Search

Flow stability plays a key role in transition to turbulence in various systems. This transition initiates with disturbances appearing in the laminar base flow, potentially amplifying over time based on flow and fluid parameters. In response to these amplified disturbances, the flow undergoes successive stages of different laminar flows, ultimately transitioning to turbulence. One influential parameter affecting flow stability is the nanoparticle volume fraction (ϕ) in nanofluids, extensively employed in thermofluid systems like cooling devices to enhance fluid thermal conductivity and the heat transfer coefficient. Focusing on the impact of nanoparticles on Jeffery–Hamel flow stability, this study assumes fluid properties are temperature- and pressure-independent, exclusively examining the momentum transfer aspect. The analysis commences by deriving the base laminar flow solution. Subsequently, linear temporal stability analysis is employed, imposing infinitesimally-small perturbations on the base flow as a modified form of normal modes. A generalized Orr–Sommerfeld equation is derived and solved using a spectral method. Results indicate that, assuming nanofluid viscosity as μ nf = μ f / (1 − ϕ) 2. 5 , nanoparticle effects on momentum transfer and flow stability hinge on the ratio of nano-solid particle density to base fluid density ( R ρ = ρ s / ρ f ). For ϕ ∈ (0 , 0. 1 ] , flow stabilization occurs with ϕ when R ρ < 3. 5000 , while destabilization is observed when R ρ > 4. 0135. Notably, nanoparticles exhibit a negligible impact on flow stability when 3. 5000 ≤ R ρ ≤ 4. 0135. • Effect of nanoparticle volume fraction on Jeffery–Hamel flow stability is investigated. • Influence of density ratio on Jeffery–Hamel flow stability is investigated. • Stability results are presented for Reynolds number based on centerline velocity and flow rate. • Results indicate that density ratio plays a decisive role in stability of Jeffery–Hamel flow. [ABSTRACT FROM AUTHOR]

Experimental and numerical investigations are conducted on a rotating disk from the perspective of convective heat transfer to understand the effect of heating on the stability of flow. A non-invasive approach with a thermal camera is employed to determine local Nusselt numbers for different rotational rates and perturbation parameters, i.e., the strength of the heat transfer. A novel transient temperature data extraction over the disk radius and an evaluation method are developed and applied for the first time for the air on a rotating disk. The evaluation method utilizes the lumped capacitance approach with a constant heat flux input. Nusselt number distributions from this experimental study show that there is a good agreement with the previous experimental correlations and linear stability analysis on the subject. A significant result of this approach is that by using the experimental setup and developed approach, it is possible to qualitatively show that instability in the flow starts earlier, i.e., an earlier departure from laminar behavior is observed at lower rotational Reynolds numbers with an increasing perturbation parameter, which is due to the strength of heating. Two experimental setups are modeled and simulated using a validated in-house Python code, featuring a three-dimensional thermal model of the disk. The thermal code was developed for the rotating disks and brake disks with a simplified geometry. Experimentally evaluated heat transfer coefficients are implemented and used as convective boundary conditions in the thermal code. Radial temperature distributions are compared with the experimental data, and there is good agreement between the experiment and the model. The model was used to evaluate the effect of radial conduction, which is neglected when using the lumped capacitance approach to determine heat transfer coefficients. It was observed that the radial conduction has a slight effect. The methodology and approach used in this experimental study, combined with the numerical model, can be used for further investigations on the subject. [ABSTRACT FROM AUTHOR]

To enhance flow stability and reduce hydrodynamic noise caused by fluctuating pressure, a quasi-periodic elastic support skin composed of flexible walls and elastic support elements is proposed for fluid noise reduction. The arrangement of the elastic support element is determined by the equivalent periodic distance and quasi-periodic coefficient. In this paper, a dynamic model of skin in a fluid environment is established. The influence of equivalent periodic distance and quasi-periodic coefficient on flow stability is investigated. The results suggest that arranging the elastic support elements in accordance with the quasi-periodic law can effectively enhance flow stability. Meanwhile, the hydrodynamic noise calculation results demonstrate that the skin exhibits excellent noise reduction performance, with reductions of 10 dB in the streamwise direction, 11 dB in the spanwise direction, and 10 dB in the normal direction. The results also demonstrate that the stability analysis method can serve as a diagnostic tool for flow fields and guide the design of noise reduction structures.

To examine the formation of tiger stripes (TS) phenomenon on the surface of the injection molding parts, a home‐made injection mold with a rapid heat exchange system and in situ measurement of pressure and temperature was designed and manufactured. Based on this special mold, the TS diagrams as a function of mold temperature and injection pressure for isotactic polypropylene (iPP) and its blend with a type of polyolefin elastomer (iPP/POE) were explored for the first time. It is found that there exists a critical mold temperature and a critical injection pressure for the TS appearance or disappearance in both systems. The critical temperatures of TS disappearance for iPP and iPP/POE are ~50 and ~ 120°C, respectively. As evidenced by the correlation between melt pressure drop and TS appearance, the wall slip mechanism is regarded as the main cause of TS formation. Compared with the critical shear stress of wall slip (σw) in totally molten PP, the lower σw value in this study further indicates that the wall slip could be facilitated by the crystallization of iPP during injection. Combined with white light interferometer, micro‐area x‐ray diffraction nanoindentation and scanning electron microscopy, it is demonstrated that the cloudy and glossy region of TS in pure iPP system could be distinguished by surface crystallinity. In the blend system, however, the rubber phase orientation as a dominant factor contributes to the glossiness of TS. [ABSTRACT FROM AUTHOR]

