Velocity Pulsations Research Articles

The impact of pulsating parameters on particle dynamics in vertical pipe during hydraulic conveying with pulsating inlet flow

This paper presents a numerical investigation of the hydraulic conveying performance of coarse particles with the inlet pulsating fluid velocity in a vertical pipe. The influence of pulsating parameters, in terms of pulsating amplitudes and frequencies, on hydraulic conveying is mainly considered. The pulsations of particle and slip velocity are as periodic and regular as the inlet pulsating velocity. The variation of the normalized force in one pulsating time cycle is also analyzed. Then the laws of particle group distribution with transient time and vertical distance are obtained: the plug phenomenon is more obvious at the larger pulsating amplitude, further leading to pipe blockage, while a larger pulsating frequency is beneficial to the weakening of the plugs. Particle relaxation time is introduced to explain the changes in particle group distribution. Additionally, the temporal variations of flow fields are also used to illustrate the propagation of kinematic waves.

The Hertzsprung progression of classical Cepheids in the Gaia era

ABSTRACT A new fine grid of non-linear convective pulsation models for the so-called ‘bump Cepheids’ is presented to investigate the Hertzprung progression (HP) phenomenon shown by their light and radial pulsation velocity curves. The period corresponding to the centre of the HP is investigated as a function of various model assumptions, such as the efficiency of superadiabatic convection, the mass–luminosity relation, and the metal and helium abundances. The assumed mass–luminosity relation is found to significantly affect the phenomenon but variations in the chemical composition as well as in the stellar mass (at fixed mass–luminosity relation) also play a key role in determining the value of the HP centre period. Finally, the predictive capability of the presented theoretical scenario is tested against observed light curves of bump Cepheids in the ESA Gaia data base, also considering the variation of the pulsation amplitudes and of the Fourier parameters R21 and Φ21 with the pulsation period. A qualitative agreement between theory and observations is found for what concerns the evolution of the light curve morphology as the period moves across the HP centre, as well for the pattern in period–amplitude, period–R21, and period–Φ21 planes. A larger sample of observed Cepheids with accurate light curves and metallicities is required in order to derive more quantitative conclusions.

Experimental and numerical research on the coupled mechanisms of cavitation effect in a Venturi-type bubble generator

Venturi type bubble generator has a broad application in various fields and its bubble performance is closely related to the strong turbulent flow field. The interaction between bubble breakup and cavitation flow is not fully explained. In present work, the influence of cavitation on the suction performance and gas-phase transport pattern of a Venturi-type bubble generator is investigated by experiments and numerical simulations. The results show that the pressure ratio of 0.58 is the critical cavitation condition and the suction of the Venturi type bubble generator reaches the limiting value. The Venturi type bubble generator almost lost its suction capacity at the pressure ratio decreased to 0.27. However, the cavitation effect has a suppressive effect on the suction volume. The quasi-periodicity of the air inlet velocity pulsation and cavity development is highly consistent. Further research found that the blocking effect formed by return jets and pressure waves from cavity collapse is the main factor in reduced suction volume. The mechanisms of cavity shedding and collapse have a similar evolutionary model to the bubble breakup mechanisms in the Venturi-type generator.

Mechanisms of Characteristic Parameters of Plasma Actuator on Film Cooling and Turbulent Transport Based on Multi-Objective Optimization

In this paper, film cooling effectiveness and pressure loss coefficient are taken as the objective functions to optimize the plasma characteristic parameters such as electrode position, spacing, length and actuation intensity for enhancing film cooling performance. Regression equations of two objective functions are accessed by the Response Surface Method, and Non-Dominated Sorting Genetic Algorithm is applied for bi-objective optimization. Two sets of Pareto optimal solutions are available via the Technique for Order Preference by Similarity to an Ideal Solution decision method. Pareto optimal solutions are calculated by Detached Eddy Simulation to clarify the turbulent transport and heat transfer characteristics induced by the plasma. Wall-averaged cooling effectiveness and pressure loss coefficient without plasma power supply are 64.3% and 3.9% superior to that without electrode, respectively. The plasma fragments the hairpin vortex into a striped structure and induces a large scale streamwise vortex behind the bare electrode. The plasma weakens the velocity diffusion and turbulent heat transport in the ionization zone. Overlapping electrodes render the root-mean-square pulsation velocity and Reynolds stress of the optimal model 1 both less than those of the optimal model 2 without overlapping electrodes, and flow field disorder is decreased. Electrode position interferes with the action location of plasma on cooling jet. Actuation intensity affects the ionization intensity and has a maximum value of 8.88 to optimize the actuator performance. Compared with no plasma actuation, wall-averaged cooling effectiveness of optimal model 1 and optimal model 2 are enhanced by 30.7% and 21.8%, and pressure loss coefficient are declined by 7.7% and 7.1%, respectively.

