Recent Volume
Vol13 , No 5 (in progress)
Effect of Guide Vane Pitch Circle Diameter on Pressure Fluctuation in Vaneless Zone of Francis Turbine
Pages : 1349-1369
Authors : zijuan wu,  Wuke Liang,  W. Dong, 
In a Francis turbine, the guide vanes are arranged in the direction of flow behind the stay vanes. And as it is generally believed that the outlet angle of water after it flows through the spiral case and stay vanes is fixed, the flow and flow circulation are altered by changing the opening of the guide vanes so as to change the output of the turbine. The rotor–stator interaction effect induced by the interaction between the guide vanes and the runner of the Francis turbine was one of the main causes of the pressure fluctuation. The effect of guide vanes placement on pressure fluctuation in vaneless zone of Francis turbine was studied. In this study, the commercial software ANSYS CFX16.0 was used for the three-dimensional numerical simulation of the whole flow passage of a Francis turbine model in a power station. The turbulence model used in the calculation was the shear stress transport (SST) model. The independence between the total number of computational meshes and the timestep was verified to ensure the reliability of the calculation results. Five schemes with different diameters of the guide vanes distribution circle were proposed including D0/D1 (guide vane pitch circle diameter /diameter of runner inlet) equaling to 1.119, 1.128, 1.138, 1.144, and 1.15. The steady calculation results showed that, when the turbine was operating under the design condition, D0/D1 increased from 1.119 to 1.15, and the turbine efficiency and output showed a monotonically increasing trend, with the efficiency increased by 0.17 percentage point and output increased by 3.91kw. Twenty monitoring points were set up in the vaneless zone between the guide vanes and the runner to collect pressure fluctuation signals in the vaneless zone. By analyzing the characteristics of unsteady pressure fluctuations in the vaneless zone under design conditions of the five schemes, the optimal position of the guide vanes was determined. The numerical results showed that the pressure fluctuation amplitude at monitoring points in the same axial direction increased gradually from the top cover to the bottom ring. When the unit operated under the design conditions, by increasing the guide vane pitch circle diameter, the rotor–stator interaction between the guide vanes and the runner domain was weakened, and the pressure fluctuation amplitude in the vaneless zone between the guide vanes and the runner was reduced, thereby the stability of unit operation was improved.
   PDF XML

Interfacial Instability of Thixotropic Fluids in Triple-Layered Channel Flow
Pages : 1363-1375
Authors : Morteza Safarifard,  Mohammad Pourjafar,  Keyvan Sadeghy, 
Interfacial stability of purely-viscous fluids is numerically investigated in channel flow. It is assumed that the main fluid (i.e., the fluid flowing through the center of the channel) is thixotropic and obeys the Moore model as its constitutive equation while the fluids flowing above and below this central (core) layer are assumed to be the same Newtonian fluids with the same thickness. Having found an analytical solution for the base-flow in all three layers, a temporal, normal-mode, linear stability analysis is employed to investigate the vulnerability of the base flow to small perturbations. An eigenvalue problem is obtained this way which is solved numerically using the pseudo-spectral collocation method. The main objective of the work is to explore the role played by the time-constant introduced through the core fluid’s thixotropic behavior on the critical Reynolds number. It is found that the thixotropic behavior of the core fluid has a stabilizing effect on the interface. An increase in the viscosity of the upper/lower Newtonian fluids is predicted to have a stabilizing or destabilizing effect on the interface depending on the parameter values of the Moore model (e.g., the ratio of the zero-shear viscosity to infinite-shear viscosity in this fluid model).
   PDF XML

Numerical Study on the Anti-Snow Performance of Deflectors on a High-Speed Train Bogie Frame
Pages : 1377-1389
Authors : G. Gao,  Qianru Chen,  J. Zhang,  Y. Zhang,  Zhen Tian,  Chen Jiang, 
In this paper, a new installation of flat plate deflector which attached on the bottom of the bogie frame is proposed and its anti-snow accumulation performance with different attack angles is numerically studied. The wind-snow two-phase flow in the bogie region is simulated based on the Reynolds Averaged Navier-Stokes (RANS) equations combined with the Realizable "k-ε" turbulence model and the Lagrangian particle phase method. The adopted numerical simulation methodology is verified and validated by comparing with previous wind tunnel tests. In this paper, three typical attack angles (30°, 60°, 90°) for deflector are studied. The results show that: the 30° case has a medium influence on the flow field and reduces snow accumulation by 35.14%; the 60° case guides the high-speed airflow downward and has the best effect with 62.46% reduction in snow accumulation; the 90° case has the smallest reduction with 20.30% in the mass. Overall, all deflectors with three different installation angles can reduce the mass of snow accumulated on the bogie surface.
   PDF XML