Issues related to transient regimes of fluid flow in channels with different cross sections are a priority when solving problems of hydrodynamics. Currently, research related to the influence of heat exchange on the stability of fluid flow in processes in which the change in viscosity with temperature cannot be neglected has become particularly relevant. This article examines some features of the loss of stability of a laminar fluid flow with an exponential dependence of viscosity on temperature in an annular channel with a given temperature regime on its walls. For this purpose, the generalized Orr–Sommerfeld equation was derived, which was eventually written in relation to the stream function. A numerical study of the corresponding boundary value problem was carried out using the spectral method based on Chebyshev polynomials. It was shown that taking into account the effect of temperature on the viscosity of the liquid, which implies its non-uniform distribution over the cross section of the channel, leads to a decrease in the critical Reynolds number, which is consistent with the results of previous studies. In particular, as previously noted, for a narrow channel and a small thermoviscosity parameter, the spectrum of eigenvalues is identical to the spectrum for an isothermal flow in a flat channel. A change in the relative channel width and an increase in the thermoviscosity parameter leads to a significant restructuring of the structure of the eigenvalue spectra of the generalized Orr–Sommerfeld equation. As a result of the studies carried out in the presented work, the dependencies of the critical Reynolds number on the exponential factor or, in other words, the thermoviscosity parameter, which characterizes the intensity of the change in viscosity with increasing temperature, and on the parameter determining the ratio of the width of the annular channel to the radius of the inner cylindrical surface were constructed. It has been established that with increasing parameter of the relative channel width, the value of the critical Reynolds number changes non-monotonically, and its minimum value depends on the specific liquid. The latter circumstance can serve as a theoretical justification for carrying out optimization calculations when modeling technological processes. The dependence of the critical Reynolds number on the thermoviscosity parameter has a form close to a decreasing exponential function for all sizes of annular channels. [ABSTRACT FROM AUTHOR]

Experimental and numerical investigations are conducted on a rotating disk from the perspective of convective heat transfer to understand the effect of heating on the stability of flow. A non-invasive approach with a thermal camera is employed to determine local Nusselt numbers for different rotational rates and perturbation parameters, i.e., the strength of the heat transfer. A novel transient temperature data extraction over the disk radius and an evaluation method are developed and applied for the first time for the air on a rotating disk. The evaluation method utilizes the lumped capacitance approach with a constant heat flux input. Nusselt number distributions from this experimental study show that there is a good agreement with the previous experimental correlations and linear stability analysis on the subject. A significant result of this approach is that by using the experimental setup and developed approach, it is possible to qualitatively show that instability in the flow starts earlier, i.e., an earlier departure from laminar behavior is observed at lower rotational Reynolds numbers with an increasing perturbation parameter, which is due to the strength of heating. Two experimental setups are modeled and simulated using a validated in-house Python code, featuring a three-dimensional thermal model of the disk. The thermal code was developed for the rotating disks and brake disks with a simplified geometry. Experimentally evaluated heat transfer coefficients are implemented and used as convective boundary conditions in the thermal code. Radial temperature distributions are compared with the experimental data, and there is good agreement between the experiment and the model. The model was used to evaluate the effect of radial conduction, which is neglected when using the lumped capacitance approach to determine heat transfer coefficients. It was observed that the radial conduction has a slight effect. The methodology and approach used in this experimental study, combined with the numerical model, can be used for further investigations on the subject.

Previous work about CAF through a T-pipe reported that CAF stability could not be maintained after passing the intersection. The current CFD study was expected to propose a strategy to recover the stability of CAF in a horizontal T-pipe without interrupting the flow process. A recirculation zone was detected in the area near the junction. Additional water with a certain design of insertion and water velocity is introduced at the intersection to support the recovery of CAF by suppressing fouling. This numerical work covered both qualitative evaluation of CAF consistency along the T-pipe and quantitative evaluation to analyze the attainable energy savings of the proposed strategy. From some candidates for water insertion geometries and velocities of additional water, the ducting insertion with a water velocity of 2 m/s showed a consistent CAF structure starting at about 200 mm from the intersection point to the final downstream exits. This proposed design was estimated to reduce pressure drop more than 98% compared to transportation without lubrication and 4% higher than design without additional water injection. The power reduction factor was computed at a value greater than 1 (30.3), indicating that the proposed system was energetically more advantageous than the single-phase oil system.

A new macroscopic traffic flow model is proposed which incorporates traffic alignment behavior at transitions. In this model, velocity is a function of the distance headway and driver response time. It can be used to characterize the traffic flow for both uniform and non uniform headways. The well-known Zhang model characterizes this flow based on driver memory which can produce unrealistic results. The performance of the proposed Khan-Imran-Gulliver (KIG) and Zhang models is evaluated for an inactive bottleneck on a 2000 m circular road. The results obtained show that the traffic behavior with the KIG model is more realistic.

The growing demand of high-quality casting is driving the use of ceramic foam filters in casting processes. The primary purpose of using a filter in a casting process is to remove impurities from the molten metal and to stabilize the flow of molten metal by slowing the flow rate. However, only a few studies have been conducted on the specific effects of ceramic foam filters on fluid flow during the casting process. In this study, a water model equipment is used to study the effect of ceramic foam filters on fluid flow, and commercial filters from three domestic and foreign companies are used. The reliability of the data obtained from the water model experiment is verified by an actual casting experiment. Finally, with the verified water model experimental data, the filter is modeled, and a computational fluid simulation is conducted to study the effect of the installation location of the filter on the casting. [ABSTRACT FROM AUTHOR]

With continuous expansion of flight speed and altitude domains, the high-temperature gas effects, especially the thermochemical non-equilibrium (TCNE) effects in hypersonic high-enthalpy boundary layers invalidated the calorically perfect gas (CPG) assumption. They can also largely influence the flow transition process. In recent years, the emergence of unstable supersonic modes in the downstream region of the second mode has been conerned. The so-called supersonic mode refers to the mode with a relative Mach number greater than one at the boundary layer edge, that is, a sound wave that propagates faster than that in the far field. Linear parabolized stability equations (PSE) theory was used to study the critical wall temperature of the supersonic mode under the condition of sharp wedge winding with Mach number 20 and half top angle of 6°. It is found that the lower the wall temperature, the more prone to unstable supersonic mode appeared. Furthermore, the supersonic mode situation and disturbance development form under different Mach number conditions in the flow of the plate boundary layer were explored. It is found that the Mach number continues to increase, the earlier the se-cond mode appears, the maximum growth rate of the second mode decreases, and the peak value of N decreases. At a speci-fic altitude of 30 km, when the Mach number exceeds a certain critical value, the disturbance growth rate and the development form of the supersonic mode are significantly different.

