Search Results (167)

This investigation is to address the aerodynamic noise generated from laminar flow over a D-shaped cylinder at a low Reynolds number (Re). Proposed is a novel assembly of arc-shaped splitter plates to effectively reduce the aeolian tone for the D-shaped cylinder. The two-dimensional flow field is simulated at an Re of 160 to investigate the mechanism of reducing the sound of the arc-shaped plates. The radiated sound has been predicted by Ffowcs Williams and Hawkings (FW-H) acoustic analogy. To verify calculations, the predicted results of a circular cylinder have been compared with the data in the literature. The results reveal that the introduction of the arc plates decreases the lift and drag fluctuations as well as the vortex shedding frequency in comparison with the no-arc plate case. The pressure and velocity fluctuations in the wake zone are reduced by the arc plates due to vortex shedding suppression. The application of the arc plates shows an effective control of sound, leading to a maximum reduction in sound pressure level (SPL) by almost 34 dB. Full article

Both experimental and numerical studies were conducted to obtain the influence laws of complex cavitation flow structures around a Clark-Y hydrofoil cascade. The similarities and differences in cavitation flow characteristics between the cascade and single hydrofoil were compared to analyze the influence of the cascade configuration on the flow field structure. This study focuses on the correlations among cavity development, lift–drag characteristics, and flow field features of the hydrofoil cascade. The results indicate significant differences in the development degree and history of cavities at different positions within the cascade. The top layer of the cascade exhibits a cavitation pattern similar to a single hydrofoil; both generate large-scale shedding vortices at the trailing edge. In contrast, the cavitation phenomena in the middle and bottom layers are similar to each other. The suction side of the top-layer hydrofoil influences the middle and bottom layers. This interaction suppresses the formation of large-scale shedding bubbles and subsequently hinders re-entrant shocks. Furthermore, the cavities in the middle and bottom layers develop more rapidly, causing the dynamic characteristics of the cascade to reach their peak values earlier. At the cloud cavitation stage, the Strouhal numbers ($St$) for cavity collapse on the top and bottom hydrofoils are approximately 0.2 and 0.3, respectively. The $St$ for the middle hydrofoil exhibits an intermediate value that decreases from 0.3 to 0.2 as the cavitation number ($\sigma$) declines, reflecting a transitional characteristic modulated by the cascade structure. Compared to a single hydrofoil, the cascade is subject to the combined effects of the three-layer hydrofoils; consequently, its lift is approximately three times that of a single hydrofoil, though its drag also increases threefold. The lift variation pattern of the top-layer hydrofoil in the cascade is similar to that of a single hydrofoil. In contrast, the middle-layer hydrofoil exhibits a more complex lift evolution, as both its suction and pressure sides are significantly influenced by the surrounding cascade structure. For the bottom-layer hydrofoil, the lift remains relatively low because no cavities are generated on its surface. Lift fluctuation frequencies that aligned with cavity collapse were identified at 45 Hz, 70 Hz, and 50 Hz across the top, middle, and bottom cascade layers, respectively. Full article

When high-altitude aircraft operate in a low-density environment, the flow instability within their internal ducts poses a severe challenge to aerodynamic design and operational safety. Especially in the intake system of the tandem dual-fan configuration, the asymmetric flow caused by rotating machinery coupled with the low-density effect exacerbates flow distortion, momentum dissipation, and efficiency loss and may even trigger system instability risks such as rotational stall or surge. To address these challenges, this paper establishes a high-fidelity dynamic model of the internal flow field of the aircraft, based on the Reynolds-averaged Navier–Stokes equations and the SST k-ω turbulence model, combined with dynamic mesh technology. It reveals the unstable mechanism caused by angular momentum accumulation under co-rotation conditions and its intrinsic correlation with the degradation of aerodynamic performance. Inspired by the concept of micro-flow regulation, an active flow control strategy integrating discrete auxiliary injection and local geometric shape optimization is proposed. Numerical results show that by reasonably arranging auxiliary injection holes in the intake duct and optimizing local geometric fillets, the uniformity of intake flow can be effectively improved, and the formation of large-scale vortex structures can be suppressed. This method increases the system’s flow capacity by approximately 47.4%, significantly improves the total pressure recovery coefficient and fan aerodynamic efficiency, and reduces the amplitude of low-frequency pressure fluctuations by approximately 23.1%. Research shows that in high-altitude low-Reynolds-number conditions, micro-flow regulation combined with geometric reconstruction can effectively suppress flow instability induced by rotating machinery. This achievement provides a theoretical basis and feasible engineering path for aerodynamic stability design and optimization of key components, such as the aircraft intake and exhaust systems and thermal management systems, and is of significant value for improving the overall performance and reliability of high-altitude long-endurance aircraft. Full article

