Analysis Of Unsteady Flow Research Articles

Fluid-structure coupling effects on periodically transient flow of a single-blade sewage centrifugal pump

A partitioned fluid-structure interaction (FSI) solving strategy that depends on problem characteristics is applied to quantitatively obtain the coupling effects of a fluid-structure system in a single-blade centrifugal pump on the unsteady flow. A two-way coupling method is employed to realize strong FSI effects in the calculation procedure. The successful impeller oscillation measurement using two proximity sensors validated the FSI simulation accuracy in a complicated and practical fluid-structure system having a rotating component. The results show that the hydrodynamic force deviation can be observed in the results for the coupled versus uncoupled cases. Additionally, the coupled unsteady pressure is larger than the uncoupled value for every monitoring point at every impeller rotation position. Comparison results for different monitoring points under an overload condition and partial-load condition display the same regularities. To some extent, this interaction mechanism would affect the accuracy and reliability of the unsteady flow and rotor deflection analysis.

One-dimensional analysis of unsteady flows due to supercritical heat addition in high speed condensing steam

Unsteadiness in supersonic flow in nozzles can be generated by the release of heat due to spontaneous condensation. The heat released is termed "supercritical" and may be responsible for turbine blades failure in turbine cascade as it causes a supersonic flow to decelerate. When the Mach number is reduced to unity, the flow can no longer sustain the additional heat and becomes unstable. This paper aims to numerically investigate the unsteadiness caused by supercritical heat addition in one-dimensional condensing flows. The governing equations for mass, momentum and energy, coupled with the equations describing the wetness fraction and droplet growth are integrated and solved iteratively to reveal the final solution. Comparison is made with well-established experimental and numerical solution done by previous researchers that shows similar phenomena.

바람에 기인하는 태양광추적구조물의 안정성 해석

A solar power generator is usually installed outdoors and it is exposed to extreme environments such as heavy fall of snow and high speed wind. Therefore, the solar tracker structure should be designed to have sufficient static and dynamic stiffness against such environmental conditions. In this paper, eigenvalue analysis of the solar tracker is carried out by varying the pose of the solar panel and unsteady flow analysis around a single tracker or multi-trackers arranged in a line is performed by varying the parameters such as wind directions, wind speeds and the pose of the solar panel to evaluate whether there exists an instability of resonance due to vortex shedding. Finite element eigenvalue analysis shows that natural frequencies and modes are almost not influenced by the pose of the solar panel and the finite element flow analysis shows that there does not exist periodic vortex shedding due to the flow around single tracker or multiple solar trackers in a line.

Numerical analysis of unsteady laminar hydromagnetic mixed convection flow in a vertical channel

The study of unsteady mixed convection flow in a vertical channel with symmetric and asymmetric wall heating conditions is considered in this article. The boundary conditions used are isothermal–isothermal, isoflux–isothermal, and isothermal–isoflux for the left–right walls of the channel. The effects of the Hartmann number, convection parameter and heat generation/absorption coefficient are included in the numerical analysis. Graphic results with various values of the parameters involved are presented. The numerical solutions of the equations representing the process are obtained by the Network Simulation method. The results obtained in the steady-state were compared with the results of Chamkha [10] and were very close. The numerical procedure employed, which satisfies the conservation law for the heat flux variable, also permits the direct visualization and evolution of the local and/or integrated transport variables at any point or section of the medium.

자동차 워셔액 가열시스템의 온도 변화 특성에 관한 연구

본 논문은 추운 겨울 자동차 앞면 유리에 생기는 성에를 제거하는 워셔액 가열시스템의 가열과 분사에 따른 온도변화 특성에 관한 연구이다. 지금까지 다른 연구에서는 워셔액 가열시스템에 대한 온도 변화 특성을 간단한 수학적 모델링을 통하여 분석하였으나 본 연구에서는 워셔액 가열시스템의 보다 더 최적화된 제어시스템 설계를 위해 워셔액 가열시스템의 시간에 따른 열유동 특성 변화를 전산유체해석(CFD)을 통해 파악하기로 한다. 이를 위해서 워셔액 가열시스템의 주요 부분인 히터와 워셔액에 대한 비정상상태 해석을 수행하고 워셔액 전체의 온도 변화 특성을 분석하였다. 이를 토대로 워셔액의 가열시간과 분사시 온도 특성을 파악하여 워셔액 가열시스템의 최적설계의 기본자료로 활용하도록 하였다. The present paper has been accomplished to elucidate the characteristics of temperature change in a car washer heater system for removal of frost formed at the surface of a car. The previous studies had used a simplified mathematical modeling to analyse the temperature change characteristics for a car washer heater system. In the present study, an unsteady computational fluid flow and heat transfer analysis for a washer heater system has been done by using computational fluid dynamic analysis method. From the present CFD analysis, the time dependent temperature change in a car washer heater system has been analysed and derived the heating time and ejection temperature of the washer liquid to establish the optimal design basis for a washer heater system.