Previous work about CAF through a T-pipe reported that CAF stability could not be maintained after passing the intersection. The current CFD study was expected to propose a strategy to recover the stability of CAF in a horizontal T-pipe without interrupting the flow process. A recirculation zone was detected in the area near the junction. Additional water with a certain design of insertion and water velocity is introduced at the intersection to support the recovery of CAF by suppressing fouling. This numerical work covered both qualitative evaluation of CAF consistency along the T-pipe and quantitative evaluation to analyze the attainable energy savings of the proposed strategy. From some candidates for water insertion geometries and velocities of additional water, the ducting insertion with a water velocity of 2 m/s showed a consistent CAF structure starting at about 200 mm from the intersection point to the final downstream exits. This proposed design was estimated to reduce pressure drop more than 98% compared to transportation without lubrication and 4% higher than design without additional water injection. The power reduction factor was computed at a value greater than 1 (30.3), indicating that the proposed system was energetically more advantageous than the single-phase oil system. [ABSTRACT FROM AUTHOR]

Natural convection of air in a tall annular cavity is explored by numerical solutions of the fully compressible Navier-Stokes equations in transient, axisymmetric (two-dimensional) form. A temperature difference is imposed on the two vertical walls, each isothermal, with the two horizontal boundaries adiabatic; the outer cylindrical surface is heated and the inner cylindrical surface cooled. Thermophysical properties of air, including density, viscosity, thermal conductivity, and specific heat are all variable with temperature. Laminar flow inside a tall annular cavity is prone to an instability which takes the form of a multicellular cat's eye pattern drifting downward in the cavity. In this study, non-Boussinesq effects on the cat's eye instability are explored by simulating flows in an annular cavity of aspect ratio A = 40 and diameter ratio η = 0.8, for Rayleigh numbers less than 14,000. A range of dimensionless wall temperature differences are investigated, 0 < ε ≤ 0.2, where ε is the temperature difference between the two vertical walls, Th – Tc, divided by the sum of the absolute temperatures, Th + Tc. The dimensionless wall temperature difference is shown to have pronounced effects on: the onset of the cat's eye instability, subsequent flow bifurcations, wave speeds, oscillation time-periods as well as local heat transfer rates. The critical Rayleigh number, Rac, signifies a transition from steady unicellular flow to transient multicellular flow; Rac is found to increase with ε. At higher values of dimensionless wall temperature difference ε, mono-periodic oscillatory flows are predicted to occur over a smaller range of Rayleigh numbers, prior to the onset of quasiperiodic flow. [ABSTRACT FROM AUTHOR]

Copyright of Large Electric Machine & Hydraulic Turbine is the property of Large Electric Machine & Hydraulic Turbine Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Foam-metal casing treatment (FMCT) has the potential to enhance the stall margin while attenuating the broadband noise of compressors. An optimized structure of FMCT backed with an air gap was developed in this study with the basis of a prediction model. The eigenvalue theory with a linear perturbation assumption was employed to judge the stability of the compression system, where the casing treatment was considered as an impedance boundary condition. The effect of this kind of casing treatment on the compression system is introduced through the equivalent surface source method. The transfer element method was applied to match the relations between perturbation parameters on the surface of each section. Prediction results showed that the double-layer configuration has better stall margin improving ability, with experimental results on the steady characteristics confirming a 39.5 % stall margin improvement. The mechanism of exerting damping on the system and thus suppressing stall precursors was proved. [ABSTRACT FROM AUTHOR]

The effect of the temperature and rate of hot deformation on the structure and strain resistance of duplex stainless steel DSS 2205 is considered. Tests for hot compression of specimens are conducted at a temperature of 1223 – 1473 K and deformation rate 0.01 – 30 sec–1 to a degree of 0.4 and 0.8. Maps of deformation mechanisms are plotted. It is shown that the strain resistance decreases after attaining a maximum value in the flow curve, and the flow stress increases with decrease of the temperature and increase of the rate of the deformation. The activation energy of the hot deformation is shown to vary in accordance with the partitioning of strain between the two phases (ferrite and austenite) in the structure of the steel. The rate and the temperature of the deformation affect the deformability of the duplex steel considerably. Continuous dynamic recrystallization of ferrite is activated strongly in the range of flow stability. The range of flow instability is characterized by weak continuous dynamic recrystallization in the ferrite and discontinuous dynamic recrystallization in the austenite. [ABSTRACT FROM AUTHOR]

The asymmetric volute structure in the turbocharger centrifugal compressor causes a static pressure circumferential double-peak distribution (PD-DP) at small flow rates and affects the circumferential stall position near the impeller inlet. In this study, a recirculation device was added to a prototype compressor, and the effect of the recirculation flow on the PD-DP was studied to reveal the difference in the PD-DP formation mechanism with and without the device. In addition, the influence of the PD-DP difference on the compressor flow stability was investigated. The results show that the PD-DP generated various pressure gradients between the front and rear slots of the recirculation device at different circumferential positions, resulting in a double-peak distribution in the recirculation flow rate. When the recirculation airflow flowed back into the impeller inlet, the two recirculation flow rate peaks were near the circumferential positions occupied by the two static pressure peaks in the prototype compressor. Therefore, the pressure distortion degree near the impeller inlet could be inhibited, weakening the difference in the deflection degree of the tip leakage flow (TLF) in each compressor passage. It should be noted that the circumferential positions with more prominent variation in the TLF corresponded to the passages with more serious flow deterioration in the prototype compressor, and the blockage of these passages near the inlet was reduced to a greater extent, which could better improve the flow stability of the compressor. [ABSTRACT FROM AUTHOR]

In the present work a detailed numerical study on the eigenvalue spectra for the stability equations of thermoviscous fluid flow in an non-isothermal annular channel was performed. The temperature dependence of viscosity had the form of exponential function that decreases with increasing temperature and was described with the single numerical value called "thermoviscous parameter". The perturbation of the temperature was taken into account and, as a result, the spectral problem was reduced to the system of two coupled ordinary differential equations for axial velocity and temperature perturbations. Eigenvalue spectra for different values of the thermoviscous parameter and the radii of outer and inner cylinders ratio were obtained. The real and imaginary parts of eigenfunctions for eigenvalues from different parts of the spectra were plotted. It was found that for narrow annular channels and rather small values of the thermoviscous parameter the eigenvalue spectra are identical to the ones for isothermal flow in a flat channel. As the thermoviscous parameter increases and the annular channel gap expands the structure of the spectra changes (namely, the spread of eigenvalues along the real axis increases) which leads to the flow stabilization. The eigenfunctions demonstare a non-trivial oscillating distribution of amplitude over the channel cross sections. It can also be mention that the eigenfunctions are not symmetrical, since the undisturbed velocity profile in annular channel also does not have symmetry. [ABSTRACT FROM AUTHOR]

