Non-uniform Flow Research Articles

A new mathematical solution of convection‐dispersion equation to describe solute transport in heterogeneous soils

AbstractThere was a long‐time debate about the validation of the convection‐dispersion equation (CDE) and its replacement with the well‐known convective lognormal transfer function model (CLT) to describe solute transport in a heterogeneous soil with uniformity at the longitudinal water flow direction and nonuniformity at the transverse direction. The objective of this study is to prove that the CDE is valid and almost identical to the CLT. Gamma probability density function (pdf) was initially assumed in this study to describe the distribution of pore‐water velocity across capillary tubes in a heterogeneous soil. The capillary bundle model was used to describe solute transport without transverse solute mixing between adjacent tubes. The inverse‐gamma function, a new mathematical solution of the CDE differential equation with scale‐dependent dispersivity, was initially derived from the capillary bundle model and the gamma pdf. The only difference between the inverse‐gamma function and the CLT is that lognormal pdf of pore‐water velocity is assumed in the CLT while the two pdfs are close to each other. The inverse‐gamma function and the CLT were tested with the published data from the miscible displacement experiments on the two repacked soils with different aggregate sizes. Results show that both the inverse‐gamma function and the CLT fit the measured breakthrough curves in the miscible displacement experiments. The estimates of the squared coefficient of variation of the pore‐water velocity in the gamma pdf were 0.314 and 0.0582 for the two soils, and they were consistent with the lognormal pdf in the CLT.

In Vitro Analysis of Left Ventricular Assist Device Outflow Graft Orientations and Their Effect on Aortic Hemodynamics.

Continuous-flow left ventricular assist devices have become an important treatment option for patients with advanced heart failure. However, adverse hemodynamic effects as consequence of an altered blood flow within the aorta and the aortic root remain a topic of concern. In this work, we investigated the influence of the outflow graft orientation on the hemodynamic profile and flow parameters within the thoracic aorta. Aortic models with different outflow graft orientations were designed and three-dimensional (3D) printed to mimic common implantation configurations and were integrated into a pulsatile mock circulatory flow loop. Assist device function was achieved using a rotary pump, replicating nonpulsatile, continuous support flows of 1-5 L/min. Flow velocity, wall shear stress, and pressure gradients were investigated for each configuration using sonography and four-dimensional (4D) flow magnetic resonance imaging. Mean wall shear stresses measured in 4D flow software were lowest for a graft inclination angle of 45°. Streamline visualization revealed areas of nonuniform, retrograde, and vortex flow in all models but most prominent for the aortic model with an outflow graft inclination of 60°. The insights gained from this research may aid in understanding clinical outcomes following assist device implantation and long-term mechanical circulatory support.

An improved body force model based on a data-driven region-segmentation combinational loss model for a transonic fan rotor under uniform and non-uniform inflow

Boundary layer ingestion (BLI) fans are required to continuously operate under distorted inflow conditions, which severely reduces the fan's efficiency. Therefore, it is necessary to focus efforts on designing a high-performance distortion-tolerant fan. During the preliminary design stage of the fan, it is inevitable to repeatedly evaluate the aerodynamic performances of different design cases through low-fidelity computational approaches. However, the predictive accuracy of these low-fidelity computational methods generally depends on the precision of the loss models integrated into them. These loss models are built using simplified physics or human experience, making it difficult to depict the complex non-linear mapping relationships between losses and their influencing factors under BLI inflow distortion. In this work, a high-accuracy data-driven based region-segmentation combinational loss model is used for the first time to attempt to improve the overall prediction accuracy of the body force model. As a low-fidelity computational approach, the body force model has the ability to predict the spatial distribution of three-dimensional, non-uniform flow under inflow distortion conditions. Various operating conditions, including different rotational speeds, radial inflow, and BLI-distorted inflow, are selected to test the predictive accuracy of this improved body force model based on a region-segmentation combinational loss model. The results show that, compared with the one based on traditional loss prediction approach, the improved body force model in this work has shown higher prediction accuracy for the fan adiabatic efficiency characteristic curve. In addition, further evaluations of the flow loss spatial distributions under BLI inflow distortion conditions indicate that the body force model based on region-segmentation combinational loss model can more accurately capture the radial loss distributions at different circumferential locations. Specifically, for the loss distribution in the tip area, it can capture the sharp increase in variation trend near the end wall, and the average prediction errors can be reduced by more than 10% at different annulus locations under various BLI inflow distortions. Meanwhile, this improved body force model also enhances the accuracy of predicting the loss distribution in the circumferential direction at different span positions, including more accurately capturing the circumferential positions corresponding to the maximum and minimum flow losses.

