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The study of the interaction between crossflow and jets has been ongoing for many years, however, the fluctuating pressure generated by jets in crossflow(JICF) is still rarely studied. In this work, the wall pressure fluctuation characteristics of JICF with three jet angles are analyzed in detail by large eddy simulation(LES). It is found that adjusting the injection angle effectively reduces the intensity and range of the fluctuating pressure. In order to reveal the mechanism of this process, the differences in the source terms of the Poisson equation under three cases are studied, and a visualization method for characterizing the source terms of partial Poisson equation by convective velocity is proposed. By analyzing the proper orthogonal decomposition (POD) modes of velocity and pressure, the reasons for the difference in the source terms of the Poisson equation are further revealed, and the noise reduction mechanism of adjusting the jet angle is explained from the perspective of eddy current transport. The results show that the main influence area of the fluctuating pressure is from 2 times the aperture upstream to 20 times the aperture downstream. The low jet angle condition has a lower mean shear-turbulence interaction(MTI) and turbulence-turbulence interaction(TTI) terms, and the MTI1 sub-term is the main contribution component of the wall fluctuating pressure. The POD spatial model shows that the vertical hanging wake vortex structure establishes the pressure relationship between the flow field and the wall surface in the process of providing vorticity for the main stage hairpin vortex. Reducing the jet angle effectively reduces the eddy current transport between the wall and the flow field, so the low jet angle condition has a lower fluctuating pressure level. • Simulation of a jet in cross-flow with different injection angles. • The relationship between vortex motion and pressure fluctuation. • The decomposition of hydrodynamic and acoustic pressures in the near field of the jet in cross-flow. • Poisson equation source term analysis of wall pressure and spatial POD modal analysis of velocity and pressure fields. [ABSTRACT FROM AUTHOR]

The paper considers a new scheme for reducing pressure fluctuations on the gas injection line in the compressor section of a hybrid power machine, highlights the main control volumes for the compressor and pump sections. Based on the General laws of conservation of energy, mass, motion and the equation of state, a mathematical model for calculating the control volumes of the compressor section of constant and variable volumes is developed. Based on the use of the equation of continuity of motion and energy, taking into account the energy loss for friction and external heat transfer, a mathematical model of the gas flow in the interstage communications of the compressor section is developed. Based on the use of Hooke’s equations of mass, volume and motion, a mathematical model of the compressor section in the pump mode is developed. Considering the fluid flow in the connecting pipelines as quasi-stationary, their calculation is carried out on the basis of the Bernoulli equations. The developed mathematical model and schematic diagram can be used in the calculation of new efficient designs of both piston compressors and hybrid power machines.

The problem of pressure fluctuations in the thermal equilibrium state of some objects is discussed, its solution being suggested via generalizing the Bogoliubov–Zubarev theorem. This theorem relates the thermodynamic pressure with the Hamilton function and its derivatives describing the object in question. It is shown that unlike to other thermodynamic quantities (e.g., the energy or the volume) the pressure fluctuations are described not only by a purely thermodynamic quantity (namely, the corresponding thermodynamic susceptibility) but also by some non-thermodynamic quantities. The attempt is made to apply these results to the relativistic ideal gases, with some numerical results being valid for the limiting ultra-relativistic or high-temperature case.

The increase in freight traffic between central and northern European countries and the countries of the Black Sea coast requires an increase in shipping along the Danube River. The Danube is the seventh international transport corridor, which unites the countries of Europe and Asia. The deepening of the Bystrii estuary of the Kiliya's delta of the Danube ensures the implementation of the transport route between the Danube and the Black Sea. The protection of the Marine channel of the Danube-Black Sea deep-sea passage from sediment during storms is provided by a protective dam. The purpose of the research is to study the peculiarities of the wave action on the fragments of an existing protective dam, determine the effective means of its protection and develop recommendations for creating a dam design that is resistant to storm loads. The results of numerical and physical simulation of wave loads on a protective dam of the Marine channel of the Danube-Black Sea deep-water passage are presented. Experimental studies were carried out to study the features of the wave action on a fragment of the dam model. The integral and spectral characteristics of the pressure field on the streamlined surface of the dam model are obtained for the conditions of the passage of storm short and long waves. The features of the wave motion transformation near the dam and the characteristic regions of the frequency spectrum of the wave pressure on its surface are shown. Studies have been carried out to determine the sustainable structure of the dam and a step-like dam with two berms that can withstand storm loads is proposed. [ABSTRACT FROM AUTHOR]

Abstract The aim of this study is to characterize the steady and unsteady flows of a high‐speed axial fan stage and assess its aerodynamic performance parameters. The flow unsteadiness is related to the stator‐rotor interaction originated mainly by two mechanisms: the first is attributed to the potential effect and the second to the wake‐blade interaction and the mixing through the blades' passages. Such effects are dependent on the circumferential positions of blades and inter‐distance between stator and rotor, which could be alleviated by understanding these flow interactions. The pressure waves' main diametrical modes (two blades interacting with two vanes) and their sequences are predicted analytically. FFT analysis of the computed static pressure fluctuations at different monitor points allowed identifying the prevailing modes of interaction and their frequencies and sequence. The assessment of inter‐distance is an important step toward identifying the detrimental effects of axial interdistance on the aerodynamic performance. The closer the blade‐rows of this fan stage can be spaced, the more compact the cooling system of an automotive could be designed.