The heat transfer characteristic of supercritical water is one of the crucial issues in SuperCritical Water-Cooled Reactors (SCWRs). The efficiency and safety of the SCWR system are largely dependent on the local heat transfer performance. This paper establishes the numerical model for supercritical water in a long vertical circular loop (inside diameter = 10 mm) and analyzes the flow and heat transfer mechanism during the transition process from subcritical to supercritical states under various heat fluxes (uniform and nonuniform). The results reveal that the difference in thermophysical properties between the boundary layer and the core region is the main reason for the heat transfer behavior, especially during the transition from subcritical to supercritical and liquidlike to gaslike. The flow structure on the buffer layer is a dominating factor for heat transfer deterioration. The cases under variable nonuniform heat fluxes have a higher heat transfer coefficient compared with uniform heat fluxes. But, this will cause large changes of the parameter locally. The dominating factors of heat transfer deterioration under these conditions are also identified. [ABSTRACT FROM AUTHOR]

We elucidate here the temperature and strain rate sensitivity of a duplex stainless steel through the development of deformation maps and by studying the flow behaviour during hot compression tests in the temperature range of 1223–1473 K and at strain rates between 0.01 and 30 s −1. The flow curves exhibited softening after attaining a peak condition and the flow stress increased with the decrease of temperature and increase of strain rate. The observed change in the activation energy is attributed to strain partitioning between the two constituent phases and the deformation maps were characterised by two regions of flow instability. On the other hand, the microstructural evolution indicated that the strain rate and deformation temperature had a significant impact on the flow behaviour. In the flow stability region, the continuous dynamic recrystallisation (DRX) of ferrite was strongly activated, while in the flow instability region, both the continuous dynamic recrystallisation in ferrite and discontinuous DRX in austenite were weak. [ABSTRACT FROM AUTHOR]

To improve the drag reduction performance of underwater vehicles, a skin composed of a porous compliant wall and micro floating raft elements is proposed. A two-dimensional stability model of boundary-layer flow over the compliant skin is established, and the effect of structural parameters on the flow stability is investigated. It is found that enlarging the spacing l attenuates the Tollmien-Schlichting Instability (TSI) mode but promotes the occurrence of the Compliance-Induced Flow Instability (CIFI) mode, while the permeability coefficient a presents an opposite effect on the instability modes. Moreover, the CIFI mode is stabilised by increasing the stiffness ratio β, damping ratio γ and intermediate mass m, within limits. The skin proposed in this paper with specific parameters can better reduce the arising of TSI and CIFI modes in boundary-layer flow compared with the corresponding rigid and compliant walls, leading to a favourable performance for delaying boundary-layer transition. [ABSTRACT FROM AUTHOR]

Three-dimensional electro-thermo-hydrodynamic (ETHD) flows of dielectric fluids driven by simultaneous Coulomb and buoyancy forces in a cubic box is numerically studied. The set of coupled equations associated with the ETHD phenomena are solved with the finite volume method. The code is first validated by comparing the numerically obtained linear critical values of the pure electro-convection and thermal convection with the previous studies. Then the neutral stability curve of the system is given and the finite amplitude instability thresholds with different Rayleigh numbers ( Ra) are presented. It is found that along the neutral stability curve, the flow strength becomes weaker with the increase of Ra and the decrease of electric Rayleigh number (T). Besides, the gap between the linear and nonlinear critical values expressed in terms of T decreases with the increase of Ra. Primarily, the distributions of charge density, temperature and velocity fields with different governing parameters near neutral stability curve are presented. The temperature and charge density profiles near the linear and nonlinear critical values are given, showing that higher T results in wider charge void region and shorter distance between the charge void region’s lower boundary and injection electrode. Finally, the symmetries of the flow patterns along the neutral stability curve are also discussed briefly. [ABSTRACT FROM AUTHOR]

• A new method to suppress transonic buzz is proposed, which can effectively avoid transonic buzz by improving the flow stability. • The suppression of transonic buzz by using shock control block (SCB), buffer vent (BB) and trailing edge deflector (TED) was studied. • BB and TED techniques can expand the buffet boundary effectively. • The mechanism of transonic buzz generation was further verified. The control surface buzz may occur when the aircraft cruise at transonic Mach numbers, which would may lead to flight accidents. Transonic buzz could be a problem in aircraft design. The triggering of transonic buzz has been investigated in the previous publications. It is essentially a single degree of freedom flutter caused by the coupling of one structural mode and one pre-instability flow mode, thus, type A/B buzz usually occurs close to the buffet onset. With this mechanism, the buzz can be suppressed by improving the flow stability and buffet onset. Three buffet passive control techniques are applied to test the suppression effect on transonic buzz. The relationship between the flow stability and buzz onset is studied to verify the mechanism of buzz. CFD simulation and global stability analysis are applied to compute the flow stability with and without control techniques. Results show that, three control techniques can improve the flow stability and postpone the buffet onset. Meanwhile the buzz can be effectively suppressed. With the aeroelastic ROM and the root loci method, the coupling process between the structural mode and the controlled flow modes are revealed, indicating the buzz suppression effects are positively correlated with its improvement of the flow stability with control. This study can help to understand the mechanism of buzz deeply and propose the new thought of buzz suppression. [ABSTRACT FROM AUTHOR]

