Analysis Of Unsteady Flow Research Articles

Unsteady Flow of Third Grade Fluid over an Oscillatory Stretching Sheet with Thermal Radiation and Heat Source/Sink

AbstractThe aim of this article is to investigate the unsteady boundary layer flow and heat transfer analysis in a third grade fluid over an oscillatory stretching sheet under the influences of thermal radiation and heat source/sink. The convective boundary condition at the sheet is imposed to determine the temperature distribution. Homotopy analysis method (HAM) is used to solve dimensionless nonlinear partial differential equations. The effects of involved parameters on both velocity and temperature fields are illustrated in detail through various plots. It is found that the amplitude of velocity decreases by increasing the ratio of the oscillation frequency of the sheet to its stretching rate and Hartmann number while it increases by increasing the third grade fluid parameter. On contrary, the temperature field is found to be a decreasing function of the third grade fluid parameter.

J0530304 ジャイロミル型風車の流体力学的考察による翼型の研究

Gyromill wind turbines with two different blade profiles were investigated experimentally and numerically in order to verify the effect of the negative camber on aerodynamic performance. Experiments were carried out using a model turbine impeller with an axial length of 200 mm and a diameter of 200 mm. The performance of the model wind turbines with two blades were measured with a wind tunnel in a wind velocity of constant 4 m/s. The results showed that the maximum power coefficient was higher for the blade with negative camber by 5.0 percentage points than the one without camber. An unsteady numerical flow analysis around the wind turbine was then conducted using a commercial code employing the finite volume method. The results showed that the power coefficient of one blade had a maximum value in the second quadrant for the two tested wind turbines. The results also showed that the blades with negative camber were advantageous for obtaining high rotational torque in the position by examination of the pressure distribution on the blade.

Heat transfer analysis of unsteady graphene oxide nanofluid flow using a fuzzy identifier evolved by genetically encoded mutable smart bee algorithm

In the current research, the unsteady two dimensional Graphene Oxide water based nanofluid heat transfer between two moving parallel plates is analyzed using an intelligent black-box identifier. The developed intelligent tool is known as evolvable evolutionary fuzzy inference system (EE-FIS) which is based on the integration of low-level fuzzy programming and hyper-level evolutionary computing concepts. Here, the authors propose the use of a modified evolutionary algorithm (EA) which is called hybrid genetic mutable smart bee algorithm (HGMSBA). The proposed HGMSBA is used to evolve both antecedent and consequent parts of fuzzy rule base. Besides, it tries to prune the rule base of fuzzy inference system (FIS) to decrease its computational complexity and increase its interpretability. By considering the prediction error of the fuzzy identifier as the objective function of HGMSBA, an automatic soft interpolation machine is developed which can intuitively increase the robustness and accuracy of the final model. Here, HGMSBA-FIS is used to provide a nonlinear map between inputs, i.e. nanoparticles solid volume fraction (ϕ), Eckert number (Ec) and a moving parameter which describes the movements of plates (S), and output, i.e. Nusselt number (Nu). Prior to proceeding with the modeling process, a comprehensive numerical comparative study is performed to investigate the potentials of the proposed model for nonlinear system identification. After demonstrating the efficacy of HGMSBA for training the FIS, the system is applied to the considered problem. Based on the obtained results, it can be inferred that the developed HGMSBA-FIS black-box identifier can be used as a very authentic tool with respect to accuracy and robustness. Besides, as the proposed black-box is not a physics-based identifier, it frees experts from the cumbersome mathematical formulations, and can be used for advanced real-time applications such as model-based control. The simulations indicate that the gradient of Nu has a direct nonlinear relation with the values of ϕ and Ec. It is also observed that an increase in the value of S decreases the value of Nu.

Simplified wave models applicability to shallow mud flows modeled as power-law fluids

Simplified wave models — such as kinematic, diffusion and quasi-steady — are widely employed as a convenient replacement of the full dynamic one in the analysis of unsteady open-channel flows, and especially for flood routing. While their use may guarantee a significant reduction of the computational effort, it is mandatory to define the conditions in which they may be confidently applied. The present paper investigates the applicability conditions of the kinematic, diffusion and quasisteady dynamic shallow wave models for mud flows of power-law fluids. The power-law model describes in an adequate and convenient way fluids that at low shear rates fluids do not posses yield stress, such as clay or kaolin suspensions, which are frequently encountered in Chinese rivers. In the framework of a linear analysis, the propagation characteristics of a periodic perturbation of an initial steady uniform flow predicted by the simplified models are compared with those of the full dynamic one. Based on this comparison, applicability criteria for the different wave approximations for mud flood of power-law fluids are derived. The presented results provide guidelines for selecting the appropriate approximation for a given flow problem, and therefore they may represent a useful tool for engineering predictions.