Sloshing Analysis of a Heterogeneous Viscous Liquid in Immovable Tank under Pitching Excitation
Pages : 1391-1405
Authors : Jalal El Bahaoui,  hilal essaouini,  larbi elbakkali, 
The effects of the heterogeneity of liquid on the tank sloshing under pitching excitation are analyzed and discussed. The time history of the free surface elevation for tank containing a homogeneous – heterogeneous liquid are recorded and discussed. Numerical simulations are performed for various functions of density using the finite-element method. A theoretical model in the case of heterogeneous viscous liquid are developed using the variation formulation based on the Navier-Stokes equations. The effect of viscosity on the responses is also discussed for each case. In each case, the time history plots for the vertical fluid displacement at a select node, and the pressure in selected elements are presented to illustrate the results of numerical simulations. The effect of heterogeneity parameter of the amplitude of liquid sloshing in a two dimensional partially filled rectangular tank under pitch excitation is conducted to investigate the effects of excitation variable density on the liquid sloshing by a series of numerical experiments. The results are compared with existing theoretical study and the comparison shows fair agreement.
   PDF XML

Numerical Investigation of the Effects of Leakage Flow From Guide Vanes of Francis Turbines using Alternative Clearance Gap Method
Pages : 1407-1419
Authors : Saroj Gautam,  Hari Prasad Neopane,  Biraj Singh Thapa,  Sailesh Chitrakar,  Baoshan Zhu, 
Flow around the Guide vanes (GV) in Francis turbine differs with the shape of hydrofoils. The difference in the pressure of fluid travelling to pressure side and suction side of GV contributes to flow behavior. This study presents the numerical technique using alternative clearance gap method to predict the flow around GV and its consequent effect on turbine performance. GV profile has a significant effect on the performance of the turbine with sediment contained fluid flow. In this paper, symmetrical NACA 0012 and cambered NACA 2412, NACA 4412 hydrofoils are studied introducing 0 mm, 2 mm, and 4 mm clearance gaps. Vortex filament can be seen when fluid leaves the clearance gap due to the leakage flow occurring through the gap. The intensity of vortex leaving clearance gap rises with an increase in the size of the clearance gap. However, in the case of asymmetrical GV profile, the velocity of fluid travelling along the vortex compared to that of symmetrical hydrofoil is lower. In case of low specific speed Francis turbines, this vortex is found to be a major reason to erode the runner surface due to high velocity of a sand particle travelling with them. With the alternative clearance gap approach, this paper compares the pressure pulsation downstream of GVs contributed by leakage flow for three NACA profiles, whose frequency is half of blade passing frequency.
   PDF XML

Kelvin Waves Structure Analysis of a Horizontal Axis Wind Turbine Tip Vortices
Pages : 1421-1435
Authors : Mohamed Mehdi Oueslati,  A. W. Dahmouni,  S. Ben Nasrallah, 
The optimization of the wind energy conversion is one of the most important domains which was widely interested researchers. The instabilities in the wind turbine wake are one of the sources of energy loss which strongly influenced the helical tube vortex structure and are generally difficult to be quantified using experimental facilities. This paper presents a numerical investigation on the wake downstream of a horizontal axis wind turbine (HAWT) model using the Fluent software. Results were validated using experimental measurements conducted in the CRTEn wind tunnel. The Kelvin wave’s theory was, also, used to analyze the deformations acting on the tip vortices. The cartography of the velocity gradient tensor components of the first tip vortex and the different families of Kelvin wave’s were studied and classified according to the azimuth wavenumber. The obtained results confirm that the tip vortices meandering correspond to the helical mode of Kelvin wave’s and the stretching-compression phenomenon is the most important deformation acting on the tip vortex tubes during the development of HAWT wake.
   PDF XML