• Re effect leads to higher frequency and stronger circumferential momentum of compressor tip leakage flow. • Impact of tip leakage flow decreases from leading edge to mid-chord, but increases from mid-chord to trailing edge at low RNI condition. • Flow instability, identified by DMD, shows growth of low-frequency stall modes and weakening of blade-passing modes. • High RNI condition exhibits non-uniform high-pressure region due to shock-TLV (tip leakage vortex) interaction, leading to delayed flow field response to blade passing. • Low RNI condition results in uniform high-pressure region due to enhanced circumferentia momentum of tip leakage flow, leading to secondary leakage formation. With the continuous increase in aircraft flight altitude, low Reynolds number effects in high-altitude environments significantly impact the stability of compressor operation. In this study, dynamic mode decomposition (DMD) of the flow field at various time instances near stall conditions in a transonic compressor was performed to investigate the influence of Reynolds number (Re) effects on critical flow characteristics and instability mechanisms. The results indicate that under different Reynolds number index (RNI) conditions near stall conditions, the frequency of tip leakage flow varies. Under high RNI condition, the influence of tip leakage flow is concentrated mainly from the leading edge to the mid-chord. The stall mode is primarily caused by the interaction of shock with the Tip Leakage Vortex, resulting in the breakdown of the vortex. The development of vortex structures inside the passage leads to a delay in the flow field response to blade passing. Under low RNI condition, the tip clearance leakage flow is stronger, with a relatively reduced influence from the leading edge to the mid-chord and an increased impact from the mid-chord to the trailing edge. The main cause of the stall mode is the circumferential movement of vortex structures, causing secondary leakage over adjacent blade tip clearances. The blockage formed by vortex structures within the passage is weaker, and the flow field response to blade passing remains consistent with the blade passing frequency. However, as stall develops further, the flow field response significantly decreases. [ABSTRACT FROM AUTHOR]

Considered a valid alternative to other receiver types, open volumetric receivers for central tower power plants have been developed and researched for multiple decades. 3D-Shaped absorber geometries, made possible by modern manufacturing technologies, particularly offer the promise of high thermal efficiencies. Yet, volumetric absorbers carry the inherent risk of flow instabilities, which can lead to a total failure of the receiver due to local overheating. Therefore, a numerical, multi-dimensional methodology for the flow stability analysis of volumetric absorbers is proposed. The first stage of the methodology applies an 1D LTNE continuum model to quantify the general pressure loss characteristic of an absorber and identifies critical pressure drop levels with potential for flow instability. These levels are tested as part of the second analysis stage using a discrete 3D CFD model against different irradiation and flow perturbations. If the absorber is able to compensate the disturbances, its flow behavior is deemed stable. Otherwise, the absorber geometry is considered susceptible to local overheating due to flow instability. The new methodology has been validated against two reference absorbers with known stable resp. unstable flow behavior. Further, the new approach was applied to analyze the flow stability of two 3D-shaped honeycomb absorber geometries. It was shown that the geometric features of 3D-shaped absorbers can directly improve the flow stability, but do not prevent instabilities automatically. Therefore, a flow stability analysis should be included in design/optimization processes of new volumetric absorbers alongside a maximization of the thermal efficiency. • A new CFD model analyzing the flow stability of volumetric absorbers was developed. • The approach combines an 1D LTNE continuum model with a discrete 3D CFD model. • The model has been successfully applied for the evaluation of 3D-shaped absorbers. [ABSTRACT FROM AUTHOR]

In order to reduce the flow resistance on the surface of ships, the skin with quasi-periodic elastic supports is designed based on the flexible skin drag reduction technology inspired by dolphin skin. In this paper, the stability characteristic equation of fluid-structure coupling system is established based on small perturbation theory and solved by MATLAB. The results indicate that the skin with quasi-periodic elastic supports has the ability of maintaining stability of turbulent boundary layer when the structural parameters are reasonable. In addition, the drag reduction performance of the skin with quasi-periodic elastic supports is studied by immersion boundary method. The analysis results demonstrate that the skin with quasi-periodic elastic supports can show better drag reduction performance than the skin with periodic elastic supports when the structural parameters q and the support spacing L are rationally set. In a certain speed range, the relative drag reduction rate is up to 72.16%. These optimal distribution parameter data of the skin under certain speed conditions obtained in this paper provide a reference for the practical application and intelligent optimization design of the skin with quasi-periodic elastic supports. [ABSTRACT FROM AUTHOR]

We use linear stability theory to investigate the effect of fluid compressibility on interface stability during a dissipative displacement (Darcy flow). We find that compressibility changes the perturbation growth rate as a function of perturbation wavenumber. Our results indicate that both favorable (less than unity) and unfavorable (greater than unity) mobility ratios will always lead to positive maximum growth rate, which traditionally is recognized as the criterion for instability. We conclude, however, that in the case of favorable mobility ratio, the maximum perturbation growth rate is always smaller than the unperturbed growth rate naturally existing in compressible displacements. The interface will still be stable because the perturbation will never exceed the background flow. Therefore, compressibility does not change the stability of displacements, which is ultimately determined by mobility ratio. [ABSTRACT FROM AUTHOR]

This study introduces the "Adjustable Oxygen Adapter," designed to address interruptions in oxygen supply for patients transitioning from oxygen spray bottles, particularly in wards with single-outlet flowmeters. Inspired by the "Three-Way Piston Rotating Adapter," this innovative design, operated by a single switch, ensures uninterrupted oxygen supply during equipment changes. Stability testing, conducted with a flow monitor by a respiratory therapist, confirms that the adapter provides a stable flow rate and oxygen concentration, offering a practical solution for seamless transitions between oxygen devices in clinical settings. [ABSTRACT FROM AUTHOR]

In the peak period of double-layer line, in the process of calculating the optimal solution of vehicle planning, assigning an optimal solution to each constraint will greatly waste computing resources. In the process of vehicle logistics route planning, the natural characteristics of unbalanced constraints will affect the flow stability of vehicles in the planning process. This paper studied the influence of unbalanced constraints on the flow stability of vehicle bilevel route planning . It constructed a two-level programming Stack elberg model for vehicle logistics lines . The upper part was mainly used to restrict the capacity and structure of vehicle logistics lines and eliminate unavaila ble routes. The lower part mainly implemented balanced distribution of vehicle logistics lines to prevent too many vehicles from crowding in the same route. It used model to integrate scheduling business into high capacity routing . It added the scheduling resource cache technology to the model to ensure the stability of the scheduling process. The experimental results show that, compared with the single model solution, using the model to schedule the business grooming can achieve a more stable performance to a large extent, and solve the flow stability problem of vehicle logistics line planning . [ABSTRACT FROM AUTHOR]