Numerical analysis of unsteady MHD flow near a stagnation point of a two-dimensional porous body with heat and mass transfer, thermal radiation, and chemical reaction

The problem of unsteady MHD flow near a stagnation point of a two-dimensional porous body with heat and mass transfer in the presence of thermal radiation and chemical reaction has been numerically investigated. Using a similarity transformation, the governing time-dependent boundary layer equations for the momentum, heat and mass transfer were reduced to a set of ordinary differential equations. This set of ordinary equations were then solved using the spectral local linearization method together with the successive relaxation method. The study made among others the observation that the local Sherwood number increases with increasing values of the unsteadiness parameter and the Schmidt number. The fluid temperature was found to be significantly reduced by increasing values of the Prandtl number and the thermal radiation parameter. The velocity profiles were found to be reduced by increasing values of the chemical reaction and the Schmidt number as well as by the magnetic parameter.

Unsteady Flow Separation and High Performance of Airfoil with Local Flexible Structure at Low Reynolds Number

AbstractThe unsteady flow separation of airfoil with a local flexible structure (LFS) is studied numerically in Lagrangian frames in detail, in order to investigate the nature of its high aerodynamic performance. For such aeroelastic system, the characteristic-based split (CBS) scheme combined with arbitrary Lagrangian-Eulerian (ALE) framework is developed firstly for the numerical analysis of unsteady flow, and Galerkin method is used to approach the flexible structure. The local flexible skin of airfoil, which can lead to self-induced oscillations, is considered as unsteady perturbation to the flow. Then, the ensuing high aerodynamic performances and complex unsteady flow separation at low Reynolds number are studied by Lagrangian coherent structures (LCSs). The results show that the LFS has a significant influence on the unsteady flow separation, which is the key point for the lift enhancement. Specifically, the oscillations of the LFS can induce the generations of moving separation and vortex, which can enhance the kinetic energy transport from main flow to the boundary layer. The results could give a deep understand of the dynamics in unsteady flow separation and flow control for the flow over airfoil.

Numerical analysis of unsteady cavitating flow in an axial inducer

This work presents the results of numerical simulation of unsteady cavitating flow through a two–bladed axial inducer. First, the analysis was carried out in a blade cascade, this two–dimensional simplified model, obtained from the studied axial inducer, was used as a test case. Later, the numerical simulations were extended to the original three-dimensional inducer. All numerical calculations were realized in cavitating flow regime. Initially, the results were obtained in steady state, and then in unsteady state.The main purpose of this study is to explore the local cavitation instabilities, such as alternate blade cavitation and rotating blade cavitation, which can appear in this type of devices when they work under certain operating conditions.The numerical results show that the fluid flow in the axial inducer is altered by the emergence of the cavitation. These vapor regions are formed, firstly near to the leading edge of each blade. The behavior of the cavitation depends on the operating conditions of the inducer, mainly by the flow rate and the suction pressure.The numerical simulation was performed using a commercial code based on a cell–centered finite–volume method. The cavitation model used for calculations assumes a thermal equilibrium between phases. It is based on the classical conservation equations of the vapor phase and a mixture phase, with mass transfer due to the cavitation appearing as a source and a sink term in the vapor mass fraction equation. The mass transfer rate is derived from a simplified Rayleigh–Plesset model for bubble dynamics.

Flow Field Numerical Simulation on the Membrane Aeration Biofilm Reactor

Based on the design of membrane bioreactor three-dimensional finite element model is established,Finite volume method was applied to sewage treatment in the process of membrane bioreactor internal flow field is unsteady flow numerical simulation and analysis. The results of numerical simulation can be seen at the bottom of the membrane module speed change is small, to purify sewage full contact with membrane module, the upper change is bigger, treated sewage can be quickly exhaust to the outside world. This study on flow field of the bioreactor of in-depth study, the new structure design and optimization has its far-reaching significance. Finite volume method was applied to sewage treatment in the process of membrane bioreactor internal flow field is unsteady flow numerical simulation and analysis. The results of numerical simulation can be seen at the bottom of the membrane module speed change is small, to purify sewage full contact with membrane module, the upper change is bigger, treated sewage can be quickly exhaust to the outside world. This study on flow field of the bioreactor of in-depth study, the new structure design and optimization has its far reaching significance.