Design of Crescent Splitting Electrodes in EWOD Device
Pages : 1437-1442
Authors : Zhaolong Wang,  Liguo Chen,  Xiongheng Bian, 
High control voltage and low success rate limit the application of droplet cutting on digital microfluidic chip, hence, the traditional square electrode was designed to crescent electrode to solve these problems in this paper. First, the relationship between the EWOD tension of micro-droplet and the chord length of effective Triple Contact Line (TCL) was analyzed based on the theory of electrowetting-on-dielectric. Then, the droplet cutting processes of different electrodes were numerically simulated and the results were analyzed. Finally, the effect of droplet cutting on four kinds of chips were tested. The results revel that the crescent electrode can decrease the applied voltage for droplet cutting and the minimum voltage required for cutting on crescent electrode (A=1.41) was at least 13.9% lower than that of square electrode. In addition, the success rates of droplet cutting on crescent electrode at different channel heights are higher than that of square electrode.
   PDF XML

Numerical Simulation of Aerodynamics of Train with Broken Windows
Pages : 1443-1452
Authors : Jian Du,  Mingzhi Yang,  Lei Zhang,  F. Wu,  Wei Huang, 
Window glass of the train was broken several times when running in the strong wind/sandy areas, causing safety risks to passengers and serious problems to the operation of the train. The aerodynamic performances of the train with broken windows in strong wind condition are uncertain. These problems remain the challenging research issues. To study these issues, the influence of the broken windows on the aerodynamic performances of the train model was analyzed using three-dimensional numerical simulation methods. The results showed that the aerodynamic forces on the second passenger car first decreased and then increased within a very short period when the two middle windows on the windward side had been broken. However, the side force and the overturning moment increased sharply when the wind angle was increasing. In addition, the number of broken window glass has significant effects on train aerodynamics when running in cross wind area, and the absolute value of the side force and of the overturning moment increased significantly with the increase in the number of broken windows on windward side.
   PDF XML

An OpenFOAM-Based Evaluation of PANS Methodology in Conjunction with Non-Linear Eddy Viscosity: Flow Past a Heated Cylinder
Pages : 1453-1469
Authors : S. Saroha,  Krishnendu Chakraborty,  S. S. Sinha,  S. Lakshmipathy, 
We evaluate the partially-averaged Navier-Stokes (PANS) methodology of turbulence computations by including non-linear eddy viscosity based closures for both turbulent stresses and thermal flux. We extract the filtered PANS version of the Shih’s quadratic model (originally proposed for the Reynolds averaged Navier-Stokes (RANS) paradigm) for arriving at a PANS closure model for the turbulent stress tensor. The unclosed thermal flux process is modeled using the gradient diffusion hypothesis, wherein we sensitize the coefficient of diffusion to the presence of non-linear stresses in the formulation. The resulting methodology is evaluated by simulating flow past a heated square cylinder. Evaluations are performed in terms of both hydrodynamic variables and heat transfer characteristics. We find that the non-linear PANS methodology shows improved results in terms of hydrodynamic quantities (coefficient of drag, pressure, velocity profiles, and high-order statistics). While the predictions of the heat transfer rate on the front face of the cylinder are similar in the linear and the non-linear PANS methodologies, in the wake region and parts of the lateral wall where shear layer detachment takes place, the non-linear PANS methodology shows improved results.
   PDF XML

Large Eddy Simulation of a Turbulent Flow over Circular and Mixed Staggered Tubes’ Cluster
Pages : 1471-1486
Authors : Mohammed Bedrouni,  Ali Khelil,  M. Braikia,  H. Naji, 
A large eddy simulation study was conducted to investigate the turbulent dynamic structure of a fluid flow in two staggered tube configurations, one is composed of all circular cylinders and the second is composed of circular and square cylinders. The present LES, based on the wall-adapting local eddy viscosity model, has been conducted for ReD = 12,858, which match available experiments. The appropriate grid has been selected using the grid convergence index method so that the wall-normal coordinate value is relevant for walls. Streamlines, turbulence kinetic energy contours, instantaneous vorticity contours computed indicate that wake patterns are more chaotic. In addition, flow coherent eddies within both configurations are identified via the -criterion. Based on the obtained findings, it can be stated that the model considered, in addition to being physically sound, demonstrated to be suitable for simulating the turbulent flow over circular and mixed staggered tube bundles with higher resolution.
   PDF XML

