Unsteady Flow Characteristics Research Articles

Entropy Generation Optimization in an Unsteady Hybrid Nanofluid Flow Over a Flat Plate with Variable Viscosity: A Cattaneo-Christov Heat Flux Model

Nanofluids are a novel and intriguing class of heat transmission fluids that can be used in place of more conventional options. They have several applications, including fuel cells, heat exchangers, and the pharmaceutical industry. When it comes to heat transport, hybrid nanofluids much outperform their mono-fluid counterparts. These are being put to use in a wide variety of areas, from solar power to air conditioning. Unsteady flow characteristics of a hybrid nanofluid (Ethylene Glycol + ZnO + TiO2) over a flat plate at varied Lorentz forces, volume fraction of nanoparticles, and thermal radiation are unknown when variable viscosity is taken into account. Paper’s novelty is addressing this gap, which is to examine the impact of variable viscosity on the hybrid nanofluid flow by a flat plate under convective boundary condition when thermal radiation and magnetic field are significant. Cattaneo and Christov’s heat flux model is used to analyse the phenomenon of heat transfer. This paper also includes an analysis of irreversibility. The equations required to represent the problem have been turned into a system, and this system has been solved using the bvp4c solver. Validated the results (of engineering parameters including friction coefficient) obtained using the bar graphs by using Multiple linear regression. It has been observed that the thermal relaxation parameter (Γ) and the unsteady parameters (S) both have similar effects on the heat transmission rate. It is found that there is an increment of 0.001278 (when 0 ≤ Γ ≤ 0.6) and 0.103865 (when 0.4 ≤ S ≤ 1) in the Nusselt number. It is clear from a closer inspection that as magnetic field parameter (Mg) and viscosity parameters (δ) are raised, the friction factor declines. It is observed that, at 0 ≤ Mg ≤ 1.8, friction factor declines at a proportion of 0.08974. It is detected that higher Brinkman number values lead to a greater rate of entropy generation and an upsurge in the (temperature-dependent) viscosity parameter contributes to a decline in the same. As the viscosity parameter is amplified, it is also seen that the fluid’s velocity declines.

Film cooling instability analyses for trailing edge cutback with a novel linear sand-dune inspired lip under mainstream pulsation

Inspired by natural linear sand-dune (LSD), the present work proposed a cutback cooling configuration with novel lip design applied in trailing edges of gas turbine blade. By time-resolved experiments and large eddy simulations, transient flow and film coverage performances of the novel configuration are investigated, and the effect of inevitable mainstream pulsation is included. Through the analyses of unsteady flow characteristics, the LSD lip design can help hinder the propagation of mainstream pulsation, and generates a series of separation wakes to destroy the highly-unsteady shedding vortices, which leads to a high film effectiveness and low level of cooling instability. Quantitatively, by the novel design, time-averaged film effectiveness can be locally increased at least 26% and the unsteadiness level can be maximally reduced by 86% compared to the baseline elliptic lip design. The analyses on probability distribution of transient film effectiveness also conform an obvious advantage of LSD design, from the perspective of statistics. Additional aerodynamic loss induced by the LSD lip is always less than 7% in both steady and pulsed conditions, which remains low and is worthy considering the significant improvement in cooling performance.

Large-Eddy Simulation of Flow Separation Control in Low-Speed Diffuser Cascade with Splitter Blades

The passive flow control technology of using splitter blades in low-speed diffuser cascade was investigated in this study. Based on the Reynolds average Navier-Stokes calculations, the arrangement parameters of the splitter blades were studied in detail to determine the optimal parameters. The large-eddy simulation was performed on the base case and the optimized splitter blade case to obtain the transient vortex structures and unsteady flow characteristics of the cascade. The results show that the aerodynamic performance of the cascade was susceptible to the position of the splitter blades. The optimal position of the splitter blades was located in the middle of the main blades near the leading edge. When the cascade was arranged with optimized splitter blades, the static pressure coefficient was improved and the stall occurrence was delayed. The scale and intensity of the separation vortices generated on the suction surface of the main blade decreased. In addition, the separation vortices of the main blade and the splitter blade interacted and rapidly decomposed into small-scale vortices downstream of the cascade, reducing the flow loss. The stability of the cascade was enhanced.