Numerical study of flow and thermal characteristics of pulsed impinging jet on a dimpled surface

This research comprehensively investigates the flow and thermal characteristics of a pulsating impinging jet over a dimpled surface. It analyzes the impact of key parameters (e.g., inlet velocity pulsation functions, pulsation frequency, amplitude, dimple pitch, dimple depth, Reynolds number) on flow patterns and heat transfer. Validated computational fluid dynamics and the Re-normalization group turbulence model are employed to accurately simulate complex turbulent flow behavior. Local and average heat transfer coefficients are calculated and compared to steady impingement cases, revealing the potential benefits of pulsation for heat transfer enhancement. The study also examines how pulsation-induced flow modulation and thermal mixing affect heat transfer mechanisms. Results indicate that combining fluctuating flow with a dimpled surface can improve heat transfer rates. In summary, increasing pulsation amplitude consistently enhances heat transfer, while the effect of frequency varies between impinging and wall jet zones.

Parametric resonance of axially functionally graded pipes conveying pulsating fluid

Based on the generalized Hamilton’s principle, the nonlinear governing equation of an axially functionally graded (AFG) pipe is established. The non-trivial equilibrium configuration is superposed by the modal functions of a simply supported beam. Via the direct multi-scale method, the response and stability boundary to the pulsating fluid velocity are solved analytically and verified by the differential quadrature element method (DQEM). The influence of Young’s modulus gradient on the parametric resonance is investigated in the subcritical and supercritical regions. In general, the pipe in the supercritical region is more sensitive to the pulsating excitation. The nonlinearity changes from hard to soft, and the non-trivial equilibrium configuration introduces more frequency components to the vibration. Besides, the increasing Young’s modulus gradient improves the critical pulsating flow velocity of the parametric resonance, and further enhances the stability of the system. In addition, when the temperature increases along the axial direction, reducing the gradient parameter can enhance the response asymmetry. This work further complements the theoretical analysis of pipes conveying pulsating fluid.

Initial stage of formation of vortex flow in an inductor with counter-rotating magnetic fields

In this article, unsteady flows of electrically conductive fluid in a cylindrical cavity under the action of differently directed rotating magnetic fields are investigated. The relevance of the study is due to the presence of such a flow in the flow meter for liquid-metal heat carrier which is currently under development. The subject of study is the process of formation of the spin-up flow stage in a combined inductor of a rotating magnetic field. The acceleration or the spin-up stage is characterized by a significant change in the angular velocity of the liquid and begins with the formation of low pressure areas due to centrifugal forces. The alternation and movement of low and high pressure areas lead to velocity pulsations. For small values of the control parameter, which is the Taylor number, the flow energy gradually increases. In this case, the steady flow consists of two azimuthal and several poloidal vortices with clearly defined structures. When the Taylor number exceeds a critical value, strong pulsations occur in the flow, resulting in the decay of large-scale vortices. The intensity of the current is characterized by the Reynolds number. A power-law relationship with an exponent of 1.57 has been established between the Reynolds and Taylor numbers. It has been found that the time of formation of a steady flow varies from several seconds to tens of seconds, depending on the value of the Taylor number. The estimates of the control parameters have been obtained for a flowmeter operating in a typical mode. It has been shown that for the correct measurement of flow velocity in the flowmeter, it is necessary that that the Taylor numbers be higher than 108. The study is carried out using mathematical modeling and the obtained results are verified by experiment. The average calculated profiles are located within the confidence intervals of the experimental profiles.

