Stability Of Flow Research Articles

Swirl flow stability: thermodynamic analysis and experiments

AbstractThe current paper presents a theoretical analysis of swirl flow stability, both inside a tube (vortex tube) and in a free annular swirl flow. The starting concept is the study of the evolution of velocity and temperature fluctuations. Methods of non-equilibrium thermodynamics are used to describe the magnitude of fluctuations and their properties. The important role of the total enthalpy follows from a variational analysis. Moreover, the thermodynamic criterion of the stability is formulated using the total enthalpy, and compared with experiments, numerical results and classical Rayleigh theory support its applicability. It was shown that the solid body vortex is at the margin of stability, which is experimentally observed. Analogously, the potential vortex is by the thermodynamic criterion stable; however, by the Rayleigh criteria it is on the onset of stability. The classical Taylor experiment of flow between two rotating cylinders is analysed from the point of view of this criterion. These results are underlined by swirl tube experiments at the Institute of Aerospace Thermodynamics at Stuttgart University and the annular nozzle experiments performed in the Institute of Thermomechanics CAS in Prague. Both independent experiments confirm the transformation of the initial annular vortex into a stable potential-type vortex. The results of this theory can also be used to explain the exceptional stability of tropical cyclones.

Stability of plane Couette flow with constant wall transpiration

Plane Couette flow with constant wall transpiration, i.e., constant blowing from below and constant suction from above, is a solution of the Navier-Stokes equations in terms of exponentials. It is characterized by two Reynolds numbers Re and ReV, where Re parametrizes the moving wall and ReV describes the influence of the transpiration. For this flow the modified Orr-Sommerfeld equation admits one of the very few exact solutions in terms of hypergeometric functions. Based on this, we solve the stability problem, though for the main part focus on temporal stability. For small wall-transpiration rates up to ReV≃6.71, the flow remains unconditionally stable, analogous to the classical Couette flow for arbitrary Reynolds numbers Re. By further increasing ReV an instability sets in and a minimum critical Reynolds number of Re=668350.491 is reached at ReV=9.799. Hence, around this point, the destabilizing effect of blowing outweighs the stabilizing effect of suction. By further increasing the transpiration rate beyond this point, the corresponding critical Reynolds number Re increases again and continues to grow. This limiting case is accompanied by the development of a strong boundary-layer-like velocity profile near the upper wall and the flow transitions to the asymptotic suction boundary layer (ASBL). Thus, the present analysis comprises the whole range from classical Couette flows extended by transpiration to the ASBL, which is known to have a strongly stabilizing effect. Published by the American Physical Society 2024

DEM investigation on flow instability of particulate assemblies under coupling between volumetric and axial strains

AbstractThe discrete element method (DEM) is employed to investigate the impact of coupling between volumetric and axial strains on the flow liquefaction vulnerability of 3D cubic particulate specimens. The virtual testing program conducted here encompasses a wide range of initial states and varying degrees of coupling between volumetric and axial strains. Utilizing data obtained from DEM simulations, the evolution of micro- and macroscale variables, including coordination number, contact fabric anisotropy, redundancy index, strong force networks, invariants of the effective stress tensor, and excess pore-water pressure, is examined. Results from DEM tests indicate that coupling expansive volumetric strain with axial strain leads to a gradual loosening of the load bearing microstructure, a decrease in coordination number, and a faster change in contact anisotropy. DEM simulations demonstrate that the triggering of flow liquefaction instability is followed by a sudden increase in contact fabric anisotropy and abrupt drops in coordination number and redundancy index. Moreover, a detailed analysis of the findings suggests that the stress ratio at the onset of post-peak softening decreases with increasing expansive volumetric strains.