Unsteady Flow Structure of Corner Separation in a Highly Loaded Compressor Cascade

Abstract Corner separation is an inherently unsteady flow feature in an axial compressor cascade, and it significantly affects the aerodynamic performance of compressors. The flow field of a highly loaded compressor cascade at the Mach number of 0.59 under the moderate separation condition is simulated based on delayed detached eddy simulation. Comparisons of averaged flow field and transient flow field show that the three-dimensional corner separation flow is highly unsteady and composed of fine-scale vortex structures. The classical recognition of corner separation structures is a consequence of time-averaging. To better understand the contribution of unsteady structures to the averaged flow structures, the evolutions of flow fields in time series and the power spectrums are analyzed. A dominant periodic flow fluctuation is caused by the development of separating vortices with a characteristic frequency around 3500 Hz or at a Strouhal number of 0.75. Further, energy scales and spatiotemporal features of these dominant unsteady behaviors are analyzed using proper orthogonal decomposition and dynamic mode decomposition methods. Results show that the low-frequency behaviors mainly caused by the passage vortex at lower-span regions govern large-scale changes of separation flow in size and intensity and act with certain intermittency. The vortex developing mode around 3500 Hz prevails at higher regions affected by the concentrated shedding vortex. As the separating vortices dissipate approaching the midspan, the effect of the vortex developing mode on axial velocity fluctuation is reduced, although it dominates the pressure fluctuation with good stability in the whole passage.

Towards the integration of distributed renewables: Operation analysis of pumped storage system under off-design condition based on CFD

Pumped storage plants are increasingly developing to cope with the rapid growth of renewable energy production. Micro-pumped storage (MPS) system is a new storage strategy for distributed energy integration. Centrifugal pump or axial pump replaces conventional pump turbines in this pumped storage system to ensure economic convenience and system flexibility. However, when MPS works in the generating operation mode, the limited high-efficiency range will result in complex hydrodynamic behavior and stability issues under off-design conditions due to renewable integration. At present, the instability mechanism of MPS under off-design conditions is not clear enough, and there is a lack of excellent research methods to analyze this problem. Therefore, this paper firstly introduces the entropy production theory considering the wall effect for visualizing the energy loss. Then the relationship between hydraulic performance and unsteady flow characteristics is established based on the loss characteristics. The results show that the entropy production theory with customized wall functions has higher accuracy and advantages in energy loss analysis compared to the default wall functions. Under partial loading conditions, the spatial-temporal instability of the flow separation point causes a rotating stall within the impeller, which in turn leads to destabilization and a significant reduction in hydraulic performance. Under overload conditions, despite slight changes in hydraulic performance, the most significant pressure fluctuation is six times stronger than the best efficiency point.

Experimental investigation of characteristics and influence of tip leakage vortex wandering in an axial compressor cascade

Vortex wandering is one of the most basic unsteady flow characteristics of the tip leakage vortex (TLV) in compressors. In this study, stereo particle image velocimetry (stereo-PIV) has been conducted in compressor cascades with various tip clearances to investigate the characteristics and influence of TLV wandering. The most effective vortex identification method for stereo-PIV data has been clarified. The wandering characteristics of the TLV are statistically analyzed, and the relationship between the vortex wandering and the dominant proper orthogonal decomposition mode is identified. The results reveal that TLV preferentially wanders along the pitchwise direction in the blade channel and gradually loses the dominant wandering direction downstream of the blade. The large displacement of the vortex center around its mean location is characterized by higher probabilities at small tip clearance size (1% chord length C). The spatial distribution characteristics demonstrate similarly concentric isocontour around the mean vortex center location for large tip clearances (3% and 5% chord length C). The effect of TLV wandering on the secondary velocity distribution, the tip flow blockage, and the distribution of Reynolds stress is explored using the vortex wandering corrected technique. The analysis verifies that the elevated turbulence kinetic energy in the core of the time-averaged uncorrected TLV is caused by vortex wandering rather than vortex deformation. The discussions of the current paper will enhance our knowledge of TLV wandering. Regardless of the reference to TLV, the interpretation of other swirling flows can benefit from the discussions presented here.