The aim of this study is to characterize the steady and unsteady flows of a high‐speed axial fan stage and assess its aerodynamic performance parameters. The flow unsteadiness is related to the stator‐rotor interaction originated mainly by two mechanisms: the first is attributed to the potential effect and the second to the wake‐blade interaction and the mixing through the blades' passages. Such effects are dependent on the circumferential positions of blades and inter‐distance between stator and rotor, which could be alleviated by understanding these flow interactions. The pressure waves' main diametrical modes (two blades interacting with two vanes) and their sequences are predicted analytically. FFT analysis of the computed static pressure fluctuations at different monitor points allowed identifying the prevailing modes of interaction and their frequencies and sequence. The assessment of inter‐distance is an important step toward identifying the detrimental effects of axial interdistance on the aerodynamic performance. The closer the blade‐rows of this fan stage can be spaced, the more compact the cooling system of an automotive could be designed. [ABSTRACT FROM AUTHOR]

Abstract The increasing demand for electric energy has pushed to rely on the microhydropower as one of the most cost‐effective technologies to improve the grid stability in rural localities. This article investigates the steady and unsteady flows in a complete small Kaplan turbine by means of the solver ANSYS‐CFX. Examination of the basic design parameters in terms of blade setting, vane opening, and interdistance has revealed the best operating parameters and the potentiality of performance improvement. Moreover, the Fast Fourier Transform of static pressure fluctuations recorded at different monitor points allowed analyzing the flow unsteadiness arising from the stator‐rotor interactions (RSI). The prevailing harmonics coinciding with the modes of RSI were determined and compared with the analytically predicted main diametrical modes and frequencies of the pressure waves. The found results in this category of low‐head small Kaplan turbine may serve in developing similar hydroturbines suitable for the microhydropower.

This study was initiated to investigate the use of a porous structure in some parts of a pipeline to protect it against water hammer pressure wave. This study was inspired by the utilization of porous breakwaters which are commonly used to protect coastal areas. To fulfill the main objective of the study, different porous structures were installed in an experimental pipeline fed by a pump. The water hammer was induced by an upstream valve. The homogeneous porous structures were made up of spherical beads or broken rocks in different sizes and filled them in pipes with different lengths and subject them under different water hammer conditions. Results obtained from this study showed that during the first cycle of water hammer wave, the ratio of maximum pressure downstream of porous structure to the corresponding value in upstream is between 0.6 to 0.95, respectively. This pressure difference decreased drastically in next cycles. Generally, it can be conclude that an insignificant protection gained from a porous structure in downstream from water hammer; however, the overall pressure fluctuations in the pipe filled with a porous structure was less than a fully open pipe under the same discharge rates and the same water hammer condition. The results showed that porous structures in particular condition could be used to control pressure oscillations in piplines and it is proposed to consider practical aspects of this idea in future researches.

• Coherent flow structures inside cyclone separators were investigated numerically. • Simulated fluctuation frequencies closely match experimental data. • Non-dimensional frequencies (Strouhal numbers) weakly depend on the Reynolds number. • Complex vortex core motion was interpreted in terms of velocity fluctuation spectra. We investigate the three-dimensional, time-dependent behaviour of coherent flow structures inside two cyclone separator geometries, a Stairmand cyclone and a swirl tube, operating in the turbulent regime. For this, large-eddy simulations (LES) with a standard Smagorinsky subgrid-scale model are carried out on the single-phase flow employing a non-orthogonal curvilinear hexahedral mesh. Reynolds numbers (Re) are in the range 9·104–4·105. The simulations successfully capture the quasi-periodic precessing vortex core (PVC) phenomenon. The spectra derived from the temporal signals of different flow variables viz. static pressure, tangential velocity, axial velocity and radial velocity in the Stairmand cyclone exhibit two distinct frequencies at all Re , whereas the spectra for the swirl tube show a single peak. These spectra strongly resemble those reported in the experimental literature. The periodicity pertaining to the inner vortex about its rotational axis is also analyzed. Next, the effect of Re on the two distinct spectral peaks have been evaluated for the Stairmand cyclone. Lastly, we present the effect of Re on mean and fluctuating fields and their variations along the geometrical axis of the cyclone. [ABSTRACT FROM AUTHOR]

• Origin of pressure fluctuations of a single bubble from formation to eruption is discussed. • Effects of density, size of particles, and injection velocity on pressure profile are investigated. • Maximum amplitude of bubble formation is related to the injection velocity and density of particles. • Amplitude of bubble eruption is related to the bubble diameter, density and size of particles. In this work, to investigate the source of pressure fluctuations, behavior of a single bubble in a two-dimensional gas–solid fluidized bed was studied. Pressure sensors located at different heights of the bed measured presure fluctuations, and simultaneously a high speed camera was used to pursue all steps from formation to eruption of bubbles. Two types of particles were applied with different sizes and densities. Experiments showed that the maximum amplitude of formation was independent of the bubble diameter. But, it depended on density of particles, velocity of injection and the distance from bed surface. When injection stopped, there was a minimum in pressure profile related to the higher dense phase voidage for a higher superficial gas velocity after injection. Also, the maximum pressure fluctuation of bubble eruptions was related to the bubble diameter, density and size of particles. It was concluded that pressure fluctuations of formation, passing and eruption of bubbles in fluidized beds are originated due to changes in dense phase voidage, bed voidage and movement of particles during bubble eruption. [ABSTRACT FROM AUTHOR]

A new parameter (degree of randomness (DR)) was defined for the identification of the main transition velocities, Utrans. The new method reconstructs the time series into multiple state vectors, thus generating non‐overlapping vector pairs and then compares the distance between them with a pre‐selected cut‐off length. The DR values were extracted from gauge and differential pressure fluctuations as well as x‐ray tomographic scans. At every Utrans value, the DR index exhibited a well‐pronounced local minimum. Three cylindrical bubble columns (BCs) with various diameters (0.1, 0.14, and 0.45 m in ID) and one rectangular BC (width = 0.2 m, depth = 0.04 m) were used. They were aerated by means of different perforated plate gas distributors. It was found that in the cylindrical BCs the disintegration of the bubbly flow regime took place always at Utrans = 0.04 m/s. In the case of the rectangular BC the first critical velocity appeared at Utrans = 0.012 m/s. The lower boundary of the churn‐turbulent regime was identified at Utrans = 0.11 m/s in the smallest cylindrical BC and at about Utrans = 0.095 m/s in the other two cylindrical BCs. In the case of the rectangular BC, the second critical velocity was identified at Utrans = 0.039 m/s. The low Utrans in the rectangular BC imply that the hydrodynamic regimes are less stable in this particular column due to higher degree of liquid turbulence. The calculated DR values from the gauge pressure fluctuations successfully distinguished the upper boundary of the gas maldistribution and the first transition sub‐regime. [ABSTRACT FROM AUTHOR]