Nonlinear Interaction of Friction and Interpolation Errors in Unsteady Flow Analyses

AbstractExplicit analytical assessments of numerical errors in solutions of decaying standing waves in pipes are obtained using analyses based on the fixed-grid method of characteristics and are used as a quantitative basis for comparing alternative grid types, sizes, and shapes. It is shown that the influences of friction and interpolation combine in a nonlinear manner, but that the combination has the form of a product so that the two effects may be considered independently to a certain extent. The analysis is presented for finite pipes, thus creating both upper and lower limits for the range of possible standing wave frequencies. It is found that frequencies in the upper half of the range are strongly damped. The performances of diamond and rectangular grids are compared, but there is no clear winner because, for equivalent computer resources, each has both advantages and disadvantages. The overall purposes of the paper are to move toward methods of (1) relieving untrained software users from tasks rel...

Effects of channel shape on propagation characteristics of flood flows through a valley

Understanding the effects of the planar, longitudinal, and cross-sectional form (channel shape) of a river channel on the propagation of a flood flow is important for proper river improvement and management. This paper shows that the propagation characteristics of a discharge hydrograph of a flood flow are closely related to temporal changes in the mean velocity. We defined the storage volume of a flood flow produced by a decrease in the mean velocity as the ‘retarding storage volume of a flood flow’. By using observed data and unsteady two-dimensional flow analysis of a flood of the Go River valley in June 1999, the propagation mechanics of the discharge hydrograph is described in relation to the channel shape, storage volume, and retarding storage volume.

Aerodynamic analysis and optimization of a transonic axial compressor with casing grooves to improve operating stability

A transonic axial compressor with circumferential casing grooves is optimized to improve operating stability. Numerical analysis is conducted by solving three-dimensional Reynolds-averaged Navier–Stokes equations using the shear stress transport turbulence model. An optimization process based on a weighted-average surrogate model and steady flow analysis are performed with three design variables defining the tip clearance, blade tip angle, and depth of the grooves. The steady stall inception point is identified from the last converged point, and values of the stall margin as the objective function are predicted using steady flow analysis at the design points sampled by Latin hypercube sampling in the design space. The surrogate model is constructed based on these objective function values. Optimization of this model found the optimum design, which yields a considerable increase in the stall margin compared to the smooth casing. To investigate unsteady behavior of the flow in the optimized compressor with casing grooves, an unsteady flow analysis is performed, and the stall inception point is re-predicted using this analysis.

Unsteady internal flow conditions of mini-centrifugal pump with splitter blades

Mini centrifugal pumps having a diameter smaller than 100mm are employed in many fields. But the design method for the mini centrifugal pump is not established because the internal flow condition for these small-sized fluid machines is not clarified and conventional theory is not suitable for small-sized pumps. Therefore, mini centrifugal pumps with simple structure were investigated by this research. Splitter blades were adopted in this research to improve the performance and the internal flow condition of mini centrifugal pump which had large blade outlet angle. The original impeller without the splitter blades and the impeller with the splitter blades were prepared for experiment. The performance tests are conducted with these rotors in order to investigate the effect of the splitter blades on performance and internal flow condition of mini centrifugal pump. On the other hand, a three dimensional unsteady numerical flow analysis was conducted to investigate the change of the internal flow according to the rotor rotation. It is clarified from the experimental results that the performance of the mini centrifugal pump is improved by the splitter blades. The blade-to-blade low velocity region was suppressed in the case with the splitter blades. In addition to that, the unsteady flows near the volute casing tongue were suppressed due to the splitter blades. In the present paper, the performance of the mini centrifugal pump is shown and the unsteady flow condition is clarified with the results of the numerical flow analysis. Furthermore, the effects of the splitter blades on the performance and the unsteady internal flow condition are investigated.