Numerical study of solution structure and nonlinear behavior of Dean flow with vortex structure in a bending square duct

Fluid flow and heat transfer in bending channels are topics of much research interest because of increasing demand in various fields, particularly in the medical and industrial arenas. This paper presents a numerical study of fluid flow and heat transfer in a bending channel with a square cross section. Numerical calculations using spectral methods were performed for a curvature of 0.001 and a Dean number (Dn) in the range of 0<Dn≤5000. A temperature difference was maintained between the horizontal walls for a Grashof number of Gr = 1000, with the bottom wall heated and the outer and inner walls thermally insulated. Applying Newton–Raphson iteration and path continuation, two branching structures of steady solutions with two to eight vortices were obtained. The first branch comprises multi-vortex up to eight and it occurs at Dn≥3500 while the second branch comprises to multi-vortex up to a maximum of four. The first branch forms symmetric solution while the second branch for its complexity forms asymmetric solutions. For unsteady solutions, time-evolution calculations were performed to investigate the nonlinear behavior, and it was found that with increasing Dn, the unsteady flow experienced various flow instabilities. The time evolution was plotted in phase space to clarify the unsteady flow characteristics. Distinctive contours of the secondary flow patterns, streamwise velocity distributions, and isotherms were also obtained, and the unsteady flow was found to comprise two to six vortices. Finally, convective heat transfer was explored by obtaining temperature contours, and the secondary flow was found to magnify the convective heat transfer significantly. Because of the increase of several secondary vortices in the chaotic solutions, heat transfer occurred markedly in the flow.

Dynamic mode decomposition for the tip unsteady flow analysis in a counter-rotating axial compressor

Counter-rotating axial compressor (CRAC) is a promising potential technology to improve the thrust-to-weight ratio of aero-engines, but its special aerodynamic layout usually causes more pronounced flow unsteadiness. Understanding the unsteady flow features and mechanism in the CRAC contributes to the aerodynamic optimization design and flow control strategy organization. A data-driven dynamic mode decomposition method is introduced to investigate the tip flow unsteadiness in a CRAC, and the unsteady features of the tip flow at the design point (DP) and near-stall point (NSP) conditions are revealed. The results show that the 1.0 times blade passage frequency (BPF) and its multi-order harmonic frequency are the dominant frequencies for both rotors at the DP condition. At the NSP condition, the 1.0 BPF is no longer the dominant frequency causing the tip flow unsteadiness, and the low frequency fluctuation of the tip leakage flow becomes the dominant frequency to induce the flow unsteadiness. In the front rotor R1, the unsteady dominant frequency is 1.0 BPF, whereas in the rear rotor R2, the frequency (0.801 BPF and 0.803 BPF) of the tip leakage flow is the dominant frequency. By reconstructing the flow field under the NSP condition, the spatiotemporal evolution of the tip flow during the unsteady stable manifests that the interference effect between the rotors is an important source of the tip flow unsteadiness. The increase in flow unsteadiness leads to an increase in the reconstruction error, indicating that more modes are required to obtain a more accurate reconstruction flow field.

Effect of water spray on tip leakage flow in a inlet stage of high-speed compressor

Water spray can reduce the inlet air temperature of the compressor inlet stage of the high-altitude high-speed turbine engine. In this paper, the Euler-Lagrange method is used to simulate the gas-liquid two-phase flow with droplet evaporation in NASA Stage 35. The effects of water spray on the heat and mass transfer characteristics of the stage, the tip leakage vortex structure, the unsteady characteristics of tip leakage flow and compressor performance are analyzed. The results show that the water spray increases the inlet mass flow and tip leakage flow rate of the stage, decreases the tip temperature. The inlet mass flow and tip leakage flow rate increase with the growth of inject mass flow rate and the decrease of droplet size. Water spray increases the scale of the main tip leakage vortex, and deflects the secondary leakage flow in the flow direction. Water spray increases the amplitude of pressure fluctuation at the blade tip. The frequency of pressure fluctuation at dry air condition is 0.2BPF, which decreases to 0.15BPF after water spraying. The leakage vortex weakens the strength of shock wave which causes a decline in the diffusing ability of blade; the leakage vortex is mixed with the main stream, resulting in a large aerodynamic loss. Although the injection of droplets increases the loss of tip leakage flow, it optimizes the performance of NASA stage 35 significantly. Spray increases the total pressure ratio of the R domain from 1.835 to 1.9; the total temperature ratio of the stage reduces from 1.225 to 1.203 and decreases the rotor loss coefficient from 0.12 to 0.08.