Influence of flow nonuniformities and real gas effects on cylindrical shock wave convergence

Convergence of cylindrical shock in argon is studied both experimentally and numerically. Shock tube experiments are conducted, where a planar shock is first transformed to a cylindrical shape and then converged to its focal axis. Numerical simulations of the converging shock using equations of state for an ideal gas and a real gas (SESAME 5173 model) are conducted and compared. High temporal resolution data of cylindrical shock convergence is presented. When comparing the trajectories of the converging shock of initial shock Mach number (MS) of 4.63, the convergence exponent (α) in experiments is found to be 0.833. This α value in experiments is higher than the value obtained from computations with argon treated as an ideal gas but agrees well with the real gas computations. It is revealed that the form of convergence varies with different MS. An asymptotic approach of α toward the self-similar solution for high MS is attributed to an earlier transition of shock motion to self-similarity, while a significantly higher α observed at lower MS is attributed to the negative influence of upstream nonuniformities and weaker initiation of the shock. It is found that even before the shock reflection, real gas effects are significant enough to affect the convergence of the shock and limit the extreme conditions predicted by the ideal gas computations. For an MS of 4.63, the maximum temperature reached is 9250 K before reflection, leading to 0.12% of the argon gas undergoing the first stage of ionization.

Power and thrust control by passive pitch for tidal turbines

Tidal turbines operate in unsteady and non-uniform flows and thus experience large load fluctuations. In this study, we explore the effects of changes in freestream velocity and yaw misalignments on the performance of a turbine equipped with passively pitching blades. We compare the experimental results from a 1.2-m-diameter turbine in a recirculating open channel facility and numerical results based on blade-element-momentum theory. The thrust remains approximately constant with increasing free stream speed, meaning that the structural load on the turbine is minimised. While not held constant, the power increases at less than half of the rate of a turbine with fixed pitch blades. This finding shows that power can be controlled effectively by changing the tip speed ratio, allowing, for example, the power to be capped at a rated value by marginally increasing the angular velocity. Furthermore, the power and thrust are found to be negligibly sensitive to yawed incoming flow compared to a fixed-pitch turbine. Overall, these results demonstrate that passive pitch is a viable alternative to active pitch, and pave the way for further development of this technology.

Numerical investigation on dimpled target average heat transfer in free-surface water jet impingement and its enhancement effect versus flat plate target

Jet impingement serves as an effective method to enhance heat transfer in diverse industrial applications, utilizing either a separate fluid from the ambient (free-surface jet) or identical fluids (submerged jet). The target surface undergoes distinct thermal processes, featuring zones like the stagnation zone, boundary layer formation, and the transition from laminar to turbulent wall flow. However, heat transfer efficiency diminishes significantly due to pressure gradient changes beyond the stagnation zone, impeding flow and boundary layer development. Addressing this decline through surface modification techniques, such as incorporating spherical dimples, emerges as an innovative approach. This study develops a numerical model via Computational Fluid Dynamics (CFD) simulations utilizing Volume of Fluid (VOF) modeling for water jet impingement on a flat plate target. The model’s validity is established by comparing hydrodynamic and thermal aspects in both laminar and turbulent scenarios against established experimental data in literature. Nine dimpled target surfaces, featuring diverse dimensional properties and positioning configurations, are examined using this model to explore potential enhancements in heat transfer intensity. The findings indicate a notable increase in the overall surface average Nusselt number, potentially reaching up to 50% by implementing dimples in the current context. Moreover, the study highlights that the radial distribution of dimples and the non-uniform flow path ahead of the incoming fluid, which differs from parallel flat flow, result in varying effects based on dimensional properties. For instance, it has been found that modifying dimensions such as deepening dimples or altering their proximity could yield contrary effects, emphasizing the need for an optimization methodology to determine the most efficient dimpled surface setup within a given geometry.