A 2D Image-Based Approach for CFD Validation of Liquid Mixing in a Free-Surface Condition
Pages : 1487-1500
Authors : Paola Elizabeth Rodríguez-Ocampo,  Michael Ring,  Jassiel Vladimir Hernández-Fontes,  Juan Alcerreca-Huerta,  Edgar Mendoza,  Gabriel G. Diez-Barroso,  Rodolfo Silva, 
This study proposes an image-based approach to evaluate the validity of numerical results for cases where the setup can be assumed to be two-dimensional (2D) and mixing between liquids of different densities occurs under a free-surface condition. The proposed methodology is based on the estimation of the relative errors of the model through density matrices generated from images of the experimental and numerical results (i.e., post-processing snapshots of the density field). To demonstrate the use of the methodology, experimental tests and numerical simulations were performed for a double-dam-break problem with two miscible liquids. For the experiments, a high-speed camera was employed to capture details of the fluid interactions after the dam breaking. For the numerical simulations, an OpenFOAM® multiphase solver was employed to reproduce the benchmarking tests. Three turbulence approaches were tested: a zero-equation RANS model, a two-equation (k-epsilon) RANS model, and a Large-Eddy Simulation (LES) model. The experimental results compared favorably against the numerical results, with averaged relative errors of ~17 and ~19 % for the zero-equation and the two-equation turbulence models, respectively, and ~14 % for the LES model. From the results obtained, it can be inferred that the two-equation (k-epsilon) model had limitations in reproducing the mixing between the liquid phases in terms of relative errors. The LES model reproduces the mixing between phases more accurately than zero and two-equation RANS models, which were seen to be more suitable for capturing the formation of large eddies in the initial phase of the experiment. The present methodology can be improved and extended for different multiphase flow configurations.
   PDF XML

Enhancement of Liquid-Solid Two-Phase Flow Through a Vertical Swirling Pipe
Pages : 1501-1513
Authors : Jie YIN,  Qunyan CHEN,  Rui ZHU,  Wenxian TANG,  Shijie SU,  Fei YAN,  Lihui WANG, 
In order to improve the transportation efficiency and safety of the vertical hydraulic transport pipe, a new type of pipeline transport system with helical blade is proposed in this paper. Based on CFD-DEM coupling method, the liquid-solid two-phase flow characteristics are analyzed for the swirling pipes and no blade pipe. The study focuses on the effect of the different helix angles of helical blade pipes in terms of the distributions of fluid velocity, the fluid vorticity, the total pressure, the particle’s local concentration, the drag force and kinetic energy of particles. Subsequently, the transport efficiency is measured based on the starting speed of particles and the particle concentration, and the safety of the particle transportation is evaluated based on the flow structure and the kinetic energy of particles. It is found that the tangential velocities of the swirling pipes are clearly larger than that of the case of no blade pipe, and the swirling number decreases as the increasing of helix angle of helical blades within swirling pipe. As the decreasing of helix angle, the vorticity magnitude increases sequentially, and the vortex core structure of the flow field is gradually enriched. Meanwhile, the total pressures for the swirling pipes decrease rapidly after the fluid enters the helical blades region, reflecting the difference energy efficiency of the swirling pipes. Furthermore, the swirling pipe accelerates the starting speed of the particle, and then increases the particle concentration in the pipe while making the particle spatial flow structure better and the particle kinetic energy larger. In general, the swirling pipe makes the particle transportation more efficient and safe.
   PDF XML

Axial Gap Studies on the Flow Behavior and Performance of a Counter Rotating Turbine
Pages : 1515-1525
Authors : Subbarao Rayapati,  Govardhan M., 
A Counter Rotating Turbine (CRT) is an axial flow turbine with a nozzle followed by two rotors that rotate in the opposite direction of each other. Axial gap is an important parameter that affects the performance of turbine stage. Current work contains computationally analyzing the flow physics and performance of CRT with the axial gaps of 15, 30, 50 and 70% of the mean axial chord. Turbine components nozzle and the two rotors are modeled for all the axial gaps of CRT. At nozzle inlet, total pressure is taken as boundary condition and at rotor 2 outlet, mass flow rate is specified. Total pressure, entropy and TKE contours plotted at the inlet and outlet of the blade rows are utilized to analyze the effect of axial gap. Mass flow average distributions of entropy, TKE and relative stagnation pressure loss drawn at rotor 1 and rotor 2 outlets estimate the changes in flow losses with respect to axial gap. The intermediate axial gap of x/a = 0.3 is found to be beneficial for CRT for most of the flow rates. Also, it is found that the smallest and the largest gap cases are showing comparable performance. Thus, results confirm the influence of axial gap on the flow behavior and performance of CRT.
   PDF XML