The runaway after pump-trip is a fast transient process, during which unit vibrations, water-hammer fluctuations, pressure pulsation changes and flow pattern transitions happen violently, endangering pump-turbine units of pumped-storage power stations. In this paper, based on validated CFD data from 1D-3D coupled simulation, we describe the evolutions of flow patterns and pressure fluctuations in a prototype pump-turbine during this working point fast sliding process, and try to reveal the influence mechanism of flow pattern transitions on pressure fluctuation changes. The results show that the general evolutions of flow patterns are dominated by the changes of the runner velocity triangles, which are controlled by the changes of the discharge and rotational speed. Inhomogeneous local flow also contributes to abundant local vortex structures, such as rotating-stall vortices in the vane and runner regions, skirt and helical vortices in the draft-tube, separation vortices in the blade channels, and the impinging jets on the blade surfaces. These unstable vortices appear, change, and vanish in a fast transitional and uneven manner, leading to great differences in pressure and runner force fluctuations in different working regions. • Evolutions of flow patterns and pressure fluctuations during runaway process were described. • Evolutions of flow patterns are dominated by changes of velocity triangles. • Inhomogeneous local flow contributes to vortex structures. • Different vortices lead to great differences in pressure fluctuations in different working regions. [ABSTRACT FROM AUTHOR]

The recent advances in centrifugal pump design do not only require a better suction performance but also there have been attempts to reduce design time at a lower cost. The traditional trial-and-error optimization design method, however, depends on the designer's experience, which requires longer cycles. This is because the computational process of calculating the net positive suction head required (NPSHr) involves several calculation steps and this consumes a lot of computational time. An investigation was therefore carried out to test a novel NPSHr prediction method in a double-suction centrifugal pump using unsteady numerical simulations. In the new approach, a new boundary pair was introduced and an algorithm was used to estimate a good value for a static pressure value that correlates to a 3% drop in pump head to determine the critical cavitation point. Experiments were conducted to validate the hydraulic performance and the cavitation model. The NPSHr and the characteristic "sudden" head-drop were very well predicted by the novel approach in only three simulation steps. The internal flow analysis showed that for 0.6 Q d , the flow around the volute tongue was uneven at NPSH = 10.06 m, inducing flow separation and recirculation at the tongue region. Attached cavities were also observed around the suction ring in the spiral suction domain. The pressure fluctuations were analyzed also and the dominant frequency at the pump outlet and tongue region was the blade passing frequency. Consequently, the novel approach proved very robust and efficient in NPSHr prediction and would be a good alternative to shorten simulation time during cavitation optimization design process in centrifugal pumps. [ABSTRACT FROM AUTHOR]

Fluidized beds are of great importance in various industries and presence of bubbles significantly influence their efficiency. In order to gain a more accurate vision on the dynamics of a single bubble in fluidized beds, formation, rising, and eruption of a single bubble in a rectangular fluidized bed were studied by CFD-DEM and validated by experimental data. Single bubble in a 2-D rectangular fluidized bed at different stages, from forming to erupting, was modeled by the CFD-DEM technique. Investigating the pressure fluctuations and bubble dynamics were the main focus of this study. Pressure fluctuations were measured using a pressure transducer and a high-speed digital camera was used to record the whole bubbling process, from bubble formation on the distributor to its eruption at the surface. Validation was performed by comparing experimental and simulated bubble diameter and pressure fluctuations. CFD-DEM results such as particles and gas velocity vectors, bed voidage and pressure contours revealed new information on characteristics of pressure fluctuations which was not possible to gain by experimental studies. In the formation stage, due to the compression wave from injection, particles accelerate and bed reaches to a packed state. This causes a pressure peak in the pressure. At the beginning of bubble formation on the distributor, pressure decreases and when the bubble detaches from the distributor, pressure reaches a minimum. While bubble is rising through the bed, pressure increases again, however, it remains below zero because of the lower voidage behind the bubble in comparison to pre-injection state and the greater velocity of the gas above and below the bubble. By bubble eruption, another small compression wave is seen in the pressure. Unlabelled Image • At bubble introduction into the bed, before bed height elevation, a maximum in pressure occurs. • Ending the injection and bubble detachment make a minimum in the pressure signal. • By bubble rising, the voidage behind the bubble is larger than pre-injection state. • Two vortices at bubble sides get larger by its rising, and particles start to fall from their border. • Bubble eruption produces another small compression wave that travels downward. [ABSTRACT FROM AUTHOR]

Vortex ropes occurred in the draft tube when hydraulic turbines operating at off-design conditions, which can generate pressure fluctuations. Cavitation vortex rope is one of the most harmful factors to the safety of hydroturbines which may induce further pressure fluctuations. A study of the multiphase flow in a model turbine is presented in this paper using the software ANSYS CFX. The main emphasis is spending on understanding the mechanism of cavitation evolution and underlying its correlation mechanism with flow instabilities. The results indicate that two types of pressure variations can be captured with a cavitation rope: 1) The type due to the vortex rope rotating whose frequency is 1/5–1/4 times runner rotating frequency; 2) The type due to cavitation volume surge, whose frequency is less than that of vortex rope rotating. The frequency of pressure fluctuation due to cavitation keep contanst as the cavitation number decreases, while the amplitude much increases. While the frequency and amplitude of the pressure variation caused by votex rope rotating increase a little. Based on vorticity transport equation, the vortex and cavitation have a close relation. Cavitation increases the vortex production as well as the pressure fluctuation frequency caused by the rotation of vortex rope. • A new multiphase flow simulation method was proposed and proved in the investigation of hydro turbine. • The evolution mechanism of the cavitation rope behavior was reflected based on the vorticity transport equation. • The correlation mechanism between cavitation rope evolution and several pressure fluctuations were revealed. [ABSTRACT FROM AUTHOR]

