Analysis of transient flow in a pump-turbine during the load rejection process

The transient flow in pump-turbines during the load rejection process is very complex. However, few studies have been conducted on three-dimensional (3-D) numerical simulation. Hence, we simulated 3-D transient turbulent flow in a pump-turbine during the load rejection process using the calculation method of coupling the flow with the rotor motion of rigid body. To simulate the unsteady boundary conditions, the dynamic closing process of the guide vanes was simulated with the dynamic mesh technology. The boundary conditions at the spiral-casing inlet and the draft tube outlet were determined using the user defined functions (UDF) according to the experimental data. The numerical results of the rotational speeds show a good agreement with the experimental data. Then, the complex transient flow in the pump-turbine during the load rejection process was analyzed based on the numerical results. The results show that there are severe unsteady vortex flows in the vaneless space near the conditions under which the hydraulic torque on the runner equals to zero. When the pump-turbine operates into the maximum reverse discharge condition in the reverse pump operating process, the unsteady vortex flows in the vaneless space are instantaneously impacted into the region between the guide vanes and the stay vanes by the sudden reverse flows. The formation and development mechanism of the unsteady vortex flow in the vaneless space is associated with the distribution characteristic of the velocity field.     

不同计算方法在轴流式风机数值模拟中的应用

基于MRF定常计算和SM非定常计算方法,对某轴流式风机进行了设计工况下的数值模拟。讨论了不同计算方法边界条件设置和收敛性判定的区别,分析了计算方法对风机全压效率的影响,研究了风机内部流场动静交界面上的静压分布、50%叶高截面静压和速度分布、叶片表面压力分布和叶轮出口动压分布等参数。数值结果表明,定常计算结果和非定常计算结果具有相同的变化趋势,但是存在一定的差异,这主要是由于定常计算没有考虑风机内部流动的非定常效应造成的;非定常计算能够更好地模拟动静交界区域的流动,比采用定常计算具有更好的数值结果。     

A study of the flow characteristics of developing turbulent pulsating flows in a curved duct

Flow characteristics of turbulent pulsating flows in a square-sectional curved duct were experimentally investigated. Experimental studies for air flow were conducted to measure axial velocity profiles, secondary flow and pressure distributions in a square-sectional 180° curved duct by using an LDV system with a data acquisition and processing system which includes a Rotating Machinery Resolve (RMR) and PHASE software. Measurements were made at the seven cross-sections from the inlet (ø=0°) to the outlet (ø=180°) of the duct with 30° intervals. Pressure was measured by using a magnetic differential pressure gage. The experiment was conducted in nineteen sections from the inlet to the outlet of the duct at 10° intervals.Velocity profiles for turbulent pulsating flows were large at the outer wall for a bend angle of ø=30° because of the centrifugal force. The velocity profiles were similar to those of turbulent steady flows. The secondary flow of the turbulent pulsating flow had a positive value at a bend angle of 150° without regarding the phase. The dimensionless value of the secondary flow became gradually weak and approached to zero in the region of a bend angle of 180° regardless of the ratio of velocity amplitude. The pressure difference of turbulent pulsating flows was the largest near the region of a bend of angle of 90° in the case of the middle region and became small beyond 90.     

Development of a 2-D flow solver on unstructured and adaptive Cartesian meshes

A two-dimensional flow solver using mixed grids has been developed for accurate and efficient simulation of steady and unsteady flow fields. The flow solver was cast to accommodate two different topologies of computational meshes: unstructured triangular meshes in the near-body region such that complex geometric configurations can be easily modeled, while unstructured adaptive Cartesian meshes are utilized in the off-body region to resolve the flow more accurately with less numerical dissipation by adopting a spatially high-order accurate scheme and solution-adaptive mesh refinement technique. The unstructured adaptive Cartesian meshes can be generated automatically and allow to handle data efficiently via quad-tree data structures. A chimera mesh approach has been employed to link the two flow regimes adopting each mesh topology. A second-order accurate vertex-centered scheme and a 3^rd- or 5^th-order accurate cellcentered WENO scheme has been utilized in the near-body region and in the off-body region, respectively. Validations were made for the unsteady inviscid vortex convection and the steady and unsteady turbulent flows over an NACA0012 airfoil, and the results were compared with other computational and experimental results.     

