Stability Of Flow Research Articles

Stability and related zero viscosity limit of steady plane Poiseuille-Couette flows with no-slip boundary condition

We study the existence and stability of smooth solutions to the steady Navier-Stokes equations near plane Poiseuille-Couette flow in the absence of any external force in two-dimensional domain Ω=(0,L)×(0,2). Under the assumption 0<L≪1, we prove that there exist smooth solutions to the steady Navier-Stokes equations which are stable under infinitesimal perturbations of plane Poiseuille-Couette flow. In particular, if the basic flow is the Couette flow, then through a formal asymptotic expansion including the Euler correctors and weak boundary layer correctors with different scales near the rigid walls y=0 and y=2, we can prove that for any finite disturbance o(1) of the Couette flow in horizontal direction, there still exist stable smooth solutions to the steady Navier-Stokes equations. Finally, based on the same linear estimates, we establish the zero viscosity limit of the solutions obtained above to the solutions of the steady Euler equations.

Hydrodynamic Analysis of Flow Stabilization Process in a Miniature Circulating Water Channel Equipped with Combined Porous Plate Structures

As a foundational platform for fluid dynamics research on underwater devices, the quality of stable and uniform incoming flow in a circulating water channel (CWC) is essential. Due to the inherent structural characteristics of porous media, their application has become widespread in offshore and marine engineering. This study presents the design of a miniature circulating water channel, alongside the development of a computational fluid dynamics (CFD) numerical model to simulate the interaction between porous media and incoming flow. By analyzing the pressure loss of fluid within the porous plates and incorporating it as a source term in the momentum equation, heavy computational costs associated with fine mesh requirements in micro porous models were avoided. Simulations were conducted to investigate the effects of combined porous plates on the flow field, and the porous media model was validated by comparisons with experimental data. The distribution of the flow field within the channel was analyzed to define the test section, which was conducive to further structural optimization. The results indicated that the combined porous plates stabilized the flow field of the CWC, and, following structural optimization, the occurrence of backflow was reduced, thereby expanding the effective test section range.

Aerodynamic characteristic of limited micro-scale Taylor–Couette flow at hyper-rotate speed

To pursue improvements in the power-to-weight ratio, cost-effectiveness ratio, and reliability in the energy industry, rotating machinery has been developed with higher rotational speeds and smaller sizes, resulting in complex aerodynamic behavior of the rotational shearing flow between rotator and stator. This study reports experimental measurements of the aerodynamic characteristics of a limited micro-scale Taylor–Couette flow (TC flow) at hyper-rotational speeds for the first time. The maximum rotational speed and eccentricity ratio are 6.0 × 104 rpm and 0.8, respectively, and the minimum average clearance height is 100 μm. The static pressure of the shearing flow is measured using a high-response micro-manometer. A large eddy simulation has been employed to unveil flow fields of the gas film between the stator and the rotator. The results show that the micrometer-scale clearance height and end leakage due to limited length dominate the micro-scale Taylor–Couette flow. Decreasing the scale of the clearance height reduces the angular distance between the peak and bottom pressures compared to that observed in the TC flow with a large-scale clearance. Furthermore, ambient pressure cannot fully permeate into central section of the eccentric Taylor–Couette flow with a micrometer-scale clearance and end leakage, and the pressure upstream near the region the minimum size of clearance decreases along the axial end. Owing to the difference in the constraint degree at both ends along the axial direction, the peak value occurs near the region of Z/L = −0.32. The maximum pressure of the limited micro-scale Taylor–Couette flow increases with increasing rotational speed and eccentricity ratio, whereas it decreases with increasing clearance height. Within the scope of the parameters in this study, the maximum pressure is 0.025 MPa. Decreasing the size of gas film restrains the instability of the shearing flow, whereas increasing the rotational speed strengthens the turbulence. In this study, the shearing flow at the middle section (Z/L = 0) is in a nearly laminar state at δ &amp;lt; 0.012, whereas it is highly turbulent near the end of the clearance and in the ambient environment. The end leakage induces additional axial flow, which is conspicuous at a large eccentricity ratio. At hyper-rotational speeds, the micro-scale Taylor–Couette flow with end leakage exhibits distinctive patterns that have never been reported in classic Taylor–Couette flows with large clearances. This work of this study is significant because it can provide theoretical guidance and technical support for the design of actual dynamic pressure gas bearings and floating ring sealing flow.

