attack on these parameters is determined for different Reynolds numbers (1000 ≤ Re ≤ 2000). When comparing the Nusselt number of a tube with/without elliptical dimpled fins, it is found that EDT 1 performed better. Lower Reynolds numbers are shown to result in a greater friction factor. Besides, elliptical dimpled fins promote flow mixing within the tube and the establishment of a thermal boundary layer. At a 135° attack angle (EDT 1), the 1.0% Al<sub>2</sub>O<sub>3</sub>/DW nanofluid is found to be the best-performing nanofluid in the dimpled tube, improving Nu by up to 44.56%. Furthermore, ff presented an increase of 29.18% when comparing ST and EDT 1 flowing 1.0% Al<sub>2</sub>O<sub>3</sub>/DW at Re = 2000, while total S<sub>gen</sub> is diminished by 37.75% in the same conditions.

A Fast Acting DC-link Voltage Controller for 3-Phase DSTATCOM to Compensate AC & DC Loads

The transient reaction of the conveyance static compensator (DSTATCOM) is vital while repaying quickly fluctuating uneven and nonlinear burdens. Any adjustment of the heap influences the dc-connect voltage straightforwardly. The unexpected evacuation of burden would bring about an expansion in the dc-connect voltage over the reference esteem, while an abrupt expansion in burden would diminish the dc-interface voltage underneath its reference esteem. The appropriate activity of DSTATCOM requires variety of the dc-connect voltage inside as far as possible. Ordinarily, a relative indispensable (PI) regulator is utilized to keep up with the dc-interface voltage to the reference esteem. It utilizes deviation of the capacitor voltage from its reference esteem as its feedback. Nonetheless, the transient reaction of the ordinary PI dc-connect voltage regulator is slow. In this paper, an effective dc-connect voltage regulator in light of the energy of a dc-interface capacitor is proposed. Numerical conditions are given to process the increases of the traditional regulator in light of effective dc-connect voltage regulators to accomplish comparable quick transient reaction. The nitty gritty reproduction and exploratory investigations are completed to approve the proposed regulator.

Higher-order mode filtering by a resistive layer.

A method for filtering higher-order acoustic modes using a resistive layer is proposed and applied to a two-dimensional rectangular waveguide with a quiescent fluid. An analogue of Cremer's criterion is discussed and used to obtain the optimal modal attenuation of the non-planar waves while the plane wave is preserved. Numerical validation of the concept is performed for a straight waveguide and an abrupt expansion in a waveguide.

FISCAL POLICY OF UKRAINE AS THE FACTOR OF THE SUSTAINABILE DEVELOPMENT OF THE TERRITORIES IN CONDITIONS OF MARTIAL LAW

It proves that the fiscal policy in the war conditions needs economic diversification with the help of the use of existing resources optimization and the search of uninvolved ones as well. The aim of the research is to deeply analyze the theoretical-methodological bases of Ukrainian fiscal policy formulation and fulfillment in conditions of martial law and its impact on the financial system of the government. Government’s revenue (taxes) and government’s expenditure are the main instruments of fiscal policy. High uncertainty in condition of full-scale war led to the reducing of the effectiveness of market instruments of fiscal policy and made it impossible in the traditional format.
 It defines that instruments of fiscal policy helped to provide some measures for designing expansionary fiscal policy in conditions of martial law. Grant funds of international partners as well as non-tax revenue at the expense of voluntary transfers from citizens, enterprises, and organizations to support Ukraine have become the ultimate source of government’s revenue from the beginning of the war. The search of instruments for financing (ensuring defense capability and security of the country; social expenses, increasing the monetary support of the military, debt service expenses) has become the key of fiscal policy.
 It explains that negative opening balance of consolidated budget of Ukraine targeted at cyclical economy position of the country proves the sharp softening of fiscal policy. The probation of such kind of fiscal policy was due to the intense growth of the budgetary expenditures on providing defense capabilities of the country and incentives to support economy in conditions of martial law.
 It proves that the abrupt deficit expansion of consolidated budget is the justified countercyclical measure of fiscal policy in conditions of martial law. The softening of fiscal policy in the country has mainly become possible because of quite balanced fiscal policy of past years and considerable international support as well.