Active control study in a flow in a model combustion chamber with LES

We conducted the mesh convergence study for an isothermal incompressible air flow in a model combustion chamber with Turbomeca swirler of D = 37 mm using Large-eddy simulations (LES). Simulation were conducted using 4 different meshes ranging from 4 to 24 × 106 cells at the Reynolds number Re = 15 632 with additional air injection of 4.4 % of the main volumetric flow rate from an circular slot. The results revealed that high spanwise slot resolution in the vicinity of the slot is crucial, particularly, for achieving convergence of radial velocity pulsations. Additionally, we analyzed the pressure signal and its spectral characteristics to demonstrate the effectiveness of the circular slot injection in reducing pressure pulsations associated with the precessing vortex core (PVC).

Novel selective adsorption cooling materials for heating cigarettes

Heated cigarettes are becoming trendy due to their advantage of not burning the tobacco material and thus reducing the amount of harmful ingredients released, while being able to leave the nicotine and tar content of the smoke untouched. However, the high temperature of the smoke as it enters the human mouth can affect the user experience. Therefore, the development of adsorption cooling materials for use in heated cigarettes is gradually becoming a hot topic. In this work, an adsorption cooling material based on embossed rod powder was prepared. It was found that compared with the commercial Yoga cooling material, the embossed rod powder based adsorption cooling material was able to reduce the maximum smoke temperature to 45.16 °C. In addition, it reduced the adsorption of nicotine by the cooling material, and increased the glycerol and nicotine content in the cooled smoke, which effectively promoted the taste of the smoke. Furthermore, the molecular simulation results found that the higher specific surface area and rich microscopic pore structure could increase the heat conduction area of the high-temperature flue gas, which enhanced the heat transfer effect of the flue gas. Finally, the periodic characteristic velocity pulsation destroyed the boundary layer formed by the fluid in the wall region, which increased the convective heat transfer effect between the fluid and the particles. The optimum sample 03 had the largest specific surface area and the lowest crystallinity, and thus had the best cooling effect. This work could provide some reference significance for the development of cooling materials for heating cigarettes.

An improved method combined modified drag model and modified erosion model based on LES for solid-liquid multiphase flow and erosion with application in a 90° elbow

In various industrial processes, solid-liquid multiphase flows involving continuous carrier and discrete particle phases have erosion challenges. In this work, a modified drag model that considers the influence of turbulence intensity and a modified erosion model that considers the effect of low-angle impact are developed. An improved method combined modified drag model and modified erosion model based on LES is proposed for numerical simulation of solid-liquid multiphase flow and erosion. The commercial software ICEM for grid generation and ANSYS FLUENT for LES are used in the present work. In validation test of velocity distribution of solid-liquid multiphase flow in a vertical pipe, the mean relative error of the liquid phase pulsation velocity is 10.01 % for modified drag model, whereas the mean relative error is 16.11 % for original drag model. In validation test of the maximum erosion depth from a submerged impact test at a tilt angle of 45°, the relative error is 0.23 % for modified erosion model, while the relative error is 13.39 % for original erosion model. Then, the improved method is employed to compute the characteristics of solid-liquid flow and erosion in a 90° elbow. Particles have a significant influence on liquid flow in the shear layer area where the inner arch separation zone interacts with the main flow, and this influence is most pronounced at the position of r* = 0.3 at yd = 0.4D. At this position, particles result in 15.27 % reduction in liquid velocity and 8.75 % increase in turbulence intensity. The mean relative error of the elbow erosion rate by using the improved method is 16.95 %, whereas it is 31.34 % of the reference work. The erosion is affected by impact velocity, angle, and particle number, and the most significant parameter is the particle number in the severe erosion area.

Seal vibrissa-based three-fourths open-jet wind tunnel nozzle optimization

This study investigated the effects of using bionics in reducing the low-frequency fluctuation phenomenon in three-fourths open-jet automobile wind tunnels. Low-frequency fluctuation in these wind tunnels can cause pulsations of velocity and pressure in the test section, leading to measurement accuracy reduction. The unique shape of the seal vibrissa had been demonstrated to effectively suppress vortex-induced vibrations. Here the wind tunnel nozzle was optimized with seal-vibrissa-shaped protrusions. The effects and corresponding mechanism of seal-vibrissa-shaped nozzle toward jet shear layer vortex structures and low-frequency fluctuation phenomenon were analyzed. A 1:15 scaled three-fourths open-jet wind tunnel was used in the study. The results indicated that after the implementation of the seal-vibrissa-shaped nozzle, the turbulence intensity was decreased at the bottom of the test section and the pressure and velocity fluctuations were effectively suppressed. The three-dimensional vortex structures separating from the surface of vibrissa destroyed the original two-dimensional vortex structures produced by the original nozzle. Many small vortex structures with dispersed frequencies and energies were formed, which decreased the low-frequency fluctuations. The study showed that the seal-vibrissa-shaped nozzle had the potential of inhibiting the large-scale vortex separation in the jet shear layer, which could effectively reduce the low-frequency fluctuations.