Stability of finite-length collapsible channel flow to spanwise perturbations

We consider flow through a finite-length flexible-walled channel formed by removing a portion from one wall of a wide rigid channel and replacing by a pre-tensioned hyperelastic sheet of finite thickness. The flow is driven by a prescribed upstream flow rate which is uniform along the channel inlet, and so with periodic boundary conditions in the transverse direction the system admits a steady state with a wall profile which is deformed in the streamwise direction but spatially uniform in the spanwise direction. Identical to the planar case, this system exhibits two stable static configurations: an upper branch where the flexible wall is mostly inflated and a lower branch where the wall is highly collapsed. We consider the stability of these steady states to spanwise perturbations, showing that both can become unstable to distinct families of self-excited oscillations. In particular, for large spanwise wavenumbers, these steady states are unstable to oscillatory normal modes where the perturbation wall profile has a single oscillating hump in the streamwise direction, not previously seen in collapsible channel flows driven by fixed upstream flux. Furthermore, for sufficiently large wavenumbers and no wall pre-tension in the spanwise direction, this system is always unstable to a new spanwise non-uniform static configuration, which arises due to the merging of a pair of unstable oscillatory normal modes. However, with non-zero spanwise wall pre-tension, there is always a region of parameter space where the spanwise uniform steady state is stable for all wavenumbers.

Suppression of melt flow instabilities by amplifying high-frequency melt waves in laser fusion cutting

High-frequency resonant melt waves locally increase the stability of melt flow and enhance cut flank quality. To amplify these waves along the melt flow, an acoustically tuned cutting nozzle, termed the cutting whistle, generates gas flow oscillations based on a resonant cavity. The nozzle cavity is designed for 6 mm thick sheets, causing the gas flow to oscillate at 15 kHz within the kerf, creating a uniform melt flow with synchronized oscillating melt waves. Performance evaluations on stainless steel sheets show that, compared to a standard conical nozzle, the cutting whistle reduces surface roughness across the entire cutting depth and prevents coarse structures near the upper sheet surface and at the bottom in burr-free cuts.

Influencing of Molten Pool Dynamic Behavior on the Weld Formation during the Laser‐Arc Hybrid Welding of 12 mm Thick Steel

A numerical simulation model is put forward to study the molten pool dynamic behavior and clarify its influencing mechanism on the undercut defects in laser‐arc hybrid welding (LAHW). The undercut defect is a common and challenging problem to address. However, a controversy exists on whether the dynamic process of molten pool can affect undercut defects, and the potential mechanism has been still unclear. The results suggest that profile variation of molten pool surface is driven by surface tension gradient, and the existence of eddy can alter the morphology and surface tension distribution of molten pool, which is the main factor to homogenize the flow field. Compared with the short‐circuit transfer mode, the molten metal flows toward the welds edge at a constant velocity (2.5–3.75 m min−1) in the spray transfer mode, which improves the stability of molten pool flow. It can be observed that the irregular fluctuation of molten metal during the solidification process can directly cause the undercut defects.