Total pressure fluctuations and two-phase flow turbulence in self-aerated stepped chute flows

Current knowledge in high-velocity self-aerated flows continues to rely upon physical modelling. Herein a miniature total pressure probe was successfully used in both clear-water and air-water flow regions of high-velocity open channel flows on a steep stepped channel. The measurements were conducted in a large size facility (θ=45°, h=0.1 m, W=0.985 m) and they were complemented by detailed clear-water and air-water flow measurements using a Prandtl-Pitot tube and dual-tip phase-detection probe respectively in both developing and fully-developed flow regions for Reynolds numbers within 3.3×10⁵ to 8.7×10⁵. Upstream of the inception point of free-surface aeration, the clear-water developing flow was characterised by a developing turbulent boundary layer and an ideal-flow region above. The boundary layer flow presented large total pressure fluctuations and turbulence intensities, with distributions of turbulence intensity close to intermediate roughness flow data sets: i.e., intermediate between d-type and k-type. The total pressure measurements were validated in the highly-aerated turbulent shear region, since the total pressure predictions based upon simultaneously-measured void fraction and velocity data agreed well with experimental results recorded by the total pressure probe. The results demonstrated the suitability of miniature total pressure probe in both monophase and two-phase flows. Both interfacial and water phase turbulence intensities were recorded. Present findings indicated that the turbulence intensity in the water phase was smaller than the interfacial turbulence intensity.     

Evaluating phase-detection-based approaches for interfacial velocity and turbulence intensity estimation in a highly-aerated hydraulic jump

Estimation of turbulence intensity within a highly-aerated turbulent flow is challenging. A possible way is to record bubble arrival information using intrusive phase-detection probes, but the derivation of velocity variation is subject to the correlation and de-correlation of the signals, especially in highly-turbulent and rapidly-varied flows with complex bubble transport such as in hydraulic jumps. Although attempts have been devoted to the improvement of data processing, it is difficult to assess the existing approaches in strong hydraulic jumps due to the lack of alternative measurement techniques applicable to the internal air-water flow region. In this letter, a substantial amount of work is devoted to manual analysis of instantaneous interfacial velocity in strong hydraulic jumps, and the velocity variation results are compared with the results of full-signal cross-correlation and adaptive-window cross-correlation approaches, to evaluate their performance in approximating the velocity turbulence. The manual results are validated in terms of the time-averaged velocity. The interfacial turbulence intensity is suggested to be greater than the water-phase velocity turbulence, typically between 20% and 40% in the unidirectional jet-shear region. Overall the manual results agree better with the calculation of adaptive-window cross-correlation technique. The relevance of the signal decomposition processing is also discussed, and it is emphasized that the phase-detection-based approaches are subject to the limitations of one-dimensional measurements in a complex three-dimensional flow.     

Numerical study on the internal flow characteristics of an axial-flow pump under stall conditions

When an axial-flow pump works in low flow rate conditions, rotating stall phenomena will probably occur, and the pump will enter hydraulic unsteady conditions. The rotating stall can lead to violent vibration, noise, turbulent flow, and a sharp drop in efficiency. This affects the safety and stability of the pump unit. To study the rotating stall flow characteristics of an axial-flow pump, the steady and unsteady internal flow field in a large vertical axial-flow pump was investigated using 3D computational fluid dynamic (CFD) technology. Numerical calculations were carried out using the Reynolds-averaged Navier–Stokes (RANS) solver and Menter's shear stress transport (SST) k-ω turbulence model. Steady flow characteristics including streamline, velocity vector, pressure and turbulent kinetic energy are presented and analyzed. Unsteady flow characteristics are described using post-processing signals for pressure monitoring points in the time and frequency domains. Using Q-criterion, the locations and evolution rules of the core region of the vortex structure in guide vanes under deep stall conditions were investigated. The reliability of the numerical simulation results was verified using the experimental prototype pressure fluctuation test. In this way, typical flow structure and pressure fluctuation characteristics in an axial-flow pump were analyzed, with contrastive analysis in design condition and stall conditions. Finally, the mechanism of low-frequency pressure fluctuation in a pump unit under the stall condition was revealed.     

