Non-uniform Flow Research Articles

Forced Vibration Induced by Dynamic Response Under Different Inlet Distortion Intensities

Boundary layer ingestion propulsion systems have attracted much attention due to their significant potential to reduce the fuel consumption of future commercial aircraft. However, the aeroelastic stability of the fan blade is affected by the continuous non-uniform incoming flow induced by the ingestion of the boundary layer. When the fan blades rotate in the junction area between the distorted area and the clean area, blade pressure fluctuations occur. This phenomenon triggers a dynamic response process in the blade. Previous numerical investigations explored the influence of the distorted inflow on the blade vibration amplitude, and found that there are two sources of low-order excitation to the blades: the distorted inflow and the dynamic response of the blade. The results show that the low-order excitation existing in the distorted inflow varies sinusoidally with the distortion extent. However, as a new source of excitation, the key influence mechanism of dynamic response is still unclear. To explore this issue, calculations and analyses were conducted for different distorted inflow intensities. The results show that the blade vibration amplitude increases with the rise in distortion intensity. The total pressure at the leading and trailing edge of the rotor blade was extracted for analysis. It was found that when the blade enters or leaves the distorted area, there is a consistent lag in the change in total pressure at the trailing edge compared to the leading edge. This lag leads to an abrupt variation in the total pressure ratio, which constitutes the dynamic response process of the rotor blade. This periodic change generates a second-order excitation that causes the blade to vibrate.

Definition of Critical Metrics for Performance Evaluation and Multiphase Flow Modeling in an Alkaline Electrolyzer Using CFD

Gas evolution and flow patterns inside an alkaline electrolyzer cell strongly affect efficiency, although such effects have not been explored in detail to date. The present study aims to critically analyze the dependence of cell performance on the multiphase flow phenomena, defining some key metrics for its assessment using CFD. Six performance indicators, involving gas accumulation, bubble coverage, and flow uniformity, are applied to a 3D CFD model of an alkaline cathodic cell, and possible optimizations of the cell geometry are evaluated. The results demonstrate the complexity of defining the optimal indicator, which strictly depends on the case study and on the analysis at hand. For the cell analyzed herein, the parameters linked to the electrode volume fraction were indicated as the most influential on the cell efficiency, allowing us to define the best geometry case during the optimization. Furthermore, a sensitivity analysis was conducted, which showed that higher mass flow rates are generally preferable as they are linked to higher bubble removal. Higher current densities, allowing enhanced gas production, are instead associated with slightly lower efficiencies and stronger nonuniformity of the electrolyte flow inside the cell.

Mechanism of instability in non-uniform dusty channel flow

Particles in pressure-driven channel flow are often inhomogeneously distributed. Two modes of low-Reynolds-number instability, absent in Poiseuille flow of clean fluid, are created by inhomogeneous particle loading, and their mechanism is worked out here. Two distinct classes of behaviour are seen: when the critical layer of the dominant perturbation overlaps with variations in particle concentration, the new instabilities arise, which we term overlap modes. But when the layers are distinct, only the traditional Tollmien–Schlichting mode of instability occurs. We derive the dominant critical-layer balance equations in this flow along the lines done classically for clean fluid. These reveal how concentration variations within the critical layer cause the two particle-driven instabilities. As a result of these variations, disturbance kinetic energy production is qualitatively and majorly altered. Surprisingly, the two overlap modes, although completely different in the symmetry of the eigenstructure and regime of exponential growth, show practically identical energy budgets, highlighting the relevance of variations within the critical layer. The wall layer is shown to be unimportant. We derive a minimal composite theory comprising all terms in the complete equation which are dominant somewhere in the flow, and show that it contains the essential physics. When particles are infinitely dense relative to the fluid, the volume fraction is negligible. But for finite density ratios, the volume fraction of particles causes a profile of effective viscosity. This is shown to be uniformly stabilizing in the present flow. Gravity is neglected here, and will be important to study in the future. So will the transient growth of perturbations due to non-normality of the stability operator, in a quest for the mechanism of transition to turbulence.