Seepage and stability analysis of hydraulically anisotropic unsaturated infinite slopes under steady infiltration

An analytical model is derived for predicting the flow field and stability of an unsaturated infinite slope subjected to steady infiltration. The proposed model is novel because it accounts for the hydraulic anisotropy of unsaturated soil. The governing equation for steady-state seepage in an infinite slope is established in terms of matric suction under a constant surface flux boundary condition. On the basis of the available experimental findings on the hydraulic anisotropy behavior of unsaturated soils, the relative hydraulic conductivity for a soil under unsaturated conditions with respect to the soil at saturation is postulated to be a direction-independent scalar. This postulation simplifies the governing equation to a form that is directly solvable via the relative hydraulic conductivity and the saturated hydraulic conductivity tensor. To enable sophisticated applications, an exponential law and a power law that are well established in the unsaturated soil literature are used to relate the relative hydraulic conductivity to the matric suction and the effective degree of saturation, respectively. Closed-form solutions are derived for the matric suction, the flow net (potential function and stream function), and the effective degree of saturation. Analytical solutions are also derived for the soil unit weight and overburden stress. These solutions are incorporated into the unsaturated infinite slope stability formula constructed on a suction stress-based effective stress failure criterion. Hydraulic anisotropy has been shown to directly affect the flow field and the change in matric suction, which, in turn, drastically affects the slope safety factor against shallow landslides. This finding demonstrates that neglecting hydraulic anisotropy can cause a considerable overestimation of the safety factor, resulting in an unsafe slope stability prediction. The proposed model is useful for preliminary evaluation of the long-term stability of unsaturated slopes during wet periods and the antecedent slope conditions for shallow landslide initiation under transient infiltration.

Effect of the flow behavior and dynamic recrystallization of EH420 marine steel on the morphology evolution of martensite substructures

The hot deformation behavior of EH420 marine steel was investigated by isothermal compression tests in the temperature range of 850~1050°C and the strain rate range of 0.01~10s−1. A high-temperature constitutive model and a hot processing map were developed. The results indicated that the microstructure of EH420 marine steel after hot deformation was mainly bainite and lath martensite. The martensite blocks gradually coarsened with the increase in temperature within the range of 950~1050°C/0.01s-1, and the average width increased from about 0.7 μm to 2.3 μm. At high strain rates, martensite blocks became coarse and disordered due to flow instability. The flow stress curve showed a single peak when the strain rate was low. There were more dynamic recrystallization (DRX) structures, resulting in fine and ordered martensite blocks. Under the deformation conditions of 0.1~10s-1/1000°C, the proportion of high-angle grain boundaries (HAGBs) decreased from 53.5% to 40.7% as the strain rate increased. Meanwhile, the hot deformation mechanism shifted from discontinuous dynamic recrystallization (DDRX) to the combined effect of DDRX and dynamic recovery (DRV) and finally transformed into DRV. The flow stress derived from the constitutive model was consistent with the experimental results. The activation energy of EH420 marine steel was 3.16×105J/mol. The optimal hot processing parameters were 950~1050°C and 0.01~0.1s-1.

Comprehensive CFD Analysis of Hypersonic Missile Nose Cone

Abstract: This study employs high-fidelity CFD to analyze a hypersonic missile nose cone's aerodynamics and boundary layer behavior (Mach &gt; 5, zero angles of attack) under idealized conditions. The simulation leverages ideal gas assumptions for simplicity while incorporating advanced turbulence models to capture hypersonic flow phenomena. The analysis focuses on pressure distribution across the nose cone to assess shockwave and streamline curvature effects. Lift and drag forces are computed to evaluate overall aerodynamic performance. Critically, the investigation delves into the boundary layer, evaluating parameters like thickness, velocity profile, and potential separation points to understand pressure and temperature variations crucial for heat transfer and flow stability

Vehicle-to-Grid Power Transfer Method for Electric Vehicles using off-board charger