Flow velocity estimation using a fish-shaped lateral line probe with product-moment correlation features and a neural network

Environmental studies on fish require measurements of highly turbulent flows in both the laboratory and in the field. A fish-shaped bioinspired flow measuring device is applied in conjunction with data processing workflow which leverages the interactions between the body and the surrounding flow field for velocity estimation in turbulent flows. Our objective is to develop a robust velocity estimation methodology relevant for studies of fish behavior using a bioinspired fish-shaped artificial lateral line probe (LLP). We show that the device is capable of covering the range of flow velocities from 0 to 1.5 m/s. Three different sets of experiments performed in a closed flow tunnel, a model vertical slot fishway and laboratory open channel flume were collected and combined to provide time-averaged flow velocity and LLP measurements under fully turbulent flow conditions. Based on the experimental results, a signal processing workflow using Pearson product-moment correlation coefficient (PCC) features in conjunction with an artificial neural network (ANN) is presented. Using PCC features provides a simple data fusion methodology exploiting the use of the LLP's as a simultaneous collocated sensing array. In this work we show that (1) the PCC-ANN workflow provides the first LLP velocity estimator without repeated calibration across the full span of 0–1.5 m/s, (2) using all pressure sensors results in the best performance with R²=0.917, but requires a PCC feature matrix of 55 dimensions and (3) a stepwise reduction of the PCC feature matrix allows for the use of as few as 11 dimensions, and results in R²=0.911, indicating that a modest reduction in LLP velocity estimation performance can be gained by a large reduction in dimensionality. A surprising finding was that after stepwise reduction, the best performing sensor pair combinations were not the expected pitot-like anteroposterior couples spanning from nose to body. Instead, it was found that optimal velocity estimation using the LLP exploited a network of sensor pairs. It is shown that the LLP can be implemented similar to an ADV for highly turbulent flows over the range of 0–1.5 m/s, and in addition provides body-centric pressure distributions which may aid in the interpretation of fish hydrodynamic preferences in future environmental studies.     

Effects of prong-wire interferences in dual hot-wire probes on the measurements of unsteady flows and turbulence in low-speed axial fans

Support needles of Dual Hot Wire (DHW) anemometers induce significant inaccuracies for flow angle and turbulence measurements in the case of X-array probes with prongs perpendicular to the flow plane. At certain angular ranges of the incident flow, a wake interference is established between the sensors which leads to a practical limitation of the device. In the case of turbomachinery environments, this is even more critical due to the inherent unsteadiness of the flow direction rotor downstream.In the present work, the measurement deviation caused by hot-wire probes operated under interference effects has been studied and evaluated, in both steady and unsteady conditions, especially for turbomachinery flows. New designs of DHW probes without prong-wire interference effects in their operative angular ranges were developed for validation. In particular, both V-type and Z-type interference-free probes are compared to a classic X-type probe susceptible for prong-wire interferences. Firstly, a steady calibration is performed to show the baseline deviation of the X-array probe in the measurement of the velocity magnitude, the flow angle and the turbulence intensity. Typical errors up to 10–13% in velocity, 5.5–7 deg in angle and 1.5–2.5 points overestimation in turbulence levels are observed. Also, unacceptable inaccuracies are found in the turbulence spectra of the measurements.Following, the impact of the interference for unsteady flow measurements is highlighted comparing the performance of the three probes within the single stage of a low-speed axial fan. The unsteady measurements of the X-array probe have revealed similar averaged discrepancies to those observed in the steady performance, but the instantaneous deviations can be as high as a 20% in velocity and 16–18 deg in flow angle in those regions (rotor wakes) with large unsteady velocity gradients and turbulence generation. Turbulence intensity measured in the rotor wakes is also excessively higher.     

