Finite Volume Computational Fluid Dynamics Research Articles

Parametric Study of an Oscillating Airfoil in a Power-Extraction Regime

A wing that is heaving and pitching simultaneously may extract energy from an oncoming flow, thus acting as a turbine. The theoretical performance of such a concept is investigated here through unsteady two-dimensional laminar-flow simulations using the commercial finite volume computational fluid dynamics code FLUENT. Computations are performed in the heaving reference frame of the airfoil, thus leaving only the pitching motion of the airfoil to be dealt with through a rigid-body mesh rotation and a circular nonconformal sliding interface. Unsteady aerodynamics basics of the oscillating airfoil are first exposed, with a description of the operating regimes. Effects of unsteadiness are stressed and the inadequacy of a quasi-steady approach to take them into account is exposed. We present a mapping of power-extraction efficiency for a single oscillating airfoil in the frequency and pitching-amplitude domain: 0 55deg in which efficiencies are higher than 20%. Results from a parametric study are then provided and discussed. It is found that motion-related parameters such as heaving amplitude and frequency have the strongest effects on airfoil performances, whereas geometry and viscous parameters turn out to play a secondary role.

Simulation of heat exposure and damage to the eye lens in a neighborhood bakery

Epidemiological studies indicated a link between high temperature environment and cataract. The purpose of the study was to investigate if the high temperature in neighborhood bakeries can cause damage to the eye lens. Measurements were done to determine the temperature and exposure time in the neighborhood bakeries during a workday. Thermal analysis was done using finite volume and finite element Computational Fluid Dynamics (CFD) codes in order to determine the temperature in the eye lens when exposed to environmental temperature fluctuations. A simulation of heat exposure was carried out using a bovine lens organ culture system. Two-hundred and seventy bovine lenses were divided into five groups. (1) Control group kept in culture for 11–14 days (2) Lenses exposed to 39.5 °C, 6 h daily starting on the second day of the culture and kept in culture for 13 days (3) Lenses exposed to 39.5 °C, 4 h daily starting on the second day of the culture and kept in culture for 11 days (4) Lenses exposed to 39.5 °C, 2 h daily for 3 days starting on the second day of the culture and kept in culture for 12 days (5) Lenses exposed to 39.5 °C, 1 h on the second day of the culture and kept in culture for 14 days. Lens optical quality was assessed during the culture period. At the end of the culture lens damage was demonstrated by inverted microscopy. Lens epithelial samples were taken for analysis of Catalase activities. Control lenses maintained their optical quality throughout the 14 days of the culture. Exposure to heat caused optical damage to the cultured lenses. The damage appeared earlier in the 6 h exposure group and progressed from the lens anterior suture to its center. Optical damage was recovered in lenses exposed 1 h to 39.5 °C, but the damage remained in the lens epithelial cells. Our study indicates that exposure to heat in bakeries can cause damage to the eye lens and that the damage is dependent on the length of exposure.

Diesel Engine Simulations with Multi-Dimensional Conditional Moment Closure

First-order elliptic Conditional Moment Closure (CMC), coupled with a computational fluid dynamics (CFD) solver, has been employed to simulate combustion in a direct-injection heavy-duty diesel engine. The three-dimensional structured finite difference CMC grid has been interfaced to an unstructured finite volume CFD mesh typical of engine modelling. The implementation of a moving CMC grid to reflect the changes in the domain due to the compression and expansion phases has been achieved using an algorithm for the cell addition/removal and modelling the additional convection term due to the CMC cell movement. Special care has been taken for the boundary conditions and the wall heat transfer. An operator splitting formulation has been used to integrate the CMC equations efficiently. A CMC domain reduction of the three-dimensional problem to two- and zero-dimensions through appropriate volume integration of the CMC equation has been explored in terms of accuracy and computational time. Additional considerations have been reported concerning the initialization of the CMC domain in conserved scalar space during transient calculations where the probability density function of the mixture fraction changes drastically with time as during fuel injection. Predictions compare favourably with the experimental pressure traces for tests at full and half load. A balance of terms in the CMC equation allows conjectures on the structure of the flame and its expansion across the spray after autoignition.