Study on Unsteady Flow Characteristics of Cooling Water Pump for Nuclear Power Plant Equipment under Low Flow Rate Conditions

During the operation of cooling water pumps, it is necessary to operate them under conditions of low flow rate. In order to improve the unstable performance of the cooling water pump under low flow rate conditions. Taking the cooling water pump as the research object, the internal flow and pressure pulsation characteristics of the cooling water pump under 0.4Q to 0.6Q conditions were investigated, and the influence of different operating conditions on the performance and vibration of the cooling water pump was analyzed. The ANSYS CFX 2022 software and the SST k-ω turbulence model were used to perform a three-dimensional numerical simulation of the cooling water pump. After analyzing the simulation results, the velocity and pressure cloud and streamline diagram within the semi-spiral suction casing and impeller were obtained. The internal flow state of the cooling water pump was then analyzed in detail under low flow rate conditions. At the same time, a series of monitoring points were set up within the impeller, and the pressure pulsation within the impeller was analyzed using the frequency domain diagram and the radial force polar coordinate diagram. The results show that at flow rates between 0.4Q and 0.6Q, a certain amount of vortex has been generated in the suction casing, which affects the flow state when entering the impeller. Furthermore, significant vortices have been generated in the middle and back part of the blade and mainly concentrated in the pump cover of the mid-open pump. At the same time, when in low flow rate conditions, the primary frequency of pressure pulsation is mainly the axial frequency, with three times the axial frequency and blade frequency following. The amplitude of the pressure surface (PS) of the blade is greater than that of the suction surface (SS) and increases as the flow rate decreases. The internal radial force corresponds with the result of pressure pulsation and exhibits a certain pattern. This study outlines the coolant pump’s internal flow and pressure pulsation characteristics under low-flow conditions. It proposes a solution to stabilize the cooling water pump at low flow rates and provides theoretical guidance for optimizing its design.

Multiscale graph neural network autoencoders for interpretable scientific machine learning

The goal of this work is to address two limitations in autoencoder-based models: latent space interpretability and compatibility with unstructured meshes. This is accomplished here with the development of a novel graph neural network (GNN) autoencoding architecture with demonstrations on complex fluid flow applications. To address the first goal of interpretability, the GNN autoencoder achieves reduction in the number nodes in the encoding stage through an adaptive graph reduction procedure. This reduction procedure essentially amounts to flowfield-conditioned node sampling and sensor identification, and produces interpretable latent graph representations tailored to the flowfield reconstruction task in the form of so-called masked fields. These masked fields allow the user to (a) visualize where in physical space a given latent graph is active, and (b) interpret the time-evolution of the latent graph connectivity in accordance with the time-evolution of unsteady flow features (e.g. recirculation zones, shear layers) in the domain. To address the goal of unstructured mesh compatibility, the autoencoding architecture utilizes a series of multi-scale message passing (MMP) layers, each of which models information exchange among node neighborhoods at various lengthscales. The MMP layer, which augments standard single-scale message passing with learnable coarsening operations, allows the decoder to more efficiently reconstruct the flowfield from the identified regions in the masked fields. Analysis of latent graphs produced by the autoencoder for various model settings are conducted using unstructured snapshot data sourced from large-eddy simulations in a backward-facing step (BFS) flow configuration with an OpenFOAM-based flow solver at high Reynolds numbers.

Fluid-induced vibration analysis of centrifugal pump including rotor system based on Computational Fluid Dynamics and Computational Structural Dynamics coupling approach

The flow-induced vibration of centrifugal pump under the effect of stator-rotor interaction are studied by Computational Fluid Dynamics method and Computational Structural Dynamics method based on the principle of one-way decomposition fluid-structure interaction. Firstly, the unsteady flow characteristics in centrifugal pump are studied by adopting the Computational Fluid Dynamics method, the numerical model of the whole flow field of the centrifugal pump is established, and the pulsating pressure characteristics of volute and impeller in the centrifugal pump are analyzed. The accuracy of the flow field modeling method and calculation method in this paper is verified by external characteristic test. Secondly, the vibration characteristic test of centrifugal pump is carried out, and the structural dynamic model and boundary conditions of centrifugal pump are verified and corrected in the modal test. Finally, the flow-induced excitation calculated by the flow field is simultaneously applied to the structural dynamics model of the centrifugal pump according to different transmission ways, and the flow-induced vibration of the whole centrifugal pump including the rotor system is calculated and analyzed by the Computational Structural Dynamics method in conjunction with rotor dynamics. Compared with the vibration test results, the validity of the flow-induced vibration calculation method of centrifugal pump in this paper is verified. The results show that under the influence of stator-rotor interaction effect, the blade passing frequency and shaft frequency are the main characteristic frequencies of pressure pulsation and dynamic response in the centrifugal pump.