Detached-eddy simulation for time-dependent turbulent cavitating flows

The Reynolds-averaged Navier-Stokes(RANS),such as the original k-ω two-equation closures,have been very popular in providing good prediction for a wide variety of flows with presently available computational resource.But for cavitating flows,the above equations noticeably over-predict turbulent production and hence effective viscosity.In this paper,the detached eddy simulation(DES) method for time-dependent turbulent cavitating flows is investigated.To assess the state-of-the-art of computational capabilities,different turbulence models including the widely used RANS model and DES model are conducted.Firstly,in order to investigate the grid dependency in computations,different grid sizes are adopted in the computation.Furthermore,the credibility of DES model is supported by the unsteady cavitating flows over a 2D hydrofoil.The results show that the DES model can effectively reduce the eddy viscosities.From the experimental validations regarding the force analysis,frequency and the unsteady cavity visualizations,more favorable agreement with experimental visualizations and measurements are obtained by DES model.DES model is better able to capture unsteady phenomena including cavity length and the resulting hydrodynamic characteristics,reproduces the time-averaged velocity quantitatively around the hydrofoil,and yields more acceptable and unsteady dynamics features.The DES model has shown to be effective in improving the overall predictive capability of unsteady cavitating flows.     

Calibration of electrodiffusion probes for turbulent flow measurements

Calibration of electrodiffusion (ED) probes with respect to the wall velocity gradient measurements has been experimentally studied in a fully developed turbulent channel flow over the Reynolds number range 14000–23000. In steady state conditions, direct calibration concerning the mean wall velocity gradient can be provided using simultaneous transient and steady state diffusion limiting current measurements. Indirect calibration of the ED probes regarding the turbulent fluctuation has been undertaken using a spectral analysis performed with probes of different size and geometry. This indirect calibration method has been supported using the measurements of thermo-dependence of molecular diffusivity. In this case, only one ED probe can be used for calibration. Dynamic calibration provides necessary information about critical longitudinal dimension of ED probes. When the longitudinal dimension of the probes exceeds the critical length, the probes becomes sensitive to the normal fluctuation as well. This fact has been confirmed using different orientation of the rectangular ED probe with respect to the flow direction.     

Unsteady Turbulent Simulation and Pressure Fluctuation Analysis for Centrifugal Pumps

The pressure fluctuation in the flow passage of both impeller and casing is addressed on design condition. The initial conditions for the unsteady turbulent simulation are resulted from the steady calculations, and the three dimensional unsteady turbulent simulation concerning the rotor-stator interaction is executed by a Navier-Stoke solver embedded with k -ε turbulence model and with appropriate moving interface boundary conditions. Detecting points are distributed in the flow passage in different radial and circumferential positions to capture the static pressure fluctuation character for one cycle of the impeller. The time-domain spectrums show that the static pressure curves are periodic and have five peaks and five valleys. With the radius increasing, the pressure fluctuation peak-to-peak values in the impeller are increasing, and reach the maximum value on the interface. In the casing flow passage, those values are about 7% of local static pressure except some ones near the tongue. The values become decreasingly in the diffuser pipe. The frequency spectrums transformed by fast Fourier transform (FFT) show that the dominant frequency is approximate with the blade passing frequency, and the pressure fluctuations in impeller passage have high frequency content while those in casing ones have no such information.     