This article explores a power transfer technique from vehicle to grid (V2G) via the construction of an off-board charger for electric cars (EVs). The charger accommodates several charging modes, such as grid-to-vehicle (G2V), vehicle-to-vehicle (V2V), and vehicle-to-grid (V2G), facilitating efficient and adaptable energy management. In G2V mode, the charger utilizes grid power to recharge electric vehicle batteries, whilst V2V mode enables direct energy transfer between electric vehicles, circumventing the grid. The novel integration of G2V and V2V modes enables the concurrent use of grid electricity and energy from other electric vehicles, therefore diminishing grid reliance and enhancing power efficiency. The system has a three-phase pulse width modulation (PWM) rectifier that sustains a constant DC link voltage and attains a unity power factor on the grid side, therefore adhering to the IEEE 519 standard for total harmonic distortion (THD). Furthermore, a half-bridge bidirectional DC/DC converter guarantees consistent charging and discharging currents, hence improving the reliability and efficiency of the charging process. This holistic strategy enhances dynamic energy flow and grid stability while providing possible economic advantages to electric vehicle owners and operators via the integration of renewable energy sources and sophisticated management algorithms for improved energy use and storage.

Transient Energy Growth in a Free Cylindrical Liquid Jet

The stability and behavior of jet flows are critical in various engineering applications, yet many aspects remain insufficiently understood. Previous studies predominantly relied on modal methods to describe small perturbations on jet flow surfaces through the linear superposition of modal waves. However, these approaches largely neglected the interaction between different modes, which can lead to transient energy growth and significantly impact jet stability. This work addresses this gap by focusing on the transient growth of disturbances in jet flows through a comprehensive non-modal analysis, which captures the short-term energy evolution. Unlike modal analysis, which provides insights into the overall trend of energy changes over longer periods, non-modal analysis reveals the instantaneous dynamics of the disturbance energy. This approach enables the identification of transient growth mechanisms that are otherwise undetectable using modal methods, which treat disturbance waves as independent and fail to account for their coupling effects. The results demonstrate that non-modal analysis effectively quantifies the interplay between disturbance waves, capturing the nonlinearity inherent in transient energy growth. This method highlights the short-term amplification of disturbances, providing a more accurate understanding of jet flow stability. Furthermore, the impact of dimensionless parameters such as the Reynolds number, Weber number, and initial wave number on transient energy growth is systematically analyzed. Key findings reveal the optimal conditions for maximizing energy growth and elucidate the mechanisms driving these phenomena. By integrating non-modal analysis, this study advances the theoretical framework of transient energy growth, offering new insights into jet flow stability and paving the way for practical improvements in fluid dynamic systems.

Linear stability and spectral modal decomposition of three-dimensional turbulent wake flow of a generic high-speed train

Linear stability and spectral modal decomposition of three-dimensional turbulent wake flow of a generic high-speed train

Zinc Oxide-Based Endodontic Sealer with Proanthocyanidin-PLGA Nanoparticles: Synthesis and Physical Characterization

This study developed a zinc oxide-based endodontic sealer incorporating proanthocyanidin nanoparticles (PAC-NPs) encapsulated in poly(lactic-co-glycolic acid) (PLGA), utilizing PAC’s proven ability to enhance dentine collagen stability. The formulation was designed to provide both antimicrobial and collagen-stabilizing benefits, aiming to improve dentine-sealer interface integrity and prolong the functional lifespan of root canal treatments. Chemical characterization was conducted using Fourier Transform Infrared (ATR-FTIR) spectroscopy, confirming the presence of Zn-O and C=O bonds, indicative of the successful integration of zinc oxide and encapsulated PAC. Physical properties, including flow, setting time, solubility, and dimensional stability, were evaluated according to ISO 6876:2012 and ANSI/ADA standards 57, with the sealer meeting all clinical performance criteria. This preliminary characterization of the physical and chemical properties establishes a basis for further in vitro studies on the mechanical and biological performance of the sealer, aimed at validating its potential to enhance dentine collagen stabilization and material durability in endodontic applications.