Finite Volume Computational Fluid Dynamics Package for Solving Convective Heat Transfer Cases

Computational fluid dynamics (CFD) has brought a step change in the field of flow analysis, particularly in the areas of fluid dynamics and heat transfer, which are intertwined. It has carved a niche in applications where, a decade back, experimentation was the only way forward. This article covers the benchmarking of convective heat transfer cases through the use of CFD software. The cases investigated are common cases of heat transfer involving free convection and one case involving forced convection. The results from CFD analysis are compared with the regressions available for each test case. The two sets of data coincide well, with a percentage difference as low as 0.4%. Information on setting up CFD analysis accurately, particularly for solving problems of natural convection, is also included.

A continuous adjoint formulation for the computation of topological and surface sensitivities of ducted flows

AbstractTopology optimization of fluid dynamic systems is a comparatively young optimal design technique. Its central ingredient is the computation of topological sensitivity maps. Whereas, for finite element solvers, implementations of such sensitivity maps have been accomplished in the past, this study focuses on providing this functionality within a professional finite volume computational fluid dynamics solver. On the basis of a continuous adjoint formulation, we derive the adjoint equations and the boundary conditions for typical cost functions of ducted flows and present first results for two‐ and three‐dimensional geometries. Emphasis is placed on the versatility of our approach with respect to changes in the objective function. We further demonstrate that surface sensitivity maps can also be computed with the implemented functionality and establish their connection with topological sensitivities. Copyright © 2008 John Wiley & Sons, Ltd.

Time-Marching Method for Computations of High-Speed Compressible Flow on Structured and Unstructured Grid

The development of Computational Fluid Dynamics (CFD) compressible codes for 2D structured quadrilateral grid and 3D unstructured hexahedral grid is described. The high-speed flow in a nozzle blade cascade is predicted numerically by solving the 2D/3D Euler Equations in a coupled manner. The new finite volume CFD solvers employ second-order accurate central differencing scheme for spatial discretization and multi-stage Runge-Kutta technique for temporal integration with flow variables stored at the vertices. Artificial dissipations with pressure sensors are introduced to control solution stability and capture shock discontinuity. The predictions have been compared with experimental measurements and good agreement has been found.

Numerical Investigation on Cooling of Small form Factor Computer Cases

In this study, cooling of small form factor computers is numerically investigated. The problem is a conjugate heat transfer problem in which ambient air is the final heat transfer medium. In modeling the problem, heat transfer using heat pipes running from the CPU to the heat exchanger in the back end of the chassis, forced convection inside the chassis, ventilation of the chassis air, conduction paths inside the chassis, and natural convection from the chassis walls to the ambient air are considered. The numerical model is analyzed using a commercial finite volume computational fluid dynamics software package. The effects of grid selection, discretization schemes and turbulence model selection on simulation results are discussed. In addition, recirculation and relaminarization are addressed briefly as examples of physical phenomena affecting cooling. For a comparison with the computational fluid dynamics results, an experiment is conducted and some temperature measurements are obtained from critical locations inside the chassis. The computational results were found to be in good agreement with the experimental ones.

Computational fluid dynamics simulation of in-cylinder flows in a motored homogeneous charge compression ignition engine cylinder with variable negative valve overlapping