Flow of a Thixotropic Bingham Fluid over a Cylinder in Stationary and Non-Stationary Regimes
Pages : 1527-1538
Authors : Cuong Bui,  Thinh Ho, 
Flow of a thixotropic Bingham liquid over a cylinder is investigated in this work. Thixotropy is a rheological characteristic associated with the microstructure of the material, which can be broken and recovered during a flowing process. Various flow characteristics such as the flow morphology, the unyielded/yielded zones, the microstructural state, and the hydrodynamic forces acting on the cylinder are analyzed at Re = 20–100 and Bn = 0.5–10. Comparison with Newtonian and non-thixotropic Bingham fluids are also carried out and discussed. Results show that, within these conditions, the flow spans from a stationary regime with or without separation to a non-stationary laminar one with von-Karman-type vortex shedding. In addition, no near-field unyielded zones are observed at Re = 100 and Bn = 0.5 and 1, probably due to the unsteady nature of the flow. At conditions where static unyielded zones exist, the wall shear stress normalized by the yield stress is minimum within these structures.
   PDF XML

Nanofluid Flow Comprising Gyrotactic Microorganisms through a Porous Medium
Pages : 1539-1549
Authors : Sohail Ahmad,  Muhammad Ashraf,  Ali Kashif, 
Researchers have significantly contributed to heat transfer field and always made out much effort to find new solutions of heat transfer augmentation. Among numerous methods which have been employed to reinforce the thermal efficiency of energy systems, one is the dispersal of gyrotactic microorganisms in commonly used nanofluids. Another way to improve the thermal efficiency is the utilization of the porous media. The present work deals with the study of nanofluid flow comprising gyrotactic microorganisms with allowance for chemical reaction through a porous medium past a stretching sheet. The nonlinear coupled ODEs are obtained after applying the persuasive tool of similarity transformation on governing model PDEs and then undertook numerically by exploiting the SOR (Successive over Relaxation) parameter method. The outcomes of assorted parameters for the flow are surveyed and discussed through graphs and tables. A comparison is correlated with the previously accomplished study and examined to be in an exceptional agreement. The culminations designate that the bioconvection Peclet number and the microorganisms concentration enhance the density of the motile microorganisms. The chemical reaction phenomenon downturns the concentration and enhances the mass transfer rate on sheet surface. The insertion of the gyrotactic microorganisms in the suspensions is widely used in the bio-microsystems. Examples include biotechnology (in order to enhance the transport phenomenon of heat and mass), enzyme biosensor and microfluidics devices like microvolumes and bacteria powered micromixers. The gyrotactic microorganisms also improve nanofluid stability. Microbial-enhanced oil recovery is also application of bioconvection phenomena where nutrients and microorganisms are inserted in oil bearing layer to maintain the variation in permeability. Our results may also be beneficial in improving the proficiency of microbial fuel cells and heat transfer devices.
   PDF XML

Numerical Analysis of the Effluent Dispersion in Rivers with Different Longitudinal Diffusion Coefficients
Pages : 1551-1559
Authors : Almir Guilherme Siqueira Lopes Ritta,  Thiago Roberto Almeida,  Julio Tomás Aquije Chacaltana,  Roger Moreira, 
The knowledge of pollutants dispersion in water bodies is a matter of concern in water quality control, especially when a new industrial development is installed e.g. near riverbanks. To predict pollutants dispersion in rivers, analytical, experimental and in-situ measurement can be performed. However, analytical estimation usually results in low accuracy, while experimental or in situ measurement are quite expensive in time and equipment. Hence, Computational Fluid Dynamics (CFD) approach is other alternative that can be used to obtain simple and accurate results for mass transport in rivers. In other words, it is a good alternative to analyse pollutants dispersion. As it is known, longitudinal diffusion coefficient (E) has strong influence on pollutants spreading into the water body. Therefore, the purpose of this paper is to analyse the effects of E on the mass transport of a conservative pollutant in rivers and channels via CFD. Contaminant dispersion is carried out by a scalar advection-diffusion transport equation that represents the conservation of mass. The velocity and pressure fields are calculated, considering an incompressible fluid, through the Navier-Stokes and the continuity equations. Numerical and analytical results, for one-dimensional (1D) flow, are compared in order to obtain the concentration field, over time and space, using different parametric equations. The concentration field showed significant differences of concentration peak and arrival time of the plume depending on the equation used to predict E. Numerical results, for two-dimensional (2D) flow, are compared with the experimental data from Modenesi et al. (2004). Such analyses are necessary to establish an appropriate correlation between simulated and real channel. The use of different parametric equations for the E in a 2D channel reveals significant differences of concentration peak and arrival time of the plume. As expected, the numerical results of the transport of pollutants show the dependence on the parameterization of the longitudinal dispersion coefficient. The one that best represents the distribution of pollutants is that proposed by Kashfipour & Falconer.
   PDF XML