The EDF aims to identify what causes fuel assembly vibrations in pressurized water reactors. The present work focuses on the validation of pressure fluctuations along the central rod of a 5 × 5 configuration for wall-modeled large eddy simulations. New experiments, called CALIFS, have been carried out by the Atomic Energy Commission (CEA) on a 5 × 5 mixing vane grid (MVG) in the framework of the Fuel Assembly EDF/CEA/FRAMATOME tripartite project. In addition to pressure drop and velocity measurements using particle image velocimetry, pressure measurements have been performed along the central rod. The computational domain is representative of a span of the experimental mock-up composed of a 5 × 5 rod bundle equipped with a split-type MVG. The hydraulic Reynolds number is equal to 66 000 and periodic boundary conditions are imposed in the streamwise direction. The mesh is fully hexahedral and conformal. Computations give very satisfactory results for the pressure drop, the mean velocity, and the Reynolds stresses at different locations. The root-mean-square of the pressure along the central rod is also compared to experimental data at different heights. The behavior is in very good agreement up to five hydraulic diameters downstream of the MVG. [ABSTRACT FROM AUTHOR]

This paper presents the experiments performed on a high head model Francis turbine during start-up. Synchronized time dependent pressure and velocity measurements were performed to investigate the instabilities in the turbine. A total of four steady state operating points, namely synchronous load, part load, best efficiency point, and high load are considered to perform the turbine start-up. The runner angular speed was observed to increase almost exponentially during the guide vane positions from completely closed to no load condition. The frequency of wave propagation due to the interaction between runner blades and guide vanes was observed to follow the trend of increase of runner angular speed. A vortex rope frequency was captured in the draft tube during synchronous load to part load of the start-up. Two different mechanisms, namely, the development of stagnation point and the available recirculation regions were observed to cause the formation of vortex rope in the draft tube. [ABSTRACT FROM AUTHOR]

In this paper, the transient turbulent cavitating flow around a marine propeller behind a ship was investigated experimentally with emphasis on how vortex generator (VG) influences propeller cavitation and hull pressure fluctuations. The experiments were carried out in China Large Cavitation Channel with the closed test section covering 10.5 m in length and cross-section of 2.2 m2 ϗ 2.0 m2. The experiment recorded instantaneous cavitation photos of the propeller and pressure fluctuations on the hull surface. The results demonstrate that without vortex generator, the sheet cavitation inception begins at −50° (310°), and then from −50° (310°) to 30°, the extent and area of sheet cavitation increase. When vortex generator installed on the hull, the sheet cavitation inception occurs in advance at −60° (300°), and the sheet cavitation expands to larger rotation range due to the vortex generator. It is shown that the vortex generator with proper geometry and installed location may contribute the highly nonuniform wake to be more uniform, and can make the change of propeller cavitation to be milder, which decrease the pressure fluctuations in this study. Further analysis indicates that the vortex generator may promote the energy distribution to be more uniform in physical and spectral space in some degree. [ABSTRACT FROM AUTHOR]

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Pressure fluctuations that cause acoustic radiation from vessel models with concentric and eccentric blunt stenoses are investigated. Large eddy simulations of non-pulsatile flow condition are performed using OpenFOAM. Calculated amplitude and spatial-spectral distribution of acoustic pressures at the post-stenotic region are compared with previous experimental and theoretical results. It is found that increasing the Reynolds number does not change the location of the maximum root mean square wall pressure, but causes a general increase in the spectrum level, although the change in the shape of the spectrum is not significant. On the contrary, compared to the concentric model at the same Reynolds number, eccentricity leads to an increase both at the distance of the location of the maximum root mean square wall pressure from the stenosis exit and the spectrum level. This effect becomes more distinct when radial eccentricity of the stenosis increases. Both the flow rate and the eccentricity of the stenosis shape are evaluated to be clinically important parameters in diagnosing stenosis. [ABSTRACT FROM AUTHOR]

The dynamic behavior of a reversible pump turbine (RPT) during the load rejection process was investigated through numerical simulation and experimental testing. The study focused on various parameters, such as the operation of guide vanes, torque, flow, head, and runner forces. The pressure fluctuations at different monitoring points were analyzed using the time-frequency method, specifically the continuous wavelet transforms (CWT). The results revealed significant pressure pulsation amplitudes in the vaneless region. Additionally, the entropy production rate was employed to quantify energy loss in different domains, including direct dissipation, turbulent dissipation, and wall shear stress. Moreover, the study utilized the vorticity transport equation to examine vortex characteristics and transport in the stay vanes, guide vanes, and runner flow channels. The vortex characteristics were classified into relative vortex stretching (RVS), Coriolis force (CORF), and viscous diffusion (VISD). The findings highlight significant energy loss in flow patterns associated with high RVS and CORF regions. This study provides valuable insights into the transition process and energy loss of RPTs during load rejection, serving as a basis for future research aiming to enhance the safety and reliability of RPTs under load rejection conditions. • Analyzing the time frequency components and dynamic behavior of a reversible pump turbine during load rejection process. • Quantifying the energy loss composition in a reversible pump turbine by entropy production rate. • Explaining the connection among relative vortex stretching, Coriolis force, and viscous diffusion components. [ABSTRACT FROM AUTHOR]

Gas-solid fluidized bed reactors are widely used in industrial operations and thus have led to a significant amount of fundamental research over decades. For better design and operation of fluidized bed systems, it is essential to understand the fluidization regime and the related transition conditions. Dr. John R. Grace, a prominent figure and pioneer in fluidization science and technology, has made significant contributions to understanding fluidized bed systems, including fluidization regimes and their transitions. This study is a tribute to his extraordinary legacy, focusing on investigations of fluidization regimes and transitions in a laboratory-scale gas-fluidized bed operating from ambient to 1600 °C. The results indicate that the fluidization regime transitions follow three distinct pathways across different temperature ranges: below 300 °C, from 300 to 1400 °C, and above 1400 °C. These distinctive transitions arise as a result of the varying significances of hydrodynamic and interparticle forces in the fluidized bed of particles investigated. [Display omitted] • Fluidized beds can operate in bubbling with dispersed or aggregated particles, slugging, and turbulent regimes. • Flow regime transitions follow three distinct paths at low, moderate, and ultra-high temperatures. • Interparticle forces at high temperatures can transform the bed from slugging to bubbling. • Ultra-high temperatures can reverse the temperature dependence of fluidization behavior. • No current regime transition correlations are adequate for fluidized beds operating at various temperatures. [ABSTRACT FROM AUTHOR]