Evaluation of Turbulence Models in Unsteady Separation

Unsteady separation is a phenomenon that occurs in many flows and results in increased drag, decreased lift, noise emission, and loss of efficiency or failure in flow devices. Turbulence models for the steady or unsteady Reynolds-averaged Navier–Stokes equations (RANS and URANS, respectively) are commonly used in industry; however, their performance is often unsatisfactory. The comparison of RANS results with experimental data does not clearly isolate the modeling errors, since differences with the data may be due to a combination of modeling and numerical errors, and also to possible differences in the boundary conditions. In the present study, we use high-fidelity large-eddy simulation (LES) results to carry out a consistent evaluation of the turbulence models. By using the same numerical scheme and boundary conditions as the LES, and a grid on which grid convergence was achieved, we can isolate modeling errors. The calculations (both LES and RANS) are carried out using a well-validated, second-order-accurate code. Separation is generated by imposing a freestream velocity distribution, that is modulated in time. We examined three frequencies (a rapid, flutter-like oscillation, an intermediate one in which the forcing and the flow have the same timescales, and a quasi-steady one). We also considered three different pressure distributions, one with alternating favorable and adverse pressure gradients (FPGs and APGs, respectively), one oscillating between an APG and a zero-pressure gradient (ZPG), and one with an oscillating APG. All turbulence models capture the general features of this complex unsteady flow as well or better than in similar steady cases. The presence, during the cycle, of times in which the freestream pressure-gradient is close to zero affects significantly the model performance. Comparing our results with those in the literature indicates that numerical errors due to the type of discretization and the grid resolution are as significant as those due to the turbulence model.

Numerical investigation of cavitating flow in centrifugal pump with improved partially-averaged Navier–Stokes method

ABSTRACT The objectives of this paper are to pursue an accurate numerical method of unsteady cavitating flow at a reasonable calculation cost and investigate the unsteady cavitating flow characteristics of a centrifugal pump. Firstly, the cavitating flow simulations of a centrifugal pump were performed using the partially-averaged Navier–Stokes (PANS) turbulence model and its improved model to evaluate the numerical methods based on experimental data. Compared with the experimental results, the improved PANS turbulence model can better predict the cavitating flows in the pump and has better applicability. Further, the unsteady cavitating flow characteristics of the centrifugal pump, such as the vapour volume fraction, the cavitation-vortex dynamics and the pressure and head fluctuations, were simulated by improved PANS model and discussed. The liquid flow vortex at the end of the cavitation in the impeller is an important reason for the periodic change of the cavitation shape. The periodic change of cavity volume has a certain impact on the pressure pulsation in the impeller and the pump head.

Experimental and numerical study on unsteady entrainment behaviour of ventilated air mass in underwater vehicles

The hydrodynamic characteristics of underwater vehicles are significantly affected by the ventilated cavity covered by the vehicle surface. In this paper, the unsteady flow characteristics of this ventilated cavity are studied using experimental and numerical methods, and the unsteady entrainment behaviour of the ventilated air mass is emphasised. The flow pattern of the ventilated air mass is recorded using a high-speed camera. The large eddy simulation turbulence model is employed for the numerical simulations, and a good agreement is observed between the experimental and numerical results. In the early stage of the formation of the ventilated air mass, the internal structure exhibits a symmetric kidney vortex system, while the ventilated cavity below the vent hole has a continuous hairpin vortex structure. The ventilated air mass experiences a growth stage, an entrainment stage, and a shedding stage. The entrainment behaviour enables the ventilated air mass to quickly fill the ventilated cavity and modifies the surface pressure distribution of the vehicle. As the cavitation number decreases, the radial size of the ventilated cavity increases, and the contact area between the cavity and the water body increases, thus enhancing the vertical drag coefficient of the vehicle.