Frequency and amplitude of flashing-induced instability in an open natural circulation loop

Several nuclear reactor designs rely on passive containment cooling systems. The so-called containment wall condenser relies on natural circulation loops to extract heat from high-temperature steam in the containment to a water tank at ambient pressure. In such passive systems, phase changes can happen and cause flow instabilities in the cooling loop. The flashing-induced instability occurs when the heated fluid in the riser suddenly vaporizes due to a hydrostatic pressure decrease. This instability causes periodic flow peaks, which are of major concern but whose characteristics have not been studied quantitatively.This paper presents two analytical models that predict the flashing frequency and a maximum flow amplitude from geometry and basic operating parameters such as power level and reservoir temperature. The expressions are derived from a physical analysis and do not involve any calibration constants. The flashing frequency appears to be driven by the power level, the inlet temperature and the riser pipe geometry. For the amplitude, the maximum flow rate can be expressed in a Froude number that depends only on the total pressure losses. These models are validated against PASI experiments and system-scale simulations with the CATHARE 3 code, both performed as part of the European Commission funded PASTELS project. Additional data from numerous experimental studies in the literature are used to extend the validity range of the frequency model.Successfully validated against experimental data and additional simulations, these models provide an explicit relationship between oscillations characteristics and design parameters, making them valuable tools for nuclear engineers.

Roles of physical collision forces and walking characteristics on pedestrian traffic flow status

The physical collision force between pedestrians affects the walking states of pedestrians. However, the relationship between physical collision force and pedestrian walking states remains unclear. To bridge this gap, a series of pedestrian-controlled experiments were designed. During the experiments, the physical collision forces were obtained using pressure sensors worn by the pedestrian. Walking characteristics, including speed and density, were obtained through video processing technologies from aerial videos taken by drones. Furthermore, the relationships between the physical collision forces and the walking characteristics were analyzed. The results show that, when the collision forces between pedestrians exceed 715 N, it will lead to the instability in the pedestrian traffic flow; when the collision forces are less than 400 N, the pedestrian walking state will remain stable; and when the collision forces are between 400 and 715 N, the pedestrian walking status may be described as in either an unstable or stable state.

Advanced Computational Framework to Analyze the Stability of Non-Newtonian Fluid Flow through a Wedge with Non-Linear Thermal Radiation and Chemical Reactions

The main idea of this investigation is to introduce an integrated intelligence approach that investigates the chemically reacting flow of non-Newtonian fluid with a backpropagation neural network (LMS-BPNN). The AI-based LMS-BPNN approach is utilized to obtain the optimal solution of an MHD flow of Eyring–Powell over a porous shrinking wedge with a heat source and nonlinear thermal radiation (Rd). The partial differential equations (PDEs) that define flow problems are transformed into a system of ordinary differential equations (ODEs) through efficient similarity variables. The reference solution is obtained with the bvp4c function by changing parameters as displayed in Scenarios 1–7. The label data are divided into three portions, i.e., 80% for training, 10% for testing, and 10% for validation. The label data are used to obtain the approximate solution using the activation function in LMS-BPNN within the MATLAB built-in command ‘nftool’. The consistency and uniformity of LMS-BPNN are supported by fitness curves based on the MSE, correlation index (R), regression analysis, and function fit. The best validation performance of LMS-BPNN is obtained at 462, 369, 642, 542, 215, 209, and 286 epochs with MSE values of 8.67 × 10−10, 1.64 × 10−9, 1.03 × 10−9, 302 9.35 × 10−10, 8.56 × 10−10, 1.08 × 10−9, and 6.97 × 10−10, respectively. It is noted that f′(η), θ(η), and ϕ(η) satisfy the boundary conditions asymptotically for Scenarios 1–7 with LMS-BPNN. The dual solutions for flow performance outcomes (Cfx, Nux, and Shx) are investigated with LMS-BPNN. It is concluded that when the magnetohydrodynamics increase (M=0.01, 0.05, 0.1), then the solution bifurcates at different critical values, i.e., λc=−1.06329,−1.097,−1.17694. The stability analysis is conducted using an LMS-BPNN approximation, involving the computation of eigenvalues for the flow problem. The deduction drawn is that the upper (first) branch solution remains stable, while the lower branch solution causes a disturbance in the flow and leads to instability. It is observed that the boundary layer thickness for the lower branch (second) solution is greater than the first solution. A comparison of numerical results and predicted solutions with LMS-BPNN is provided and they are found to be in good agreement.