In-cylinder air motion is one of the most important factors that control the degree of mixture preparation and thus is fundamental to improvements in the combustion process and overall engine performance. The major aim of this paper is to elucidate, through a predictive study, the main features of in-cylinder flow fields in a motored homogeneous charge compression ignition (HCCI) engine cylinder with variable negative valve overlapping (NVO). A commercial finite-volume computational fluid dynamics (CFD) package was used in the programme of simulation. The computational model was validated through a qualitative comparison between CFD results and the available experimental data. Thus one of the main developments presented in this study is the investigation of the intake process of the HCCI engine with various valve strategies, and it is perhaps the first time (to the current authors' best knowledge) that a direct comparison has been made of the results obtained in the same HCCI NVO motored engine using modelling and experimental approaches. The comparison illustrated a fair agreement between both sets of results, with some differences. A parametric predictive study of the effects of variable valve timings on the in-cylinder air motion has then been carried out. Three different sets of valve timings have been applied to the intake and exhaust valves to generate NVO of 70, 90, and 110 degrees of crank angle (°CA). The NVO was controlled by adjusting the times of exhaust valves closing (EVC) and intake valves opening (IVO) while keeping the times of exhaust valves opening (EVO) and intake valves closing (IVC) unchanged. The predicted results show a noticeable modification of the strength and the global direction of the in-cylinder charge motion as a result of increasing the magnitude of NVO. Modifications of in-cylinder swirl and tumble motions obtained by applying higher degrees of NVO are expected to have a considerable effect on the air-fuel mixture preparation process as well as the actual in-cylinder conditions at the end of the compression stroke.

Numerical simulation of air-water two-phase flow over stepped spillways

Stepped spillways for significant energy dissipation along the chute have gained interest and popularity among researchers and dam engineers. Due to the complexity of air-water two-phase flow over stepped spillways, the finite volume computational fluid dynamics module of the FLUENT software was used to simulate the main characteristics of the flow. Adopting the RNG k - e turbulence model, the mixture flow model for air-water two-phase flow was used to simulate the flow field over stepped spillway with the PISO arithmetic technique. The numerical result successfully reproduced the complex flow over a stepped spillway of an experiment case, including the interaction between entrained air bubbles and cavity recirculation in the skimming flow regime, velocity distribution and the pressure profiles on the step surface as well. The result is helpful for understanding the detailed information about energy dissipation over stepped spillways.

Lift and drag forces on a sphere attached to a wall in a Blasius boundary layer

Lift and drag forces on a sphere attached to a planar wall, over which a laminar flat plat boundary layer flows, are examined numerically in this study. Particle Reynolds number ranged from 0.1–250, which represents steady, laminar flow about the sphere, and the plate Reynolds number was held constant at 32 400. A finite-volume computational fluid dynamics program was utilised. Simulation results were validated against analytical results for drag and lift in creeping flow and against experimental results available in the literature for lift at higher particle Reynolds number. The model results were curve-fitted and interpolating drag and lift coefficient functions are reported. The lift and drag results are shown to be weakly dependent upon plate Reynolds number. The resulting correlations are expected to be useful in the development of particle impending motion and aerosol entrainment predictions of particles adhering to planar walls.

Computational Fluid Dynamics for the Prediction of Temperature Profiles and Hygienic Design in the Food Industry

Many factors need to be considered to ensure that food process lines consistently deliver microbiologically safe products. These factors include process and equipment design, and in particular the interaction between the product and the equipment geometry. Computational fluid dynamics (CFD) has recently been used within the food industry in a number of applications. In this work a commercial finite volume CFD code, FLUENT, has been used to predict those parameters critical to the prediction of velocity and temperature profiles for low viscosity fluids in typical, complex equipment geometries and compared these with experimental data. A comparison of the temperature distribution using predictive modelling and experiments is encouraging, and qualitatively satisfactory and reliable for laminar and turbulent flows. Application of this approach will enable the interaction between food products and equipment geometries to be predicted, and provide an improved method of assessing the implications for the microbiological safety of foods.

A three-dimensional CFD model of direct ethanol fuel cells: Anode flow bed analysis

This paper presents a study of the flow field and residence times in the anode flow bed of a pilot direct ethanol fuel cell (DEFC) using 3-D numerical flow modelling. The flow characteristics in the pilot DEFC anode flow bed are discussed with regard to the available data. Calculations of the flow field characteristics and residence times in a pin-type fuel cell with a modified version of an existing finite-volume computational fluid dynamics code are presented. The numerical results are in good agreement with other computational and experimental studies for the residence times and flow distribution inside the anode flow bed. The present model provides results that can be of good assistance for the improvement of specific flow bed designs.