Steady and Unsteady Flow Characteristics inside Short Jet Self-Priming Pump

Due to their great efficiency and minimal loss, self-priming jet pumps are frequently employed in a variety of sectors for sustainable development. A short jet self-priming pump’s steady and unsteady flow characteristics are investigated numerically in this study using a standard k-ε turbulence model. The precision and dependability of the numerical calculations used in this work are demonstrated by the less than 2% difference between the pump performance data from the numerical calculation and the external characteristics test results for each flow condition. It was found that due to the perpendicularity of the nozzle axis to the impeller axis, the high-flow velocity zone in the nozzle gradually deviates to the side away from the impeller under high-flow conditions. Backflow is generated on the side close to the impeller, where eccentric vortices are created. As time progresses, the asymmetry of the low-pressure zone within the impeller becomes more pronounced under high-flow conditions, and the fluid is unable to form a stable vortex structure at a specific location. This is an important cause of impeller vibration and noise. The nonlinear vibration at the impeller inlet is less periodic, while the increase in flow rate can make the nonlinear vibration generated within the impeller more regular and stable. This reflects the fact that the fluid flow at small flow rates is more likely to be affected by the blade configuration and the shape of the flow channel, which leads to fluid instability and discontinuity. For various flow rates, the main frequency of the pressure pulsation is higher at the impeller intake (W1) than it is in the impeller channel (W2~7). Additionally, the pressure pulsation is more frequent before 10 times the rotational frequency, with no significant regularity. This suggests that the impeller and injector rear chamber dynamic and static interference impacts may have some bearing on the pressure pulsation. The pressure pulsation coefficients (W2~7) in the impeller at different flow rates show an exponentially decreasing trend with the increase of multiples of five in the rotation frequency. The equations for the relationship between CP and 5NF were obtained, respectively: CP-Q1 = 0.07044 × exp(−0.2372NF), CP-Q3 = 0.06776 × exp(−0.2564 NF), CP-Q5 = 0.07005 × exp(−0.2884 NF). The findings of this study contribute to understanding the flow inhomogeneities inside the pump as well as the analysis of the internal pump vibration, enhancing the jet pump’s efficiency and lifespan.

Investigation of unsteady flow mechanisms and modal behavior in a compressor cascade

This paper utilizes experimentally validated Large Eddy Simulation method and the standard Dynamic Mode Decomposition (DMD) analysis technique to investigate the unsteady flow characteristics on a compressor cascade under two Reynolds number (Re) conditions (1.1 × 106 in ground state and 8.0 × 104 in high altitude state). The results demonstrate that at low Re, large-scale vortices are formed on the surface of compressor blades, exhibiting a stable rotating state and dominating the flow. Kelvin-Helmholtz instability serves as the determining factor, inducing chaotic and unstable behavior in the separation shear layer, leading to turbulence. Furthermore, DMD modal decomposition reveals a dominant mode at low Re, gradually amplifying and significantly contributing to the overall flow field, while showing strong correlations with other major modes. Conversely, at high Re, vortices on the blade surface become dispersed and smaller, resulting in intricate interactions between vortices of different scales. Harmonic instability, alongside Kelvin-Helmholtz instability, plays a significant role in accelerating boundary layer transition, heightening flow complexity and instability. Additionally, pronounced interaction and energy transfer between different modes occur at high Re, yielding a relatively balanced energy distribution. These findings have important implications for optimizing blade design and enhancing compressor performance by providing a deeper understanding of vortex dynamics and dynamic modal characteristics in the compressor blade flow field.

Performance and unsteady flow characteristic of forward-curved impeller with different blade inlet swept angles in a pump as turbine