Pathline glyphs

AbstractVisualization of pathlines is common and highly relevant for the analysis of unsteady flow. However, pathlines can intersect, leading to visual clutter and perceptual issues. This makes it intrinsically difficult to provide expressive visualizations of the entire domain by an arrangement of multiple pathlines, in contrast to well‐established streamline placement techniques. We present an approach to reduce these problems. It is inspired by glyph‐based visualization and small multiples: we partition the domain into cells, each corresponding to a downscaled version of the entire domain. Inside these cells, a single downscaled pathline is drawn. On the overview scale, our pathline glyphs lead to emergent visual patterns that provide insight into time‐dependent flow behavior. Zooming‐in allows us to analyze individual pathlines in detail and compare neighboring lines. The overall approach is complemented with a context‐preserving zoom lens and interactive pathline‐based exploration. While we primarily target the visualization of 2D flow, we also address the extension to 3D. Our evaluation includes several examples, comparison to other flow visualization techniques, and a user study with domain experts.

Unsteady Seepage Applied to Lining Design in Stilling Basins

AbstractCases of damages experienced in stilling basins and on the toe of chute spillways are recognized to be due to global instantaneous uplift force resulting from imbalance between the instantaneous pressure distribution both above and below slabs. The latter is due to pressure fluctuation propagation forced though unsealed slab joints. In the past, several theoretical approaches were developed to estimate the uplift contribution due to pressure transmission underneath the slabs. In the paper, a model is presented, based on unsteady flow analysis of seepage through porous media, that shows that the propagation models proposed in literature are particular cases of this more comprehensive formulation. In addition, the use of the unsteady seepage model makes it possible to account for finite thickness foundation layers, typical in the cases of earth dams, rock-fill dams, and in other dam types, whenever the slabs are posed on alluvial substrates. Experimental results obtained in laboratory facilities, as...

Numerical analysis of unsteady cavitating flow by using a modification based on an assumption of apparent phase equilibrium

The prediction accuracy of cavitation by CFD is still not so high even in a simplest flow field around a single hydrofoil especially in transient condition at higher angle of attack, which is common problem in both commercial software and in-house solvers. In the transient condition, unsteady cavitation occurs, in which sheet cavity breaks off and cloud cavity sheds downstream periodically. At that time, the sheet cavity length tends to be underestimated in usual CFD. In the present study, modification for the phase change model is suggested, which is based on an idea of apparent phase equilibrium on gas-liquid interface with unsteady and disturbed flow. At first, a preliminary experiment has been done for evaporation on two gas- liquid interfaces with and without flow, the result contributes the evidence of the idea of apparent phase equilibrium with flow. In the result, the pressure around gas-liquid interface with flow was higher than that without flow on the occasion of evaporation, it means flow accelerates evaporation. I treat the gap of the pressure as a gap of phase equilibrium pressure macroscopically. Then, numerical simulation of cavitating flow around a hydrofoil is performed with a modification of phase change model in the transient condition at higher angle of attack which is most difficult to predict by the present solvers. In the modification, the gap of the pressure with and without flow is taken into account according to a value of a local variation of velocity in the cavitating flow filed. The formulation is similar to the PDF model for phase change model in cavitation by Singhal. The numerical results by the present modification are compared among few pressure variation components which are assumed to accelerate the evaporation in transient cavitation.