Theoretical analysis on detonation initiation induced by thermal nonuniformity in a supersonic flow

Theoretical analysis on detonation initiation induced by thermal nonuniformity in a supersonic flow

Effects of Non-Uniform Center-Flow Distribution and Cavitation on Continuous-Type Pintle Injectors

In this paper, the flow characteristics of a continuous-type liquid–liquid pintle injector are described, focusing on the differential impact of a non-uniform center-flow distribution on single- and bi-injection methodologies as well as the cavitation effect on the spray angle. Using cold-flow experiments, jet-type flows of the center propellant caused by a non-uniform flow distribution were observed during a single injection. This resulted in an augmented pressure drop, as opposed to the flow characteristics of uniform single-film injection. By contrast, bi-injection modalities exhibited a substantial reduction in the pressure drop of the center propellant, underscoring a more equitable flow distribution. Moreover, the occurrence of cavitation in the center propellant was found to markedly affect the spray angle. By considering the injection exit area reduction caused by cavitation, the spray-angle prediction accuracy increased. The findings of this study are expected to reveal the interplay between flow distribution and pressure drop as well as that between cavitation and the spray angle in pintle injectors. Through this understanding, this study provides crucial considerations for the development of more efficient propulsion systems.

Numerical simulation and experimental study on guidance performance of hypersonic seeker under aerodynamic optical transmission effects

When a hypersonic seeker flies at high speed within the atmosphere, intense interaction with the incoming flow gradually develops into a complex turbulent flow field. This interaction results in complex thermal responses at the seeker window, causing aerodynamic optical effects such as image shift, jitter, and blur of the target image, thereby restricting the seeker's detection capability and accuracy. This paper uses a numerical simulation model for the guidance performance of a hypersonic seeker under aerodynamic optical transmission effects. The study focuses on an ellipsoidal seeker, with its supersonic flight simulation on the basis of the Reynolds-Averaged Navier-Stokes (RANS) equations to get a non-uniform gradient flow field. The correctness of the flow filed results can be verified by wind tunnel experiments. The transient temperature field of the seeker is solved using an unsteady thermal conduction-radiation coupled fluid-solid heat transfer method. Finally, the guidance performance of the hypersonic seeker under aerodynamic optical effects is predicted using the ray tracing method, which employs wavefront aberration, point spread function, degraded images, and image shift.

Investigations of inlet arrangement strategies and particle parameters on thermochemical performance in falling particle solar reactors

The reformation of methane to hydrogen and steam in a falling particle solar reactor (FPSR) is a promising method to use the solar energy, which converts solar energy into chemical energy. FPSR has higher operation temperature, higher pressure bearing capacity and more direct heat energy storage than those of traditional reactor. However, due to the short residence time of the particles, hydrogen production efficiency is reduced. Thus, inlet arrangement strategies and particle catalysts should be carefully designed to improve the thermochemical reaction performance of FPSR. In this contribution, the Eulerian-Eulerian model in CFD program (Fluent 2020 R2) is used to explore the effects of different particle inlet arrangement strategies, non-uniform particle mass flow inlet and particle size on the performance enhancement of methane steam reforming reaction. The results indicate that the two arrangement strategies show different trends in influencing hydrogen production efficiency. Besides, non-uniform particle mass flow rate also has a significant impact on hydrogen production efficiency, with the optimal arrangement conditions increasing the hydrogen production rate by 2.4831 mol/m−3(−|-) s−1. Additionally, smaller particle size demonstrates better hydrogen production efficiency, with a 0.15 mm particle size catalyst showing a 14.02 % improvement in hydrogen production efficiency compared to a 0.75 mm particle size. This contribution can provide a guidance for optimizing the heat and mass transfer performance of the falling particle solar reactor.

Deflagration-to-detonation transition and detonation propagation in supersonic flows with hydrogen injection and downstream ignition

This study investigates the mechanisms of flame acceleration and deflagration-to-detonation transition (DDT) in supersonic flows using transverse hydrogen injection and downstream ignition. Utilizing the graphics processing unit accelerated adaptive mesh refinement approach, we examine the influence of downstream ignition jet pressure on DDT through high-resolution computational simulations. Our results indicate that the transverse injection of hydrogen into the supersonic mainstream generates strong turbulence and numerous vortices due to Kelvin–Helmholtz instability, enhancing fuel mixing efficiency along the flow but deviating from the ideal premixed state. Following the injection of the downstream ignition jet into the supersonic main flow, initial flame acceleration is less effective than in the premixed state due to the non-uniformity of the incoming flow. However, within the boundary layer, the flame remains stable, and the intense turbulence fosters shock–flame interactions. The convergence of multiple compression waves into a shock wave facilitates energy deposition, coupling with the flame to trigger local detonation via the reactive gradient mechanism. The detonation wave exhibits complex wavefront structures, including vertical and oblique fronts induced by boundary layer interactions. Ignition jet pressure significantly impacts the DDT process and detonation wave characteristics, reducing ignition time and affecting the detonation temperature, pressure, and propagation speed. This study provides valuable insights into the dynamics of flame acceleration and DDT in supersonic flows with non-uniform fuel distribution and downstream jet ignition. The findings highlight the critical role of ignition jet pressure in optimizing ignition and detonation processes, offering new perspectives for achieving low-energy, rapid detonation initiation within the tube.