Experimental investigation on combined flow and temperature non-uniformity for three-fluid compact heat exchanger with cross flow configuration

ABSTRACT Non-uniformities in flow and temperature are the most conspicuous reason behind diminishing efficacy in heat exchangers. Cross-flow three-fluid heat exchanger being a pivotal component in cryogenics, is experimentally investigated under imposed flow and temperature non-uniformities in central hot fluid. The interaction of combined flow and temperature non-uniformity invariably precipitates a degradation in the efficacy of both adjacent fluids. About 7.3% and 6.2% of efficacy degradation is encountered in fluid “a” (for C 2 configuration) and fluid “c” (for C 1 configuration), respectively, for F N 1 + T N III model. In many cases, C 1 configuration and T N III model is inimical to fluid “c” performance.

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.

Modular Flat Structure with Miura Origami for Space Solar Power Station

To address the challenges associated with existing space solar power station (SSPS) concepts, including noncompact structural design, nonuniform solar energy flow density, and orbital deployment complexities, an integrated, highly modular, flat functional structure based on the Miura origami pattern is proposed. The flat functional structure consists of a flat quadrilateral Fresnel concentrator for solar energy collection, a photovoltaic array for photoelectric conversion, and a transmitting array for microwave power transmission. To optimize space utilization and enhance the deployment ratio, the Miura origami pattern is introduced, and the subarray composed of multiple flat functional structures can be packaged in a payload capsule before launch and unfold as a standard rectangle with uniform thickness and a flat surface in orbit, improving the transportability of the launch vehicle. Additionally, to solve the nonuniform solar energy flow density, a Fresnel concentrator was employed to achieve uniform irradiation, while the transmitting array was designed to achieve good electrical performance. Finally, the structural parameters and efficiency of the SSPS system composed of multiple subarrays are presented in detail.

Response surface optimisation on Non-Uniform shapes ternary hybrid nanofluid flow in stenosis artery with motile gyrotactic microorganisms

This work used ternary hybrid nanofluids containing motile gyrotactic microorganisms and irregularly shaped platelet, cylindrical, and spherical nanoparticles to evaluate heat transport in a stenosis artery with volume fractions of Cobalt φ1=0.01, Silver φ2=0.01, and Gold φ3=0.01. The proper self-similarity variables are used to convert the fluid transport equations into ordinary differential equations., which the BVP4C then solves in MATLAB. We analyse the effects of various parameters, including curvature, magnetic intensity, thermal radiation, and non-Newtonian behaviour, regarding Nusselt numbers, temperature profiles, skin friction, and velocity distribution. The study reveals that higher curvature enhances convective heat transfer despite initial resistance due to flow constriction, while magnetic fields stabilise flow patterns and improve heat transfer via nanoparticle alignment. Thermal radiation amplifies heat transfer by reducing boundary layer thickness and enhancing energy absorption. The non-linear relationship between magnetic intensity, thermal radiation, and the Eckert number that our results reveal emphasizes the need for more vital magnetic fields to sustain stability and effective heat transfer as thermal radiation rises. This work offers valuable information for improving nanofluid, automotive, and biomedical engineering heat transfer mechanisms. It can improve heat therapy, targeted medication administration, and diagnostic imaging in biomedicine. It provides advancements in gasoline additives, lubricants, and engine cooling systems for the automotive industry. It can improve solar energy systems, microfluidics, and heat transfer systems in nanofluid engineering.

Numerical study on inter-particle effects for multiple reacting biomass and coal particles based on Micro-CT morphology

Co-firing of biomass with coal combines the environmental benefits of renewable biomass with the high energy content of coal. Although the common numerical simulation treats the biomass and coal particles with ideal morphology, real particles often demonstrate nonsmoothed surface and irregular shape. To understand the impact of particle morphology in a group of biomass and coal particles co-firing together and to inform simple models appropriate, this study investigated the interparticle effects among particles using realistic particle morphology, focusing on fluid dynamics such as temperature distribution, flow patterns and drag coefficients. Particle-scale computational fluid dynamics (CFD) simulations using micro-CT imaging showed that realistic particle shapes resulted in nonuniform flow fields and temperature distributions with different reaction intensities due to the species transportation. It is in contrast to traditional ideal shape models, which often rely on simplified spherical representations of particles and cannot capture the intricacies of real particle shapes. Realistic models revealed more complex surfaces, highly irregular particle structures, and varied reaction zones that affected the overall dynamics. In addition, changing the orientation of one particle affects the combustion characteristics of neighboring particles. This effect is also not captured while using the spherical structure. These differences underscore the critical impact of particle morphology on drag and heat transfer, thereby challenging conventional spherical models. The findings of this study advocate for a paradigm shift in CFD modeling approaches, emphasizing the importance of realistic particle representation to improve the accuracy of predictions and enhance the co-firing system efficiency.