홍수시 둔치구간의 수리해석을 위한 2차원 모형 비교

2000년대 후반 이래로 홍수터를 활용한 생태공원이나 체육시설 등과 같은 다양한 친수시설들이 조성되었다. 또한 최근 강우강도 및 홍수 빈도의 증가로 인해 관련 피해가 급증하고 있으며, 이는 홍수터에 설치된 공원과 같은 친수시설들의 침수피해와 직접적으로 연관되므로, 극한강우 시 홍수터에서의 수치해석이 필요하다고 판단된다. 본 연구에서는 홍수터에서의 수위상승과 하강에 따른 수리학적 영향을 분석하기 위해 마름/젖음 모의가 가능한 준부정류 흐름해석모형인 FaSTMECH와 부정류 흐름해석모형인 Nays2D 모형을 이용하여 각 모델간의 태풍 사상시 시간에 따른 침수심 및 유속분포를 비교 분석하였다. 태풍 사상시 홍수터에서의 유속분포는 주수로에서의 유속에 비해 매우 낮은 유속을 보이고 있었으며, 홍수터에서는 침식보다는 토사퇴적 문제가 더 높을 것이라 판단된다. In the flood plain, river facilities such as sports facilities and ecological park are builded up since the late 2000s. The recent increase of rainfall intensity and flood frequency results in the immersions of parks and river facilities located in the flood plain. Therefore it is necessary to perform the numerical analysis for the extreme rain storm in the flood plain. In this study, to analyze the hydraulic impact by lowering and rising of the water level at flood plain, Both the FaSTMECH, which is a quasi-unsteady flow analysis model to be used for simulating the wet and dry, and the Nays2D, which is unsteady flow analysis model, are used in this study. Also, the flow velocity distribution and the inundation are compared over a period of the typhoon. As a result, the flow velocity distribution at flood plain showed very low values compared to the flow rate in the main channel. This means that the problem of sedimentation is more important than that of erosion in the flood plain.

Computation of unsteady viscous flow around a locally flexible airfoil at low Reynolds number

A numerical method for fluid–structure interaction is presented for the analysis of unsteady viscous flow over a locally flexible airfoil. The Navier–Stokes equations are solved by ALE–CBS algorithm, coupling with a structural solver with large deformation. Following the validation of the method, a numerical example for the flight of micro-air vehicles at low Reynolds number is chosen for the computation. The coupling effect of flexible structure with different elastic stiffness on aerodynamic performance is demonstrated. A noticeable camber effect is induced by the deflection of the structure as the elastic stiffness of the structure goes smaller. Moreover, when the vibrating frequencies of the structure with smaller elastic stiffness have a close correlation with the shedding frequencies, the positive impact of the vibration of local flexible surface on the lift of the airfoil is highlighted, which results from the formation of the coherent vortices.

재생형 송풍기의 고효율 저소음 설계를 위한 통합형 최적설계 프로그램 개발

A multi-objective optimization of a regenerative fan for enhancing the aerodynamic and aeroacoustic performance was carried out using an integrated fan design system, namely, Total FAN-Regen<SUP>ⓡ</SUP>. The Total FAN-Regen<SUP>ⓡ</SUP> was developed for non-specialists to carry out a series of design process, viz., computational preliminary design, three-dimensional aerodynamic and aeroacoustic analyses, and design optimization, for a regenerative fan. An aerodynamic analysis of the regenerative fan was conducted by solving three-dimensional Reynolds-averaged Navier-Stokes equations using the shear stress transport turbulence model. And, an aeroacoustic analysis of the regenerative fan was implemented in a finite/infinite element method by solving the variational formulation of Lighthill’s analogy based on the results of the unsteady flow analysis. An optimum shape obtained by Total FAN-Regen<SUP>ⓡ</SUP> shows the enhanced efficiency and decreased sound pressure level as much as 1.5 % and 20.0 dB, respectively, compared to those of the reference design. The performance test was carried out for an optimized regenerative fan<BR>to validate the performance of the numerically predicted optimal design.

NATURAL CONVECTION FLOW PAST AN OSCILLATING PLATE WITH NEWTONIAN HEATING

An exact analysis of unsteady natural convection flow of viscous fluid past an oscillating plate with Newtonian heating is presented. The mathematical model with radiation effect in the energy equation is reduced to a system of linear partial differential equations. These equations are solved exactly using the Laplace transform technique. Expressions for the velocity and temperature are obtained. They satisfy all imposed initial and boundary conditions. As a special case, these solutions can be reduced to similar solutions in the existing literature. The skin friction and Nusselt number are evaluated analytically as well as numerically in a tabular form. Numerical results for velocity and temperature are shown graphically for various parameters of interest and the physics of the problem is well explored.

Analysis of Unsteady MHD Thin Film Flow of a Third Grade Fluid with Heat Transfer down an Inclined Plane

An investigation is made for unsteady MHD thin film flow of a third grade fluid down an inclined plane with no slip boundary condition. The governing nonlinear partial differential equations involved are reduced to linear partial differential equations using regular perturbation method. The resulting equations were solved using method of separation of variable and eigen functions expansion. The solutions obtained were examined and discussed graphically. It is interesting to find that the variation of the velocity and temperature profile with the magnetic field parameter and gravitational parameter depends on time.