Gas saturation degree and flow patterns are key factors for modeling and designing multiphase systems and operations. In turn, these factors are strongly related to the flow dynamics, which are controlled largely by the fluid and media properties. However, predicting the gas distribution and flow pattern remains elusive. Data from 11 series of steady air injection experiments into initially water‐saturated granular media, conducted with different media and flow geometries, are analyzed to identify the main factors governing air saturation and flow patterns. For air injection into otherwise water‐saturated granular media, the flow pattern is affected mainly by the ratio between the Capillary number (ratio of viscous to capillary forces) and the Bond number (ratio of gravitational to capillary forces), that is, by the ratio of viscous to gravitational forces. Moreover, the meta‐analysis presented here indicates that the steady air saturation degree is correlated strongly to flow velocity and grain diameter. Furthermore, analysis of experimental results from different studies of air injection into coarse, homogeneous, granular media (glass beads, sand) also suggests a significant effect of inertial forces. Both viscous and buoyancy forces increased air saturation, while capillary forces decreased the saturation degree. The ratio between capillary and buoyancy forces determines the air flow pattern during air injection into otherwise water‐saturated media. Key Points: Capillary, viscous, gravity, and inertial forces control air flow regime and content, the latter increases with velocity and grain sizeA new phase diagram depicts the effect of Ca and Bo on air flow patternsThe effect of the background saturation degree cannot be easily decoupled from the Bo‐Ca‐flow pattern phase diagram [ABSTRACT FROM AUTHOR]

The filling valve requires high efficiency and stability in the working process, which is closely related to the distribution of the flow characteristics inside the valve. In this study, computational fluid dynamics (CFD) methods were present to analyse the pressure, velocity, and other characteristics of the filling valve under different inlet pipe angles and valve openings. The flow characteristics, which are experimentally measured, are consistent with CFD simulation results. Research shows that the increase in the angle of the inlet pipe can improve the uniformity of the flow field inside the valve, produce a better core flow pattern at the valve outlet, and have better fluid stability. Based on the flow characteristics of the valve, to obtain good filling efficiency and stability, a moderate valve opening of 9 mm should be selected, and the flow coefficient can reach 1.08, which is 1.6 times higher than that of 3 mm, and the angle of the inlet pipe should be appropriately increased. This study provides a reference for the use and optimization of filling valves. • Structural optimization of filling valves. • Improved filling efficiency of filling valves. • CFD simulation improves the smoothness of the filling process of the filling valve. [ABSTRACT FROM AUTHOR]

Stable delivery of liquids to microfluidic systems is essential for their reproducible functioning, especially when supplying flows to organs-on-chips – delicate living models that recreate human physiology on the microscale and thus can be used to reduce the need for animal testing. Most flow control systems are unable to sustain a robust and stable flow in longer experiments (>1 week), particularly those based on the ubiquitous syringe pump. Though easy to use, syringe pumps have no mechanism for actually measuring flow, let alone flow regulation with sensor feedback. We have developed a liquid delivery system based on the generation of flow by applying a constant air pressure to liquids in sealed containers. A flow of liquid is monitored by accurate measurement of mass flows (mg/min) using downstream Coriolis-based mass flow sensors. Measured mass flows provide fast feedback to integrated valves, with valves opening or closing slightly to increase or decrease solution flows to the organs-on-chips as required. This mass flow sensing principle is not affected by changes in the density, temperature, and viscosity of the liquids being displaced. This is in contrast to systems that use volumetric flow sensors, which require recalibration when these parameters change. The rationale behind using this principle for organs-on-chips, is that the stability provided by this flow control system allows for more control over growth of these mini-organs. We demonstrate the functionality of this system with three examples: 1) Fast stabilization (within seconds) under changing physical conditions; 2) Short-term stability (minutes to hours) of delivered flows in a microreactor with interconnected inlets; and 3) Long-term stability (>1 week) of cell medium flows to a living organ-on-a-chip. Two categories of organs-on-chips (OOCs) can be distinguished: 1) solid OOC are designed for three-dimensional cell or tissue constructs that interact with each other and their surroundings, and 2) barrier-type OOC contain a selective cellular barrier between two compartments as do many barriers in the body. The latter of these two types is the most challenging to culture and maintain as they are very sensitive to variations in flow and pressure surges. The flow control system presented in this work provides a great improvement compared to the use of syringe pumps and volumetric flow sensors in OOC studies. The novelty of this work lies in the long-term stability use of this system for organs-on-chips, maintaining stability for short to very long periods of time without compromising the barrier function of the organ-on-chip by pressure surges, bacterial contamination, or other undesired effects from the flow delivery system. • Stable flows are required for long-term (>1 week) organ-on-chip experiments. • This paper describes a flow control system based on Coriolis mass flow sensors. • Stable flows for long operation times, with varying conditions (viscosity, density). • Flow stability needed for optimal control of cell culture conditions. [ABSTRACT FROM AUTHOR]

• Assess how cyberattacks influence the stability, energy efficiency, and volatility of cooperative driving. • Investigate how sustainability expressed through traffic stream stability and volatility is affected by cyberattacks. • Detect anomalous behavior in real time for resilient operation of cooperative driving in smart cities. Increasing levels of communication and automation has led to the development of cyber physical systems applications known as cooperative driving automation. While such applications provide enormous benefits in improving transportation systems operation and environmental sustainability, they also introduce new risks to smart cities. These systems provide wider access to adversaries and increase the attack surface to compromise the operation of smart cities. This study develops a vehicle to infrastructure (V2I) based cooperative driving automation platform to understand and analyze the cyber risks in these systems from a V2I perspective in a representative traffic environment. Three cyberattacks (Fake BSMs, replay and denial of service) were emulated as a cooperative game to assess the impact on volatility, energy efficiency and traffic flow stability. The critical nature of cooperative driving automation is revealed by this research and flow stability, and energy efficiency of cooperative driving automation are severely impacted by the three categories of cyberattacks. Further, a 35 %-40 % increase in volatility in acceleration-deceleration regimes along with 20 % increase in emission is observed as opposed to the baseline case with no cyberattacks. The proposed long-short term memory neural networks (LSTM) showed an average detection accuracy of around 98 % for the analyzed cyberattacks. The research findings would foster the development of algorithms to continuously monitor the state of cooperative automated driving environment for anomalous behavior and promote resilient and environmentally sustainable cities in the future. [ABSTRACT FROM AUTHOR]