双流道泵内非定常流动数值模拟及粒子图像测速测量

为探讨双流道泵内部的非定常流动机理,采用Fluent软件,基于滑移网格技术、 湍流模型计算了一双流道泵在不同工况下的内部流动,并将计算结果与粒子图像测速仪(Particle image velocimeter, PIV)实测结果进行比较。结果表明：计算所得双流道泵内部流场符合叶轮机械内部流动的一般规律,且与PIV实测结果总体变化趋势一致;由于双流道泵结构特殊,其进口处的流动状态与普通叶轮相差较大,出口处的流动状态与普通叶轮类似;叶轮进口处,流体基本沿流道吸力面流动,流道工作面上的相对速度很小,存在严重的脱流和旋涡;叶轮出口处,压力面和吸力面的速度趋于相等,射流—尾迹现象并不明显;由于叶轮—蜗壳动静干涉,两个叶轮流道内的静压分布有所不同;同一流道内,静压随着半径的增加而逐步增大,压力面侧静压大于吸力面侧;蜗壳流道内静压随半径增大,最大静压值在隔舌处。此项研究不仅加深了人们对双流道泵内非定常流动图画的理解,从而进一步完善双流道泵设计方法,同时也可为其他类型泵的内流研究提供借鉴。     

Application of video imagery techniques for low cost measurement of water surface velocity in open channels

Developments in digital video recording technology make the video imagery tools more popular for velocity measurement in water flows. This has especially been of large interest due to its inherent advantage of non-contact nature which is quite handy in extreme flow conditions. Particle Image Velocimetry (PIV), Particle Tracking Velocimetry (PTV) and Large Scale Particle Tracking Velocimetry (LSPTV) are applied to free surface channel flow for water surface velocity measurement. Experiments are conducted to measure either a single point velocity applying PTV or velocity profiles across the channel width applying PIV on the water surface in a rectang typical velocities of nearly 1 andular tilting flume for various flow conditions. Technical issues regarding tracer particle size and type, travel distance, lighting, recording speed, camera position, image distortion and state of flow are discussed. Measured data is compared to computational results obtained from a numerical model involving a non-linear turbulence model capable of predicting turbulence driven secondary flows. Confirmation of reasonable match between computational and experimental results whereby applying mutual collaboration of them for discharge measurement has been attested. In addition to discharge, boundary roughness has also been predicted as an outcome of the numerical solution.     

Laminar flow past a sphere rotating in the transverse direction

Laminar flow past a sphere rotating in the transverse direction is numerically investigated in order to understand the effect of the rotation on the characteristics of flow over the sphere. Numerical simulations are performed at Re = 100, 250 and 300, where the Reynolds number is based on the free-stream velocity and the sphere diameter. The rotational speeds considered are in the range of 0 ≤ ω* ≤ 1.2, where ω* is the maximum velocity on the sphere surface normalized by the free-stream velocity. Without rotation, the flow past a sphere experiences steady axisymmetry, steady planar-symmetry, and unsteady planar-symmetry, respectively, at Re = 100, 250 and 300. With rotation, however, the flow becomes planar-symmetric for all the cases investigated, and the symmetry plane of flow is orthogonal to the rotational direction. Also, the rotation affects the flow unsteadiness, and its effect depends on the rotational speed and the Reynolds number. The flow is steady irrespective of the rotational speed at Re = 100, whereas at Re = 250 and 300 it undergoes a sequence of transitions between steady and unsteady flows with increasing ω*. As a result, the characteristics of vortex shedding and vortical structures in the wake are significantly modified by the rotation at Re = 250 and 300. For example, at Re = 300, vortex shedding occurs at low values of ω*, but it is completely suppressed at ω* = 0.04 and 0.6. Interestingly, at ω* = 1 and 1.2, unsteady vortices are newly generated in the wake due to the shear layer instability. The critical rotational speed, at which the shear layer instability begins to occur, is shown to be higher at Re = 250 than at Re = 300. This paper was recommended for publication in revised form by Associate Editor Dongshin Shin Dongjoo Kim is an associate professor in the School of Mechanical Engineering at Kumoh National Institute of Technology. His research interests include computational fluid dynamics, bluff-body wakes, and control of turbulent flows. He has a PhD in mechanical engineering from Seoul National University. He is a member of the American Physical Society and the American Institute of Aeronautics and Astronautics.     