Enhanced flow boiling by manipulating two-phase flow in Tesla channel heat sink using HFE-7100

Flow boiling of dielectric fluids in copper microchannel heat sinks is highly desirable for cooling large-sized insulated gate bipolar transistor (IGBT) power electronic modules. However, dielectric fluids present challenges in flow boiling because of their unfavorable thermophysical properties. These factors make it difficult to enhance critical heat flux (CHF) without precooling. To address this, we developed a large copper heat sink (10 cm × 5 cm) with Tesla microchannels designed to suppress vapor backflow and promote intense fluid mixing. The microchannels have a high length to hydrodynamic diameter ratio of approximately 220, significantly higher than those in previous studies. Flow boiling experiments using HFE-7100 were conducted for both Tesla and plain-wall microchannels. Tesla channels demonstrated a 26.2 % increase in CHF and a 120 % improvement in heat transfer coefficient (HTC). These enhancements are attributed to the vapor backflow suppression and improved fluid mixing. Moreover, the standard deviation of wall temperature in plain-wall microchannels was 10 times higher than in Tesla channels, highlighting the effectiveness of the periodic Tesla valves in reducing two-phase flow instabilities. Flow pattern visualization was conducted to further understand the mechanism behind vapor regulation, clarifying the role of Tesla valves in controlling vapor backflow. This study demonstrates the potential of dielectric fluids in Tesla microchannels for flow boiling applications, offering a promising solution for cooling large electronics.

Comparative study of turbulence models in CFD for transonic flow over the ONERA M6 wing

Transonic flight presents significant challenges due to complex phenomena such as shock waves and turbulent boundary layer interactions, often leading to flow separation and instabilities. This study aims to evaluate the performance of various turbulence models in predicting these phenomena using Computational Fluid Dynamics (CFD) with ANSYS-Fluent 2020R1. The ONERA M6 wing model is analyzed under specific flow conditions (angle of attack, α = 3.06°, Mach number, M∞ = 0.8395, and Reynolds number, Re = 11.72 × 10^6) to assess five turbulence models: Spalart–Allmaras, k-ε Standard, k-ε Realizable, k-ω Standard, and k-ω SST. Results from the simulations closely matched experimental data, with all models demonstrating a margin of error within 5% compared to NASA’s CFD benchmarks. Notably, the Spalart–Allmaras and k-ω SST models showed superior accuracy in predicting shock formation and pressure distribution, highlighting their potential for enhanced predictive capabilities in aerodynamic analysis. This study demonstrates the effectiveness of ANSYS-Fluent in aerodynamic performance evaluation and suggests pathways for advancing aircraft design efficiency and safety through improved turbulence modeling.

Effects of Lane Imbalance on Capacity Drop and Emission in Expressway Merging Areas: A Simulation Analysis

Lane imbalance does not provide sufficient space for merging vehicles to adjust their speed and change lanes smoothly. This leads to improper driving behavior that disrupts mainline traffic flow stability, resulting in capacity drops and increased vehicle emissions. However, quantitative analyses, specifically the effects of lane imbalance on capacity and emissions, remain limited. Existing traffic simulation platforms struggle to capture the effects of geometric design changes on capacity. To address these gaps, we developed a simulation method incorporating interactions between geometric design and traffic flow demand into an XGBoost model, enhancing the predictive accuracy for driving behavior parameters. Implemented within the TESS NG platform, this model enables real-time adjustments in driving behavior parameters as traffic demand varies under different lane balance conditions. The simulation results indicated a 42.4% capacity drop and a 34.9% increase in CO2 emissions when the balanced merging area was shifted to lane imbalance. Conversely, shifting to lane balance increases capacity by 8.2% and reduces CO2 emissions by 39.8% under severe congestion conditions. Under lane imbalance, vehicle speeds are lower across all traffic demand levels. When the demand exceeds 1300 pcu/h/ln, lane changes occur closer to the end of the acceleration lane, with higher speed differentials. These insights underscore the potential of lane balance optimization to mitigate capacity drops and emissions, providing a valuable simulation approach for the design and evaluation of merging areas.