FORMULATION AND EVALUATION OF RED GINGER RHIZOME EXTRACT SOAP AS AN ANTIBACTERIAL

Objective: Current research work aimed to develop a formulation of red ginger into a stable soap preparation, effective as an antibacterial and safe for long-term use. Methods: The maceration method was used to extract red ginger rhizome. In the formulation, there are variations in the concentration of the extract, where Formulation I (FI) 3%, Formulation II (FII) 5%, Formulation III (FIII) 7%, and Formulation IV (FIV) 0%. The evaluation of the preparation includes tests such as sensory evaluation, homogeneity, pH, viscosity and flow properties, density, foam height, and stability, as well as antibacterial activity using the agar diffusion method against Staphylococcus aureus ATCC 25932 and Escherichia coli ATCC 25922. Results: The percentage of extract yield obtained was 2.38%. All three formulations of soap have shown good stability during the 28-day evaluation (significant>0.05), indicating no significant changes during storage, and the inhibitory power of Staphylococcus aureus ATCC 25923 bacteria in the three successive formulations was 9.03±0.4; 12.21±0.3; 15.26±0.4 (mm) respectively, while that of Escherichia coli ATCC 25922 was 6.01±0.6; 10.32±0.4; 12.58±0.6 (mm). Conclusion: The evaluation results, all formulations have good stability during storage. The variation in concentration of red ginger extract will affect the inhibitory power against test bacteria. F III, with an extract content of 7% has better antibacterial activity compared to other formulations.

Differential Impact of Surface Conduction and Electroosmotic Flow on Ion Transport Enhancement by Microscale Auxiliary Structures.

Our research investigates the impact of auxiliary structures on ion transport in electrochemical systems such as batteries and microscale desalination units, whose importance for sustainable development has increased dramatically in recent decades. The electrochemical systems typically feature ion-selective surfaces, such as electrodes and ion exchange membranes, where ion depletion can cause performance issues including metal dendrite formation and flow instability. Recent research has shown that auxiliary structures in these electrochemical systems can enhance ion transfer near ion-selective surfaces, thereby resolving the instability problem and improving the energy conversion efficiency of the system. Our study leverages recent advancements in nanoscale electrokinetics to model these auxiliary structures as pillar arrays near an ion exchange membrane in a microchannel. We examine how these structures enhance ion transports relative to the characteristic length scale of microchannel depth and pillars' proximity to the ion-selective surface. Results show that the effect of the pillars varies significantly with their placement. Specifically, in deeper microchannels, where electrokinetic convection is stronger, the closer the auxiliary structure is to the ion-selective membrane, the better the ion transfer. However, in the thinner microchannel, the proximity of the auxiliary structure to the ion selective membrane has a less significant correlation with the ion transfer. Therefore, this finding highlights the importance of spatial arrangement of the auxiliary structures in improving the performance of electrochemical devices. Conclusively, this study can help to better understand energy conversion systems such as fuel cells, salinity gradient power generation systems, and electrochemical desalination systems, where auxiliary structures can be used in the vicinity of ion-selective surfaces. Especially, our fundamental electrokinetic study provides an effective means for designing the efficient electrochemical platforms utilizing micro/nanofluidics.

The burnt gas flow stability of limit flames in vertical tubes

Stability of the gas flows generated by steady near-limit flames propagating in vertical tubes is studied numerically. Basic scenarios of the burnt gas flow evolution are identified in relation to the thermal gas expansion parameter and the normal flame speed. It is shown that the realization of specific scenario essentially depends on the stagnation zone width as well as on the distance travelled by the gas from the flame front to the tube end. In particular, it is found that for sufficiently short distances, the burnt gas flows are stable provided that the stagnation zone width is less than half the tube diameter. Otherwise, an unstable flow evolution can lead to the appearance of recirculation domains and acoustic perturbations.