RANS Study of Flows across an Abrupt Change in Surface Roughness
Pages : 1561-1573
Authors : Wenye Li,  Chun-Ho Liu,  Wai Chi Cheng, 
Flows across an abrupt change in surface roughness lead to the development of an internal boundary layer (IBL). In this paper, the effect of surface discontinuity on the structure of flow and turbulence is unveiled by the Reynolds-averaged Navier-Stokes (RANS) turbulence model. Three configurations of smooth-to-rough transition, which are fabricated by sinusoidal wavy surfaces, are examined to contrast the flow adjustment. After the change in (increasing) surface roughness, the flows decelerate and the downward momentum flux ( ) increases to overcome the increasing drag. The changes in friction velocity (uτ,2/uτ,1) and roughness length (z0,2/z0,1) follow the conventional power law. The developments of roughness sublayer (RSL) and inertial sublayer (ISL), which characterize the flows adjustment, are clearly observed. The flow structure after the roughness transition is also defined quantitatively, through which the interaction among IBL, RSL and ISL is elucidated. The growth of IBL and ISL signifies that the influence from the upstream (smoother) surface is being weakened while the flows are developing in equilibrium with the downstream (rougher) surface. Finally, the winds over complex terrain (Hong Kong Island) are modelled to demonstrate the sea-land effect on atmospheric flows. The results show that the flow dynamics and structure over natural topography are consistent with those over idealised surfaces.
   PDF XML

Numerical Study of the Hydrodynamic Pressure Field Generated due to Ship Motion at Different Speeds
Pages : 1575-1586
Authors : Amir Nasseroleslami,  Ali sarreshtehdari,  mahmoud salari, 
Ship movement in the shallow seas creates a significant hydrodynamic pressure field about the ship that has effect on the environmental structures such as waterway beds, stationary or moving neighbored vessels, and can also affect marine life. Therefore, the study of this phenomenon is very important in many applications. The present study investigated the hydrodynamic pressure field caused by an oil tanker with 247 m long, 53 m wide and 17 m draft moving at different speeds of 10, 15 and 20 knots on a sea level with a depth of 80 m. The fluid flow governing equations including the continuity equation, the momentum equations, and the K-ε turbulence model are solved numerically and the SIMPLE algorithm is used to correlate the pressure and velocity fields. An accurate Trimmer's structured mesh has been utilized to discrete the studied domain around the ship. To validate the methodology, the obtained dimensionless velocity field is compared with those presented by other works a good consistency is observed. As expected, the magnitude of the hydrodynamic pressure field varied as a function of the distance to the body of the vessel, ship's traveling velocity and magnitude of the draft. In this study, the minimum effects of the pressure were for the case of 10 knots (the minimum working velocity of heavy vessels) and 80 m of depth with a maximum pressure of 980 Pa. The results show that the pressure field dissipation occurs more rapidly in close distances to the vessel, and the pressure field domain decreases with a lower slope in far away from of the body. a hydrodynamic pressure correlation is obtained based on the depth and ship's velocity. Two and three-dimensional hydrodynamic pressure contours are also presented for different depths and velocities. Moreover, he hydrodynamic pressure increments in 12 and 7 m drafts are investigated and that shows after the 3/4 height of the bulbous bow lies below the sea surface, the increase in draft has little effect on the hydrodynamic pressure field.
   PDF XML