Numerical investigation of the effect of the non-uniform intake flow on the unsteady flow and the excitation of the pump in a water-jet propulsion

Abstract Water-jet propulsions are widely used in high-performance ships. This study conducts numerical study on the unsteady flow of a complete water-jet propulsion. The internal flow field of the propulsion pump under the conditions of no installation of the shaft sleeve (and installation of the shaft sleeve. Spectrum analysis is used to compare the time-varying external characteristic and excitation characteristic under different conditions. Then, Dynamic Mode Decomposition technique is used to extract the high energy flow structures and corresponding frequencies. The results indicated that a large-scale vortex roll up above the driving shaft under the condition of rotational shaft, a pair of anti-rotation vortices appear behind the driving shaft under the condition of stationary shaft. These shaft induced vortices develop into unstable flow structures in the impeller region which moves relative to the impeller blades at rotational frequency. They further affect the unsteady flow in the guide vane region. The shedding vortices induced by the rotor-stator interaction are strengthened, while the passage vortices are weakened. Moreover, the dominant frequency of the passage vortex has decreased from 1.68 times the rotational frequency to 1.50 times the rotational frequency.

Particle migration due to non-uniform laminar flow

Abstract Using methods of granular mechanics in the quasi-static limit, with inter-particle interactions derived from the lubrication limit, the intensity of velocity fluctuations in the slurry is associated with fluctuations in the local distribution of inter-particle distances. These are shown to consist of a vector intensity and a scalar intensity; the former couples to the first velocity gradient, the latter (which is associated with solidosity fluctuations) couples to the second velocity gradient. Rheologies for both are presented, as is the rheology that links the particle pressure to the intensity of the velocity fluctuations (also known as the ‘granular temperature’) to the dispersive pressure. The rheologies are informed by experimental results. The granular temperature profile, modified from previous work, is responsible for axial particle migration (Bagnold effect). Two broad categories are assessed: symmetrical vertical and non-symmetrical lateral flow. For the latter the roughness of the boundary walls and a non-zero density contrast are important; this case is studied for a system in which flow effects are confined to the immediate vicinity of the boundary. Sensitivity analysis reveals several key variables including the parameters that control a slipping boundary condition and the mean solidosity in the conduit. For lateral flow, a sedimentary deposit with a solidosity profile may develop near the upper or lower boundary. The theory predicts an approximate relation between the fluid-particle density contrast and sediment thickness as a function of the mean flow rate, conduit width, the mean particle diameter and fluid viscosity that has utility in a range of engineering and geological situations where particulate matter is transported in the laminar flow regime.

Numerical and experimental study of the cavitation performance of a composite propeller

Cavitation, which is harmful and unavoidable, has troubled people for a long time and it is urgent to be solved. A composite propeller has the potential ability to improve the cavitation performance because of its distinctive hydro-elastic characteristic, but the related research is limited. In this paper, the cavitation performance of a composite propeller under uniform and non-uniform wake flow is investigated numerically and experimentally. A cavitation coupling simulation method is presented for predicting the cavitation performance of a composite propeller. Cavitation tests are conducted at a cavitation tunnel. The calculated and tested results of cavitation hydrodynamic performance and cavitation patterns are discussed and analyzed. The results show that they are quite consistent, more importantly, cavitation has a significant impact on the hydro-elastic response of a composite propeller.

Investigation on the hydraulic loss in the vertical inlet and outlet of a pumped storage power station

Abstract The vertical inlet/outlet is commonly adopted in the upper reservoir of pumped-storage power stations, in which the internal flow patterns exert a critical influence on the safety and operational efficiency of the entire power station. Based on the standard k-ε turbulence model coupled with the revised wall roughness theory, this investigation presented a numerical model of a vertical inlet/outlet in a pumped-storage upper reservoir and the validity and accuracy of the numerical simulation are validated through comparison well with the results of the model experiments. The results suggest that the emergence of vortices of varying intensities near the inlet during power-generation scenarios at dead water levels and at 3.5 times of the rated flow rate (389.6 m3/s). The bend section experiences fairly nonuniform flow velocity, with the highest average flow hydraulic loss in the entire vertical pipe inlet/outlet system. These insights provide a substantial utility for operational strategies and optimization designs in related engineering projects.