This paper examines the effects on a Clark-y three-dimensional hydrofoil of wavy leading-edge protuberances in a quantitative and qualitative way. The simulation is accompanied by a hybrid RANS-LES model in conjunction with Zwart-Gerber–Belamri model. Detailed discussions of the stable no-cavitating, unsteady cavitating flow fields and the control mechanics are involved. The force characteristics, complicated flow behaviors, cavitation–streamwise vortex interactions, and the cavitating flow instability are all presented. The results demonstrate that protuberances acting as vortex generators produce a continuous influx of boundary-layer vorticity, significantly enhancing the momentum transfer of streamwise vortices and therefore improving the hydrodynamics of the hydrofoil. Significant interactions are described, including the encouragement impact of cavitation evolution on the fragmentation of streamwise vorticities as well as the compartmentation effect of streamwise vorticities binding the cavitation inception inside the troughs. The variations in cavitation pressure are mainly due to the acceleration in steam volume. In summary, it is vital for new hydrofoils or propeller designs to understand in depth the effects of leading-edge protuberances on flow control.

This paper proposes an original approach to investigate the influence of the geometry of Francis turbines draft tube on pressure fluctuations and energy losses in off-design conditions. It is based on Design of Experiments (DOE) of the draft tube geometry and steady/unsteady Computational Fluid Dynamics (CFD) simulations of the draft tube internal flow. The test case is a Francis turbine unit of specific speed Ns=120 m-kW which is required to operate continuously in off-design conditions, either with 45% (part-load) or 110% (full-load) of the design flow rate. Nine different draft tube geometries featuring a different set of geometrical parameters are first defined by an orthogonal array-based DOE approach. For each of them, unsteady and steady CFD simulations of the internal flow from guide vane to draft tube outlet are performed at part-load and full-load conditions, respectively. The influence of each geometrical parameter on both the flow instability and resulting pressure pulsations, as well as on energy losses in the draft tube, are investigated by applying an Analysis of Means (ANOM) to the numerical results. The whole methodology enables the identification of a set of geometrical parameters minimizing the pressure fluctuations occurring in part-load conditions as well as the energy losses in both full-load and part-load conditions while maintaining the requested pressure recovery. Finally, the results of the CFD simulations with the final draft tube geometry are compared with the results estimated by the ANOM, which demonstrates that the proposed methodology also enables a rough preliminary estimation of the draft tube losses and pressure fluctuations amplitude.

Background. Numerical simulation and experimental research have been used as powerful tools to understand and predict the behavior and mechanics of the operation of natural heart valves and their prostheses in natural and patho­logical conditions. Such studies help to evaluate the effectiveness of the valves, their design and the results of surgical procedures, to diagnose healthy and impaired function of the heart valves. There is an actual problem in creating more reliable methods and tools for the operation diagnostics of mechanical heart valves. Objective. The aim of the research is to investigate the effect of fluid viscosity on the hydroacoustic characteristics of jets that flow from a semi-closed and open mechanical bileaflet heart valve. To study the possibility of using hydro­acoustic measuring instruments as diagnostic equipment for determining the working conditions of the bileaflet pro­sthe­tic heart valve. Methods. The experimental research was carried out by means of hydroacoustic measurements of the hydrodynamic noise in the near wake of the side and central jets of the glycerin solution and the pure water flow downstream of the prosthetic bileaflet heart valve. Results. The effect of fluid viscosity on the hydroacoustic characteristics of the jets that flow from a semi-closed and open mechanical bileaflet heart valve has been experimentally determined. Integral and spectral characteristics of the hydrodynamic noise of jets of the glycerin solution and the pure water flow downstream of the bileaflet mitral heart valve for different fluid rate were detected. Conclusions. In the stream conditions of pure water, the integral characteristics of the pressure field are lower than in stream conditions of the aqueous glycerin solution. As the glycerin concentration in the solution increases, increase average pressures and especially RMS pressure fluctuations. The spectral levels of the hydrodynamic noise in the near wake of the side jet of the glycerin solution are lower than for water flow in the frequency ranges from 1 to 7-8 Hz and from 100 to 1000 Hz for fluid rate 5 l/min. For higher fluid rates, the spectral components of the hydrodynamic noise in the near wake of the side jet of the glycerin solution of the semi-closed mitral valve are higher than that for the pure water. The greatest difference (1.5–1.8 times) in the spectral levels is observed in the frequency range from 10 to 100 Hz for the fluid rate 15 l/min.

The stability of pump-turbines is of great importance to the operation of pumped storage power (PSP) stations. Both hydraulic instabilities and operational instabilities have been reported in PSP stations in China. In order to provide a reference to the engineers and scientists working on pump-turbines, this paper summarizes the hydraulic instabilities and performance characteristics that promote the operational instabilities encountered in pump-turbine operations in China. Definitions, analytical methods, numerical and experimental studies, and main results are clarified. Precautions and countermeasures are also provided based on a literature review. The gaps between present studies and the need for engineering practice are pointed out.

In this work, the feasibility of flow pattern and oil hold up the prediction for vertical upward oil–water two–phase flow using pressure fluctuation signals was experimentally investigated. Water and diesel fuel were selected as immiscible liquids. Oil hold up was measured by Quick Closing Valve (QCV) technique, and five flow patterns were identified using high-speed photography through a transparent test section with Inner Diameter (ID) of 0.0254 m. The observed flow patterns were Dispersed Oil in Water (D O/W), Dispersed Water in Oil (D W/O), Transition Flow (TF), Very FineDispersed Oil in Water (VFD O/W) and a new flow pattern called Dispersed Oil Slug & Water in Water (D OS& W/W). The pressure fluctuation signals were also measured by a static pressure sensor and decomposed at five levels using wavelet transform. Then, standard deviation values of decomposition levels were used as input parameters of a Probabilistic Neural Network (PNN) to train the network for predicting the flow patterns. In addition, some considered numerical values for actual flow patterns together with the signal energy value of each level were used as input parameters of a MultiLayer Perceptron (MLP) network to estimate the oil holdup. The results indicated good accuracy for recognition of the flow patterns (accuracy of 100% and 95.8% for training data and testing data, respectively) and oil holdup (AAPE=9.6%, R=0.984 for training data and AAPE=8.07%, R=0.99 for testing data).