Optimization design of an innovative francis draft tube: Insight into improving operational flexibility

Hydropower has transitioned from base-load generation into an energy regulator in the hydro-wind-solar integrated systems, resulting in prolonged turbine operation under off-design conditions. Inspired by the new requirements of operation mode, an innovative Francis draft tube with inclined conical diffuser is presented to mitigate the pressure fluctuations and fatigue damage induced by flow instabilities. The design is optimized and comprehensively investigated to improve the operational flexibility. Computational fluid dynamics combined with design of experiments, artificial neural network and multi-objective optimization process lead to two optimal designs denoted as Opt-1 and Opt-2 from the Pareto fronts. Corresponding maximum pressure fluctuation amplitude is reduced by 15 % and 20 % compared to the traditional, respectively. An 81 % decrease of the fluctuating pressure recovery for the modified draft tube is obtained, leading to a steadier runner torque and consequently lower power swing. The asymmetric design of inclined daft tube is mainly targeted at the mitigation of the synchronous component as well as the attenuation of interaction between the swirling flow and elbow, accompanied with the mitigated energy of first-order vortex structure. The proposed innovative draft tube and optimization technique are found useful to broaden the operating range of hydraulic turbine in a computationally affordable way.

Flow instabilities driven by Prandtl number effect and rotation-depth coupling effect in the cylinder with a top disk

We employed linear stability analysis to investigate the Prandtl number (Pr) effect and rotation-depth coupling effect on the complex flow instability in a cylinder with a top disk. The dependence of the critical Rayleigh number on the Pr number, as well as the relationship between the critical disk rotation rate and the aspect ratio at Pr = 6.7, were obtained. Results reveal that the flow stability increases with increasing Pr number, and the convection instability stems from the inertial mechanism for 0.011 ≤ Pr ≤ 0.0258 and thermal buoyancy mechanism for 1.4 ≤ Pr ≤ 28.01. Regarding the rotation-depth coupling effect for the melt with Pr = 6.7, a decreasing melt level leads to a general increase in the critical disk rotation rate. Furthermore, within the interval of aspect ratio (ratio of depth to cylinder radius) 0.7 ≤ Γ ≤ 1.62, multiple transitions of the flow state from stable to unstable and then back to stable were observed with increasing disk rotation rate before finally becoming unstable. Energy analysis reveals that multiple transitions in the flow state are attributed to the competition between thermal buoyancy and inertial mechanisms.

Free surface water waves generated by instability of an exponential shear flow in arbitrary depth

The stability of an exponential current in water to infinitesimal perturbations in the presence of gravity and capillarity is revisited and reformulated using the Weber and Froude numbers. Some new results on the generation of gravity-capillary waves are presented, which supplement the previous works of Morland et al. [“Waves generated by shear layer instabilities,” Proc. Math. Phys. Sci. 433, 441–450 (1991)] and Young and Wolfe [“Generation of surface waves by shear-flow instability,” J. Fluid Mech. 739, 276–307 (2014)] on finite depth. To consider perturbations at much larger scales, special attention is given to the stability of exponential currents only in the presence of gravity. More precisely, the present investigation reveals significant insights into the stability of exponential shear currents under different environmental conditions. Notably, we have identified that the dimensionless growth rate increases with the Froude number, providing a deeper understanding of the interplay between shear layer thickness and surface velocity. Furthermore, our analysis elucidates the dimensional wavelength of the most unstable mode, emphasizing its relevance to the characteristic shear layer thickness. Additionally, within the realm of gravity-capillary instabilities, we have established a sufficient condition for the stability of exponential currents based on the Weber number. Our findings are supported by stability diagrams at finite depth, showing how the size of stable domains correlates with the characteristic thickness of the shear layer. Moreover, we have explored the stability of a thin film of liquid in an exponential shearing flow, further enriching our understanding of the complex dynamics involved in such systems.

Traffic flow impact of mixed heterogeneous platoons on highways: an approach combining driving simulation and microscopic traffic simulation

The connected automated vehicle (CAV) platoons are expected to improve the operational performance of the transportation system. Existing platoon testing processes have used the format of mixed heterogeneous platoons (MHPs), where the leader vehicles are connected human-driven vehicles (CHDVs) and the follower vehicles are CAVs. The impact of this type of MHP on existing highway traffic flow is more complex due to the presence of human drivers, and this impact has yet to be analyzed and quantified. Therefore, this paper takes this type of MHP as the research object and conducts experiments by combining driving simulation and microscopic traffic simulation. The driving behavior of the driver of the MHP's leader vehicle is incorporated into the car-following model by means of a car-following model calibration, and an impact analysis is carried out in terms of multiple dimensions of stability, efficiency, and ecological aspects of the mixed traffic flow under different MHP market penetration rates (MPRs). The results show that MHPs weaken the oscillatory effect of mixed traffic flow and improve mixed traffic flow stability. The MPRs of MHPs are proportional to the road capacity and average speed and antipodal to the average travel time. Also, the MHPs help reduce fuel consumption and pollutant emissions (CO2, CO, NOx, HC, and PMx) during traffic operations. This study can provide theoretical support for the future large-scale application and management of MHPs.