A study of the effects of housing tongue design on radial turbine performance

The nozzleless volute housings can generate non-uniform flow distributions to the downstream radial turbine rotors, which not only affects turbine aerodynamic performance but also poses a threat to the fatigue life of the rotors. It is believed that the housing tongue is the main factor in generating this non-uniformity. Tongue design is based on experience and the geometric parameters of tongue, such as the distance between the tongue and rotor, the thickness of the tongue, and tongue shape are to be specified in the design. A few studies on housing tongue can be found in the literature, but in these investigations of the influences of tongue geometric parameters, factors such as mass flow rate and volute A/R distribution were not kept constant, thus introducing additional factors and reducing the persuasiveness of the results. Therefore, three new housings with different tongue-rotor distances are designed with a two-dimensional method for comparative CFD studies. The results indicate that when the mass flow of the turbines is kept, the distance between the tongue and the rotor has little impact on the turbine efficiency (about 0.1%), while the blade excitation forces from the housing increase with the decrease of the distance by more than 50%. Two further new housings are then created by modifying the housing inlet pipe to minimize tongue thickness, one of the housings is more practical with a short and thin tongue. CFD results indicate 0.2%∼0.4% improvement of turbine efficiency and reduction of housing excitation forces by more than 50% using these two thin tongue housings.

Non-uniform flow and heat transfer characteristics and parametric study of generator hydrogen double-flow ring seal

Non-uniform flow and heat transfer characteristics and parametric study of generator hydrogen double-flow ring seal

Efficient navigation of cargo-towing microswimmer in non-uniform flow fields

The vision of deploying miniature vehicles within the human body for intricate tasks holds tremendous promise across engineering and medical domains. Herein, optimal navigation of a cargo-towing swimmer under an applied zig-zag flow is studied by employing direct numerical simulations coupled with a deep reinforcement learning algorithm. Tasks include navigation in flow and shear-gradient directions. We initially explore combinations of state inputs, finding that optimal navigation necessitates swimmers to perceive hydrodynamics and alignment, surpassing reliance solely on hydrodynamic signals while considering their memories. Next, we study combinations of action spaces, allowing dynamic changes in swimming and/or rotational velocities by tuning B1 and C1 parameters of the squirmer model, respectively. By keeping both parameters fixed, cargo-towing swimmers demonstrate superior performance in the flow direction compared to swimmers without load due to tumbling movements influenced by shear flow. In the shear-gradient direction, swimmers without load outperform cargo-towing swimmers, with performance decreasing as load length increases. Across the combination of allowing B1 and C1 to change, the policies from solely dynamic B1 actions demonstrate superior navigation. The policies are then used as a showcase against naive cargo-towing and inert colloidal chains. A t-distributed stochastic neighbor embedding analysis reveals the complex interplay between perceived hydrodynamic signals and swimmer position. In the flow direction, swimmers align effectively with regions of maximum velocity, while in the shear-gradient direction, periodic transitions from minimum to maximum state values occur. Comparing pullers, pushers, and neutral swimmers, cargo-towing swimmers show a reversal in swimming velocity trends, with pullers outpacing neutral and pusher swimmers, irrespective of load lengths. Published by the American Physical Society 2024

Design and aerodynamic stability improvement study of a non-axisymmetric stator fan in a distorted insertion plate configuration

The design of anti-distortion fans must ensure the reliable operation and flight safety of propulsion systems under inlet distortion conditions. This research uses an insertion plate to obtain the inlet distortion flow field for a specific type of turbofan engine. Based on the characteristics of distortion effects, a novel strategy is proposed to rapidly implement a non-axisymmetric stator (NAS) quasi-three-dimensional design to create anti-distortion fan designs and improve the performance and stability of the fan in distortion environments. Numerical studies show that under three rotational speed operating conditions, the NAS effectively improves the total pressure ratio and efficiency characteristics of the fan under inlet distortion conditions. Compared to the prototype, the NAS can significantly increase the stability margins by 10.31 %, 8.31 %, and 7.39 % under the three different speed operating conditions, which effectively expands the stability boundaries. The NAS design directly affects the distortion region, effectively improves the incidence angle of stator vanes in the distortion region, suppresses the development and migration of vortices inside the stator passage, and significantly reduces the stator vane diffusion factor. Because it effectively eliminates corner separations and makes the incidence angle distribution of stator vanes relatively circumferentially uniform, the internal flow field of the fan's stator vanes and overall aerodynamic performance of the fan improve. Furthermore, the NAS design can improve the internal flow field downstream of the fan, direct the nonuniform flow field at the inner bypass outlet toward circumferential uniformity, and effectively improve flow field uniformity.