Online detection of hydrodynamics of gas-solid fluidized bed was characterized using pressure fluctuations by cross recurrence plot (CRP) and cross recurrence quantification analysis (CRQA). Experiments were conducted in a lab scale fluidized bed of various particle sizes 150 μm, 280 μm and 490 μm at different gas velocities. Firstly, pattern changes of cross recurrence plot were discussed and then reference states was selected. Afterwards, cross average diagonal line (CLave) of other states corresponding to reference states were obtained. It was found that cross average diagonal line of non-normalized data initially decreases and then increases with increasing the gas velocity. When the signal is initially normalized, cross average diagonal line does not change with the superficial gas velocity. It was concluded that cross average diagonal line could be used for detecting small changes of particle size and if a proper reference state is chosen, it can be perceived as a powerful index for detecting changes in the size of particles in a fluidized bed.

The article presents the results of experimental studies of pressure fluctuations in the pipes of piston pumps. The relevance of these studies is due to the need to reduce the pressure fluctuations that create a positive displacement pumps, due to their negative impact not only on the reliability of the pump, but the accuracy of flow measurement and pressure of the medium supplied to such pumps. Aim: The aim of this study was to investigate the hydraulic characteristics of pulsating flows in pressure lines piston pumps of liquefied gas, as well as the study of the effectiveness of the dampers of pressure pulsations and conformity of their parameters to the calculation. Materials and Methods: As a drive used the piston pumps − single-line and trilinear. In the tests recorded pressure fluctuations in the pressure line. For this purpose the low-inertia pressure sensors 15.0 MPa working complete with strain test station, which allows registering the pressure fluctuations at frequencies up to 10 kHz. Strain test station output signal fed to the input of the oscilloscope operating in memory mode. In all tests the pressure sensor is mounted at three points - after the piston group on the pressure line before the damping device and after it. In the experiment, three different damper was used - two new, designed by the author's method and one regular damper, which are equipped with serial piston pumps. Dampers installed vertically, the flow entering to the lower cap, and an output through the side surface. Results: The experimental results confirmed the general position of the greater efficiency of complex composite filters and the correctness of chosen method of their calculation, proposed earlier by the authors. In particular, the actual level of weakening of pressure fluctuations on developed damper with a high degree coincided with the calculated results.

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Abstract In order to study pump turbine stability in view of flow induced noise, this paper focuses on two-part operation of one high head reversible turbine: transient load rejection process and intermittent vane closing conditions. Both above simulations are solved by DES turbulent model, additionally, the flow field results are regarded as sound source and analyzed comparatively by LMS software. Except for flow field interpretation, some developing rules of flow induced noise during the period of pump turbine load rejection are found. The result indicates that flow induced noise radiation is consistent with internal fluid characteristics, and the developing trends are similar which acquired from these two part calculations, whereas the values are distinct. This work provides a reference for the study of pump turbine operating stability from the acoustic aspect, and for pump turbine design work as well. Highlights • Transient results show that rotor-rotor interaction influence on flow field is stronger than that of rotor-stator interaction during the continuous load rejection period. • The flow induced noise radiation is consistent with fluid characteristics during the period of pump turbine load rejection. Its radiation level is determined by the pressure fluctuating frequency and shell natural frequency. • The shape of sound distribution likes ∞ and sound level in different directions and faces are symmetrical. • PSD at homologous locations of transient load rejection is with one more magnitude than that of fixed vane conditions. • Both fixed vane calculations and continuous vane closure obtained similar flow induced noise developing tendency, even the acoustic pressure levels are different. [ABSTRACT FROM AUTHOR]

• Hydrodynamics of gas- solid semi-cylindrical bed was investigated. • Minimum fluidization velocity is not affected by the cross section of bed. • No significant difference exists between hydrodynamics of semi-cylindrical and cylindrical beds. • Number of large bubbles increases faster in the semi-cylindrical bed in a bed of coarser particles. Hydrodynamic characteristics of a gas-solid semi-cylindrical fluidized bed was experimentally investigated and compared with that of a cylindrical bed by analysis of pressure fluctuations. Pressure fluctuations were analyzed in time and frequency domains using standard deviation, power spectral density function and discrete wavelet transform methods. Experiments were carried out in two semi-cylindrical and cylindrical fluidized beds of 14 cm in diameter each, operating in the bubbling fluidization regime at ambient pressure and temperature. Both beds were filled with glass beads of various sizes (120, 290 and 450 µm). The superficial gas velocity was varied in the range of 0.2–0.8 m/s. Results showed that although the minimum fluidization velocity is influenced by the particle size, it is not affected by the geometry of the bed. It was shown that the hydrodynamics of both beds are very similar and the difference is negligible. Number of large bubbles is slightly larger in the semi-cylindrical bed as compared with the cylindrical bed. Also, increase in the particle size and superficial gas velocity result in a greater difference between the number of large bubbles in both beds and the number of large bubbles in the semi-cylindrical bed increases slightly faster than in the cylindrical bed. [ABSTRACT FROM AUTHOR]

In centrifugal pumps, it is important to select appropriate turbulence model for the numerical simulation in order to obtain reliable and accurate results. In this work, ten turbulence models in 3-D transient simulation for the centrifugal pump are chosen and compared. The pump performance is validated with experimental results. The numerical results reveal that the SST turbulence model was closer to the experimental results in predicting head. In addition, the pressure variation trend for the ten models is very similar which increases and then decreases from the inlet to outlet of the pump along the streamline. The SST k-ω model predicts the performance of the pump was more accurately than other turbulent models. Furthermore, the results also found that the error is the least at design operation condition 300(l/min), which is around 1.98 % for the SST model and 2.14 % and 2.38 % for the LES and transition omega model. Within 7.61 %, the errors at higher flow rate 350(l/min) for SST. The error for SST model is smaller as compared to different turbulent models. For the Realizable k-ɛ model, the errors fluctuate were more high than other models. [ABSTRACT FROM AUTHOR]