Hydrodynamic study of a rotating MHD flow in a cylindrical cavity by ultrasound Doppler shift method

An experimental study of a steady laminar magnetohydrodynamic (MHD) flow driven by a rotating disk at the top of a cylindrical cavity filled with water or mercury is presented. The velocity distributions were analysed using the ultrasound velocity (UVP) measuring technique. The uniform and constant applied magnetic field is directed along the axis of the cavity. The measurements were compared with results obtained from a numerical model based on a finite volume computational fluid dynamics (CFD) model. The effects of the magnetic field, the fluid and wall electrical conductivities, and the wall thickness are investigated through the conductance ratio k which characterises the influence of the wall on the closure of the electric current distribution. The other relevant parameters are the Hartmann number M, and the Reynolds number Re. The study was performed essentially for different values of Re ⩽ 30,000 and M ⩽ 260. There were close agreement between numerical results, the present ultrasonic measurements and other reported experimental and numerical works. The experiments have revealed something that has not been predicted numerically, the sidewall layer is unstable for special conditions of Hartmann and Reynolds numbers.

Thermal analysis of a storage cask for 24 spent PWR fuel assemblies

The purpose of this paper is to perform a thermal analysis of a spent fuel storage cask in order to predict the maximum concrete and fuel cladding temperatures. Thermal analyses have been carried out for a storage cask under normal, off-normal and accident conditions. The environmental temperature is assumed to be 27°C under the normal condition. The off-normal condition has an environmental temperature of 40°C. An additional off-normal condition is considered as a partial blockage of the air inlet ducts. Four of the eight inlet ducts are assumed to be completely blocked. The accident condition is defined as a 100% blockage of air inlet ducts. The storage cask is designed to store 24 PWR spent fuel assemblies with a burn-up of 55,000 MWD/MTU and a cooling time of 7 years. The decay heat load from the 24 PWR assemblies is 25.2 kW. Thermal analyses of the ventilation system have been carried out for the determination of the optimum duct size and shape. The finite-volume computational fluid dynamics code FLUENT was used for the thermal analysis. From the results of the analysis, the maximum temperatures of the fuel rod and concrete overpack were lower than the allowable values under the normal, off-normal and accident conditions.

Three-Dimensional Numerical Simulation and Performance Study of an Industrial Helical Static Mixer

AbstractIn many branches of processing industries, viscous liquids need to be homogenized in continuous operations. Consequently, fluid mixing plays a critical role in the success or failure of these processes. Static mixers have been utilized over a wide range of applications such as continuous mixing, blending, heat and mass transfer processes, chemical reactions, etc. This paper describes how static mixing processes of single-phase viscous liquids can be simulated numerically, presents the flow pattern through a helical static mixer, and provides useful information that can be extracted from the simulation results. The three-dimensional finite volume computational fluid dynamics code used here solves the Navier-Stokes equations for both laminar and turbulent flow cases. The turbulent flow cases were solved using k-ω model and Reynolds stress model (RSM). The flow properties are calculated and the static mixer performance for different Reynolds numbers (from creeping flows to turbulent flows) is studied. A new parameter is introduced to measure the degree of mixing quantitatively. Furthermore, the results obtained by k-ω and RSM turbulence models and various numerical details of each model are compared. The calculated pressure drop is in good agreement with existing experimental data.

Application of Simultaneous Perturbation Stochastic Approximation Method for Aerodynamic Shape Design Optimization

Aerodynamic shape design optimization problems, such as inverse and constrained airfoil design and axisymmetric nozzle design, are investigated by applying the simultaneous perturbation stochastic approximation (SPSA) method to objective functions that are estimated during each design iteration using a finite volume computational fluid dynamics technique for solving the compressible Navier‐Stokes equations. The SPSA method has been demonstrated in the literature as having significant advantages over stochastic global optimization methods such as the simulated annealing (SA) method. In this work the SPSA is compared with SA method for a class of twodimensional and axisymmetric aerodynamic design optimization problems. The numerical studies show that the SPSA method is robust in reaching optimal aerodynamic shapes, is easy to implement, and is highly efficient. The SPSA method can also decrease the computational costs significantly compared with the SA method.