316 集水装置を有する軸流水車の非定常流れ解析(OS3-(4),OS3 オーガナイズドセッション《熱・流体・エネルギー変換工学の進展》)

A hydraulic turbine for an open channel can generate power at low head and doesn't affect the environment because it doesn't have incidental equipment. However, because the rotational speed is low, the hydraulic turbine outer diameter becomes large. Therefore, we proposed an axial flow hydraulic turbine with collection device, using kinetic energy of open channel flow. The performances and flow fields of runner only, collection device only and combination of runner and collection device were clarified. It was checked the velocity increasing of collection flow in collection device by analysis result of collection device only and analysis result of output of combination of runner and collection device is larger than analysis result of runner only. However, the collection device used in these studies is non-axial symmetry. To study the effect of the blade position, we conducted the unsteady flow analysis for the combination of the runner and the collection device, compared with the steady flow analysis result. As a result, effect of the blade position on the performance of this hydraulic turbine was clarified.

Unsteady Magnetohydrodynamic Free Convection Flow of a Radiative Fluid Past an Infinite Vertical Plate with Constant Heat and Mass Flux

Theoretical analysis of unsteady magnetohydrodynamic free convection flow of a viscous incompressible radiative fluid past an infinite vertical plate with constant heat and mass flux is presented. The dimensionless governing linear partial differential equations have been solved using the Laplace transform technique. The exact solutions for the velocity, temperature and concentration fields are derived. The effects of radiation, magnetic field and buoyancy ratio parameters on the velocity and temperature fields are discussed through graphs. It is found that the velocity increases with increasing radiation parameter whereas it decreases with increasing magnetic field parameter for buoyancy assisted flows.

Numerical investigation of unsteady airflow in subway influenced by piston effect based on dynamic mesh

The piston effect has a significant influence on unsteady airflows in subway stations and tunnels. This study uses in situ experimental data and a computational fluid dynamics (CFD) method to analyze the three-dimensional unsteady air flow in a subway station and tunnel. An experimental analysis of train-induced unsteady flow was measured in an actual station with platform bailout doors (PBD), and air velocity variations were recorded at regular time intervals. The unsteady numerical analysis uses a dynamic mesh method for the full-scale model. The results indicate that Standard k–ε and RNG k–ε equations are both appropriate for simulating the high Reynolds numbers in tunnel and station airflow because these equations coincide with the experimental data. Specific diversion and suction ratios exist in each channel of the airflow for piston wind. The proportions between bypass ducts and platforms are stable no matter in open or close systems. And the draught relief shaft located before station plays more important role for piston wind than the one located after the station.

Three-dimensional large eddy simulation and vorticity analysis of unsteady cavitating flow around a twisted hydrofoil

Large Eddy Simulation (LES) was coupled with a mass transfer cavitation model to predict unsteady 3-D turbulent cavitating flows around a twisted hydrofoil. The wall-adapting local eddy-viscosity (WALE) model was used to give the Sub-Grid Scale (SGS) stress term. The predicted 3-D cavitation evolutions, including the cavity growth, break-off and collapse downstream, and the shedding cycle as well as its frequency agree fairly well with experimental results. The mechanism for the interactions between the cavitation and the vortices was discussed based on the analysis of the vorticity transport equation related to the vortex stretching, volumetric expansion/contraction and baroclinic torque terms along the hydrofoil mid-plane. The vortical flow analysis demonstrates that cavitation promotes the vortex production and the flow unsteadiness. In non-cavitation conditions, the streamline smoothly passes along the upper wall of the hydrofoil with no boundary layer separation and the boundary layer is thin and attached to the foil except at the trailing edge. With decreasing cavitation number, the present case has σ = 1.07, and the attached sheet cavitation becomes highly unsteady, with periodic growth and break-off to form the cavitation cloud. The expansion due to cavitation induces boundary layer separation and significantly increases the vorticity magnitude at the cavity interface. A detailed analysis using the vorticity transport equation shows that the cavitation accelerates the vortex stretching and dilatation and increases the baroclinic torque as the major source of vorticity generation. Examination of the flow field shows that the vortex dilatation and baroclinic torque terms increase in the cavitating case to the same magnitude as the vortex stretching term, while for the non-cavitating case these two terms are zero.