Orbit determination for the binary Cepheid V1344 Aql

Context. Binary Cepheids play an important role in the investigation of the calibration of the classical Cepheid period-luminosity relationship. Therefore, a thorough study of individual Cepheids belonging to binary systems is necessary. Aims. Our aim is to determine the orbit of the binary system V1344 Aql using newly observed and earlier published spectroscopic and photometric data. Methods. We collected new radial velocity observations using medium resolution (R ≈ 11 000 and R ⪅ 20 000) spectrographs, and we updated the pulsation period of the Cepheid based on available photometric observations using an O − C diagram. Separating the pulsational and orbital radial velocity variations for each observational season (year), we determined the orbital solution for the system using χ2 minimisation. Results. The updated pulsation period of the Cepheid estimated for the epoch of HJD 2458955.83 is 7.476826 days. We determined orbital elements for the first time in the literature. The orbital period of the system is about 34.6 yr, with an eccentricity of e = 0.22.

Numerical Simulation of Radiatively Driven Convection in a Small Ice-Covered Lake with a Lateral Pressure Gradient

The results of a numerical simulation of radiatively driven convection (RDC) in a small ice-covered lake with a lateral pressure gradient are shown. RDC influences aquatic ecosystems as convective flow transfers heat and dissolved and suspended matter through the water column. There is a hypothesis that a continuum of convective cells with areas of ascending and descending water flows exists in a convective mixed layer (CML). Until now, little has been known about how the structure of the CML changes in lakes with lateral transport. In this work, the evolution of the CML in the computational domain with a lateral pressure gradient over several days is reproduced using an Implicit Large Eddy Simulation. We show that after a few days of lateral pressure gradient occurrence, convective cells are replaced by rolls oriented along the lateral transport direction. The change in the CML’s turbulence patterns under a lateral pressure gradient is confirmed by Anisotropic Invariant Map analysis. The study revealed a heterogeneity of pulsations of the horizontal and vertical velocity components over the entire depth of the CML and showed that when a horizontal gradient is present, the velocity pulsations generally increase.

Phase change mechanism of periodic cavitation evolution in turbulent cavitation boundary layer

The instantaneous flow features of a partial cavity were investigated using a mass transfer cavitation model and large eddy simulations around a three-dimensional symmetric hydrofoil at Reynolds number Re=1×106, cavitation number σ = 0.43, and attack angle α = 0°. The phase change mechanism of periodic cavitation evolution, that is, energy conservation and transformation between turbulent fluctuation kinetic energy and water-vapor volume fraction ∝vapor, was identified in the turbulent cavitation boundary layer. The streamwise velocity Vx and pulsation Vx′,Vx′Vx′ play the dominant role in the cavitation evolution, whereas the spanwise and normal magnitudes and their pulsations are all less than 1–2 orders of magnitude. Because of the flow stratification phenomenon, the boundary layer is divided into the inner (below y+=40), transition (within a range of y+=50–80), and outer zones (beyond y+=90) in the normal y+ direction. Along all chordwise points, cavitation initiates entirely within the inner zone of y+=12–30, where Vx′Vx′ is strongest at the water saturation pressure. In the inner zone of y+=5–8 and the outer zone, cavitation initiation is delayed with a low ∝vapor. The expression between the instantaneous ∝vapor and Vx′Vx′ was improved by adding a correction term (γ*Sc) to validate the rationality of the phase change mechanism.