A Study on the Effect of Dredging and Operation of Weirs on Hydraulic Characteristics in Nakdong River

수리구조물의 효과적 운영과 홍수기 피해저감을 위해서는 하천수리특성에 대한 적절한 이해가 필수적이기 때문에 본 연구에서는 낙동강 수계 금호강 유입지점 하류를 대상으로 준설 및 수리시설물 운영에 따른 수리특성 영향을 분석하였다. 수리특성 분석을 위해 HEC-RAS 모형을 이용하였으며, 부정류 흐름 해석을 통해 흐름의 천이를 반영하고 수리시설물 운영을 위한 다기능보 제원을 적용하였다. 조도계수와 조고값의 관계, 정류와 부정류 흐름해석, 사업 전과 후 하천 단면 그리고 다기능보 운영에 따른 하천 흐름에 대해 다각도로 연구를 하였다. 낙동강 수계 주요 관심구간을 중심으로 급격한 지형변화 및 수리구조물 운영에 의한 홍수에 대비하고 주요지점별 유량 전달에 의한 홍수파에 대해 분석된 자료는 합리적인 하천관리를 위한 기초자료로 활용될 수 있을 것이다. It is essential to understand the hydraulic characteristics in accordance with the changes in the river environment and operating hydraulic structures for increasing flood-control capacity. The objective of this study is to analysis the effect of dredging and operation of weirs on hydraulic characteristics in Nakdong River(Seongju~Imhaejin). HEC-RAS model has been used to examine the hydraulic characteristics with considering the flow transition through the unsteady flow analysis and the information about the specifications of multi-functional weirs. The relationship between Manning n and roughness height k, the effect of weir installation and operation on the flow and stage, and the bed change resulted from the river treatment project have been examined by steady and unsteady flow analysis. It is expected that the result from this study can be the basic data for the river treatment and management.

MHD transient free convection and chemically reactive flow past a porous vertical plate with radiation and temperature gradient dependent heat source in slip flow regime

The present paper deals with the analysis of unsteady free convection heat and mass transfer flow through a non-homogeneous porous medium with variable permeability bounded by an infinite porous vertical plate in slip flow regime taking into account the radiation, chemical reaction and temperature gradient dependent heat source. The flow is considered under the influence of magnetic field applied normal to the flow. The permeability of the porous medium and the suction velocity at the plate decrease exponentially with time about a constant mean. Approximate solutions for velocity, temperature and concentration fields are obtained using perturbation technique. The expressions for skin-friction and rate of heat transfer and rate of mass transfer are also derived. The results obtained are discussed for cooling case (Gr>0) of the plate. The effects of various physical parameters, encountered into the problem, on the velocity field are numerically shown through graphs while the effects on skin-friction and rate of heat and mass transfer are numerically discussed with the help of tables.

Visco-Elastic Unsteady Mhd Flow Between Two Horizontal Parallel Plates With Hall Current

An analysis of unsteady MHD flow of an electrically conducting visco-elastic fluid confined between two horizontal parallel non conducting plates in presence of a transverse magnetic field and Hall current is presented. The lower plate is a stretching sheet while the upper one is an oscillating porous plate, which is oscillating in its own plane. The motion of the fluid is produced by the stretching of the lower plate. A constant suction is applied at the upper plate and the stretching velocity is taken to be a linear function of distance along the channel. The equations governing the flow field are solved by perturbation technique. Expressions for velocity distribution of the flow field and non-dimensional skin-friction coefficient are obtained and presented graphically to observe the visco-elastic effect in combination of other flow parameters involved in the solution. The flow field is observed to be considerably affected by the visco-elastic parameter.