Pump-turbines are critical for maintaining power grid stability, but they frequently suffer from flow instabilities induced by cavitation due to frequent operating condition changes. This study employs numerical simulations to systematically analyze the internal flow characteristics and changes in runner forces within a model pump-turbine under varying guide vane openings and cavitation coefficients. Results indicate that, under low opening conditions, a spiral vortex rope forms within the draft tube, inducing significant low-frequency pressure fluctuations. As cavitation intensifies, the vortex rope undergoes substantial expansion. At guide vane openings of 30.6 degrees and 37.3 degrees, the draft tube vortex rope exhibits a straight conical shape, with its dimensions increasing as flow rate rises. Additionally, the radial force on the runner is dominated by low-frequency fluctuations generated by the draft tube at low opening conditions, shifting to high-frequency characteristics caused by rotor–stator interaction at high opening conditions. Meanwhile, the expansion and contraction of the cavity volume induce low-frequency fluctuations in the axial force on the runner. These findings reveal the mechanism of vortex rope evolution on runner forces, emphasizing the impact of cavitation on the flow characteristics and force characteristics of the unit. Full article

In recent years, flow batteries have emerged as a crucial technological solution for large-scale energy storage, leveraging their unique power-capacity decoupling characteristics and long cycle life to demonstrate significant potential in applications such as renewable energy integration and grid frequency regulation. Based on differences in electrolyte systems, mainstream flow battery technologies are primarily categorized into three types: all-vanadium redox flow batteries (VRFBs), iron-chromium redox flow batteries (ICFBs), and zinc-based redox flow batteries (ZRFBs). However, each of these technologies faces critical challenges in practical commercialization: VRFBs are constrained by cost pressures due to fluctuations in vanadium resource prices and relatively low energy efficiency; ICFBs require urgent solutions to issues such as hydrogen evolution side reactions at the negative electrode and the sluggish kinetic responses of the Cr³⁺/Cr²⁺ redox couple; while ZRFBs grapple with safety concerns such as zinc dendrite growth and morphology instability. To overcome these technical bottlenecks, extensive innovative research has been conducted in key materials (electrodes, ion-exchange membranes, electrolytes). Against this backdrop, this paper systematically reviews recent advances in the modification and optimization of flow battery technologies and conducts an extended discussion on the emerging organic redox flow batteries in recent years. Full article

Coalbed methane (CBM) hosted by multiple (>20) thin (<2 m) seams in South China represents an important unconventional gas supplement. In the Yanjiao composite syncline, high-frequency sea-level fluctuations produced widely distributed thin seams (20–60 layers), with tidal-flat coal groups I (No. 2–No. 9) and III (No. 20–No. 35) as primary targets. Variable magma intrusion drives the present coal-rank partitioning (1.8–4.3% R_o) and pronounced reservoir heterogeneity. A basalt floor (>200 m) and the Lower Triassic Feixianguan caprock (~100 m) confine the Longtan strata into an independent hydrodynamic unit. Groundwater migrates from syncline wings to the axial domain and seals CBM in stagnant zones, resulting in higher gas contents toward the axis. Deep CBM is constrained by high in situ stress and low permeability (typically <0.1 mD below 600 m) and a relatively uniform and low-abnormal pressure system. The syncline is divided into four parts: Part I is the most favorable, where staged fracturing of closely spaced (<60 m) coal group III achieved a maximum production rate of 2400 m³/d and a stabilized rate of 2100 m³/d, whereas Part IV (depth > 1000 m) records a peak daily gas rate of 512–654 m³/d and shows no stabilized-production stage. Full article