Experimental investigation of the 3D unsteady flow field downstream of axial fans

This experimental study, applied to axial fans with three different types of sweep (backward, radial and forward), aims at quantifying the 3D structure of the flow field from unsteady velocity measurements. The hot-wire anemometry is used to measure the velocity components in the nearfield, downstream of the fans. The data analysis leading to averaged and turbulent velocities, the components of the Reynolds’ stress tensor and the turbulent kinetic energy is presented in order to illustrate the influence of the sweep. A spectral analysis is also performed.     

A method for measuring the dynamics of velocity vector fields in a turbulent flow using smoke image-visualization videos

A new optical method for determining flow-velocity vector fields that allows investigations of unsteady and fast processes is described. The method is based on measuring the displacements of turbulent structures, which are visualized in a light sheet, within a fixed time interval between consecutive video frames. The method was tested using measurements of pulsating-flow velocity. It was shown that the difference between the mean flow velocity in the investigated flow measured using this method and a hot-wire anemometer does not exceed 1%, while the deviation of the rms velocity pulsations is within 4.5%.     

Experimental Investigation on Cavitating Flow Shedding over an Axisymmetric Blunt Body

Nowadays,most researchers focus on the cavity shedding mechanisms of unsteady cavitating flows over different objects,such as 2D/3D hydrofoils,venturi-type section,axisymmetric bodies with different headforms,and so on.But few of them pay attention to the differences of cavity shedding modality under different cavitation numbers in unsteady cavitating flows over the same object.In the present study,two kinds of shedding patterns are investigated experimentally.A high speed camera system is used to observe the cavitating flows over an axisymmetric blunt body and the velocity fields are measured by a particle image velocimetry(PIV)technique in a water tunnel for different cavitation conditions.The U-type cavitating vortex shedding is observed in unsteady cavitating flows.When the cavitation number is 0.7,there is a large scale cavity rolling up and shedding,which cause the instability and dramatic fluctuation of the flows,while at cavitation number of 0.6,the detached cavities can be conjunct with the attached part to induce the break-off behavior again at the tail of the attached cavity,as a result,the final shedding is in the form of small scale cavity and keeps a relatively steady flow field.It is also found that the interaction between the re-entrant flow and the attached cavity plays an important role in the unsteady cavity shedding modality.When the attached cavity scale is insufficient to overcome the re-entrant flow,it deserves the large cavity rolling up and shedding just as that at cavitation number of 0.7.Otherwise,the re-entrant flow is defeated by large enough cavity to induce the cavity-combined process and small scale cavity vortexes shedding just as that of the cavitation number of0.6.This research shows the details of two different cavity shedding modalities which is worthful and meaningful for the further study of unsteady cavitation.     

Modeling and simulation for PIG with bypass flow control in natural gas pipeline

This paper introduces modeling and simulation results for pipeline inspection gauge (PIG) with bypass flow control in natural gas pipeline. The dynamic behaviour of the PIG depends on the different pressure across its body and the bypass flow through it. The system dynamics includes: dynamics of driving gas flow behind the PIG, dynamics of expelled gas in front of the PIG, dynamics of bypass flow, and dynamics of the PIG. The bypass flow across the PIG is treated as incompressible flow with the assumption of its Mach number smaller than 0.45. The governing nonlinear hyperbolic partial differential equations for unsteady gas flows are solved by method of characteristics (MOC) with the regular rectangular grid under appropriate initial and boundary conditions. The Runge-Kuta method is used for solving the steady flow equations to get initial flow values and the dynamic equation of the PIG. The sampling time and distance are chosen under Courant-Friedrich-Lewy (CFL) restriction. The simulation is performed with a pipeline segment in the Korea Gas Corporation (KOGAS) low pressure system, Ueijungboo-Sangye line. Simulation results show us that the derived mathematical model and the proposed computational scheme are effective for estimating the position and velocity of the PIG with bypass flow under given operational conditions of pipeline.