Unsteady flow and heat transfer analysis on a Sisko fluid

Unsteady flow and heat transfer analysis on a Sisko fluid

J013023 主成分分析による流体解析結果評価を支援する可視化ソフトウェアの開発([J013-02]流体情報学とデータ解析(2))

A visualization software for evaluating computational fluid dynamics (CFD) results was developed. Recently, progress of computer performance and CFD software enables to execute large-scale computation. However, the results of these computations tend to be enormous and difficult to be evaluated in detail. In order to reduce those results, proper orthogonal decomposition (POD) has been investigated. A small set of simulation results is analyzed by POD and grouped by the indicator of eigenvector. In this study, a visualization software for POD results was developed and applied to the unsteady flow analysis results of a optical disc drive model. The original results were reduced to 30% of original data and efficiently visualized by using this software.

Numerical analysis of unsteady cavitating turbulent flow and shedding horse-shoe vortex structure around a twisted hydrofoil

Cavitating turbulent flow around hydrofoils was simulated using the Partially-Averaged Navier–Stokes (PANS) method and a mass transfer cavitation model with the maximum density ratio (ρl/ρv,clip) effect between the liquid and the vapor. The predicted cavity length and thickness of stable cavities as well as the pressure distribution along the suction surface of a NACA66(MOD) hydrofoil compare well with experimental data when using the actual maximum density ratio (ρl/ρv,clip=43391) at room temperature. The unsteady cavitation patterns and their evolution around a Delft twisted hydrofoil were then simulated. The numerical results indicate that the cavity volume fluctuates dramatically as the cavitating flow develops with cavity growth, destabilization, and collapse. The predicted three dimensional cavity structures due to the variation of attack angle in the span-wise direction and the shedding cycle as well as its frequency agree fairly well with experimental observations. The distinct side-lobes of the attached cavity and the shedding U-shaped horse-shoe vortex are well captured. Furthermore, it is shown that the shedding horse-shoe vortex includes a primary U-shaped vapor cloud and two secondary U-shaped vapor clouds originating from the primary shedding at the cavity center and the secondary shedding at both cavity sides. The primary shedding is related to the collision of a radially-diverging re-entrant jet and the attached cavity surface, while the secondary shedding is due to the collision of side-entrant jets and the radially-diverging re-entrant jet. The local flow fields show that the interaction between the circulating flow and the shedding vapor cloud may be the main mechanism producing the cavitating horse-shoe vortex. Two side views described by iso-surfaces of the vapor volume fraction for a 10% vapor volume, and a non-dimensional Q-criterion equal to 200 are used to illustrate the formation, roll-up and transport of the shedding horse-shoe vortex. The predicted height of the shedding horse-shoe vortex increases as the vortex moves downstream. It is shown that the shape of the horse-shoe vortex for the non-dimensional Q-criterion is more complicated than that of the 10% vapor fraction iso-surface and is more consistent with the experiments. Further, though the time-averaged lift coefficient predicted by the PANS calculation is about 12% lower than the experimental value, it is better than other predictions based on RANS solvers.

Finite Element Analysis of Unsteady Seepage Flow through the Earth Dam of Huangbizhuang Reservoir

The unsteady seepage through earth dams is analyzed with the finite element method in this paper.Using this method, the seepage through Huangbizhuang earth dam is calculated. Many reasonable suggestions are posed to help the management and reinforcement of Huangbizhuang reservoir earth dam.

공력음향학을 이용한 축류홴의 삼차원 소음 해석

This paper presents a systematic procedure for three-dimensional noise analysis of an axial-flow fan by using computational aero-acoustics based on Ffowcs Williams-Hawkings equation. Flow-fields of a basic fan model are simulated by solving three-dimensional, unsteady, Reynolds-averaged Navier-Stokes equations using the commercial code ANSYS CFX 11.0. Starting with steady flow results, unsteady flow analysis is performed to extract the fluctuating pressures in the time domain at specified local points on the blade surface of the axial flow fan. The perturbed density wave by rotating blades reaches at the observer position, which is simulated by an in-house noise prediction software based on Ffowcs Williams-Hawkings equation. The detailed far-field noise signatures from the axial-flow fan are analyzed in terms of source types, field characteristics, and interpolation schemes.