Transportation characteristics analysis of the dense-phase gas-solid two-phase flow based on phase space reconstruction

The operational efficiency and safety of the dense phase pneumatic conveying system is deeply influenced by the transportation characteristics of the dense-phase gas-solid two-phase flow in conveying pipelines. This paper presents a framework for analyzing the transport characteristics of the gas-solid two-phase flow based on the phase space reconstruction. The framework explores the transport characteristics by reconstructing the state evolution trajectories of the two-phase flow system in state space. The main goal is to enhance the interpretability of feature analysis. By employing the phase space reconstruction method to process the output signals of an electrostatic sensor, the evolution trajectory of the dynamic state of the gas-solid two-phase flow system was reconstructed in phase space.This provides an effective entry point for extracting the essential dynamic characteristics of the gas-solid two-phase flow system. Furthermore, a series of feature values were proposed, including the average speed of the trajectory, Shannon entropy of the trajectory, and the average distance between state points in phase space, which were used to respectively evaluate important transportation characteristics such as flowability, stability, and particle aggregation degree of the gas-solid two-phase flow. Moreover, through comparative analysis with visualization images that represent actual conveying conditions, the effectiveness of these proposed feature values in evaluating these transportation characteristics was verified.

Analysis of the Effect of Structural Parameters on the Internal Flow Field of Composite Curved Inlet Body Hydrocyclone

Analysis of the Effect of Structural Parameters on the Internal Flow Field of Composite Curved Inlet Body Hydrocyclone

Experimental and numerical studies on enhanced flow boiling in tube with superwetting micro-finned surfaces

Due to the high latent heat from evaporation, flow boiling has been applied in a variety of industrial applications. However, the occurrence of “annular bubble” will dramatically increase the thermal resistance near the wall, causing decreases both in heat transfer coefficient (HTC) and critical heat flux (CHF). To tackle this issue, we propose the micro-finned surfaces with strong capillary force to enhance the heat transfer coefficient and critical heat flux for flow boiling. With deionized water as the coolant, we investigate the effects of fin width, fin height, fin pitch, coolant flow rate and heat flux on flow boiling through experiment and numerical simulation. The results show that the wider the fin width, the lower the fin height, and the smaller the fin pitch, the higher the convective heat transfer coefficient. In addition, the higher the coolant flow rate leads to higher heat transfer coefficient and lower flow boiling instability. When the heat flux increases, the convective heat transfer coefficient and flow instability increase. Among them, the microchannel with fin width of 20 μm, fin height 40 μm, and fin pitch 40 μm obtains the maximum convective heat transfer coefficient, 59.3 % higher than that of the smooth microchannel.

A novel stochastic second-order macroscopic continuum traffic flow model for traffic instability

Dispersion of vehicle speed often causes traffic instability, which affects traffic efficiency and safety. Previous studies have demonstrated that fluctuations near the steady-state equilibrium of traffic flow can be attributed to the stochasticity of traffic system leading by large vehicle speed dispersion. However, the intrinsic mechanisms between fluctuations of traffic flow and vehicle speed dispersion need further discussion. Therefore, a novel stochastic extended speed gradient model based on the fast-and-low speed vehicles is proposed to describe their relationships in this paper. Firstly, a speed-dependent stochastic process is characterized for the stochastic deviation of velocity dispersion, and a stochastic high-order macroscopic traffic flow model is constructed for fast and low speed vehicles. Secondly, using Lyapunov stability theorem and its derivatives, the linear stability criterion of the proposed model is derived. The theoretical results also indicate that the steady-state equilibrium of traffic flow is profoundly affected by the traffic flow initial density, penetration rate of fast and low speed vehicles, and noise intensity. Hence, simulations are implemented from the above three parameters. It can be found that the critical values of initial density affecting traffic flow stability are pursued, and some detailed tendencies of traffic flow are explored for different parameters. Finally, the model is also calibrated and validated through real data, and the impact of fast-and-low-speed vehicles on traffic instability at ramp-merging area is analyzed. Numerical experiments show that the stochastic traffic flow model proposed in this paper well reproduces the traffic oscillations occurring in real traffic.