Pulsation curves strengthen the high gradient magnetic separation process Ⅱ: Consideration of separation performance

The use of pulsating slurry has been found to be effective in overcoming matrix blockage and improving separation selectivity in high gradient magnetic separators (HGMS). The velocity of the pulsating slurry is regulated by a sinusoidal cycle function, which causes the downward velocity to be much greater than the upward velocity due to the superposition of the sinusoidal pulsation velocity and the feeding velocity. However, the unbalanced velocity configuration in the up-downward stroke limits separation efficiency. This paper investigates the use of asymmetrical pulsation curves in a lab-scale HGMS to balance the velocity configuration and consequently improve separation performance. Magnetic field strength, feed velocity, and pulsating frequency were varied in separate experiments to determine the effect of pulsation curves on separation performance. The asymmetrical pulsation curve with a faster upward stroke and slower downward stroke was found to achieve a TiO2 concentrate grade of 23.94 %, a recovery of 66.78 %, and a separation efficiency of 50.46 % for the separation of ilmenite. When the concentrate grade is slightly higher, this new curve shows an increase of 5.68 % in recovery and 5.05 % in separation efficiency compared to the conventional sinusoidal pulsation curve. This improvement is attributed to a simultaneous strengthening of the material loose-adsorption process by separately regulating the pulsating upward and downward strokes.

Effect of pulsation parameters on the spatial and temporal variation of flow and heat transfer characteristics in liquid metal cross flow the in-line tube bundle

Liquid metal has a broad application prospect in tube bundle heat exchangers because of its excellent thermal conductivity. However, conventional in-line tube bundle arrangements often exhibit limited secondary flow, so new methods are urgently needed to improve heat transfer. The topic of this paper is to combine two methods, liquid metal cross flow tube bundle and pulsating flow, to improve the heat transfer performance in-line tube bundles. Firstly, the applicability of the model was validated through experimental data from existing literature, encompassing overall flow dynamics, heat transfer performance, and circumferential pressure distribution around the tubes. Subsequently, numerical simulations are conducted involving liquid metal cross flow in-line tube bundles at varying pulsating frequencies, amplitudes, and Reynolds numbers. Circumferential pressure distributions and temperature profiles for individual tubes were analyzed and compared. The amplitude of reduction in thermal resistance compared to the no-oscillation condition increases gradually from a minimum of 8 % in the first row to a maximum of 40 % in the third row, while the reduction in the rear row of tubes is similar to that of the third row. Remarkably, a reversal in the trend of thermal resistance for the first three rows of tubes under pulsating flow were observed, contrasting conventional flow except for the case with an amplitude of 0.1. By analyzing the local transient flow field distribution near single tube and the corresponding circumferential heat transfer performance, the influence of the vortex evolution characteristics on the heat transfer performance at different phases of the pulsating velocity is revealed, which transient Nu is up to three times that of flow with no pulsating. Finally, a series of coherence analyses reveal that as frequency rises, turbulence exhibits a richer spectrum of small-scale vortex structures, intricately linked to enhanced energy cascade efficiency. Amplitude augmentation intensifies velocity fluctuations, particularly at high amplitudes, resulting in substantial energy peak amplification. The integrated heat transfer performance PEC factor varies in the range of 1.25 to 1.51 for all the conditions simulated in this paper, which indicates that the liquid metal coupled pulsating flow approach can significantly increase the comprehensive performance of the tube bundle heat exchangers. These findings hold significant promise for advancing heat exchanger design and enhancing thermal efficiency, with implications for various engineering applications.

Parametric resonance for pipes conveying fluid in thermal environment

Parametric resonance is a kind of specific fluid-induced vibration for pipes conveying fluid. This paper focuses on revealing the qualitative characteristics of parametric resonances of the pipe with pulsating fluid speed in thermal environment, and compares the differences between the resonance characteristics in subcritical and supercritical regions. According to the generalized Hamilton's principle, the partial-differential-integral governing equation of a straight pipe is established. Under simply-supported boundary conditions, the non-trivial equilibrium configuration is obtained analytically. The governing equation of a supercritical pipe is derived by coordinate substitution on the basis of the new equilibrium configuration. The approximate analytical solution of parametric resonance for a pipe conveying fluid in a thermal environment is obtained by using the direct multi-scale method. The approximate analytical solution is verified to be reliable by using the Runge-Kutta method. The stability bounds of the parameters inducing parametric resonance are investigated via the introduced analytical method. The results show that the influence of temperature increment on sub-harmonic resonance is non-monotonic. In the subcritical region, when the temperature increment increases, the unstable region widens. This means that the system is more prone to parametric resonance, and the response amplitude decreases. In the supercritical region, when the temperature increment increases, the unstable bandwidth decreases, and the response amplitude increases. When viscous damping is increased or the average velocity is decreased, the sub-critical instability region decreases, and the supercritical instability region increases. With the increase in the pulsation velocity, the unstable bandwidth of subcritical and supercritical regions will increase, and parametric resonance is more likely to occur.