Analysis of Unsteady Flows Past Horizontal Axis Wind Turbine Airfoils Based on Harmonic Balance Compressible Navier-Stokes Equations With Low-Speed Preconditioning

This paper presents the numerical models underlying the implementation of a novel harmonic balance compressible Navier-Stokes solver with low-speed preconditioning for wind turbine unsteady aerodynamics. The numerical integration of the harmonic balance equations is based on a multigrid iteration, and, for the first time, a numerical instability associated with the use of such an explicit approach in this context is discussed and resolved. The harmonic balance solver with low-speed preconditioning is well suited for the analyses of several unsteady periodic low-speed flows, such as those encountered in horizontal axis wind turbines. The computational performance and the accuracy of the technology being developed are assessed by computing the flow field past two sections of a wind turbine blade in yawed wind with both the time-and frequency-domain solvers. Results highlight that the harmonic balance solver can compute these periodic flows more than 10 times faster than its time-domain counterpart, and with an accuracy comparable to that of the time-domain solver.

Hydromagnetic Unsteady Mixed Convection Flow Past an Infinite Vertical Porous Plate

An approximate analysis of unsteady mixed convection flow of an electrically conducting fluid past an infinite vertical porous plate embedded in porous medium under constant transversely applied magnetic field is presented here. The periodic transverse suction velocity is applied to the surface due to which the flow becomes unsteady. The surface is kept at oscillating wall temperature. Analytical expressions for the transient velocity, temperature, amplitude and phase of the skin-friction and the rate of heat transfer are obtained and discussed in detail with the help of graphs, under different parameter values.

개수로에서의 부정류 수문곡선 재현을 위한 유량공급장치의 개발 및 정확도 분석

시간에 따른 하도의 수위 및 유량 변화에 영향을 많이 받는 수리구조물의 설계에 있어서 부정류 흐름 해석은 반드시 필요하다. 일반적으로 부정류 흐름 해석에는 수치모형이 많이 활용되고 있으나 수치모형의 검 보정을 위한 현장 자료의 획득이 어려운 경우가 많다. 또한 자료구축이 가능하더라도 인력과 비용이 많이 소모되며, 측정 정확도를 신뢰하기 어려운 경우가 많다. 이러한 경우 수치모형의 검 보정을 위해 부정류 수리실험을 통해 획득되는 자료를 활용하는 것이 대안이 될 수 있다. 따라서 본 연구에서는 다양한 형태의 부정류 수문곡선을 실험에서 재현할 수 있는 유량공급장치를 개발하고자하며, 개발된 부정류 유량공급장치를 이용하여 수리실험 수로에서 재현되는 수문곡선과 목표 수문곡선을 비교 분석함으로써 재현 정확도를 정량적으로 평가하고자 한다. 본 연구에서는 유량이 급격하게 증가 또는 감소하는 사각형 형태, 첨두유량 발생 시간이 짧은 삼각형 형태 및 일반적인 홍수 수문곡선 형태의 종(bell) 형태 수문곡선을 대상으로 재현 오차 및 Root Mean Square Error (RMSE)를 분석하였다. 재현 정확도 분석 결과, 사각형 형태의 수문곡선 재현 오차는 약 59% 정도로 가장 크게 나타났으며, 삼각형 형태의 수문곡선은 단일첨두와 이중첨두 형태 모두 약 10% 정도의 재현 오차가 나타났지만 홍수 수문곡선 형태인 종 모양의 수문곡선의 재현 오차는 최대 2% 이내인 것으로 나타났다. The analysis for unsteady flow is necessary to design the hydraulic structures affected by water level and discharge changes through time. The numerical model has been generally used for unsteady flow analysis, however it is difficult to acquire field data to calibrate and validate the numerical model. Even though it is possible to collect field data for some case, high cost and labor are required and sometimes it is considered that the confidence of measured data is very low. In this case, the experimental data for unsteady flow can be used to calibrate and validate the numerical model as an alternative. Therefore, the discharge-supply system which could generate various type of unsteady flow hydrograph was developed in this study. Also, the accuracy of the unsteady flow hydrograph generated by developed dischargesupply system in the experiment was evaluated by comparing with target hydrograph. Accuracy errors and Root Mean Square Error (RMSE) were analyzed for the rectangular-type hydrograph with sudden changes of flow, triangular-type hydrograph with short peak time, and bell-type flood hydrograph. As a result, the generating error of the discharge-supply system for the rectangular-type hydrograph was about 59% which was maximum error among various types. Also, it was represented that RMSE for the triangular-type hydrographs with single and double peaks were approximately corresponding to 10%. However, RMSE for the bell-type flood hydrograph was lower than 2%.