Dynamics of bubble rising in extremely high and low viscous fluids

Bubble dynamics is a captivating field of study due to its complexity and wide-ranging applications. The present research focuses on bubble behavior in fluids with very high and low viscosities. Our study also focuses on understanding the effect of gas flow rate and needle diameter on bubble size. This research is essential because of scientific disagreement in this area, and our revisit adds new findings to the problem. We perform experiments in liquids with extreme viscosity differences using needles ranging from 0.84 mm to 1.54 mm and gas flow rates from 5 ml/min to 80 ml/min. In low-viscosity fluid, bubble necking time is constant with the flow rate, and secondary necking leads to daughter bubble formation. In high-viscosity fluid, bubble formation and rising differ, with smaller bubbles sliding over larger ones and merging symmetrically along their axis. The symmetry of contact decides sliding or coalescence. We also noticed that bubbles formed pairs (doublets) when the gas flow rate decreased with increasing needle size. The consecutive bubble diameter in both fluids is explored. The contrasting characteristics in low-viscosity environments (such as deionized water) and high-viscosity environments (like glycerol) highlight their respective monotonic and non-uniform flow behaviors.

Enhancing flow uniformity and reducing turbulent kinetic energy in refrigerator freezers through magnetic resonance velocimetry-based structural modifications

High energy efficiency and low operational noise are increasingly demanded in premium household appliances. Magnetic resonance velocimetry (MRV) has recently emerged as a versatile flow visualization technology, particularly suited for the efficient design of such appliances. This study conducted a comprehensive analysis of a 3/5 scale freezer model, incorporating the cooling system, compartment, and cabinets, all fabricated using stereolithography three-dimensional (3D) printing. By focusing on flow characteristics, 3D mean velocity and turbulent kinetic energy (TKE) fields were measured, identifying regions of non-uniform flow and elevated TKE. To address these issues, structural modifications were introduced in an improved model. These modifications included refining the central structure of the fan chamber, altering inlet geometries, and adding a fillet at the inlet edge. The results were significant: a more uniform flow distribution was achieved, with a 15 percentage-point increase in the effective flow rate through the evaporator's finned area, a reduction in secondary flow energy in the fan chamber, and a substantial decrease in TKE. Consequently, the improved model demonstrated enhanced energy efficiency and quieter operation. These findings highlight the potential of MRV as an effective tool for analyzing complex flow systems in appliance design.

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.

Bifurcation control and analysis of traffic flow model based on driver prediction effect.

Based on the measured data, in this paper, we find that there are significant differences in the ideal speed of drivers with different attributes during their driving process. Through wavelet analysis, it is proved that the regularity of the car-following behavior of the driver is poor in a short time, and it cannot be predicted and analyzed at a small time scale. As one of the important participants in the traffic system, it is necessary to consider the driving behavior factors in the traffic flow model. Therefore, in this paper, we propose a nonuniform continuous traffic flow model. Based on the difference in the expected headway of the driver, the model considers the self-stabilization effect of the prediction time of the driver on the optimal speed difference and considers control theory to further improve the model. Through this model, the bifurcation theory can be applied to the stability analysis of the traffic system to study the stability mutation behavior of the traffic system at the bifurcation point. Through linear and nonlinear analysis methods, the stability conditions of the model can be derived, and the type of equilibrium point of the model can be judged. In addition, the random function is applied to the traffic model, and the bifurcation control is carried out for the bifurcation behavior in the traffic; that is, the bifurcation characteristics of the traffic system are changed by designing linear and nonlinear random feedback controllers. In this paper, we theoretically prove the existence conditions and types of Hopf bifurcation at the equilibrium point of the model and analyze the internal causes of the stability mutation of the traffic flow through the Hopf bifurcation point. Then a feedback controller is designed to control the amplitude of the Hopf bifurcation and the limit cycle formed by the Hopf bifurcation. Finally, the theoretical derivation results are verified by experimental numerical simulation. In this paper, we show that, by adjusting the control parameters of the feedback controller, the Hopf bifurcation of the stochastic system can be delayed or even eliminated, and the amplitude of the limit cycle formed by the Hopf bifurcation can be adjusted to achieve the purpose of controlling the stability of the traffic system and prevent or alleviate traffic congestion.

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

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.

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.