Forward and inverse river contaminant transport modeling using group preserving scheme

Environmental concerns have necessitated the development of computational models predicting pollutant dispersion in natural water systems. Due to the ill-posed nature of the inverse contaminant transport equation, solving this equation using all stable and convergent inverse methods is impossible. Factors such as river geometry, unsteady and non-uniform flow, and tidal influences add to the complexity of the inverse problem. These factors have prompted the evaluation of Group Preserving Scheme for environmental applications. The inverse solution method derives a general equation for solving ordinary differential equations by addressing a dynamical system at negative time steps, ensuring convergence. Three test cases have been presented to evaluate Backward Group Preserving Scheme (BGPS). These have included validation using observational data from Missouri River, inverse simulation in a tidal river, and sensitivity analysis of parameters such as pollutant patterns, advection, dispersion, and decay coefficients. The dataset includes calibrated data from Missouri River, which demonstrated high accuracy, with Mean Relative Error (MRE) ranging from 2.8% to 5.0% for the inverse model. Under tidal conditions, accuracy decreases over time but remains robust, with a Nash–Sutcliffe efficiency of 0.77–0.96 and an MRE of 0.9%–5.9%. Sensitivity analysis revealed optimal model performance for Péclet numbers greater than 500. The model performed best with gradual, wide-peaked pollutant patterns and moderate decay rates (Damköhler number between 5 and 10). BGPS proves effective for transport simulation and concentration history reconstruction in complex rivers, including those with tidal influences, offering a robust tool for water contamination analysis in various flow conditions.

Flow Nonuniformity and Energy Dissipation in Moderate-Sloped Stepped Chutes with a Labyrinth Crest

Flow Nonuniformity and Energy Dissipation in Moderate-Sloped Stepped Chutes with a Labyrinth Crest

Fast flow field prediction based on E(2)-equivariant steerable convolutional neural networks

In the field of flow field reconstruction, traditional deep learning models predominantly rely on standard convolutions, but their predictive accuracy remains limited. To address this issue, we explore the potential of E(2)-equivariant convolutions to enhance the predictive accuracy of deep learning models for fast flow field prediction. Unlike conventional convolutions, E(2)-equivariant convolutions offer a richer representation capability by better capturing geometric and structural information. Our neural network integrates an attention mechanism that leverages the signed distance function (SDF) to encode geometric details and an indicator matrix to incorporate boundary conditions. The model predicts velocity and pressure fields as outputs. We conducted experiments specifically targeting non-uniform steady laminar flows, and the results show a 16.1% reduction in overall error compared to models based on traditional convolutions while maintaining high efficiency. These findings indicate that E(2)-equivariant convolution, coupled with an attention mechanism, significantly improves flow field prediction by focusing on critical information and better representing complex geometries.

An adjustable vane-shaped nozzle-based modulated pre-swirl system for gas turbine aero-engine: Structure parameters and aerodynamic performance analysis

The pre-swirl system of an aero engine provides cooling air with suitable temperature and pressure for turbine blades, rarely contributing to thrust. However, the traditional pre-swirl system with a fixed geometry leads to significant wastage of cooling air during part-load conditions, such as subsonic cruising, resulting in a decrease in engine performance. Unlike the existing approach, which uses valves to close a part of the pipeline, a novel modulated pre-swirl system based on the adjustment of the vane-shaped nozzle is proposed to overcome the flow nonuniformity in the circumferential air supply. The effects of the number of pre-swirl nozzles and receiver hole parameters (number, length, and structure) on the system performance are discussed in design and subsonic cruising conditions. The results demonstrate that increasing the number of pre-swirl nozzles significantly improved the flow adjustment efficiency and performance of the modulated pre-swirl system. A larger number and shorter length of receiver holes resulted in a higher system temperature drop in both design and subsonic cruising conditions. With regard to the configurations of the receiver holes, the vane-shaped receiver hole exhibited optimal aerodynamic performance. Compared to the baseline case, the system with Np = 60, NRec = 60, lRec = 3 mm, and vane-shaped receiver hole showed a 27.6 % and 26.7 % improvement in the temperature drop efficiency in the design and subsonic cruising conditions, respectively.