A new curved road lattice model integrating the multiple prediction effect under V2X environment

In order to explore the driver's multiple prediction effects of flux changes (MPEFC) on traffic flow on a curved road, parameters of the weighted flow gradient of multiple lattices ahead are introduced to characterize the MPEFC effect on a curved road. On this basis, combined with the characteristics of a curved road, a lattice model considering the drive's MPEFC effects is proposed under the V2X environment. The stability criterion is obtained. Through the reduced perturbation method, the density wave equation and its solution used to describe the propagation law of traffic congestion near the critical point are derived. The results show that the stability of traffic flow on a curved road increases along with the increase in radian, and that considering the MPEFC effects simultaneously has a significant stabilizing effect for traffic flow on a curved road. It is also discovered that under the V2X environment, the traffic flow is optimally stabilized by considering the information of just two lattices ahead.

Characteristics and mechanism of the shift in condition- and time-dependent spray patterns induced by oxygenated fuel addition

As renewable energy sources, ethanol and butanol are usually added to fuels to reduce the fossil fuel consumption and soot emissions. Gasoline direct injection with multi-hole injectors provides accurate plume control, enabling ejection toward the desired location, while spray pattern shift leads to deviation in the injection direction. The spray shift needs to be modulated to realize the desired spray pattern, which is complicated by oxygenated fuel addition. To illustrate the shift induced by oxygenated fuel addition, numerical simulations and experimental studies were performed. The macroscopic and microscopic characteristics of the spray under various operating conditions were obtained via the diffuse backlight illumination and phase Doppler anemometry methods and the simulation of the internal three-phase flow. Moreover, typical single-component and binary fuels were employed to define the general envelope of the multi-component nature for auxiliary analysis. The shift in condition- and time-dependent spray patterns under both flashing and non-flashing conditions were illustrated. The spray morphologies were characterized by rectangular- and trapezoid-like trends. The addition of oxygenated fuels resulted in a smoother trend due to the change of the vapor pressure and surface tension of the mixed fuel. Moreover, it resulted in the variation of the critical width, which is crucial for the spray pattern shift. Additionally, the correlation between the critical width and ambient pressure was obtained. For the non-flashing spray, the internal flow and jet instabilities both influenced the spray width, and the correlation between gas ingestion and turbulence characteristics was obtained. This study aimed to provide guidance for better utilization of oxygenated fuels.

CFD modelling of Close-Coupled gas atomisation process by employing the Euler-Lagrange approach to understand melt flow instabilities

CFD modelling of Close-Coupled gas atomisation process by employing the Euler-Lagrange approach to understand melt flow instabilities

Characterizing rheological behavior and fluidization of highly refined furnishes for process optimization

In this work, highly refined softwood bleached kraft pulp (SWBKP) furnishes, referred to here as XFC, were studied from the perspective of fiber suspension handling in processing. The rheology of the furnishes was studied with a rotational rheometer using a non-standard flow geometry to understand the viscosity development at different consistencies and the impact of temperature. For fluidization analysis during pipe flow, two optical methods were implemented; namely, optical coherence tomography (OCT) and high-speed video (HSV) imaging. The OCT was used to determine the small-scale floc structures near the pipe wall where the shear stress is highest, and the HSV imaging was applied for observing flow instabilities and XFC suspension uniformity at the pipe scale. All these issues can be significant in deciding the minimum flow rate required for a process pipe to get sufficient fluidization of XFC suspensions.