Low-frequency oscillatory flow is a long-standing instability in cryogenic jet condensation systems and is closely associated with abnormal pressure fluctuations in propulsion pipelines. While previous studies mainly focused on operating conditions, the role of injector orifice layout in triggering low-frequency oscillations remains unclear. In this work, a three-dimensional numerical investigation was conducted to examine the effect of orifice layout on condensation-induced oscillatory flow in an oxygen jet condensation system. A curvature-coupled mass transfer model is employed, in which the interfacial mass transfer rate is dynamically linked to local vapor–liquid interfacial curvature, enabling accurate representation of interfacial evolution. A series of numerical cases are designed by varying the number, arrangement, and diameter of orifices under different combinations of mass rate, mass flux, and total injection area. Two distinct condensation patterns are identified: suck-back chugging and weak pulsation. Pronounced low-frequency oscillations are observed only for specific orifice layouts. When the total injection area and gaseous oxygen mass rate are maintained, chugging persists under different layouts, producing dominant frequencies of approximately 10~11 Hz and pressure amplitudes of about 80~120 kPa. Once either the total area or mass rate is altered, the system transitions to weak pulsation with pressure fluctuations below 3 kPa. These results demonstrate that low-frequency oscillatory flow is a layout-enabled instability rather than a mass-flux-controlled phenomenon, highlighting the importance of injector geometric design in regulating condensation-induced oscillations. Full article

Amid China’s pursuit of the “dual carbon” goals, the development and large-scale integration of renewable energy have become a core pillar of the power system transition. However, the intermittency and uncontrollability of wind and photovoltaic (PV) power have intensified peak-regulation conflicts after large-scale grid integration. Traditional coal-fired units lack sufficient flexibility to accommodate renewable energy fluctuations, while their willingness to participate in deep peak shaving remains low due to high associated costs. Addressing these challenges requires both enhanced system-level peak-regulation flexibility and effective market incentives for thermal units. Motivated by the limitations of existing studies that often consider individual flexibility resources or deterministic market mechanisms in isolation, this study investigates a coordinated multi-resource peak-regulation framework combined with an optimized market-clearing mechanism for deep peak-shaving ancillary services. First, flexibility resources are classified, and the peak-regulation mechanisms of source–load–storage coordination and auxiliary service markets are analyzed. Second, a wind–PV–thermal–storage operation cost model is established, followed by a two-layer peak-regulation market-clearing model that explicitly accounts for wind–PV uncertainty. The upper-level model minimizes total system operating costs through the coordinated dispatch of demand response and energy storage, while the lower-level model minimizes power purchase costs under a unified marginal clearing price. In addition, an uncertainty modeling framework based on Information Gap Decision Theory (IGDT) is introduced to manage renewable generation uncertainty and support decision-making under different risk preferences. Case studies are conducted to verify the effectiveness of the proposed framework. The results show that: (1) synergistic peak shaving through energy storage and demand response reduces the system peak–valley difference from 460 MW to 387.87 MW and decreases wind–PV curtailment costs from 355,000 yuan to 15,700 yuan, thereby alleviating thermal unit pressure and improving renewable energy accommodation; (2) the unified marginal clearing price mechanism reduces total system operating costs by 41.07% and significantly lowers the frequency of deep peak shaving for thermal units, enhancing their participation willingness; and (3) the IGDT-based model effectively addresses wind–PV uncertainty by providing optimistic and pessimistic scheduling strategies under different deviation coefficients. These results confirm that the proposed framework offers an effective and flexible solution for coordinated peak shaving in power systems with high renewable energy penetration. Full article

Complex hydraulic transients induced during load adjustment of turbine units in long tailrace tunnels pose significant threats to the safety and stability of tailwater systems. In view of this, based on VOF multiphase flow and compressible water–air models, a three-dimensional full-flow-channel numerical model of long tailrace system incorporating surge shaft and downstream river channel was developed using computational fluid dynamics (CFD) software to explore the transient impact of load changes on flow rate, water level, and pressure pulsations under different flow regimes in the tailrace tunnel, including open channel flow, pressurized flow, and free-surface-pressurized flow. The results indicate that the discharge at the outlet of the tailrace tunnel exhibits attenuated oscillations in response to load variations, with the most severe fluctuations occurring due to the intense air–water interface mixing during free-surface-pressurized flow. Flow regime transitions are accompanied by air pocket phenomena, resulting in significant fluctuations in air volume fraction. Pressure pulsations show periodic variations, with energy gradually dissipating as they propagate downstream. Open channel flows predominantly feature high-frequency waves, while pressurized flows exhibit intense low-frequency pulsations. Additionally, load changes in one unit have an ultra-low-frequency impact on another unit sharing the same tailrace tunnel, with high-frequency waves being filtered out by the surge shaft. Full article