In this work, structural parameters and performance prediction of fan nozzles for powder spraying are investigated. Computational Fluid Dynamics (CFD) was used to simulate the working process of the grooved fan nozzle. A Box-Behnken experimental model was developed to analyze the effects of the structural parameters on the nozzle air flow stability and working performance, taking the nozzle structural parameter contraction angle (α), the ratio of outlet to inlet diameter (d/D), the ratio of outlet diameter to length (L 2 /d), and the half angle of the "V"– shaped groove (β). The air flow stability and working performance of nozzle were evaluated through coefficient of variation (C v) and standard deviation (S D). Using multiple regression analysis, the prediction model of C v and S D on the structural parameters is established. It was obtained that: with the increase of the diameter and length of the nozzle outlet, the spray angle of the nozzle decreases. As the "V"– shaped groove angle increases, the powder spray angle decreases. The effects of structural parameters on C v are d/D , α , β , and L 2 /d in ascending order, and on S D are d/D , α , β , and L 2 /d in ascending order. In addition, the influence of the interaction of each structural parameter on C v and S D was obtained. This study can provide a reference for the design and structural optimization of fan nozzles for powder spraying. [Display omitted] • The performance of gas-solid nozzles is evaluated and predicted. • The effects of structural parameters on nozzle performance were obtained. • Regression models for C v and S D were fitted and obtained. • The interaction between structural parameters was discussed. [ABSTRACT FROM AUTHOR]

In some thermally driven two-phase natural circulation systems, bubble pumps serve as the key driving powers for the cycles. Recently a type of distribute heating bubble pump generator (BPG) is gradually receiving attention due to its compact structure and great potentials to utilize solar energy and low-grade waste heat recovery. The BPG provides a variety of promising features (e.g., passive heat transfer, enhanced reliability), which can benefit the advancing of heat transfer technology. For the primary study, we performed an experiment in a distributed heating BPG. Through utilizing multiple lift tubes and partial visualization configurations, it provides accesses to observe the flow pattern transition and monitor the flow instability, and thus to explore some of the underlying mechanisms affecting BPG performance. Results showed that heat input and immersion height were crucial parameters to enable the operation of distribute heating BPG. With low heat input or high inlet water subcooling level, the flow within the pump was unstable with intermittent flow interruptions. As the heat input increased, the fluid flow became more stable, the vapor generation increased linearly, while the lifted liquid flow rate initially increased then decreased. Correspondingly, the flow pattern at the outlet section of lift tubes gradually changed from slug flow to churn flow, and then to annular flow. The higher of the immersion was, the higher heat input was needed for the flow pattern transition. It was in the churn flow regime at the outlet of lift tubes for the BPG to lift a maximum liquid. At lower immersion level, liquid reflux in the lift tubes was obvious and affected the flow stability as well as the lifting performance. At higher immersion level, the fluid flow was more stable and faster, which lifted more liquid while generated less vapor depending on the inlet subcooling. In general, the BPG showed better performance (both the lifted liquid and vapor generation increased) at smaller inlet subcooling level or lower system pressure. This study highlights the flow pattern evolution and flow stability, which is helpful to the reliable design and effective operation of the distributed heating BPG. [ABSTRACT FROM AUTHOR]

Leidenfrost drops were recently found to host strong dynamics. In the present study, we investigate both experimentally and theoretically the flow structures and stability inside a Leidenfrost water drop as it evaporates, starting with a large puddle. As revealed by infrared mapping, the drop base is warmer than its apex by typically 10 °C, which is likely to trigger bulk thermobuoyant flows and Marangoni surface flows. Tracer particles unveil complex and strong flows that undergo successive symmetry breakings as the drop evaporates. We investigate the linear stability of the base flows in a non-deformable, quasi-static, levitating drop induced by thermobuoyancy and the effective thermocapillary surface stress, using only one adjustable parameter. The stability analysis of nominally axisymmetric thermoconvective flows, parametrized by the drop radius R, yields the most unstable, thus, dominant, azimuthal modes (of wavenumber m). Our theory predicts well the radii R for the mode transitions and cascade with decreasing wavenumber from m = 3, m = 2, down to m = 1 (the eventual rolling mode that entails propulsion) as the drop shrinks in size. The effect of the escaping vapour is not taken into account here, which may further destabilize the inner flow and couple to the liquid/vapour interface to give rise to motion (Bouillant et al. Nat. Phys., vol. 14 (12), 2018, pp. 1188-1192; Brandão & Schnitzer Physical Review Fluids, vol. 5 (9), 2020, 091601). [ABSTRACT FROM AUTHOR]

This study introduces the "Adjustable Oxygen Adapter," designed to address interruptions in oxygen supply for patients transitioning from oxygen spray bottles, particularly in wards with single-outlet flowmeters. Inspired by the "Three-Way Piston Rotating Adapter," this innovative design, operated by a single switch, ensures uninterrupted oxygen supply during equipment changes. Stability testing, conducted with a flow monitor by a respiratory therapist, confirms that the adapter provides a stable flow rate and oxygen concentration, offering a practical solution for seamless transitions between oxygen devices in clinical settings.

