Stability Of Flow Research Articles

Analysis and Optimization of the Performances of the Tandem Blade Radial Compressor Using the CFD

Centrifugal compressors are frequently used in both military and commercial areas because they can be easily manufactured and reach high-pressure compression ratios. The factor that limits the performance and operating range of compressors is flow instability. Many ideas have been put forward for performance improvement, but tandem blade radial compressors, which do not require an extra air system, have attracted the most interest. In this study, computational fluid dynamics (CFD) analyses were carried out on various parameters of the tandem blade (TB) radial compressor, and an optimization study was carried out to find the best design using a genetic algorithm on a whole operating curve. It was investigated how these parameters affected the efficiency and total pressure ratio between the determined lower and upper limits. The numerical analyses of the optimum design obtained as a result of the iterations were carried out. As a result of the iterations, three optimum designs were obtained and numerical analysis was carried out according to one of them, and then they were compared with the results in the literature. The general agreement of computational fluid dynamics and the literature data served as a validation for the computational approach. The error rates between the numerical analysis results and the experimental results in the literature were calculated for different flow rates and were found to be 1.98% as the highest and 0.35 as the lowest. The work carried out in this article will provide a valuable reference for future advanced tandem blade compressor designs.

Brattkus–Davis modes in the solution to the linear stability of plane stagnation-point flow

Brattkus–Davis modes in the solution to the linear stability of plane stagnation-point flow

Asymptotic Stability of Couette Flow in a Strong Uniform Magnetic Field for the Euler-MHD System

Asymptotic Stability of Couette Flow in a Strong Uniform Magnetic Field for the Euler-MHD System

Surface Nanostructures Promote Droplet Splash on Hot Substrates.

Splash, one of the most visually apparent droplet dynamics, can manifest on any surface above a certain impact velocity, regardless of surface wettability. Previous studies demonstrate that elevating the substrate temperature can suppress droplet splash, which is unfavorable for many practical applications, such as spray cooling and combustion. Here, we report that the suppression effect of substrate temperature on splash is nullified by utilizing surfaces with nanostructures. By manipulating air evacuation time through surface nanostructures, we have identified a pathway for precise control over the splash threshold and the ability to tailor the dependence of the splash onset on surface temperature. We further propose a theoretical criterion to determine different splash regimes by considering the competition between air evacuation and the development of flow instabilities. Our findings underscore the crucial role of nanostructures in splash dynamics, offering valuable insights for the control of splash in various industrial scenarios.

Концентрационная конвекция в замкнутой области пористой среды при заданном вертикальном перепаде концентрации и учете иммобилизации примеси

This work is devoted to the study of stability of horizontal filtration flow of a mixture through a closed porous domain taking into account impurity immobilization. The instability arises due to the vertical concentration difference of heavy impurities, which creates unstable density stratification. A general mathematical model describing the transport of an impurity through a porous medium is presented. The equations are simplified for the case of low impurity concentration. Simplification made it possible to analytically obtain the solution corresponding to homogeneous horizontal filtration and to study its stability. It is known that, in the narrow regions of porous medium and at weak intensities of the external flow, convection is excited in a monotonous manner. On the contrary, in the case of an infinite horizontal layer, oscillatory instability is observed. A study of the transition between the instability modes is presented. It is shown that the oscillatory regime is observed in long regions or at significant intensity of the external horizontal flow. At low flow intensities, convective cells do not move relative to the region and, hence, there is no reason for oscillations. It has been established that the range of flow intensity values, in which oscillations are observed, grows with increasing length of the domain. Impurity immobilization leads to the stabilization of horizontal filtration with respect to convective perturbations. Critical curves and stability maps are obtained in a wide range of problem parameters and then analyzed. For limiting cases, a comparison is made with the known results obtained for an infinite layer.

Swirl flow stability: thermodynamic analysis and experiments

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

Stability of plane Couette flow with constant wall transpiration

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

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

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

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

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

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

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

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

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

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

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

CFD modelling of Close-Coupled Gas Atomisation (CCGA) process by employing the Euler-Lagrange approach to understand melt flow instabilities

This paper presents a CFD analysis of the melt and flow-field instabilities in a close-coupled gas atomisation process (CCGA) caused by the gas–melt interactions. The melt mass flow is coupled to the atomiser internal pressure, which varies over time due to the unsteady flow field. A two-phase flow of Argon gas and melt particles is modelled using a coupled Euler-Lagrange framework. Three different initial gas-to-melt ratios (GMRs) of 5.5, 2.6 and 1.32 are considered to study the gas–melt interaction. The results show that in the case GMR = 1.32, sustained instability both in the melt and in the flow-field are distinctly observed, resembling physical atomisation process. This melt fluctuation corresponded to an alternating flow-field fluctuation; alternating between open- to closed-wake condition, where the local maxima of the melt corresponded to an open-wake condition, and the local minima of the melt corresponded to a closed-wake condition.

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

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

The burnt gas flow stability of limit flames in vertical tubes

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

Research on characteristic of flow instability in a two-phase natural circulation loop with parallel once-through steam generators

When there are two sets of parallel once through-steam generators(OTSG) in the natural circulation loop, the thermal-hydraulic characteristics are complicated. Flow instability often occurs in such loops. In order to alleviate the harm caused by flow instability and enhance the natural circulation capacity, it is necessary to study the characteristics of flow instability in depth. In this paper, a thermal-hydraulic computational model is developed for a two-phase natural circulation loop with two parallel OTSGs based on the RELAP5 program. And the model is validated. The thermal-hydraulic parameters of the loop were calculated for different decay heat conditions. The flow instability of the loop is found to be characterized by a clear division into three zones in the decay heat interval. The concept of "load-flow ratio" is proposed to analyze the critical point of flow instability under different operating conditions. The reason for the flow instability zones of the loop is explained. The conclusions of the relevant research can provide a basis for an in-depth study of the characteristics of flow instability and the development of operational schemes to mitigate flow instability.

Heat transfer and flow analysis of a novel particle heater using CFD-DEM

CFD-DEM coupled simulations of a unique gravity-fed particle heater are performed. Physical and heat transfer properties of the solid particles and walls are varied to determine their relative impact on the local heat transfer coefficient along the heater surfaces. The varied parameters are the coefficient of restitution, particle-particle and particle-wall friction coefficients, particle size distribution, normal spring constant, particle conductivity, particle specific heat, and the numerical lens radius. The local heat transfer coefficient increases when the local void fraction along the heater surface decreases. In addition to analyzing thermal performance, flow instabilities are also identified under certain conditions. The outlet particle flow oscillates when the top of each heater is rounded. For heaters with sharp top edges, the local heat transfer coefficient and void fraction profiles change dramatically. The sharp corners remove the particle stagnation point, and the particles adhere to the surface better which increases the heat transfer.

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

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

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

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

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

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