Numerical investigation on the evolution of cavitating dynamics and energy loss around a hydrofoil in fluoroketone under different temperatures

To investigate the effects of variable free-stream temperatures on cavitation evolution and entropy production in fluoroketone cavitation flows. The Zwart cavitation model with the thermodynamic effect and the modified RNG k-ε turbulent model with the compressibility of the gas-liquid mixed phase are used to run the simulation. Numerical results demonstrate that the intensity of cavitation decreases as free-stream temperatures rises, resulting in the cavitation thermal effect. Because cavitation shedding caused a negative pressure gradient, the main flows on the hydrofoil pressure surface at the trailing edge went back along the suction surface. The re-entrant at the trailing edge is caused by the interaction between returned flows and the process of cavitation growth. The process of cavitation interphase mass transfer alters the internal temperature field. In addition to the local temperature decrease induced by evaporation, condensation causes a local temperature increase. And, evaporation is more intense than condensation. The entropy production caused directly by temperatures is very small, and primarily caused indirectly by the velocity pulsation in the flow field. This study could help to comprehend the unsteady cavitation flow in fluoroketone while accounting for the thermodynamic effect, as well as the rational design of fluid machinery for transporting thermo-sensitive fluids.

Study on the combined influence of aerodynamic sweep and casing treatment in a transonic compressor rotor

Casing treatment and three-dimensional blade design are effective techniques to remedy the deficiency of compressor stability margin, yet their combined effects on compressor performance are seldom studied. With the help of URANS simulations, this paper explored the combined influence of aerodynamic sweep and casing treatment in a transonic compressor rotor. Compared with the configurations of single sweep and single casing treatment, the combined configuration presented an outstanding advantage for improving compressor performance. For compressor overall performance, the stall margin improvement of combined configuration was up to 11.8%, larger than single forward sweep and single casing treatment. Meanwhile, peak efficiency penalty of combined configuration (−1.02%) was significantly lower than that of single casing treatment. Under the effect of casing treatment, the results showed that the pulsating axial velocity could effectively delay the interface of the tip leakage flow and main flow, preventing the overflow of tip leakage flow. The distributions of blockage coefficient and loading coefficient indicated that combined configuration was more effective due to tip unload by casing treatment and blockage resistance ability by forward sweep. Furthermore, the spatial and temporal evolutions of axial velocity at slot opening surface were discussed in detail. Compared with the single casing treatment, higher positive axial velocity excitation and smaller range of negative axial velocity were introduced by combined configuration, which contributed to better stability enhancement.

Investigation of Harmonic Response in Non-Premixed Swirling Combustion to Low-Frequency Acoustic Excitations

The propagation mechanism of flow disturbance under acoustic excitations plays a crucial role in thermoacoustic instability, especially when considering the effect of non-premixed combustion on heat release due to reactant mixing and diffusion. This relationship leads to a complex coupling between the spatial distribution of the equivalence ratio and the propagation mechanism of flow disturbance. In the present study, the response of a methane-air non-premixed swirling flame to low-frequency acoustic excitations was investigated experimentally. By applying Proper Orthogonal Decomposition (POD) analysis to CH* chemiluminescence images, the harmonic flame response was revealed. Large Eddy Simulation (LES) was utilized to analyze the correlation between the vortex motion within the shear layers and the harmonic response under non-reacting conditions at excitation frequencies of 20 Hz, 50 Hz, and 150 Hz. The results showed that the harmonic flame response was mainly due to the harmonic velocity pulsations within the shear layers. The acoustically induced vortices within the shear layer exhibited motion patterns susceptible to harmonic interference, with spatial distribution characteristics closely related to the oscillation modes of the non-premixed combustion.