Highlights • The dynamics of disturbed droplet pinch-off depends on initial perturbations. • Pressure-fluctuation-induced nonlocal flow velocity is responsible for the variations in disturbed pinch-off dynamics. • A dimensionless perturbation parameter was derivated to characterize the disturbed pinch-off. • We applied the pressure fluctuation to engineering pinch-off of droplets with highly predictable dynamics. Abstract Dynamics of Newtonian fluid pinch-off is universal, excluding the possibility of manipulating pinch-off behavior by varying initial and boundary conditions which is desirable in applications such as inkjet printing and microfluidics. Here we show that the dynamics of two-fluid pinch-off with disturbed inlet pressure (such as the profile of the conical liquid neck, cone slope, and neck thinning rate) depends on initial perturbations. The nonuniversality arises from pressure-fluctuation-induced nonlocal flow velocity that stretches the axial length scale of the pinch-off region. We renormalize the disturbed pinch-off using the linear ratio, (1 + βε), with β being the empirical constant and ε the dimensionless pressure fluctuation. We further apply the pressure fluctuation in engineering pinch-off where the disturbed and undisturbed systems have identical pinch-off dynamics but distinct material properties. Our results could provide useful guidelines for controlling the breakup of liquid threads with extreme physical properties, such as ultrahigh viscosity and ultralow interfacial tension, in inkjet printing and microfluidics for a range of applications. [ABSTRACT FROM AUTHOR]

Highlights • LES simulations are conducted to study wastewater treatment via pump-turbine. • Sufficient amount of dissolved oxygen is obtained for wastewater treatment. • Peripheral aeration is much more efficient for oxygen dissolution. • Aeration results in a minor penalty on the power generation. • Flow induced vibration is reduced significantly with all aeration techniques. Abstract Large eddy simulations of a ventilated pump-turbine were conducted using LES-mixture model. Peripheral and central aeration were applied to study the effectiveness of oxygen dissolution and the pump-turbine performance. The peripheral draft tube aeration was performed by air injection through (1) a continuous orifice and (2) a series of discrete orifices along the surface of the draft tube. The simulations were performed when the system is operating in the turbine mode. Peripheral aeration was more effective for the achieved dissolved oxygen level and the oxygen dissolution efficiency. The mean dissolved oxygen concentration and the dissolution efficiency inside the draft tube were predicted to be 1.8 mg/l and 80% using the continuous or discrete peripheral aeration and 1.4 mg/l and 25% using the central aeration. A mean value ranging from 1.0 mg/l to 2.0 mg/l was considered to be sufficient for aerobic bacteria to treat the wastewater. Aeration resulted in a minor penalty on the power generation, while it provided a significant reduction in the flow-induced vibration. The central aeration was more effective in reducing the amplitude of pressure fluctuations and yielded more stable turbine operating conditions. [ABSTRACT FROM AUTHOR]

Fluid-flow and pressure fluctuation are main causes of vibration and noise in liquid-conveying pipeline. As a crucial auxiliary component of liquid-conveying pipeline, the bladder pressure fluctuation attenuator plays an important role in absorbing fluctuations. According to the working condition, reasonable selection of attenuator parameters can give full play to the maximum efficiency of the attenuator so as to reduce pressure fluctuation of the pipeline. Based on the stress model of the attenuator, the mathematical and simulation models of the attenuation device are established by suitable hypothesis and simplification, and based on the calculation and physical models provided by FLUENT software, the movement of bladder was simulated by dynamic mesh and UDF (user-defined function). The absorption effect of attenuator on pipeline pressure fluctuation was investigated under different inflating pressures and frequencies. The results show that reasonable selection of initial pressure of the attenuator can effectively reduce the pressure fluctuation of pipeline fluid. Compared with the theoretical analysis results, the simulation results are more close to the experimental results, and the validity of the simulation model is verified, and a method and new idea are provided for the future study of the parameters of the bladder pressure fluctuation attenuator. [ABSTRACT FROM AUTHOR]

Abstract Turbulent flow over the DrivAer fastback vehicle model is investigated using large-eddy simulation with particular emphasis on flow separation, vortical structures and unsteady quantities. A systematic and detailed analysis of the flow field is made considering rotating wheels and moving ground floor. Overall features of vortical structures at the cowl top, behind side mirrors, near front and back wheels, at A-, B- and C-pillars and behind the rear end of the vehicle are revealed by investigating velocity and vorticity fields. The rear end is identified to be the main contributor to the pressure force acting on the vehicle, followed by back and front wheels and side mirrors. The resulting pressure force on the upper part of the vehicle, including A-, B-, and C-pillars but excluding the cowl top and side mirrors, is found to be only slightly higher than the contribution of the gap in the cowl top. Flow separation and resulting vortices do not only have an impact on automotive drag, it is also pointed out how unsteadiness in the flow field affects pressure fluctuations. High levels of surface pressure fluctuations are found near side mirrors and front wheels. Similarities in distributions of pressure fluctuations and turbulent kinetic energy are found. Highlights • Turbulent flow over the DrivAer fastback vehicle model is investigated in detail using large-eddy simulation. • Flow separation and resulting vortices at various regions and their impact on automotive drag have been identified. • High levels of surface pressure fluctuations are found near side mirrors and front wheels. [ABSTRACT FROM AUTHOR]