To investigate the hydraulic instability mechanisms of low-specific-speed pump–turbines operating in turbine mode, this study experimentally characterized the pressure distribution and pulsation evolution on the guide vanes of a model unit (ns = 28) using an embedded sensor technique. By overcoming the accessibility limitations of traditional measurement methods, this research reveals the distinct pressure response mechanisms on the guide vane Front Side (upstream-facing) and Back Side (runner-facing). The results demonstrate that the time-averaged pressure distribution is highly sensitive to the Guide Vane Opening (GVO). Specifically, pressure on the Front Side increases with GVO, dominated by the improvement of flow pattern and stagnation effect, whereas pressure on the Back Side decreases monotonically, governed by the Bernoulli effect. Increasing the GVO significantly improves pressure uniformity, reducing the surface pressure gradient by 55%. Regarding dynamic characteristics, pressure fluctuation intensity on the Back Side is significantly higher than that on the Front Side. Furthermore, fluctuations are notably amplified near the tongue, confirming that flow distortion induced by the tongue is a key factor driving circumferential non-uniformity. Spectral analysis identifies the Blade Passing Frequency (BPF) as the dominant frequency, verifying Rotor–Stator Interaction (RSI) as the primary excitation source, while the guide vane channel exhibits a significant low-pass filtering effect on high-order harmonics. These findings provide a solid theoretical foundation and data support for the optimal design and stability control of pump–turbine guide vanes. Full article

Tubular pump systems (TPSs) represent a critical class of large-scale turbomachinery for low-head water transport, where mechanical reliability is often challenged by complex internal flow dynamics. Pressure pulsations in pump systems induce vibrations that adversely affect performance, emphasizing the need for effective control mechanisms to ensure stable operation. In tubular pumps, unsteady pressure pulsations are typically driven by rotor–stator interactions; however, the behavior of these pulsations in the absence of impeller rotation remains poorly understood. In this study, a novel comparative investigation is conducted to elucidate the effect of impeller rotation on pressure pulsations characteristic by examining two scenarios: normal impeller operation at rated speed and a completely stationary (zero-speed) impeller condition. Experiments were performed on a model low-head tubular pump, measuring dynamic pressures at four key locations across a range of flow rates. Time–frequency analysis using the continuous wavelet transform (CWT) and the wavelet coherence transform (WTC) was applied to delineate the unsteady pressure features. The results demonstrate that under normal rotation, pressure pulsations are dominated by pronounced periodic components at the impeller’s rotational frequency and its harmonics, with the strongest fluctuation amplitudes observed near the impeller outlet region. In contrast, with the impeller held stationary, these distinct periodic peaks vanish, replaced by broadband, irregular fluctuations. Crucially, WTC analysis revealed that significant coherence between the two operational states was confined to low frequencies (≈16.7–50 Hz), particularly at the impeller inlet, highlighting the presence of low-frequency dynamics likely associated with system-scale hydraulic compliance or inlet flow non-uniformity, independent of impeller rotation. These findings confirm the pivotal role of impeller rotation in generating periodic pressure pulsations while providing new insight into the underlying unsteady flow mechanisms in tubular pumps. Full article

Unsteady flow-induced pressure fluctuations and the consequent dynamic stresses in pump-turbines are critical determinants of their operational reliability and fatigue resistance. This investigation systematically examines the spatiotemporal propagation of Rotor–Stator Interaction (RSI)-induced pressure pulsations and evaluates the corresponding dynamic stress mechanisms based on a phase-resolved fluid–structure interaction strategy. The results reveal a significant hydrodynamic duality: RSI pressure waves manifest as convective traveling waves on the pressure side but as modal standing waves on the suction side. Crucially, a severe spanwise phase mismatch is identified between the hub and shroud streamlines, which induces a periodic hydrodynamic torsional moment on the blade. Due to the rigid constraint at the blade–crown junction, this torsional tendency is restricted, resulting in high-amplitude constrained tensile stresses at the root. This explains why the stress concentration at the crown inlet is significantly higher than in other regions. Additionally, the stress spectrum shows strong load dependence, characterized by low-frequency modulations on the suction side under high-load conditions. Full article