Control of convergence in a computational fluid dynamics simulation using ANFIS

Adaptive-network-based fuzzy inference system (ANFIS) was used to control convergence of a computational fluid dynamics (CFD) algorithm. Normalized residuals were used to select the relaxation factors on each iteration of the computation. The ratio between the residual of the current iteration and of the previous iteration was used as a control input. ANFIS was designed to keep the tuning index at or near one throughout the fluid dynamics simulation. The finite volume CFD algorithm SIMPLER was used as the platform for this study. Four different benchmark cases were examined.

Three-dimensional model of a complete polymer electrolyte membrane fuel cell – model formulation, validation and parametric studies

A steady-state, three-dimensional model of a complete polymer electrolyte membrane fuel cell (PEMFC), including both the anode and cathode, is formulated and solved using a finite volume computational fluid dynamics (CFD) code, Fuel3D, developed at Loughborough University. The model is first validated against data obtained from the literature on a global basis. It is further validated on a local basis using experimental data obtained from a segmented cell. Excellent agreement is obtained. The validated model is then used to study the effect of electro-osmotic drag and diffusion of water across the membrane. Overall transport of water across the membrane is seen to take place from the anode to the cathode side. Finally, the model has been used to carry out some parametric studies, such as variation of electrode thickness, shoulder width, degree of permeability and oxidant concentration, to provide a clearer understanding on how changes in parameters affect the cell performance.

유체-고체 상호작용 (FSI)기법을 이용한 이엽기계식 인공심장판막을 지나는 혈액유동과 판첨거동에 관한 수치해석적 연구

Bileaflet mechanical valves have the complications such as hemolysis and thromboembolism, leaflet damage, and leaflet break. These complications are related with the fluid velocity and shear stress characteristics of mechanical heart valves. The first aim of the current study is to introduce fluid-structure interaction method for calculation of unsteady and three-dimensional blood flow through bileaflet valve and leaflet behavior interacted with its flow, and to overcome the shortness of the previous studies, where the leaflet motion has been ignored or simplified, by using FSI method. A finite volume computational fluid dynamics code and a finite element structure dynamics code have been used concurrently to solve the flow and structure equations, respectively, to investigate the interaction between the blood flow and leaflet. As a result, it is observed that the leaflet is closing very slowly at the first stage of processing but it goes too fast at the last stage. And the results noted that the low pressure is formed behind leaflet to make the cavitation because of closing velocity three times faster than opening velocity. Also it is observed some fluttering phenomenon when the leaflet is completely opened. And the rebounce phenomenon due to the sudden pressure change of before and after the leaflet just before closing completely. The some of time-delay is presented between the inversion point of ventricle and aorta pressure and closing point of leaflet. The shear stress is bigger and the time of exposure is longer when the flow rate is maximum. So it is concluded that the distribution of shear stress at complete opening stage has big effect on the blood damage, and that the low-pressure region appeared behind leaflet at complete closing stage has also effect on the blood damage.

A new method of modeling the conditional scalar dissipation rate

A new method for calculating the conditional scalar dissipation rate 〈N|η〉 is derived from the probability density function (pdf) transport equation for the conserved scalar Z. Two different formulations are obtained. One is the result of direct integration of the pdf transport equation and the second is further developed assuming a two-parameter presumed form for the pdf. A linear model is used for the conditional velocity. The model is compared with a direct numerical simulation (DNS) of inhomogeneous turbulent mixing. The results are in very good agreement with the DNS and perform better than Girimaji’s model which is based on homogeneous flow properties. Further validation with some experimental data would be useful. The new method has also the potential of being easily implemented in a finite-volume computational fluid dynamics code.