Numerical Investigation into the Effects of Tip Clearance on the Performance of a Counter-Rotating Axial Flow Compressor
Pages : 1587-1599
Authors : Tian Liang,  B. Liu,  Stephen Spence,  Xiaochen Mao,  Hao Cheng, 
The impact of varying the tip clearance of each rotor on the performance of a counter-rotating axial compressor has been investigated based on numerical simulations. The main purpose was to investigate the sensitivity to the tip clearance of each of the two individual rotors and the corresponding aerodynamic mechanisms associated with the performance variation in this compressor. The results indicated that both the total pressure ratio and the efficiency decreased as the tip clearance was increased, and the sensitivity curve for peak efficiency for both rotors was found to be an approximately linear negative relationship with increasing tip clearance. The variations of peak efficiency and stability margin of Rotor 2 were more sensitive to changing tip clearance than Rotor 1. An optimum combination of tip gaps existed for this compressor, i.e. 0.5τ for Rotor 1 and 0.25τ for Rotor 2 (where τ represents the nominal tip clearance value). At this optimum configuration, the peak efficiency and stability margin were improved by 0.63% and 29.4%, respectively. The location of the onset of the tip leakage vortex was found to be shifted downstream when the tip clearance increased. The nature of the tip leakage flow for each rotor was found to be influenced by the variation of tip clearance in the other rotor. Rotor 2 showed a more significant impact on Rotor 1. Additionally, varying the combination of tip clearances changed which of the two rotors was the first to stall.
   PDF XML

Numerical Simulation of Supersonic Flow through Scramjet Intake with Concavity in Cowl Surface
Pages : 1601-1610
Authors : Senthilkumar Paramasivam,  T.M. Muruganandam, 
Scramjet intake usually employs shock waves to reduce the flow velocity and increases the static pressure of the flow. However, this causes flow separation and multiple reflections of shock waves, which result in total pressure loss for the flow. This paper discusses the performance enhancement of scramjet intake through the implementation of a concavity along the cowl surface. The baseline intake model used here is the same as that reported in Emami et al. (1995) Two models with the concavities of depth 0.05 and 0.1 inches on cowl inner surface are numerically simulated at Mach number 4.03, and compared with the base model. An improvement in the performance is investigated in terms of total pressure and flow separation. Present study shows that a concavity on cowl surface reduces the flow separation on the ramp wall and increases the total pressure when compared to the base case. This is achieved by expansion fans produced at the beginning of the concavity. These expansion fans weaken the cowl lip shock and suppress the separation size. Further, it turns the shock waves along the flow, decreasing the number of shock wave reflections in the isolator. Thus, increase in total pressure at the exit of the isolator is observed. It is found that there is a marginal increase in Mach number for both the concavity cases without any change in mass flow rate. There was a minor flow distortion observed, which may be corrected by changing the isolator length. This study demonstrates the scope of overall improvement in scramjet engine performance by implementing concavity along the cowl surface.
   PDF XML

A Semi-Coupled Model for Morphological Flow Simulation in River Bend
Pages : 1611-1622
Authors : Kaushik Bora,  Hriday Mani Kalita, 
This paper presents a new efficient and robust numerical model for morphological flow simulation in river bends. The hydrodynamic model is developed by solving the two dimensional (2D) shallow water equations using a total variation diminishing (TVD) MacCormack predictor corrector scheme. The present TVD method is very simple and provides accurate results free from numerical oscillations near sharp gradient. The effective stresses are modeled by using a constant eddy viscosity model. The sediment transport model solves the Exner equation using a simple forward time and central space (FTCS) finite difference algorithm. The sediment transport model incorporates the helical flow and the transverse bed slope effects on the sediment direction computation. These models are coupled using the semicoupled approach. The present semicoupled model is used to replicate two popular experimental test cases of both tight and loose channel bends. The obtained results in terms of bed level variation reveal that the model can accurately simulate several features of the bed changes such as oscillations of the transverse bed profile with the formation of point bars and pools along the banks. The values obtained for three widely used statistical parameters show the applicability of the present model for this type of complex scenarios.
   PDF XML