This study aims to demonstrate the presence of a pronounced coseismic increase in atmospheric methane concentrations during the 2024 Noto Peninsula Earthquake and to examine whether this increase may have originated from underground natural gas release. By analyzing hourly CH₄ data from the Ministry of the Environment’s monitoring network, this study shows that significant methane increases occurred only in areas with seismic intensity of 6– or greater, and that an exceptional anomaly—reaching 29 times the standard deviation of the past year—was recorded at the Nanao station. The validity of this anomaly was confirmed through consultation with local atmospheric officer, and high-time-resolution data (6 min values) were provided, verifying continuous instrument operation. Detailed analysis further shows that two major methane peaks occurred, each rising not immediately after the main shock but synchronously with two large aftershocks approximately 8 and 44 min later. Geological and hydrogeological information indicates the presence of water-soluble gas and unsaturated hydrocarbons beneath the Nanao region, suggesting that seismic shaking may have ruptured clay layers and released accumulated gas. Analyses of public reports and interviews with local officials show that alternative explanations—such as fire smoke, pipeline rupture, instrument malfunction, and gas-cylinder damage—were unlikely. These findings indicate that the observed methane anomaly was most likely caused by earthquake-synchronous underground gas release, suggesting that methane-release risk should be considered in post-earthquake fire-hazard assessments. Full article

With the advancement of welding technology, the demand for 70S-6 welding wire steel has steadily increased in industries such as construction, automotive, pressure vessels, and line pipe manufacturing. To optimize the production process of the target material, this study utilized the finite-element software ABAQUS to numerically simulate the multi-pass finish rolling process of 70S-6 welding wire steel. The study investigates the effects of the key rolling parameters—pass distribution (8/10/12 passes), finish rolling temperature (860/880/900 °C), and rolling speed (0.5 Vp/1.0 Vp/1.5 Vp, here Vp denotes the reference industrial rolling speed) on the rolling force, temperature field, and equivalent stress/strain during finish rolling. The results show that the increased number of passes homogenizes deformation, reduces local stress concentration and enhances mechanical properties. Specifically, 12 passes reduce the peak rolling force from 250,972 N to 208,124 N, significantly enhancing stress and temperature uniformity across the section. Increasing the finish rolling temperature lowers the pass-averaged flow stress and attenuates rolling-force fluctuations. At 880 °C, the simulated core–surface temperature gradient is minimal (50 °C), whereas at 900 °C the gradient increases (80 °C) but the rolling-force histories exhibit a lower peak level and smaller low-frequency oscillations; thus 880 °C is preferable when through-thickness thermal uniformity is targeted, while 900 °C is more suitable when a smoother load response is required. Increasing the finish rolling speed from 0.5 Vp to 1.5 Vp reduces the peak rolling force from 233,165 N to 183,665 N and significantly damps low-frequency load oscillations. However, it concurrently intensifies stress localization at the outer-surface tracking points P3/P4, which are in direct contact with the rolls, where the local equivalent stress approaches 523 MPa, even though the overall strain distribution along the bar length becomes more uniform. Overall, the optimal processing window is identified as a 12-pass schedule, a finish rolling temperature of 880–900 °C, and a rolling speed of 1.0–1.5 Vp, which can improve both rolling quality and temperature and stress and strain uniformity. Full article

Against the backdrop of the carbon peaking and neutrality (“dual-carbon”) goals and evolving new-type power system dispatch, the share of pumped-storage hydropower (PSH) in power systems continues to increase, imposing stricter requirements on units for higher cycling frequency, greater operational flexibility, and rapid, stable startup and shutdown. Focusing on the entire hot-standby-to-generation transition of a PSH plant, a full-flow-path three-dimensional transient numerical model encompassing kilometer-scale headrace/tailrace systems, meter-scale runner and casing passages, and millimeter-scale inter-component clearances is developed. Three-dimensional unsteady computational fluid dynamics are determined, while the surge tank free surface and gaseous phase are captured using a volume-of-fluid (VOF) two-phase formula. Grid independence is demonstrated, and time-resolved validation is performed against the experimental model–test operating data. Internal instability structures are diagnosed via pressure fluctuation spectral analysis and characteristic mode identification, complemented by entropy production analysis to quantify dissipative losses. The results indicate that hydraulic instabilities concentrate in the acceleration phase at small guide vane openings, where misalignment between inflow incidence and blade setting induces separation and vortical structures. Concurrently, an intensified adverse pressure gradient in the draft tube generates an axial recirculation core and a vortex rope, driving upstream propagation of low-frequency pressure pulsations. These findings deepen our mechanistic understanding of hydraulic transients during the hot-standby-to-generation transition of PSH units and provide a theoretical basis for improving transitional stability and optimizing control strategies. Full article