The influence mechanisms of re-fused scanning on the surface roughness, microstructural evolution and mechanical properties of laser powder bed fusion processed AlMgScZr alloy

Laser powder bed fusion (LPBF) stands out as the foremost method for achieving intricate structure with superior design flexibility among additive manufacturing technologies. Within the LPBF process, surface roughness serves as a critical metric governing both fabrication precision and build quality. This study delves into the evolution of the top and side surface roughness as re-fused scanning times increase in LPBF-fabricated AlMgScZr alloy. Concurrently, the investigation evaluates alterations in microstructural characteristics and mechanical properties. Various characterization techniques, including confocal laser scanning microscope, scanning electron microscope, X-ray diffraction, electron backscatter diffraction, as well as tensile and microhardness tests, are employed. The findings concerning the top surface reveal that the initial application of re-fused scanning contributes to enhanced surface roughness. This improvement stems from refined grain structures and induced lattice distortions, facilitated by rapid heat dissipation and thermal recycling within the solidified layer. Consequently, microhardness increases. However, introducing the subsequent re-fused scanning exacerbates surface roughness and instigates Marongoni flow instability, magnesium depletion, and alterations in lattice structures. Moreover, with increasing the re-fused times, there is a noticeable enhancement in preferential crystallite growth orientation along the (200)α-Al crystal plane. Regarding the side surface, escalating re-fused times correlate with deteriorating roughness and alterations in molten pool shape and size, attributable to successive energy input. Despite these surface variations, minimal differences are observed in yield strength, tensile strength, and elongation. In essence, this research offers valuable insights into selecting optimal scanning methodologies during the LPBF process, thus guiding improvements in fabrication precision and component quality.

Dynamic Simulation of the Stability of Annular Flows in Gas Wells

This paper provides a holistic view of the development and progression of multiphase gas-liquid annular flow in gas wells. With the help of a momentum balance, the stability of a fully developed annular flow is defined. The Euler-Euler model was used to model the multiphase flow system, while the Reynolds-Averaged Navier–Stokes (RANS) model was used to depict a time-dependent solution for a vertical fluid flow system, considering acceleration due to gravity and fluid compressibility. The resultant flow regime was tested for stability by gradually reducing the field velocity of flow and observing the effects using a computational fluid dynamics (CFD) software. The principal parameters considered are the superficial gas velocity and liquid film flow rate. The properties of the region where the flow becomes unstable are documented and related to the incidence of liquid loading in a typical gas well. The effect are critically observed and compared to flow regimes at lower superficial gas velocity. The resultant flow regime configurations are analyzed and compared to the annular flow system. This study will help researchers understand the behaviour of gas wells that have potential in developing liquid loading problems and the movement of liquid film on the wall of the tubing when in annular flow regime.

Solar Influences on Winter Atmospheric Circulation Over the Ural, North Atlantic and North Pacific Regions: Pitchfork‐Like Relationships

AbstractLi's previous study found a one‐to‐two correspondence relationship between the total solar irradiance (TSI) and monthly anomalies of the Ural atmospheric circulation during winter months. This relationship roughly meets the so‐called supercritical pitchfork bifurcation model, hence termed a pitchfork‐like (PL) relationship. Here, we extend this study and provide more evidence to support previous findings. By analyzing data from 192 winter months and estimating the joint probability density function, we find that over the Ural, North Atlantic, and North Pacific regions, there are all PL relationships between the TSI and geopotential height anomalies at pressure levels no higher than 700 hPa. When the TSI gradually decreases and passes a critical threshold of 1360.9 W m−2, the preferred circulation patterns undergo a regime transition from one regime to two others. When TSI < 1360.9 W m−2, positive and negative geopotential height anomalies occur at nearly equal frequencies within the same TSI range. This quantitative relation is met in almost the whole troposphere in each region. We suggest that the occurrence of PL relationships may be associated with the interaction between the westerly flow and topographic barriers; The decrease in TSI can regulate this interaction by reducing the stability of background westerly flow over all three regions, via increasing the land‐ocean thermal contrast and altering the planetary wavenumber‐2 wave at the mid‐high latitudes. Our work highlights the same quantitative relation between the TSI and the monthly mean geopotential height anomalies of three regions in winter, which has important implications for seasonal climate prediction.