Numerical simulation of mechanical interaction between pellet and zircaloy-4 cladding under reactivity-initiated accident conditions

Reactivity-initiated accident (RIA) is one of the accidents considered by fuel safety criteria. The power pulse caused abrupt expansion of the pellet, leading to pellet and cladding mechanical interaction (PCMI). Cladding ductility is gradually decreased in the process of oxidation and hydrogen uptake during irradiation. As a result, the extrusion of the pellet to the cladding causes the cladding to produce greater hoop stress and hoop strain, which leads to damage. A simulation of PCMI using ABAQUS was designed to analyze cladding mechanic properties. By comparing the calculation results of the simulation scheme with the data obtained in the reactor experiment, it is proved that the simulation calculation results are in good agreement with the experimental data in the reactor, and the rationality of the simulation scheme is verified. Based on the out-pile mandrel equipment simulated PCMI, the corresponding finite element model was established to study related test parameters.By analyzing the results of the simulation calculation outside the reactor, it is found that the pellet material, cladding temperature, compression rate, tensile load, friction coefficient and the cladding hoop stress and hoop strain have a certain relationship: (1) The choice of pellet material affects the reactor. The overall law of hoop stress-strain in the external simulation results basically has no effect. (2) The temperature of the cladding has effect on the hoop stress peak. (3) The compression rate has effect on the time for the cladding to reach the hoop peak stress. (4) Tensile load will affect the starting position of the hoop stress-strain of the cladding. (5) The friction coefficient has a significant influence on the stress and strain trend of the cladding. The outside experiment can be carried out more effectively, and it can provide guidance and a basis for the outside experiment.

Effect of ground tire rubber media's viscoelasticity and flow passage geometry on the abrasive flow finishing of helical gear

This work aims to elucidate the behavior of viscoelastic ground tire rubber media (GTRM) under abrupt contraction and expansion of the media at the entry and exit of the gear flow channel and its influence on polishing results. Due to the inherent nature of the abrasive flow finishing (AFF) process, the media flow converges at the entrance of the gear tooth as it flows from a larger fixture cross-section to a narrower volumetric space between gear teeth. The material removal and subsequent surface roughness rely heavily on the flow's pressure and velocity. A numerical simulation was performed to obtain the pressure and velocity along the gear flank face width (FFW). To investigate the effect of variation in the media deformation rate along the gear flow channel on AFF results, the material removal height and improvement in average surface roughness at different locations along the tooth flank face width were measured. AFF results reveal that the material removal near the two edges of the gear tooth is higher and has an inferior surface finish compared to the tooth's center. Due to the abrupt compression of the media at the flow channel entrance, the media act elastically dominant, resulting in high pressure and flow velocity near the tooth edge (flow channel's entry). Additionally, the effect of the helix angle on the AFF of various helical gear (HG) was investigated. Among the three HGs chosen, the 30° HG produces the lowest surface roughness under moderate pressure and high velocity. The microhardness and fretting wear analyses further confirm that the AFF polishing increases the microhardness and wear resistance of the gear tooth surface as a result of work hardening and minimizing surface defects.

Phylogeography of alpine plant Parnassia wightiana (Celastraceae)

Parnassia wightiana Wall. ex Wight & Arn., a perennial herb, is an alpine plant distributed across three biodiversity hotspots in China. This species offers an excellent study system to analyze the distribution pattern and genetic structure of high mountainous plant populations. Three chloroplast DNA (cpDNA) regions ( rpl32- t rnL, trnL-F, and trnS-G) from 442 individuals of 39 populations and internal transcribed spacers (ITS) from 418 individuals of 38 populations were sequenced and analyzed. We identified a total of 26 haplotypes based on the concatenated sequences of cpDNA and a total of 36 based on ITS sequences. A hierarchical analysis of molecular variance indicated significant phylogeographic structure among populations. Mismatch analysis of multipeak and Tajima’s D neutral test demonstrated Parnassia wightiana populations within China had not experienced abrupt expansion recently. The allopatric fragmentation resulted in the geographic isolation due to environmental heterogeneity, producing a high proportion of private haplotypes in the distribution of Parnassia wightiana within China.