Abstract Pressure fluctuations in double-suction centrifugal pumps are greatly affected by the flow rate and rotational speed. An experimental investigation was carried out to determine effects of the flow rate and rotational speed for optimizing the hydraulic design and realizing stable operation. Pressure fluctuation signals were obtained at five measurement points along the suction chamber and four measurement points along the discharge chamber by means of pressure transmitters. The flow rates ranged from 0% to 150% of the design flow rate, and the rotational speed was reduced to 850 rpm from the rated speed of 1450 rpm. Results show that the third harmonic of rotational frequency is detected as the dominant frequency in the upper part of the suction chamber at low flow rates. At zero flow rate, the pressure fluctuations increase by 200% above that at the design flow rate. The pressure fluctuations decrease by 50% as the speed is reduced from the rated speed of 1450 rpm to 1250 rpm. At 850 rpm, the pressure fluctuations at partial flow rates are larger than those at higher rotational speeds, and the maximum efficiency is 5% less than that at the rated speed of 1450 rpm. Highlights • The third harmonic of rotational frequency is detected in suction chamber. • Pressure fluctuations increase by 200% at zero flow rate compared with design flow rate. • Pressure fluctuations decrease by 50% at 1250 rpm compared with rated speed (1450 rpm). • Lower rotational speed (850 rpm) results in the raise of pressure fluctuations. [ABSTRACT FROM AUTHOR]

Highlights • An LES study is carried out on a centrifugal pump with a vaned diffuser. • The influence of flow-rate and diffuser geometry on pressure fields is assessed. • At off-design increased pressure fluctuations at the impeller/diffuser interface. • Lower pressure fluctuations for larger radial gaps between impeller and diffuser. • Limited effect on pressure field within the impeller (especially suction side). Abstract Large Eddy Simulations (LES) were carried out on a centrifugal pump in order to investigate the effect on pressure fluctuations through rotating and stationary channels by both flow-rate conditions and orientation of the diffuser blades. Computations were performed for Reynolds numbers, based on the inflow velocity and outer impeller radius, equal to R e = 1.5 × 10 5 (design flow-rate) and R e = 6 × 10 4 (off-design flow-rate, equivalent to 40% of the nominal one). Results show that sensitivity to both working conditions and setting angle of the diffuser blades is much stronger for the stationary vanes, compared to the rotating channels. At reduced flow-rates pressure fluctuations are higher, with more energy at frequencies lower than that of passage of the impeller blades. However, rotating stall phenomena were not observed. A reduction of the geometric inlet angle of the diffuser vanes produces an increase of the incidence angle on the diffuser blades at the nominal flow-rate and a decrease at the lower flow-rate. Pressure rise is negatively impacted in the former case, while the effect is beneficial in the latter one. In contrast, pressure fluctuations decrease for both flow-rates, mainly due to the wider gap between trailing edge of impeller blades and leading edge of diffuser blades. With the modified diffuser geometry, at the design flow-rate the role of the low frequencies keeps negligible and at the reduced flow-rate they become also less energetic, thanks to improved diffuser inflow and weaker separation phenomena. [ABSTRACT FROM AUTHOR]

To fully understand the evolution of flow structures and pressure fluctuations in the S-shaped region of pump-turbines, three dimensional (3D) numerical simulations were carried out for a model pump-turbine at four different guide-vane openings (GVOs). We found that special backflow vortex structures (BFVSs) form around the runner inlet. The backflow location shifts from the hub side to the shroud side and then to the midspan, as the operating point moves from the best efficiency point to the zero discharge point along the S-shaped characteristic curve. At large GVO, if the discharge is relatively large, the flow separations at the runner inlet are primarily driven by the local adverse pressure gradient, resulting in hub and shroud side backflows. At small GVO or small discharge, the separations are mainly dominated by the centrifugal force, resulting in backflow at the guide-vane midspan section. The locally distributed BFVSs enhance the local rotor-stator interaction (RSI) in the vaneless space, leading to different pressure fluctuating intensities along the spanwise direction for high frequency components. On the other hand, the rotating stall is the main cause of the uneven distributions of low frequency pressure fluctuations along the vaneless space between runner and guide-vanes. [ABSTRACT FROM AUTHOR]

In this paper, the unsteady cavitating turbulent flow around a marine propeller is simulated based on the unsteady Reynolds averaged Navier-Stokes (URANS) with emphasis on the hull-propeller interaction by an integral calculation approach, which means the propeller and hull are treated as a whole when the cavitating flow is calculated. The whole calculational domain is split to an inner rotating domain containing a propeller and an outer domain containing a hull. And the two split sections are connected together in ANSYS CFX by using the GGI interfaces and the transient rotor stator frame change/mixing model. The alternate rotation model is employed for the advection term in the momentum equations in order to reduce the numerical error. Comparison of predictions with measurements shows that the propeller thrust coefficient can be predicted satisfactorily. The unsteady cavitating flow around the propeller behind the ship hull wake shows quasi-periodic features including cavity inception, growth and shrinking. These features are effectively reproduced in the simulations which compare well to available experimental data. In addition, significant pressure fluctuations on the ship hull surface induced by the unsteady propeller cavitation are compared with experimental data at monitoring points on the hull surface. The predicted amplitudes of the first components corresponding to the first blade passing frequencies match well with the experimental data. The maximum error between the predictions and the experimental data for the pressure pulsations is around 8%, which is acceptable in most engineering applications. [ABSTRACT FROM AUTHOR]

• Statistical, Hurst, Shannon entropy and chaos analysis methods are used. • Flow regime transitions in an aerated stirred tank are identified. • Standard deviation and largest Lyapunov exponent are effective indicators. • The relationship between the flow regime and the degree of chaos is discussed. A novel method was proposed to determine the flow regime based on the bottom pressure fluctuations of the stirred tank. Statistical, Hurst, Shannon entropy and chaos analysis methods were used to analyze the pressure time series in an aerated stirred tank with a Rushton impeller, and the relationship between the characteristic parameters and the flow regimes was established. The results showed that the bubble motion in the loading regime was obviously stronger than that in the flooding regime, and the homogeneity of the system was rapidly improved. Compared with the first two stages, the chaotic extent of the complete dispersion regime was slightly weaker, and the benefit generated by continuously increasing the speed was limited at this time. In addition, considering the sensitivity of pressure pulsations to the gas–liquid dispersion state, the transition points of the three flow regimes were successfully identified by the standard deviation and the largest Lyapunov exponent. The data were well consistent with the measurements from the visual method and power method, with a maximum deviation of 16%. This demonstrated that it was a reliable non-invasive method for flow regime identification in gas–liquid stirred tanks. [ABSTRACT FROM AUTHOR]