As an economical energy recovery device, a pump as turbine (PAT) is widely used in micro hydropower and energy recovery fields. Compared with an original impeller, a special impeller with forward-curved blades can dramatically enhance a PAT's efficiency and broaden its high-efficiency range because the blade angles created by the latter more effectively match the PAT's operating mode. The shape of blade curve and sweep has obvious influence on the performance of a PAT. To investigate how the inlet swept angle of forward-curved blade influences the performance of PAT, five different forward-curved impellers with different inlet swept angle blades were designed and numerically investigated based on a verified computational fluid dynamics (CFD) method. The effects of blade inlet swept angles on the external characteristics, energy loss and unsteady characteristics of PAT are analyzed. Compared with the blade whose inlet swept angle is 0°, the maximum efficiency of back-swept PAT with swept angle of −8° increases by 0.58% while the head decreases by 0.49 m. However, the efficiency of forward-swept PAT of 8° reduces by 0.9%, and the head increases by 0.68 m. The maximum reduction rates of the main frequency amplitude of pressure pulsation in the volute, impeller and draft tube of back-swept PAT are 35.2%, 45.47% and 67.24%, respectively, indicating that the back-swept blade can contribute to improve the operation stability of PAT. Moreover, the vortex in PAT with back-swept blade is greatly improved under partial and design flow rate, which can improve the unsteady flow characteristics of PAT.

Hydrodynamic Characteristics of Strong, Unsteady Open-Channel Flow

Due to climate change, the intensity of extreme rainfall has been observed to increase with a shorter duration, causing flash floods (strong, unsteady flow) that lead to serious loss of life and economic damage all over the world. In this study, by repeating the same flume experiments twenty times over a bare bed or with a submerged vane installed, the hydrodynamic characteristics of a strong, unsteady open-channel flow were investigated. Acoustic Doppler velocimetry (ADV) was used to measure the instantaneous three-dimensional velocity, and the ensemble average method was then adopted to obtain the time-varying mean flow velocities. Reynolds decomposition was applied to disintegrate the instantaneous velocity to time-varying average velocity and fluctuating velocity. Turbulence characteristics such as turbulent intensity, turbulent bursting, and power spectral density (PSD) were analyzed against water depth variations. The results show that the loop pattern of the streamwise velocity against the water depth variations could significantly affect the turbulence characteristics of unsteady flow. Near the bed, the peaks of the turbulent intensity and the TKE lag behind the peak of the water depth. The PSD revealed that the turbulent energy increases at the rising and falling stages were due to the generation of small-scale turbulence vortices or eddies. As a submerged vane was present, the increase in the angle of attack caused the increase in the turbulent intensity and TKE, which means that the induced vortex became stronger and the wake region was larger. When the angle of attack was equal to 20°, the TKE abruptly enlarged in the falling stages, implying the breaking-up of the induced vortex. The PSD of the transverse fluctuation velocity showed multiple spikes at the high-frequency part, possibly denoting the shedding frequency from the induced vortex. Further downstream, behind the submerged vane, the peak frequencies of the PSD became imperceptible, likely because of the induced vortex decays or other factors such as the turbulence generated from the free surface or the channel bed mixing with the turbulence from the induced vortex.

Numerical study on unsteady cavitation flow and vortex dynamics characteristics around the Delft Twist 11 hydrofoil

The objective of this study is to explore the unsteady cavitation characteristics and the mechanism of cavitation-vortex interaction around a twist hydrofoil. The cavitation conditions were simulated using the Partially-Averaged Navier-Stokes (PANS) method in conjunction with a homogeneous flow cavitation model. The numerical method employed in this study successfully captures the intricate shedding process of cavitation on the hydrofoil surface. Furthermore, the cavity morphology obtained at various time instants exhibits a remarkable agreement with the experimental results. The simulation results of the unsteady characteristics of cavitation shedding on the hydrofoil surface revealed that the shedding process could be divided into primary and secondary shedding phases. The primary shedding happened as a result of the re-entrant jet flow generated by the adverse pressure gradient at the cavitation tail. Notably, the secondary shedding was predominantly caused by the combined effects of the re-entrant jet and the side re-entrant jet flow. During the primary shedding process, the cavitation experienced disturbances from the side re-entrant jets, causing it to roll and be drawn upwards into a U-shape cavity structure. Subsequently, as the secondary shedding happened, the side re-entrant jets contributed to the formation of a secondary U-shape cavity at the cavitation tail. The investigation on the cavitation-vortex interaction around the hydrofoil showed a strong consistency between the distribution of vortices and cavitation. The foot of the primary U-shape vortex was attached to the hydrofoil surface, while the shedding vortex tail on both sides exhibited secondary U-shape vortex structures. The primary U-shape vortex formation resulted from the combined effects of suction and rotation generated by the side re-entrant jets and re-entrant jets on both sides of the hydrofoil. The secondary U-shape vortex was induced by secondary shedding cavitation which produced by rotational effects at the stable cavitation tail.