The paper presents a problem of one-dimensional flow of water and steam separated with an interface; the flow is arranged in an inclined porous layer and subjected to lateral heat flux. The analytical stationary solution was found and the properties analyzed. Instability of flow for a certain region of parametric space was found and the nature of instability was described. [ABSTRACT FROM AUTHOR]

Along with the rapid growth of cutting-edge petrochemical technology and the pressing demand for efficiency improvement, evaluation of the performance characteristics of high-speed pump is becoming increasingly important. In this paper, numerical simulation is presented on the flow instability of a 16 straight-blade highspeed centrifugal pump with flow rate of 3 m3/h and rotating speed of 8500 rpm. Combined with the analysis of flow stability, the entropy production method is introduced to evaluate regions of high mechanical energy loss and its distribution at different flow rates. Results show that approximately 96% of the energy loss of the pump is produced in the volute, gap, and front and back chambers. Large energy loss is observed near the trailing edge of the blade and volute tongue, which are caused by the small region including both the high and low pressure gradients and large momentum exchange by the flow separation, respectively. Moreover, the rotor–stator interaction causes much energy loss at the wall of the volute and front and back chambers. Owing to the circumferential pressure gradient and the 90° leading edge of the straight blade, the fluid tends to form counter-rotating recirculation vortices. The large number of blades narrows the passage and limits the formation of large vortices in flow channels, thus the backflow phenomenon seems not to worsen with the rise of flow rates. Hence, the entropy production in most of the flow parts are insensitive to flow rates. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The influence of different diffuser configurations on the flow stability and aerodynamic performance of a centrifugal compressor stage with a mass flow coefficient of 0.196 is numerically investigated. Research results show that the performance of a traditional full-height vaned diffuser (TVD) deteriorates rapidly, and a shroud-side partial vaned diffuser (SVD) displays better adaptability in off-design conditions. SVD can suppress the development of vortices generating at the diffuser leading-edge. Therefore, it can reduce the flow loss inside the stage and improve the flow stability of the stage at low mass flow rates. The unsteady analysis for TVD and SVD shows that the stall cell propagates at about 35.7% of impeller rotational speed in the semi-vaneless space and diffuser passages. Furthermore, the internal flow in TVD and SVD is studied by employing the proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) methods. The flow loss and instability mechanism in the stage are consequently revealed more comprehensively. [ABSTRACT FROM AUTHOR]

This study investigated wind-induced vibrations in square cylinders, commonly encountered in practical engineering. Passive-jet flow control was achieved by arranging a hollow pipe with openings on a square cylinder surface, thereby suppressing the wake and wind-induced vibrations. Large eddy simulation was used to determine a reasonable grid and time-step scheme and an optimal chamfered scheme. The effects of various parameters such as the hollow pipe height, radial thickness, opening number, and opening height on the wake vortex state and aerodynamic forces of the cylinder were investigated at R e D = 50 , 000 based on the side length (D) of the cylinder. The results indicate that the passive-jet flow control method could effectively suppress the wake and reduce the aerodynamic forces acting on the cylinder. The optimal control effect was achieved when the hollow pipe height, radial thickness, opening number, and opening height were 0.6 D , 0.05 D , six per side, and 0.4 D , respectively. Compared to the uncontrolled chamfered square cylinder, the lift coefficient fluctuation and mean drag coefficient decreased by 89.2 % and 19.2 %, respectively. • Passive-jet flow control method suppresses wake and reduces aerodynamic forces. • Passive-jet hollow pipe control can blow the main vortices farther downstream. • The optimal opening parameters of the hollow pipe is determined. • Flow state in the near wake region is more stable under the hollow pipe control. [ABSTRACT FROM AUTHOR]

The article presents and analyzes the results of an experimental study of the aerodynamics of the flow in the working volume of cyclone chambers of a large relative length, significantly exceeding the length of the chambers that were used in the previous studies. Air supply to the chamber swirler was carried out tangentially from diametrically opposite sides by two inlet channels. The air discharge from the working volume of the chamber was made from the opposite end of the chamber through a round axisymmetric hole. The values of the area of the inlet channels and the diameter of the outlet were varied with replaceable incrustations and clamps. The experiments were performed with the use of laser Doppler anemometry. As a result, previously unknown features of flow formation in the working volume of relatively long cyclone chambers have been determined. The main characteristic values of the working volume flow have determined as well. In particular, the determining influence of the flow core characteristics on its structure in relatively long cyclone chambers has been discovered. The calculated ratios have been chosen to determine these values depending on the geometrical characteristics of the chamber under study. The boundary of the near-wall flow region, in which favorable conditions for the flow instability are created, is determined. Numerical simulation of the flow in the ANSYS Fluent software has been performed. Based on its results, a comparison of the results of numerical simulation, calculated dependencies and experimental data is presented. A comparison of the results demonstrated a completely satisfactory coincidence. Data obtained in the process of research and calculated ratios can be used in engineering practice and are of an interest from the point of view of further study of aerodynamics in a highly swirled flow of cyclone devices in order of to improving the methods of their thermal and aerodynamic calculations.

This review surveys the dramatic variations in wake structures and flow transitions, in addition to body forces, that appear as the motion of bluff bodies through a fluid occurs increasingly closer to a solid wall. In particular, we discuss the two cases of bluff bodies translating parallel to solid walls at varying heights and bluff bodies impacting on solid walls. In the former case, we highlight the changes to the wake structures as the flow varies from that of an isolated body to that of a body on or very close to the wall, including the effects when the body is rotating. For the latter case of an impacting body, we review the flow structures following impact and their transition to three-dimensionality. We discuss the issue of whether there is solid–solid contact between the bluff body and a wall and its importance to body motion. [ABSTRACT FROM AUTHOR]

Subject of Research.The paper deals with the problem of viscous film evolution on a surface of rotating cylinder. The objective of the work is stability analysis of such system in the cases when the cylinder axis is nearly vertical.Stability of uniform film on a strictly vertical cylinder is analyzed, and the system dynamics under small time-periodic deviations of cylinder axis is studied.Methods. Initial Navier-Stokes equations, which describe dynamics of viscous fluids, are considered with introduced small parameter – the ratio of fluid layer thickness to cylinder radius. This fact allows applying of asymptotic methods and helps to derive a simplified equation on the film thickness. The obtained equation is a non-linear partial-derivative equation and cannot be solved analytically in general form. Perturbation theory is applied for the subsequent analysis. Linear stability analysis for uniform film on the vertical cylinder surface is carried out. Response to time-periodic perturbation of the cylinder tilt is studied. Main Results. In the case of strictly vertical cylinder, capillary forces imply instability of axisymmetric perturbations with a wavelength exceeding critical value. The obtained critical wavelength value is proportional to the cylinder radius. It is shown that when a cylinder axis oscillates around the vertical, non-uniform fluid layer appears with diametrically opposite thickening and thinning. In the case when the axis oscillation frequency is equal to the rotation frequency, a resonance appears.Practical Relevance. The obtained results might be used for better understanding and further improvement of existing manufacturing processes.