Comparison Aerodynamic Performance and Power Fluctuation Between Darrieus Straight-Bladed and Gorlov Vertical Axis Wind Turbines
Pages : 1623-1633
Authors : Mahdi Moghimi,  Hadi Motawej, 
All human actions necessitate energy resources. Currently, a major part of our energy requirements is supplied by fossil fuels, which are faced with uncertainties concerning their availability in the forthcoming decades. However, the combustion of fuels results in adverse environmental aftermaths. Energy of wind falls into one of the clean and renewable energy resources. Wind power is generated using horizontal and vertical axis wind turbines (HAWTs & VAWTs). VAWTs operate appropriately under low wind velocity conditions to generate power in small scales. On the other hand, numerous VAWT designs have been presented to enhance their performance in such circumstances. This study is aimed at developing cost-effective aerodynamic calculations models for both Gorlov and Darrieus straight-bladed VAWT types. Thus, DMST (double multiple stream tube) models, which act on the basis of BEM (blade element momentum) theory, have been designed for Gorlov and Darrieus VAWTs. By comparison of the results obtained with those available in the literature, the models developed are validated. Additionally, the performance of the Darrieus-type straight-bladed VAWTs was compared to that of the Gorlov VAWTs. According to the findings, although the peak power coefficient (C_P) decreased slightly for Gorlov VAWTs in comparison with the Darrieus straight-bladed type, the Gorlov rotor showed improved performance in terms of fluctuation and effectiveness based on the torque coefficient curve of helical blades.
   PDF XML

A Numerical Approach for Simulating a High-Speed Train Passing through a Tornado-Like Vortex
Pages : 1635-1648
Authors : Renze Xu,  F. Wu,  Weihua Su,  Jiangfeng Ding,  Dmitri Vainchtein, 
Tornados are one of the most common natural disasters, but their occurrence can be sudden and unpredictable. For trains operating in the areas where tornadoes frequently happen, the operation safety is challenged. Tornado generator was recently proposed as a method of numerical investigation of tornado-like vortex flows. This paper presents a numerical approach for the simulation of train passing through a tornado-like vortex on realistic scale. It is found that the tornado-like vortex causes appearance of localized regions of a negative pressure on the train and transient variations of the aerodynamic loads acting on the train. As a result, the tornado-like vortex causes swings on the lateral force, and subsequently on the rolling moment, which affect the passenger comfort and operation safety of the train. The method presented herein can be further applied to the study of train behavior and real time response while encountering tornadoes of different types and strength, which is significant for evaluating the operation safety of high-speed trains.
   PDF XML

Non-orthogonal Multiple-Relaxation-Time Lattice Boltzmann Simulation of Mixed Convection in Lid-Driven Porous Cavity with an Isothermally Heated Block
Pages : 1649-1662
Authors : ying zhang,  Jin Bao,  Mengjun Yao,  Yuxian Xie,  Yichen Huang,  peisheng li, 
Laminar mixed convection in porous cavity with an isothermally heated block had been investigated numerically by using Non-orthogonal multiple-relaxation time lattice Boltzmann method (MRT-LBM). The effects of six different arrangements of the cold sources on the characteristics of fluid flow and heat transfer had been studied. Another important influencing factor was the direction of lid-driven. We investigated the effects of four different lid-driven directions on fluid flow and heat transfer when the top and bottom walls of the cavity maintained constant cold temperature. The results show that different arrangements of the cold sources produce different numbers of vortices with the Richardson number increases. As for Top-Left, Top-Right and Top-Bottom arrangements, these three arrangements always show high heat teansfer level. Additionally, the right-moving top and bottom walls exhibits best heat transfer characteristic than other three cases when Ri≤1, and the case of top and bottom walls moves in the opposite directions has best heat transfer performance than other three cases when Ri>1. When the cold sources are arranged on the upper wall of the cavity, it shows better heat transfer performance.
   PDF XML

Concentration Modulation Effect on Weakly Nonlinear Thermal Instability in a Rotating Porous Medium
Pages : 1663-1674
Authors : Palle Kiran, 
The present article is to study mass transfer in a rotating porous layer subjected to imposed time-periodic solutal boundaries. A weakly nonlinear analysis is applied to investigate mass transfer in a porous medium. The mass transfer coefficient is calculated by cubic Ginzburg Landau (GLE) amplitude equation. In this article the stationary convection is discussed in the presence of rotating solutal Rayleigh number. The amplitude equation (GLE) is solved numerically to calculate finite temporal convective amplitude. This amplitude is used to find Sherwood number in terms of the various system parameters. The effect of individual parameters on mass transport is discussed in detail in the presence of lower rotational rates. The onset of convection is discussed through the stability curves for stationary and oscillatory solutal critical Rayleigh number as a function of wavenumber. Further, it is found that the mass transfer enhances for modulated system than un-modulated system. Internal solutal number Si is to enhances for higher values and diminishes the mass transfer for lower values. Finally, it is also found that rotation and solutal modulation can be effectively used to enhance or diminish the mass transfer.
   PDF XML